Abstract:
Numerous observations of anomalously cold temperatures have been made on freshly recovered gas-rich continental margin cores collected during the Deep Sea Drilling Project and the Ocean Drilling Program (ODP). To date, core temperatures have only been measured on the catwalk using thermistors inserted several minutes after core retrieval. These catwalk temperatures show that many core sections arrived on deck at distinctly lower temperatures (5-10 degrees C cooler) than other cores recovered from the same site, and in some sections the pore water was frozen when the core arrived on deck. These observations clearly show that there are significant thermal differences between adjacent cores apparently produced during the coring process. Modeling indicates that the observed thermal anomalies can be explained by a combination of the cooling effects caused by gas hydrate decomposition, gas exsolution, and gas expansion.To better quantify the thermal changes that occur in gas-rich cores, we are developing a tool to continuously monitor the temperature, pressure, and conductivity (TPC) changes during ODP coring. The TPC sensors are located on the face of the standard ODP advanced piston corer's piston and the data logging electronics are embedded within the piston. This system is designed to operate passively and to require little shipboard attention. By establishing families of ascent curves comprising data from successive cores, the stratigraphic variations in the relative amounts of gas stored in sediments can be determined at individual sites and variations between sites can be assessed. Models indicate that these data also will provide information on whether gas hydrate was present in the sediment before core retrieval.