Sulfate concentration and redox state control the pyrite formation and sulfur cycle in a T-OAE lake, Sichuan Basin, China

Abstract

The sedimentary pyrite formation and sulfur cycle have been extensively investigated, especially in marine systems. The Early Jurassic Toarcian Oceanic Anoxic Event (T-OAE; ∼183 Ma) was marked by significant shifts in Earth’s climate and paleoceanographic conditions, and its global carbon cycle perturbations have been extensively studied. However, constraints on terrestrial sulfur cycling within the Earth system during this period are underexplored. To fill the knowledge gap, this study presents new constraints on the Da'anzhai Member of the Sichuan Basin in South China, a terrestrial record of the T-OAE, to explore the sedimentary pyrite formation and sulfur cycle in ancient lake environments. Results show that pyrite sulfur contents (Spyr) of most samples are <0.1 wt.%, and the stable sulfur isotopic compositions of pyrite (δ34Spyr) change from a low negative value of −11.5‰ to a positive value of 20.3‰ before the T-OAE. This variation was interpreted to be influenced by the oxygenated water and a relatively high sedimentation rate; the former confined microbial sulfate reduction (MSR) to the porewater space, while the latter reduced the efficiency of diffusive resupply of porewater sulfate. The T-OAE was associated with a marked increase in Spyr values (mean = 0.52 wt%), and δ34Spyr values fluctuated by ∼10‰ (5.4‰–15.6‰), consistent with higher sulfate reduction rates driven by increased organic carbon contents and sulfate concentration ([SO42−]) during this stage. After the T-OAE, the Spyr contents were < 0.1 wt% and δ34Spyr values were ∼10‰. The δ34Spyr values correlated well with the salinity proxy Sr/Ba ratios, suggesting that sulfate levels on system openness were likely the dominant way controlling δ34Spyr. Compared with the marine systems, relatively lower sulfate concentrations are the main limiting factor on pyrite formation in freshwater lacustrine systems, and the sedimentation rate and sulfate contents may be the main factors responsible for the higher δ34Spyr values in lacustrine systems. During the T-OAE, elevated atmospheric pCO2 and global climate warming triggered a series of chemical changes in lake systems (particularly in [O2] levels), which in turn significantly disrupted lacustrine sulfur cycling. This represents a notable terrestrial response to the T-OAE. This study highlights the dynamic control of sedimentary environmental factors on lacustrine sulfur cycling and the broader impact of the T-OAE on lacustrine environments by focusing on the first documented case of a large lake system.