Barbara Donner

Temperature:δ18O relationships of planktonic foraminifera collected from surface waters

Abstract Most of the isotopic paleotemperature equations used for paleoceanographic reconstructions have been derived from culture experiments or inorganic precipitates of calcium carbonate. To test these equations in the modern ocean, we measured the oxygen isotope composition of planktonic foraminifera (Globigerinoides ruber, Globigerinoides sacculifer, Globigerina bulloides and Neogloboquadrina pachyderma) collected from Atlantic and Southern Ocean surface waters, and added published plankton tow data from the Pacific, Indian and Arctic Oceans. The resulting species-specific regression equations of the temperature:δ18O relationships for G. ruber, G. sacculifer and G. bulloides are statistically indistinguishable. The equations derived for G. sacculifer and G. bulloides agree with relationships obtained from laboratory experiments, in which these species were cultured at pH values close to modern surface waters. The equation derived from N. pachyderma has a significantly lower slope and offset than the other three species but produces a regression equation that is nearly identical to the one for the epifaunal benthic foraminifer Cibicides sp. Our work on plankton tow and pumped samples indicates that culture-derived equations appear to be more appropriate for predicting the absolute δ18O of the species examined compared to equations derived from inorganic precipitates. However, over the oceanic temperature range, the slopes of the equations we derive for living species agree with the slopes obtained from inorganic precipitates.

Distinct year-to-year particle flux variations off Cape Blanc during 1988–1991: Relation to δ 18 O-deduced sea-surface temperatures and trade winds

Particle fluxes measured from 1988 to 1991 adjacent to a coastal upwelling site off Cape Blanc showed significant interannual variability of fluxes and sea-surface temperatures (SST) deduced from stable oxygen isotope analysis of the planktonic foraminifera Globigerinoides ruber and, partly, of the pteropod Limacina inflata. For the duration of the study period, a decrease in the seasonality of SSTs was observed, as well as a significant decrease in the average annual SST from 24.4° to 20.8°C. This cooling trend was mainly the effect of a drastic decrease in the summer to fall SST (from 27.2° to 21.8°C). In comparison, the winter-spring SST decreased only slightly from 20.3° in 1988 to 19.8°C in 1991. Concomitantly, we measured decreasing annual total, carbonate, biogenic opal and lithogenic fluxes and, in contrast, increasing marine organic carbon fluxes. During 1991, when cold SSTs prevailed and the trade winds were rather high throughout, annual biogenic and lithogenic fluxes (except organic carbon) were lower by approximately a factor of two compared to the other years. Colder SSTs, generally corresponding to stronger trade winds and upwelling intensity, did not result in increased biogenic opal and lithogenic matter sedimentation ; but higher marine organic carbon fluxes were recorded. Decreasing summer-fall SST from 1988 to 1991 coincided with decreased carbonate sedimentation maxima which generally occurred during the warm summer season. In the summer of 1989, when SSTs were the highest of the four-year sampling period and upwelling was less intense due to weak spring-summer trades, a large sedimentation pulse of pteropod shells was observed. Our data set does not yet provide conclusive evidence that the observed year-to-year flux and SST variations represent larger-scale, periodically occurring climatic variations in the eastern Atlantic but it offers insight into the prevailing large variability in biochemical cycles and processes in the eastern Atlantic.

Late Quaternary Surface Circulation of the South Atlantic: The Stable Isotope Record and Implications for Heat Transport and Productivity

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).

Mid-Pleistocene environmental change in tropical Africa began as early as 1.05 Ma

Palynological records from the Congo fan reveal environmental change in equatorial Africa occurring 1.05 Ma ago, 100 k.y. before the mid-Pleistocene climatic shift at 0.9 Ma. Prior to 1.05 Ma, a glacial-interglacial rhythm is not obvious in the African vegetation variation. Afterwards, Podocarpus spread in the mountains of central Africa mainly during glacials and Congo River discharge decreased. The sequence of vegetation variation associated with the mid-Pleistocene glacials and interglacials differed from that observed during the late Pleistocene. Between 0.9 and 0.6 Ma, interglacials were characterized by warm dry conditions and glacials were characterized by cool humid conditions, while during the past 0.2 Ma glacials were cold and dry and interglacials warm and humid. Our data indicate that before the Northern Hemisphere ice caps dramatically increased in size (0.9‐0.6 Ma), low-latitude climate forcing and response in the tropics played an important role in the initiation of 100 k.y. ice-age cycles. During the mid to late Pleistocene, however, the climate conditions in the tropics were increasingly influenced by the glacialinterglacial variations of continental ice sheets.

Sea surface temperatures in the equatorial and South Atlantic Ocean during the Last Glacial Maximum (23–19 ka)

[1] We used planktic foraminiferal assemblages in 70 sediment cores from the tropical and subtropical South Atlantic Ocean (10°N-37°S) to estimate annual mean sea surface temperatures (SSTs) and seasonality for the Last Glacial Maximum with a modified version of the Imbrie-Kipp transfer function method (IKTF) that takes into account the abundance of rare but temperature sensitive species. In contrast to CLIMAP Project Members [1981], the reconstructed SSTs indicate cooler glacial SSTs in the entire tropical/subtropical South Atlantic with strongest cooling in the upwelling region off Namibia (7-10°C) and smallest cooling (1-2°C) in the western subtropical gyre. In the western Atlantic, our data support recent temperature estimates from other proxies. In the upwelling regions in the eastern Atlantic, our data conflict with SST reconstructions from alkenones, which may be due to an environmental preference of the alkenone-producing algae or to an underestimation of foraminiferal SSTs due to anomalous high abundances of N. pachyderma (sinistral).

Flux and stable isotope composition of Neogloboquadrina pachyderma and other planktonic foraminifers in the Southern Ocean(Atlantic sector).

Planktonic foraminifers were collected with time-series sediment-traps in Bransfield Strait, Northern Weddell Sea, and near Maud Rise. Seasonal flux patterns of planktonic foraminifers are similar to total flux patterns. The dominant species was the left-coiling Neogloboquadrina pachyderma. At the Maud Rise, in an open-ocean polynya,Turborotalita quinqueloba, Globigerinita glutinata and Globigerinoides ruber (white) also were abundant. δ18O values of N. pachyderma ranged from 3 to 3.6%, corresponding to an apparent temperature range of about 3°C. The smaller size classes showed the lower δ18O values. Oxygen isotope variability between size classes and during the year are explained by seasonal changes in temperature and vital effect. δ13C values of the foraminifer shells ranged from 0.1 to 1.1%. The largest sizes had the highest and the smallest sizes the lowest values. Changes in the deviation from thermodynamic equilibrium during bio-mineralization seem to be involved in this pattern of 13C/12C ratios.

Seasonal productivity dynamics in the pelagic central Benguela System inferred from the flux of carbonate and silicate organisms

Flux of bulk components, carbonate- and silicate-bearing skeleton organisms, and the d 15 N-isotopic signal were investigated on a 1-year time-series sediment trap deployed at the pelagic NU mooring site (Namibia Upwelling, ca. 29jS, 13jE) in the central Benguela System. The flux of bulk components mostly shows bimodal seasonality with major peaks in austral summer and winter, and moderate to low export in austral fall and spring. The calcium carbonate fraction dominates the export of particulates throughout the year, followed by lithogenic and biogenic opal. Planktonic foraminifera and coccolithophorids are major components of the carbonate fraction, while diatoms clearly dominate the biogenic opal fraction. Bulk d 15 N isotopic composition of particulate matter is positively correlated with the total mass flux during summer and fall, while negatively correlated during winter and spring. Seasonal changes in the intensity of the main oceanographic processes affecting the NU site are inferred from variations in bulk component flux, and in the flux and diversity patterns of individual species or group of species. Influence from the Namaqua (Hondeklip) upwelling cell through offshore migration of chlorophyll filaments is stronger in summer, while the winter flux maximum seems to reflect mainly in situ production, with less influence from the coastal and shelf upwelling areas. On a yearly basis, dominant microorganisms correspond well with the flora and fauna of tropical/ subtropical waters, with minor contribution of near-shore organisms. The simultaneous occurrence of species with different ecological affinities mirrors the fact that the mooring site was located in a transitional region with large hydrographic variability over short-time intervals. D 2002 Elsevier Science B.V. All rights reserved.

Linking desert evolution and coastal upwelling: Pliocene climate change in Namibia

A late Pliocene high-resolution pollen record from the southeast Atlantic (Ocean Drilling Program Site 1082) registers vegetation development in southwest Africa. The marine record is continuous, ranges from 3.5 to 1.7 Ma, and has a millennial resolution between 2.9 and 1.9 Ma. Changes in climate and vegetation correspond to the Matuyama diatom maximum of the Namibian upwelling system and seem to be highly susceptible to latitudinal shifts in the Polar Front Zone of the Southern Ocean. A northward advance of the polar fronts is connected with an increase in winter rainfall in southwest Africa. Rapid desiccation in Namibia at 2.2 Ma is associated with increasing upwelling and decreasing sea- surface temperatures along the coast.

Pleistocene variations in dust input and marine productivity in the northern Benguela Current: Evidence of evolution of global glacial-interglacial cycles

Abstract We infer variations in paleoproductivity and eolian input at ODP Site 1082 in the Walvis Basin from stable oxygen isotope compositions of the planktonic foraminifera Globorotalia inflata, total organic carbon mass accumulation rates (TOC MAR), and X-ray fluorescence analyses of Fe content. The most pronounced paleoclimatic changes correspond to the time at about 0.9 Ma, when glacial conditions in the northern hemisphere (NH) led to the onset of pronounced 100-kyr glacial–interglacial cycles. We used Fe intensity as a proxy for eolian terrigenous input, and TOC MAR as a paleoproductivity indicator. Paleoproductivity and eolian input show generally higher-amplitude variations of glacial–interglacial cyclicity from 1.5 to 0.58 Ma, indicating pronounced variations in upwelling-favorable winds in this area. At 0.58 Ma, paleoproductivity and eolian input shifted abruptly to lower-amplitude variations with a periodicity of 100 kyr while δ18O values show a trend toward more negative isotope values for the past 0.65 Myr. Especially during glacial periods, oxygen isotope values indicate increasingly warmer sea-surface temperatures toward the end of the Pleistocene. To evaluate the relative influences of NH glaciation and southern hemisphere (SH) insolation as potential forcing mechanisms for variations of eolian input and productivity in the northern Benguela system, we filtered our proxy records at orbital frequencies. The filtered records of Fe intensity and TOC MAR indicate a strong influence of the 100-kyr and 41-kyr frequency bands, supporting our assumption that strong ice buildup in the NH is the dominant trigger for climate changes on the continent and probably in trade-wind intensity. SH insolation and low-latitude precession-related insolation changes were important for paleoproductivity variations in the northern Benguela system, modifying the nutrient supply by southern ocean intermediate waters and the zonal direction of upwelling-inducing trades by the African monsoon system, respectively.

The South Atlantic Oxygen Isotope Record of Planktic Foraminifera

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 δ18O of planktic foraminifera are temperature, the δ18O and the pH of ambient seawater. Seasonal and vertical calcification weight the mean δ18O of a foraminiferal population towards the hydrographic conditions in the preferred ecological niche. After deposition, the δ18O 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 modem 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.