Glacial to Interglacial Changes in the Isotopic Gradients of Southern Ocean Surface Water

Abstract

The Atlantic sector of the Southern Ocean is a region characterized by intense horizontal gradients in surface water properties, including the stable isotopic content. These gradients are climatically sensitive and can be exploited for paleoceanographic purposes, provided there is a means of recording their variability. Here we show that core top values of δ ¹⁸O and δ ¹³C in the planktic foraminiferal species Neoghboquadrina pachyderma l.c. follow those expected for calcite precipitated in equilibrium with surface waters. The entire equator-to-pole gradient in δ ¹⁸Ocalcite for surface waters is roughly 6‰, and the δ ¹⁸O of core top N. pachyderma records fully half of this gradient, increasing by 3‰ over a latitudinal range of 41° – 60°S. Meanwhile, the geographic pattern of δ ¹³O of core top N. pachyderma is similar to observed trends in δ ¹³C of surface ΣCO₂. Highest values are recorded near the present Antarctic Polar Front (APF), where gas exchange rates are the highest; lower values occur both to the south, in the Weddell Gyre, and to the north, near the Subtropical Convergence. Thus, the isotopic composition of N. pachyderma from Quaternary sediments may serve as an effective tracer of the paleochemistry of Southern Ocean surface waters.

The δ ¹⁸O and δ ¹³C of glacial-age N. pachyderma recovered from a transect of South Atlantic cores provide a dynamic contrast to the present surface stable isotopic distribution. While the latitudinal δ ¹⁸O gradient in glacial N. pachyderma is virtually the same as the recent, δ ¹³C values from cores both north and south of the present APF are reduced by an average of 0.8‰. The highest glacial δ ¹³C values (in absolute terms) are observed in sites roughly 5° north of the present APF, suggesting a possible northward translation of the zone of maximum CO₂ exchange between surface waters and the atmosphere.

If in fact the δ ¹³C of N. pachyderma reflects the δ ¹³C of surface ΣCO₂, the overall reduction of values represents a change in the wrong direction for models which explain lower atmospheric CO₂ during glacial periods by increased Southern Ocean productivity. This reduction is. however, compatible with documented changes in Circumpolar Deep Water (CPDW), whose δ ¹³C was affected by varying contributions of North Atlantic Deep Water. Since CPDW is the ultimate source for most of the Southern Ocean surface waters, the most obvious explanation of the surface water δ ¹³C shift involves a direct transferral of water mass properties from upwelling CPDW.

The strong sub-Antarctic isotopic gradients complicate generalized interpretations of the glacial circulation, because even slight frontal movements can have a significant effect on foraminiferal isotopic values. By the same token, however, the N. pachyderma isotopic anomalies may be among the most quantitative and reliable measures of Southern Ocean frontal shifts.