Key Laboratory of Petroleum Geochemistry, Research Institute of Petroleum Exploration and Development, China National Petroleum Corporation, Beijing, China.
Department of Geoscience and Natural Resource Management, University of Copenhagen, Copenhagen K, Denmark.
Geobiology. 2019 May;17(3):225-246. doi: 10.1111/gbi.12337. Epub 2019 Mar 6.
The Mesoproterozoic Era (1,600-1,000 million years ago, Ma) geochemical record is sparse, but, nevertheless, critical in untangling relationships between the evolution of eukaryotic ecosystems and the evolution of Earth-surface chemistry. The ca. 1,400 Ma Xiamaling Formation has experienced only very low-grade thermal maturity and has emerged as a promising geochemical archive informing on the interplay between climate, ecosystem organization, and the chemistry of the atmosphere and oceans. Indeed, the geochemical record of portions of the Xiamaling Formation has been used to place minimum constraints on concentrations of atmospheric oxygen as well as possible influences of climate and climate change on water chemistry and sedimentation dynamics. A recent study has argued, however, that some portions of the Xiamaling Formation deposited in a highly restricted environment with only limited value as a geochemical archive. In this contribution, we fully explore these arguments as well as the underlying assumptions surrounding the use of many proxies used for paleo-environmental reconstructions. In doing so, we pay particular attention to deep-water oxygen-minimum zone environments and show that these generate unique geochemical signals that have been underappreciated. These signals, however, are compatible with the geochemical record of those parts of the Xiamaling Formation interpreted as most restricted. Overall, we conclude that the Xiamaling Formation was most likely open to the global ocean throughout its depositional history. More broadly, we show that proper paleo-environmental reconstructions require an understanding of the biogeochemical signals generated in all relevant modern analogue depositional environments. We also evaluate new data on the δ Mo of Xiamaling Formation shales, revealing possible unknown pathways of molybdenum sequestration into sediments and concluding, finally, that seawater at that time likely had a δ Mo value of about 1.1‰.
中元古代（16 亿至 10 亿年前，Ma）的地球化学记录较为稀少，但对于厘清真核生物生态系统的演化与地球表面化学演化之间的关系至关重要。大约 1400 Ma 的下马岭组仅经历了非常低的热成熟度，是一个很有前景的地球化学档案，可以提供有关气候、生态系统组织以及大气和海洋化学之间相互作用的信息。事实上，下马岭组部分地层的地球化学记录已经被用来对大气氧浓度施加最低限制，以及气候和气候变化对水化学和沉积动力学的可能影响。然而，最近的一项研究认为，下马岭组的一些地层是在高度受限的环境中沉积的，作为地球化学档案的价值有限。在本研究中，我们全面探讨了这些论点以及在使用许多用于古环境重建的示踪剂时所涉及的基本假设。在这样做的过程中，我们特别关注深海氧气最小层环境，并表明这些环境产生了被低估的独特地球化学信号。然而，这些信号与被解释为最受限的下马岭组部分的地球化学记录是兼容的。总体而言，我们得出的结论是，下马岭组在其沉积历史的大部分时间里很可能与全球海洋相连通。更广泛地说，我们表明，正确的古环境重建需要了解所有相关现代类似物沉积环境中产生的生物地球化学信号。我们还评估了下马岭组页岩中 δMo 的新数据，揭示了钼进入沉积物的可能未知途径，并最终得出结论，当时的海水 δMo 值可能约为 1.1‰。
