Episodic methane release events from Last Glacial marginal sediments in the western North Pacific

TitleEpisodic methane release events from Last Glacial marginal sediments in the western North Pacific
Publication TypeJournal Article
Year of Publication2004
AuthorsUchida, M, Shibata, Y, Ohkushi, K, Ahagon, N, Hoshiba, M
JournalGeochemistry Geophysics Geosystems
Date PublishedAug 19
ISBN Number1525-2027
Keywordsatmospheric composition and structure : geochemical cycles, atmospheric methane, carbon isotopes, carbon isotopic fractionation, climate-change, cold seeps, diploptene, Foraminifera, gas-hydrate, geochemistry : isotopic composition/chemistry, geochemistry : organic geochemistry, hydrocarbons, interglacial, intermediate water, methane hydrate, santa-barbara basin, sea sediments, subduction zone, western north pacific

[1] According to recent observations of anomalous bottom-simulating reflections (BSR),the northwest Pacific marginal sediments around Japan main islands bear large abundances of methane hydrate [Satoh, 2002]. During the Last Glacial, direct and indirect evidence accumulated from geochemical data suggests that methane episodically released from hydrate trapped in the seafloor sediments [ Dickens et al., 1995; Hinrichs et al., 2003; Kennett et al., 2000]. Here we show that marginal sediments from the western North Pacific contain a hopanoid 17alpha( H), 21beta(H)-hop-22(29)-ene ( diploptene) derived from the activity of methanotrophic bacteria in water column and/or surface sediment during a warming period (Interstadial 3) in the Last Glacial. The carbon isotopic compositions of diploptene range between - 41.0parts per thousand and - 27.9parts per thousand ( relative to PDB). In the horizon indicative of a contribution of methanotrophic bacteria, foraminiferal isotope signals were also found with highly depleted C-13 compositions of planktonic foraminifera ( similar to - 1.9parts per thousand, PDB) and benthic foraminifera ( similar to - 0.8parts per thousand, PDB), suggesting indirect records of enhanced incorporation of C-13-depleted CO2 formed by methanotrophic process that use C-12-enriched methane as their main source of carbon. From combined isotopic data of molecular ( diploptene) and foraminifera, the most prominent signal of methane release was detected in the sediments deposited around 25.4 cal. kyr BP ( similar to 100 year time span), corresponding to the Interstadial 3. This is the first evidence of methane hydrate instability in the open western North Pacific during the Last Glacial. Considering the glacial-interglacial hydrographic conditions in this region, the instability of methane hydrate may be modulated by intermediate water warming and/or the lowering of sea level. Our results suggest that the western North Pacific marginal regions may be a profound effect on rapid global warming climate changes during the Last Glacial.