Abstract
The Holocene sediments of the eastern margin of the Japan Sea are characterized by high total organic carbon (TOC) and total nitrogen (TN) contents, low TOC/TN and TS/TOC values with enriched δ13Corg signatures, as a result of high marine productivity during present oxic highstand. On the other hand, the LGM sediments are characterized by low TOC and TN contents, high TOC/TN and TS/TOC values with depleted δ13Corg signatures, characteristic of C3-derived terrestrial organic matter input during that anoxic lowstand. However, at the top of mounds at the seafloor, where gas hydrate and authigenic carbonate nodules occur, the host sediments have a mixture of both Holocene and LGM geochemical signatures. Both gas hydrate and authigenic carbonate, formed by the anaerobic oxidation of methane, increased the sedimentary volume and caused an uplift of older sediments, inducing mound formation. The thickness of the Holocene sediments over mounds is very small or absent exposing the last glacial maximum (LGM) sediments to the seafloor. The uplift of the LGM sediments within mounds is estimated to be >2 m. We conducted geochemical analysis to detect such sediment movement, using samples collected by shallow cores in the Joetsu Basin, eastern margin of the Japan Sea.
Highlights
Japan Sea is a typical back-arc basin formed behind the Japanese island-arc system initiated by the rifting of the eastern margin of the Eurasian Continent at around 25 Ma [1]
During the Middle Pliocene, the tectonic style changed from the extensional to compressive [4] and a series of NNE-SSW trending folds were formed along the eastern margin of the Japan Sea [5], where an incipient subduction zone extends throughout the western side of the Japanese island-arc system [6]
Unit 5 is at the bottom and is characterized by light gray bioturbated silty mud belonging to the early last glacial maximum (LGM) sediments [13, 14]
Summary
Japan Sea is a typical back-arc basin formed behind the Japanese island-arc system initiated by the rifting of the eastern margin of the Eurasian Continent at around 25 Ma [1]. Associated to the gas chimneys, Matsumoto [12, 15] observed several mounds and pockmarks at the seafloor According to his studies, mounds were formed by the crystallization of gas hydrates, while pockmarks were the result of intense gas hydrate dissociation during the LGM, when the sea level dropped ca. Mounds were formed by the crystallization of gas hydrates, while pockmarks were the result of intense gas hydrate dissociation during the LGM, when the sea level dropped ca. The main propose of this study is to detail the influence of both gas hydrate and authigenic mineral formation for the growth of mounds related to gas seepages Both processes within the porous space increase the sedimentary volume and induce the formation of mounds by an uplift of old sediments associated with high methane flux venting.
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