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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 δ13C 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.

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 phytodetritus-induced 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.

Understanding spatial and temporal changes in oceanic carbonate dissolution and preservation patterns is of key importance for testing models which seek to explain past changes in atmospheric pCO2 and surface water PCO2 through changes in the global carbon cycle. As part of the South Atlantic Dissolution Experiment, three deep-sea transects covering areas above and below the calcite lysocline into the Brazil and through the Cape Basin were investigated. Our work includes (1) determination of sediment surface assemblages of coccolithophores and planktic foraminifera; (2) SEM ultrastructure analysis of the planktic foraminifera Globigerina bulloides; and (3) comparative assessment of different carbonate dissolution proxies. We find that all dissolution proxies are able to distinguish the area above the calcite lysocline from the area below. Moreover, some parameters are qualified to distinguish the upper continental margin of upwelling areas from the open ocean. Regarding three different oceanographic regimes, only the carbonate ion content and the percentage of sediment carbonate content put us in the position to determine the total scale of the calcite transition zone. If these parameters are not available, a combination of the Globigerina bulloides Dissolution Index, the Calcidiscus leptoporus — Emiliania huxleyi Dissolution Index, and the rain ratio give the best approach to the authentic conditions.

The central problem of late Quaternary circulation in the South Atlantic is its role in transfer of heat to the North Atlantic, as this modifies amplitude, and perhaps phase, of glacial- interglacial fluctuations. Here we attempt to define the problem and establish ways to attack it. We identify several crucial elements in the dynamics of heat export: (1) warm-water pile-up (and lack thereof) in the western equatorial Atlantic, (2) general spin-up (or spin-down) of central gyre, tied to SE trades, (3) opening and closing of Cape Valve (Agulhas retroflection), (4) deepwater E-W asymmetry. Means for reconstruction are biogeography, stable isotopes, and productivity proxies. Main results concern overall glacial-interglacial contrast (less pile-up, more spin-up, Cape Valve closed, less NADW during glacial time), dominance of precessional signal in tropics, phase shifts in precessional response. To generate working hypotheses about the dynamics of surface water circulation in the South Atlantic we employ Croll’s paradigm that glacial - interglacial fluctuations are analogous to seasonal fluctuations. Our general picture for the last 300 kyrs is that, as concerns the South Atlantic, intensity of surface water (heat) transport depends on the strength of the SE trades. From various lines of evidence it appears that stronger SE trades appeared during glacials and cold substages during interglacials, analogous to conditions in southern winter (August).

Carbonate dissolution data (sand contents) and δ13C records of the epibenthic foraminifer Cibicides wuellerstorfi from 12 gravity cores are used to reconstruct the history of deep water circulation in the South Atlantic for the last 360,000 years. The cores were selected from depth-sections in four basins (Brasil-, Guinea-, Angola- and Cape Basins) in water depths between 2900 m and 4600 m. The depth-transect approach allows removal of mean global shifts as well as local productivity effects from the paleo-property records and extraction of variations which are due to changes in deep water chemistry and/or circulation in the South Atlantic. As a result of the reduction of NADW during the last glacial maximum the Southern Component Water was higher in the water column and extended farther north than it does today. This glacial water mass can be divided into an upper part (USCW) with δ13C values between 0.2%o and 0.7%o and a lower part (LSCW) characterized by values of -0.2%o to 0.2%o. The boundary, marked also by the calcite lysocline, was at 3800 m water depth near the equator and rose slightly toward the Southern Ocean. The asymmetry observed in bottom water circulation today (LCDW in western basins and in the Cape Basin, NADW in eastern basins below 4000 m) was not present. From comparison to a deep western Pacific core (ODP 806B; Bickert et al. 1993) there is evidence that the nutrient-enriched but oxygen-depleted LSCW resembles the glacial Pacific Deep Water. This is also true for the older glacial stages 4, 6, 8 and 10.