Neogene aridification and lake development in the Issyk‐Kul basin, Kyrgyzstan

Abstract

Uplift of the Tian Shan range modified regional climate during Cenozoic aridification in Central Asia. This study presents facies analyses and Neogene oxygen and carbon isotopic records from magnetostratigraphically dated terrestrial sedimentary sections on the southern side of the intermontane Issyk-Kul basin in the Kyrgyz Tian Shan and 26 Al/ 10 Be isochron burial ages from the southern and eastern sides of the basin. The δ 18 O and δ 13 C data show a positive ca. 2‰ shift in values between ca. 8 and 7 Ma and a change from a negative to a positive trend. This change is attributed to the upwind growth of the Kyrgyz, Kungey and Trans Ili (Zaili) ranges, which diverted the westerlies, thereby changing the Issyk-Kul basin from a windward to a leeward position, enhancing aridification and establishing the modern-day spring and summer precipitation regime within the basin. Two 4 to 5 Ma 26 Al/ 10 Be isochron burial ages constrain the onset of Sharpyl Dak deposition on the eastern side of the basin; southward paleocurrent directions there suggest the eastward growth of the Kungey range in the Pliocene. Increased subsidence on the southern side of the basin and local tectonically 1194 | EAGE KUDRIAVTSEVA et al.

Impact of Quaternary Glaciations on Denudation Rates in North Pamir—Tian Shan Inferred From Cosmogenic 10Be and Low‐Temperature Thermochronology

“…The three samples used to determine our 10 Be‐derived paleo‐denudation rates were collected from rare and thin fine‐grained lenses in conglomerates from the Sharpyl Dak sedimentary group, which was deposited above finer‐grained sediments and is analogous to the Xiyu formation conglomerates in the Chinese Tian Shan (Abdrakhmatov et al., 2001; J. Chen et al., 2002; Heermance et al., 2007). The deposition of the Sharpyl Dak and Xiyu conglomerates is diachronous across the Tian Shan, varying from mid‐Miocene to Pleistocene (Charreau et al., 2009; J. Chen et al., 2002; Heermance et al., 2007; Kudriavtseva et al., 2023), and is considered to mark either the intensification of tectonic activity or the onset of Plio‐Pleistocene glaciation in the region (Zhao et al., 2021). Samples AKT‐U (2.71 ± 0.37 Ma) and PET‐QTS‐PIT (2.75 ± 0.15 Ma) were taken from the basal parts of the conglomerates, whereas sample PET‐QTS‐L (2.08 ± 0.14 Ma) was collected from the youngest outcropping Sharpyl Dak conglomerates (Kudriavtseva et al., 2023).…”

“…Paleo‐denudation rates were calculated using three buried amalgamated sand samples (250–500 μm grain size) from south of lake Issyk‐Kul (Figure 1a), taken from the same stratigraphic layers as the individual clasts collected for isochron burial dating by Kudriavtseva et al. (2023). The 26 Al and 10 Be concentrations measured in the sand samples plot near (sample AKT) or intersect (samples PET‐QTS) the lines defined by the isochron clast samples (Figure S2 in Supporting Information S1), indicating that the sands and clasts have experienced similar exposure and burial histories.…”

“…The 26 Al and 10 Be concentrations measured in the sand samples plot near (sample AKT) or intersect (samples PET‐QTS) the lines defined by the isochron clast samples (Figure S2 in Supporting Information S1), indicating that the sands and clasts have experienced similar exposure and burial histories. We calculate paleo‐denudation rates by first correcting the 10 Be concentrations measured in the sand samples for radioactive decay using the isochron burial ages calculated for each location (see Table S2 in Supporting Information S2 and Kudriavtseva et al., 2023). To account for uncertainties related to the average elevation of the sediment's source areas and to the possibility of incomplete burial or lengthy exposure to cosmic radiation prior to sampling, we express paleo‐denudation rates for each of the three localities as a range of values in addition to a central value obtained by assuming complete and continuous burial (Table S2 in Supporting Information S2).…”

“…At two sampling locations we also collected material in the 8-16 mm (samples KYR16-01B and KYR16-02B) and 16-32 mm (sample KYR16-01C) grain size ranges. Sampling of buried sediments for calculation of 10 Be-derived paleo-denudation rates and calculation of ages using isochron burial dating (Figure 1a, Table S2 in Supporting Information S2) are described in Kudriavtseva et al (2023). Our sample inventory includes one modern river sediment sample (KYR-DRMS1) previously reported by Landgraf et al (2016).…”

“…The Cenozoic climate in Central Asia has been arid and mainly influenced by the northern mid‐latitude westerlies, which transport moisture eastward across Eurasia (e.g., Caves et al., 2015, 2016). Aridification was likely caused by a decrease in the westerlies' moisture flux, which resulted from Cenozoic global cooling and Parathethys retreat (Miao et al., 2012; Ramstein et al., 1997), as well as the creation of orographic barriers due to surface uplift of the Tibetan Plateau, the Tian Shan, and the Pamir mountains (Caves Rugenstein & Chamberlain, 2018; Kent‐Corson et al., 2009; Kudriavtseva et al., 2023; X. Wang et al., 2020). The Tian Shan has interacted with the westerlies since the late Oligocene (X. Wang et al., 2020).…”