Planktonic foraminiferal depth habitat and δ 18 O calibrations: Plankton tow results from the Atlantic sector of the Southern Ocean

Plankton tows conducted in the Atlantic sector of the Southern Ocean allow analysis of the influence of water column structure on planktonic foraminiferal abundance and δ 18 O composition. Foraminiferal abundance varies by several orders of magnitude across a large gradient in sea surface temperature and other hydrographic features, demonstrating high sensitivity of foraminiferal populations to regional differences in water properties. The depth of maximum abundance for key species such as Globigerina bulloides and Neogloboquadrina pachyderma is not constant from station to station. The pattern suggests that their abundance and shell chemistry are tied to density horizons or other conditions (such as food availability) that become more sharply defined with depth in the northern subantarctic. The consistent observation of Globorotalia inflata and Globoratalia truncatulinoides as relatively deep‐dwelling species confirms their utility as indicators of upper thermocline properties. In δ 18 O all species are observed to be isotopically lighter than predicted from water properties, but the species‐specific offset is fairly uniform at all stations. These observations define the utility of multispecies δ 18 O for reconstructing temperature and density stratification from past surface oceans.