地球表層炭素循環モデリングに基づく暁新世/始新世境界温暖化極大の発生原因の再検討  [in Japanese] Re-examination of the cause of the Paleocene-Eocene thermal maximum, based on global carbon-cycle modeling  [in Japanese]

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Abstract

約55 Maに生じた暁新世/始新世境界温暖化極大(PETM)では,炭素同位体比負異常(CIE)を伴う4℃以上の全球的温暖化が生じた.これは<sup>12</sup>Cに富む温室効果ガスの放出を示唆しているが,その起源は現在も特定されていない.本稿ではこの問題を,シンプルな炭素循環モデリングにより再検討した.PETMのトリガーとしては,同時期に北大西洋で起こった大規模火成活動が最も妥当と考えられる.計算の結果,この火成活動により生成した2,200 Gt-Cの熱分解起源メタンと,海底メタンハイドレートの分解に伴う700-2,800 Gt-Cの微生物起源メタンの放出により,-3‰の全球的なCIEを説明しうることがわかった.必要な微生物起源メタン量の不確実性は暁新世末期の大気中CO<sub>2</sub>濃度の見積もり幅に由来する.一方,計算された温度上昇幅は最大で1.4℃に留まり,地質記録から推定される温暖化を再現することはできなかった.

One of the most dramatic global warming events in the Earth’s history occurred at the Paleocene-Eocene boundary (ca. 55 Ma). During the Paleocene-Eocene thermal maximum (PETM), the global temperature increased by more than 4°C within a few thousand years, accompanied by an abrupt negative carbon isotope excursion (CIE) in both the marine and terrestrial environments. Although this excursion implies a massive and rapid addition of <sup>13</sup>C-depleted carbon to the oceans and atmosphere, the source of the massive carbon injection during the PETM remains uncertain.<br>To provide some constraints on the cause of the PETM, we re-examined the observed magnitude of the CIE, and then reconstructed the perturbation of the global carbon cycle during the PETM, using a simple one-box global carbon-cycle model.<br>The eruption of the North Atlantic Igneous Province (NAIP) appears to be the most plausible candidate for a triggering mechanism of the PETM. Our model indicates that the CIE of –3‰ is best explained by inputs of 2,200 Gt-C of thermogenic methane, produced by NAIP volcanism, and 700–2,800 Gt-C of biogenic methane produced by the subsequent dissociation of seafloor gas hydrates within 10 kyr. The uncertainty in the mass of released biogenic methane is attributed mainly to high variability in atmospheric <i>p</i>CO<sub>2</sub> reconstructed for the late Paleocene (ranging from <300 to >2,000 ppm). However, the global temperature rise calculated by our model is 1.4°C at most, and we cannot reconstruct a temperature anomaly exceeding 4°C.

Journal

  • The Journal of the Geological Society of Japan

    The Journal of the Geological Society of Japan 116(8), 418-436, 2010

    The Geological Society of Japan

Codes

  • NII Article ID (NAID)
    130000413567
  • NII NACSIS-CAT ID (NCID)
    AN00141768
  • Text Lang
    JPN
  • ISSN
    0016-7630
  • NDL Article ID
    10814285
  • NDL Source Classification
    ZM49(科学技術--地球科学--地質)
  • NDL Call No.
    Z15-174
  • Data Source
    NDL  J-STAGE 
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