Mechanism and geological significance of anomalous negative δ13Ckerogen in the Lower Cambrian, NW Tarim Basin, China

Abstract

The Lower Cambrian black strata with an anomalous negative excursion of carbon isotope in organic carbon are widely developed in many continents, contracting few attentions from scholars around the world. Based on the detailed investigations for the strata from the Xiaoerbulake and Shiairike outcrop sections located in the Aksu area, northwest Tarim Basin, multiple test and analysis methods including the total organic carbon (TOC), carbon isotope of kerogen (δ13Ckerogen), aryl isoprenoids and major/trace elements were systematically performed to reveal the mechanism and developmental model for the anomalous negative δ13Ckerogen. This anomalous negative excursion was caused by the coupling of paleoweathering and hydrothermal activities. Euxinic bottom water created by hydrothermal activities made the organic carbon (CO2 or HCO3-) from the decomposing organisms as the additional carbon source of phytoplankton photosynthesis, resulting in the development of 13C-depleted original organisms. Insufficient availability of essential nutrient (particularly Fe) caused by paleoweathering enhanced the low growth rate, resulting in the development of more 13C-depleted original organisms during the deposition of the organic-poor siliceous shales compared to that of the organic-rich shales. Besides, TOC was mainly affected by the dilution of high concentrations of hydrothermal SiO2, resulting in the organic-rich shales under weak hydrothermal conditions, while the organic-poor siliceous shales under strongly hydrothermal condition. Importantly, the production of anomalous negative δ13CKerogen triggered the occurrence of diagnostic biomarkers, aryl isoprenoids, indicating the photic zone euxinia. These contributes allow for potential application for deep petroleum exploration in the Lower Paleozoic in the Tarim Basin and insights for biological evolution.