Abstract
We report 4 years of temperature profiles collected from May 2014 to May 2018 in Integrated Ocean Drilling Program Hole U1364A in the frontal accretionary prism of the Cascadia subduction zone. The temperature data extend to depths of nearly 300 m below seafloor (mbsf), spanning the gas hydrate stability zone at the location and a clear bottom-simulating reflector (BSR) at ∼230 mbsf. When the hole was drilled in 2010, a pressure-monitoring Advanced CORK (ACORK) observatory was installed, sealed at the bottom by a bridge plug and cement below 302 mbsf. In May 2014, a temperature profile was collected by lowering a probe down the hole from the ROV ROPOS. From July 2016 through May 2018, temperature data were collected during a nearly two-year deployment of a 24-thermistor cable installed to 268 m below seafloor (mbsf). The cable and a seismic-tilt instrument package also deployed in 2016 were connected to the Ocean Networks Canada (ONC) NEPTUNE cabled observatory in June of 2017, after which the thermistor temperatures were logged by Ocean Networks Canada at one-minute intervals until failure of the main ethernet switch in the integrated seafloor control unit in May 2018. The thermistor array had been designed with concentrated vertical spacing around the bottom-simulating reflector and two pressure-monitoring screens at 203 and 244 mbsf, with wider thermistor spacing elsewhere to document the geothermal state up to seafloor. The 4 years of data show a generally linear temperature gradient of 0.055°C/m consistent with a heat flux of 61–64 mW/m2. The data show no indications of thermal transients. A slight departure from a linear gradient provides an approximate limit of ∼10−10 m/s for any possible slow upward advection of pore fluids. In-situ temperatures are ∼15.8°C at the BSR position, consistent with methane hydrate stability at that depth and pressure.
Highlights
We report 4 years of temperature profiles collected from May 2014 to May 2018 in Integrated Ocean Drilling Program Hole U1364A in the frontal accretionary prism of the Cascadia subduction zone
The stability of methane hydrates in continental margin sediments has long been known to be strongly dependent on temperature and pressure (e.g., Kvenvolden and Barnard, 1983; Hyndman et al, 1992), such that seismically determined depths to bottom-simulating reflectors (BSRs) that often mark the base of gas hydrate stability (BGHS) have been used by many authors to estimate heat flux through the sedimentary section above the BSR (e.g., Yamano et al, 1982; Davis et al, 1990; Hyndman et al, 1992)
The best previous example of the latter was at Ocean Drilling Program (ODP) Site 892 at Hydrate Ridge in the southern Cascadia accretionary prism, where thermal results were enigmatic with respect to inferred depths of methane hydrate stability (Davis et al, 1995)
Summary
The stability of methane hydrates in continental margin sediments has long been known to be strongly dependent on temperature and pressure (e.g., Kvenvolden and Barnard, 1983; Hyndman et al, 1992), such that seismically determined depths to bottom-simulating reflectors (BSRs) that often mark the base of gas hydrate stability (BGHS) have been used by many authors to estimate heat flux through the sedimentary section above the BSR (e.g., Yamano et al, 1982; Davis et al, 1990; Hyndman et al, 1992). This paper describes 4 years of temperature time-series data in the northern Cascadia accretionary prism, at Integrated Ocean Drilling Program (IODP) Site U1364, spanning a clear BSR and the GHSZ. These data are consistent with and resolve more clearly the previously interpreted temperature gradient and temperature at the BGHS at the site. They improve the estimate of heat flux at depth and provide an upper limiting constraint on the rate of any vertical pore-fluid expulsion
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