Impact of High Methane Flux on the Properties of Pore Fluid and Methane-Derived Authigenic Carbonate in the ARAON Mounds, Chukchi Sea

Abstract

We investigated the pore fluid and methane-derived authigenic carbonate (MDAC) chemistry from the ARAON Mounds in the Chukchi Sea to reveal how methane (CH4) seepage impacts their compositional and isotopic properties. During the ARA07C and ARA09C Expeditions, many in situ gas hydrates (GHs) and MDACs were found near the seafloor. The fluid chemistry has been considerably modified in association with the high CH4 flux and its related byproducts (GHs and MDACs). Compared to Site ARA09C-St 08 (reference site), which displays a linear SO42- downcore profile, the other sites (e.g., ARA07C-St 13, ARA07C-St 14, ARA09C-St 04, ARA09C-St 07, and ARA09C-St 12) that are found byproducts exhibit concave-up and/or kink type SO42- profiles. The physical properties and fluid pathways in sediment columns have been altered by these byproducts, which prevents the steady state condition of the dissolved species through them. Consequently, chemical zones are separated between bearing and non-bearing byproducts intervals under non-steady state condition from the seafloor to the sulfate-methane transition (SMT). GH dissociation also significantly impacts pore fluid properties (e.g., low Cl-, enriched δD and δ18O). The upward CH4 with depleted δ13C from the thermogenic origin affects the chemical signatures of MDACs. The enriched δ18O fluid from GH dissociation also influences the properties of MDACs. Thus, in the ARAON Mounds, the chemistry of the fluid and MDAC has significantly changed, most likely responding to the CH4 flux and GH dissociation through geological time. Overall, our findings will improve the understanding and prediction of the pore fluid and MDAC chemistry in the Arctic Ocean related to CH4 seepage by global climate change.

Exploring the pore fluid origin and methane-derived authigenic carbonate properties in response to changes in the methane flux at the southern Ulleung Basin, South Korea

“…However, this fluid cannot account for the observed high Clconcentration in pore fluid at this site. Such characteristics have usually been reported in the bottom water and/or pore fluid in Arctic regions (Alexeev and Alexeeva, 2003;Charkin et al, 2017;Kim et al, 2022) as a result of the formation of submarine ice and/or permafrost within the sediments. Since the water and sediment columns in the southern Ulleung Basin have not been frozen annually, these mechanisms are not attributed to high Clconcentration with low dD and d 18 O values in pore fluid observed at Site 16GH-P4.…”

“…3), suggesting that pore fluid with a similar d 18 O value to that of present-day bottom seawater was involved during MDAC precipitation. For Site 16GH-P3, it is likely that the enrichment in 13 C for MDACs from above the SMT (> -34.3‰) compared to those from below the SMT (< -40%) is caused by incorporation of 13 (Kastner et al, 1991;Kim et al, 2013;Kim et al, 2021;Kim et al, 2022), are not exhibited in fluids from Site 16GH-P3 (Figures 3, 7; Supplementary Table 1).…”

“…in pore fluid produced by the reactions provides favorable conditions for the precipitation of methane-derived authigenic carbonates (MDACs; Equation 3) in the sediment and/or water column, which is characterized by the low carbon isotopic value inherited from CH 4 (Peckmann et al, 2001;Luff et al, 2004;Torres et al, 2020). Gas hydrates (GHs) have also been found at cold seeps located in the Arctic regions, Cascadia Margin, and South China Sea (e.g., Han et al, 2008;Malinverno et al, 2008;Wang et al, 2014;Hong et al, 2017;Kim et al, 2020;Kim et al, 2022).…”