Burial of seawater–rock interaction-derived pyrites in altered oceanic crust: Implication for Phanerozoic oceanic sulfur cycle

Abstract

The oceanic sulfur cycle is intimately linked to the cycles of organic matters and oxygen on the Earth's surface. However, controversy exists on the dominant pathway (pyrite vs. evaporite burial) of Phanerozoic oceanic sulfur cycle and its influence on atmospheric oxygen concentrations. This controversy arises from controversial pyrite burial flux (F_py = 2.40 vs. 31.5 × 10¹¹ mol yr⁻¹), sparked by only counting sedimentary pyrites without seawater–rock interaction (SRI)-derived ones and by underestimating evaporite burial flux. To address these issues, we present in-situ sulfur isotopic data for SRI-derived pyrites from Hole U1502B in the South China Sea and calculate the burial flux (F_SRI-py) of SRI-derived pyrites by employing a new equation without using evaporite burial flux. The studied pyrites exhibit positive δ³⁴S values (1.93–5.96 ‰), high contents of fluid-mobile elements (Pb of 5,810–8,870 ppm and Mo of 8,260–10,240 ppm) and temperature-sensitive elements (Co up to 1,761 ppm and Cu up to 798 ppm), indicating a hydrothermal origin. The values of F_SRI-py (7.23–14.9 × 10¹¹ mol yr⁻¹), estimated from the S isotopic data of this study and compiled data for SRI-derived pyrites, are similar to the burial flux of sedimentary pyrite, highlighting the essential role of SRI in shaping the Phanerozoic oceanic sulfur cycle. The calculated high total F_py (1.40–2.91 × 10¹² mol yr⁻¹) and pyrite burial fraction (ƒ_py = 47–97 %), incorporating both sedimentary and SRI-derived pyrites, suggest that pyrite burial was the dominant pathway of Phanerozoic oceanic sulfur cycle, and actively regulated atmospheric oxygen concentrations. Moreover, the abrupt increases in Phanerozoic F_py and atmospheric oxygen concentrations were potentially associated with supercontinent assembly.