Headspace temperature curves obtained using the TPC tool are the first systematic attempt to record the temperature inside a coring system during core recovery. Some ascent curves show thermal phenomena that would be anticipated for gas-bearing cores. Understanding the source of this gas is critical for interpreting the TPC data. A geological source for the observed gas is excluded because sediment gas data for Site 1226 show no indication of significant quantities of methane gas in these sediments. Methane concentrations in the uppermost 100 mbsf at Site 1226 are <2 µM and sulfate concentrations span from 22 to 30 mM (D'Hondt, Jørgensen, Miller, et al., 2003). These concentrations are consistent with low to modest levels of microbial activity and do not suggest that any substantial accumulations of methane should be expected in this sedimentary section. Thus, the gas detected by the TPC tool was introduced at the surface.
Conductivity measurements indicate that, in every case, gas is trapped in headspace at the face of the TPC tool during the initial deployment of the tool downhole. In many cases, the conductivity sensor indicates that the gas headspace disappeared during descent and reappeared during ascent, albeit at a slightly shallower depth. This suggests that gas completely dissolved during descent and then exsolved during ascent. There are no cases where the initial headspace did not return during ascent, whereas in some cases the gas headspace never completely disappeared. However, the conductivity data for three cores that have the earliest disappearance and latest reappearance of a gas headspace (Cores 201-1226E-7H, 9H, and 11H) indicate very little gas was entrained under the face of the TPC tool during these particular runs. The ascent temperature profiles for these curves (Fig. F5B) lack any signal related to gas exsolution or expansion. Thus, the size of the initial trapped gas headspace is quite variable, but it is always present during APC coring.
The trapping of gas headspace at the face of the TPC tool when the tool is first deployed is most likely the natural consequence of the coring process. While the drill pipe is opened for insertion of the APC coring string, the level of seawater in the pipe drains down from the rig floor to sea level, a distance of 11.2 m. When the drill pipe is reassembled, ~100 L of atmospheric gas is trapped within the drill pipe. This trapped gas is circulated down the hole by the mud pumps and eventually should exit at the seafloor. However, the conductivity data suggest sufficient amounts of gas remain trapped and/or dissolved in the seawater adjacent to the TPC tool to affect the conductivity sensor. Undoubtedly, this gas contributes to the thermal signal measured by the TPC thermistor (Fig. F5). This entrainment of atmospheric gases into the coring system is also the most likely explanation for some runs of the Pressure Core Sampler (PCS) during Leg 201 coming up with pressurized air (J. Dickens, pers. comm., 2003; Dickens et al. 2003). Changes in coring protocols will be necessary to alleviate this inherent flaw in pressure coring techniques.
The TPC data obtained from Site 1226 establish a baseline for the types of effects that would be expected during APC coring of known gas-rich sites. At gas-rich sites, the addition of gas from sedimentary sources should create a different pattern of conductivity changes. A conductivity change during core ascent at depths greater than those for core descent would be a clear indication of gas addition. Because gas was initially entrained in the coring system during its descent in the drill pipe, the temperature anomalies measured at Site 1226 during core ascent define the limits on the range of thermal changes that can be expected for cores collected at gas-rich sites. Data from Site 1226 have shown that there are wide variations in what happens with gas in cores, and more data are required to determine why these variations occur. Cores that remained at the bottom of the borehole the longest before being pulled out had the greatest depth difference between the disappearance and reappearance of a gas headspace. Mud pump circulation, which is always maintained while a coring string is in a borehole, may have swept out some of the gas entrained by the APC coring system.
Relatively small (<8 bar) pressure anomalies were produced within the core liner of cores collected at Site 1226. In numerous cases, there was a pressure increase when the APC was shot into the sediment and a pressure decrease when the core was pulled out. The core pullout caused gas expansion within the core liner, rapidly creating a relatively large (up to 2°C) temperature anomaly that decayed quickly as the core ascended to the surface.
A benefit derived from the temperature and pressure data collected by the TPC tool is improved understanding of the core recovery process. Collecting the APC core creates temperature and pressure anomalies that have the potential to alter the physical state of the core material and may lead to expanded core recovery statistics (>100%). When more data become available from sites with different lithologies, we may learn more about how the coring process could be modified to enhance core recovery and quality.