Diagenetic analysis of shallow and deep-seated gas hydrate systems from the Bay of Bengal

Abstract

This is the first systematic high-resolution rock magnetic study complemented by mineralogical and petrological observations conducted on sediment cores representing shallow (active cold seep; SSD-45/Stn-4/GC-01) and deep-seated (NGHP-01-15A) gas hydrate systems in the Bay of Bengal which addresses two key questions: (i) how magnetic minerals respond to the geochemical environment at two different diagenetic settings experiencing variable fluid sequence, and (ii) elucidates the control on the magnetite and greigite authigenesis in sulfidic, methanic, and gas hydrate bearing sediments. Titanomagnetite is the dominant detrital magnetic mineral identified in the studied cores from the Krishna-Godavari (K-G) basin along with diagenetic (pyrite) and authigenic (magnetite, greigite) minerals. Large detrital titanomagnetite and silicate-hosted (fine-grained) magnetic inclusions survived diagenetic dissolution during high sedimentation events. Magnetic proxies (ARM/SIRM and SIRM/Chi 1f) provided useful insights on the diagenetic and authigenic formation and preservation of magnetic particles in the studied cores. Preferential diagenetic dissolution of finer (detrital) magnetic particles in sulfidic and authigenic formation of magnetite in methanic environment is clearly evident in the ARM/SIRM record of cores from both sites. Elevated values of SIRM/Chi 1f in cores SSD-45/Stn-4/GC-01 (sulfidic and hydrate bearing) and NGHP-01-15A (methanic) indicate formation and preservation of ferrimagnetic authigenic (SP) greigite particles. Based on the magnetic proxies, we demonstrated that the formation of authigenic magnetite in the methanic environment of both sites is tightly linked with the microbial iron-reduction process. Multiple occurrences of authigenic carbonate provided evidence on the episodic intensification of anaerobic oxidation of methane (AOM) at active seep and silicate weathering coupled to microbial methanogenesis at deep-seated gas hydrate site respectively.