Stable carbon and oxygen isotope analyses of Neogloboquadrina pachyderma

Detailed records of the carbon and oxygen isotopic ratios of Neogloboquadrina pachyderma are compared between nine high-latitude sediment cores, from the Northern and Southern Hemispheres, covering the last 140000 yrs. The strong analogies between the delta13C records permit to define a delta13C stratigraphic scale, with three clear cut transitions simultaneous with the oxygen isotopic transitions 6/5 (125 kyrs.), 5/4 (65 kyrs.), and 2/1 (13 kyrs.). The delta13C records of N. pachyderma in the high-latitude cores, which follow the changes in delta13C of the surface water TCO2 near areas of deep water formation present trends similar to the benthic foraminifera delta13C records in cores V19-30 and M12-392, although amplitudes of the isotopic shifts are different. This implies that a large part of the observed variations represents global changes in the carbon distribution between biosphere and ocean. The 13C/12C ratios of N. pachyderma in the North Atlantic cores display larger regional variations at 18 kyrs. B.P. than at present. To explain these differences, we have plotted the 18 kyrs. B.P. delta13C values of N. pachyderma from 17 cores distributed N of 40°N. Comparison with published surface water temperature distribution at 18 kyrs. B.P. indicates that a strong divergent cyclonic cell, centered approximatively 55°N and 15°W, was active during most of the last ice-age maximum. This hydrology, analogous to the present Weddell Sea, explains the published evidences of bottom water formation, if located on the northern flank of the gyre, and the strong polar front on the southern flank, probable location of intermediate water formation.

Stable isotope and Cd/Ca ratios of Neogloboquadrina pachyderma in high-latitude sediments

Recent studies have stressed the role of high latitude nutrient levels and productivity in controlling the carbon isotopic composition of the deep sea and the CO2 content of the atmosphere. We undertook a study of the chemical composition of the polar planktonic foraminifer Neogloboquadrina pachyderma (s., sinistral coiling) from 30 late Holocene samples and 49 down core records from the high-latitude North and South Atlantic Oceans to evaluate the history of sea surface chemical change from glacial to interglacial time. Stable isotopic analysis of coretop samples from the Atlantic, Pacific and Southern Oceans shows no significant correlation between the delta13C of N. pachyderma and either delta13C or PO4 in seawater. Conversely, Cd/Ca ratios in planktonic foraminifera are consistent with the PO4 content of surface waters. The level of maximum glaciation (18,000 yr B.P.), identified by CLIMAP and delta18O, was chosen for mapping. Isopleths of delta18O on N. pachyderma (s.) in the North Atlantic reveal a pattern largely influenced by sea surface temperature (S.S.T.) and generally support the S.S.T. reconstruction of CLIMAP. Differences between the two suggest significantly lower salinity in North Atlantic surface waters at high latitudes than in lower latitudes. Down core delta13C records of N. pachyderma confirm that low delta13C values occurred in the northeast Atlantic during the latest glacial maximum (Labeyrie and Duplessy, 1985, doi:10.1016/0031-0182(85)90069-0). However, a map of delta13C for the 18,000 yr B.P. level for a much larger region in the North Atlantic shows that minimum N. pachyderma delta13C occurred in temperate waters. N. pachyderma delta13C decreased toward the southwest, reaching a minimum of -1 per mil at 37°N. Despite the variability seen in delta13C records of N. pachyderma, none of our cores show significant temporal variability in Cd/Ca. From the combined Cd/Ca and delta13C data we can see no evidence for an upwelling gyre in the eastern North Atlantic during the latest glacial maximum, nor evidence that the southern and northern oceans had significantly different levels of preformed nutrients than today.