Investigating how shallow water carbonate systems record the carbon cycle

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The history and future of life on Earth is intricately tied to the coevolution of the biosphere, oceans, atmosphere, and lithosphere over billions of years. Ancient sediments are the fragmented record of these systems, and detailed reconstructions of key moments in the development of our planet offer a critical perspective on the rarity and role of life in our universe. The long term goal of the proposed research program is to document past environmental and Earth systems change from the carbonate sedimentary record of Earth history, with particular focus on the coevolution of climate and life. The carbonate sedimentary record of Earth offers a window to investigate the shallow seas of the ancient Earth. These sedimentary rocks are stacked in layers that provide a relative chronology for reconstructing the past. For more than half a century, Earth scientists have been documenting the carbon isotopic composition of ancient carbonates from intervals of time scattered throughout Earth history. These records form the backbone of our best guesses about the evolution of atmospheric oxygen and carbon-dioxide on Earth. However, these guesses generally assume that shallow-marine carbonates perfectly record the average carbon isotopic composition of ancient global ocean. Modern datasets demonstrate that shallow-marine carbonates today can have carbon isotopic composition significantly different than the global ocean, and we do not fully understand the mechanisms driving these variations. To constrain the mechanisms controlling the isotopic composition of carbonates in coastal waters, we will collect a suite of water property measurements and sedimentary chemistry from sites where carbonate sediment is actively accumulating. The sites have been selected to maximize the potential signal of coastal carbon cycling processes. Moreover, the selected sites are immediately adjacent to rocky outcrops of analogous sedimentary deposits from the recent past. When sediments turn into rock (lithify), the isotopic chemistry of the rocks can differ from the original sediment. To assess the impact of this lithification process we plan to map the carbon isotopic variability of the lithified deposits and compare those variations to the observations of the analogous modern sediments and coastal sea-water. To understand how changes in open-marine carbon chemistry are recorded by shallow systems, we also plan to investigate much older shallow-carbonate rocks from a time interval where open-marine carbon isotopic changes are independently known from marine cores. Well studied shifts in open-marine isotopic composition offer opportunity to specifically target how and where these isotopic changes are recorded in the coeval shallow carbonate system. Because most of the geologic record lacks deep marine records of changes in ocean chemistry, insights gained from this work can be widely applied to interpreting the vast shallow carbonate record of Earth history.