How do shallow-water carbonates record seawater chemistry? Insights from the Mesoproterozoic Belt-Purcell Supergroup

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The carbonate rock record forms a 3500 million-year-long history of the physical, biological and chemical conditions of marine environments. Deep-sea carbonates, composed of microfossils and deposited on oceanic crust, are available to study time periods <250 million years old (Ma.). Owing to plate tectonics, the only carbonates older than 250 Ma. that exist today were deposited in shallow water, formed on continental crust flooded with marine waters. This distinction is critical, because shallow marine waters can be only tenuously connected to the open ocean, meaning that their chemistry often diverges from the global ocean. For example, the carbon isotopic composition of dissolved inorganic carbon (d13CDIC) in these settings can be very different from open ocean d13CDIC, influenced by local processes such as river outflow, evaporation and high local rates of photosynthesis. Carbonate d13C values (d13Ccarb) are one of the most common measurements in Earth history research, used as proxies to study the global carbon cycle in deep time. However, this approach assumes that d13Ccarb reflects the d13CDIC of the global ocean. Extreme variability in d13Ccarb in shallow-water carbonates, especially in the Precambrian, broadly co-occur with momentous changes in Earth history, such as atmospheric oxygenation and the radiation of animal life, leading to proposed roles for an anomalous global carbon cycle in these events. However, recent work challenges these common interpretations. Based on the dependence of measured carbonate geochemistry on depositional environment and early diagenetic history, these papers argue that extreme d13C values are a result of local processes occurring in shallow marine waters. I propose to unravel the origin of anomalous variability in d13C values by studying why it is absent for almost a quarter of Earth's history, between ~2000 and 1000 Ma. If the geochemistry of shallow-water carbonates are dominated by local processes, which should always be active, why is there muted variability in d13Ccarb for 1000 million years? Or do shallow-water carbonates of this age truly record global carbon cycle stasis, and thus record global processes (and if so, why at this time, but not others)? The ~1400 Ma. Lower Belt Group of western Canada and US, with its range of paleo-water depths and early diagenetic histories, offers an ideal study region to disentangle global phenomena from local processes in shallow-water carbonates by integrating sedimentology with geochemical measurements. If local processes dominate, how we interpret the 3500 million-year-long record of d13Ccarb will be transformed. Proposed mechanisms to interpret ancient carbonate geochemistry would have to shift from global cycles towards how changing boundary conditions on Earth's surface influence shallow marine environments. The record will remain an invaluable archive of an evolving Earth system, but one that encodes processes different from canonical interpretations.