[M-IS19] Paleoclimatology and paleoceanography

[MIS19-21] Os isotope record of the Quaternary seawater derived from pelagic sediment in the Lau Basin, western South Pacific Ocean, and its implications for global climate changes

*Yusuke Kuwahara¹, Kazutaka Yasukawa^1,2, Koichiro Fujinaga^2,3, Tatsuo Nozaki^4,3,5,2, Junichiro Ohta^2,6,3, Honami Sato^2,4, Jun-Ichi Kimura⁶, Kentaro Nakamura¹, Yasuhiro Kato^3,1,2 (1.Department of Systems Innovation, School of Engineering, The University of Tokyo, 2.Ocean Resources Research Center for Next Generation, Chiba Institute of Technology, 3.Frontier Research Center for Energy and Resources, School of Engineering, The University of Tokyo, 4.Research and Development Center for Submarine Resources, Japan Agency for Marine-Earth Science and Technology , 5.Department of Planetology, Graduate School of Science, Kobe University, 6.Department of Solid Earth Geochemistry, Japan Agency for Marine-Earth Science and Technology)

Keywords:Glacial−interglacial cycle, Global warming, Osmium isotopic ratio, Carbon cycle, Hydrothermal sediment

Silicate weathering on land has been considered to play an important role in controlling global climate change through the consumption of atmospheric CO₂ on long timescales (>10⁶ yr). This process has been working as a negative feedback mechanism, which has prevented the Earth system from runaway warming or cooling. However, whether this system works on even shorter-term global climate changes on the timescale of 10⁴−10⁵ yr such as Quaternary glacial-interglacial cycles is still controversial. In order to constrain the response time of the solid earth to global climate change, we employed the marine Os isotopic ratios (¹⁸⁷Os/¹⁸⁸Os) as a proxy of continental weathering. The marine Os isotopic ratio reflects the relative intensity of two dominant influxes to the ocean; radiogenic continental-derived materials (¹⁸⁷Os/¹⁸⁸Os = 1.0−1.4) and unradiogenic mantle-like materials (i.e. hydrothermal fluids and cosmic dust) (¹⁸⁷Os/¹⁸⁸Os = ~0.12). Owing to the contrasting ¹⁸⁷Os/¹⁸⁸Os values between these two influxes, and short residence time of Os in the ocean (~10⁴ yr), the variability in ¹⁸⁷Os/¹⁸⁸Os of seawater constitutes a sensitive tracer for mantle and continental input into the marine environment. By assuming that the fluxes of mantle-derived materials into the ocean were constant during the Quaternary, we can obtain the fluctuation in the intensity of continental silicate weathering on glacial-interglacial timescales from the marine Os isotope record. Here, we report the marine Os isotopic variations during the glacial-interglacial cycles derived from the pelagic sediment in the Lau Basin, western South Pacific Ocean. In the present work, we targeted metalliferous carbonate as the most suitable media preserving marine Os isotope records at the timing of deposition. This sediment is enriched in hydrothermal Fe-oxyhydroxides that absorb Os in seawater efficiently and deposits rapidly without significant contamination of detrital materials from continents. As the result, we observed a clear fluctuation of the marine ¹⁸⁷Os/¹⁸⁸Os value along with the glacial−interglacial cycles. Based on the marine ¹⁸⁷Os/¹⁸⁸Os record, we also present the fluctuation of riverine Os flux calculated by simple 1-box mass-balance modeling, and its implications for the Quaternary climate change.