Organic geochemistry of ODP Site 128-799 (Table 1)

Samples of Quaternary through Miocene diatomaceous sediment and siliceous sedimentary rock from the Japan Sea (ODP Leg 128, Site 799) were examined to determine the origin of bitumen encountered during drilling, whether previously published studies using Rock-Eval pyrolysis have accurately assessed organic matter type and maturity, and if evidence exists to postulate the presence of a Kuroko-type massive sulfide deposit at depth or along depositional strike. Whole-rock total organic carbon:total nitrogen ratios have values ranging from 31 to 4. Elemental atomic H:C and O:C kerogen ratios decrease from ca 1.3 and 0.3, respectively, in Quaternary and Pliocene samples to 1.2 and 0.1, respectively, in samples obtained from the bottom of the hole. The bitumen ratio of these rocks increases from 50 mg bitumen/g OC at 700 mbsf to 150 mg bitumen/g at 1000 mbsf. The kerogen in these samples comprises a mixture of marine and terrigenous organic material that reaches the earliest stages of catagenesis in the bottom part of this hole. The bitumen is dominated by asphaltenes, resins and polar components. Because these compounds have very high boiling points, Rock-Eval pyrolysis does not assess the thermal maturity, or organic matter type accurately in the early stages of catagenesis. The bitumens have a very low thermal maturity and the observed degree of thermal alteration is consistent with the modern geothermal gradient in the Kita-Yamato trough.