ABSTRACT

Two piston cores (06GHSA P1 and 06GHSA P6) taken from the Ulleung Basin of the Japan Sea/East Sea were examined for the presence of an organic compound, diploptene (hopanoid 17α(H),21β(H)-hop-22(29)-ene). Carbon isotopes of diploptene (δ¹³C_dip) were analyzed to determine their origins and relationship with methane hydrate dissociation. The presence of diploptene was identified, and its concentrations in the two cores exhibited spatiotemporal variations, ranging from 1.7 to 632.1 ng/g (average, 87 ng/g) in core 06GHSA P6 and from 1.4 to 42.2 ng/g in core 06GHSA P1 (average, 20.1 ng/g). In both cores, δ¹³C_dip exhibited significant variations, ranging from -22.3 to -61.3‰. In core 06GHSA P6, extreme δ¹³C_dip depletion was evident in the sediments deposited under glacial conditions, whereas relatively enriched values were determined in Holocene sediments. High diploptene concentrations correspond to depleted δ¹³C_dip, indicating that carbon was, in part, derived from methanotrophic bacterial activities, whereas low diploptene concentrations may reflect a mainly cyanobacterial origin. The presence of depleted δ¹³C_dip and its fluctuation may record the dissociation of gas hydrates and methane seepage during glacial intervals for core 06GHSA P6. The release of methane hydrate release may have been triggered by the collapse of the stability zone. Several factors associated with the hydrate stability include sea-level fluctuations. Lowered sea levels during glacial periods may cause the collapse of hydrate stability zone, which could be the source of dissociated of methane hydrate and methane seepage. This study documents the evidence of methane hydrate instability, inferred from compound-specific analysis, of the Japan Sea/East Sea sediments.