The dynamical response of the tropical thermocline to continued greenhouse gas forcing represented one of the primary feedbacks influencing the future evolution of ocean warming, and understanding of past variability of this response based on paleo records is crucial for accurate simulation of future changes by global ocean/atmosphere models. The impact of the Indonesian Throughflow on the western Indian Ocean thermocline is rarely considered explicitly in climate models, despite the fact that it represents a substantial transfer of heat and energy. This project is aimed at analyzing trends in stable isotopes, trace elements, and radiocarbon in multiple coeval sclerosponge specimens to arrive at a robust, biennially resolved reconstruction of western tropical Indian Ocean thermocline change across the 20th century. The comparison of the radiocarbon trends with stable isotopic/trace element trends will help determine the extent to which the characteristics or strength of the Indonesian Throughflow (ITF) were responsible for the inferred thermocline changes. The research will generate valuable baseline data for assessing present and future impacts of anthropogenic environmental change on deep reef fauna. The award supports a graduate student's dissertation research, as well as outreach efforts in collaboration with the Birch Aquarium at Scripps.
In this project, we developed and analyzed the record of 20th century variability in the Indian Ocean thermocline, afforded by fossil sclerosponge skeletons that were collected fortuitously in a series of sea floor dredges spanning 70-120 meters water depth off the southeastern Seychelles Bank. Sclerosponges represent a relatively novel paleoceanographic archive that can sample an oceanographic province that is otherwise practically inaccessible with either instrumental observations or with other historical archives such as deep sea sediments. Sclerosponges grow their skeletons slowly, but continuously, and without the same metabolic complications that affect the chemistry of surface dwelling corals. Thus, their skeletons faithfully record continuous snapshots of ambient seawater conditions wherever they are found. Here we sought to produce robust and credible continuous reconstructions from the sclerosponges to test the involvement of thermocline variability in governing surface ocean and climate variability on interannual and decadal timescales. Two alternative mechanisms for why one might observe thermocline variability include i.) wind-induced changes in vertical mixing ,or ii.) changes in the "pre-formed" properties of the water mass, which in this region are strongly affected by the characteristics of the Indonesian Throughflow water. These hypothesized mechanisms for thermocline variability have very different implications for the propagation of variability throughout the global oceans, as well as the long term trends in ocean heat storage. Thus, the exact timing of fluctuations in the sclerosponges, as well as the vertical coherence of the excursions, could help distinguish which, if either of these mechanisms played out over the 20th century. This goal involved not only producing credible paleoceanographic "proxy" records from the sclerosponge specimens, but also dating the specimens precisely enough to fix the timing of excursions to within a year or so. This dating was a daunting challenge, but we succeeded through a two-step approach. First, continuous radiocarbon dating showed that three separate sclerosponge specimens spanned all or part of the interval associated with the nuclear bomb testing of the late 1950's and 1960's and subsequent invasion of bomb-produced radiocarbon into the upper ocean. The appearance of the bomb radiocarbon transient in the sclerosponge skeletons fixed their chronology to within ~5 years. To achieve better accuracy, we found that excursions in the skeletal properties of the sclerosponges corresponded with the same properties in a surface dwelling coral record from nearby St. Joseph atoll (which we also developed as part of this project.) The key here is that the coral is annually banded and was cored while still living; therefore, the age of the top surface of the coral core is known, and its chronology is fixed by counting annual bands. Once the detailed chronology for the sclerosponge records was established, the reproducible trends in skeletal chemistry could be compared to known modes of ocean/atmosphere variability. We found that fluctuations in the Indian Ocean thermocline inferred from the sclerosponges corresponded with variability in the so-called Indian Ocean Dipole phenomenon. This correspondence, along with the vertical coherence of the fluctuations, strongly suggest that wind-induced vertical mixing was responsible for variability observed--as opposed to changes in the Indonesian Throughflow properties or its influence. This inference then further implies that the thermocline variability must be considered in the evolution of interannual-decadal variability in the tropical Indian Ocean. Finally, models of the ocean carbon cycle often rely on the penetration of the bomb-produced radiocarbon for calibration. If the detailed chronology we established is accurate, then there was a lag of about 3 years between the rise of bomb radiocarbon in the Indian Ocean surface ocean and its appearance in the Indian Ocean thermocline.
