Joachim Schönfeld

North Atlantic sea-surface temperature changes and the climate of western Iberia during the last deglaciation; a marine palynological approach

Sea-surface temperature (SST) variations based on δ18O and dinoflagellate cyst records in core SO75-6KL indicate that shifts in the position of the North Atlantic polar front during the last deglaciation led to sudden changes of the SST in the offshore area of Portugal. Our data show that the onset of the two major cooling events, attributed to Heinrich event 1 (H1) and the Younger Dryas (YD), occurred within 600 and 400 years, respectively. The first-order land–sea correlation, provided by the pollen record of SO75-6KL, enables a detailed evaluation of the response of the vegetation on land to altered heat and moisture transport from the North Atlantic Ocean toward SW Europe. The expansion of aridity-tolerant vegetation, as reflected in the pollen record of steppe taxa, occurred within 350 and 180 years from the onset of the cooling events connected to H1 and the YD, respectively. n nThe inception of the warmer interval assigned to the Bolling-Allerod Interstadial shows a less sudden response, probably due to competition or to the lower migration rates for deciduous trees such as Quercus compared to most steppe taxa.

Paleo-export production, terrigenous flux and sea surface temperatures around Tasmania: Implications for glacial/interglacial changes in the Subtropical Convergence zone

The Tasmanian Gateway, focus of ODP Leg 189, is a key oceanographic area within the Southern Ocean. Our investigations concentrate on the last -500,000 years of sedimentation at Sites 1168 (western Tasmanian margin), 1170 and 1171 (Tasman Rise), and 1172 (East Tasman Plateau). A suite of geochemical proxy data reflecting paleo-export production, terrigenous flux, and sea surface temperature, constrain temporal and spatial variations in surface water masses and oceanographic frontal systems over these sites. Interglacial periods were commonly of low productivity and less affected by terrigenous matter supply, suggesting that the position of the Subtropical Convergence remained south of Tasman Rise. Only during early MIS 11 and MIS 9 over the southern Tasman Rise, and during MIS 7 over the northern Tasman Rise, did enhanced marine productivity, combined with an enhanced terrigenous flux, indicate waxing influence of subantarctic waters. During glacial MIS 2, marine productivity and terrigenous flux increased significantly at Sites 1168, 1170, and 1171 implying that the Subtropical Convergence moved northward to ∼42°S west of Tasmania. East of Tasmania, the presence of the East Australian Current caused the Subtropical Convergence to remain south of East Tasman Plateau. Glacial MIS 6 appears to have been different from MIS 2. The Subtropical Convergence stayed north of East Tasman Plateau, but clearly south of Site 1168 on the western Tasmanian margin. Strongly enhanced marine productivity and terrigenous flux during MIS 10 and 12 at Sites 1168, 1170, and 1172 suggest the dominant influence of subantarctic waters and the position of the Subtropical Convergence north of East Tasman Plateau. At South Tasman Rise, in contrast, the reduced terrigenous flux implies that Site 1171 moved outside the belt of westerly winds. Marine productivity ceased at that time mainly due to iron limitation.

Coupling of oceanic carbon and nitrogen facilitates spatially resolved quantitative reconstruction of nitrate inventories

Anthropogenic impacts are perturbing the global nitrogen cycle via warming effects and pollutant sources such as chemical fertilizers and burning of fossil fuels. Understanding controls on past nitrogen inventories might improve predictions for future global biogeochemical cycling. Here we show the quantitative reconstruction of deglacial bottom water nitrate concentrations from intermediate depths of the Peruvian upwelling region, using foraminiferal pore density. Deglacial nitrate concentrations correlate strongly with downcore δ13C, consistent with modern water column observations in the intermediate Pacific, facilitating the use of δ13C records as a paleo-nitrate-proxy at intermediate depths and suggesting that the carbon and nitrogen cycles were closely coupled throughout the last deglaciation in the Peruvian upwelling region. Combining the pore density and intermediate Pacific δ13C records shows an elevated nitrate inventory of >10% during the Last Glacial Maximum relative to the Holocene, consistent with a δ13C-based and δ15N-based 3D ocean biogeochemical model and previous box modeling studies.Understanding controls on past nitrogen budgets can improve predictions for future global biogeochemical cycling. Here, using foraminiferal pore density and δ13C, the authors present a quantitative record of deglacial nitrate from the intermediate Pacific and infer close coupling between carbon and nitrogen cycles.

Mg / Ca and δ 18 O in living planktic foraminifers from the Caribbean, Gulf of Mexico and Florida Straits

Past ocean temperatures and salinities can be approximated from combined stable oxygen isotopes (δ18O) and Mg /Ca measurements in fossil foraminiferal tests with varying success. To further refine this approach, we collected living planktic foraminifers by net sampling and pumping of sea surface water from the Caribbean Sea, the eastern Gulf of Mexico and the Florida Straits. Analyses of δ18O and Mg /Ca in eight living planktic species (Globigerinoides sacculifer, Orbulina universa, Neogloboquadrina dutertrei, Pulleniatina obliquiloculata, Globorotalia menardii, Globorotalia ungulata, Globorotalia truncatulinoides and Globorotalia tumida) were compared to measured in situ properties of the ambient seawater (temperature, salinity and δOseawater) and fossil tests of underlying surface sediments. “Vital effects” such as symbiont activity and test growth cause δ18O disequilibria with respect to the ambient seawater and a large scatter in foraminiferal Mg /Ca. Overall, ocean temperature is the most prominent environmental influence on δOcalcite and Mg /Ca. Enrichment of the heavier 18O isotope in living specimens below the mixed layer and in fossil tests is clearly related to lowered in situ temperatures and gametogenic calcification. Mg /Ca-based temperature estimates of G. sacculifer indicate seasonal maximum accumulation rates on the seafloor in early spring (March) at Caribbean stations and later in the year (May) in the Florida Straits, related to the respective mixed layer temperatures of ∼ 26 C. Notably, G. sacculifer reveals a weak positive linear relationship between foraminiferal derived δOseawater estimates and both measured in situ δOseawater and salinity. Our results affirm the applicability of existing δ18O and Mg /Ca calibrations for the reconstruction of past ocean temperatures and δOseawater reflecting salinity due to the convincing accordance of proxy data in both living and fossil foraminifers, and in situ environmental parameters. Large vital effects and seasonally varying proxy signals, however, need to be taken into account.

Pleistocene to Holocene benthic foraminiferal assemblages from the Peruvian continental margin

The benthic foraminiferal inventory and their assemblage composition was documented nalong five sediment cores from the Peruvian margin between 3°S and 18°S at nwater depths of 500 to 1250 m, covering the lower boundary of today’s Oxygen Minimum nZone (OMZ). Emphasis was given to certain time intervals during the last 22 nthousand years when different climatic and oceanographic conditions prevailed than ntoday. In total three agglutinated and 186 calcareous species were recognised. Bolivina ncostata, Bolivinita minuta, Cassidulina delicata and Epistominella exigua were most nabundant. The foraminiferal distributions revealed a marked change in assemblage ncomposition particularly at the deeper cores during and after the deglaciation. The ndiversity declined and Bolivina species became dominant. These changes took place ngradually over several millennia, and high-frequency fluctuations were not recorded. nThis pattern provides evidence for rather stable ecological conditions and sluggish nchanges in bottom water circulation during the last deglaciation.