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

碳酸盐岩记录形成了长达35亿年的海洋环境物理、生物和化学条件的历史。深海碳酸盐岩由沉积在洋壳上的微体化石组成，可用于研究距今2.5亿年的时间段。由于板块构造作用，仅有的碳酸盐岩年龄大于250 Ma。今天存在的沉积物沉积在浅水中，形成于被海水淹没的大陆地壳上。这种区别是至关重要的，因为浅海水域只能与开阔的海洋微弱地相连，这意味着它们的化学物质往往与全球海洋背道而驰。例如，在这些环境中，溶解无机碳的碳同位素组成(D13CDIC)可能与开阔海域的d13CDIC有很大不同，受河流外流、蒸发和高局部光合作用等局部过程的影响。碳酸盐d13C值(D13Ccarb)是地球历史研究中最常用的测量方法之一，可作为研究深部全球碳循环的指标。然而，这种方法假设d13Ccarb反映了全球海洋的d13CDIC。浅水碳酸盐中d13Ccarb的极端变异性，特别是在前寒武纪，与地球历史上的重大变化，如大气氧化和动物生命的辐射，广泛地共存，导致提出了在这些事件中异常的全球碳循环的作用。然而，最近的研究对这些常见的解释提出了挑战。根据测得的碳酸盐地球化学对沉积环境和早期成岩历史的依赖关系，本文认为，d13C值的极端值是浅海水域局部作用的结果。我建议通过研究为什么在大约2000 Ma到1000 Ma之间，地球历史上几乎四分之一的时间里没有d13C值的异常变化，来揭开它的起源。如果浅水碳酸盐的地球化学性质是由局部过程主导的，而局部过程应该总是活跃的，那么为什么d13Ccarb在1000百万年里会有微弱的变异性呢？或者，这个时代的浅水碳酸盐真的记录了全球碳循环停滞，从而记录了全球过程(如果是这样的话，为什么是在这个时候，而不是其他时候)？约1400 Ma。加拿大西部和美国的下带群，具有广泛的古水深度和早期成岩历史，通过将沉积学和地球化学测量相结合，提供了一个理想的研究区，可以将浅水碳酸盐中的全球现象与局部过程区分开来。如果本地过程占主导地位，我们解释d13Ccarb长达35亿年的记录的方式将会改变。提出的解释古代碳酸盐地球化学的机制必须从全球循环转向地球表面不断变化的边界条件如何影响浅海环境。这份记录仍将是地球系统演化的无价档案，但它编码的过程不同于规范的解释。

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.