The Carbon Rocks of Oman

太古のマントル岩石にCO2封印 中東オマーンで実験開始

By Douglas Fox D. フォックス
English 日本語 日本語
Wadi Lawayni is a remote desert valley in the interior Al-Hajar Mountains of Oman, east of Saudi Arabia. A visitor gets there by following a lonely dirt road that dwindles to tire tracks running through a gravelly wash. Groundwater in this region occasionally surfaces in small pools that have a bluish tint—saturated with alkaline salts and sometimes so full of hydrogen gas that the liquid fizzes like champagne when it’s raised out of a well.  ワジ・ラワイニは,サウジアラビアの東,オマーンのハジャール山脈の内陸側にある砂漠の谷だ。そこを訪れるには,人けのない未舗装の道を進み,最後には砂利だらけの低地に刻まれたわだちをたどらなければならない。この地域の地下水は時おり染み出し,青みがかった小さな水たまりを形成している。塩基性の塩で飽和した地下水は水素ガスを大量に含んでいることが多く,井戸から汲み上げるとシャンパンのように泡立つ。
The valley, sparsely dotted with thorny shrubs, is ringed by worn pinnacles of faded brown stone that rise hundreds of meters into the air. The rock is an anomaly of minerals that are chemically unstable on Earth’s surface. It may have formed dozens of kilometers below the surface, within the mantle—the middle layer of our planet, which humans have never directly seen—far deeper than any oil well or diamond mine. The rock was shoved to the surface through an accident of plate tectonics around 80 million years ago, and now that it is exposed to the elements, it is undergoing a smoldering, flatulent geochemical decay.  とげのある低木がまばらに生える谷は,高さ数百mの薄茶色の岩の峰々に囲まれている。この岩石は地表では化学的に不安定な鉱物からなり,どの油井やダイヤモンド鉱山よりもはるかに深い地下数十kmのマントル(地殻と地核の間に存在する層で,直接見ることはできない)で形成されたのだろう。プレート運動によって約8000万年前に地表に押し出された岩石は風雨にさらされ,膨張を伴う地球化学的風化を起こしている。
Peter Kelemen thinks this geologic oddity might help humans change the course of the climate emergency. He introduced this vision to me one afternoon in January 2018 as we sat in camp chairs in Wadi Lawayni in the tattered shade of a scraggly acacia tree. A hundred meters away, under a canopy, was a makeshift outdoor laboratory with tables, chemicals and a specialized scanner for examining rock samples. Now 65, Kelemen is a geologist at the Lamont-Doherty Earth Observatory at Columbia University, with cropped gray hair and skin tanned by decades of working outside. Leathery dollops of camel dung were strewn in the gravel at our feet. Kelemen motioned to the wall of rock behind us, made of brownish, weathered mantle rock called peridotite. When rain percolates through cracks in the rock, it brings dissolved oxygen and carbon dioxide from the air. The water and gases react with the rock, forming solid veins of new minerals that dig, like tree roots, ever deeper into the stone. The rock was crisscrossed with these creamy-white veins. Kelemen pointed to one a centimeter across composed of magnesium carbonate. “That’s about 50 percent CO2,” he said. When I tapped it with a pebble, it emitted a glassy clang.  この地質学的に特異な岩石が気候の非常事態を変えるのに役立つ可能性があると,コロンビア大学ラモント・ドハティ地球観測研究所の地質学者ケレメン(Peter Kelemen)は考えている。ケレメンは現在65歳,白髪を短く刈り込み,長年のフィールド研究で日焼けしている。
Kelemen and his colleagues estimate that Oman’s exposed mantle rocks are absorbing and petrifying up to 100,000 metric tons of CO2 every year. That’s roughly one gram of the greenhouse gas per cubic meter of stone. “If you [enhance] that by a factor of a million”—something Kelemen thinks is doable with a bit of engineering—“then you end up with a billion tons of CO2 per cubic kilometer of rock per year,” he says.  ケレメンのチームはオマーンの露出したマントル岩石が毎年10万トンのCO2を吸収・固定していると推定している。岩石1m3あたり約1gの温室効果ガスを封じ込めている計算だ。「このプロセスを100万倍促進できれば(ちょっとした技術を用いれば実行可能だとケレメンは考えている),岩石1km3あたり年間10億トンのCO2を封印できる」と彼は言う。