Spatiotemporal variation of organic geochemical properties since the mid-Miocene in the deep South China Sea (IODP Expedition 349)

We performed an extensive survey of total organic carbon (OC) and stable OC composition of in the deep South China Sea sediments from the International Ocean Discovery Program (IODP) Expedition 349 Sites U1431, U1433 and Ocean Drilling Program (ODP) Leg 184 Site 1148. Our results reveal dramatic spatial and temporal variations in organic geochemical characteristics among the three Sites. Before similar to 8 Ma, OC deposited at Site U1431 may be predominantly terrigenous, whereas OC deposited at Sites U1433 and 1148 is mainly of marine origin. After similar to 8 Ma, OC accumulation at the three Sites progressively increases, mainly due to enhancement of marine primary productivity and input of terrestrial organic matter; the three sites receive mainly marine OC, whereas Sites U1433 and 1148 receive relatively more terrestrial OC compared with Site 1431. Additionally, methane concentrations vary from similar to 20 to similar to 100,000 ppmv at Site U1433 and from similar to 10 to similar to 600 ppmv at Site 1148 with higher geothermal gradients (78 and 84 degrees C/km, respectively). In contrast, methane concentrations are < 5 ppmv at Site U1431 with lower geothermal gradient (14.8 degrees C/km). Such high methane concentrations and ratios of methane to ethane at Site U1433 suggest that the gaseous hydrocarbons in the southwestern sub basin (deep water depth, > 4000 km) are primarily sourced from microbial production. Our findings suggest that the sources, trajectories, and fates of OC buried in the deep South China Sea since the mid-Miocene are spatially and temporally variable, controlled by sedimentological processes in the sub-basins, and regional tectonic evolution and long-term paleoenvironmental changes.