Enhanced continental weathering and its marine environmental effects in the late Devonian: Constraints from strontium isotopes of carbonate rocks in South China

Abstract

The Frasnian-Famennian extinction in the late Devonian was one of the largest mass extinction during Earth's history, which was believed to be caused by the plant landing-indued intensification of terrestrial chemical weathering and the consequent hypoxia of marine water. However, direct evidence remains limited. Here, we apply strontium (Sr) concentrations and isotopes (87Sr/86Sr) as well as uranium-thorium ratios (U/Th) of shallow marine carbonate rocks that are widely developed in the Guangxi area to explore the rate of continental chemical weathering and the redox condition of seawater from the middle devonian to lower Carboniferous. The results show that, the intensification of terrestrial chemical weathering in the late Devonian (early Famennian) caused the increase of Sr concentration and the enrichment of heavy Sr isotopes in seawater. The low U/Th ratio in carbonate also suggests that the water was highly hypoxia during this period. Uranium in seawater was reduced and deposited in sediments, resulting in the decrease of U concentration in seawater. The intensification of chemical weathering on land surface has a significant influence on the fluctuation of redox conditions and the extinction of marine organisms in the late Devonian.

Keywords

• weathering rate
• hypoxia of water
• plant landing
• mass extinction
• carbonate rock
• strontium isotope