Middle Permian palaeoclimatic-palaeoceanographic evolution and its controls on organic matter accumulation in the Lower Yangtze upwelling region

Abstract

For more than 10 million years (∼272–260 Ma), the Middle Permian underwent periodic episodes of vigorous upwelling and massive organic matter burial along the west coasts of continents, as testified by the abundant black chert-mudstone records. However, less attention has been paid to the palaeoclimatic-palaeoceanographic evolution in this upwelling area and its relationship with the mechanisms of coeval organic matter accumulation. To better understand this scenario, multiple geochemical analyses were performed on organic-rich deposits of the Gufeng Formation of the Lower Yangtze region. Our results showed that the Middle Permian climatic conditions gradually shifted from warm-humid to hot-humid. Comprehensive geochemical analyses using multiple proxies suggested that the study area was characterised by a hybrid palaeoceanographic system influenced by both upwelling and restriction. The coupled climatic-oceanic evolution went through four stages. The Roadian stage was characterised by gradually enhanced upwelling but relatively oxic conditions and low productivity; the Wordian-early Capitanian stage was marked by the alternating development of perennial and seasonal upwelling with higher productivity and anoxic-euxinic conditions; the early-middle Capitanian stage was distinguished by an intermittent seasonal upwelling with relatively decreased productivity and weak anoxic conditions; the middle Capitanian stage immediately after the deposition of the Gufeng Formation transitioned from a transient-restricted condition to a weak upwelling setting and was dominated by extremely low productivity and suboxic conditions. Further studies on these organic-rich sediments show that the integration of biological productivity, redox conditions, detrital inputs, and hydrographic restrictions under coupled climatic-oceanic forcing could have had a complex influence on their formation. In this upwelling setting, organic matter accumulation is dominated by changes in both upwelling-driven primary productivity and oxygen minimum zone-related reducing conditions, but to a lesser extent by the dilution of climate-influenced detrital inputs.