Proto-kerogens in Holocene sediments of ODP Hole 165-1002C

Proto-kerogens were isolated, by extraction and HF/HC1 treatment, from core samples of Holocene sediments of the Cariaco Trench, with interpolated ages of 900, 2850 and 6000 years, and examined via a combination of microscopic, spectroscopic and pyrolytic methods. It appears that these proto-kerogens were chiefly formed from phytoplanktonic components via the degradation-recondensation pathway. The natural sulfurisation pathway only afforded a minor contribution, in spite of the conditions prevailing in the water column and sediments that correspond to those generally considered as especially favourable for the formation of sulfurised organic matter. Proto-kerogen formation via sulfurisation, i.e. the endpoint of the continuum leading to insoluble high molecular weight structures cross-linked by sulfur and resistant to acid hydrolysis, is therefore a rather slow process under these conditions. However, the contribution of sulfurised moieties to the total proto-kerogen substantially increased with depth due to continuous sulfurisation in the time/depth interval, whereas formation through degradation-recondensation is almost complete for the 900 years old sample onwards. Proto-kerogen formation via carbohydrate sulfurisation is faster than lipid sulfurisation and only sulfurised carbohydrates were detected in the shallowest sample. In contrast, sulfurised lipids occur in the other two proto-kerogens. Moreover, their contribution relative to sulfurised carbohydrates increases with depth, probably due to the higher resistance of lipids to mineralisation compared to carbohydrates.原干酪根（Proto-kerogens）通过萃取与氢氟酸/盐酸（HF/HCl）处理，从卡里亚科海槽全新世沉积物岩芯中分离得到，其插值年龄分别为900年、2850年与6000年；随后采用显微、光谱学与热解技术联用对其开展表征分析。研究结果显示，此类原干酪根主要由浮游植物组分经降解-再缩合途径形成。尽管该区域水柱与沉积物环境普遍被认为是硫化有机质形成的最优条件之一，但自然硫化途径仅起到次要贡献。由此可见，在该环境条件下，经由硫化作用形成原干酪根的过程相对缓慢——此类硫化作用是指通过连续反应生成硫交联、抗酸解的不溶性高分子量结构的过程终点。不过，由于在时间-深度区间内持续发生硫化作用，硫化组分在总原干酪根中的占比随埋藏深度增加而显著提升；而通过降解-再缩合途径的原干酪根形成过程，在900年年龄的沉积物样品中已基本完成。相较于脂质硫化，碳水化合物硫化的反应速率更快，且在最浅的沉积物样品中仅检测到硫化碳水化合物。与之相对，另外两份原干酪根样品中均检出硫化脂质。此外，硫化脂质相对于硫化碳水化合物的占比随埋藏深度增加而升高，这可能是因为脂质相较于碳水化合物具有更强的抗矿化能力。