Inference of GIA-based mantle viscosity from the last and Marinoan snowball deglaciations

• Project/Area Number: 16K05543
• Research Category: Grant-in-Aid for Scientific Research (C)
• Allocation Type: Multi-year Fund
• Section: 一般
• Research Field: Solid earth and planetary physics
• Research Institution: Kyushu University
• Principal Investigator: Nakada Masao 九州大学, 理学研究院, 教授 (50207817)
• Project Period (FY): 2016-04-01 – 2019-03-31
• Project Status: Completed (Fiscal Year 2018)
• Budget Amount: ¥2,470,000 (Direct Cost: ¥1,900,000、Indirect Cost: ¥570,000); Fiscal Year 2018: ¥780,000 (Direct Cost: ¥600,000、Indirect Cost: ¥180,000); Fiscal Year 2017: ¥780,000 (Direct Cost: ¥600,000、Indirect Cost: ¥180,000); Fiscal Year 2016: ¥910,000 (Direct Cost: ¥700,000、Indirect Cost: ¥210,000)
• Keywords: 海水準変動 / マントルレオロジー / 氷床変動 / snowball Earth / 粘性率の時間変化 / Snowball Earth / マントル粘性率 / 地球回転変動 / 全球凍結 / 粘性率 / 地球回転

Outline of Final Research Achievements

We examine the glacial isostatic adjustment (GIA) due to the last deglaciation and the Marinoan (~635 Ma) snowball Earth meltdown to infer the viscosity structure of Earth’s mantle. The viscosity structure inferred from the GIA data sets for the last deglaciation, relative sea level (RSL) changes and secular variations in zonal harmonics of Earth’s geopotential for degree 2~6, is as follows: ~10e22 Pa s at 670 km depth, smooth depth variation in the lower mantle viscosity and ~10e23 Pa s in the deep mantle. The RSL changes due to the Marinoan deglaciation suggest the upper part lower mantle viscosity of ~5x10e21 Pa s and ~5x10e22 Pa s in the deep mantle. The average temperature of the mantle at 0.6~0.7 Gyr may be ~50 K higher than at present, suggesting that the Neoproterozoic mantle viscosity is approximately half of the present one. This is consistent with the viscosity structures inferred from the GIA-based data sets for the last deglaciation and Marinoan deglaciation.

Academic Significance and Societal Importance of the Research Achievements

地球マントルの粘性率構造は、地球内部ダイナミクスの研究において最も重要な要素の一つである。粘性率は強い温度依存性を持ち、地球進化に伴い地球内部温度が減少すると粘性率は時間と共に増加する。つまり、第四紀とマリノアン期の氷床変動・固体地球変動より推定された粘性率構造は、地球熱史研究にも強い制約を与える。岩石学的に推定されたマリノアン期のマントル平均温度は現在に比べほぼ50K高い。このことは、当時の粘性率は現在に比べほぼ半分であることを示唆し、本研究成果と調和的である。つまり、本研究成果はマントルダイナミクス・地球進化の研究に大きな制約を与え、学術的・社会的に重要な意義を持つと考えられる。

Research Products

• [Journal Article] Inference of viscosity jump at 670 km depth and lower mantle viscosity structure from GIA observations (2017)
  • Author(s): Nakada Masao、Okuno Jun’ichi、Irie Yoshiya
  • Journal Title: Geophysical Journal International Volume: 212 Issue: 3 Pages: 2206-2225
  • DOI: 10.1093/gji/ggx519
  • Peer Reviewed / Open Access / Int'l Joint Research
• [Journal Article] Secular variations in zonal harmonics of Earth’s geopotential and their implications for mantle viscosity and Antarctic melting history due to the last deglaciation (2017)
  • Author(s): Nakada, M., Okuno, J.
  • Journal Title: Geophysical Journal International Volume: 209 Issue: 3 Pages: 1660-1676
  • DOI: 10.1093/gji/ggx116
  • Peer Reviewed / Int'l Joint Research
• [Journal Article] Total meltwater volume since the Last Glacial Maximum and viscosity structure of Earth’s mantle inferred from relative sea level changes at Barbados and Bonaparte Gulf and GIA-induce J2-dot (2016)
  • Author(s): Masao Nakada, Jun’ichi Okuno, Yusuke Yokoyama
  • Journal Title: Geophysical Journal International Volume: 204 Issue: 2 Pages: 1237-1253
  • DOI: 10.1093/gji/ggv520
  • Peer Reviewed / Acknowledgement Compliant
• [Journal Article] Inference of mantle viscosity for depth resolutions of GIA observations (2016)
  • Author(s): Nakada, M., Okuno, J.
  • Journal Title: Geophysical Journal International Volume: 207 Issue: 2 Pages: 719-740
  • DOI: 10.1093/gji/ggw301
  • Peer Reviewed
• [Presentation] Viscosity structure of Earth’s mantle inferred from glacial isostatic adjustment (2018)
  • Author(s): Nakada, M., Okuno, J., Yokohama, Y., Irie, Y., Lambeck, K. and Purcell, A.
  • Organizer: 日本地球惑星科学連合 2018年大会
• [Presentation] Sea level change due to Marinoan snowball deglaciation (2018)
  • Author(s): Irie, Y., Nakada, M., Okuno, J. and Bao, H.
  • Organizer: 日本地球惑星科学連合 2018年大会
• [Presentation] Predicting relative sea level changes at the aftermath of Marinoan (635) Snowball meltdown (2016)
  • Author(s): Irie, Y., Nakada M., Okuno, J. and Bao, H.
  • Organizer: American Geophysical Union, Fall Meeting
  • Place of Presentation: サンフランシスコ
  • Year and Date: 2016-12-12
  • Int'l Joint Research