Geochemistry of carbon at DSDP Legs 58 and 59 Holes

I obtained 68 quarter sections of cores from the JOIDES Organic Geochemistry Panel for studying type, distribution, and stages of organic diagenesis of sedimentary organic matter in the West Philippine and Parece Vela basins and Mariana Trough area (Figure 1). The present chapter compares (1) 11 geochemical parameters used to determine organic source and its stage of genesis within the 9 site locations in this study area and (2) compares these 11 with the same parameters reported from Leg 56, outer trench slope of the Japan Trench, and Leg 60, Mariana Trough and Trench (Schorno, in press a, b). Even though these sediments are considered pelagic, the organic content in most of the core sections appears to be hemipelagic. The sedimentary organic matter in these cores is believed to be in an early stage of diagenesis. Both conclusions are based primarily on the fl-alkane distribution within the organic matter. This particular parameter, I note later, has a major weakness. As Hunt (Hunt, 1979) and I (in press b) observed, marine organisms synthesize /z-alkanes with distributions containing neither odd nor even preferences. Thus those sediments that did contain w-alkane distributions with OEP near 1, suggesting a late stage of catagenesis, may in actuality be immature marine sediments.

本研究从JOIDES有机地球化学小组（JOIDES Organic Geochemistry Panel）处获取了68个岩心四分样段，用于研究西菲律宾海盆、帕雷塞维拉海盆及马里亚纳海槽区域内沉积有机质的类型、分布特征与成岩演化阶段（图1）。本章节开展两项对比：其一，针对本研究区内的9个站位，对比用于确定有机质来源及其成因演化阶段的11项地球化学参数；其二，将上述11项参数与日本海沟外斜坡第56深海钻探航次（Leg 56）以及马里亚纳海槽与海沟第60深海钻探航次（Leg 60）所报道的同类参数进行对比（Schorno，待刊a、b）。尽管这些沉积物通常被归类为远洋沉积，但多数岩心样段中的有机质组分实际呈现半远洋沉积特征。本次研究认为，这些岩心中的沉积有机质处于成岩作用早期阶段，上述两项结论主要基于有机质中正构烷烃的分布特征。需指出，该参数存在显著局限性。正如Hunt（1979）以及笔者（待刊b）所观察到的，海洋生物合成的正构烷烃其碳数分布既无奇数优势也无偶数优势。因此，那些呈现奇偶优势指数（Odd-Even Predominance, OEP）接近1的正构烷烃分布、被认为处于热演化晚期阶段的沉积物，实际可能属于未成熟的远洋沉积物。