Reduced advection into Atlantic Ocean deep eastern basins during last glaciation maximum

Abstract

Causes of change in deep water δ¹³C can be either global or local in extent. Global causes include (1) climatically-induced changes in the amount of terrestrial biomass which alter the average carbon isotopic composition of the oceanic reservoir¹, and (2) erosion and deposition of organic-rich, continental shelf sediments during sea level fluctuations which change the mean oceanic carbon: phosphorus ratio². Regional gradients of δ¹³C are created by remineralization of organic detritus within the deep ocean itself thus reflecting the distribution of water masses and modern thermohaline flow. Changes in a single geological record of benthic foraminiferal δ¹³C can result from any combination of these global and abyssal circulation effects. By sampling a large number of cores collected over a wide bathymetric range yet confined to a small geographical region we have minimized the ambiguity. We can assume that each δ¹³C record was equally affected by global causes of δ¹³C variation. The differences seen between the δ¹³C records must, therefore, reflect changes in the distribution of δ¹³C in the deep ocean. We interpret these differences in distribution in terms of changes in the ocean's abyssal circulation. Benthic foraminiferal carbon isotopic evidence from a suite of Sierra Leone Rise cores indicates that the deeper parts of the eastern Atlantic basins underwent a reduction in [O₂] during the maximum of the last glaciation. Reduced advection of O₂-rich deep water through low-latitude fracture zones, associated with increased delivery of organic matter to the deep ocean, lowered the δ¹³C of deep water ΣCO₂ at all depths below the sill separating the eastern and western Atlantic basins.³ This decreased advection into the eastern Atlantic Ocean coincides with the overall decrease in deep water production in the North Atlantic during the last glacial maximum^4–7.