Sedimentary specimens of the planktonic foraminifera Globorotalia
inflata can provide much needed information on subsurface conditions of
past oceans. However, interpretation of its geochemical signal is
complicated by possible effects of cryptic diversity and encrustation.
Here we address these issues using plankton tow and sediment samples
from the western South Atlantic, where the two genotypes of G. inflata
meet at the Brazil-Malvinas Confluence Zone. The 18O and δ13C of
encrusted specimens from both genotypes from a core within the
confluence zone are indistinguishable. However, we do find a large
influence of encrustation on δ18O and Mg/Ca. Whereas crust Mg/Ca ratios
are at all locations lower than lamellar calcite, the crust effect on
δ18O is less consistent in space. Plankton tows show that encrusted
specimens occur at any depth and that even close to the surface crust
Mg/Ca ratios are lower than in lamellar calcite. This is inconsistent
with formation of the crust at lower temperature at greater depth.
Instead we suggest that the difference between the crust and lamellar
calcite Mg/Ca ratio is temperature-independent and due to the presence
of high Mg/Ca bands only in the lamellar calcite. The variable crust
effect on δ18O is more difficult to explain, but the higher incidence of
crust free specimens in warmer waters and the observation that a crust
effect is clearest in the confluence zone, hint at the possibility that
the difference reflects advective mixing of specimens from warmer and
colder areas, rather than vertical migration.