Nd-isotope ratios of Cretaceous sediment samples

The role that meridional overturning circulation (MOC) patterns played in poleward heat transport during the extreme warmth of the Early to Late Cretaceous is a fundamental and unresolved question in climate dynamics. In order to address this question we must determine where deep waters formed, and how they may have circulated during periods of extreme warmth. Here we present late Albian through Maastrichtian (105 to 65 Ma) Nd isotope records from Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sites in the proto-Indian Ocean and the tropical Pacific. Comparison of these data with previously published records indicates deep-water formation in the Indian sector of the Southern Ocean began at least ?105 Ma, extending the record of high-latitude convection back into the Early Cretaceous prior to the peak warmth of the mid-Cretaceous. The growing body of data supports a mode of MOC in part characterized by high-latitude downwelling during the peak of greenhouse warmth of the Mesozoic and Cenozoic. However, this mode of MOC likely was characterized by numerous locations of deep convection that were regionally important, but not significant in terms of a globally overturning circulation due to paleogeographic and bathymetric barriers.

Seawater Nd and Pb isotopic record in central Pacific and Caribbean sediments

The shoaling and final closure of the Central American Seaway (CAS) resulted in a major change of the global ocean circulation and has been suggested as an essential driver for strengthening of Atlantic Meridional Overturning Circulation (AMOC). The exact timing of CAS closure is key to interpreting its importance. Here we present a reconstruction of deep and intermediate water Nd and Pb isotope compositions obtained from fossil fish teeth and the authigenic coatings of planktonic foraminifera in the eastern equatorial Pacific (Ocean Drilling Program (ODP) Site 1241) and the Caribbean (ODP Sites 998, 999, and 1000) covering the final stages of CAS closure between 5.6 and 2.2 Ma. The data for the Pacific site indicate no significant Atlantic/Caribbean influence over this entire period. The Caribbean sites show a continuous trend to less radiogenic Nd isotope compositions during the Pliocene, consistent with an enhancement of Upper North Atlantic Deep Water (UNADW) inflow and a strengthening of the AMOC. Superimposed onto this long-term trend, shorter-term changes of intermediate Caribbean Nd isotope signatures approached more UNADW-like values during intervals when published reconstructions of seawater salinity suggested complete closure of the CAS. The data imply that significant deep water exchange with the Pacific essentially stopped by 7 Ma and that shallow exchange, which still occurred at least periodically until approximately 2.5 Ma, may have been linked to the strength of the AMOC but did not have any direct effect on the intermediate and deep Caribbean Nd isotope signatures through mixing with Pacific waters.

Sr and Nd composition, stable oxygen isotope record and lithogenic flux in eastern Mediterranean sediments

Isotopic ratios of Sr and Nd from lithogenic components of three isochronous core sections recovered from an east-west transect in the Eastern Mediterranean Sea (EMS) have been analyzed. The data are used for a quantitative estimate of the temporal and spatial variation of detrital flux to the EMS, assuming Saharan dust and Aegean/Nile particulate matter as dominant end members. It was established that the carbonate-free Saharan dust flux during deposition of the nonsapropel layers of marine oxygen isotope stage 5.4 (MIS 5.4) was similar to the present flux. During the deposition of sapropels S5 and S6, however, the Saharan dust input was drastically reduced and was not balanced by a change in the riverine influx at this time. Denser vegetation cover during more humid conditions may have reduced physical erosion and sediment removal in the source area. During marine oxygen isotope stage 6.2 (MIS 6.2) a pronounced increase of Saharan dust and detrital influx from the Aegean region is evident and implies more arid conditions in the southern and northern catchment areas. During this period, intersite variations are interpreted in terms of their geographic location relative to the seaways connecting the Aegean Sea and EMS. The width of the straits and hence the amount of sediment entering the eastern basins may have been affected by a low sea level that impeded interbasin sediment dispersal.

Nd, Pb and Sr isotopes in sediment cores from the Gulf of Cadiz and the Portuguese margin

Mediterranean Outflow Water (MOW) is characterised by higher temperatures and salinities than other ambient water masses. MOW spreads at water depths between 500 and 1500 m in the eastern North Atlantic and has been a source of salinity for the Atlantic Meridional Overturning Circulation in the North Atlantic. We used high-resolution Nd and Pb isotope records of past ambient seawater obtained from authigenic ferromanganese coatings of sediments in three gravity cores at 577, 1745 and 1974 m water depth in the Gulf of Cadiz and along the Portuguese margin complemented by a selection of surface sediments to reconstruct the extent and pathways of MOWover the past 23 000 years. The surface and downcore Nd isotope data from all water depths exhibit only a very small variability close to the present day composition of MOW but do not reflect the present day Nd isotopic stratification of the water column as determined from a nearby open ocean hydrographic station. In contrast, the Pb isotope records show significant and systematic variations, which provide evidence for a significantly different pattern of the MOW pathways between 20 000 and 12 000 years ago compared with the subsequent period of time.