Iron speciation in sediment cores near the Jiulong Methane Reef and its implication

Variability in iron species in marine deposits are excellent geochemical archives for exploring variations in redox conditions during early diagenesis and seepage dynamics over time. Two sediment cores (DS07 and DS08)were collected in the Jiulong Methane Reef, a typical cold seep site that is located in the South China Sea (SCS), and analyzed for iron species using Mössbauer spectrometry and magnetic procedures. Excluding the surface layer (0-10 cm)in DS07, Fe²⁺/Fe³⁺ ratios indicated that core sediments were under anoxic conditions. The Fe²⁺/Fe³⁺ and frequency-dependent susceptibility (χ_fd)at this site were higher while the total organic matter content (TOC)and magnetic susceptibility (χ)were significantly lower at approximately 170 cm depth. Combined with chromium reducible sulfur (CRS), Fe/Al and sulfur to organic content (S/C)ratios suggested that there was a sulfate-methane transition zone (SMTZ)at approximately 170 cm. Moreover, thermomagnetic analyses indicated that magnetite was the dominant ferrimagnetic mineral at 21–23 cm, 165–185 cm, 213–233 cm and 285–295 cm depths, however, magnetite content was relatively lower between the 165 and 185 cm depth, which suggested that magnetite had been largely transformed to paramagnetic iron minerals during the anaerobic oxidation of methane (AOM)process. The DS08 sediment core was also mostly anoxic, however, no obvious methane seepage activity was observed because the maximum values of Fe²⁺/Fe³⁺, Fe/Al, CRS, S/C and the minimum values of TOC and χ did not simultaneously occur at the same depth. The study revealed that the speciation of Fe in sediment cores varied under different redox conditions which was of interest when utilizing iron speciation and magnetic minerals as indicators of historical environmental conditions and seepage activities.