Revisiting the Paleogene Climate Pattern of East Asia: A Synthetic Review

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East Asian Paleogene climates have long been regarded as controlled by the planetary wind system, which might result in a climate pattern with three latitudinally distributed zones. Two humid zones located separately in the north and south were lithologically designated by coals and oil shales, while an arid zone in the middle was represented by red beds and evaporites. Because the middle arid zone was located along ~. 30° N paleolatitude, its presence had been further linked with a then subtropical high. However, this long-standing model has recently been challenged by growing evidence from petrology, sedimentology, paleontology, paleobiogeography, paleoclimatology, and climate modeling. Here we review the primary data from these disciplines and reinterpret their climate significances to revisit the East Asian climate pattern during the Paleogene. Petrologically, while the occurrence of coals and/or oil shales is accepted as an indicator for overall humid climates, that of red beds and/or evaporites is highly equivocal to exclusively indicate perennial arid climates unless their origins are carefully investigated. In reality, generic red beds merely represent an oxidizing environment, not essentially associated with a single specific climate type. Meanwhile evaporites, although typically precipitated in arid environments, may be deposited in either perennial dry or seasonal/monsoonal climates. There is no solid evidence so far to convincingly support that the landscape of the so-called middle arid zone was dominated by desert and/or steppe under a then subtropical high during most of the Paleogene. The plant function type study additionally suggests that the "middle arid zone" appears to be lack of xerophytic vegetation, even though some xerophytic or sclerophyllous plant taxa did sporadically occur. Interestingly, paleozoological data show that the Paleogene mammalian faunas were somewhat equably distributed over East Asia, strongly suggesting the evident absence of a critical biogeographical or climatic barrier stretched across the "middle arid zone" as the planetary wind model implied. In contrast to the planetary wind model, monsoonal or monsoon-like Paleogene climates have been broadly reported from the northern, middle, and southern East Asia, as well as adjacent regions of Russia and Kazakhstan. If only the indicators for humid climates are considered, simply due to the uncertainty of those for perennial arid climates, East Asia must have had a relatively dry region in the continental interior during the late Eocene to Oligocene transition, likely caused by the continentality and/or the rain shadow effect along with the global cooling. The monsoonal interpretation is highly in agreement with the evidence from floras, faunas, basin analyses, and modeling experiments, and well explicates the Paleogene climate distribution and seasonal dynamics of East Asia. However, further studies will be largely needed to verify whether, uniformly according to the modern criteria, the Paleogene climates of the East Asia interior can be accurately attributed to the arid category.