[U-02] 人新世・第四紀の気候および水循環

[U02-10] Asian monsoon response to meltwater events in the North Atlantic

★Invited Papers

*Jasper Alexander Wassenburg^1,2、Hubert B. Vonhof³、Hai Cheng^4,5,6、Alfredo Martinez-Garcia³、Pia-Rebecca Ebner³、Xianglei Li⁷、Haiwei Zhang⁴、Lijuan Sha⁴、Ye Tian⁴、R. Lawrence Edwards⁷、Jens Fiebig⁸、Gerald Haug^3,9 (1.Center for Climate Physics, Institute for Basic Science、2.Pusan National University、3.Climate Geochemistry Department, Max Planck Institute for Chemistry、4.Institute of Global Environmental Change, Xi'an Jiaotong University、5.State Key Laboatory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences、6.Key Laboratory of Karst Dynamics, MLR, Institute of Karst Geology, CAGS、7.Department of Earth and Environmental Sciences, University of Minnesota、8.Institute of Geosciences, Goethe University、9.Department of Earth Sciences, ETH Zurich)

キーワード：Asian monsoon, Ice Age Termination, TII, Heinrich event, speleothem, Greenland

Anthropogenic warming is amplified in the Arctic region causing the Greenland ice sheet to melt, which is increasing freshwater discharge into the North Atlantic Ocean. The melt water may weaken the Atlantic meridional overturning circulation (AMOC), ultimately reducing the oceanic northward heat transport and potentially affecting atmospheric circulation patterns. Observations of independent AMOC indices indeed show evidence it has already weakened over the last 100 years¹. To predict the effects of Greenland melt water discharge on the Earth’s future climate we mostly rely on water hosing experiments with climate models performed under different greenhouse gas emission scenarios². However, looking back at past changes in AMOC strength and proxy-based paleo-climate reconstructions can be extremely informative. During the last 150,000 years many melt water events in the North Atlantic have been recorded by the presence of ice rafted debris in North Atlantic marine sediments. Ice rafted debris was deposited during so called Heinrich events, which were associated with surges of massive ice bergs.

Asian monsoon speleothems (secondary cave carbonates) have shown to register these events in their CaCO₃oxygen isotope compositions (d¹⁸O_Cc). It is commonly assumed that d¹⁸O_Cc reflects the oxygen isotope variability of the rainfall above the cave site (d¹⁸O_rain). However, the d¹⁸O_Cc is affected by multiple parameters including cave air temperature, rainfall or parent water d¹⁸O, as well as potential kinetic isotope effects. Here we present a unique speleothem record from southwest China³. The combination of three novel techniques allows to quantify cave air temperature variations, reconstruct summer monsoon d¹⁸O_rain and completely deconvolve the d¹⁸O_Cc for the period from 150,000 years before present (BP) to 125,000 years BP. This interval includes multiple large melt water events and ice age termination II. We show that Asian summer monsoon intensity was only affected by the largest melt water pulse, annual mean temperature responded sensitively to each ice rafted debris event and our speleothem d¹⁸O_Cc is best explained by changes in the balance between winter and summer monsoon rainfall. Although it is unclear if the entire glacial – interglacial temperature amplitude was recorded, we observe a rapid increase in temperature of a minimum of 4-5°C.

1 Boers, N. Observation-based early-warning signals for a collapse of the Atlantic Meridional Overturning Circulation. Nat. Clim. Chang. 11, 680-688 (2021). https://doi.org:10.1038/s41558-021-01097-4
2 Chemison, A., Defrance, D., Ramstein, G. & Caminade, C. Impact of an acceleration of ice sheet melting on monsoon systems. Earth Syst. Dynam. 13, 1259-1287 (2022). https://doi.org:10.5194/esd-13-1259-2022
3 Wassenburg, J. A. et al. Penultimate deglaciation Asian monsoon response to North Atlantic circulation collapse. Nat. Geosci., 937-941 (2021). https://doi.org:10.1038/s41561-021-00851-9