Shallow stratification prevailed for ~1700 to ~1300 Ma ocean: Evidence from organic carbon isotopes in the North China Craton

The Late Paleoproterozoic to Early Mesoproterozoic (from ~1700 Ma to ~1300 Ma) was highlighted by the assembly of the Nuna supercontinent, expansion of euxinic marine environments and apparent stasis in the diversity of eukaryotes. The isotopic composition of carbonate carbon (δ¹³C_carb) was surprisingly constant during this interval, but little is known about the secular variation in the organic carbon isotopic composition (δ¹³C_org). Here we report δ¹³C_org data from the latest Paleoproterozoic (>1650 Ma) to Early Mesoproterozoic (~1300 Ma) succession in North China. The δ¹³C_org values range from -25‰ to -34‰, and are dependent on sedimentary facies. In subtidal and deeper environments δ¹³C_org values are low and constant, ca. -32‰, but relatively enriched and more variable in shallower intertidal and supratidal environments. We attribute the facies-dependent variation in δ¹³C_org to the presence of a shallow chemocline. A probable result of a shallow chemocline is that it supported significant contributions of organic matter produced by chemoautotrophic and/or anaerobic photoautotrophic microbes in relatively deep environments from the latest Paleoproterozoic to Early Mesoproterozoic continental shelf of North China.