How much did Glacial North Atlantic Water shoal?

Abstract

Observations of δ13C and Cd/Ca from benthic foraminifera have been interpreted to reflect a shoaling of northern source waters by about 1000 m during the Last Glacial Maximum, with the degree of shoaling being significant enough for the water mass to be renamed Glacial North Atlantic Intermediate Water. These nutrient tracers, however, may not solely reflect changes in water mass distributions. To quantify the distribution of Glacial North Atlantic Water, we perform a glacial water mass decomposition where the sparsity of data, geometrical constraints, and nonconservative tracer effects are taken into account, and the extrapolation for the unknown water mass end-members is guided by the modern-day circulation. Under the assumption that the glacial sources of remineralized material are similar to that of the modern day, we find a steady solution consistent with 241 δ13C, 87 Cd/Ca, and 174 δ18O observations and their respective uncertainties. The water mass decomposition indicates that the core of Glacial North Atlantic Water shoals and southern source water extends in greater quantities into the abyssal North Atlantic, as previously inferred. The depth of the deep northern-southern water mass interface and the volume of North Atlantic Water, however, are not grossly different from that of the modern day. Under this scenario, the vertical structure of glacial δ13C and Cd/Ca is primarily due to the greater accumulation of nutrients in lower North Atlantic Water, which may be a signal of the hoarding of excess carbon from the atmosphere by the glacial Atlantic.