Organic petrology of Neogene sediment from ODP Leg 117

Neogene sediments from three areas of the Northern Indian Ocean (Indus Fan, Owen Ridge, Oman Margin, ODP Leg 117) were studied in order to determine the amount, type, and preservation of organic matter as functions of the environments encountered. The work consisted of geochemical analyses on whole sediment (Total Organic Content and Rock Eval pyrolysis) and of petrographic studies on isolated organic matter by optical and scanning electron microscopy. In Indus Fan sediments, organic matter is present in low amounts, mainly as lignaceous fragments. A contrasting situation exists in Oman Margin sediments which are generally rich in amorphous autochtonous organic matter. Owen Ridge, located between Indus fan and Oman Margin areas, shows two phases of organic sedimentation as a consequence of the uplift of the ridge. The older phase (Oligocene to early or middle Miocene) is strongly influenced by detrital supply from the Indus, while the younger phase (middle Miocene to Pleistocene) is characterized by relatively high amounts of autochtonous organic matter. From a general point of view it appears that high amounts of organic matter are mainly due to good preservation of marine amorphous organic matter, such as in Oman Margin sediments and in upper pelagic levels of Indus Fan and Owen Ridge deposits. Low total organic carbon contents are correlated with low proportions of amorphous material in the total organic matter due to oxidizing conditions. This leads to a relative enrichment in components derived from resistant materials (lignin, chitin, or other resistant biopolymers) such as lignaceous fragments (Indus Fan) and/or fragments from benthic organisms and alveolate microplankton (Oman Margin).本研究针对北印度洋三个区域（印度扇、欧文海岭、阿曼陆缘，对应大洋钻探计划（Ocean Drilling Program, ODP）第117航次）的新近纪沉积物展开分析，旨在明确有机质的含量、类型及保存特征随沉积环境的变化规律。本次研究涵盖全沉积物的地球化学分析（总有机碳含量（Total Organic Content）与Rock Eval热解分析），以及通过光学显微镜与扫描电子显微镜对分离有机质开展的岩石学研究。印度扇沉积物中的有机质含量较低，且主要以木质碎屑形式产出。阿曼陆缘沉积物则呈现截然相反的特征，整体富含原地成因非晶质有机质。位于印度扇与阿曼陆缘之间的欧文海岭，受海岭隆升作用影响，发育两期有机质沉积：早期（渐新世至早-中中新世）强烈受印度河碎屑供给的影响，而晚期（中中新世至更新世）则以相对富集的原地成因有机质为典型特征。 总体而言，高有机质含量主要源于海相非晶质有机质的良好保存，例如阿曼陆缘沉积物，以及印度扇、欧文海岭沉积物的远洋上层沉积层中。总有机碳含量偏低则与总有机质中非晶质组分占比低密切相关，该现象多由氧化环境所导致。在此情况下，源自抗降解物质（如木质素、甲壳素或其他抗降解生物聚合物）的组分相对富集，例如印度扇的木质碎屑，以及/或者底栖生物与囊泡类微浮游生物的碎屑（阿曼陆缘）。