A CONSERVATIVE TRACER FOR GLACIAL OCEAN CIRCULATION FROM CARBON-ISOTOPE AND PALAEO-NUTRIENT MEASUREMENTS IN BENTHIC FORAMINIFERA

THE ratio of cadmium to calcium (Cd/Ca) and the carbon isotope ratio ( delta(13)C) in, the calcite tests of benthic foraminifera both record nutrient distributions in the ocean(1,2). Strict interpretation of bot h delta(13)C and Cd as nutrient tracers has led to conflicting views o f glacial ocean circulation(3-5). Here we show that, when one takes in to account the fact that delta(13)C reflects air-sea exchange as web a s nutrient distributions, these two proxies can provide complementary information about the movement of deep water in the glacial ocean. We use the Cd concentration (assumed to be controlled primarily by biolog ical cycling) to infer the age history of glacial deep water, and dedu ce the sources of deep water from the carbon isotope air-sea exchange signature, a conservative tracer that we construct using both Cd and d elta(13)C 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 hav e originated in the Pacific Ocean. As well as demonstrating the potent ial of this approach, our findings provide further support for a Pacif ic glacial deep water source, evidence for which has until now been bo th scarce and conflicting(3,5-12).