Contribution of oxygenic photosynthesis to palaeo-oceanic organic carbon sink fluxes in Early Cambrian Upper Yangtze shallow sea: Evidence from black shale record

Abstract

The extensive transgression that occurred on the Yangtze Plate in Early Cambrian led to a massive organic carbon pool in the Niutitang Formation. A black shale core section from 3 251.08 to 3 436.08 m beneath the Earth’s surface was studied to estimate the contribution of oxygenic photosynthesis to organic carbon sink fluxes in Early Cambrian Upper Yangtze shallow sea. Results indicate that the oxygenic photosynthesis played the most important role in carbon fixation in Early Cambrian. Organic carbon sink was mainly contributed by photosynthetic microorganisms, e.g., cyanobacteria, algae and archaea. The Niutitang Formation was formed in a deep anoxic marine shelf sedimentary environment at a sedimentation rate of ~0.09±0.03 mm/yr. The initial TOC abundance in Niutitang shale ranged from 0.18% to 7.09%, with an average of 2.15%. In accordance with the sedimentation rate and initial TOC abundance, organic carbon sink fluxes were calculated and found to range from 0.21 to 8.10×103 kg/km2·yr-1, especially the organic carbon sink fluxes in depth between 3 385 and 3 470 m range from 3.80 to 8.10×103 kg/km2·yr-1, with an average of ~6.03×103 kg/km2·yr-1, which is much higher than that of contemporary marine sediments. The organic carbon sink fluxes of Niutitang shale are equal to 0.56 to 21.61×103 kg/km2·yr-1 net oxygen emitted into the Early Cambrian ocean and atmosphere, this emitted oxygen may have significantly promoted the oxygen level of the Earth’s surface and diversification of metazoans.