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

• Published in: Frontiers in Marine Science Vol. 10
• Main Authors: Kim, Ji-Hoon, Park, Myong-Ho, Ryu, Jong-Sik, Jang, Kwangchul, Choi, Jiyoung, Park, Sanghee, Song, Yungoo, Yi, Bo-Yeon, Joo, Young Ji, Kim, Tae-Hoon, Hur, Jin
• Format: Journal Article
• Language: English
• Published: Lausanne Frontiers Research Foundation 17-01-2024; Frontiers Media S.A
• Subjects: Alkalinity; Aragonite; Calcite; Carbon; Carbon cycle; Carbonates; Chemical precipitation; Chemistry; chimney structure; Climate change; Cold; Continental margins; Dissociation; Fluids; gas hydrate; Gases; Geochemistry; High density polyethylenes; marine silicate weathering; MDACs; Methane; microbial CO2 reduction; Reproducibility; Seawater; Sediments; Seepages; Ulleung Basin; South Korea
• ISSN: 2296-7745; 2296-7745
• DOI: 10.3389/fmars.2023.1156918

We investigated the geochemistry of gas, pore fluid, and methane-derived authigenic carbonate (MDAC) from four sites in the southern Ulleung Basin, South Korea. In contrast to Sites 16GH-P1 and 16GH-P5, Sites 16GH-P3, and 16GH-P4 are characterized by acoustic chimney structures associated with gas flux. The composition of gas and isotopic signatures of methane (CH
4
) (C
1
/C
2+
> 300, δ
13
C
CH4
< -60‰, δD
CH4
≤ -190‰) indicate microbial source CH
4
at all sites. The upward migration of CH
4
can affect the chemical and isotopic properties of pore fluid and gas-related byproducts (e.g., gas hydrate (GH) and MDAC) within the shallow sediments including the current sulfate-methane transition (SMT) (< 5 meters below seafloor). Although no GH was found, elevated Cl
-
concentrations (maximum = 609 mM) with low δD and δ
18
O values in Site 16GH-P4 pore fluids delineate the influence of massive GH formation in deeper sediment. In contrast, relatively constant Cl
-
, δD, and δ
18
O values in fluids from Sites 16GH-P1, 16GH-P3, and 16GH-P5 indicate a predominant origin from seawater. Pore fluids also exhibit higher concentrations of H
4
SiO
4
, B, Mg
2+
, and K
+
, along with increasing alkalinity compared to seawater. These observations suggest that marine silicate weathering alters fluid chemistry within the sediment, affecting element and carbon cycles. High alkalinity (up to 60 mM) and Mg
2+
/Ca
2+
ratios (> 6) alongside decreasing Ca
2+
and Sr
2+
concentrations imply carbonate precipitation. MDACs with diverse morphologies, mainly composed of aragonite and magnesian calcite, and characterized by low carbon isotopic values (δ
13
C
MDAC
< -31.3‰), were found at Sites 16GH-P3 and 16GH-P4. Interestingly, δ
13
C
MDAC
values at Site 16GH-P3 are clearly differentiated above and below the current SMT. High δ
13
C
MDAC
values above the SMT (> -34.3‰) suggest the combined influence of seawater and CH
4
migrating upward on MDAC precipitation, whereas low δ
13
C
MDAC
values below it (< -41.6‰) indicate a predominant impact of CH
4
on MDAC formation. Additionally, the vertical variation of δ
18
O
MDAC
values at Site 16GH-P4, compared to the theoretical values, reflects an association with GH dissociation and formation. Our findings improve the understanding of fluid, gas, and MDAC geochemistry in continental margin cold seeps, providing insights into global carbon and element cycles.