Formation of methane-derived dolomite concretions in Okinawa-Jima, Southwest Japan during the Early Pleistocene “Chinen Disturbance”

Abstract

A thick deposit of the muddy Shimajiri Group (Upper Miocene-Early Pleistocene) is overlain by the calcareous sandy Chinen Formation (Pleistocene) in southern Okinawa-Jima, Southwest Japan. The abrupt facies change has been explained by a significant unconformity resulted from subaerial erosion (the Shimajiri disturbance) or the consequence of rapid and regional uplift leading to an abrupt environmental change (the Chinen disturbance). We identified sandy mass transfer deposits (MTD) directly overlying the Shinzato Formation-the uppermost part of the Shimajiri Group-in the large Urizun outcrop. The Shinzato-Chinen transition zone yields numerous dolomite concretions. The dolomite is strongly depleted in ¹³C, with δ¹³C values of −23.77‰ to −31.50‰, suggesting that the concretions were derived from anaerobic oxidation of methane (AOM). Zonation in δ¹⁸O values across the concretions, from 3.89‰ in the centers to 2.25‰ at the rims, suggest that the precipitation temperature increased from 12.3±2.0 to 19.5±2.0°C, corresponding to a shallowing of the water depth from 440±100 to 190±60 m. The estimate is consistent with the analysis of benthic foraminiferal assemblage. Given the wide distribution of the Bottom Simulating Reflector (BSR) offshore of Okinawa-Jima, methane hydrates are assumed to have persisted in the Shimajiri Group throughout the Pliocene-Pleistocene. Abrupt uplift and relative sea-level fall during the Chinen disturbance at 2 Ma must have triggered abrupt changes in facies and a massive methane hydrate dissociation. This disturbance resulted in a high methane flux, increased AOM, and the formation of the dolomite concretions in the Shinzato-Chinen transition zone. Methane-derived dolomite concretions are thus a geochemical record of an abrupt tectonic disturbance and hydrate dissociation offshore of Okinawa-Jima.