Description: A down-core 231Pa/230Th record has been measured from the southwestern Indian Ocean to reconstruct the history of deep water flow into this basin over the last glacial-interglacial cycle. The (231Paxs/230Thxs)0 ratio throughout the record is nearly constant at approximately 0.055, significantly lower than the production ratio of 0.093, indicating that the proxy is sensitive to changes in circulation and/or sediment flux at this site. The consistent value suggests that there has been no change in the inflow of Antarctic Bottom Water to the Indian Ocean during the last 140 ka, in contrast to the changes in deep circulation thought to occur in other ocean basins. The stability of the (231Paxs/230Thxs)0 value in the record contrasts with an existing sortable silt (SS) record from the same core. The observed equation image variability is attributed to a local geostrophic effect amplifying small changes in circulation. A record of authigenic U from the same core suggests that there was reduced oxygen in bottom waters at the core locality during glacial periods. The consistency of the (231Paxs/230Thxs)0 record implies that this could not have arisen by local changes in productivity, thus suggesting a far-field control: either globally reduced bottom water oxygenation or increased productivity south of the Opal Belt during glacials.

Description: 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 d13C, neodymium isotopes (ENd) 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 to ~0.055 during interglacial periods and are 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 ENd and lower benthic d13C 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.