Collaborative Research: Southeast Pacific and Southern Ocean Seawater Isotopes Determined from US GEOTRACES GP17-OCE and GP17-ANT Samples

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The stable isotopic composition of the oxygen atom in seawater (d18O) is controlled by evaporation, precipitation, and riverine or glacial meltwater input. Most of the deep interior water masses in the ocean are formed or modified in the Southern Ocean and sink to fill the deep layers that circulate throughout the world’s oceans. The d18O signatures in seawater can be used as tracers for these sub-surface water masses once they sink to depth. The Southern Ocean and South Pacific are critical locations for water mass formation and thus, for understanding global overturning circulation, carbon cycling and climate dynamics. However, data on the d18O of seawater, particularly in the sub-surface, is virtually nonexistent for the South Pacific and Southern Ocean. Moreover, our understanding of past temperature and oceanic processes based on the d18O signature in marine carbonates (e.g. corals and foraminifera) have relied on the assumption that variations in the oxygen isotopic composition of the sub-surface water masses are nominal through time, which can be directly tested on a wider basis using new technologies during this project. Investigators will conduct analyses of seawater d18O from samples collected from depth transects during the US GEOTRACES science expedition to the South Pacific (GP17-OCE) and the Amundsen Sea sector of the Antarctic continental margin (GP17-ANT). This ocean region is of particular significance because it is experiencing rapid environmental changes in the past few decades, including the fastest melting of ice shelves around the entire Antarctic. This project will support the development of diverse involvement in teaching and outreach efforts in a primarily undergraduate and Hispanic serving institution. The outreach activities will provide ample opportunities to engage students from underrepresented groups in both formal and informal settings. It is well-established from first principles that d18O and d2H of seawater have a positive relationship to salinity, but regional variations in the isotopic relationship of seawater to salinity are poorly constrained. Temperature reconstructions relying on d18O in calcite microfossils biologically precipitated in equilibrium with seawater and preserved in marine sediments are only quantitative if the seawater d18O (d18Osw) in which that carbonate was precipitated is known. Despite the reliance of these paleo-estimations on the d18O of modern seawater, surface measurements are few and far between and sub-surface data in the South Pacific and Southern Oceans is even more limited. Filling this fundamental gap in data to quantify the influence of precipitation, evaporation and glacial melt water on the d18O and d2H of interior seawater masses is globally relevant to our understanding of ocean circulation and broader climate dynamics. The development and improvement of new laser-based spectroscopy techniques such as off-axis integrated cavity output spectroscopy (OA-ICOS) and cavity ring-down spectroscopy, allows for seawater d18O and d2H analyses to be run quickly and cost effectively compared to traditional IRMS (Isotope Ratio Mass Spectrometry). A core aim of this project is to compare and contrast IRMS and OA-ICOS analyses to address accuracy, precision and offsets between methods. Obtaining d18Osw directly contributes to the GP17-OCE stated aims of characterizing near- and far-field trace element and isotope (TEI) inputs and to the GP17-ANT stated aims of quantifying gradients in TEI distributions and characterizing, glacial meltwater inputs in the Amundsen Sea and Southern Ocean.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.