ALBERT

Description: This file contains benthic foraminiferal oxygen and carbon isotopes for IODP Expedition 356 Site U1463. The data were used to construct the age model for the site for the Plio-Pleistocene. Therefore the file also contains depth within core section, and at CSF-A and CCSF (= composite mcd) scales, and the ages for the respective samples.

Description: Benthic foraminiferal d18O and Mg/Ca of sediment cores off tropical NW Africa are used to study the properties of Atlantic central waters during the Last Glacial Maximum (LGM) and Heinrich Stadial 1 (HS1). We combined our core top data with published results to develop a new Mg/Ca-temperature calibration for Planulina ariminensis, which shows a Mg/Ca-temperature sensitivity of 0.19 mmol/mol per °C. Estimates of the LGM and HS1 thermocline temperatures are comparable to the present-day values between 200 and 400 m water depth, but were 1.2-1.5°C warmer at 550-570 m depth. The HS1 thermocline waters (200-570 m depth) did not show any warming relative to the LGM. This is in contrast to previous climate model studies, which concluded that tropical Atlantic thermocline waters warmed significantly when Atlantic meridional overturning circulation was reduced. However, our results suggest that thermocline temperatures of the northeastern tropical Atlantic show no pronounced sensitivity to changes in the thermohaline circulation during glacial periods. In contrast, we find a significant increase in thermocline-water salinity during the LGM (200-550 m depth) and HS1 (200-400 m depth) with respect to the present-day, which we relate to changes in the wind-driven circulation. We infer that the LGM thermocline (200-550 m depth) and the HS1 upper thermocline (200-400 m depth) in the northeastern tropical Atlantic was ventilated by surface waters from the North Atlantic rather than the southern-sourced waters. This suggests that the frontal zone between the modern South Atlantic and North Atlantic Central Waters was probably shifted southward during the LGM and HS1.

Description: In order to investigate a possible connection between tropical northeast (NE) Atlantic primary productivity, Atlantic meridional overturning circulation (AMOC), and drought in the Sahel region during Heinrich Stadial 1 (HS1), we used dinoflagellate cyst (dinocyst) assemblages, Mg/Ca based reconstructed temperatures, stable carbon isotopes (d13C) and geochemical parameters of a marine sediment core (GeoB 9508-5) from the continental slope offshore Senegal. Our results show a two-phase productivity pattern within HS1 that progressed from an interval of low marine productivity between ~ 19 and 16 kyr BP to a phase with an abrupt and large productivity increase from ~ 16 to 15 kyr BP. The second phase is characterized by distinct heavy planktonic d13C values and high concentrations of heterotrophic dinocysts in addition to a significant cooling signal based on reconstructions of past sea surface temperatures (SST). We conclude that productivity variations within HS1 can be attributed to a substantial shift of West African atmospheric processes. Taken together our results indicate a significant intensification of the North East (NE) trade winds over West Africa leading to more intense upwelling during the last millennium of HS1 between ~ 16 and 15 kyr BP, thus leaving a strong imprint on the dinocyst assemblages and sea surface conditions. Therefore, the two-phase productivity pattern indicates a complex hydrographic setting suggesting that HS1 cannot be regarded as uniform as previously thought.

Description: Sedimentary specimens of the planktonic foraminifera Globorotalia inflata can provide much needed information on subsurface conditions of past oceans. However, interpretation of its geochemical signal is complicated by possible effects of cryptic diversity and encrustation. Here we address these issues using plankton tow and sediment samples from the western South Atlantic, where the two genotypes of G. inflata meet at the Brazil-Malvinas Confluence Zone. The δ18O and δ13C of encrusted specimens from both genotypes from a core within the confluence zone are indistinguishable. However, we do find a large influence of encrustation on δ18O and Mg/Ca. Whereas crust Mg/Ca ratios are at all locations lower than lamellar calcite, the crust effect on δ18O is less consistent in space. Plankton tows show that encrusted specimens occur at any depth and that even close to the surface crust Mg/Ca ratios are lower than in lamellar calcite. This is inconsistent with formation of the crust at lower temperature at greater depth. Instead we suggest that the difference between the crust and lamellar calcite Mg/Ca ratio is temperature-independent and due to the presence of high Mg/Ca bands only in the lamellar calcite. The variable crust effect on δ18O is more difficult to explain, but the higher incidence of crust free specimens in warmer waters and the observation that a crust effect is clearest in the confluence zone, hint at the possibility that the difference reflects advective mixing of specimens from warmer and colder areas, rather than vertical migration.

Description: Modern variability in upwelling off southern Indonesia is strongly controlled by the Australian-Indonesian monsoon and the El Niño-Southern Oscillation, but multi-decadal to centennial-scale variations are less clear. We present high-resolution records of upper water column temperature, thermal gradient and relative abundances of mixed layer- and thermocline-dwelling planktonic foraminiferal species off southern Indonesia for the past two millennia that we use as proxies for upwelling variability. We find that upwelling was generally strong during the Little Ice Age (LIA) and weak during the Medieval Warm Period (MWP) and the Roman Warm Period (RWP). Upwelling is significantly anti-correlated to East Asian summer monsoonal rainfall and the zonal equatorial Pacific temperature gradient. We suggest that changes in the background state of the tropical Pacific may have substantially contributed to the centennial-scale upwelling trends observed in our records. Our results implicate the prevalence of an El Niño-like mean state during the LIA and a La Niña-like mean state during the MWP and the RWP.

Description: The Southern Westerly Winds (SWW) exert a crucial influence over the world ocean and climate. Nevertheless, a comprehensive understanding of the Holocene temporal and spatial evolution of the SWW remains a significant challenge due to the sparsity of high-resolution marine archives and appropriate SWW proxies. Here, we present a north-south transect of high-resolution planktonic foraminiferal oxygen isotope records from the western South Atlantic. Our proxy records reveal Holocene migrations of the Brazil- Malvinas Confluence (BMC), a highly sensitive feature for changes in the position and strength of the northern portion of the SWW. Through the tight coupling of the BMC position to the large-scale wind field, the records allow a quantitative reconstruction of Holocene latitudinal displacements of the SWW across the South Atlantic. Our data reveal a gradual poleward movement of the SWW by about 1-1.5° from the early to the mid-Holocene. Afterwards variability in the SWW is dominated by millennial-scale displacements in the order of 1° in latitude with no recognizable longer-term trend. These findings are confronted with results from a state-of-the-art transient Holocene climate simulation using a comprehensive coupled atmosphere-ocean general circulation model. Proxy-inferred and modeled SWW shifts compare qualitatively, but the model underestimates both orbitally forced multi-millennial and internal millennial SWW variability by almost an order of magnitude. The underestimated natural variability implies a substantial uncertainty in model projections of future SWW shifts.

Description: The use of planktonic foraminifera in paleoceanographic studies relies on the assumption that morphospecies represent biological species with ecological preferences that are stable through time and space. However, genetic surveys unveiled a considerable level of diversity in most morphospecies of planktonic foraminifera. This diversity is significant for paleoceanographic applications because cryptic species were shown to display distinct ecological preferences that could potentially help refine paleoceanographic proxies. Subtle morphological differences between cryptic species of planktonic foraminifera have been reported, but so far their applicability within paleoceanographic studies remains largely unexplored. Here we show how information on genetic diversity can be transferred to paleoceanography using Globorotalia inflata as a case study. The two cryptic species of G. inflata are separated by the Brazil-Malvinas Confluence (BMC), a major oceanographic feature in the South Atlantic. Based on this observation, we developed a morphological model of cryptic species detection in core top material. The application of the cryptic species detection model to Holocene samples implies latitudinal oscillations in the position of the confluence that are largely consistent with reconstructions obtained from stable isotope data. We show that the occurrence of cryptic species in G. inflata, can be detected in the fossil record and used to trace the migration of the BMC. Since a similar degree of morphological separation as in G. inflata has been reported from other species of planktonic foraminifera, the approach presented in this study can potentially yield a wealth of new paleoceanographical proxies.