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.

原生干酪根（Proto-kerogens）通过萃取与氢氟酸/盐酸（HF/HCl）处理，从卡里亚科海槽（Cariaco Trench）全新世沉积物岩芯样品中分离得到，其插值年龄分别为900年、2850年和6000年，并通过显微、光谱及热解方法联用进行了表征。研究表明，此类原生干酪根主要通过降解-再缩合途径由浮游植物组分形成；尽管水柱与沉积物环境普遍被认为是形成硫化有机质的有利条件，但天然硫化途径仅起到次要作用。因此，在该环境条件下，通过硫化作用形成原生干酪根——即经由连续反应生成由硫交联、抗酸水解的不溶性高分子量结构的终产物——是一个相对缓慢的过程。不过，随着埋藏深度增加，硫化组分在总原生干酪根中的占比显著提升，这是因为在时间-深度区间内持续发生硫化作用；而通过降解-再缩合途径形成的干酪根，在900年及以上年龄的样品中已基本完成形成过程。相较于脂类硫化作用，碳水化合物硫化作用的速率更快，且在最浅部样品中仅检测到硫化碳水化合物。与之相对，另外两份原生干酪根样品中则存在硫化脂类。此外，硫化脂类相对于硫化碳水化合物的占比随深度增加而升高，这可能是因为脂类相较于碳水化合物具有更强的抗矿化能力。