Paleoenvironment evolution prior to the Carnian Pluvial Episode: Implications for organic matter enrichment in the Qiangtang Basin (Tibetan Plateau)

Abstract

The Carnian Pluvial Episode (CPE; ca. 234–232 million years ago) is an interval with increase in humidity and siliciclastic input, which is linked to global warming, enhanced hydrological cycle, water mass hypoxia caused by the eruption of the Wrangellia Large Igneous Province (Wrangellia LIP). Interestingly, preceding the CPE, there were discernible shifts in paleoenvironmental conditions, manifested through fluctuations in detrital, freshwater, and terrigenous higher plant inputs during that period. This investigation employed various methodologies, including organic geochemistry, elemental geochemistry, and mineralogy analysis of the Bagong and Boli La Formation in the Qiangtang Basin (Tibetan Plateau), to study paleoenvironmental evolution preceding the CPE. The aim was to investigate how these conditions influenced the accumulation of organic matter during this timeframe. Oraganic and elemental geochemical proxies indicated a gradual increase in detrital input prior to the CPE. Combined with decreased paleosalinity indicators, we proposed that enhanced continental inputs prior to the CPE could be associated with an intensified hydrological cycle. Framboidal pyrite morphology and size along with redox sensitive trace element content and ratio indicated anoxic to euxinic bottom water conditions during deposition. Persistently low P/Ti and Ni/Al ratios supported a low biotic productivity, which is adverse for organic matter production. The weak relationship between TOC and redox and productivity proxies indicated that organic matter accumulation was not mainly controlled by reducing condition and low primary productivity. We proposed that three-stage (Intervals A, B, and C) paleoenvironmental evolution before the CPE controlled the accumulation of organic matter in the Bagong Formation. This study provides new insights to paleoenvironmental evolution prior to the CPE, which is of paramount interest for a comprehensive understanding of the CPE.