University of Cambridge > > Institute for Energy and Environmental Flows (IEEF) > The link between the global carbon cycle and the interplay between calcium and sulfur in marine pore fluid

The link between the global carbon cycle and the interplay between calcium and sulfur in marine pore fluid

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The removal of carbon from the surface of the planet is a critical component of the long-term carbon cycle; this removal is through the deposition and subsequent burial of carbonate minerals in the ocean. Sedimentary, post-depositional processes play a key role in the global carbon cycle because much of the organic carbon in sediments is microbially oxidized back to dissolved inorganic carbon. The oxidation of organic carbon in anaerobic sediment through the reduction of sulfate can consume protons, raising pH, and leading to some of the dissolved inorganic carbon precipitating as in situ, or authigenic carbonate. Because this authigenic carbonate has a different carbon isotope composition to marine carbonate, it may influence the overall carbon isotope balance at Earth’s surface and critically, our interpretation of global shifts in the carbon cycle over geological time. In today’s ocean the two main processes that produce alkalinity and drive authigenic carbonate formation are the organiclastic reduction of sulfate and the anaerobic oxidation of methane, coupled to microbial sulfate reduction. However, each of these produces authigenic carbonate with a different carbon isotope composition, and it is unclear how important these processes were in past oceans with limited sulfate concentrations. I will discuss how we can identify the processes that are linked to the precipitation of authigenic carbonate, and how we can utilise the global ODP /IODP pore fluid database to examine these processes on a global scale. Here we concentrate on the relative change in alkalinity, methane and sulfate concentrations to predict regions of anaerobic methane oxidation versus organiclastic sulfate reduction and the relative decrease in calcium concentration to predict the amount of authigenic carbonate formed. Idealized carbon isotope profiles for dissolved inorganic carbon then allow us to determine the global carbon isotopic composition of the authigenic carbonate sink today. I will present a machine-learning approach to identify the distribution of these processes in the modern ocean tuned with chlorophyll, temperature, water depth and distance from the coast to predict the extent of anaerobic methane oxidation versus organiclastic sulfate reduction to extend the sites with observations to those areas that have none.

This knowledge is then applied to a simple steady-state model to discuss how the formation of authigenic carbonate would have been impacted by the lower sulfate and higher calcium concentrations found in Cretaceous seawater.

This talk is part of the Institute for Energy and Environmental Flows (IEEF) series.

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