Occurrence and formation mechanism of silicate-interlayered iron minerals in hydrate-bearing sediment

Abstract

In this investigation, the geochemical properties of carbonates and iron sulfides in bulk sediment were systematically investigated to elucidate the original depositional characteristics and post-seepage alterations in hydrate-bearing sediments from the mid-continental slope of the Shenhu area, South China Sea. The paleo-sulfate methane transition zone at site W07B was delineated using proxies for carbonate (elevated dolomite levels and δ¹³C-TIC exhibiting negative bias) alongside indicators for iron sulfide (enhanced pyrite levels and δ³⁴S-pyrite showing positive bias) between 132 and 151mbsf. Beyond the typical marine sediment inclusion such as framboid pyrite and foram-shell-filled pyrite, this research reveals a notable occurrence of interlayered silicate pyrite and interlayered siderite, as revealed by EDS and Raman spectrum, marking the novel documentation of such iron-rich secondary minerals in hydrate-bearing sediments. The formation mechanisms and conditions for these authigenic, iron-rich minerals are explored, with their formation linked to the microbial weathering of iron-rich silicate minerals during methane metabolism by entities like methanogens and sulfate-reducing bacteria. The genesis of these minerals is fundamentally driven by the presence of methane-rich fluids and microbial activities. Additionally, the weathering of silicates results in unique secondary mineral assemblages within various chemical zones, where interlayered pyrite appears in the sulfate reduction zone, and interlayered siderite forms within the Fe-Mn reduction zone and/or methanogenic zone. This study shed light on the complex interactions between microbial activities and sediment geochemistry in hydrate-bearing marine settings.