Mineralogy and geochemical data

Description

Authigenic pyrite, a characteristic mineral product of the anaerobic oxidation of methane coupled with sulfate reduction (AOM-SR) process in cold seeps, serves as a key indicator for tracing methane seepage and environmental evolution. To elucidate the mechanisms by which methane flux and benthic communities influence its morphological differentiation and evolution, we conducted a systematic study of sediment samples from the Wenhai I cold seep in the Qiongdongnan Basin, integrating gas isotope geochemistry, mineralogy, and AMS ¹⁴C dating. Results revealed a microbial-dominated mixed methane gas derived from terrigenous humic organic matter, exhibiting dual methanogenic pathways via CO₂ reduction and acetate fermentation (δ¹³C-CH₄ = -109.5‰ to -62.0‰), with ≥22 kyr of sustained seepage history forming a distinct methane flux gradient from vent centers to background areas. Authigenic pyrite across stations was confirmed as methane-derived with AOM-SR as the primary sulfur source, displaying a spatial pattern of high abundance/large grain size in mussel-bleached zones, medium values in mussel-thriving zones, low abundance/small grain size in tubeworm zones, and low abundance/narrow distribution in seep venting zones. The complete framboidal-overgrowth-euhedral pyrite evolutionary sequence was identified, demonstrating that pyrite formation and evolution are dominantly controlled by methane flux-benthic community coupling. The tripartite coupling model of methane flux-benthic community-pyritization established in this study further elucidates synergistic biogeochemical mechanisms in cold-seep systems, providing a theoretical foundation for quantitatively assessing ecological impacts during future energy resource exploration and development.

Steps to reproduce

Pyrite morphology and particle size analysis were conducted at the Laoshan Laboratory. Sediment samples were soaked in distilled water, sieved through a 0.063-mm mesh, air-dried, and framboidal pyrite aggregates were manually picked (Sun, 2024). Morphological and geochemical characteristics of pyrite aggregates were observed using energy-dispersive spectroscopy (EDS) mode on an EM30N high-resolution benchtop scanning electron microscope (SEM), with framboidal pyrite size distributions measured from SEM images and geometric mean diameters calculated following Rickard (2019) methodology. Headspace gas composition analysis was performed using a Thermo Fisher Ultra Trace gas chromatograph (GC-TCD) at the Testing Center of Qingdao Institute of Marine Geology. The analysis employed a Pora Plot Q capillary column (30 m × 0.32 mm × 20 μm) with column temperature set at 60°C, vaporization chamber at 150°C, TCD detector at 200°C, and filament voltage at 10 V; high-purity helium served as carrier gas at 3 mL/min flow rate with 10:1 split ratio and 50 μL injection volume, while reference and makeup gas flow rates were 12 mL/min and 10 mL/min respectively. Concentrations of CH4 and CO2 in headspace gases were converted to molar concentrations per unit volume of interstitial water (mmol/L or mM) based on average sediment porosity at each station, with analytical error <3%. Carbon and hydrogen isotopic compositions of CH4 and CO2 in sediment headspace (δ13C- CH4, δ13C-CO2, δD-CH4) were analyzed using a Thermo Fisher MAT 253 gas chromatography-isotope ratio mass spectrometer (GC-GCIsolink-IRMS) with operational parameters following He et al. (2012). For carbon isotope analysis: the initial column temperature was maintained at 50°C for 3 min, ramped at 15°C/min to 190°C and held for 5 min; high-purity He (99.999%) served as carrier gas at 1.5 mL/min flow rate with injection port temperature of 100°C; split injection was employed with split ratio and injection volume adjusted in real-time based on CH4 and CO2 concentrations, combustion furnace temperature set at 1000°C, and IRMS monitoring m/e 44, 45, 46 ions. Hydrogen isotope analysis utilized a pyrolysis furnace temperature of 1420°C with IRMS monitoring m/e 2, 3 ions. Isotopic results were calculated as δ‰ = [(Rsample/Rstandard) - 1] × 1000, referenced to VPDB for carbon and SMOW for hydrogen, achieving analytical precisions better than 0.20‰ and 2.0‰ respectively. Quality control was implemented throughout testing using in-house CH4 and CO2 isotopic reference materials (CH4-1#, CH4-2#, CO2-1#, CO2-2#) and NIST standards (RM8562, RM8563, RM8564).

Institutions

• Laoshan Laboratory

  Shandong, Qingdao

Categories

Licence