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

Abstract

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 17α(H), 21β(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 −41.0‰ and −27.9‰ (relative to PDB). In the horizon indicative of a contribution of methanotrophic bacteria, foraminiferal isotope signals were also found with highly depleted 13C compositions of planktonic foraminifera (∼−1.9‰, PDB) and benthic foraminifera (∼−0.8‰, PDB), suggesting indirect records of enhanced incorporation of 13C‐depleted CO2 formed by methanotrophic process that use 12C‐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 (∼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.