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 atmosphere^1–3. 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 waters^4–14. 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.