Geochemistry of surface sediment cores (Table 4)

The mineralogy and geochemistry of a suite of nine manganese nodules from the South Atlantic have been determined. The Ce/La ratios of the nodules were investigated to see if they could be used as redox indicators to trace the oxygen content of the ambient water mass and the flow path of the Antarctic Bottom Water as has previously been successfully carried out in the Pacific Ocean. The Ce/La ratios of the nodules decrease in the sequence Lazarev Sea, Weddell Sea (10.4 and 9.7)>East Georgia Basin (6.5 and 7.1)>Argentine Basin (5.0), but then increase in the Brazil Basin (6.2) and Angola Basin (9.8 and 15.1). A further decrease was observed in the Cape Basin (7.6). An extremely high Ce/La ratio of 24.4 had already been determined for nodules sampled north of the Nares Abyssal Plain in the western North Atlantic. These data reflect the more complicated pattern of bottom water flow in the South Atlantic than in the South Pacific. The penetration of more oxygenated North Atlantic Deep Water into the South Atlantic accounts for the higher Ce/La ratios in the nodules from the Angola and Brazil basins. Based on this study, the flow path of the Antarctic Bottom Water could only be traced as far north as the Argentine Basin. The unique geochemistry of nodules from the central Angola Basin (high Mn/Fe and Ce/La ratios, high contents of Ni, Cu, Zn and Mo) appears to be a function of the nature of the overlying water mass and of the multiple diagenetic sources of metals to the nodules.

Cadmium analytical results from sediment and foraminifera shells

There has been a major contradiction between benthic foraminiferal Cd/Ca and d13C data concerning the labile nutrient chemistry of the Southern Ocean during the Last Glacial Maximum (LGM). Cd data indicates that LGM South Atlantic nutrient concentrations were as low as they are today, indicative of a persistent influx of nutrient-depleted North Atlantic Deep Water (NADW). d13C data indicates that LGM South Atlantic nutrient concentrations were much higher than at present (even higher than anywhere else in the ocean at that time), and these data have been interpreted as signifying the complete shutdown ofthe export of NADW into the global ocean. This paper examines both true geochemical differences and various confounding foraminiferal artifacts for both tracers. While many different processes and artifacts affect both tracers in the margin, we conclude the discrepancy is mainly due to the "Mackensen Effect" of low foraminiferal d13C as a result of high carbon flux to the sediments, and that LGM Atlantic Sector Southern Ocean nutrient concentrations remained similar to the levels encountered today.

Standing stocks and carbon isotopes of live benthic foraminifera from the South Atlantic

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 (d13CDIC) in ambient water masses, we compiled new and previously published d13CDIC 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 d13C 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 per mil. 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) d13C values of endobenthic Globobulimina affinis, Fursenkoina mexicana, and Bulimina mexicana consistently are by between -1.5 and -1.0 per mil VPDB more depleted than d13C 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 d13CDIC values without significant offset, whereas L. lobatula and C. pachyderma values deviate from bottom water values by about -0.4 per mil and -0.6 per mil, respectively. This adds to the growing amount of data on contrasting cibicid d13C values which on the one hand support the original 1:1-calibration of F. wuellerstorfi and bottom water d13CDIC, 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 per mil between bottom water d13CDIC and d13C values of infaunal living Bulimina aculeata in contrast to about -0.6 +/- 0.1 per mil 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 13C 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, d13C 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 d13C values are strongly influenced by 13C enrichment probably due to carbonate-ion undersaturation, whereas above this sedimentary carbonate threshold endobenthic d13C values reflect depleted pore water d13CDIC values.

(Appendix A) Abundance of calcareous dinoflagellates of sediment core GeoB2204-2

The environmental preferences of calcareous dinoflagellates have been investigated over the last 140 ka by comparing material from two sediment cores: one from the highly productive equatorial divergence of the eastern Atlantic Ocean and the other from the low productivity western tropical Atlantic Ocean. Pronounced differences in palaeoproductivity between the two sediment cores are indicated by high and variable organic carbon accumulation rates in the east, in contrast to relatively constant and low values in the west. Calcareous dinoflagellates show just the opposite pattern: high accumulation rates in the west and lower in the east. At the equatorial divergence, temporal variations of calcareous dinoflagellate and organic carbon accumulation rates show, for the most part, an inverse relationship. High calcareous dinoflagellate content coincides with low organic carbon accumulation rates and vice versa. In the investigated region and time interval, enhanced production of calcareous dinoflagellates can be correlated to periods of reduced palaeoproductivity probably related to relatively stratified conditions of the upper water column.

Bulk 15N, and 13Corg ratios, and calculation of unutilized nitrate of surface samples off the southwest coast of Africa (Table 1)

Bulk delta15N values in surface sediment samples off the southwestern coast of Africa were measured to investigate the biogeochemical processes occurring in the water column. Nitrate concentrations and the degree of utilization of the nitrate pool are the predominant controls on sedimentary delta15N in the Benguela Current region. Denitrification does not appear to have had an important effect on the delta15N signal of these sediments and, based on delta15N and delta13C, there is little terrestrial input.

G. bulloides dissolution index of surface sediments

The Atlantic is regarded as a huge carbonate depocenter due to an on average deep calcite lysocline. However, calculations and models that attribute the calcite lysocline to the critical undersaturation depth (hydrographic or chemical lysocline) and not to the depth at which significant calcium carbonate dissolution is observed (sedimentary calcite lysocline) strongly overestimate the preservation potential of calcareous deep-sea sediments. Significant calcium carbonate dissolution is expected to begin firstly below 5000 m in the deep Guinea and Angola Basin and below 4400 m in the Cape Basin. Our study that is based on different calcium carbonate dissolution stages of the planktic foraminifera Globigerina bulloides clearly shows that it starts between 400 and 1600 m shallower depending on the different hydrographic settings of the South Atlantic Ocean. In particular, coastal areas are severely affected by increased supply of organic matter and the resultant production of metabolic CO2 which seems to create microenvironments favorable for dissolution of calcite well above the hydrographic lysocline.

Distribution of coccoliths in surface sediments of the south-eastern South Atlantic Ocean

Recent coccoliths from 52 surface sediment samples recovered from the south-eastern South Atlantic were examined qualitatively and quantitatively in order to assess the controlling mechanisms for their distribution patterns, such as ecological and preservational factors, and their role as carbonate producers. Total coccolith abundances range from 0.2 to 39.9 coccoliths*10**9/ g sediment. Four assemblages can be delineated by their coccolith content characterising the northern Benguela, the middle to southern Benguela, the Walvis Ridge and the deeper water. Distinctions are based on multivariate ordination techniques applied on the relative abundances of the most abundant taxa, Emiliania huxleyi, Calcidiscus leptoporus, Gephyrocapsa spp., Coccolithus pelagicus and subtropical to tropical species. The coccolith distribution seems to be temperature and nutrient controlled co-varying with the seaward extension of the upwelling filament zone in the Benguela. A preservation index (CEX') based on the differential dissolution behaviour of the delicate E. huxleyi and Gephyrocapsa ericsonii versus the robust C. leptoporus is applied in order to detect the position of the coccolith lysocline. Although some samples were recognised as dissolution-affected, the distribution of the coccoliths in the surface-sediments reflects the different oceanographic surface-water conditions. Mass estimations of the coccolith carbonate reveal coccoliths to be only minor contributors to the carbonate preserved in the surface sediments. The mean computed coccolith carbonate content is 17 wt.%, equivalent to a mean contribution of 23% to the bulk carbonate.