The influx of reducing (i.e., anoxic) fluids through deep-sea and shallow-sea hydrothermal systems into the ocean plays a major role in determining ocean chemistry and its evolution over geologic history. The interface between hydrothermal fluids and the overlying cool and well-oxygenated seawater is characterized by intense thermodynamic disequilibrium, driving a suite of biogeochemical reactions, which in turn further shape ocean chemistry. These reactions generate strong geochemical redox gradients transitioning from the marine water column into the sediments beneath along the interface with reducing hydrothermal fluids. These gradients provide a framework where microbial communities live, evolve, shape, and are shaped by their local geochemical environment. However, the link between the metabolic activity of these micron-sized microorganisms and the resulting geochemical signatures (typically sampled at approximately cm-scale resolution) remains elusive, in large part because of the difficulty of extracting sufficient material for geochemical analyses at small scales (~1-100 microns). Yet, it is precisely this fine-scale geochemical variability that directly impacts biogeochemical activity. This research takes advantage of recent advances in analytical techniques that allow for geochemical and stable isotopic measurements at a spatial resolution as small as ~1-10 microns. Samples come from accessible shallow-sea hydrothermal systems in the Mediterranean Sea, where diverse sulfur cycling has been previously identified and steep redox gradients make it possible for micron-scale measurements to sample across a wide range of geochemical environments. In addition, a 20+-year record of observations allows the results to be put into a rich geochemical context. Goals are to generate an unparalleled high-resolution geochemical and isotopic characterization of sulfur cycling in shallow-sea hydrothermal systems and use the resulting data to calculate the thermodynamic drivers for diverse biogeochemical reactions. The data will also allow the evaluation of the role of ambient geochemistry on isotopic fractionation during sulfur biogeochemical cycling and application of results to understanding variations in the isotopic composition of sulfur species in marine sediments through time because sulfur isotopes are one of the principle means to reconstruct paleoenvironmental conditions over Earth history. Broader impacts of the work include undergraduate and graduate student training in the field and laboratory. Cross training in different laboratories will also deepen the integration of research and education. This project also involves international collaboration with Sicilian and German scientists.
