Inverse Modeling of Particulate Organic Carbon Fluxes in the South Atlantic

R. Schlitzer^1*, R. Usbeck¹ and G. Fischer²

¹ Alfred Wegener Institute for Polar and Marine Research, Columbusstrasse, D-27568 Bremerhaven, Germany

² Universität Bremen, Fachbereich Geowissenschaften, Postfach 33 04 40, D-28334 Bremen, Germany

* corresponding author (e-mail): rschlitzer@awi-bremerhaven.de

Abstract: The biological production of particulate material near the ocean surface and its subsequent remineralization during sinking and after deposition on the seafloor strongly affect the distribution of oxygen, dissolved nutrients and carbon in the ocean. Dissolved nutrient distributions therefore reveal the underlying biogeochemical processes, and these data can be used to determine rates of production, remineralization and accumulation with the aid of inverse techniques. Here, an ocean circulation, biogeochemical model that exploits the existing large sets of hydrographic, oxygen, nutrient and carbon data is presented and results for the export production of particulate organic matter, vertical fluxes in the water column, and sedimentation rates are presented. In the model, the integrated export flux of particulate organic carbon (POC) in the South Atlantic amounts to about 1300 Tg C yr-1 (equivalent to 1.3 Gt C yr-1), most of which occurring in the Benguela/Namibia upwelling region and in a zonal band following the course of the Antarctic Circumpolar Current (ACC). Remineralization of POC in the upper water column is intense, and only about 7% of the export reaches a depth of 2000 m. Comparison of modeled particle fluxes with sediment trap data suggests that shallow traps tend to underestimate the downward flux, whereas the deep traps seem to be affected by the lateral input of material and apparently overestimate the vertical flux. These findings are consistent with recent radionuclide studies. The rapid degradation of POC with depth produces geographical patterns of POC fluxes to the seafloor and POC accumulation in the sediment that are very different from the pattern of surface productivity, because of the modulation with varying bottom depth. Whereas there is significant surface production in deep-water, open-ocean regions, the benthic fluxes occur predominantly in coastal and shelf areas.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 1-19

Transfer of Particles into the Deep Atlantic and the Global Ocean: Control of Nutrient Supply and Ballast Production

G. Fischer^1*, G. Wefer¹, O. Romero¹, N. Dittert², V. Ratmeyer¹ and B. Donner¹

¹ Universität Bremen, Fachbereich Geowissenschaften, Klagenfurter Strasse, 28359 Bremen, Germany

² Institute Universitaire Européen de la Mer, Technopôle Brest-Iroise, Place Nicolas Copernic, F-29280 Plouzané, France

* corresponding author (e-mail): gerhard.fischer@rcom-bremen.de

Abstract: Particle fluxes from 20 trap sites in the Atlantic/Southern Ocean have been compiled to study the regional variations in comparison with important environmental variables. In turn, these results have been compared to other study sites from the world ocean, mainly regarding the relationship between bulk fluxes/various flux ratios to nutrient supply. It is shown that the supply of dissolved silicic acid to the surface waters (the 'silicate pump', Dugdale et al. 1995) plays a central role in opal fluxes, BSi:C_org ratios, BSi:carbonate ratios, and thus carbon rain ratios. The mean annual BSi:C_org ratio (mol/mol) normalized to 1000 m was 0.05 in the Atlantic, 0.4 in the Indian, 0.5 in the Pacific, and 0.1-3 in the Southern Ocean and follows the general path of the conveyor belt (Ragueneau et al. 2000). A shift in the primary producer community from coccolithophorids to diatoms, reflected by an exponential increase of the annual BSi:carbonate flux ratios, occurs above a molar Si:N_(250m) nutrient threshold of about 1.7. The surface sediment opal:carbonate ratios (%) versus the Si:N_(250m) nutrient values produce a threshold of 2-2.5, however, this value may be biased by opal dissolution during early diagenesis. We also tested the most recent findings about particle ballast which presume that carbonate is most important for the rapid downward transport of organic particles to bathypelagic depths. Our compilation of global flux data confirms such a general relationship. However, at certain sites and in particular years/seasons, other minerals may serve as ballast for organic carbon. Off NW Africa, for instance, lithogenic components were the major particle carriers. There, relationships between carbonate/lithogenic/total ballast fluxes versus daily organic carbon fluxes may even vary from year to year. Off Cape Blanc, the carbonate-C_org-relationship is highly significant during a strong coccolithophorid bloom in 1991, probably resulting in an efficient downward transfer of organic carbon. Interannual variation of fluxes was highest in high production systems combined with high seasonality of fluxes. We obtained ca. 20% variability in oligotrophic regions and up to 100% in the Southern Ocean where seasonality is most pronounced.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 21-46

Radionuclides as Tracers for Particle Flux and Transport of Water Masses in the Atlantic Sector of the Southern Ocean

M. Rutgers van der Loeff^1,2, J. Friedrich^2*, W. Geibert², C. Hanfland², H. Höltzen, I. Vöge² and H.J. Walter

¹RIKZ, P.O.Box 20907, 2500 Ex Den Haag, The Netherlands

²Alfred-Wegener-Institut für Polar- und Meeresforschung, Columbusstraße, 27515 Bremerhaven, Germany

* corresponding author (e-mail): jfriedrich@awi-bremerhaven.de

Abstract: The natural uranium decay series provide a suite of tracers to study transport processes in the ocean. We have used nuclides of the particle-reactive elements Th, Pa, Pb and Po for studies of particle flux in the Southern Ocean, whereas isotopes of the elements Ra and Ac served as tracers for the transport of water masses. Here we summarize the specific aspects of the behaviour of these nuclides in the Southern Ocean and give some examples of their application. We review the important influence of exchange between ocean basins by advection and upwelling on the long-lived nuclides. We show how the distribution of 234Th in surface waters across the ACC represents the export production, whereas in the benthic nepheloid layer this tracer is used to illustrate how the resuspension regime in the ACC is linked to the position of the oceanographic fronts.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 47-63

Heterotrophic Particle-Associated Bacteria from the South Atlantic: A Community of Marine Microorganisms with a High Organic Carbon Degradation Potential

I. Berkenheger^*, A. Heuchert, S. de Silva and U. Fischer

Universität Bremen, Fachbereich Biologie/Chemie, Zentrum für Umweltforschung und Umwelttechnologie (UFT), Abt. Marine Mikrobiologie, Leobener Straße, 28359 Bremen, Germany

* corresponding author (e-mail): imke@biotec.uni-bremen.de

Abstract: Samples of the Equatorial Atlantic (EA) and the South Atlantic/Antarctica (SA) were taken during two cruises with RV Meteor in 1996 and 1997 and one cruise with RV Polarstern in 1998 in order to study the bacterial communities attached to organic particles. Ten heterotrophic bacterial strains, isolated from particles of the EA, and 11 strains, isolated from particles of the SA, were chosen for further investigations. All isolates are Gram-negative rods, which differ strongly in their ability to metabolize high and low molecular weight organic compounds (polysaccharides, di- and monosaccharides, organic acids). The phylogenetic composition of the bacterial communities on sinking particles was investigated by random amplified polymorphic DNA (RAPD), amplified ribosomal DNA restriction analysis (ARDRA), fluorescence in situ hybridization (FISH), and 16S rDNAsequencing. In both investigation areas, members of the α- and γ-subclass of Proteobacteria and also of the Cytophaga/Flavobacteria-cluster were detected. The genus Sulfitobacter was present in both investigation areas, whereas other genera such as Marinobacter and Psychrobacter could each be found on only one sampling site.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 65-79

Contribution of Calcareous Plankton Groups to the Carbonate Budget of South Atlantic Surface Sediments

K.-H. Baumann^*, B. Böckel, B. Donner, S. Gerhardt, R. Henrich, A. Vink, A. Volbers, H. Willems and K.A.F. Zonneveld

Universität Bremen, Fachbereich Geowissenschaften, Postfach 330440, D-28334 Bremen, Germany

* corresponding author (e-mail): baumann@uni-bremen.de

Abstract: A total of more than 400 surface sediment samples from the equatorial, central and subpolar South Atlantic Ocean were investigated for their carbonate content as well as for the carbonate contribution from various calcareous plankton groups. The modern pattern of marine carbonate production is exemplified by comparing two sediment traps located in different domains of the South Atlantic. In addition, this paper presents new carbonate calculations for the content of coccoliths, calcareous dinocysts, planktic foraminifera, and pteropods in surface sediments. In general, carbonate input of the different organism groups is highly variable although dominated by both planktic foraminifera and coccolithophorids. Whereas coccolith carbonate dominates the oligotrophic gyres of the South Atlantic, carbonate derived from planktic foraminifera is much more important in more fertile, mesotrophic areas, such as the equatorial divergence zone. In contrast, calcareous dinocysts only supply a minor proportion of calcium carbonate to the sediments. The aragonite content, mainly derived from pteropod shells, is of regional importance at the continental margin of the western South Atlantic. Here, aragonite contents of up to 50 wt-% of the total sediments were measured. Carbonate dissolution has a major effect below the lysocline depth, but also in highly productive areas (supralysoclinal dissolution). Foraminiferal carbonate is much more affected by dissolution than either coccolith or calcareous dinocyst carbonate. Preservation of pteropod shells is restricted to relatively shallow parts of the ocean distant from continental margins, as aragonite is much more susceptible to dissolution than calcite. As a result, the maximum aragonite content is observed at an intermediate depth, i.e. between 2000 to 3000m.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 81-99

Coccolithophorid and Dinoflagellate Synecology in the South and Equatorial Atlantic: Improving the Paleoecological Significance of Phytoplanktonic Microfossils

A. Vink^*, K.-H. Baumann, B. Böckel, O. Esper, H. Kinkel, A. Volbers, H. Willems and K.A.F. Zonneveld

Universität Bremen, Fachbereich Geowissenschaften, Postfach 330 440, D-28334 Bremen, Germany

* corresponding author (e-mail): vink@micropal.uni-bremen.de

Abstract: Individual planktonic microfossil species, or assemblage groups of different species, are often used to, qualitatively and/or quantitatively, reconstruct past (sub)surface-water conditions of the world's oceans and seas. Until now, little information has been available on the surface sediment distribution patterns and paleoenvironmental reconstruction potential of coccolith, calcareous dinoflagellate cyst and organic-walled dinoflagellate cyst assemblages of the South and equatorial Atlantic, especially at the species level. This paper (i) summarizes the distributions of these three phytoplanktonic microfossil groups in numerous Atlantic surface sediments from 20°N–50°S and 30°E–65°W and determines their relationship with the physicochemical and trophic conditions of the overlying (sub)surface-waters, and (ii) determines the synecology of the three phytoplankton groups by carrying out statistical analyses (i.e. detrended and canonical correspondence analyses) on all groups simultaneously. Ecological relationships are additionally strengthened by statistically comparing the distribution patterns of the phytoplankton groups with those of planktonic foraminifera (Pflaumann et al. 1996; Niebler et al. 1998), as the ecological preferences of the latter are much better known. Many of the analyzed phytoplanktonic microfossil species or groups of species in the surface sediments do show restricted distributions which primarily reflect the environmental conditions of the upper water masses above them (e.g. sea-surface temperature, productivity, stratification). The acquired 'reference' data sets are large and diverse enough to allow future development of transfer functions for the reconstruction of past surface-water conditions, and show that there is still an enormous paleoenvironmental reconstruction potential concealed in many fossil coccolith and dinoflagellate cyst assemblages.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 101-120

The South Atlantic Oxygen Isotope Record of Planktic Foraminifera

S. Mulitza^*, B. Donner, G. Fischer, A. Paul, J. Pätzold, C. Rühlemann and M. Segl

Universität Bremen, Fachbereich Geowissenschaften, Klagenfurter Strasse, D-28359 Bremen, Germany

* corresponding author (e-mail): smulitza@uni-bremen.de

Abstract: This paper reviews the recording of oxygen isotope ratios in planktic foraminifera and summarizes recent results of the application of oxygen isotopes in paleoceanographic studies of the South Atlantic. The most important factors controlling the d¹⁸O of planktic foraminifera are temperature, the d¹⁸O and the pH of ambient seawater. Seasonal and vertical calcification weight the mean d¹⁸O of a foraminiferal population towards the hydrographic conditions in the preferred ecological niche. After deposition, the d¹⁸O signal is affected by bioturbation and dissolution. Despite many influence factors, the composition of oxygen isotopes in fossil tests of planktic foraminifera provides important constraints on variations of the surface water hydrography of the South Atlantic and the Southern Ocean throughout the past 20,000 years. During the last glacial maximum, the Polar Front remained close to its modern position or shifted only slightly towards the north. In the tropics, oxygen isotopes indicate only a moderate glacial cooling of 2-3°C. During deglaciation, oxygen isotope ratios in the eastern boundary currents of the subtropical South Atlantic decreased asynchronously relative to those in the eastern North Atlantic, with the highest interhemispheric contrasts during the Younger Dryas and the Heinrich Event 1. This pattern is consistent with a redistribution of heat within the Atlantic Ocean in response to a weakening of the thermohaline circulation. The slowdown of deglacial overturning was associated with a southward displacement of the thermal equator and the Intertropical Convergence.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 121-142

Nitrogen Isotopes in Sinking Particles and Surface Sediments in the Central and Southern Atlantic

M.E. Holmes, G. Lavik, G. Fischer^* and G. Wefer

Universität Bremen, Fachbereich Geowissenschaften, Klagenfurter Strasse, 28359 Bremen, Germany

* corresponding author (e-mail): gerhard.fischer@rcom-bremen.de

Abstract: This manuscript provides an overview of sedimentary nitrogen isotope records in the tropical and southern Atlantic Ocean. Sedimentary δ¹⁵N in most of this region reflects the extent of surface water nitrate depletion. Nitrogen isotopes in sediments from the coastal upwelling regions of the Angola Basin and Benguela region off of Africa ranged from 5 to 12‰ and were negatively correlated with averaged near surface (0 - 50 m) historical nitrate concentrations. Coincidence of low δ¹⁵N in sinking particles (2 - 5‰) with low sea surface temperatures and high fluxes confirm the importance of relative nitrate utilization for the isotopic composition of organic matter in the Benguela during modern times. Off the Brazilian coast, isotope ratios were 5 - 7‰ and showed weak correspondence to surface nitrate concentrations, which are low. The expected relationship between nitrate and δ¹⁵N may be obscured here and in other oligotrophic regions because the δ¹⁵N of the small pool of nitrate can be readily altered by the advection or diffusion of even low levels of nitrate with a different isotopic composition. In the tropical and central South Atlantic, sediment isotopic values were between 6 and 11‰. The lack of any apparent relationship between δ¹⁵N and surface nitrate may be partly due to a paucity of nitrate concentration data, but δ¹⁵N near the equator also may be influenced by nitrogen fixation. In these oligotrophic waters, the input of iron via aeolian transport of African dust may support the fixation of N₂ into organic matter, thereby lowering δ¹⁵N. Higher input of dust to the surface waters north of the equator is hypothesized to be the cause of the southward δ¹⁵N increase in sinking particles revealed by a north-south transect of moored sediment traps. In the southern South Atlantic, relative nitrate utilization is evidently the main control on sedimentary nitrogen isotopes between around 35°S and 50°S, where average near surface nitrate concentrations were strongly correlated with δ¹⁵N. South of the Polar Front, at around 50°S, this relationship is not observable in our data and there is an apparent switch from nitrate-based primary production to production based on ammonium. Sinking particles at the Polar Front are enriched in ¹⁵N in austral winter and show δ¹⁵N minima when fluxes are high, but because of the consistently low relative nitrate utilization in the Southern Ocean, this pattern is likely caused by changes in the plankton community or to increased degradation during times of low flux. δ¹⁵N values throughout the tropical and southern Atlantic are correlated with sediment organic carbon content and also generally mirror primary productivity patterns, with low δ¹⁵N associated with areas of high productivity and vice versa. Sediment trap data indicate that sediments are enriched in ¹⁵N relative to sinking particles by up to 4‰. The offset (-2.0 to 4.3‰) does not vary greatly between the Polar Front, the productive Benguela upwelling area and the oligotrophic tropical Atlantic, in spite of the vastly different environmental conditions prevailing in these three regions.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 143-165

C37-Alkenones as Paleotemperature Tool: Fundamentals Based on Sediment Traps and Surface Sediments from the South Atlantic Ocean

P. J. Müller^* and G. Fischer

Universität Bremen, Fachbereich Geowissenschaften, Postfach 33 04 40, D-28334 Bremen, Germany

* corresponding author (e-mail): pmueller@uni-bremen.de

Abstract: The alkenone paleotemperature method has gained wide acceptance, but questions remain concerning the water depth and seasonality of alkenone production or the temperature calibration of the U^K'₃₇ unsaturation index. In this paper, we summarize alkenone results from the South Atlantic Ocean which were obtained within the scope of the collaborative research project SFB 261 at Bremen University. We present sediment trap time-series from the eastern Equatorial Atlantic, the Northern and Southern Benguela, the Polar Frontal Zone and the Antarctic Zone, and compare the U^K'₃₇ records to concurrent temperature variations in the surface waters (Reynolds and Smith 1994). To convert U^K'₃₇ into temperature, we used the Emiliania huxleyi calibration of Prahl et al. (1988). In addition, we recapitulate surface sediment results and provide an update of the global core-top calibration. Our sediment trap results confirm earlier conclusions deduced from surface sediments that U^K'₃₇ principally reflects mixed-layer temperatures in the eastern South Atlantic. A shallow alkenone source is indicated, for example, by coinciding SST and U^K'₃₇ records, comparable temperature amplitudes and identical flux-weighted SST and U^K'₃₇ values within ±1°C. The sediment traps further reveal that seasonal variations in alkenone production have little effect on the overall U^K'₃₇ signal exported out of the euphotic zone. Canonical spring-autumn blooms as observed in the Northern Benguela and episodic flux events prevailing in filamentous upwelling regions produce average U^K'₃₇ signals not significantly different from the annual mean SST. Additional interannual variations weaken seasonal effects. In the Polar Frontal Region, where the dominant alkenone flux occurred in late winter and spring, the flux-weighted U^K'₃₇ signal was lower by about 1°C compared to the mean SST in the collection period. Only at site BO1 south of the Polar Front, did the U^K'₃₇ time series fail to reproduce the annual SST cycle. Relatively low alkenone temperatures (-0.4° to 0°C) obtained for the productive summer season at this site may be attributed to the calibration, although other factors cannot be ruled out. Altogether, our sediment trap and sediment results suggest that U^K'₃₇ reflects the mean annual temperature of the mixed layer in most regions of the South Atlantic. An exception is the western Argentine Basin, where the sedimentary U^K'₃₇ ratios appear to be biased by offshore and northward redistribution processes. An update of the global core-top calibration (n=518) using annual mean SST data of World Ocean Atlas 1994 yields exactly the same relationship as before (U^K'₃₇ = 0.033 SST + 0.044; Müller et al. 1998). A slightly different equation is obtained using temperature data of World Ocean Atlas 1998 (U^K'₃₇ = 0.032 SST + 0.073) but both relationships yield similar temperature estimates (within 1°C) as the Prahl et al. (1988) calibration. Core-top as well as sediment trap results do not indicate a systematic deviation from linearity at the warm and cold ends of the calibrations. The linear relationships may therefore be used to determine paleotemperatures in the range from 0 to 29°C, with an uncertainty of about ±1°C. They also produce reasonable temperature estimates for periods that predate the first occurrence of E. huxleyi, suggesting that the Gephyrocapsa species contributing alkenones to Quaternary and Pliocene sediments responded similarly to temperature changes.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 167-193

Stable Carbon Isotopic Composition of the C_37:2 Alkenone: A Proxy for CO₂(aq) Concentration in Oceanic Surface Waters?

S. Schulte^1,2,*, A. Benthien^1,3, N. Andersen^1,4, P.J. Müller¹, C. Rühlemann¹ and R.R. Schneider¹

¹Universität Bremen, Fachbereich Geowissenschaften, Postfach 330 440,D-28334 Bremen, Germany

²Institut für Chemie und Biologie des Meeres (ICBM), Universität Oldenburg, Postfach 2503, D-26111 Oldenburg, Germany

³Alfred Wegener Institut für Polar- und Meeresforschung, Postfach 120161, D-27515 Bremerhaven, Germany

⁴ETH Zürich, Geologisches Institut, CH-8092 Zürich, Switzerland

* corresponding author (e-mail): sschulte@uni-bremen.de

Abstract: We tested the applicability of the carbon isotopic composition of C_37:2 alkenones (δ¹³C_37:2) as a proxy for dissolved carbon dioxide CO2(aq) in oceanic surface waters. For this purpose we determined δ¹³C_37:2 in suspended particulate organic matter (POM) and surface sediments from the South Atlantic. In opposite of what would be expected from a diffusive CO2 uptake model for marine algae we observed a positive correlation between 1/[CO2(aq)] and the isotopic fractionation (ε_p) calculated from δ¹³C_37:2. This clearly demonstrates that CO2(aq) is not the primary factor controlling ep at the sites studied. On the other hand we found a negative correlation between ε_p and the phosphate concentration in the surface waters (0-10 m) supporting the assumption of Bidigare et al. (1997) that ε_p is primarily related to nutrient-limited algal growth rather than to [CO2(aq)]. Reconstructing past CO2(aq) levels from δ¹³C_37:2 thus requires additional proxy information in order to correct for the influence of haptophyte growth on the isotopic fractionation. In the eastern Angola Basin, we previously used δ¹⁵N of bulk organic matter as proxy for nutrient-limited growth rates. As an alternative the Sr/Ca ratio of coccoliths has been recently suggested as growth-rate proxy which should be tested in future studies.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 195-211

Late Quaternary Terrigenous Sedimentation in the Western Equatorial Atlantic South American versus African Provenance Discriminated by Magnetic Mineral Analysis

U. Bleil* and T. von Dobeneck

Universität Bremen, Fachbereich Geowissenschaften, Postfach 33 04 40, D-28334 Bremen, Germany

* corresponding author (e-mail): bleil@geomarin.uni-bremen.de

Abstract: Magnetic mineral accumulation at the Ceará Rise has been studied with the aim to discriminate and reconstruct fluvial South American and eolian African terrigenous fluxes to the late Quaternary western Equatorial Atlantic. Seven sediment series recovered along two bathymetric transects were investigated with standard environmental magnetic techniques. Climatically controlled fluctuations in continental detrital discharge and marine biogenic carbonate fluxes strongly modulate the susceptibility records. Their coherent precessional and higher-frequent signal components could be used to establish a high-resolution age framework for these sediments. According to a partial susceptibility analysis, on average 79 % of the susceptibility signal originates from magnetite of different grain size, 13 % from hematite and 8 % from paramagnetic matrix compounds. In terms of absolute concentrations this implies that hematite is almost twenty times more abundant than magnetite, because of its orders of magnitude lower intrinsic susceptibility. The longitudinal gradients of their respective accumulation rates document a delivery from two major sources characterized by largely different magnetite to hematite ratios (about 1:12 versus 1:50). A mixing model of this scenario provided detailed insight into the past variability of the separate magnetic mineral fluxes and their most probable provenance. Overall about 56 % of hematite and 84 % of magnetite were transported in the Amazon fluvial load. Their accumulation is closely related to sea level changes, reaching highest (lowest) rates, when most South American shelf areas fell dry (were flooded) before and after Termination I and II. Hematite and magnetite of African provenance, 44 and 16 %, respectively, follow a distinctly different accumulation pattern with prominent maxima during cold intervals of glacial periods. By statistically linking these trace minerals to total lithogenic fluxes, we find that during the last 200 kyr, on average 79 % of total terrigenous material in the Ceará Rise area originates from South American sources in the Amazon River catchment, while African dust sources contributed 21 %.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 213-236

Integrated Rock Magnetic and Geochemical Quantification of Redoxomorphic Iron Mineral Diagenesis in Late Quaternary Sediments from the Equatorial Atlantic

J.A. Funk^1*, T. von Dobeneck^1,2 and A. Reitz³

¹Universität Bremen, Fachbereich Geowissenschaften, Postfach 33 04 40, D-28334 Bremen, Germany

²Paleomagnetic Laboratory 'Fort Hoofddijk', Faculty of Earth Sciences Utrecht University, Budapestlaan 17, 3584 CD Utrecht, The Netherlands

³Geochemistry Department, Faculty of Earth Sciences Utrecht University, PO Box 80 021, 3508 TA Utrecht, The Netherlands

* corresponding author (e-mail): funk@geomarin.uni-bremen.de

Abstract: Rock magnetic and geochemical data logged by fast, non-destructive X-ray fluorescence and susceptibility half core scanning techniques have been combined to create high-resolution records of redoxomorphic iron mineral diagenesis in suboxic marine sediments. The great potential of this approach and advantage to standard single sample methods is demonstrated on two Late Quaternary sequences from the central Equatorial Atlantic (GeoB 2908-7 and 4317-2). Reductive dissolution of ferric minerals, most prominently magnetite (Fe₃O₄) and hematite (Fe₂O₃), induced by organic carbon degradation is shown to represent a gradual, mineral- and grain-size selective process. Proportionality of Fe, Ti and magnetite concentrations in the unaltered sections lead us to define proxy parameters for magnetite depletion (Fe/Κ_nd) below and precipitation (Κ_nd/Ti) above the modern and numerous fossil redox boundaries, while iron relocation was detected on basis of the Fe/Ti ratio. By calibrating all three ratios internally, we reconstruct and quantify primary deposition and secondary change of both, magnetite and total Fe profiles. Fine-scaled C_org variations (0.1 to 0.6 %) and susceptibility losses (up to 200 · 10-6 SI) show high signal resemblance and appear to be equivalent signatures of cyclic productivity pulses in the study area. Some minor suboxic events are still expressed in the rock magnetic proxy signal, but are not accompanied by residual C_org enrichments.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 237-260

Magnetic Signals in Plio-Pleistocene Sediments of the South Atlantic: Chronostratigraphic Usability and Paleoceanographic Implications

F. Schmieder

Universität Bremen, Fachbereich Geowissenschaften, Postfach 33 04 40, D-28334 Bremen, Germany

e-mail: schmiede@uni-bremen.de

Abstract: The origin of the magnetic signals used to build age models for marine sediments recovered in the framework of the long-term Quaternary South Atlantic research project SFB 261 is twofold. Conventional magnetostratigraphy makes use of well-dated polarity reversals of the Earth's magnetic field recorded in the natural remanent magnetization (NRM) of the sediments. In addition, magnetic cyclostratigraphy has been successfully established as a very efficient dating tool for marine sediment sequences during recent years. In the oligotrophic South Atlantic, confirmation of orbital forcing of magnetic susceptibility records made it possible to establish high-resolution age models, by tuning the respective components to astronomical variations. A set of twelve individually tuned and wellcorrelated Pleistocene magnetic susceptibility records were stacked within the stratigraphic network SUSAS and can now be used as a correlation reference for other cores recovered in this region. The suitability of this target curve for age control is tested against paleomagnetic ages. Pattern correlation is possible for nine of ten selected cores recovered in the oligotrophic South Atlantic between 15°S and 35°S, but seems to be only partly successful for sediments from the Congo Basin. In the Pleistocene sequences, the magnetic age models provide further evidence for the simultaneous deposition of previously reported unusual diatom ooze layers between 23°S and 33°S at approximately 540 – 530 ka, at the end of the Mid-Pleistocene climate transition (MPT). The age models also indicate enhanced carbonate dissolution during the MPT interim state (920 – 640 ka). The concept of tuning magnetic susceptibility records to orbital variations is extended to the late Pliocene and reveals characteristics obviously related to the rearrangement of ocean circulation as Northern Hemisphere glaciation intensified. Enhanced carbonate preservation since approximately 3.0 Ma and the establishment of obliquity-driven dissolution cycles since about 2.5 Ma document increasing influx of North Atlantic Deep Water (NADW) into the subtropical South Atlantic. In a deep core from the Rio Grande Rise area, an abrupt change from red deep-sea clay to carbonaceous sediments is recorded at 2.73 Ma, exactly the time proposed for the major intensification of Northern Hemisphere glaciation.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 261-277

Congo Fan Neogene and Quaternary Sedimentation: Interplay of Riverine and Current Induced Deposition

G. Uenzelmann-Neben^* and H. Miller

Alfred-Wegener-Institut für Polar- und Meeresforschung, Postfach 120161, 27515 Bremerhaven, Germany

* corresponding author (e-mail): uenzel@awi-bremerhaven.de

Abstract: The incorporation of information regarding sedimentation rates and lithology from ODP Leg 175 Sites 1075, 1076 and 1077 into the analysis and interpretation of high-resolution seismic reflec-tion data led to the revision and refinement of a depositional model for the upper Congo Fan area presented earlier by Uenzelmann-Neben (1998). For four time slices since the Eocene (Late Oligocene - Miocene/Pliocene, Pliocene - 600 ky, 600 ky - ~160 ky, ~160 ky - Recent) the main sed-iment contributor to the upper fan was determined. Thus we can say that in the Late Paleogene input of sediments from the north dominated the area by either a south setting current or the Kouilou/Niari River. This situation continued to the period Pliocene - 600 ky when southern sediment sources (the Congo River and upwelling) became dominant with the material being deflected to the north by the Benguela Current. Upwelling as a sediment source on the upper fan became even more important after 600 ky while the main sediment load of the Congo River is guided to the middle and lower fan via the Congo Canyon.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 279-293

Terrigenous Signals in Sediments of the Low Latitude Atlantic – Implications for Environmental Variations during the Late Quarternary: Part I: Organic Carbon

T. Wagner^1,2*, M. Zabel², L. Dupont², J. Holtvoeth² and C.J. Schubert³

¹Woods Hole Oceanographic Institution, Marine Chemistry and Geochemistry Department, USA

² Universität Bremen, Fachbereich Geowissenschaften, Klagenfurter Strasse, 28359 Bremen, Germany

³AWAG, Limnological Research Center, Seestrasse 79, 6047 Kastanienbaum, Switzerland

* corresponding author (e-mail): twagner@whoi.edu

Abstract: The established view of a marine-dominated organic signature of modern and late Quarternay deep ocean sediments is challenged by recently performed organic geochemical, petrolocical, and palynological investigations. This study reviews multidisciplinary concepts that were developed over the last decade in Bremen and have been successfully applied to modern and late Quarternary sediments from the low latitude Atlantic. Relative proportions and compositional variations of terrigenous OM are deduced from macerals (organic particles), freshwater diatoms, phytoliths, pollen grains, lignin signatures, and carbon isotopic compositions of bulk organic matter as well as from higher plant-derived long-chain n-alkanes. For their variety of depositional settings and their close location next to each other the dust-influenced central Equatorial Atlantic and the West-African continental margin are examined. To assess environmental variations during the late Quarternary, terrigenous organic records from the central Atlantic to the low latitude West-African continental margin and the Congo deep-sea fan are discussed with regard to the paleoclimatic evolution of central African dust source areas, continental run off and vegetational changes in the Congo catchment area. Additionally, the influence of degradation processes and/or selective preservation, both on short and long time scales, of non-reactive (mostly terrigenous) organic matter is investigated.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 295-322

Terrigenous Signals in Sediments of the Low-Latitude Atlantic - Indications to Environmental Variations during the Late Quaternary: Part II: Lithogenic Matter

M. Zabel^1*, T. Wagner¹ and P. deMenocal²

¹Universität Bremen, Fachbereich Geowissenschaften, Postfach 330 440, D-28334 Bremen, Germany

²Lamont Doherty Earth Observatory, Columbia University, PO Box 1000, Palisades NY 10964

* corresponding author (e-mail): mzabel@uni-bremen.de

Abstract: The inorganic terrigenous fraction of marine sediments offers a great number of different and well established proxy parameters to investigate the development of Earth's climate. This study presents a synthesis of multidisciplinary investigations which have been applied to late Quaternary sediments recovered from the low-latitude Atlantic during the Bremen Special Reseach Project 261. In the equatorial Atlantic terrigenous matter is supplied by eolian and fluvial pathways. In addition to the dust input from African deserts, the catchment areas of the three major rivers Amazon, Niger and Zaire (Congo) are the dominant sources. Small river systems are of local importance. Terrigenous records from near-continental and open pelagic depositional settings are discussed. The main questions we focused on are a) the control of climate change and b) the identification and timing of rapidly occurring events. Results from the low- latitude Atlantic support the suggestion that both high-latitude and low-latitude forcing influence tropical climate and marine sedimentation. Apparently, the frequency of climate variability in the tropics during the late Quaternary is controlled by the precessional insolation cycle, whereas amplitudes and timing of climate change are mainly determined by the high latitudes in the Northern Hemisphere. Within the phase relationships, however, regional differences arise. Furthermore, there is evidence for climate instability during glacials and interglacials which probably occurred on decadal to centennial time scales.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 323-345

Surface Sediment Bulk Geochemistry and Grain-Size Composition Related to the Oceanic Circulation along the South American Continental Margin in the Southwest Atlantic

M. Frenz^*, R. Höppner, J.-B.W. Stuut, T. Wagner and R. Henrich

Universität Bremen, Fachbereich Geowissenschaften, Klagenfurter Strasse, 28359 Bremen, Germany

* corresponding author (e-mail): mfrenz@uni-bremen.de

Abstract: Surface sediments from the South American continental margin surrounding the Argentine Basin were studied with respect to bulk geochemistry (CaCO₃ and C_org) and grain-size composition (sand/silt/clay relation and terrigenous silt grain-size distribution). The grain-size distributions of the terrigenous silt fraction were unmixed into three end members (EMs), using an end-member modelling algorithm. Three unimodal EMs appear to satisfactorily explain the variations in the data set of the grain-size distributions of terrigenous silt. The EMs are related to sediment supply by rivers, downslope transport, winnowing, dispersal and re-deposition by currents. The bulk geochemical composition was used to trace the distribution of prominent water masses within the vertical profile. The sediments of the eastern South American continental margin are generally divided into a coarse-grained and carbonate-depleted southwestern part, and a finer-grained and carbonate-rich northeastern part. The transition of both environments is located at the position of the Brazil Malvinas Confluence (BMC). The sediments below the confluence mixing zone of the Malvinas and Brazil Currents and its extensions are characterised by high concentrations of organic carbon, low carbonate contents and high proportions of the intermediate grain-size end member. Tracing these properties, the BMC emerges as a distinct north-south striking feature centered at 52- 54°W crossing the continental margin diagonally. Adjacent to this prominent feature in the southwest, the direct detrital sediment discharge of the Rio de la Plata is clearly recognised by a downslope tongue of sand and high proportions of the coarsest EM. A similar coarse grain-size composition extends further south along the continental slope. However, it displays better sorting due to intense winnowing by the vigorous Malvinas Current. Fine-grained sedimentary deposition zones are located at the southwestern deeper part of the Rio Grande Rise and the southern abyssal Brazil Basin, both within the AABW domain. Less conspicuous winnowing/accumulation patterns are indicated north of the La Plata within the NADW level according to the continental margin topography. We demonstrate that combined bulk geochemical and grain-size properties of surface sediments, unmixed with an end-member algorithm, provide a powerful tool to reconstruct the complex interplay of sedimentology and oceanography along a time slice.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 347-373

Terrigenous Sediment Supply in the Polar to Temperate South Atlantic: Land-Ocean Links of Environmental Changes during the Late Quaternary

B. Diekmann^1,2*, D.K. Fütterer¹, H. Grobe¹, C.D. Hillenbrand¹, G. Kuhn¹, K. Michels¹, R. Petschick² and M. Pirrung¹

¹Alfred-Wegener-Institut für Polar- und Meeresforschung, Columbusstraße, 27515 Bremerhaven, Germany

²Alfred-Wegener-Institut für Polar- und Meeresforschung, Forschungsstelle Potsdam,Telegrafenberg A43, 14473 Potsdam, Germany

³Geologisch-Palaeontologisches Institut, Universität Frankfurt,Senckenberganlage 32-34, 60054 Frankfurt/Main, Germany

* corresponding author (e-mail): bdiekmann@awi-potsdam.de

Abstract: Terrigenous sediment parameters in modern sea-bottom samples and sediment cores of the South Atlantic are used to infer variations in detrital sources and modes of terrigenous sediment supply in response to environmental changes through the late Quaternary climate cycles. Massaccumulation rates of terrigenous sediment and fluxes of ice-rafted detritus are discussed in terms of temporal variations in detrital sediment input from land to sea. Grain-size parameters of terrigenous mud document the intensity of bottom-water circulation, whereas clay-mineral assemblages constrain the sources and marine transport routes of suspended fine-grained particulates, controlled by the modes of sediment input and patterns of ocean circulation. The results suggest low-frequency East Antarctic ice dynamics with dominant 100-kyr cycles and high rates of Antarctic Bottom Water formation and iceberg discharge during interglacial times. In contrast, the more subpolar ice masses of the Antarctic Peninsula also respond to short-term climate variability with maximum iceberg discharges during glacial terminations related to the rapid disintegration of advanced ice masses. In the northern Scotia Sea, increased sediment supply from southern South America points to extended ice masses in Patagonia during glacial times. In the southeastern South Atlantic, changes in regional ocean circulation are linked to global thermohaline ocean circulation and are in phase with northernhemispheric processes of ice build-up and associated formation of North Atlantic Deep Water, which decreased during glacial times and permitted a wider extension of southern-source water masses in the study area.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 375-399

Fluxes at the Benthic Boundary Layer - A Global View from the South Atlantic

C. Hensen^1*, K. Pfeifer², F. Wenzhöfer³, A. Volbers⁴, S. Schulz², J. Holstein², O. Romero² and K. Seiter²

¹GEOMAR, Forschungszentrum für Marine Geowissenschaften, Wischhofstr. 1-3, 24148 Kiel, Germany

²Universität Bremen, Fachbereich Geowissenschaften, Klagenfurter Strasse, 28359 Bremen, Germany

³Marine Biological Laboratory, University of Copenhagen, Strandpromenaden 5, 3000 Helsingør, Denmark

⁴Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, 30655 Hannover, Germany

* corresponding author (e-mail): chensen@geomar.de

Abstract: Fluxes between the ocean waters and the sediments are key regulation processes for the marine biogeochemical cycles and, thus, their quantification is of crucial importance. At this transition it is ultimately determined how much of a primary particulate signal is preserved or mineralized and hence recycled. Our review summarizes two major approaches how to use spatial information obtained from surface sediments: (1) In the first part we summarize the state-of-the-art regarding the use of biogenic barium as a proxy for primary productivity. We discuss the possibilities and limitations of this approach mainly based on the results of a recent study in the South Atlantic. The general outcome of this study was that the spatial pattern of primary productivity can well be traced back by calculating (sub-)recent accumulation rates of biogenic barium and applying available and newly formulated empirical equations. Most of those equations, however, fail to give the really observed magnitude of today's productivity values. The main reasons for this are mostly the uncertainty of the C_org/Ba_bio depth relation, which differs between distinct ocean regions, dynamic sedimentary processes at ocean margins combined with badly constrained values of terrigeneous barium input, and the effect of barite dissolution due to subsequent anoxic diagenesis. To improve the quality of prognoses for past productivity multi proxy approaches are recommended to bypass the uncertainty in predictions from a single proxy. (2) The more extensive second part is based on the large amount of studies that aimed at the quantification of benthic fluxes of nutrients and oxygen, which are good measures for the amount of reactive particulate material being mineralized at the seafloor and thus returned into the marine cycle. Those results enabled us to give profound calculations of the benthic oxygen consumption and the release of nitrate, phosphate, and silicate at the seafloor of the South Atlantic and give upscaled estimates for the global area of the sea floor. Additionally, we discuss more detailed studies focusing on control parameters for benthic fluxes like primary production and lateral advection along the ocean margins off Southwest Africa and Argentina. A very conspicuous result was obtained by calculating mass balances for biogenic opal in those regions indicating a dramatic underestimation of accumulation fluxes of opal by "conservative" methods, which is believed to be of global significance. The last section mainly focuses on the effect of benthic mineralization on the dissolution of calcium carbonate even above the chemical lysocline. This process is in discussion since more than two decades. A number of studies have been performed, mainly using in situ devices, to determine CaCO₃ dissolution. We summarize and discuss the results obtained from the South Atlantic and use a recently developed empirical algorithm to show the worldwide distribution of supralysoclinal CaCO₃ dissolution fluxes in marine surface sediments and give an estimate of their total amount. Finally, a table for benthic fluxes of major constituents is provided on ocean wide and global scales.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 401-430

Processes and Signals of Nonsteady-State Diagenesis in Deep-Sea Sediments and their Pore Waters

S. Kasten^1*, M. Zabel¹, V. Heuer¹ and C. Hensen^1,2

¹ Universität Bremen, Fachbereich Geowissenschaften, Postfach 33 04 40, 28334 Bremen, Germany

² GEOMAR – Forschungszentrum für Marine Geowissenschaften, Wischhofstr. 1-3, 24148 Kiel, Germany

* corresponding author (e-mail): skasten@uni-bremen.de

Abstract: Nonsteady-state conditions – induced by changes in the fluxes of electron donors and acceptors and environmental conditions – are shown to have been and to be still widespread in sediments of the equatorial and South Atlantic Ocean. Typical diagenetic phenomena initiated under such nonsteady-state conditions comprise the fixation and downward progression of redox boundaries and reaction fronts. Intervals most severely altered by diagenetic overprint often occur cyclically within the sedimentary record and are mostly associated with full glacial/interglacial transitions. The extent of post-depositional oxidation of organic carbon as well as the dissolution and re-precipitation of minerals across these glacial terminations was shown to depend on the overall sedimentation rate and the magnitude of change encountered in the various depositional and geochemical factors. A sedimentation rate of about 2 cm/kyr was confirmed to be the critical value below which no significant amounts of non-refractory organic carbon are preserved. The influence of climatically induced variations in environmental conditions is not restricted to the geochemical boundaries in the vicinity of the sediment surface (e.g. oxic/post-oxic and Fe redox boundary) but well extends into much deeper sediment sections – namely into the zone of anaerobic oxidation of methane (AOM). In this way, processes within the zone of AOM can produce a further profound diagenetic alteration of the sediment composition up to hundreds of thousands of years after initial deposition and thus a significantly delayed chemical log-in. The long-term utility of all primary and secondary signals – also those formed and initially preserved across the oxic/post-oxic and Fe redox boundaries – is ultimately controlled by the geochemical processes within and below the sulfate/methane transition (SMT). While dissolution of authigenic and productivity-related barite takes place in sulfate-depleted sediment sections, iron sulfides as well as sulfurized organic matter and associated trace elements have a high potential to survive burial below the SMT. Nonsteady-state diagenesis can be triggered not only by changes in conditions at the sediment/water interface like TOC input, sedimentation rate or O₂ content of bottom water but also by processes in the underlying sediment – namely the formation and/or liberation of methane. Apart from the distinct alteration of the solid-phase composition, variations in the upward flux of methane also have a considerable impact on the shape of sulfate pore water profiles. Modelling the effects of such variations in methane flux on sulfate profiles has illustrated that considering possible nonsteady-state situations in the sediment/pore water system is of utmost importance for the interpretation of pore water data.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 431-459

Late Quaternary Sedimentation and Early Diagenesis in the Equatorial Atlantic Ocean: Patterns, Trends and Processes Deduced from Rock Magnetic and Geochemical Records

J.A. Funk^1*, T. von Dobeneck^1,2, T. Wagner³ and S. Kasten¹

¹Universität Bremen, Fachbereich Geowissenschaften, Postfach 33 04 40, D-28334 Bremen, Germany

²Paleomagnetic Laboratory 'Fort Hoofddijk', Faculty of Earth Sciences Utrecht University, Budapestlaan 17, 3584 CD Utrecht, The Netherlands

³Woods Hole Oceanographic Institution, Marine Chemistry and Geochemistry Dept., Fye Laboratory (MS#4), 360 Woods Hole Rd, Woods Hole, MA 02543-1543, USA

* corresponding author (e-mail): funk@uni-bremen.de

Abstract: This is an interdisciplinary and synoptic study of Equatorial Atlantic sediment formation in the Late Quaternary aimed at untangling the interlaced signatures of terrigenous and biogenous deposition and early diagenesis. It is based on a stratigraphic network of 16 gravity core records arranged along one meridional and three zonal transects (4°N, 0° and 4°S) crossing the Amazon and Sahara plumes as well as the Equatorial Divergence high productivity region. All newly introduced sediment sequences are collectively dated by their coherent CaCO₃ content profiles and two available δ¹⁸O age models. To infer proxy records indicative of individual fluxes and processes, we analyze environmental magnetic parameters describing magnetite concentration, magnetic grain sizes and magnetic mineralogy along with CaCO₃, C_org, Fe, Mn, Ba and color data. Diagenetically affected layers are identified by a newly introduced Fe/κ index. Reach and climatic variability of the major regional sedimentation systems is delimited from lithological patterns and glacial/interglacial accumulation rate averages. The most prominent regional trends are the N-S decrease in terrigenous accumulation and the Equatorial Divergence high in glacial Corg accumulation, which decays much faster south- than northwards. Glacial enrichments in Corg and proportional depletions in CaCO₃ content appear to reflect sedimentary carbonate diagenesis more than lysoclinal oscillations and dominate temporal lithology changes. Suboxic iron mineral reduction is low at Ceará Rise and Sierra Leone Rise, but more intense on both flanks of the Mid-Atlantic ridge, where it occurs within organic rich layers deposited during oxygen isotope stages 6, 10 and 12, in particular at the terminations. To the equator, these zones reflect a full precessional rhythm with individual diagenesis peaks merging into broader magnetite-depleted zones. Rock magnetic and geochemical data show, that the depths of the Fe³⁺/Fe²⁺ redox boundary in the Equatorial Atlantic are not indicative of average productivity and were frequently shifted in the past. They are now located just above the topmost preserved productivity pulse. At 4°N, this organically enriched layer coincides with glacial stage 6, at 0° with glacial stage 2. Subsequent oxic and suboxic degradation of organic material entails stratigraphically coincident carbonate and magnetite losses opening new analytical perspectives.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 461-497

The Late Pleistocene South Atlantic and Southern Ocean Surface - A Summary of Time-Slice and Time-Series Studies

R. Gersonde^1*, A. Abelmann¹, G. Cortese¹, S. Becquey¹, C. Bianchi¹, U. Brathauer¹, H.-S. Niebler^1,2, U. Zielinski¹ and J. Pätzold²

¹Alfred Wegener Institut für Polar- und Meeresforschung, Columbusstrasse, 27515 Bremerhaven, Germany

²Universität Bremen, Fachbereich Geowissenschaften, Klagenfurter Strasse, 28359 Bremen, Germany

* corresponding author (e-mail): rgersonde@awi-bremerhaven.de

Abstract: Central to global climate evolution is the paleoceanographic development of the South Atlantic as it represents the passageway for inter-hemispheric heat exchange within global thermohaline circulation (THC). Processes in the adjacent Southern Ocean regulate the heat import into the South Atlantic via the Agulhas "warm water route"(WWR) and the Drake Passage "cold water route"(CWR), and amplify climate change through various feedback mechanisms and teleconnections. For paleoceanographic reconstruction an inventory of new data sets and methods is now available, allowing for the estimation of Pleistocene sea-surface water temperatures and sea-ice distribution on time-slices and time-series based on the calcareous and siliceous microfossil record. Reconstruction of the Last Glacial Maximum (LGM) reveals distinct cooling in the Southern Ocean (up to 4 - 6 °C) accompanied by an expansion of winter and summer sea ice, cooling in the African upwelling regimes (up to 10°C) and in the Equatorial Atlantic (4 - 5 °C), but the Subtropical Gyre region remains relatively warm and unchanged compared with the present. While the WWR was not strongly altered during the LGM, heat transport via the CWR was most probably much weaker. The reconstruction of time-slices representing a warm climate end-member at the onset of the last climate cycle documents a distinct lead of southern high-latitudes in global climate development that also affects the south-west African upwelling regions. It is at the Marine Isotope Stage (MIS) 6/MIS 5 transition when Southern Ocean surface temperatures reach maximum values and sea ice is at a minimum, marking a period of South Atlantic heat piracy. During the isotopic minimum of MIS 5.5, the tropical South Atlantic was slightly colder than at present, likely the result of an enhanced poleward heat export. Time-series studies from key areas document that climate variability related to orbital forcing is overprinted by THC changes driven by meltwater injections into the North Atlantic and the Southern Ocean, changes in atmospheric circulation and greenhouse gas concentration, as well as sea ice that amplify climate change at global, hemispheric and regional scales. The study of centennial-scale variability during interglacial optima, such as MIS 5.5 and MIS 11, suggests that the presence of large ice sheets, meltwater events, changes in greenhouse gas concentration and seaice distribution are not the only prerequisite to trigger millennial-centennial-scale variability, but that another external agent, changes in solar irradiance, must be considered as an important factor in climate development.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 499-529

The Atlantic Ocean at the Last Glacial Maximum: 1. Objective Mapping of the GLAMAP Sea-Surface Conditions

C. Schäfer-Neth^* and A. Paul

DFG Forschungszentrum Ozeanränder, Universität Bremen, Postfach 33 04 40, 28334 Bremen, Germany

* corresponding author (e-mail): csn@uni-bremen.de

Abstract: Recent efforts of the German paleoceanographic community have resulted in a unique data set of reconstructed sea-surface temperature for the Atlantic Ocean during the Last Glacial Maximum, plus estimates for the extents of glacial sea ice. Unlike prior attempts, the contributing research groups based their data on a common definition of the Last Glacial Maximum chronozone and used the same modern reference data for calibrating the different transfer techniques. Furthermore, the number of processed sediment cores was vastly increased. Thus the new data is a significant advance not only with respect to quality, but also to quantity. We integrate these new data and provide monthly data sets of global sea-surface temperature and ice cover, objectively interpolated onto a regular 1°x1° grid, suitable for forcing or validating numerical ocean and atmosphere models. This set is compared to an existing subjective interpolation of the same base data, in part by employing an ocean circulation model. For the latter purpose, we reconstruct sea surface salinity from the new temperature data and the available oxygen isotope measurements.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 531-548

The Atlantic Ocean at the Last Glacial Maximum: 2. Reconstructing the Current Systems with a Global Ocean Model

A. Paul^* and C. Schäfer-Neth

DFG Forschungszentrum Ozeanränder, Universität Bremen, Postfach 33 04 40, 28334 Bremen, Germany

* corresponding author (e-mail): apaul@palmod.uni-bremen.de

Abstract: We use a global ocean general circulation model (OGCM) with low vertical diffusion and isopycnal mixing to simulate the circulation in the Atlantic Ocean at present-day and the Last Glacial Maximum (LGM). The OGCM includes δ¹⁸O as a passive tracer. Regarding the LGM sea-surface boundary conditions, the temperature is based on the GLAMAP reconstruction, the salinity is estimated from the available δ¹⁸O data, and the wind-stress is derived from the output of an atmospheric general circulation model. Our focus is on changes in the upper-ocean hydrology, the large-scale horizontal circulation and the δ¹⁸O distribution. In a series of LGM experiments with a step-wise increase of the sea-surface salinity anomaly in the Weddell Sea, the ventilated thermocline was colder than today by 2–3°C in the North Atlantic Ocean and, in the experiment with the largest anomaly (1.0 beyond the global anomaly), by 4–5°C in the South Atlantic Ocean; furthermore it was generally shallower. As the meridional density gradient grew, the Antarctic Circumpolar Current strengthened and its northern boundary approached Cape of Good Hope. At the same time the southward penetration of the Agulhas Current was reduced, and less thermocline-to-intermediate water slipped from the Indian Ocean along the southern rim of the African continent into the South Atlantic Ocean; the 'Agulhas leakage' was diminished by up to 60% with respect to its modern value, such that the cold water route became the dominant path for North Atlantic Deep Water (NADW) renewal. It can be speculated that the simulated intensification of the Benguela Current and the enhancement of NADW upwelling in the Southern Ocean might reduce the import of silicate into the Benguela System, which could possibly resolve the 'Walvis Opal Paradox'. Although δ¹⁸O_w was restored to the same surface values and could only reflect changes in advection and diffusion, the resulting δ¹⁸O_c distribution came close to reconstructions based on fossil shells of benthic foraminifera.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 549-583

Inverse Modelling of the Glacial Atlantic Circulation under Geostrophic Side Conditions

R. Schlotte^1* and B. Grieger²

¹Universität Bremen, Fachbereich Geowissenschaften, Klagenfurter Straße, 28359 Bremen, Germany

²Max-Planck-Institut für Aeronomie, Max-Planck-Str. 2, 37191 Katlenburg-Lindau, Germany

* corresponding author (e-mail): r.schlotte@science-computing.de

Abstract: The inverse ocean model 2RAIOM is presented. It is designed to determine the mean circulation of the glacial ocean from observations of temperature and salinity. Derived from a model code suited for application to the present day ocean, new terms have been added to the objective function to enhance the model's performance if temperature data are sparse. The effects of the new objective function are studied with present day temperature and salinity data. The model is then applied to the Atlantic Ocean during the Last Glacial Maximum. Although the performance of the new model version is improved considerably, the amount of available data is only sufficient to reconstruct the ocean circulation during the Last Glacial Maximum qualitatively.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 585-599

Palaeoceanographic Changes in the Northern Benguela Upwelling System over the last 245.000 Years as Derived from Planktic Foraminifera Assemblages

A.N.A. Volbers^1,3*, H.-S. Niebler³, J. Giraudeau², H. Schmidt³ and R. Henrich³

¹ Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, 30655 Hannover, Germany

²DGO - UMR 5805 EPOC, Université Bordeaux I, Avenue des Facultés, 33405 Talence, France

³Universität Bremen, Fachbereich Geowissenschaften, Klagenfurter Strasse, 28359 Bremen, Germany

* corresponding author (e-mail): a.volbers@bgr.de

Abstract: Planktic foraminiferal records from six sediment cores recovered from the Walvis Ridge and the northern Cape Basin indicate changes in the spatial and temporal variability in the degree of upwelling during the past 245 kyrs. During periods of intensified upwelling, northern Benguela upwelling cells were displaced westward and increased in size, covering areas at least three times larger than present day. Distinct upwelling events were recognized during oxygen isotopic stage (OIS) 2 and 3 and oxygen isotopic event (OIE) 4.2, 5.2, 5.4, 5.53, 6.2, 6.4/6.5, and 7.4. During OIS 3, OIE 5.4 and 7.4 the maximum upwelling was recorded around the Namibia/Walvis Bay cells and during OIE 3.1, 5.4, and 6.2 at around Walvis Bay/Lüderitz. During OIE 5.1 and 5.51, upwelling was at its minimum. A good correlation between upwelling events in the northern Benguela region and increases in equatorial seasonality implies that both regions respond to the same mechanism, i.e. probably changes in the trade wind intensity.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 601-622

Carbon Isotopes of Live Benthic Foraminifera from the South Atlantic: Sensitivity to Bottom Water Carbonate Saturation State and Organic Matter Rain Rates

A. Mackensen^* and L. Licari

Alfred Wegener Institute for Polar and Marine Research, Columbusstrasse, 27568 Bremerhaven, Germany

* corresponding author (e-mail): amackensen@awi-bremerhaven.de

Abstract: Live (Rose Bengal stained) and dead benthic foraminifera of surface and subsurface sediments from 25 stations in the eastern South Atlantic Ocean and the Atlantic sector of the Southern Ocean were analyzed to decipher a potential influence of seasonally and spatially varying high primary productivity on the stable carbon isotopic composition of foraminiferal tests. Therefore, stations were chosen so that productivity strongly varied, whereas conservative water mass properties changed only little. To define the stable carbon isotopic composition of dissolved inorganic carbon (δ¹³C_DIC) in ambient water masses, we compiled new and previously published δ¹³C_DIC data in a section running from Antarctica through Agulhas, Cape and Angola Basins, via the Guinea Abyssal Plain to the Equator. We found that intraspecific δ¹³C variability of all species at a single site is constantly low throughout their distribution within the sediments, i.e. species specific and site dependent mean values calculated from all subbottom depths on average only varied by ±0.09 ‰. This is important because it makes the stable carbon isotopic signal of species independent of the particular microhabitat of each single specimen measured and thus more constant and reliable than has been previously assumed. So-called vital and/or microhabitat effects were further quantified: (1) δ¹³C values of endobenthic Globobulimina affinis, Fursenkoina mexicana, and Bulimina mexicana consistently are by between -1.5 and -1.0 ‰ VPDB more depleted than δ¹³C values of preferentially epibenthic Fontbotia wuellerstorfi, Cibicidoides pachyderma, and Lobatula lobatula. (2) In contrast to the Antarctic Polar Front region, at all stations except one on the African continental slope Fontbotia wuellerstorfi records bottom water δ¹³C_DIC values without significant offset, whereas L. lobatula and C. pachyderma values deviate from bottom water values by about -0.4‰ and -0.6‰, respectively. This adds to the growing amount of data on contrasting cibicid δ¹³C values which on the one hand support the original 1:1-calibration of F. wuellerstorfi and bottom water δ¹³C_DIC, and on the other hand document severe depletions of taxonomically close relatives such as L. lobatula and C. pachyderma. At one station close to Bouvet Island at the western rim of Agulhas Basin, we interpret the offset of -1.5 ‰ between bottom water δ¹³C_DIC and δ¹³C values of infaunal living Bulimina aculeata in contrast to about -0.6 ± 0.1 ‰ measured at eight stations close-by, as a direct reflection of locally increased organic matter fluxes and sedimentation rates. Alternatively, we speculate that methane locally released from gas vents and related to hydrothermal venting at the mid-ocean ridge might have caused this strong depletion of δ¹³C in the benthic foraminiferal carbon isotopic composition. Along the African continental margin, offsets between deep infaunal Globobulimina affinis and epibenthic Fontbotia wuellerstorfi as well as between shallow infaunal Uvigerina peregrina and F. wuellerstorfi, δ¹³C values tend to increase with generally increasing organic matter decomposition rates. Although clearly more data are needed, these offsets between species might be used for quantification of biogeochemical paleogradients within the sediment and thus paleocarbon flux estimates. Furthermore, our data suggest that in high-productivity areas where sedimentary carbonate contents are lower than 15 weight %, epibenthic and endobenthic foraminiferal δ¹³C values are strongly influenced by ¹³C enrichment probably due to carbonate-ion undersaturation, whereas above this sedimentary carbonate threshold endobenthic δ¹³C values reflect depleted pore water δ¹³C_DIC values.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 623-644

Carbonate Preservation in Deep and Intermediate Water Masses in the South Atlantic: Evaluation and Geological Record (a Review)

R Henrich^*, K.-H. Baumann, S. Gerhardt, M. Gröger and A. Volbers

Universität Bremen, Fachbereich Geowissenschaften, Postfach 330440, D- 28334 Bremen, Germany

* corresponding author (e-mail): henrich@uni-bremen.de

Abstract: Evaluation of conventional dissolution proxies in South Atlantic surface sediments revealed broad applicability only in far offshore, rather oligotrophic regimes in the western basins. In contrast, they fail or produce misleading and incorrect results in the more productive eastern South Atlantic basins, due to the combined effects of variable dilution by non-carbonate material and fluctuating ecological conditions. Much more promising are the results from new dissolution proxies on the planktic foraminifer Globigerina bulloides (BDX') and the pteropod Limacina inflata (LDX) which were calibrated with carbonate saturation as indicated by GEOSECS data. In the western South Atlantic, the sedimentary calcite lysocline is encountered by the BDX' at the transition between AABW and LNADW. However, it rises up into the LNADW close to the equator due to additional supralysoclinal dissolution. In the eastern South Atlantic basins, supralysoclinal dissolution results in an elevation of the sedimentary calcite lysocline of several hundred metres to a maximum of 1600 m as compared to the position of the hydrographic lysocline, with aragonite preservation in the eastern South Atlantic being even poorer. At most sites investigated, the surface sediments are void of pteropods and thus LDX failure is indicated. However, in the western South Atlantic the LDX displays a double lysocline for aragonite, the upper lysocline at a water depth of 750 m and the lower at 2500 m. Aragonite and calcite preservation profiles indicate much weaker stratification of the water during the LGM. With 3200 m, the position of the calcite lysocline is encountered at the same level in the southern parts of the eastern and western basins dropping to 4000 m near the equator. Along the western continental margin no indication for aragonite-corrosive glacial AAIW was found, providing clear evidence for a strengthened GNAIW flow along the Brazil margin. The long-term history of carbonate dissolution in the equatorial Atlantic was reconstructed by a multiproxy approach combining benthic foraminifer stable isotopes and new proxies from silt analysis. For the first time, this allows a reconstruction of the chemical (nutrient content, carbonate corrosiveness) and physical (bottom current strength) properties of deep and intermediate water masses. The terrigenous silt records of ODP Site 927 at the Ceara Rise show rapid shifts from low to very high bottom-currents speeds for nearly all the isotopic transitions in the Brunhes epoch, indicating subsequent phases of shutdown and rapid reinstatement of LNADW circulation. A drastic reduction of glacial bottomcurrent strength at Site 927 is inferred after 2.75 Ma, synchronous with the first occurrence of larger continental ice shields and with a drastic decrease in deep convection in the Norwegian-Greenland Sea. After the mid-Pleistocene climate transition, progressively weaker bottom currents and poorer carbonate preservation during glacials indicate a progressive reduction of LNADW from the Late Pliocene to Pleistocene. On the contrary, an opposite trend with progressive improvement of preservation during glacials from Late Pliocene to the Pleistocene is observed in the Caribbean at Site 999. This indicates a contemporaneous progressive increase in the contribution of UNADW to the Atlantic in glacial periods. Altogether, a progressive weakening of the circulation in the LNADW loop and a contemporaneous strengthening of the UNADW loop are evident since the mid Pleistocene transition.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 645-670

Last Glacial to Holocene Changes in South Atlantic Deep Water Circulation

T. Bickert^1* and A. Mackensen²

¹ Universität Bremen, Fachbereich Geowissenschaften, Klagenfurter Strasse, 28359 Bremen, Germany

² Alfred-Wegener-Institut für Polar- und Meeresforschung, Columbusstrasse, 27515 Bremerhaven, Germany

* corresponding author (e-mail): bickert@uni-bremen.de

Abstract: A set of 55 benthic foraminiferal stable carbon and oxygen isotope time series, including 28 new records, is presented from the South Atlantic Ocean between 6°N and 47°S. We compiled these records with published data of the eastern North Atlantic to reconstruct the Atlantic deepwater circulation for the Last Glacial Maximum (19-23 ka) and the Late Holocene (0-4 ka) times. To better understand the spatial distribution of deep and bottom water masses, we assigned these records to three North-South sections representing the western South Atlantic, the central Atlantic east of the Mid-Atlantic Ridge, and the eastern marginal Atlantic. Corrections of up to +0.4‰ are suggested for several benthic δ¹³C values of cores located in high-productivity areas, to adjust for phytodetritusinduced depletion of especially glacial values. As a result of this new compilation, no shift of NADW to intermediate depth during the last glacial maximum is evident in the eastern and western marginal Atlantic. Instead, the core of an ¹³C-enriched water mass spreading southward to at least 30°S between 1200 and 1900 m points to a source of this water mass close to the Isthmus of Gibraltar, indicated by δ¹³C-values of up to 1.8‰. Therefore, we interpret this layer as an extended tongue of the Mediterranean Outflow Water. Below, a layer of glacial NADW is shown to flow southward at about the same depth interval or even deeper than it does today, although slightly depleted in ¹³C and less extended in water column. The admixing of NADW into the circumantarctic deepwater belt occurred a few degrees farther north than today, marked by a steep gradient in glacial δ¹³C between 30° and 40° S. From these gradients we derive a local formation of Southern Ocean deep water in the zone of extended winter sea-ice coverage south of the polar front. The spreading of this newly formed water mass, however, is restricted to the Atlantic basins south of Walvis Ridge and Rio- Grande Rise, where only a small amount of nutrient-enriched deep water passes across these barriers into the northern basins. Converted into nutrient concentrations, the new carbon isotope data set gives only a slight increase in the nutrient inventory of the deep Atlantic, in good agreement with previously published Cd/Ca data.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 671-695

Last Glacial d13C Distribution and Deep-Sea Circulation in the Atlantic Ocean: A Model - Data Comparison

M. Matthies^*, T. Bickert and A. Paul

Universität Bremen, Fachbereich Geowissenschaften, Klagenfurter Straße, 28334 Bremen, Germany

* corresponding author (e-mail): mmat@palmod.uni-bremen.de

Abstract: We used a carbon cycle model (HAMOCC2) coupled to a general ocean circulation model (LSG) to explore the δ¹³C distribution in the glacial Atlantic Ocean. We compared the simulated δ¹³C pattern with a new data set of benthic carbon isotopes of the Western and Eastern Atlantic from the Last Glacial Maximum (18,000 to 20,000 14C years or 21,000 - 23,500 calendar years before present). The model output fits the d13C distribution derived from sediment samples, when the glacial export of NADW to the Southern Ocean was reduced by 50 % and the inflow of glacial AABW was held constant. In most cases, the modeled δ¹³C pattern matched the paleodata within a range of ±0.2 ‰. Furthermore, the asymmetry between the glacial NADW distribution in the South Atlantic basins was reproduced by the coupled ocean circulation and carbon cycle models. No additional increase of the nutrient inventory in the deep ocean was necessary to reproduce the paleodata. Hence we conclude that a significant increase in biological pumping during glacials may not be necessary to explain the reconstructed δ¹³C distribution in this region. The results are discussed with respect to other scenarios for the decrease of global atmospheric pCO₂.

From WEFER G, MULITZA S, RATMEYER V (eds), 2003, The South Atlantic in the Late Quaternary: Reconstruction of Material Budgets and Current Systems. Springer-Verlag Berlin Heidelberg New York Tokyo, pp 695-722