At 80km off the coast at Hachinohe on the Shimokita Peninsula the seafloor is at a 1,180 m depth. Below the seafloor, at a depth of a further 2,000 m, coal strata were found.
The organic matter released by this coal is used by microorganisms that live underground. This is the deep coalbed biosphere, and we don’t know what specifically this biosphere is without directly reaching it. The only ship with the ability to examine this is Deep Sea Scientific Drilling Vessel CHIKYU.
Its 2006 shakedown cruise was however cut short by an approaching explosive low pressure system.
The 2011 expedition was hit by the Tohoku Earthquake and had to be postponed.
The drilling survey conducted from August to September 2012 was its third attempt, and turned out to be a series of difficulties and unexpected discoveries.
（Published online February 2013）
Interviewees.
 |
Fumio Inagaki Group Leader, Senior Scientist, Geomicrobiology Group, Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), Co-chief Scientist IODP Expedition 337 |
 |
Masafumi Murayama Professor, Graduate School, Kochi University |
 |
Yuki Morono Senior Scientist, Geomicrobiology Group, Kochi Institute for Core Sample Research, Japan Agency for Marine-Earth Science and Technology (JAMSTEC) |
Continuing challenges
Off the coast at Hachinohe on the Shimokita Peninsula, coal layers were thought to lie at a depth of over 2,000 m below the sea floor. Coal originates from the remains of plant life on land. Coal strata which were buried deep beneath the seafloor with the changes in the earth’s crust, are in other words “the forests beneath the seafloor”. As part of the process where the remains of plant matter change into coal, inorganic energy substrates are released such as organic compounds or hydrogen, and microorganisms that live on that exist beneath the seafloor. Microorganisms living off buried plant matter is also something that happens on land, but in the case of the subseafloor forests this is happening on a scale of tens of millions of years. To investigate this, the CHIKYU drills down to the coal strata using a riser drilling system and obtains core samples. That was the objective of the survey conducted in 2012.
“Our first concern was the metal cap we used to cover the hole we drilled in 2006,” tells Fumio Inagaki, who lead the survey as Co-chief Scientist. Because of an approaching explosive low pressure system, the CHIKYU was forced to seek shelter away from the drill site in 2006 after covering the hole that had been drilled up to 650 m beneath the seafloor. More than six years had passed since. What to do if the metal cover had rusted and wouldn’t come away?
Tells Inagaki, thinking back, “Fortunately it was in immaculate condition. Now it was adjusting the blow-out-preventer (BOP) however that ended up taking up time.” It is blowouts that are the frightening aspect of drilling beneath the seafl When gas or water in underground and its pressure are tremendously high, they could erupt through the hole. To prevent this, a BOP reaching a weight of 380 tons is installed in the hole’s opening.
Explains Inagaki, “This piece of equipment is needed to ensure safety. If we can’t confirm on board ship that all functions work normally, we can’t lower it to the seafloor. This takes time, and one day after the other of the expedition passes by in doing so.” When the BOP was finally installed on the deep sea floor and drilling began, a problem occurred in the drill hole after drilling to about 1,200 meters deep.
“In drilling at great depths, a riser drilling system is used that circulates drilling fluid in the drill hole which is adjusted for specific gravity and viscosity, but the problem of it not circulating properly occurred,” remembers Inagaki. Underground were layers that were apt to absorb the drilling fluid, such as sand strata or cracks. This poses a complicated problem which in technical terms is called ‘lost circulation’. If the drilling fluid can’t circulate it becomes impossible to drill deeper and samples can’t be collected either. If that doesn’t change, then that is the end of it. Lost circulation materials are therefore brought into the drill hole to plug the zones of lost circulation, which can be likened to black holes. Little by little the circulation of drilling fluid returned, and when drilling could be resumed a casing was lowered to protect the drill hole wall. This was further reinforced with cement and the problem could finally be overcome.
Flags in the cores
A line-up of collected cores which have been cut open lengthwise. The little flags have been put in by the researchers to mark parts required for the research
The forest discovered underground
Coal retrieved from beneath the sea floor off the Hachinohe. Although it shows up inky-black in the photo, it is in fact brown coal (lignite) that hasn’t matured yet. Because of its immaturity it is still rich in inorganic energy substrates, potentially forming an important food source for underground microorganisms
