Abstract The search for hydrocarbon molecular biomarkers in Archean metasediments is of prime importance for deciphering the early evolution of life. Suitable criteria are required to identify promising targets for further molecular biomarkers. Possible criteria include the Hydrogen-to-Carbon (H/C) atomic ratio used as a proxy of the aliphatic content of the kerogen matrix. However, H/C ratio values exhibit large variation in Archean kerogens and their significance remains poorly understood. In this study, we thus investigate the significance of the H/C ratios of Archean kerogens by combining elemental analyses, Nanoscale Secondary Ion Mass Spectrometry (NanoSIMS), Rock-Eval pyrolysis and Raman spectroscopy. First, NanoSIMS investigations show the H/C ratio of kerogen can be compromised by residual minerals. In addition, Rock-Eval pyrolysis underlines the fact that thermal cracking of Archean kerogens does not just release hydrocarbon covalently linked to the macromolecular network but also a complex mixture of organic pools distinguished by their thermal maturity. Therefore, the H/C ratio alone cannot be used to probe the preservation of aliphatic compounds bound to kerogen since it can be biased by the presence of (i) residual bitumen, as well as (ii) refractory organic matter in secondary hydrothermal veins whose syngenecity is debatable. Rock-Eval pyrolysis then provides a useful and complementary method to check the significance of H/C atomic ratio as a proxy for hydrocarbon preservation in Archean kerogens.

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太古代有机质中确定的氢碳比是什么意思？

摘要 在太古代变沉积物中寻找碳氢化合物分子生物标志物对于破译生命的早期进化至关重要。需要合适的标准来为进一步的分子生物标志物确定有希望的目标。可能的标准包括氢碳 (H/C) 原子比，用作干酪根基质中脂肪族含量的代表。然而，H/C 比值在太古代干酪根中表现出很大的变化，其意义仍然知之甚少。因此，在这项研究中，我们通过结合元素分析、纳米级二次离子质谱 (NanoSIMS)、Rock-Eval 热解和拉曼光谱来研究太古代干酪根 H/C 比的重要性。首先，NanoSIMS 研究表明干酪根的 H/C 比会受到残余矿物的影响。此外，Rock-Eval 热解强调了这样一个事实，即太古代干酪根的热裂解不仅释放与大分子网络共价连接的碳氢化合物，而且还释放出以热成熟度为特征的有机池的复杂混合物。因此，单独的 H/C 比不能用于探测与干酪根结合的脂肪族化合物的保存情况，因为它可能会受到 (i) 残留沥青​​以及 (ii) 次生热液脉中难熔有机质的影响而产生偏差。同质性值得商榷。然后，Rock-Eval 热解提供了一种有用的补充方法来检查 H/C 原子比作为太古代干酪根中碳氢化合物保存代理的重要性。