Stable isotopes and Mg/Ca measurements of two sediment cores in the northeastern tropical Atlantic

Two SST records based on Mg/Ca of G. ruber (pink) from the continental slope off West Africa at 15°N and 12°N shed new light on the thermal bipolar seesaw pattern in the northeastern tropical Atlantic during periods of reduced Atlantic Meridional Overturning Circulation (AMOC) associated with Heinrich stadials H1 to H6. The two records indicate that the latitudinal position of the bipolar seesaw's zero-anomaly line, between cooling in the North and warming in the South, gradually shifted southward from H6 to H1. A conceptual model is presented that aims to provide a physically consistent mechanism for the southward migration of the seesaw's fulcrum. The conceptual model suggests latitudinal movements of the Intertropical Convergence Zone, driven by a combination of orbital-forced changes in the meridional temperature gradient within the realm of the Hadley cell and the expansion of the Northern Hemisphere cryosphere, as a major factor.

Planktonic foraminifera Mg/Ca-based temperatures and planktonic foraminiferal cenus counts of core GeoB10065-7 (Lombok Basin, Indonesia)

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.

Oxygen and carbon isotopic composition in Eocene and Oligocene planktonic and benthic foraminifera from DSDP sediments

Oxygen and carbon isotope ratios in Eocene and Oligocene planktonic and benthic foraminifera have been investigated from Atlantic, Indian, and Pacific Ocean locations. The major changes in Eocene-Oligocene benthic foraminiferal oxygen isotopes were enrichment of up to 1 per mil in 18O associated with the middle/late Eocene boundary and the Eocene/Oligocene boundary at locations which range from 1- to 4-km paleodepth. Although the synchronous Eocene-Oligocene 18O enrichment began in the latest Eocene, most of the change occurred in the earliest Oligocene. The earliest Oligocene enrichment in 18O is always larger in benthic foraminifera than in surface-dwelling planktonic foraminifera, a condition that indicates a combination of deep-water cooling and increased ice volume. Planktonic foraminiferal d18O does not increase across the middle/late Eocene boundary at our one site with the most complete record (Deep Sea Drilling Project Site 363, Walvis Ridge). This pattern suggests that benthic foraminiferal d18O increased 40 m.y. ago because of increased density of deep waters, probably as a result of cooling, although glaciation cannot be ruled out without more data. Stable isotope data are averaged for late Eocene and earliest Oligocene time intervals to evaluate paleoceanographic change. Average d18O of benthic foraminifera increased by 0.64 per mil from the late Eocene to the early Oligocene d18O maximum, whereas the average increase for planktonic foraminifera was 0.52 per mil. This similarity suggests that the Eocene/Oligocene boundary d18O increase was caused primarily by increased continental glaciation, coupled with deep sea cooling by as much as 2°C at some sites. Average d18O of surface-dwelling planktonic foraminifera from 14 upper Eocene and 17 lower Oligocene locations, when plotted versus paleo-latitude, reveals no change in the latitudinal d18O gradient. The Oligocene data are offset by ~0.45 per mil, also believed to reflect increased continental glaciation. At present, there are too few deep sea sequences from high latitude locations to resolve an increase in the oceanic temperature gradient from Eocene to Oligocene time using oxygen isotopes.

Physical and chemical analysis of firn cores from Ronne Ice Shelf, Antarctica

This data on the distribution of the accumulation rate and 18O content of near-surface layers in the eastern part of the Ronne Ice Shelf, Antarctica, were derived from an analysis of 16 firn cores. The firn cores were drilled along the traverse route of the Filchner-V-Campaign in 1995. The traverse followed an ice flowline of the Foundation Ice Stream and reached the margin of the inland ice, an area which has not yet been investigated. On the ice shelf the accumulation rates decrease with distance from the coast. Ascending to the inland ice the accumulation rates again reach almost coastal values. This regional distribution is in agreement with the temperature gradient along the traverse. The 18O content of the near-surface layers is closely related to the 10 m firn temperature. They strongly decrease from the grounding line towards the inland ice. At the southernmost site at 1100 m a.s.l., the mean d18O value of the firn decreases to -40?. Ice with that isotopic signature was found in cores from the central part of the Ronne Ice Shelf just above the marine ice layer, indicating that it originates from this area. All ice deposited as snow further south was melted beneath the ice shelf after passing the grounding-line area. The time series of accumulation rate and 18O content reveal no climatic trend during the last 30-50 years.