Refining the stable isotope budget for Antarctic Bottom Water: New foraminiferal data from the abyssal southwest Atlantic

摘要

Stable isotope tracer budget results suggest the transport to vertical diffusivity ratio for Antarctic Bottom Water (AABW) in the Atlantic was higher at the Last Glacial Maximum (LGM). Reduced mixing across the upper boundary of AABW is consistent with movement of this surface away from the seafloor and may be a factor in sequestering CO _2 in the abyssal Atlantic. Two key unknowns in the budget are the isotopic composition of AABW and the spatial representativeness of isolated vertical profiles of δ ~(18)O and δ ~(13)C. Due to a lack of core material below 3 km water depth, Lund et al. (2011) based their Holocene budget on water column data and their LGM budget on extrapolation of isotopic trends from shallower cores. Here we determine δ ~(18)O and δ ~(13)C for AABW using new isotopic records from 3 to 4 km water depth at the Brazil Margin. The core top data yield tracer budget parameters consistent with water column data in the broader Southwest Atlantic. At the LGM, benthic δ ~(18)O reaches 4.9‰ at 4 km water depth, the highest LGM δ ~(18)O value in the published literature. The corresponding δ ~(13)C of-0.2‰ is less depleted than expected and >0.5‰ greater than δ ~(13)C in the Southeast Atlantic. Our Peclet number estimates suggest δ ~(13)C acted conservatively during both the Holocene and LGM. Both δ ~(18)O and δ ~(13)C imply the transport to vertical diffusivity ratio for AABW was an order of magnitude larger during the LGM, due to enhanced AABW transport or reduced mixing across its upper boundary.