Chemical composition of carbonate hardground cements

Trace metal and isotopic ratios, including some rare earth elements, Mg/Ca, manganese and strontium concentrations, δ¹⁸O, δ¹³C, and ⁸⁷Sr/⁸⁶Sr, were analyzed in the carbonate cements from 17 Phanerozoic carbonate hardgrounds. The sensitivity of the geochemical signal to alteration depends on the geochemical analysis in question and the environmental water-rock ratio. Of these samples, only our modern sample has measurements consistent with primary precipitation from seawater; all other samples precipitated from chemically evolved seawater or were influenced by meteoric water, even if only minimally changed. The more recent samples from the Cenozoic had seawater ⁸⁷Sr/⁸⁶Sr. The Mesozoic samples, in contrast, did not preserve seawater ⁸⁷Sr/⁸⁶Sr, even though the Mg/Ca, δ¹⁸O, and δ¹³C values were consistent with precipitation from seawater. Finally, the Paleozoic samples preserved expected seawater ⁸⁷Sr/⁸⁶Sr, though REE and δ¹⁸O suggest primary precipitation was from evolved seawater. Additionally, we place our results in the context of open vs. closed system precipitation using transects of the Mg/Ca ratios across individual cements. Overall, we stress that one geochemical measurement provides only a partial record of fluid composition, but multiple measurements allow a potential understanding of the seawater geochemical signal.

本研究对17个显生宙（Phanerozoic）碳酸盐硬底（carbonate hardgrounds）中的碳酸盐胶结物（carbonate cements）开展了地球化学分析，分析指标涵盖痕量金属（trace metal）与同位素比值，包括部分稀土元素（Rare Earth Elements, REE）、Mg/Ca比值、锰（manganese）与锶（strontium）浓度、氧同位素δ¹⁸O、碳同位素δ¹³C以及锶同位素⁸⁷Sr/⁸⁶Sr。地球化学信号对蚀变作用的敏感性，取决于所针对的地球化学分析对象与环境中水岩比（water-rock ratio）。在所获全部样品中，仅现代样品的测试结果符合海水原生沉淀的特征；其余所有样品要么形成于经历化学演化的海水，要么受到大气降水（meteoric water）的影响，即便蚀变程度极低。新生代（Cenozoic）的新近样品，其⁸⁷Sr/⁸⁶Sr比值与海水同位素组成一致。与之相反，中生代（Mesozoic）样品未能保留海水的⁸⁷Sr/⁸⁶Sr比值，尽管其Mg/Ca、δ¹⁸O及δ¹³C数值均符合海水沉淀的特征。最后，古生代（Paleozoic）样品保留了预期的海水⁸⁷Sr/⁸⁶Sr比值，但稀土元素与δ¹⁸O数据表明，其原生沉淀形成于演化后的海水。此外，本研究通过单颗碳酸盐胶结物的Mg/Ca比值剖面（transects），将研究结果置于开放体系（open system）与封闭体系（closed system）沉淀的研究框架中进行探讨。综上，本研究强调：单项地球化学测试仅能反映流体组成的部分信息，而多指标测试则可帮助我们潜在地还原海水的地球化学信号。