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

• Published in: Paleoceanography Vol. 27; no. 1
• Main Authors: Hoffman, J. L., Lund, D. C.
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-03-2012
• Subjects: Antarctic Bottom Water; Bottom water; Budgets; Carbon dioxide; Carbon isotopes; Holocene; Isotopes; Marine; Ocean floor; Physical oceanography; stable carbon isotopes; Stable isotopes; stable oxygen isotopes; tracer budget; Water column; Water depth; ASW, Atlantic; ASE, Atlantic; AS, Equatorial Atlantic, Antarctic Bottom Water; A, Atlantic
• ISSN: 0883-8305; 2572-4517; 1944-9186; 2572-4525
• DOI: 10.1029/2011PA002216

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 CO2 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 δ18O and δ13C. 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 δ18O and δ13C 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 δ18O reaches 4.9‰ at 4 km water depth, the highest LGM δ18O value in the published literature. The corresponding δ13C of −0.2‰ is less depleted than expected and >0.5‰ greater than δ13C in the Southeast Atlantic. Our Peclet number estimates suggest δ13C acted conservatively during both the Holocene and LGM. Both δ18O and δ13C 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.
Key Points
Brazil Margin stable isotope profiles are representative of the SW Atlantic
LGM oxygen and carbon isotopes indicate LGM Psi/kappa was larger than today
The upper boundary of AABW was located at ~2.5 km water depth during the LGM