A 210,000-year record of barium variability in the deep northwest Atlantic Ocean

Abstract

THE deep ocean is the storehouse for most of the carbon and nutrients in the ocean–atmosphere system; together with its out-crops at high latitudes, the deep ocean is of great importance in driving glacial–interglacial changes in the carbon dioxide content of the atmosphere1–3. The chemical and physical structure of the deep glacial oceans can be reconstructed by means of palaeochemical tracers; for example, the cadmium and carbon isotope contents of benthic foraminifera have been used to reconstruct the phosphate (or labile nutrient) compositions of deep waters4–14. Recently, it was proposed15 that barium in benthic foraminifera could be used to reconstruct the distribution of refractory, deep-regenerated chemical properties in the water masses of the glacial oceans. A reconstruction of barium concentra-tions in the oceans at the time of the Last Glacial Maximum revealed the greatest changes in the North Atlantic, where barium concentrations in deep water were up to 50% higher than at present16. Here we present a record of the benthic Ba/Ca ratio in the deep northwest Atlantic stretching back to interglacial stage 7 (∼210 kyr ago), encompassing two full glacial cycles. A comparison of the barium record with that of the other nutrient-like tracers lends confidence in the general agreement between these tracers while showing some spectral variability unique to barium, thus underscoring its utility as an additional palaeoceanographic tracer.