Enhanced terrestrial organic matter burial in the marine shales of Yangtze platform during the early Carboniferous interglacial interval

Abstract

The supply and preservation of terrestrial organic matter (TOM) during early Carboniferous glacial-interglacial intervals in South China were investigated with respect to their roles in paleo-environmental variation and organic matter (OM) accumulation in black shales. The marine redox proxies such as V/Cr, Corg/P ratios, and Mo–U covariation indicate a fully oxic water condition during the earliest Visean, while dominant oxic with occasional suboxic water conditions prevailed during the early to middle Visean. High sedimentary δ15N values (1.9–6.3‰) suggest the existence of enhanced water-column denitrification in the expanded oxygen minimum zones (OMZs) during the earliest Visean, whereas lower δ15N values (avg. 2.6‰) during the early to middle Visean indicate a decreasing water column denitrification in the reduced OMZs. Both the organic matter compositions and the relationship between Corg/N ratios and δ13Corg values indicate a significant contribution of terrestrial OM in the marine sediments, and the low abundance of marine OM is mainly attributed to the low paleoproductivity as evidenced by low P/Ti and Ba/Al ratios. One reason for such a high contribution of TOM is that a large amount of plant debris and wildfire-derived OM was preserved during Tournaisian glacier time and then released and buried rapidly in the marine sediments during the interglacial interval of Visean due to the temperature-driven glacial melting. The rapid transportation of large quantity of TOM from land to ocean may have resulted in remarkable drawdown of atmospheric pCO2 and subsequent cool climate, as evidenced by the decoupled relationship between δ13Corg and δ13Ccarb values. In summary, the sinking of TOM flux during the sea level lowstand and highstand may be the controlling factor for the paleo-environmental variation as well as OM accumulation in the Visean black shales.