UC San Diego

International Ocean Discovery Program (IODP) Site U1477, located in the Mozambique Channel, provides a uniquely high-resolution archive that proves to be extremely insightful for studying Indian Ocean–Atlantic exchange, regional hydroclimate, and biogeochemical variability locally and globally across glacial–interglacial timescales. Complementary geochemical proxies demonstrate that the site records tightly coupled physical and biogeochemical responses to large-scale climate events during the late Pleistocene. I first demonstrate that benthic–planktonic foraminiferal radiocarbon age differences in U1477 are remarkably stable over the past 30 kyr. Given that bottom waters at this site are a direct reflection of the subantarctic surface ocean, this result indicates that the extent of subantarctic surface radiocarbon disequilibrium with the atmosphere—the “reservoir age” —likewise remained relatively constant over the last deglaciation. This result is contrary to previously assumed values of subantarctic surface reservoir age, with important implications for radiocarbon ventilation ages throughout the Southern Ocean. Secondly, I use the foraminiferal-bound nitrogen isotope (FB-δ¹⁵N) tracer in U1477 to develop a unique perspective into the biogeochemical cycling of the Mozambique Channel, a region that is a “hotspot” for nitrogen fixation in the modern ocean. The planktonic foraminiferal FB-δ¹⁵N analyses from U1477 document an intensification of regional nitrogen fixation in association with millennial-scale global climate events known as Heinrich Stadial Events—a result that serves to not only pinpoint the underlying drivers for regional nitrogen fixation, but also highlight the potential for the use of FB-δ¹⁵N records as a monitor of inter-ocean exchange. Finally, I sought to determine the extent to which millennial-scale excursions in benthic foraminiferal oxygen isotopes in U1477 were affected by changes in temperature or salinity, by making Mg/Ca and Mg/Li analyses—ostensibly tracers of temperature alone—in benthic foraminifera. Bottom-water temperature reconstructions obtained from benthic Mg/Ca and Mg/Li ratios were highly variable, not reproducible within or between different foraminiferal tests, and generally unrealistic, despite aggressive culling of analyses for possible metal contamination. Although there were large analytical uncertainties that were faced, all the records analyzed and studied establish the Mozambique Channel as a key gateway for tracing the past Indian Ocean–Atlantic exchange and provide an essential framework for evaluating how physical and biological processes responded to climate variability over the last glacial cycle.