A conservative tracer for glacial ocean circulation from carbon isotope and palaeo-nutrient measurements in benthic foraminifera

Abstract

THE ratio of cadmium to calcium (Cd/Ca) and the carbon isotope ratio (δ¹³C) in the calcite tests of benthic foraminifera both record nutrient distributions in the ocean^1,2. Strict interpretation of both δ¹³C and Cd as nutrient tracers has led to conflicting views of glacial ocean circulation^3–5. Here we show that, when one takes into account the fact that δ¹³C reflects air–sea exchange as well as nutrient distributions, these two proxies can provide complemen-tary information about the movement of deep water in the glacial ocean. We use the Cd concentration (assumed to be controlled primarily by biological cycling) to infer the age history of glacial deep water, and deduce the sources of deep water from the carbon isotope air–sea exchange signature, a conservative tracer that we construct using both Cd and δ¹³C measurements. Our analysis suggests that there were at least two sources of glacial deep water: a less dense component originating in the North Atlantic Ocean, and a more dense component which may have originated in the Pacific Ocean. As well as demonstrating the potential of this approach, our findings provide further support for a Pacific glacial deep water source, evidence for which has until now been both scarce and conflicting^3,5–12.