Constant bottom water flow into the Indian Ocean for the past 140 ka indicated by sediment Pa-231/Th-230 ratios

摘要

A down-core (231)pa/(230) Th 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 (Pa-231(xs)/Th-230(xs))(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 (Pa-231(xs)/Th-230(xs))(0) value in the record contrasts with an existing sortable silt ((SS) over bar) record from the same core. The observed SS 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 (Pa-231(xs)/Th-230(xs))(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.