Abstract
Diagenetically formed magnetic minerals at marine methane seep sites are potential archive of past fluid flow and could provide important constraints on the evolution of past methane seepage dynamics and gas hydrate formation over geologic time. In this study, we carried out integrated rock magnetic, and mineralogical analyses, supported by electron microscope observations, on a seep impacted sediment core to unravel the linkage between greigite magnetism, methane seepage dynamics, and evolution of shallow gas hydrate system in the K-G basin. Three sediment magnetic zones (MZ-1, MZ-2, and MZ-3) have been identified based on the down-core variations in rock magnetic properties. Two events of intense methane seepage are identified. Repeated occurences of authigenic carbonates throughout the core indicate the episodic intensification of anaerobic oxidation of methane (AOM) at the studied site. Marked depletion in magnetic susceptibility manifested by the presence of chemosynthetic shells (Calyptogena Sp.), methane-derived authigenic carbonates, and abundant pyrite grains provide evidences on intense methane seepage events at this site. Fracture-controlled fluid transport supported the formation of gas hydrates (distributed and massive) at this site. Three greigite bearing sediment intervals (G1, G2, G3) within the magnetically depleted zone (MZ-2) are probably the paleo-gas hydrate (distributed-type vein filling) intervals. A strong linkage among clay content, formation of veined hydrate deposits, precipitation of authigenic carbonates and greigite preservation is evident. Hydrate crystallizes within faults/fractures formed as the methane gas migrates through the gas hydrate stability zone (GHSZ). Formation of authigenic carbonate layers coupled with clay deposits restricted the upward migrating methane, which led to the formation of distributed-type vein filling hydrate deposits. A closed system created by veined hydrates trapped the sulfide and limited its availability thereby, causing arrestation of pyritization and favored the formation and preservation of greigite in G1, G2, G3.
Highlights
Cold seep ecosystems are unique ecosystem characterized by ebullition of methane-rich fluids through the seafloor (Suess, 2014)
MZ-1 showed higher values of χlf, anhysteretic remanent magnetization (ARM) and saturation IRM (SIRM) suggesting high concentration of magnetic minerals while lower values are observed in MZ-2 and show uniform values except at certain intervals (Figures 2A–C)
We demonstrate that combination of rock magnetic and mineralogical (SEM-energy dispersive X-ray spectroscopy (EDS), X-ray Diffraction (XRD)) data together with authigenic carbonates are useful to identify the present and past sulfate-methane transition zone (SMTZ) fronts fueled by methane seepages
Summary
Cold seep ecosystems are unique ecosystem characterized by ebullition of methane-rich fluids through the seafloor (Suess, 2014). The hydrogen sulfide facilitate dissolution of primary detrital magnetic minerals and transform them into stable non-magnetic pyrite or intermediate ferrimagnetic greigite; thereby, generating secondary magnetic signals in the host sediments (Housen and Musgrave, 1996; Passier et al, 1998; Kasten et al, 2003; Neretin et al, 2004; Riedinger et al, 2005; Musgrave et al, 2006; Larrasoaña et al, 2007; Roberts et al, 2015) These magnetic minerals serve as an excellent geological archive to record past methane seepage, and help in understanding the geochemical processes that favor diagenesis of magnetic mineral and may influence the gas hydrate dynamics (formation and dissociation) (Housen and Musgrave, 1996; Aharon et al, 1997; Musgrave et al, 2006; Larrasoaña et al, 2007; Bayon et al, 2013; Feng and Chen, 2015). Ferrimagnetic iron sulfide minerals (greigite, pyrrhotite) formed during such processes are regarded as potential markers of fossil gas hydrate deposits, and identification of such magnetic minerals may help in deciphering paleo-gas hydrate deposits in marine sediments (Larrasoaña et al, 2007)
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