Drilling into the coal strata

　“From then onwards progress was fast. When we drilled to around 2,000 m and retrieved inky-black coal, we were very relieved. We anticipated there to be coal based on previous investigations, but until you actually dig down you don’t know whether it is really there. I remember thinking that this truly is black diamonds.” Inagaki happily explains, his expression clearly reflecting his feelings at the time.

　The previous record depth in scientific ocean drilling was set at 2,111 m by the American JOIDES Resolution. On September 6, 2012 the CHIKYU broke this record off the coast of Hachinohe. The world below that point is virgin territory for the scientific community. The coal continued, alternating with several sand strata. Passing the scheduled depth of 2,200 m, drilling continued following the coal to further depths still at 2,400 m. Inagaki, hitting an unexpected coal stratum, tells how he contacted the support unit on shore in the middle of the night to consult about extending the drilling depth. This was because he thought that a more complete sample could be obtained if they also drilled through this unexpected layer, which was extremely thin. The depth ultimately reached was 2,466 m, which at this point in time is a drill-depth record for scientific ocean drilling.

Surprising temperatures

　Since this is scientific drilling it is naturally not only about setting new records. Equipment is lowered into the drill hole and a variety of data is recorded for several depths. Surprisingly, the temperature in the strata where the coal lies buried was even in the deepest areas around 60℃. This was a surprise to everybody. Typically, the temperature of the sediments off the continental

　“coast rises by about 1℃ for every 30 cm. A simple calculation shows that for a depth of over 2,400 m the temperature should be over 80 ℃. Masafumi Murayama, who is professor of Kochi University and took part in the expedition as a sedimentologist points out a possibility: “The area off the coast of the Shimokita Peninsula is an area where a cold oceanic plate starts to subduct under the North American Plate. Perhaps the temperature is low because the underlying area is cold.”

　“Coal matures through the working of geothermal heat. Probably because of the low temperature of 50 ℃, the last coal stratum that was drilled through had a low level of maturity and still had large amounts of nutritional components. In other words, it had a great deal of food for microorganisms, which is good news for the exploration of the deep biosphere.