Spatial and vertical gradients in the stable carbon isotope composition of Lower Circumpolar Deep Water over the last 900 thousand years

Abstract:

Changes in stable carbon isotopes (δ¹³C) recorded in benthic foraminiferal calcite reflect that of the dissolved inorganic carbon (DIC) of ambient seawater, and thus are used to reconstruct past changes in water mass mixing. Records of benthic foraminiferal δ¹³C from the Atlantic Ocean have revealed the development of a sharp vertical δ¹³C gradient between 2300-2500m water depth during successive glacial periods throughout the Late Quaternary, with extremely negative δ¹³C values recorded below this depth. It had been hypothesised that this gradient resulted from an increased stratification of water masses within the glacial Atlantic Ocean, and that these extreme δ¹³C values originated in the Southern Ocean. However the mechanisms behind the formation of this gradient and extreme δ¹³C depletion have remained unclear. This is in part due to the poor preservation of calcareous microfossils in the corrosive waters below 2500-3000m found in the Southern Ocean, which hampers our understanding of this key region.

Here we present a unique new δ¹³C deep water record measured on benthic foraminifera (Cibicidoides spp.) from a sediment core recovered from 2100m water depth in the Amundsen Sea, south-eastern Pacific sector of the Southern Ocean. The site is bathed in Lower Circumpolar Deep Water (LCDW) today, and combined palaeomagnetic and oxygen isotope stratigraphy show that the sediments continuously span at least the last 890 ka. A comparison of this new δ¹³C data with other LCDW records from ODP Sites 1089/1090 in the South Atlantic and ODP Site 1123 in the Southwest Pacific demonstrate a clear spatial gradient in circum-Antarctic LCDW during glacial periods. The pool of extremely depleted glacial deep marine δ¹³C is restricted to the Atlantic Sector of the Southern Ocean, with increasingly positive δ¹³C values found in the Southwest Pacific and the south-eastern Pacific sector of the Southern Ocean. This implies that the δ¹³C depletion in the deep glacial Atlantic was sourced in the Atlantic sector of the Southern Ocean, and remained limited to this sector. This finding indicates either increased supply of relatively more positive δ¹³C deep waters or increased vertical mixing in the Indian and Pacific sectors of the glacial Southern Ocean.