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

Abstract

The 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 (CaSO 4 content, carbonate δ 13 C and δ 18 O) from the endorheic alluvial-lacustrine Aktau succession (Ili Basin, southeast 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. Plain Language Summary The global climate changed from an exceptional warm to a colder state in the middle Miocene epoch, representing a milestone in the evolution of today's climate. This study focuses on the, so far fragmentary, understanding of the Central Asian climate response to this global climate transition by investigating deposits of a former (salt) lake in the Ili Basin, southeast Kazakhstan. Regular sediment alternations represent cycles of low and high water level, overprinted by a long-term lake expansion. Time series analysis of climate sensitive geochemical and environmental parameters, together with the determination of absolute rock ages, enabled the identification of sedimentary cycles (405 ka and 1.2 Ma long), which are equivalent to climate influencing variations of the Earth's orbit and tilt angle. We conclude that water level maxima are linked to periods of low seasonal climate differences reoccurring every 405 ka. The lake expansion is caused by more precipitation due to strengthened westerly winds, in response to global cooling. Westerly winds were transiently weakened during periods of low variability of the Earth's tilt angle, promoting high evaporation and salinization. Our results emphasize the impact of climate change on the westerlies' strength and thus on Central Asian moisture supply.

Depositional age models in lacustrine systems from zircon and carbonate U‐Pb geochronology

“…Although the interplay between these factors remains unclear, carbonates precipitated from lacustrine water, meteoric water and groundwater are particularly amenable to dating (e.g. Rasbury & Cole, 2009; Drost et al ., 2018; Liivamägi et al ., 2018; Frisch et al ., 2019; Parrish et al ., 2019; Woodhead & Petrus, 2019; Kurumada et al ., 2020; Nicholson et al ., 2020; Roberts et al ., 2020a, 2021; Brigaud et al ., 2021; Hoareau et al ., 2021; Montano et al ., 2021; Nuriel et al ., 2021; Rasbury et al ., 2021).…”

“…In spite of this, LAcarb dating may provide ages consistent, within uncertainties, with biostratigraphic and radiometric constraints. This has been shown for meteoric cements in speleothems (Hopley et al ., 2019; Woodhead & Petrus, 2019; Nicholson et al ., 2020) and hardgrounds (Liivamägi et al ., 2018; Scardia et al ., 2019; Kurumada et al ., 2020; Brigaud et al ., 2021; Środoń et al ., 2022), lacustrine tufa, micrites and associated cements (Drost et al ., 2018; Frisch et al ., 2019; Parrish et al ., 2019; Hoareau et al ., 2021; Montano et al ., 2021; Rasbury et al ., 2021), marine cements (Meinhold et al ., 2020; Hoareau et al ., 2021) and bioclasts (Drost et al ., 2018). Despite these remarkable results, no study has attempted to define, solely based on LAcarb, a robust depositional age depth model along an entire stratigraphic section, which was also dated by zircon geochronology.…”

“…However, absolute radiometric constraints are often absent in most continental settings. Therefore, in such settings, a complex multi‐proxy analysis based on sedimentology, stratigraphy, magnetostratigraphy and diagenesis is usually needed to reconstruct the depositional history and the sedimentation rate evolution over time (Schumer & Jerolmack, 2009; Cartier et al ., 2018; Frisch et al ., 2019).…”

Drivers for Asynchronous Patterns of Dust Accumulation in Central and Eastern Asia and in Greenland During the Last Glacial Maximum

“…During times of low obliquity, relatively more radiation reaches the midlatitudes and relatively less reaches the poles, creating a steeper meridional insolation gradient (Cruz et al., 2006; Loutre et al., 2004). Lower obliquity during the early LGM than during the late LGM means that there was a higher meridional insolation gradient between northern mid and high latitudes (Berger et al., 2006), which would enhance the westerlies in the northern hemisphere (Frisch et al., 2019). A strong obliquity signal has been revealed in geological proxy to reflect the intensity of northern westerlies over the last 180 ka (von Suchodoletz et al., 2010).…”

Middle Miocene Intensification of South Asian Monsoonal Rainfall

“…This led to a progressive global reorganization of the atmospheric circulation as a result of increasing latitudinal thermal gradients during the MMCT. An increasing SAM during the middle Miocene may also explain a stronger westerly jet stream over Central Asia during and after the MMCT (Frisch et al, 2019). Modern observations show that a stronger than average summer SAM strengthens the westerly jet stream north of the Tibetan Plateau (Schiemann et al, 2009).…”