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Tectonic controls on greenhouse gas flux to the Paleogene atmosphere from the Gulf of Alaska accretionary prism

¹ Applied Geology, 1432 Fox Hollow Road, Sequim, Washington 98382, USA
² U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA

The late Paleocene to early Eocene (ca. 61–56 Ma) was a period of long-term global warming, perhaps the warmest in the Cenozoic. Recent modeling suggests that methane loading of the atmosphere, and related development of polar stratospheric clouds, could have been an important forcing mechanism for this period of warm climate. The Gulf of Alaska accretionary prism contained 6 x 10⁶ km³ of siliciclastic sediments deposited in trench and slope settings along Alaska's Maastrichtian and Paleogene continental margin. These sediments underwent complex deformation, accretion, and unusual high heat flow soon after deposition. Accretion processes thermally overmatured the sediments during a time that overlaps the 61–56 Ma period of long-term global warming. Assuming a modest average organic carbon content of 0.3 wt% in these sediments, an estimated 8.35 x 10¹⁵ kg of methane were generated in the accretionary prism over an 5 m.y. period. This methane was not effectively trapped, and migration pathways to the atmosphere were developed through complexly deformed and emergent continental borderlands. The Gulf of Alaska accretionary prism is a possible source of the atmospheric methane needed to force Paleocene and early Eocene global warming and an example of how tectonic processes can significantly recycle carbon from the geosphere.

Key Words: methane • Gulf of Alaska • Paleogene • global warming • metamorphism • greenhouse gas

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