Geochemical Analysis as a Complementary Tool to Estimate the Uplift of Sediments Caused by Shallow Gas Hydrates in Mounds at the Seafloor of Joetsu Basin, Eastern Margin of the Japan Sea
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 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 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. 1. Introduction 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]. The opening was almost completed before 15？Ma [2, 3]. 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]. As a result, several potential hydrocarbon traps were formed during this period and continuous subsidence created kitchen areas with mature source rocks [5]. Joetsu Basin is located southwest of Sado Island (Figure 1) and was formed during the Miocene [5, 7]. A favorable source rock was developed by high production of organic matter under anoxic conditions in the Nanatani Fm. (<12.5？Ma) and Teradomari Fm. (12.5~5.5？Ma) [5]. Oil generation occurred in the Miocene and a 15？m oil column was confirmed in tuffaceous sandstones in the lower part of the Shiiya Fm. (5.5~3.5？Ma) [5]. The top of the Nishiyama Fm. (3.5~1.3？Ma) is characterized by a “domino style” with several horsts and grabens and both normal and reverse faults are observed, reflecting the complex stress field involved
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