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.

In this study, we investigated the effect of burial diagenesis on the microstructure and isotopic (δ¹³C and δ¹⁸O) and chemical compositions of modern brachiopod (Terebratulina crossei) shells through controlled artificial diagenesis experiments. The shells were placed in sediment and artificial seawater mixtures and subjected to experimental conditions of 125°C and 75 MPa for 720 h. Statistically significant changes were observed in the isotopic and chemical compositions of the shells before and after the artificial experiments. Notable findings include decreases in δ¹⁸O values under all four experimental conditions; increases in Mn concentration in the carbonate powder-artificial seawater mixture, quartz powder-artificial seawater mixture, and artificial seawater; and decreases in the δ¹³C values in the carbonate powder-artificial seawater mixture and sandstone powder-artificial seawater mixture. The observed δ¹⁸O variations were predominantly influenced by temperature rather than by the isotopic and chemical compositions of the ambient sediments and fluids. The Mn concentration increased when the shells were placed in materials relatively poor in Mn (i.e., carbonate, quartz, and artificial seawater). This suggests that Mn originated from organic matter within the shells. The decrease in δ¹³C values is likely attributable to the thermal degradation of organic matter in the shells. Scanning electron microscopy (SEM) revealed minimal evidence of shell microstructure degradation and destruction due to the experiments. However, transmission electron microscopy (TEM) revealed traces of dissolution that were not discernible using conventional SEM. These findings underscore the importance of nanoscale analysis in future investigations of brachiopod shell-based paleoenvironmental reconstructions.