ISSN 2330-717X

Can East Asian Monsoon Enhancement Induce Global Cooling?

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The study of the orogenic effects of the Tibetan Plateau uplift on global climate during the Cenozoic has focused almost exclusively on the India-Asia collision zone, the Himalayas. The strong erosion in the Himalayas was assumed to be a primary driver of Cenozoic atmospheric CO2 decline and global cooling predominantly through accelerating silicate chemical weathering in the India-Asia collision zone or through effective burial of organic carbon in the nearby Bengal Fan in South Asia. 

 However, the size of the India-Asia collision and the associated closure of the Tethys Ocean had a prominent effect on the reorganization of the climatic patterns beyond the collision zone. In an article coauthored with Yibo Yang and Albert Galy at Institute of Tibetan Plateau Research, Chinese Academy of Sciences and Centre de Recherches Pétrographiques et Géochimiques, CNRS-Université de Lorraine, and other ten colleagues, these researchers stated: “the Oligocene-Miocene boundary Asian climatic reorganization linked to the northward migration of the East Asian monsoon into subtropical China is a potentially important but poorly constrained atmospheric CO2consumption process.”

These twelve scholars performed a first-order estimation of the difference in CO2 consumption induced by silicate weathering and organic carbon burial in subtropical China related to the monsoon advance around the late Oligocene. They revealed in the study, which was published in the Science China Earth Sciences, that the northward advance of the East Asian monsoon on tectonically inactive subtropical China induced globally significant silicate weathering atmospheric CO2 sink. That is, an increase in long-term CO2consumption by silicate weathering varies from 0.06 to 0.87×1012 mol·yr-1 depending on erosion flux reconstructions, with an ~50% contribution of Mg-silicate weathering since the late Oligocene. The organic carbon burial flux is approximately 25% of the contemporary CO2 consumption by silicate weathering.

The first-order calculation of CO2 consumption highlighted the very significant role of the weathering of the Mg-rich Yangtze craton and surrounding terranes because the unusual Mg-rich nature of eroded crust not only enhances the tectonic forcing of climate but also can contribute to the rise in the Mg content of the ocean during the Neogene. 

The study provided a novel perspective on the Cenozoic carbon cycle linked to the Mg-rich nature of the crust affected by such uplift-driven climatic change and illustrated how complex can be the perturbations of global climate and atmospheric CO2 levels by orogenic uplift, and how important is the nature of the crust not only involved in the collision but also around the collision. In past decades, the role of the heterogeneity of the crust and/or the lithosphere has been highlighted in other geosciences disciplines for a long time, and the distinction between mantle-derived and upper crustal rocks was already well integrated in the long-term climate science community.

“But to our knowledge”, wrote the twelve researchers, “the key findings of this study (the importance of the composition of the crust, and the spatial extent of the perturbations of global climate and atmospheric CO2 levels by orogenic uplift) suggests that the tectonics affects Cenozoic cooling via modulation of the geological carbon cycle in diverse ways, and such forcing might not be fully extrapolated to older global-scale orogeny.”

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