Coupled C–S–Fe geochemistry in a rapidly accumulating marine sedimentary system: Diagenetic and depositional implications

Abstract

In the present study, we have investigated the C–S–Fe systematics in a sediment core (MD161‐13) from the Krishna‐Godavari (K‐G) basin, Bay of Bengal. The core covers the late Holocene period with high overall sedimentation rate of ∼573 cm kyr−1. Pore fluid chemical analyses indicate that the depth of the present sulfate methane transition zone (SMTZ) is at ∼6 mbsf. The (ΔTA + ΔCa + ΔMg)/ ΔSO42− ratios suggest that both organoclastic degradation and anaerobic oxidation of methane (AOM) drive sulfate reduction at the study site. The positive correlation between total organic carbon content (TOC) and chromium reducible sulfur (CRS) content indicates marked influence of organoclastic sulfate reduction on sulfidization. Coupled occurrence of 34S‐enriched iron sulfide (pyrite) with 12C‐enriched authigenic carbonate zones is the possible records of paleo‐sulfate methane transition zones where AOM‐driven‐focused sulfate reduction was likely fueled by sustained high methane flux from underlying gas‐rich zone. Aluminum normalized poorly reactive iron (FePR/Al) and La/Yb ratios suggest increasing contribution from Deccan basalts relative to that of Archean‐Proterozoic granitic complex in sediment flux of Krishna‐Godavari basin during the last 4 kyr.

Using sediment geochemistry to infer temporal variation of methane flux at a cold seep in the South China Sea

“…4), like most values for continental margin sediments, are the result of early sulfur cycling close to the sediment water interface where the diffusion rate of sulfate from bottom water into the underlying pore water exceeds the rate of sulfate consumption (Peketi et al, 2015). Large S isotopic fractionation of 39e53.6‰ from 0 to 400 cm relative to the average sea water sulfate d 34 S of~21‰ (Rees et al, 1978), is likely related to the microbial sulfur disproportionation of sulfur intermediates eS , that are produced during the oxidative part of sulfur cycling (Canfield and Thamdrup, 1994;Peketi et al, 2012).…”

“…34 S enrichments have been observed in cold seep areas of Blake Ridge, Black Sea, and Bay of Bengal (Jørgensen et al, 2004;Peketi et al, 2012Peketi et al, , 2015Borowski et al, 2013). Therefore, the high values of d 34 S CRS combined with the high CRS contents in Unit II and Unit III, as well as the presence of authigenic carbonates suggest the occurrence of AOM and past locations of the SMTZ at about 740e820 cm and 1000e1150 cm, respectively.…”

“…Large S isotopic fractionation of 39e53.6‰ from 0 to 400 cm relative to the average sea water sulfate d 34 S of~21‰ (Rees et al, 1978), is likely related to the microbial sulfur disproportionation of sulfur intermediates eS , that are produced during the oxidative part of sulfur cycling (Canfield and Thamdrup, 1994;Peketi et al, 2012). Sulfur disproportionation is a particular type of redox reaction in which sulfur is simultaneously reduced and oxidized to form two different products (Peketi et al, 2015). However, up to 70‰ 34 S depletion C age, the CALIB 6.1.0 program (Stuiver and Reimer, 1993), and the Marine 13 calibration data (Reimer et al, 2013).…”

“…¼ 1950 AD. without disproportionation of sulfur intermediates may also be caused by dissimilatory sulfate reduction in marine sediments (Wortmann et al, 2001;Brunner and Bernasconi, 2005;Sim et al, 2011;Peketi et al, 2012Peketi et al, , 2015 (Borowski et al, 1996(Borowski et al, , 2013Jørgensen and Parkes, 2010;Peketi et al, 2012Peketi et al, , 2015.…”

“…Moreover, low dissolved methane content of fluids, high bioturbation rates, and/or high sedimentation rates can also result in the absence of authigenic carbonates in marine sediments (Luff et al, 2004;Bayon et al, 2007). In fact, numerous recent studies reveal that seep-impacted sediment cores generally have relatively complete sequence and may serve as another important archive to reconstruct the evolution of past seepage activities (Bayon et al, 2007;Peketi et al, 2015).…”