Page 21 - i1052-5173-27-2

MWoeisstteurrley GobiLDimesietrotmf moiosntusroeonal Hemisphere cooling. We attribute this
decrease to a combination of sufficiently
Tibetan high topography and an equator-ward shift
Plateau of the mid-latitude jet during cooling,
which interact to create the modern spring
MMooisntsuoroenal MoMnsooisotnuarel Zays&anLBakaesin and fall precipitation regime in
Kazakhstan. The resulting climatic effects
Altai reorganized climate in Central Asia, fur-
ther drying interior China and establish-
Lake Balkash Saur-Manrak ing the modern seasonal precipitation
Almaty JunBgasgianr regime in Central Asia.
Dzhungar
GEOLOGIC AND CLIMATIC
Urumqi SETTING

Tian ShanTaklamakanTDareismerBtasim Ordos Basin The Zaysan Basin (48°N; 84°E) lies in
eastern Kazakhstan, bordered to the north
Issyk Kul by the Altai and to the south and east by
the Saur-Manrak ranges, which separate
Geologic Sections and Sampling Locations Elevation (m) the Zaysan Basin from the Junggar Basin
(Fig. 1). These ranges are the northern end
Zaysan (This study) Water Samples 0 1000 2000 3000 of the Dzhungar Mountains, which splay
Macauley et al. 2016 Charreau et al. 2012 northward off the Tian Shan northeast of
Caves et al. 2014 Other studies Almaty. As Russell and Zhai (1987, p. 158)
note, “Perhaps nowhere in Asia … is there
Figure 1. Location map. Inset: yellow arrow—westerly moisture; blue arrows—monsoonal moisture. a better sequence of continental Tertiary
Dashed blue line—approximate, modern-day inland extent of monsoonal moisture (Araguás-Araguás et sediments than that found in the Zaysan
al., 1998). Basin.” Though more recent work sug-
gests substantial unconformities, sedi-
Stable isotope locality Locality with spring/fall precipitation ments in the basin represent nearly every
epoch since the Late Cretaceous (Lucas et
AB al., 2009). The Paleogene and early
Neogene are primarily lacustrine, which
Altai Altai transitions to pedogenic redbeds by the
late Neogene (Lucas et al., 2000).
Tian Shan Tian Shan
Tibetan Plateau Tibetan Plateau Climatically, the Zaysan Basin is excep-
tionally continental, with wintertime
Average Annual Precipitation (mm) Fraction of Precipitation in Spring and Fall (DJF) temperatures less than -15 °C and
summertime (JJA) mean daily tempera-
0 200 400 600 800 1000 tures nearing 20 °C (Schiemann et al.,
2008) (Fig. 3). Moisture is supplied
Figure 2. (A) Average annual precipitation. (B) Fraction of precipitation that falls in spring and fall versus entirely by the mid-latitude westerlies
total spring-summer-fall precipitation. Data from the Global Precipitation Climatology Center (Meyer- (Fig. 4), because high ranges to the south,
Christoffer et al., 2015). Gray points—Neogene sedimentary stable isotope localities; gray stars—sites including the Tian Shan, Pamir, and
impacted by spring and fall precipitation seasonality; black line—smoothed 2000 m contour. Major Hindu Kush, block subtropical air from
ranges are labeled. penetrating into Kazakhstan (Schiemann
et al., 2008). Notably, the Zaysan Basin
and Tian Shan (Fig. 1). Oxygen isotopes which starkly contrasts with nearly all lies on the border between two contrasting
(d18O) are particularly sensitive to the other records of sedimentary d18O from precipitation regimes: Kazakhstan
effects of orographically forced rainout on downwind localities in interior China and receives the majority of its precipitation in
the windward flanks of ranges (Mulch, Mongolia that in general are constant or the spring (MAM) and fall (SON),
2016; Winnick et al., 2014), suggesting increasing during the Neogene (Caves et whereas interior China and Mongolia
that any interaction between the Tian Shan al., 2015). The timing of the d18O decrease experience dominantly JJA precipitation
and Altai and climate should be detectable in the Zaysan Basin is broadly synchro- (Baldwin and Vecchi, 2016) (Fig. 2B).
in the Zaysan Basin. We find a 4‰ nous with both accelerated uplift of the This pronounced difference in precipita-
decrease in d18O in the late Neogene, Tian Shan and Altai and Northern tion seasonality is a consequence of the
annual migration of the Northern
Hemisphere mid-latitude jet, which
swings northward in April and returns

www.geosociety.org/gsatoday 21