On the abyssal circulation in the glacial Atlantic

An inverse method is used to evaluate the information contained in sediment data for the Atlantic basin during the Last Glacial Maximum (defined here as the time interval 18-21 kyr before present). The data being considered are an updated compilation of the isotopic ratios O-18/O-16 (delta O-18) and C-13/C-12 (delta C-13) of fossil shells of benthic foraminifera (bottom-dwelling organisms). First, an estimate of the abyssal circulation in the modern Atlantic is obtained, which is consistent with (i) climatologies of temperature and salinity of the World Ocean Circulation Experiment, (ii) observational estimates of volume transport at specific locations, and (iii) the statements of a finite-difference geostrophic model. Second, estimates of water properties (delta O-18 of equilibrium calcite or delta O-18(c) and delta C-13 of dissolved inorganic carbon or delta C-13(DIC)) derived from sediment data are combined with this circulation estimate to test their consistency with the modern flow. It is found that more than approximately 80% of water property estimates (delta O-18(c) or delta C-13(DIC)) are compatible with the modern flow given their uncertainties. The consistency of glacial delta C-13(DIC) estimates with the modern flow could be rejected after two assumptions are made: (i) the uncertainty in these estimates is +/- 0.1% (this uncertainty includes errors in sediment core chronology and oceanic representativity of benthic delta C-13, which alone appears better than this value on average); and (ii) delta C-13(DIC) in the glacial deep Atlantic was dominated by a balance between water advection and organic C remineralization. Measurements of delta C-13 on benthic foraminifera are clearly useful, but the current uncertainties in the distribution and budget of delta C-13(DIC) in the glacial Atlantic must be reduced to increase the power of the test.