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.
The oceanic thermocline is an integral component of the Earth’s climate system, especially in the tropical Pacific Ocean (TPO) and tropical Indian Ocean (TIO). It is widely known that TPO thermocline dynamics are tightly related to El Niño-Southern Oscillation (ENSO) events. In the Indian Ocean, there is a considerable paucity of data that makes drawing any concrete conclusions about regional climate and dynamics difficult. It is clear that a solid understanding of the TIO thermocline is necessary to explain and predict Indian Ocean climate variability on interannual to decadal timescales. A paleoclimate proxy archive could play the crucial role of filling in the gaps in the instrumental record, yet the most common proxies are either confined to surface waters (coral) or accumulate in sediment at much too low of a resolution (foraminifera). This project was, using 14C, to establish the chronology of a novel sclerosponge archive to assess tropical thermocline variability within the Indian Ocean. A companion project is exploring the isotope geochemistry of the sclerosponges. In 2007, a series of dredges were conducted off of the Seychelles Bank (roughly 5.5°S, 57°E) which brought up a variety of carbonate material from depths ranging from 60-150m. A large portion of this material was made up of sclerosponges of genus Acanthochaetetes—biologically simple organisms that precipitate high-Mg calcite in apparent isotopic equilibrium with seawater. Sclerosponges are found throughout the tropics at depths of 0-500 meters and are slow growing at rates up to 2mm/year. This collection presents an ideal opportunity to assess TIO thermocline variability and determine its forcing mechanisms throughout the 20th and likely into the 19th century. The Seychelles Bank, located in the western TIO, is characterized by stepped terrace and drowned coral reef morphology, associated with sea level changes from the last deglaciation. The stepped morphology provides strong vertical constraints on dredge depth and, therefore, the depth at which individual sclerosponges grew. The majority of samples collected were concentrated within 3 individual dredges covering water depths of 73-92m, 95-119m, and 102-165m. Although no living sponge specimens were collected, the roughly 20m, 20m, and 60m depth ranges (respectively) provide good vertical control on the sponge’s actual living habitat, as any specimen that may have fallen downslope would not have been transported more than 15m according to the local bathymetry. While the sponges do not exhibit the concentric density banding that most corals do as a means to provide an easily recognizable growth axis, their mushroom-like growth form does allow for a reasonable guess of their growth orientation (see image). The sponges have a clear basal stem, representing the oldest material, and grow upwards and radially away from the stem. Although the lack of growth banding presents some difficulty in determining the ideal transect for analysis, the general direction of sponge growth is not difficult to define. The best sponge specimens from our collection range in size from about 3-7cm which, paired with an estimated average growth rate of 1.5 mm/year, corresponds to sponges spanning up to 50 years. There was no evidence of living tissue on any specimen, but radiocarbon (Δ14C) data place many of the sponges within the 20th century and potentially as old as the early 1800s. Two sponges span the 14C bomb spike of the early 1960s, one is clearly post-bomb, and the rest that have been analyzed are pre-bomb. The combination of the terrain’s stepped morphology, the uniform condition of all sponge samples, and Δ14C of a handful of other sponges implies that the entire collection likely spans the 20th century and into the 19th century.
