Diagenesis of Magnetic Minerals in Active/Relict Methane Seep: Constraints From Rock Magnetism and Mineralogical Records From Bay of Bengal

Abstract

In this study, we conducted a comprehensive investigation of rock magnetic, mineralogical, and sedimentological records of sediment cores supplemented by a high resolution seismic data to elucidate the controls of structural and diagenetic (early vs. late) processes on the sediment magnetism in active and relict cold seep sites in the Bay of Bengal. Two distinct sediment magnetic zones (Z-I and Z-II) are defined based on the down-core variations in rock magnetic properties. The sediment magnetism is carried by complex magnetic mineral assemblages of detrital (titanomagnetite, titanohematite) and authigenic (fine-grained greigite) minerals. Overall, the magnetic susceptibility varies over one order of magnitude with highest values found in relict core. Uppermost sediment magnetic zone (Z-I) is characterized by higher concentration of magnetite as seen through elevated values of magnetic susceptibility (χlf) and saturation isothermal remanent magnetization (SIRM). A systematic gradual decrease of χlf and IRM1T in Z-I is attributed to the progressive diagenetic dissolution of iron oxides and subsequent precipitation of iron sulfides. Magnetic grain size diagnostic (ARM/IRM1T) parameter decreases initially due to the preferential dissolution of fine-grained magnetite in the sulfidic zone (Z-I), and increases later in response to the authigenic formation of magnetite and greigite in methanic zone (Z-II). Distinct low S-ratio and χlf values in methanic zone of relict core is due to increased relative contribution from highly preserved coercive magnetic (titanohematite) grains of detrital origin which survived in the diagenetic processes. A strong linkage between occurrence of authigenic carbonates and greigite formation is observed. Two plausible mechanisms are proposed to explain the formation and preservation of greigite in Z-I and Z-II: 1) decline in methane flux due to massive hydrate accumulation within the active fault system and formation of authigenic carbonate crust in the sub-surface sediments hindered the supply of upward migrating fluid/gas; thereby limiting the sulfide production which preferentially enhanced greigite formation in Z-I and 2) restricted supply of downward diffusing sulfide by the carbonate layers in the uppermost sediments created a sulfide deficient zone which inhibited the pyritization and favoured the formation of greigite in the methanic zone (Z-II).

Isolating Detrital and Diagenetic Signals in Magnetic Susceptibility Records From Methane‐Bearing Marine Sediments

“…Our IRM and thermal demagnetization results are consistent with the extensive work of Badesab et al (2017Badesab et al ( , 2019Badesab et al ( , 2020, Dewangan et al (2013), Gaikwad et al (2021), João et al (2021), and Usapkar et al (2014) who used IRM and additional rock magnetic parameters (e.g., anhysteric remanent magnetization (ARM), and S-ratio) coupled with XRD and SEM EDS to document a dominant presence of titanomagnetite and minor amounts of hematite and goethite in the magnetic assemblages of the Krishna-Godavari Basin, with distinct intervals bearing magnetic iron sulfides and pyrite as diagenetic overprints. These comprehensive rock magnetic studies include Site MD-161 (Dewangan et al, 2013) which is about 100 m from NGHP-01 Site 10, and NGHP-01 Site 07 (Badesab et al, 2019), which is 8 km to the south of NGHP-01 Site 16.…”

“…These studies document losses in fine‐grained titanomagnetite, precipitation of pyrite and greigite/pyrrhotite, and relative increases in high‐coercivity minerals in zones affected by H 2 S‐related diagenesis. In addition, numerous gravity cores collected throughout the KG‐Basin show titanomagnetite with reduced magnetic susceptibility or greigite presence at seep sites (Gaikwad et al., 2021; Usapkar et al., 2014).…”

Effect of Hydrocarbon Presence and Properties on the Magnetic Signature of the Reservoir Sediments of the Catcher Area Development Region, UK North Sea

“…This observation is in agreement with available thermodynamic models (Machel, 1995), that suggest the preservation and/or precipitation of iron-oxides in sediments that are located around hydrocarbon accumulations, and the instability of these minerals in the reservoir itself. Also, the requirements for greigite's preservation are high concentrations of reactive iron, "small" amounts of organic carbon present and low levels of sulphides relative to the amounts of reactive iron (Kao et al, 2004;Gaikwad et al, 2021) and these will more likely be met in the "proximal" shales. Lepidocrocite was found to co-exist with greigite (Figure 9E) reiterating what has been found in another study (Minyuk et al, 2013).…”

“…Many studies have shown that hydrocarbon migration into sedimentary rocks results in magnetic mineral diagenesis (e.g., Elmore et al, 1987;Machel, 1995;Liu et al, 2006a;Badejo et al, 2021c;Gaikwad et al, 2021), however, the product of this process has revealed significant variability in minerology and susceptibility along this migration pathway. Enhancement and destruction of magnetic signature has been reported for different environments, e.g., the formation of magnetite resulting in over an order increase in magnetization in the hydrocarbon saturated sediments of the Ordovician Arbuckle Group in South Oklahoma (Elmore et al, 1987), the replacement of authigenic iron oxides by pyrite resulting in diminished magnetization in the Cenozoic sandstones of south west Texas (Reynolds et al, 1993), and the formation of hydrocarbon related authigenic maghemite in the Mawangmiao Oil Field, Jianghan Basin, China (Liu et al, 2006a).…”

Mineralogia autigênica em sedimentos quaternários da bacia de Santos: evidências de emanações de metano no Oceano Atlântico Sudoeste

“…As emanações de metano provocam a dissolução redutiva dos minerais magnéticos detríticos primários convertendo-os em sulfetos de ferro, e consequentemente causando uma diminuição no sinal magnético dos sedimentos (Canfield e Berner, 1987;Gaikwad et al, 2021).…”