Investigating how shallow water carbonate systems record the carbon cycle

地球上生命的历史和未来与数十亿年来生物圈、海洋、大气和岩石圈的共同进化紧密相连。古代沉积物是这些系统的碎片记录，而对地球发展关键时刻的详细重建，为我们了解宇宙中生命的稀缺性和作用提供了一个关键的视角。该研究计划的长期目标是从地球历史的碳酸盐沉积记录中记录过去的环境和地球系统变化，特别关注气候和生命的共同进化。地球的碳酸盐沉积记录为研究古代地球的浅海提供了一个窗口。这些沉积岩层层叠叠，为重建过去提供了一个相对的年表。半个多世纪以来，地球科学家一直在记录古代碳酸盐的碳同位素组成，这些碳同位素组成来自地球历史上分散的时间间隔。这些记录构成了我们对地球大气中氧气和二氧化碳演化的最佳猜测的基础。然而，这些猜测通常假设浅海碳酸盐完美地记录了古代全球海洋的平均碳同位素组成。现代数据集表明，今天的浅海碳酸盐的碳同位素组成可能与全球海洋有很大不同，我们并不完全了解驱动这些变化的机制。为了限制沿海水域碳酸盐同位素组成的控制机制，我们将从碳酸盐沉积物活跃积聚的地点收集一套水性质测量和沉积化学。这些地点的选择是为了最大限度地发挥沿海碳循环过程的潜在信号。此外，所选地点与最近的类似沉积矿床的岩石露头直接相邻。当沉积物变成岩石（岩石化）时，岩石的同位素化学成分可能与原始沉积物不同。为了评估这一岩化过程的影响，我们计划绘制岩化矿床的碳同位素变化图，并将这些变化与类似的现代沉积物和沿海海水的观测结果进行比较。为了了解浅层系统是如何记录开放海相碳化学变化的，我们还计划研究更古老的浅层碳酸盐岩，这些浅层碳酸盐岩来自一个时间间隔，在这个时间间隔中，开放海相的碳同位素变化是由海洋岩心独立知道的。对开放海相同位素组成变化的充分研究，为我们提供了一个机会，可以专门研究这些同位素变化是如何以及在哪里被记录在同时期的浅层碳酸盐岩体系中。由于大多数地质记录缺乏海洋化学变化的深海记录，因此从这项工作中获得的见解可以广泛应用于解释地球历史的大量浅层碳酸盐记录。

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.