The origin of decoupled carbonate and organic carbon isotope signatures in the early Cambrian (ca. 542-520 Ma) Yangtze platform

The early Cambrian (ca. 542-520 Ma) strata in South China record two prominent negative carbonate carbon isotope (delta C-13(carb)) excursions of early Nemakit-Daldynian (N-D) and early Tommotian ages. Across each of these excursions, carbonate and organic carbon isotopes (delta C-13(carb) and delta C-13(org)) are strongly decoupled. Regional correlation across a shelf-to-basin transect shows lateral heterogeneity of delta C-13(org) during the early-middle N-D but more homogenized delta C-13(org) values across the basin during the late N-D and Tommotian. The temporal and lateral variations in delta C-13(org) suggest that decoupled delta C-13(carb) and delta C-13(org) across the N-D delta C-13(carb) excursion were possibly caused by diagenetic alteration of organic matter and/or amplification of detrital organic carbon isotope signature ture in low-TOC carbonates. In contrast, decoupled delta C-13(carb) and delta C-13(org) of the upper N-D and Tommotian were likely resulted from chemoautotrophic-methanotrophic biomass contribution to TOC in organic-rich black shale and carbonates. The decoupled delta C-13(carb)-delta C-13(org) pattern from the lower N-D strata (ca. 542 Ma) shows striking similarities with those from the basal (ca. 635 Ma) and upper (ca. 551 Ma) Doushantuo Formation. In all three cases, decoupled delta C-13(carb)-delta C-13(org) are seen in organic-poor carbonates (TOC <= 0.1 parts per thousand) and coupled delta C-13(carb)-delta C-13(org) occur in organic-rich black shale and carbonates at the end of the negative delta C-13(carb) excursion. These similarities suggest that the shift from decoupled to coupled delta C-13(carb)-delta C-13(org) has no causal link with the terminal oxidation of a large oceanic DOC reservoir. Given the pervasive anoxia/euxinia in Ediacaran-early Cambrian oceans, local DOC-rich environments may have been common, but a large oceanic DOC reservoir capable of buffering the delta C-13 of marine organic matter requires independent evidence. (C) 2011 Elsevier B.V. All rights reserved.