Isotopic composition of organic and inorganic carbon from the Mesoproterozoic Jixian Group, North China: Implications for biological and oceanic evolution

Analyses of marine carbon isotope profiles have provided much of our current understanding of the evolution of Earth surface environments, particularly in the latter portion of the Proterozoic Eon. Earlier Mesoproterozoic successions, however, have received comparatively little attention due to the relatively subdued nature of carbon isotope variation. In this study, we present high-resolution isotopic profiles from three sections in the Yanshan Basin, North China craton that, combined, comprise the entirety of the early Mesoproterozoic (1600-1400 Ma, Calymmian period) Jixian Group. High-resolution profiles of both carbonate and organic carbon provide critical data for global comparison and permit us to better constrain both the pattern and origin of isotopic variation in the Mesoproterozoic. Marine carbonate rocks of the Jixian Group show generally muted isotopic variation with average values near 0 parts per thousand, consistent with previous observations from the early Mesoproterozoic. Data furthermore record an increase in isotopic variation through the succession that is interpreted to reflect a long-term decrease in pCO(2) and, consequently, in the isotopic buffering capacity of marine dissolved inorganic carbon (DIC). By contrast, the isotopic composition of marine organic matter suggests facies-dependent differences in carbon cycling. Organic carbon compositions suggest a dominance of autotrophic carbon fixation and aerobic decomposition in shallow-water environments, and increased remineralization by anaerobic heterotrophs in deeper-water environments. Correlation between organic carbon composition and depositional environment are interpreted to reflect differences in carbon cycling within benthic microbial mats under low oxygen conditions and dynamically maintained stratification of marine waters. (c) 2012 Elsevier B.V. All rights reserved.