Journal Article

Biogeochemical Reconstruction of Authigenic Carbonate Deposits at Methane Seep Site off Krishna‐Godavari (K‐G) Basin, Bay of Bengal

Sadique, Mohd, Peketi, A., Zatale, Anjali, Mishra, S., Mazumdar, A., Sivan, K., Pillutla, S. P. K., Verma, Swati

2024

Overview

Active and relic marine methane‐seep sites are widely distributed globally and are distinguished by distinctive geology, biogeochemistry, and ecosystems. The discovery of methane‐seep sites in the Krishna‐Godavari (K‐G) basin has created exciting new opportunities for methane‐seep research in the Bay of Bengal. In this study, we document the occurrence of authigenic carbonates, including micro‐crystalline aragonite crust (arg‐crusts) admixed with chemosynthetic shells and high‐magnesium carbonate tubular structures (HMC‐tube), from the methane‐seep site SSD‐045/4 in the K‐G basin. The δ13C values of HMC‐tubes (−54.5 to −46.2‰) and arg‐crusts (−57.6 to −34.8‰) indicate biogenic methane as the likely carbon source. Enhanced porewater alkalinity driving carbonate precipitation may be attributed to microbial‐mediated SO₄2−‐AOM processes. Additionally, δ13C values (−35.2 ± 8‰) of the residual organic matter within the carbonates suggest a contribution of methanotrophic bacterial biomass. The δ18Ocarb values of HMC and aragonite indicate methane hydrate degassing and crystallization pathways, respectively. Pelloid‐filled burrows suggest the reworking of shallow HMC deposit by burrowing organisms, whereas the polyphase cementations (aragonite and HMC) within burrows indicate early and burial diagenetic pathways. The wide range in ΣLREE/ΣHREE ratios and Ceanom values in arg‐crusts reflect micro‐spatial variations in redox conditions, likely due to cementation occurring in both open and closed diagenetic systems. In contrast, more constrained Ceanom values and ΣLREE/ΣHREE ratios in HMC tubes suggest persistent sulfidic conditions. Overall, these findings provide insights into the pathways of carbonate formation at the K‐G basin methane‐seep site, highlighting the complex interplay of microbial processes, fluid dynamics, and diagenetic alterations. Plain Language Summary Methane cold‐seeps are regions on the seabed where methane (CH4) gas oozes out across the sediment‐water interface into the water column. Both active and inactive seepage sites are known across the oceans. Active methane seep sites expel significant quantities of methane into the seawater which influences the oceanic methane budget as well as the carbon cycle. In contrast, the preserved authigenic carbonate deposits and chemosynthetic fauna at the relic seep sites are archives of past seepage activities. The seep ecosystem comprises a variety of chemosynthetic and non‐chemosynthetic organisms, whereas the geological features include mounds, mud volcanoes, carbonate crusts, chimneys, etc. formed due to methane expulsion processes. Here, we investigate the origin of calcium carbonate structures formed in a cold seep region off the east coast of India. The carbonate structures include tubular‐shaped high magnesian carbonates (HMC) and crusts composed of aragonite minerals. These tubes and crusts archive biological and geological processes, including past variations in methane effusion and diffusion through the sediment. Tubular structures are formed by gas flow through the sediments as chimneys and by burrowing activities of benthic organisms. On the other hand, the aragonite crusts are formed where methane flow is high across the seabed. Key Points Relics of methanogenic high magnesian calcite tubes and aragonite crust recovered from cold seep site K‐G basin east coast of India HMC precipitation is confined to sulfidic conditions in contrast to a wider redox range for aragonite precipitation Reworking of unconsolidated HMC produced the pelloid rich tubular structures, whereas aragonite crust formed at high CH4 flow conditions

• Publisher: John Wiley & Sons, Inc,Wiley
• Subject: Alkalinity / anaerobic oxidation of methane / Aragonite / authigenic carbonates / Benthos / Biogeochemistry / Burrowing organisms / Calcium / Calcium carbonate / Calcium carbonates / Carbon cycle / Carbon sources / Carbonates / Cementation / Coastal zone / Concretions / Crystallization / Degassing / Diagenesis / Fluid dynamics / Geological processes / high magnesian calcite / Hydrodynamics / Magnesium / Magnesium carbonate / Methane / Methane budget / Methane hydrates / methane seep / Mounds / Mud volcanoes / Ocean floor / Oceans / Oozes / Organic matter / Pore water / Seawater / Sediment / Sediments / Structures / Volcanoes / Water column