Deep Circulation Changes In The Central South Atlantic During The Past 145 kyrs Reflected In A Combined 231Pa/230Th, Neodymium Isotope And Benthic δC13 Record
Faculty of Health, Engineering and Science
School of Natural Sciences / Centre for Marine Ecosystems Research
Previous work showed that South Atlantic sediments have lower glacial than Holocene 231Pa/230Th, which was attributed to a switch in the flow direction of Atlantic deep-water. Debate exists, however as to the degree to which two processes – circulation and scavenging – determine sedimentary 231Pa/230Th, making this interpretation contentious. Here we address this issue using 145-kyr records of paleocirculation proxies. Benthic foraminiferal δC13, neodymium isotopes (εNd) and sedimentary 231Pa/230Th were all measured in a single sediment core from the South Atlantic subtropical gyre. This site largely excludes the influence of local productivity changes on 231Pa/230Th records. Measured 231Pa/230Th ranges between ∼0.041 during glacials and ∼0.055 during interglacial periods and is consistently lower than the production ratio, indicating export of 231Pa from the central South Atlantic for the entire duration of the record. The lower glacial 231Pa/230Th is regionally consistent, suggesting that basin-scale oceanographic processes cause the decrease. In turn, less radiogenic εNd and lower benthic δC13 confirm the classical picture of an increase in Southern Component Water (SCW) influence in the Atlantic during glacial periods and point to a circulation control on the observed 231Pa/230Th decrease rather than a local productivity change. We suggest that associated with this change in water mass distribution the dominant sink for 231Pa shifted from the margins of the South Atlantic and/or the Southern Ocean during interglacials, to the North Atlantic during glacial periods. Indeed, elevated 231Pa/230Th in the deep North Atlantic during glacials supports this mechanism of northward transport of 231Pa by SCW.