Meridional circulation during the Last Glacial Maximum explored through a combination of South Atlantic δ¹⁸O observations and a geostrophic inverse model

The vertical profile of meridional transport in the South Atlantic is examined by combining paleoceanographic observations with a geostrophic circulation model using an inverse method. δ 18 O calcite observations along the margins of the South Atlantic show that upper‐ocean cross‐basin differences were weaker during the Last Glacial Maximum (LGM) than the Holocene. The δ 18 O calcite observations can be explained by a shift of water‐mass properties without any change in the overturning circulation. Alternatively, they may indicate a reduced LGM cross‐basin density difference and, via the thermal wind relation, a reduced vertical shear. Model inversions of δ 18 O calcite are found to require meridional transports different from the modern only after three assumptions are made: temperature and salinity distributions are spatially smooth, the relationship between salinity and δ 18 O water is linear and spatially invariant, and LGM temperatures are known to within 1°C along the margins. The last assumption is necessary because an independent constraint on temperature or salinity is required to determine density from δ 18 O calcite observations. δ 18 O calcite observations are clearly useful, but before any firm constraints can be placed on LGM meridional transport, it appears necessary to better determine the relationship between δ 18 O calcite and density.