Preprint Article Version 1 Preserved in Portico This version is not peer-reviewed

Differences in the Sulfate-Methane Transitional Zone in Coastal Pockmarks in Various Sedimentary Environments

Version 1 : Received: 21 November 2020 / Approved: 23 November 2020 / Online: 23 November 2020 (09:04:12 CET)

A peer-reviewed article of this Preprint also exists.

Cao, C.; Cai, F.; Qi, H.; Zhao, S.; Wu, C. Differences in the Sulfate–Methane Transitional Zone in Coastal Pockmarks in Various Sedimentary Environments. Water 2021, 13, 68. Cao, C.; Cai, F.; Qi, H.; Zhao, S.; Wu, C. Differences in the Sulfate–Methane Transitional Zone in Coastal Pockmarks in Various Sedimentary Environments. Water 2021, 13, 68.

Journal reference: Water 2020, 13, 68
DOI: 10.3390/w13010068

Abstract

Different types of pockmarks, including single pockmarks, circular pockmarks, elongated pockmarks, chain-type pockmarks and compound pockmarks, were identified in coastal areas around Fujian, China. The sediments associated with pockmarks were mainly silty clay to clay, with a small quantity of silt with fine sand. The sulfate content in the pore water in the sedimentary layers associated with pockmarks decreased with depth from the surface, whereas the free methane content increased with depth. The interaction between sulfate and methane is well known, but differences in the sulfate-methane transitional zone (SMTZ) were observed in different areas with different hydrologic characteristics. The sedimentary SMTZ of the offshore Zhe-min mud wedge was shallow, at 50–70 cm below the seafloor. The sedimentary SMTZ was moderately deep (90–115 cm) in the central bay area and deep (180–200 cm) in the sandy area offshore. This variability in SMTZ depth reflects different amounts of free methane gas in the underlying formations, with a shallower SMTZ indicating a higher free methane content. The free methane had δ13C values of -26.47‰ to -8.20‰ and a biogenic-hybrid genetic type. The flux of sedimentary gas from the pockmark surfaces, calculated according to Fick’s formula, was 2.89 to 18.85 l/m2·a. The shape, size and scale of the pockmarks are directly related to the substrate type and the gas production of the underlying strata and thus vary with the sedimentary environment and development stage. Therefore, different types of pockmarks, in various phases of development, are associated with different sedimentary and dynamical conditions. A single circular pockmark forms by strong methane flux. As the intensity of methane flux weakens, the pockmark becomes elongated in the direction of the water flow because of long-term erosion induced by regular hydrodynamic forces. Finally, under weak intensity of methane flux and the influence of complex hydrodynamic conditions, pockmarks merge to form large-scale, compound pockmarks.

Subject Areas

pockmark; sulfate and methane transitional zone; various sedimentary environments; pore water and sediment

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