Provenance discrimination of the last glacial sediments from the northeastern South China Sea and its paleoenvironmental indications

A total of 849 core samples collected from two drilling core STD235 and ZSQD289 in the northeastern South China Sea were analyzed for elemental composition including total organic carbon (TOC), total nitrogen (TN), stable carbon and nitrogen isotopic composition (δ13C, δ15N), and clay mineral composition. Based on clay mineral assemblages and organic geochemistry characteristics, it is supposed that the terrestrial source for sediments at STD235 and ZSQD289 is mainly from southwestern Taiwan. Moreover, the sedimentary organic matter of site ZSQD289 was probably directly inputted from southwestern Taiwan submarine canyon, while the sedimentary organic matter of core STD235 was probably transported by deep water current. The chronology suggested that it covered the record since ~19.7 and 34.6 ka BP in site STD235 and ZSQD289 respectively. Before the Last Glacial Maximum (LGM), high TOC/TN ratios with low δ13C and δ15N values might indicate that terrestrial organic input and nitrogen fixation was enhanced. During the LGM, gradually increased TOC, TN, δ13C, and δ15N values might result from the enhanced burial of organic carbon and higher marine primary production constrained by the strong winter monsoon. TOC/TN ratios and δ13C values generally present a decreased trend from LGM to Holocene (18 - 11 cal ka BP), strongly correlated to the terrestrial organic input and marine primary production reduced during this period. Since Holocene, the terrestrial organic input to both sites further reduced and maintained a low level, in according with a high sea level and a gradual intensified summer monsoon

Geochemical Evolution of Occluded Hydrocarbons inside Geomacromolecules: A Review

Within geomacromolecules, such as kerogen, asphaltene, and solid bitumen, other compounds can be adsorbed and even occluded as free molecules. The occluded components have been well preserved by the macromolecular structure, and retain some of the primary geochemical information. In this work we try to probe the geochemical evolution of occluded hydrocarbons inside geomacromolecules associated with the geomacromolecule evolution from kerogen -> asphaltene -> solid bitumen. The results show that occluded hydrocarbons can be transferred steadily from kerogen -> asphaltene -> solid bitumen. Later-evolved geomacromolecules not only inherit the occluded hydrocarbons from the former ones, but can also occlude some new free molecules. Occluded hydrocarbons are subject to a relatively independent thermal evolution, whereas the evolution of adsorbed molecules is constrained by other factors besides thermal stress. Elucidation of the geochemical evolution of occluded hydrocarbons inside geomacromolecules will be helpful in oil (bitumen) source correlation, identification of mixed-source reservoirs, and characterization of hydrocarbon accumulation and evolution