Hans-Stefan Niebler

Oxygen Isotope Values of Planktic Foraminifera: A Tool for the Reconstruction of Surface Water Stratification

The δ18Ocalcite composition of planktic foraminiferal shells in 59 samples collected from surface sediments between 34°S (south-eastern Subtropical Gyre) and 56°S (Antarctic Zone of the Antarctic Circumpolar Current) and additionally from 20°S (Walvis Ridge) in the South Atlantic Ocean documents the apparent calcification depth of 24 planktic foraminiferal taxa. Based on the δ18Ocalcite signal of their shells, four depth related groups of foraminifera are distinguished, calcifying in a depth between 0 and 50 m (Sphaeroidinella dehiscens, Pulleniatina obliquiloculata, Orbulina bilobata, Globigerinoides sacculifer (with and without sac), Globigerinella aequilateralis, Globigerinoides ruber (white), Globigerina bulloides); 0 and 200 m (itGlobigerinoides conglobatus, Orbulina universa, Neogloboquadrina dutertrei, Turborotalita quinqueloba, Neogloboquadrina pachyderma} (dextral and sinistral)); 100 and 250 m (Globorotalia tumida, Globorotalia menardii, Globigerinella calida, Globorotalia inflata); and deeper than 250 m (Globorotalia hirsuta, Globorotalia scitula, Globorotalia crassaformis, Globorotalia truncatulinoides (dextral and sinistral)). We propose a method to reconstruct temperature gradients between various depths in the water column, but most specifically between 30 m and 250 m, using the δ18Ocalcite composition of foraminiferal calcite. Changes in past sea-surface water stratification in the temperate southern South Atlantic Ocean and in the Antarctic Circumpolar Current can be reconstructed in particular with G bulloides, G truncatulinoides andG inflata. Modern temperature gradients between 30 m and 250 m water depth are reconstructed accurately within a range between 1.7 and 9.4 °C with a correlation coefficient of 0.77 and a standard deviation of ±1.3 °C.

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).

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).

Primary production in the Arabian Sea during the last 135 000 years

Abstract Variations in primary productivity (PP) have been reconstructed in eutrophic, mesotrophic and oligotrophic parts of the Arabian Sea over the past 135 000 years applying principal component analysis and transfer function to planktic foraminiferal assemblages. Temporal variation in paleoproductivity is most pronounced in the mesotrophic northern (NAST site) and oligotrophic eastern (EAST site) Arabian Sea, and comparatively weak in the western eutrophic GeoB 3011-1 site in the upwelling area off Oman. Higher PP during interglacials (250–320 g C m−2 year−1) than during cold stages (210–270 g C m−2 year−1) at GeoB 3011-1 could have been caused by a strengthened upwelling during intensified summer monsoons and increased wind velocities. At NAST, during interglacials, PP is estimated to exceed 250 g C m−2 year−1, and during glacials to be as low as 140–180 g C m−2 year−1. These fluctuations may result from a (1) varying impact of filaments that are associated to the Oman coastal upwelling, and (2) from open-ocean upwelling associated to the Findlater Jet. At EAST, highest productivity of about 380 g C m−2 year−1 is documented for the transition from isotope stage 5 to 4. We suggest that during isotope stages 2, 4, 5.2, the transition 5/4, and the end of stage 6, deep mixing of surface waters was caused by moderate to strong winter monsoons, and induced an injection of nutrients into the euphotic layer leading to enhanced primary production. The deepening of the mixed layer during these intervals is confirmed by an increased concentration of deep-dwelling planktic foraminiferal species. A high-productivity event in stage 3, displayed by estimated PP values, and by planktic foraminifera and radiolaria flux and accumulation rate, likely resulted from a combination of intensified SW monsoons with moderate to strong NE monsoons. Differential response of Globigerina bulloides, Globigerinita glutinata and mixed layer species to the availability of food is suited to subdivide productivity regimes on a temporal and spatial scale.

The South Atlantic Carbon Isotope Record of Planktic Foraminifera

We reviewed the paleoceanographic application of the carbon isotope composition of planktic foraminifera. Major controls on the distribution of δ13C of dissolved CO2 (δ13C∑CO2) in the modern ocean are photosynthesis-respiration cycle, isotopic fractionation during air-sea exchange, and circulation. The carbon isotope composition of surface waters is not recorded without perturbations by planktic foraminifera. Besides δ13C∑CO2 of the surrounding seawater, the δ13C composition of planktic foraminifera is affected by vital effects, the water depth of calcification and postdepositional dissolution. We compared several high-resolution (>10cm/ka) carbon isotope records from the Southern Ocean, the Benguela upwelling system, and the tropical Atlantic. In the Southern Ocean, carbon isotope values are about 1.2 %0 lower during the LGM and up to 1.7 %0 lower during the last deglaciation, when compared to the Holocene. These depletions might be explained with a combination of a subsurface nutrient enrichment and reduced air-sea exchange due to an increased stratification of surface waters. In the Benguela Upwelling system, waters originating in the south are upwelled. While the deglacial minimum is transferred and recorded in its full extent in the δ13C record of Globigerina bulloides, glacial values show only little changes. This might suggest, that the lower glacial δ13C values of high-latitude surface waters are not upwelled off Namibia, or that G. bulloides records post-upwelling conditions, when increased seasonal production has already increased surface-water δ13C. Synchronous to the δ13C depletions in high latitudes, low δ13C values were recorded in Globigerinoides sacculifer during the LGM and during the last deglaciation in the nutrient-depleted western equatorial Atlantic. Hence, part of the glacial-interglacial variability presumably transferred from high to low latitudes seems to be related to changes in thermodynamic fractionation. The variability in δ13C is lowest in the northernmost core M35003-4 from the eastern Caribbean, implying that the Antarctic Intermediate Water might have acted as a conduit to transfer the deglacial minimum to tropical surface waters.

Calibration of Cycladophora davisiana events versus oxygen isotope stratigraphy in the subantarctic Atlantic Ocean - a stratigraphic tool for carbonate-poor Quaternary sediments

We calibrated the Cycladophora davisiana abundances versus oxygen isotope stratigraphy back to220ka for the subantarctic Atlantic Ocean. The relative abundances of C. davisiana and 18Omeasurements of benthic and planktic foraminifera have been determined in two sediment cores.Oxygen isotope stratigraphy has been used to date the C. davisiana records and to assignSPECMAP ages to the C. davisiana events. Comparisons with an existing calibration from thesubantarctic Indian Ocean show, that the C. davisiana events `b2, c1, c2, d, e1, e2, e3, f, h, i1 and i2occur synchronous within the errors of the oxygen isotope stratigraphy in the Indian and the Atlanticsectors of the Southern Ocean. Larger deviations occur only for events `b1 and `g. Furthermore,the long-term fluctuations in C. davisiana abundances have been studied in a sediment core coveringthe last 700kyr. Based on biostratigraphic extinction levels, ages for early Brunhes C. davisianaevents have been estimated. Major C. davisiana abundance maxima occur approximately every100ka in conjunction with glacial/interglacial cycles over the entire record.

The late Pleistocene South Atlantic and Southern Ocean surface waters - A summary of time slice and time series studies

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 sea-ice 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.