Influence of dissolved organic carbon on the Cambrian Ocean: Evidence from the carbon isotopes of the sedimentary rocks of the Eastern Tarim Basin, North-west China

In recent years, the typical crude oil from the Cambrian Lower Ordovician source rocks has been found in the Tarim Basin, and it is characterized by C-13 enrichment. The stable carbon isotope of the oil is concentrated at approximately -28 parts per thousand, which is 3 parts per thousand-6 parts per thousand heavier than that of the crude oil from the Middle and Upper Ordovician source rocks. The stable carbon isotope between crude oil and its kerogen is characterized by inversion, with an amplitude of approximately 3 parts per thousand-4 parts per thousand. However, the origin of this phenomenon remains controversial, which restricts the next exploration of crude oil in the Cambrian-Lower Ordovician in the Tarim Basin. Samples were collected from seven outcrop profiles in the Kuruketage region. The basic geochemical features of the Cambrian strata, including total organic carbon abundance, delta C-13(org), delta C-13(carb), and delta O-18(carb), were studied and compared with those of well TD2. The results show that the lithologies, isotope stratigraphic evolutional curves, and correlations between the organic carbon and inorganic carbon of the profiles from the northern Kuruketage region are successively comparable with those of the profiles from the southern Kuruketage region and the well TD2. In this study, the carbon isotope records are supposed to indicate a vertical gradient Cambrian oceanic mode, including the shallow decoupled isotope zone, middle gradient-coupled isotope zone, and deep isotope-decoupled zone. The discovery of this isotope record reflects the formation of stable carbon and oxygen isotope stratified oceans during the Cambrian period. Additionally, it provides important insights into the origin of the C-13 enrichment observed in some Lower Cambrian crude oils in the Tarim Basin.