Collaborative Research: Investigating the Fate of Carbon at an Ultraslow Spreading Center

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Knowing the cycle of carbon in the Earth and how biological and abiological processes control the composition of the resulting carbon-bearing phases and how those change over time is a major scientific goal that has important implications for our understanding of both ecosystems and global warming. The circulation of seawater through deep-sea hydrothermal vent systems results in the extensive transformation of both inorganic and organic carbon compounds and may have a significant impact on the deep ocean carbon cycle. To date, most reports of organic compounds in deep-sea hydrothermal fluids have focused on methane and light hydrocarbons. The distribution and abundance of other types of organic compounds, however, remains poorly known. As a result, the contribution of hydrothermal systems to organic carbon in the deep ocean and the biogeochemical processes responsible for this input, remain highly uncertain. This research conducts a comprehensive study of the composition and abundance of carbon compounds from fluids discharging from two very different, but closely located, hydrothermal vents on the ultraslow spreading mid-Cayman rise in the Caribbean Sea. The two vent sites are located in substantially different host rocks and occur at very different water depths. They also have very different vent fluid temperatures. Fluids will be collected using multiple, complementary sampling systems and analyzed by various mass spectrometric and other analytical techniques. Comparison between these two sites allows carbon fluxes and chemical transformations to be determined over a range of physical and chemical conditions. Broader impacts of the work include training of graduate and undergraduate students in state-of-the-art geochemical methods and provide them with an opportunity to gain experience and training in sea-going research in a strongly interdisciplinary subject. A graduate and several undergraduate students from the University of South Carolina, an EPSCoR state (i.e., a state that does not receive significant federal dollars); Bridgewater State University, which serves economically disadvantaged communities in Massachusetts; and the University of Colorado at Boulder will be involved in the research and play key roles in both the scientific cruise and supporting laboratory activities.In ocean crust, extensive transformations occur to the carbon compounds present in both the fluids and solids on and below the seafloor due to processes such as heating, precipitation of minerals, injections of magma, and the activity of microorganisms. These processes have consequences for global elemental cycling in the present day and over geologic time. To date, most reports of organic compounds in deep-sea hydrothermal fluids have focused on small, volatile organic molecules such as methane. However, the abundance and fate of other types of organic compounds remains poorly known. To better understand these transformations and the amount of carbon in seafloor hydrothermal systems and their potential impact on deep sea microbes and ecology, samples of hydrothermal fluids emanating from two compositionally and thermally different seafloor vent systems on the mid-Cayman rise (Piccard and von Damm) will be collected on a 16-day oceanographic cruise using a robotic vehicle. Samples will be analyzed for their carbon content and carbon compounds using mass spectrometric and other analytical tools. Goals of the project are to addresses issues such as: (1) the relative contribution of abiotic synthesis, thermal alteration of biomass, fluid-rock interaction, and microbial activity to the inventory of carbon compounds in ridge-crest hydrothermal fluids, (2) how host rock, style of venting, and temperature influence the generation and stability of reduced carbon compounds in axial hot-springs, (3) the fate of seawater-derived dissolved organic carbon during circulation through the oceanic lithosphere, and (4) which organic compounds and signatures are the best geochemical indicators of fluid reaction history. Three different sampling devices will be employed to ensure the full range of carbon containing compounds is captured. Collected samples will be analyzed for bulk characteristics (dissolved organic carbon concentrations and isotopes) and for the composition and isotope characteristics of individual compounds, ranging from volatile (e.g. methane and light hydrocarbons) to non-volatile (e.g. fatty acids), and from labile (e.g. amino acids) to refractory (e.g. polycyclic aromatic hydrocarbons). These data will permit the calculation of carbon and isotope mass balances at each location. From this, a comparison of the fate of carbon in both systems will be carried out to evaluate the importance of rock type, temperature, and style of venting on the carbon-bearing compounds present in seafloor hydrothermal vents. Additional measurements will be made with non-targeted, broad spectrum organic compound analyses to identify alteration trends and carbon compound evolution.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.