Barium peaks at glacial terminations in sediments of the equatorial Atlantic Ocean—relicts of deglacial productivity pulses?

Abstract

In sediment cores recovered from oligotrophic settings of the western and eastern north-equatorial Atlantic Ocean, Ceará Rise and Sierra Leone Rise, distinct peaks of excess barium were detected at glacial/interglacial transitions. These Ba maxima are unrelated to any other potential productivity proxy, e.g. organic carbon, carbonate or opal. Moreover, they coincide with minima in organic carbon contents. Despite the lack of correlation between excess barium and organic carbon, we ascribe these Ba spikes to pulses of productivity that occurred during glacial/interglacial transitions. The discrepancy between barium and organic carbon in these transitional sediment intervals is attributed to the action of downward-progressing oxidation fronts during deglacial nonsteady-state depositional conditions. The oxidation fronts were initiated due to the overall low sedimentation rates prevailing in the study areas and an increase in bottom water oxygen concentrations at the onset of enhanced NADW production during interglacial periods. The fronts led to a very efficient oxidation of the organic carbon initially present leaving peaks in solid phase barium as relicts of these short-time productivity events. This assumption is supported by the depth arrangement of the solid phase peaks of Ba, Fe, and Mn along the 6/5 oxygen isotope stage boundary in sediments of the Ceará Rise which show striking similarities to the distribution of these elements in oxidized sapropel intervals in eastern Mediterranean sediments. It remains unclear whether productivity was proportional to the magnitude of the Ba excess maxima. Possibly, higher dissolved Ba concentrations in intermediate and deep waters during glacial/interglacial transitions caused the precipitation of barite to increase out of proportion of the (postulated) deglacial productivity pulses.