Constraints on the glacial operation of the Atlantic Ocean's conveyor circulation

Circulation in the Atlantic Ocean is currently dominated by a northward flow of upper waters balanced by a return flow of deep water (i.e., the conveyor). Paleoproxies tell us that, unlike today, during the glacial age the deep Atlantic was stratified. Rather than being flooded with one nearly homogeneous water mass, there were two distinctly different ones. In this paper, the paleoproxy results are analyzed in an attempt to constrain the sources and ventilation rate of the deeper of these two glacial Atlantic water masses. Taken together, the cadmium and carbon isotope measurements on benthic foraminifera and the radiocarbon measurements on coexisting benthic and planktonic foraminifera appear to require a conveyor-like circulation no weaker than half of today's. This conclusion is at odds with geostrophic reconstructions. This seeming disagreement could be eliminated if, as suggested by Keigwin and Schlegel, the radiocarbon measurements by Broecker et al. significantly underestimate the difference between the C-14 to C ratio for glacial-age surface water and deep water in the equatorial Atlantic.