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Palaeo-environmental evolution of Central Asia during the Cenozoic: New insights from 1 the continental sedimentary archive of the Valley of Lakes (Mongolia)

27 The Valley of Lakes basin (Mongolia) contains a unique continental sedimentary archive, 28 suitable for constraining the influence of tectonics and climate change on the aridification of 29 Central Asia in the Cenozoic. We identify the sedimentary provenance, the (post)depositional 30 environment and the palaeo-climate based on sedimentological, petrographical, mineralogical 31 and (isotope) geochemical signatures recorded in authigenic and detrital silicates as well as soil 32 carbonates in a sedimentary succession spanning ~34 to 21 Ma. The depositional setting was 33 characterized by an ephemeral braided river system draining prograding alluvial fans, with 34 episodes of lake, playa or open steppe sedimentation. Metamorphics from the northern adjacent 35 Neoarchean to late Proterozoic hinterlands provided a continuous influx of silicate detritus to 36 the basin, as indicated by K-Ar ages of detrital muscovite (~798-728 Ma) and discrimination 37 function analysis. The authigenic clay fraction is dominated by illite-smectite and “hairy” illite 38 (K-Ar ages: ~34-25 Ma), which formed during coupled petrogenesis and precipitation from 39 hydrothermal fluids originating from major basalt flow events (~32-29 Ma and ~29-25 Ma). 40 Changes in hydroclimate are recorded in δ 18 O and δ 13 C profiles of soil carbonates and in silicate 41 mineral weathering patterns, indicating comparatively humid to semi-arid conditions prevailed 42 in the late(st) Eocene, changing into arid conditions in the Oligocene and back to humid to 43 semi-arid conditions in the early Miocene. Aridification steps are indicated at ~34-33 Ma, ~31 44 Ma, ~28 Ma and ~23 Ma and coincide with some episodes of high-latitude ice sheet expansion 45 inferred from marine deep-sea sedimentary records. This suggests long-term variations of the 46 ocean/atmosphere circulation patterns due to p CO 2 fall, re-configurations of ocean gateways 47 and ice-sheet expansion in Antarctica could have impacted the hydroclimate and weathering 48 regime in the basin. We conclude that the aridification in Central Asia was triggered by reduced moisture influx by


52
The Cenozoic Era (66 Ma to the present day) saw several dramatic changes of the marine and 53 continental ecosystems (e.g., evolution of large plankton feeders such as baleen whales, shift 54 towards cold-water, high nutrient plankton assemblages at high latitude, expansion of terrestrial 55 mammals) major tectonic events (e.g., opening of Southern Hemisphere Oceanic gateways, 56 shift to the 4-layer structure of the modern ocean, collision of the African-Arabian-Eurasian 57 plates, uplift of the Alpine and Himalayan mountain belt) and global climate forcing (e.g., 58 change from greenhouse to icehouse conditions) (Cerling, 1997   Samples with a high clay mineral content based on an initial mineralogical inspection were 168 selected further for an identification of the clay mineral suite, which is defined here as < 2 µm 169 size fraction (Rafiei et al., 2020). As for the clay mineral separation, 5 g of the bulk material 170 was reacted with 5 % HCl for 10 min to remove the carbonates, followed by standard Atterberg 171 sedimentation and subsequent collection and drying of the < 2 µm size fraction at 40 °C. Fast The major, minor and trace element composition of a sub-set of samples (91 in total) was 180 analyzed via a Philips PW2404 wavelength dispersive X-ray fluorescence (XRF) spectrometer.

181
Fine powdered samples (0.8 g) were heated to 1050 °C to remove the volatile components 182 (CO2, H2O, etc.), following determination of the loss on ignition (LOI) by gravimetric analysis.

183
The residuals were fused at 1200 °C using LiBO2 (4 g) as the fluent agent. The standard glass 184 tablets were analyzed together with a set of USGS standards (analytical error: ± 0.5 wt% for below). The weathering conditions of the source areas were identified further using Al2O3 -

201
The mineralogical composition of all bulk samples was determined by Rietveld-based analysis 202 of X-ray diffraction (XRD) patterns recorded on a PANalytical X'Pert PRO diffractometer 203 (Co-Kα; 40 kV and 40 mA) equipped with a high-speed Scientific X'Celerator detector. The   The δ 13 C and δ 18 O isotopic composition of the carbonate fraction was analyzed to constrain the 248 palaeo-climatic trends recorded in the paleosols. In a previous study (Richoz et al., 2017) it 249 was shown that the soil carbonates (calcrete nodules, lenses and crusts) mostly record pristine   greyish-black color. The basalt I and basalt II horizons are exposed at ~40-41 m and at ~94-278 100 m and interfinger with the sediments from the Hsanda Gol Formation (Fig. 2b,d).

317
Calichized areas are cemented by calcite spar, which appears as crypto-to microcrystalline 318 material with aggregate particle sizes in the micrometer to millimeter range (Fig. S3d). All 319 these components are covered or intergrown by fine hematite particles (Fig. S3e), although silt-320 size hematite grains are also observable. Coarse chlorite flakes as well as tiny, rounded to 321 vermiform kaolinite particles are barely seen (Fig. S3f). Indeed, the clay mineral suite is 322 dominated by two types of illite and one type of Ilt-Smc. SEM-EDX analysis suggests the illites 323 have higher contents of Al2O3 and K2O, but lower contents of SiO2 and Na2O than the Ilt-Smc.

14
The illites occur either as micrometer-sized particles with platy or pseudohexagonal forms 325 being evenly dispersed throughout the matrix (type 1: Fig. 4a,c,e,g) or as long (micrometer-326 scale), but thin laths and fibers, which grow into the open pore space (type 2: Fig. 4b,d,f,g,h).

327
The latter type of illite is often referred to as "hairy illite" (Güven et al., 1980;Rafiei et al., 328 2020). The Ilt-Smc is a nanometer-sized material with flaky to irregular particle forms, which 329 covers detrital grains or grows into the open pore space (type 3: Fig. 4b,d,h). 330 When viewed together with the results of the illite polytype analysis and measured K-Ar ages 331 (Table 1), all sub-samples represent physical mixtures of detrital 2M1 illite/muscovite (type 1), 332 authigenic 1Md/1M illite (type 2) and authigenic 1Md Ilt-Smc (type 3). Accordingly, the plot 333 of the proportion of 2M1 illite/muscovite against the K-Ar age of a given sub-sample (  Table S1 and Fig. 3).  The plot of the chemical data in the A-CN-K ternary diagram (Fig. 6) shows the samples fall  (Table S3).   The major oxide compositions of the sediments from the Valley of Lakes mainly plot in the 392 "P4-quartzose sedimentary provenance" field and only a few samples plot into the "P1-mafic 393 igneous provenance" field in the Roser and Korsch (1988) discrimination diagram (Fig. 7).   (Miall, 1996). Imbrications of pebbles, cobbles and clasts within these beds suggest a 433 palaeo-current direction from north to south (Höck et al. 1999), which is consistent with major 434 sediment source areas in the northern Burdgol Zone. In contrast to Badamgarav (1993) and temporarily affected by ephemeral river and playa lake sedimentation (Miall, 1996), as it can   (Fig. 4) and/or the hydrothermal alteration of pre-existing pedogenic 506 Ilt-Scm to hairy illite . This mechanism is applicable to explain the 507 shift of the chemical data towards the K pole in the A-CN-K ternary diagram (Fig. 6).