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Volume 27 Issue 2 (February 2017)

GSA Today

Article, pp. 19-26 | Abstract | PDF (2.2MB)

Late Miocene Uplift of the Tian Shan and Altai and Reorganization of Central Asia Climate

Introduction

Research on tectonic-climatic coupling in Asia has focused primarily on the importance of the Tibetan Plateau in strengthening monsoonal circulation and in aridifying Central Asia (Zhisheng et al., 2001; Zhang et al., 2007). However, recent work places Plateau uplift in the Paleogene (Rowley and Currie, 2006), challenging the mechanisms that couple Plateau orography with increasing Central Asian aridity in the Neogene (Molnar et al., 2010; Caves et al., 2016). In contrast, many of the high ranges north of the Plateau—including the Tian Shan and Altai—appear to have uplifted more recently (Charreau et al., 2009; De Grave et al., 2007), suggesting that they may play a role in the Neogene paleoclimatic history of Central Asia (Fig. 1). Today, these ranges cast substantial rain shadows, with substantially more precipitation on their windward flanks than in the lee deserts of the Gobi and Taklamakan (Fig. 2A). Further, these ranges demarcate a stark precipitation seasonality boundary, with dominantly spring and fall precipitation to the west and primarily summertime precipitation to the east (Fig. 2B) (Baldwin and Vecchi, 2016).

Unfortunately, there is little paleoclimatic data that unequivocally indicate when the surface topography of these ranges became sufficiently prominent to impact climate in Asia. Basin analysis and sedimentological data indicate that residual topography has continuously separated many of the inward-draining basins in Central Asia since the Mesozoic (Carroll et al., 2010). Thermochronological and sedimentological studies demonstrate that reactivation of Paleozoic structures in the Tian Shan and Altai began in the late Paleogene and accelerated and/or propagated northward by the late Miocene (Charreau et al., 2009; De Grave et al., 2007). However, most paleoclimatic records in Asia come from the arid expanses of interior China, where the competing influences of uplift, Paratethys retreat, and global climate are difficult to deconvolve (Sun et al., 2013; Tang et al., 2011; Molnar et al., 2010). General circulation models (GCMs) configured with paleo-boundary conditions also produce equivocal results because their relatively low resolution hinders proper treatment of Tian Shan and Altai uplift. Therefore, most work has focused on Tibetan Plateau uplift or westward retreat of the Paratethys (Zhisheng et al., 2001; Zhang et al., 2007).

To test when the Altai and Tian Shan became sufficiently prominent to impact climate, we collected pedogenic and lacustrine carbonates for isotopic analysis from the Zaysan Basin in Kazakhstan, a long-lived basin that lies windward of the Altai and Tian Shan (Fig. 1). Oxygen isotopes (δ¹⁸O) are particularly sensitive to the effects of orographically forced rainout on the windward flanks of ranges (Mulch, 2016; Winnick et al., 2014), suggesting that any interaction between the Tian Shan and Altai and climate should be detectable in the Zaysan Basin. We find a 4‰ decrease in δ¹⁸O in the late Neogene, which starkly contrasts with nearly all other records of sedimentary δ¹⁸O from downwind localities in interior China and Mongolia that in general are constant or increasing during the Neogene (Caves et al., 2015). The timing of the δ¹⁸O decrease in the Zaysan Basin is broadly synchronous with both accelerated uplift of the Tian Shan and Altai and Northern Hemisphere cooling. We attribute this decrease to a combination of sufficiently high topography and an equator-ward shift of the mid-latitude jet during cooling, which interact to create the modern spring and fall precipitation regime in Kazakhstan. The resulting climatic effects reorganized climate in Central Asia, further drying interior China and establishing the modern seasonal precipitation regime in Central Asia.

Geologic and Climatic Setting

The Zaysan Basin (48°N; 84°E) lies in eastern Kazakhstan, bordered to the north by the Altai and to the south and east by the Saur-Manrak ranges, which separate the Zaysan Basin from the Junggar Basin (Fig. 1). These ranges are the northern end of the Dzhungar Mountains, which splay northward off the Tian Shan northeast of Almaty. As Russell and Zhai (1987, p. 158) note, “Perhaps nowhere in Asia … is there a better sequence of continental Tertiary sediments than that found in the Zaysan Basin.” Though more recent work suggests substantial unconformities, sediments in the basin represent nearly every epoch since the Late Cretaceous (Lucas et al., 2009). The Paleogene and early Neogene are primarily lacustrine, which transitions to pedogenic redbeds by the late Neogene (Lucas et al., 2000).

Climatically, the Zaysan Basin is exceptionally continental, with wintertime (DJF) temperatures less than −15 °C and summertime (JJA) mean daily temperatures nearing 20 °C (Schiemann et al., 2008) (Fig. 3). Moisture is supplied entirely by the mid-latitude westerlies (Fig. 4), because high ranges to the south, including the Tian Shan, Pamir, and Hindu Kush, block subtropical air from penetrating into Kazakhstan (Schiemann et al., 2008). Notably, the Zaysan Basin lies on the border between two contrasting precipitation regimes: Kazakhstan receives the majority of its precipitation in the spring (MAM) and fall (SON), whereas interior China and Mongolia experience dominantly JJA precipitation (Baldwin and Vecchi, 2016) (Fig. 2B). This pronounced difference in precipitation seasonality is a consequence of the annual migration of the Northern Hemisphere mid-latitude jet, which swings northward in April and returns south in October (Schiemann et al., 2009). During this biannual migration, cyclones originating to the west interact with the Tian Shan and Altai, producing orographic precipitation (Schiemann et al., 2008, 2009; Baldwin and Vecchi, 2016).

To characterize the modern precipitation δ¹⁸O (δ¹⁸O_p) in the Zaysan Basin, we rely upon three methods (Fig. 3). First, we interpolate estimates of δ¹⁸O_p from the SWING2 (Stable Water Isotope Inter­comparison Group 2) database, which uses isotopically enabled atmospheric GCMs (n = 6) to estimate δ¹⁸O_p at ~2.5° × 2.5° resolution (Risi et al., 2012). Second, we use the Online Precipitation Isotope Calculator (Bowen et al., 2005). Third, we calculate flux-weighted δ¹⁸O_p from the Urumqi, China, GNIP (Global Network of Isotopes in Precipitation) station (IAEA/WMO, 2016). All three methods produce a similar seasonal cycle, with the lowest δ¹⁸O_p during DJF and the highest during JJA (Fig. 3). Average MAM and SON δ¹⁸O_p is lower relative to JJA by 3.6 ± 2.3‰ and 6.8 ± 1.8‰ (1σ), respectively (gray points, Fig. 3). This seasonal cycle is characteristic of the continental air masses of Central Asia (Araguás-Araguás et al., 1998).

Geologic Sampling and Methods

We collected 77 pedogenic carbonate samples from the type-section in the Zaysan Basin along the Kalmakpay River in the southeast of the basin (47.4°N; 84.4°E). The sediments in the basin are classified into svitas (Sv.; similar to, but not strictly identical to formation) (Borisov, 1963). The upper Neogene comprises ~200 m of pedogenically altered, carbonate-rich redbeds, including the lower Kalmakpay Sv. and the upper Karabulak Sv. Kalmakpay Sv. sediments are primarily mudstones that dip ~15° NE and are separated from the yellowish sands of the underlying, carbonate-poor Sarybulak Sv. by an unconformity (Fig. 5). The overlying Karabulak Sv. is coarser, with multiple pebble conglomerates and cross-bedded sandstones that cut into pedogenic facies, and dips ~10° NE. Throughout the basin, this sequence is capped by the “Gobi Conglomerate” (Berkey and Morris, 1923), which is a Quaternary deposit (Lucas et al., 2009), from which we collected five samples of carbonate-rich cement. We also collected 54 samples from the lower Miocene, carbonate-rich, lacustrine Akzhar Sv. from the Tayzhugen and Kyzylkain River sections (Lucas et al., 2009) in the southwest of the basin. We measured carbonate δ¹⁸O and δ¹³C (δ¹⁸O_c and δ¹³C_c) on a Finnigan MAT Delta+ XL mass spectrometer at the Stable Isotope Biogeochemistry Laboratory (Stanford University, California, USA; see the GSA Supplemental Data Repository¹ for full methods).

¹ GSA Supplemental Data Repository Item 2017024, containing methods, additional climatology, sedimentary, and diagenetic background and discussion, figures and tables, is online at www.geosociety.org/datarepository/2017/. If you have questions, please email gsatoday@geosociety.org.

The ages of these sediments are constrained by biostratigraphy and magnetostratigraphy. The Akzhar Sv. is assigned to the Shanwangian Asian Land Mammal Age (ALMA) (16.9–13.7 Ma), and the overlying Sarybulak Sv. is assigned to the Tunggurian ALMA (13.7–11.1 Ma) (Kowalski and Shevyreva, 1997). The Karabulak Sv. contains abundant “Hipparion” fauna, which is correlated with the latest Turolian (8.7–5.3 Ma; latest Baodean ALMA) (Sotnikova et al., 1997; Vangengeim et al., 1993; Lucas et al., 2009). The intervening Kalmakpay Sv. is not as clearly constrained by mammal fossils, yet is a separate unit from the overlying Karabulak Sv. Following Vangengeim et al. (1993) and Lucas et al. (2009), we therefore place the lower boundary in the late Sarmatian (12.7–11.6 Ma) and the upper boundary in the lower Baodean ALMA. To account for the uncertainty in using this chronologic scheme, we present the data against both height (Fig. 5) and plotted against age (Fig. 6), where we bin the data by svita and assume that the sediments in each svita could have been deposited at any interval during the ALMA.

We also collected 29 stream and well-water samples to characterize the δ¹⁸O of modern waters in Kazakhstan. Stream waters integrate δ¹⁸O across storm and snowmelt events and are used to characterize modern water δ¹⁸O where δ¹⁸O_p data are sparse (Hoke et al., 2014). We measured water δ¹⁸O on a Los Gatos Research TWIA-45EP liquid isotope water analyzer at Santa Clara University, California, USA.

Results

The mean Kalmakpay Sv. δ¹⁸O_c (20.6‰) is significantly 4‰ higher than the mean δ¹⁸O_c of the Karabulak Sv. (16.8‰) (Fig. 5). Similarly, there is a slight, but significant, increase in the mean δ¹³C_c from the Kalmakpay Sv. (−5.8‰) to the Karabulak Sv. (−5.2‰). Using an estimate of the mean value (solid, red line in Fig. 5), δ¹³C_c increases 2.5‰ from the base of the section to the top. The overlying Gobi Conglomerate samples have equivalent δ¹⁸O_c as the underlying Karabulak samples, but higher δ¹³C_c.

Samples from the Akzhar Sv. show the greatest spread in δ¹³C_c and δ¹⁸O_c and the mean δ¹⁸O_c (24.7‰) is significantly higher than the overlying svitas (see GSA supplemental data Table S1). Given the correlation between δ¹⁸O_c and carbonate content (supplemental data Fig. S2), we treat these δ¹⁸O_c data as evaporatively enriched. Following other studies in Central Asia (Rowley and Currie, 2006), we therefore consider the minimum δ¹⁸O_c value as closest to the δ¹⁸O_p value.

Stream/well water δ¹⁸O ranges from −15.5‰ to −11.3‰ (mean = −13.7‰) (supplemental data Table S2). This mean value is less than the estimated value of MAM δ¹⁸O_p (−12.3‰) and substantially less than the estimated value of JJA δ¹⁸O_p (−8.6‰). These samples were collected from streams that drain the 2500+ m Manrak-Saur-Dzhungar ranges and therefore likely integrate δ¹⁸O in catchments potentially still influenced by snowmelt and with large variations in elevation (Hoke et al., 2014).

Discussion

The most prominent trend in our record is the 4‰ decrease in δ¹⁸O_c that occurs in the late Neogene. This trend is opposite to nearly all other δ¹⁸O_c records in Central Asia that lie downwind of the Zaysan Basin and either increase or remain approximately constant over the Neogene (Fig. 6) (Caves et al., 2015). Notably, the only other records that decrease during the Neogene are those that also lie on the windward flank of the Tian Shan (Issyk Kul) and/or within the modern-day spring and fall precipitation regime (Junggar Basin) (gray stars; Fig. 2B).

What mechanisms might explain a simultaneous decrease in δ¹⁸O on the windward flanks of the Tian Shan and Altai, while permitting constant or increasing δ¹⁸O in their lee?

One potential mechanism is that increased orographic precipitation due to uplift might shift windward δ¹⁸O to lower values, particularly as high-elevation precipitation is increasingly captured (Mulch, 2016). This hypothesis has been used to explain the decrease in δ¹⁸O at both Issyk Kul and the Junggar Basin (Charreau et al., 2012; Macaulay et al., 2016). However, increased orographic precipitation appears unlikely to explain the full decrease in the Zaysan Basin. The long-term increase in δ¹³C in the Zaysan Basin—at a location well north of where abundant C₄ vegetation is typically found (supplemental data Fig. S3)—indicates that soil respiration rates dropped, suggesting a decline in productivity and, hence, precipitation (Breecker et al., 2009; Caves et al., 2016). Further, increased orographic precipitation should also result in coupled δ¹⁸O decreases in the lee of these ranges, which is not observed, though δ¹⁸O_c in lee basins is complicated by evaporative effects (Mulch, 2016). An additional mechanism could be an increase in the formation temperature of pedogenic carbonate, given that the fractionation of 18O between water and calcite decreases as temperature increases (~−0.2‰/°C) (Kim and O’Neil, 1997). However, an increase in temperature is unlikely because global climate cooled in the late Neogene following the Miocene Climatic Optimum (Zachos et al., 2001). Further, if changes in global temperature were the dominant factor driving changes in δ¹⁸O_c, one would expect similar decreases across much of northern Central Asia, which are not observed (Caves et al., 2015).

In contrast to the above two mechanisms, shifting precipitation seasonality in Kazakhstan might have a substantial effect on δ¹⁸O_c given the large seasonal δ¹⁸O_p changes observed in Central Asia (Fig. 3) (Araguás-Araguás et al., 1998). Pedogenic carbonates are seasonally biased recorders of δ¹⁸O_p, typically recording wet-season δ¹⁸O_p (Breecker et al., 2009). Thus, a shift toward dominantly spring and fall precipitation might change the timing of pedogenic carbonate formation and decrease δ¹⁸O_c, without necessarily increasing total precipitation. Such a shift may have also lowered the temperature of carbonate formation, raising δ¹⁸O_c, and partially offsetting the true change in precipitation-weighted δ¹⁸O_p; this implies that the change in precipitation seasonality may have been substantial. Importantly, this change in precipitation seasonality would decouple windward δ¹⁸O_c records (Zaysan, Issyk Kul, and Junggar) from leeward δ¹⁸O_c records in interior China and Mongolia, which receive dominantly JJA precipitation.

Implications

The modern spring and fall precipitation regime in Kazakhstan is a result of the interaction between cyclones routed along the mid-latitude westerly jet as it migrates seasonally north and south and the high topography of the Tian Shan and Altai (Schiemann et al., 2009). Thus, a change in precipitation seasonality would imply either a shift of the mid-latitude jet, the development of high topography, or both. The mean position of the jet is thought to have been farther northward during the late Miocene given warmer Arctic temperatures (Micheels et al., 2011), suggesting that cooling into the late Miocene may have contributed to a southward shift of the jet.

Further, northward growth of high topography in Asia would result in increased interaction between cyclones routed along the jet and this topography, resulting in orographic precipitation during the spring and fall (Baldwin and Vecchi, 2016). There is substantial evidence that deformation from the collision of India-Asia has propagated northward during the Cenozoic, resulting in—most recently in the latest Miocene—uplift of the Altai range (De Grave et al., 2007). The Altai rain shadow likely formed by the latest Miocene, as indicated by a substantial increase in pedogenic carbonate δ¹³C in the lee of the Altai (Caves et al., 2014) (Fig. 6). In the northern Tian Shan, Charreau et al. (2009) found increased sedimentation rates in the Junggar Basin at ca. 11 Ma, which they proposed resulted from an acceleration of uplift. Though the individual uplift histories of the local Saur-Manrak ranges are unknown, but are likely linked to uplift in the Altai or northern Tian Shan (Campbell et al., 2013), we propose that the timing of this accelerated uplift is broadly synchronous with our observed decrease in δ¹⁸O_c. Northward-propagating deformation might also explain why oxygen isotopic records from Issyk Kul, which lies 5° south, decline starting in the early Miocene, reflecting earlier growth of high topography to the south (Macaulay et al., 2016).

Our new oxygen isotope record from the windward side of the Tian Shan and Altai ranges indicates that these ranges were sufficiently elevated by the late Miocene to impact climate in Central Asia. Most notably, this interaction resulted in a substantial reorganization of Central Asia climate, creating a stark precipitation seasonality boundary between eastern and western Central Asia. Such high topography would have further blocked moisture from reaching downwind Central Asia, contributing to the increasingly arid conditions and formation of the Taklamakan and Gobi deserts in the late Miocene (Caves et al., 2016; Sun et al., 2009). The Altai are the largest source of lee cyclones in Asia (Chen et al., 1991), and the combined interaction of high topography with the jet’s oscillations as it moves northward in the spring (Schiemann et al., 2009) creates the dust storms that blanket the Loess Plateau (Shao and Dong, 2006; Roe, 2009). Thus, acceleration of loess deposition in the late Miocene (Zhang et al., 2014; Rea et al., 1998) may be a consequence of increasing cyclogenesis as a result of upwind topographic growth (Caves et al., 2014; Shi et al., 2015) (Fig. 6). The subsequent Pliocene decline in eolian accumulation rates may reflect a northward shifted jet during the mid-Pliocene Warm Period, which precluded interaction with the Altai until Quaternary cooling shifted the jet southward again.

We conclude that uplift of the Tian Shan and Altai reorganized late Neogene climate in Central Asia, establishing a prominent seasonal precipitation boundary as a result of increasing interactions between the mid-latitude jet and high topography. Thus, paleoclimatic changes in Central Asia in the late Neogene may not be due solely to changes in the extent or height of the Tibetan Plateau. In contrast, the timing of uplift and consequent climatic impacts of Asia’s northern bounding ranges are critical to unraveling the paleoenvironmental history of Central Asia. Ultimately, future work to test this interpretation must include westward/upwind records of δ¹⁸O combined with GCM studies that resolve the impact of Tian Shan and Altai uplift on Asian climate.

Acknowledgments

We thank our field expedition members, including B. Dautaliev, G. Nazyanbeteva, S. Samzatbaeva, J. Buztsev, and S. Bayshashov, and N. Ballantine, D. Moragne, and P. Blisniuk for assistance with lab work. We also thank two anonymous reviewers and editor G. Dickens for comments that improved the manuscript. A GSA Student Research grant (Caves), three Stanford University McGee grants (Caves, Ritch, and Ibarra), and NSF grants EAR-1009721 and EAR-1423967 (Chamberlain) funded this work. Caves is also funded by an NSF Graduate Research Fellowship (DGE-1147470) and a Stanford Graduate Fellowship.

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