Effect of Paleosalinity and Paleoclimate on Organic Matter Accumulation in Lagoonal Environments: Insights From the Biluo Co Jurassic Black Shales in the Qiangtang Basin

ABSTRACT The lagoonal environment is a favourable facies for organic matter (OM) enrichment. However, the controls on OM accumulation in such a setting remain unclear. Based on high‐resolution organic and inorganic geochemical data of the Jurassic lagoonal OM‐rich black shales from the Biluo Co section in the Qiangtang Basin, the paleosalinity, paleoclimate, redox and biotic productivity were investigated to reveal the OM accumulation mechanism in a lagoonal environment. High B/Ga ratios, coupled with low Mo/TOC and Cd/Mo ratios, indicate that the studied shales were deposited under a highly restricted hypersaline environment. These conditions are compatible with a barrier‐lagoon setting in the study area. Sr/Cu and Rb/Sr ratios indicate a semi‐humid to semi‐arid climate. Enrichment factors of redox‐sensitive trace elements (i.e., Mo, U and V) as well as Th/U and C org /P ratios reveal an anoxic to euxinic bottom water column. High ratios of P/Ti, Ni/Al, Zn/Al and Cd/Al imply a high surficial bioproductivity level. The strong positive correlations among redox conditions, biotic productivity and OM contents indicate that the first two factors were the primary controls on OM enrichment. Paleosalinity and paleoclimate during the black shale deposition indirectly govern OM accumulation via different positive feedback processes. Hypersaline‐induced water‐column stratification fostered reducing benthic conditions, thereby creating favourable preservation conditions for OM accumulation. Under semi‐humid to semi‐arid climates, enhanced fluvial delivery of nutrients to the lagoonal area led to eutrophic waters and fueled biotic productivity, ultimately resulting in high OM production. Our findings underscore that OM enrichment in the Biluo Co lagoonal black shales is interpreted as the combined influence of paleosalinity and paleoclimate, rather than the individual effect of either factor.