UC San Diego

The tropical Indian Ocean (TIO) thermocline is a dynamic oceanographic system, the mechanics of which are intrinsically related to various climatic phenomena. Unfortunately, variability within the TIO at depth—in truth, the Indian Ocean as a whole—is not well-characterized relative to the other ocean basins, primarily because of a notoriously sparse observational record. Data reanalysis studies that attempt to recreate the natural state of thermocline variability are inconsistent and fraught with uncertainty. In order to fill in and extend the modern record of the TIO at depth, the use of reliable, continuous paleoarchives is necessary. In 2007, the SIO-led KNOX10RR expedition aboard the R/V Roger Revelle collected water samples, sclerosponge specimens, and a well-resolved sediment core, all off of the eastern Seychelles Bank. This dissertation is principally focused on the analysis of these geochemical archives as a means of assessing thermocline dynamics on modern and glacial scales.

The modern theme of Chapters 1 and 2 centers on the oxygen isotope (δ18O) and radiocarbon (∆14C) characterization of the sclerosponge archive in conjunction with water cast samples and a previously collected surface coral record. This examination allows us to determine a chronology for the non-annually banded sclerosponges to within a 1-year window. Further, a comparison of sclerosponge and surface coral ∆14C creates a unique opportunity to evaluate the vertical transport skill of CCSM and ECCO—two modern ocean circulation models—showing that ECCO is fairly realistic. We then utilize sclerosponge δ18O as a proxy for thermocline variability from 1930-2005; though its resolution precludes us from observing an annual cycle, we see strong interannual variability within the archive and no multidecadal trends throughout its timespan. Chapter 3 assesses thermocline variability at the millennial resolution by analyzing time-constrained population statistics of individual foraminiferal δ18O. This analysis shows reduced seasonal and increased interannual variability during the Last Glacial Maximum, overall enhanced variability 8.0-10.2ka ago, and a moderate state of variability throughout the late Holocene. We draw a connection between monsoon-induced wind stress variability and the TIO thermocline seasonal cycle, and suggest that assessments of seasonality are related to monsoon activity.