Geochemistry and ecology of polar planktonic foraminifera, and applications to paleoceanographic reconstructions

The geochemistry of polar planktonic foraminifera has long been used to monitor paleoclimatic changes in polar surface oceans. Plankton tow, sediment trap, and surface sediment samples of Neogloboquadrina pachyderma (s.) from the polar Atlantic Ocean are used to study how temperature, salinity, carbonate chemistry, diet, and nutrient conditions affect calcification in N. pachyderma (s.). At Arctic sites, plankton tow samples demonstrate that separate processes control foraminiferal productivity and shell chemistry. Highest abundances of N. pachyderma (s.) are found in association with maximum chlorophyll concentrations (20-80 m). However, calcite crust addition triples shell mass at the depth of the main pycnocline (100-200 m). In response to local water column gradients at the Arctic sites, the {dollar}deltasp{lcub}18{rcub}O{dollar} of N. pachyderma (s.) increases by {dollar}1.5perthous{dollar} between 0 and 200 m and no longer represents surface ocean conditions. At the South Atlantic sites where strong vertical gradients are absent, the {dollar}deltasp{lcub}18{rcub}O{dollar} of N. pachyderma (s.) is a faithful recorder of surface ocean conditions. The ability of {dollar}deltasp{lcub}18{rcub}O{dollar} of N. pachyderma (s.) to record surface ocean conditions depends on vertical water column gradients, as evidenced by the different core-top calibrations in the North and South Atlantic Oceans.; The carbon isotope composition of N. pachyderma (s.) is frequently used as a proxy for surface water nutrient conditions in the Southern Ocean. However, carbon isotope disequilibrium between surface water {dollar}rm TCOsb2{dollar} and N. pachyderma (s.) from surface sediments increases from 1 to {dollar}3perthous{dollar} between {dollar}rm 70spcirc S{dollar} and {dollar}rm 40spcirc S,{dollar} and is well-correlated with several oceanographic variables. This carbon isotope disequilibrium in N. pachyderma (s.) is a function of shell size, dietary {dollar}deltasp{lcub}13{rcub}C,{dollar} carbonate ion concentrations, and calcification temperature. Once correction factors for these variables are applied to the {dollar}deltasp{lcub}13{rcub}C{dollar} of N. pachyderma (s.), the {dollar}deltasp{lcub}13{rcub}C{dollar} of {dollar}rm TCOsb2{dollar} in the South Atlantic can be more accurately reconstructed. The corrected {dollar}deltasp{lcub}13{rcub}C{dollar} of N. pachyderma (s.) from glacial South Atlantic sediments suggests that the glacial {dollar}deltasp{lcub}13{rcub}C{dollar} of {dollar}rm TCOsb2{dollar} (0-200 m) was comparable to today between {dollar}rm 55spcirc S{dollar} and {dollar}rm 40spcirc S.{dollar} However, the magnitude of the corrections required to reconstruct the {dollar}deltasp{lcub}13{rcub}C{dollar} of {dollar}rm TCOsb2{dollar} from N. pachyderma (s.) is 2.5-7 times larger than the glacial-to-interglacial signal, and the correction factors to N. pachyderma (s.) require verification in laboratory culture experiments.