Deep sea bottom-simulating-reflectors: calibration of the base of the hydrate stability field as used for heat flow estimates

Abstract

Ocean Drilling Program and Deep Sea Drilling Project downhole data from three areas, the southwestern Japan Nankai margin, the continental slope off Peru, and the Blake-Bahama Outer Ridge, provide temperature calibrations for bottom simulating reflectors (BSR) that mark the base of a clathrate hydrate stability field. The inferred temperatures at BSRs provide an important reference for the mapping of geothermal gradient and heat flow from subduction zone accretionary sedimentary wedges. The borehole results provide information on which stability field is applicable for the BSRs and thus calibrate the heat flow estimates. While an ideal calibration has not been possible, the BSR temperatures at the three sites in the temperature range 25–27°C, have been estimated with uncertainties of ±0.7 to ±2.0°C. The temperatures correspond closely to the laboratory dissociation temperatures for pure water-pure methane hydrate at equivalent pressures. No laboratory data are available for seawater salinity and methane at equivalent pressures, but extrapolation from lower pressures gives temperatures 1–2°C lower, which is just significantly different. The data also could be explained by the stability curve for seawater salinity and methane with about 7% CO2, or with a small amount of higher hydrocarbons, but most hydrate samples that have been recovered by deep sea drilling have contained almost pure methane. The uncertainties in the temperature at the BSR should contribute no more than ±5% error in heat flow estimates from BSR depths if the pure water-methane stability field is used.