Abyssal Atlantic circulation during the Last Glacial Maximum: Constraining the ratio between transport and vertical mixing

The ocean's role in regulating atmospheric carbon dioxide on glacial‐interglacial timescales remains an unresolved issue in paleoclimatology. Reduced mixing between deep water masses may have aided oceanic storage of atmospheric CO2 during the Last Glacial Maximum (LGM), but data supporting this idea have remained elusive. The δ13C of benthic foraminifera indicate the Atlantic Ocean was more chemically stratified during the LGM, but the nonconservative nature of δ13C complicates interpretation of the LGM signal. Here we use benthic foraminiferal δ18O as a conservative tracer to constrain the ratio of meridional transport to vertical diffusivity in the deep Atlantic. Our calculations suggest that the ratio was at least twice as large at the LGM. We speculate that the primary cause was reduced mixing between northern and southern component waters, associated with movement of this water mass boundary away from the zone of intense mixing near the seafloor. The shallower water mass boundary yields an order of magnitude increase in the volume of southern component water, suggesting its residence time may have increased substantially. Our analysis supports the idea that an expanded volume of Antarctic Bottom Water and limited vertical mixing enhanced the abyssal ocean's ability to trap carbon during glacial times. Key Points Foraminiferal d18O can be used as a watermass tracer The upper boundary of AABW was at 2 km water depth during the LGM The ratio of transport to vertical mixing for AABW increased at the LGM