Accumulation rates and composition of organic matter in sediments off Peru

Deep-sea sediment samples from three Ocean Drilling Program (ODP) Leg 112 sites on the Peru continental margin were investigated, using a number of organic geochemical and organic petrographic techniques, for amounts and compositions of the organic matter preserved. Preliminary results include mass accumulation rates of organic carbon at Site 679 and characteristics of the organic facies for sediments from Sites 679, 681, and 684.
Organic-carbon contents are high, with few exceptions. Particularly high values were determined in the Pliocene interval at Site 684 (4%-7.5%) and in the early Pliocene to Quaternary section of Hole 679D (2%-9%). Older sediments at this site have distinctively lower organic-carbon contents (0.2%-2.5%). Mass accumulation rates of organic matter at Site 679 are 0.02 to 0.07 g carbon/cm**2/k.y. for late Miocene to early Pliocene sediments and higher by a factor of 5 to 10 in the Quaternary sediments. The organic matter in all samples has a predominantly marine planktonic and bacterial origin, with minor terrigenous contribution. Organic particle sizes are strikingly small, so that only a minor portion is covered by visual maceral analysis.
Molecular organic-geochemical data were obtained for nonaromatic hydrocarbons, aromatic hydrocarbons (including sulfur compounds), alcohols, ketones, esters, and carboxylic acids. Among the total extractable lipids, long-chain unsaturated ketones from Prymnesiophyte algae strongly predominate among the gas chromatography (GC) amenable components. Steroids are major constituents of the ketone and free- and bound-alcohol fractions. Perylene is the most abundant aromatic hydrocarbon, whereas in the nonaromatic hydrocarbon fractions, long-chain n-alkanes from higher land plants predominate, although the total terrigenous organic matter proportion in the sediments is small.

本研究针对秘鲁大陆边缘大洋钻探计划（Ocean Drilling Program, ODP）第112航次的3个站位深海沉积物样品，采用多种有机地球化学与有机岩石学分析技术，对其中保存的有机质的含量与组成展开系统调查。初步研究成果涵盖679站位的有机碳质量堆积速率，以及679、681与684站位沉积物的有机相特征。
有机碳含量整体偏高，仅少数例外情况。其中684站位上新统地层（4%~7.5%）与679D孔早上新世至第四纪沉积段（2%~9%）的有机碳含量尤为突出。该站位较老沉积物的有机碳含量则显著偏低（0.2%~2.5%）。679站位晚中新世至早上新世沉积物的有机质质量堆积速率为0.02~0.07 g碳/cm²/千年，第四纪沉积物的该速率较前者高出5~10倍。所有样品中的有机质主要源自海洋浮游生物与细菌，仅伴随少量陆源贡献。有机质颗粒尺寸普遍极小，仅能通过显微组分分析识别出极小一部分颗粒。
研究还获取了非芳香烃、芳香烃（含含硫化合物）、醇类、酮类、酯类及羧酸类物质的分子有机地球化学数据。在可通过气相色谱（Gas Chromatography, GC）检测的组分中，源自定鞭藻门藻类的长链不饱和酮类占据绝对主导地位。甾类化合物是酮类组分以及游离与结合态醇类组分的主要构成成分。苝是丰度最高的芳香烃，而在非芳香烃组分中，尽管沉积物中陆源有机质占比偏低，但高等陆生植物来源的长链正构烷烃仍占据主导地位。