Formation of the authigenic pyrite in the gas hydrate-bearing layer of the Shenhu region, northern South China Sea: Constraints from geochemical and sulfur isotope compositions

Abstract

Authigenic pyrite plays a vital role in understanding diagenetic processes, growth mechanisms within marine gas hydrate-bearing sediments, and the global sulfur cycle. This study investigates the elemental and sulfur isotopic composition of authigenic pyrites within the gas hydrate-bearing layer of a drilling core retrieved from the Shenhu region, northern South China Sea. A suite of in-situ analytical techniques, including electron microprobe analysis (EMPA), laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and laser ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS), were employed to characterize the pyrites. Two distinct pyrite groups were identified based on their morphology and geochemical signatures: (1) framboidal pyrite and (2) subhedral pyrite. Group 1 framboidal pyrites exhibit negative δ³⁴S_V-CDT values (-24.5‰ to -7.1‰) and elevated concentrations of Cr, Mn, Co, Zn, As, Mo, Sb, Ba, and Pb. Conversely, Group 2 subhedral pyrites display positive δ³⁴S_V-CDT values (2.7‰ to 41.4‰) and lower concentrations of the aforementioned trace elements. Integrating the textural and geochemical variations observed in the two pyrite groups, we propose a growth model for authigenic pyrite within a closed system influenced by gas hydrates. The transition from framboidal to subhedral pyrite morphology likely reflects evolving sediment geochemical conditions. The high abundance and variable sulfur isotopic and elemental compositions of the pyrites suggest elevated reaction rates associated with sulfate-driven anaerobic oxidation of methane (SD-AOM) and pyritization within the gas hydrate-bearing layers. The elevated abundance and unique geochemical feature of pyrites within the gas hydrate-bearing layer could potentially serve as an indicator of pre-existing gas hydrate. This study provides valuable insights into the growth processes of authigenic pyrite within a gas hydrate-driven closed system and elucidates the relationship between authigenic pyrite formation and gas hydrate occurrence, which will be beneficial to the prospecting for the gas hydrate reservoir.