Abstract:
Variation in the oxygen and carbon isotopic composition of two species of benthic foraminifera (the epifaunal P. wuellerstorfi and the infaunal U. peregrina) were determined through a Plio-Pleistocene section cored at Site 959C in the eastern equatorial Atlantic. At 2102m water depth, this site lies on the present transition between deep and intermediate waters. Our objectives were to establish a high resolution oxygen-18 time stratigraphy, use the carbon-13 composition of P. wuellerstorfi to measure the relative influence of intermediate and deep waters, and contrast the carbon-13 chemistries of P. wuellerstorfi with U. peregrina to track any changes in the sediment porewater carbon isotope gradient. Variation in the oxygen-18 of seawater overlying this site recorded by two contemporaneous benthic foram species should be parallel. Ours is not, likely because mixing and some combination of mixing and variation in foram relative abundances produce random differences and systematic offsets between the two records. Even with the uncertainties these introduce, it is clear that the association of higher carbonate sediments with interglacial periods that is observed throughout most of the Atlantic is not found at this site. Rather the reverse is seen. Apparently, during glacials local carbonate productivity exceeded the terrigenous dilution observed elsewhere. As for the oxygen-18, variation in the carbon-13 of seawater overlying this site should produce parallel records, however any changes in the porewater carbon-13 gradient will be seen as additional variability in the record of the infaunal species. Assuming that shared variation reflects changes in seawater carbon-13, any differences are attributed to changes in the pore water gradient. Variation in seawater carbon-13 at this site is similar to that seen at other deep water sites throughout the Atlantic, indicating that intermediate waters here remained shallower than 2100m throughout the Plio-Pleistocene.