Late quaternary paleochemistry of high-latitude surface waters

Abstract

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 CO 2 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 δ 13C of N. pachyderma and either δ 13C or PO 4 in seawater. Conversely, Cd/Ca ratios in planktonic foraminifera are consistent with the PO 4 content of surface waters. The level of maximum glaciation (18,000 yr B.P.), identified by CLIMAP and δ 18O, was chosen for mapping. Isopleths of δ 18O 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 δ 13C records of N. pachyderma confirm that low δ 13C values occurred in the northeast Atlantic during the latest glacial maximum (Labeyrie and Duplessy, 1985). However, a map of δ 13C for the 18,000 yr B.P. level for a much larger region in the North Atlantic shows that minimum N. pachyderma δ 13C occurred in temperate waters. N. pachyderma δ 13C decreased toward the southwest, reaching a minimum of −1‰ at 37°N. Despite the variability seen in δ 13C records of N. pachyderma, none of our cores show significant temporal variability in Cd/Ca. From the combined Cd/Ca and δ 13C 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.