Stratigraphic variability in the geochemistry of sedimentary rocks provides critical data for interpreting paleoenvironmental change throughout Earth history. However, the vast majority of pre-Jurassic geochemical records derive from shallow-water carbonate platforms that may not reflect global ocean chemistry. Here, we used calcium isotope ratios (δ⁴⁴Ca) in conjunction with minor-element geochemistry (Sr/Ca) and field observations to explore the links among sea-level change, carbonate mineralogy, and marine diagenesis and the expression of a globally documented interval of elevated carbon isotope ratios (δ¹³C; Hirnantian isotopic carbon excursion [HICE]) associated with glaciation in Upper Ordovician shallow-water carbonate strata from Anticosti Island, Canada, and the Great Basin, Nevada and Utah, USA. The HICE on Anticosti is preserved in limestones with low δ⁴⁴Ca and high Sr/Ca, consistent with aragonite as a major component of primary mineralogy. Great Basin strata are characterized by lateral gradients in δ⁴⁴Ca and δ¹³C that reflect variations in the extent of early marine diagenesis across the platform. In deep-ramp settings, deposition during synglacial sea-level lowstand and subsequent postglacial flooding increased the preservation of an aragonitic signature with elevated δ¹³C produced in shallow-water environments. In contrast, on the mid- and inner ramp, extensive early marine diagenesis under seawater-buffered conditions muted the magnitude of the shift in δ¹³C. The processes documented here provide an alternative explanation for variability in a range of geochemical proxies preserved in shallow-water carbonates at other times in Earth history, and challenge the notion that these proxies necessarily record changes in the global ocean.

中文翻译：

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海平面，碳酸盐岩矿物学和早期成岩作用控制了上奥陶统碳酸盐岩中的δ13 C记录

沉积岩地球化学中的地层变化为解释整个地球历史中的古环境变化提供了重要数据。但是，侏罗纪前的地球化学记录绝大多数来自可能无法反映全球海洋化学的浅水碳酸盐岩平台。在这里，我们使用钙同位素比率（δ ⁴⁴与次要元素地球化学的（Sr / Ca），且实地观察相结合的Ca）探讨中海平面变化，碳酸矿物学，和海洋成岩和记录的全局表达的链接升高的碳同位素比率的间隔（δ ¹³C; 与加拿大安蒂科斯蒂岛，美国内华达州和犹他州大盆地的上奥陶纪浅水碳酸盐岩层冰化作用有关的赫南特同位素碳偏移[HICE]。上安蒂科斯混合动力内燃机是保存在石灰岩低δ ⁴⁴ Ca和高的Sr / CA，与文石作为主要矿物学的一个重要组成部分是一致的。在δ大盆地地层的特征在于横向梯度⁴⁴ Ca和δ ¹³ Ç反映在整个平台早期海洋成岩的程度的变化。在深斜坡设置，期间synglacial海平面低位和随后的冰后期洪水沉积增加的文石签名的保存与升高的δ ¹³在浅水环境中产生的碳。与此相反，在中期和内坡道，海水缓冲条件下广泛早期海洋成岩静音移位的量值δ ¹³ C的过程在这里记载在浅保存的范围地球化学替代的提供可变性的另一种解释碳酸盐在地球历史上的其他时间，并挑战了这些代理必定记录了全球海洋变化的观念。