Long‐Period Astronomical Forcing of Westerlies' Strength in Central Asia During Miocene Climate Cooling

Konstantin Frisch,Erwin Appel,Yuki Weber,Jacek Raddatz,Sietske J Batenburg,Richard Albert,Thomas Voigt,Silke Voigt,Iris Arndt,Verena Verestek,Axel Gerdes

doi:10.1029/2019pa003642

Abstract

AbstractThe continental expression of global cooling during the Miocene Climate Transition in Central Asia is poorly documented, as the tectonically active setting complicates the correlation of Neogene regional and global climatic developments. This study presents new geochemical data (CaSO4 content, carbonate δ13C and δ18O) from the endorheic alluvial‐lacustrine Aktau succession (Ili Basin, south‐east Kazakhstan) combined with findings from the previously published facies evolution. Time series analysis revealed long‐eccentricity forcing of the paleohydrology throughout the entire succession, split into several facies‐dependent segments. Orbital tuning, constrained by new laser ablation U‐Pb dates and a preexisting magnetostratigraphy, places the succession in a 5.0 Ma long interval in the middle to late Miocene (15.6 to 10.6 Ma). The long‐term water accumulation in the Ili Basin followed the timing of the Miocene Climate Transition, suggesting increased precipitation in the catchment area in response to climate cooling and stronger westerly winds. This was paced by minima of the 2.4 Ma eccentricity cycle, which favored the establishment of a discharge playa (~14.3 Ma) and a perennial lake (12.6 to 11.8 Ma). Furthermore, low obliquity amplitudes (nodes) caused a transient weakening of the westerlies at ~13.7 to 13.5 Ma and at ~12.7 Ma, resulting in negative hydrological budgets and salinization. Flooding of the windward Ili Basin coeval with aridification in the leeward basins suggests that the Tian Shan was a climate boundary already in the middle Miocene. Our results emphasize the impact of climate fluctuations on the westerlies' strength and thus on Central Asian hydrology.

Highlights

The global Miocene climate evolution is characterised by a period of pronounced cooling during the Miocene Climate Transition (MCT), which terminated the exceptional warmth of the Miocene Climatic Optimum (MCO) and marked the beginning of a sustained climate deterioration, culminating in the modern ice-house mode (e.g. Zachos et al, 2001)
This study presents the effects of orbital forcing and global cooling on the hydrological evolution of the alluvial-lacustrine Ili Basin in south-east Kazakhstan, during a period of steady basin subsidence in the Middle to Late Miocene
Results of this study show a strong impact on the hydrology of the closed Ili Basin by the long eccentricity cycle, which was recorded in the sedimentary archive by eccentricity modulated precession

Summary

Introduction

The global Miocene climate evolution is characterised by a period of pronounced cooling during the Miocene Climate Transition (MCT), which terminated the exceptional warmth of the Miocene Climatic Optimum (MCO) and marked the beginning of a sustained climate deterioration, culminating in the modern ice-house mode (e.g. Zachos et al, 2001). The MCT is well documented in marine sediments by a series of positive shifts in the δ18O of benthic foraminifera (Mi3a, Mi3b and Mi4), which correspond to glaciation events on Antarctica (Miller et al, 1991, 1996; Flower & Kennett, 1994; Westerhold et al, 2005; Holbourn et al, 2013) This Middle Miocene cooling has been recognized in numerous continental climate archives from the Northern Hemisphere and likely contributed to the step-wise process of the Neogene mid-latitude aridification in North America and Eurasia (Flower & Kennett, 1994; Eronen et al, 2012; Miao et al, 2012; Pound et al, 2012; Frisch et al, 2019). This led to a differentiation into locally diverging precipitation trends (Miao et al, 2012; Caves Rugenstein & Chamberlain, 2018)

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