A Test of the Temperature, Pressure, and Conductivity Tool at a Gas-Poor Background Site

A tool to continuously monitor temperature, pressure, and conductivity (TPC) changes during Ocean Drilling Program (ODP) coring was tested at ODP Site 1226. TPC sensors are located on the face of the standard ODP advanced piston corer piston, and the data logging electronics and batteries are embedded within the piston. This tool operates autonomously and requires little shipboard attention. The objective is to monitor the TPC changes that occur in gas-rich and gas hydrate– bearing cores and to learn about the processes that occur during core collection. Gas evolution during core recovery alters the temperature and pressure conditions within the core barrel. By establishing families of ascent curves comprising TPC data from successive cores, variations in the relative amounts of gas and gas hydrates stored in sediments can be determined at individual sites and variations between sites can be assessed. Here, the performance of the TPC tool and the response of the tool at a site without significant quantities of sediment gas are described. INTRODUCTION The temperature, pressure, and conductivity (TPC) tool was developed to measure the effects of expanding gas within the Ocean Drilling Program (ODP) advanced piston corer (APC) assembly during core recovery. In this chapter, we describe the observations that stimulated the 1Ussler, W., III, Paull, C.K., McGill, P., Schroeder, D., and Ferrell, D., 2006. A test of the temperature, pressure, and conductivity tool at a gas-poor background site. In Jorgensen, B.B., D’Hondt, S.L., and Miller, D.J. (Eds.), Proc. ODP, Sci. Results, 201, 1–21 [Online]. Available from World Wide Web: . [Cited YYYYMM-DD] 2Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing CA 95039-9964, USA. Correspondence author: methane@mbari.org 3Integrated Ocean Drilling Program, 1000 Discovery Drive, College Station TX 77845-9547, USA. Initial receipt: 26 July 2004 Acceptance: 7 June 2005 Web publication: 18 January 2006 Ms 201SR-114 WILLIAM USSLER III ET AL. TPC TOOL TESTING AT A GAS-POOR BACKGROUND SITE 2 development of the tool, its design, and initial tests of the tool conducted at ODP Site 1226. During the more than a quarter century of Deep Sea Drilling Project (DSDP) and ODP drilling, vigorous gas expansion and anomalously cold temperatures in cores from continental margins have been commonly observed. Thermal observations have included cores feeling cold (e.g., Leg 146, Site 889 [Westbrook, Carson, Musgrave, et al., 1994]) and the presence of frozen pore waters along the interior wall of the core liner (e.g., Leg 164 [Paull, Matsumoto, Wallace, et al., 1996]). A few investigations have quantified these observed thermal anomalies by inserting thermistors into the core after it has been removed from the core barrel and delivered to the catwalk (Leg 164, Site 994 [Paull, Matsumoto, Wallace, et al., 1996]) or to the core laboratory immediately after core splitting (temperatures as low as –2°C have been recorded) (e.g., Leg 66, Site 490 [Watkins, Moore, et al., 1982]; Leg 146, Site 889 [Westbrook, Carson, Musgrave, et al., 1994]) and by scanning core liner on the catwalk with infrared imaging cameras (e.g., Leg 201 [Ford et al., 2003]; Leg 204 [Trehu et al., 2004]). These “catwalk core temperature” measurements show that some core sections arrived on deck at distinctly lower temperatures (5°–10°C cooler) than other cores recovered from the same drill site. As part of the shipboard sampling protocol used during DSDP and ODP, core gas samples have been collected soon after the core arrived on deck for routine gas chromatographic analysis of the low molecular weight gases, including methane. Gas composition data clearly indicate that core gas is usually dominated by methane. However, because most of the dissolved and interstitial gas contained in sediment cores is lost during their ascent to the surface (e.g., Paull and Ussler, 2000), very little is known about how much methane was actually in these sediments before recovery. This gas loss and the thermal anomalies observed soon after core recovery suggest that the temperature history of a gas-rich sediment core may provide information about the amount of gas originally contained in the sediment prior to recovery. There are three endothermic processes that can cause a temperature decrease in sediment cores during their ascent to the sea surface: (1) gas expansion, (2) gas exsolution, and (3) gas hydrate decomposition. Predictions about the temperature changes that will occur in gassy sediments (with or without gas hydrates) during core recovery based on simple thermodynamic calculations (Ussler et al., 2002) indicate that temperature changes caused by a combination of one or more of these endothermic processes are of the right order of magnitude and direction (i.e., 1°–10°C colder than gas-free sediment). Thermal modeling has shown that cores which evolve gas during their ascent to the surface will have distinct ascent temperature profiles (Fig. F1) (Ussler et al., 2002). Ascent temperature profiles will track profiles for gas-free cores until gas saturation occurs and exsolution commences. Free gas coming out of solution along the length of the core will buoyantly rise within the core barrel (Fig. F2). As soon as free gas is trapped at the top of the core barrel, temperatures should drop as a result of gas exsolution and gas expansion (Fig. F1B). Most of the gas exsolution and expansion, and thus the largest temperature changes, will occur in the upper water column. In situ gas concentration can be calculated using the pressure and temperature of the inflection in ascent temperature profiles and the methane gas solubility model of Duan et al. (1992), assuming that no gas was introduced during the coring process. F1. Radial heat transfer models of ascent temperatures, p. 11. -10 0 10 20 30 40 50 0 10 20 30 40 50 0

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