Climate Change and Equestrian Empires in the Eastern Steppes: New Insights from a High-resolution Lake Core in Central Mongolia

The repeated expansion of East Asian steppe cultures was a key driver of Eurasian history, forging new social, economic, and biological links across the continent. Climate has been suggested as important driver of these poorly understood cultural expansions, but paleo-climate records from the Mongolian Plateau often suffer from poor age control or ambiguous proxy interpretation. Here, we use a combination of geochemical analyses and comprehensive radiocarbon dating to establish the rst robust and detailed record of paleo-hydrological conditions for Lake Telmen, Mongolia, covering the past ~4000 years. Our record shows that humid conditions coincided with solar minima, and hydrological modelling conrms the high sensitivity of the lake to paleo-climate changes. Careful comparisons with archaeological and historical records suggest that in the vast semi-arid grasslands of eastern Eurasia, solar minima led to reduced temperatures, less evaporation, and high biomass production, expanding the power base for pastoral economies and horse cavalry. Our ndings suggest a crucial link between temperature dynamics in the Eastern Steppe and key social developments, such as the emergence of pastoral empires, and fuel concerns that global warming enhances water scarcity in the semi-arid regions of interior Eurasia.


Introduction
The rise of transcontinental, pastoral empires linking eastern and western Eurasia across the steppes had a tremendous transformative effect on human societies, facilitating the spread of people, goods, and ideas -as well as organisms like domestic animals, plants, and catastrophic disease [1][2][3][4] . The Mongolian steppe was rst occupied by pastoral people ca. 3000 BCE, when early herders appear to have migrated to the region from western Asia [5][6][7] . Around 1200 BCE, domestic horses were used rst for transport by mobile herders of the Deer Stone-Khirgsuur complex (DSK) and other Bronze Age culture groups [8][9][10][11] . The emergence of horse culture changed mobility of the steppe cultures, leading to the rise of important nomadic polities like the Xiongnu (ca. 200 BCE -100 CE) and the Great Mongol Empire, who rose to global dominance under Genghis Khan in the early 13th century CE 10,12 . For these pastoral empires, extensive and productive grasslands provide the engine for both economic and political power 13,14 . Yet, particularly in the dry and harsh steppes of eastern Eurasia, minor climate variations can have large impacts on the water balance, biomass production, and ecosystem carrying capacity [15][16][17] . The close coupling between precipitation and temperature regimes and domestic animal productivity has inspired hypotheses that climate changes may have played an important role for network formation and human history in Central Asia 17 . While social-economic changes such as the emergence of social inequality in pastoral societies can be well inferred from historical and archaeological records 11,17,18 , potential climatic controls can only be assessed by high-resolution paleoclimate records, which are currently rare for the Late Holocene in Mongolia.
Paleoclimate information derived largely from lake sediments [19][20][21][22][23][24][25][26] and tree-rings 13,27−29 , suggest a possible link between the onset of wetter conditions and social integration among Mongolian pastoralists. For northern and Central Mongolia, many records indicate a shift to humid conditions with the onset of the Late Holocene. Increasing moisture availability around 1000 BCE 19,30 was suggested to favor the expansion of nomadic tribes 31 . For the past ~1500 years high resolution tree-ring records show short-term temperature uctuations [27][28][29]32 , that can be attributed to volcanic forcing 33,34 . Pederson et al. 13 identi ed more persistent droughts during the Medieval Climate Anomaly (MCA; 850 -1300 CE), followed by warmer and more humid conditions between 1211 and 1225 CE, which could have favored the expansion of the Mongol Empire.
Despite these tantalizing links between climate changes and pastoral dynamics, the paleoclimatic andenvironmental records for Mongolia suffer from poor temporal resolution, age uncertainties, and/or ambiguous proxy interpretation. Oftentimes, chronological frameworks designed for geological research questions (with wide error ranges) are applied to archaeological timescales, meaning that the same dataset can be used to draw widely differing conclusions 35 . Anthropogenic impacts related to herding have also drastically impacted Mongolia's landscape -meaning that pollen and any other biological data, for example, might be affected by human land-use since 1200 or even 3000 BCE 8, 30 . In order to provide a more convincing link between climate and human history, more robust and well-dated highresolution paleo-climate records are needed. Here, we report a single well-dated paleohydrological record that spans the whole timeline of Mongolian pastoral history and prehistory from the late Bronze Age, allowing the rst proper opportunity to test for a causal link between climate dynamics and pastoral empires in the eastern Steppe.
In this study, we investigated a 161 cm long sediment core from Lake Telmen, an endorheic lake in Zavkhan province in semi-arid central Mongolia, along the western edge of the Khangai Mountains ( Supplementary Fig. 1). We applied radiocarbon dating on bulk TOC and molecular markers to establish a robust chronology, and we combined compound-speci c δ 2 H and δ 13 C on individual n-alkanes, bulk δ 13 C and δ 18 O on carbonates, with elemental and inorganic geochemical and sedimentological analyses to establish a detailed paleo-environmental record and to precisely constrain the regional hydrological history. Moreover, the lake's sensitivity to changes in temperature and precipitation was evaluated by a hydrological water balance model, which enables an identi cation of relevant forcings.

Sediment core chronology
The lowermost and oldest radiocarbon age from our sediment core is 2300 ± 170 BCE, while a presentday water plant reveals a hard-water effect (Δ HW ) of 190 ± 83 years (Supplementary Tab. 1). We established an age-depth model (ADM) using seven Δ HW -corrected bulk 14 C ages and two compoundclass n-alkane 14 C ages ( Supplementary Fig. 2). The 14 C chronology is stratigraphically very consistent ( Supplementary Fig. 2, Supplementary section S2). We further re ned the ADM using nine tie points that we identi ed by comparison with total solar irradiance (TSI) 36 (Fig. 1, Supplementary Fig. 3, see "Method section"). The (Δ HW -corrected) 14 C ages overlap with the 95% con dence interval of the tie-point ADM, and the median ages of both ADMs differ by no more than 246 years.

Sedimentological and geochemical analyses
Our sediment core is nely laminated, mainly consists of silty siliciclastic components (≥ 66%) and is characterized by high amounts of total organic carbon (TOC: 5.4 -12.3%) and carbonates (total inorganic carbon, TIC: 3.2 -6.3%) ( Supplementary Fig. 4) Supplementary Fig. 5). PC1 can be interpreted to re ect allochthonous input related to weathering and erosion processes in the catchment 37 , whereas PC2 characterizes the autochthonous production ( Supplementary Fig. 5). With regard to our geochemical data, the most relevant paleo-hydrological information is inferred from the Ca/Al ratio, which ranges from 4.7 to 23. Wide ratios indicate enhanced autochthonous production and carbonate precipitation during probably dry and warm periods around 1500 BCE and again around 1000 CE (Fig.  2a). Narrow ratios indicate more allochthonous input in between 1200 BCE and 700 CE, as well as after 1300 CE, which can be interpreted to document humid conditions with elevated runoff.

Isotope analyses, evaporation index (E I ), and paleohydrology
n-Alkanes were present in all samples in su cient amounts for compound-speci c isotope analyses ( Supplementary Fig. 7). A differentiation of allochthonous and autochthonous compounds can be made on the basis of n-alkane chain lengths. n-C 31 -Alkanes are predominantly synthesized by higher terrestrial plants and are of allochthonous origin 37,38 . Therefore, our δ 2 H n−C31 record mainly re ects changes in the isotopic signature of precipitation 39 . Changes were minor over the past 4000 years, since δ 2 H n−C31 is relatively constant, ranging from -219 ± 2.0 to -193 ± 2.1‰ (Fig. 2b). n-C 23 -Alkanes, on the other hand, are predominantly synthesized by aquatic plants 37 . Thus, our δ 2 H n−C23 record re ects the isotopic signature of lake water and its evaporative 2 H enrichment [40][41][42][43] . δ 2 H n−C23 ranges widely from -180 ± 3.5 to -139 ± 2.7‰ and indicates lake water 2 H enrichment in distinct periods, particularly before 1500 BCE (Fig. 2c).
Signi cant co-variation between δ 18 O carb and δ 13 C carb (r = 0.61, p = 1.06e −17 ) re ects evaporation under equilibrium conditions of dissolved and atmospheric CO 2 and indicates the paleo-hydrological sensitivity of both isotopes [44][45][46] . δ 13 C carb ranges from 1.3 to 3.1‰ with maximum values again before 1500 BCE, but also relatively high values around 1000 CE (Fig. 2e). δ 2 H n−C23 , δ 18 O carb , and δ 13 C carb show similar down-core trends and primarily re ect the evaporative enrichment of lake water. We therefore combined all three proxies into a normalized Evaporation Index (E I ; Supplementary Fig. 8). Positive E I values document enhanced evaporative lake water enrichment under dry conditions, which also leads to reduced moisture effectively available for the ecosystems at our study site. We refer to this as "dry" in the following. By contrast, negative E I values document reduced evaporative enrichment due to colder and/or more humid conditions, which leads to increased effective moisture in this ecosystem (referred to as "humid").
As illustrated in Figure 3f, the E I reveals dry conditions for the early Late Holocene until ca. 1200 BCE, followed by a long-lasting humid period from 1200 BCE to 700 CE. From 700 to 1300 CE, regional climate conditions became drier again, temporally coinciding with the MCA. With the onset of the Little Ice Age (LIA) around 1300 CE, regional climate tended to be more humid. This dry-humid-dry-humid pattern perfectly agrees with the Ca/Al ratio, corroborating the interpretation and robustness of the various proxies.
Our results show a transition from dry to humid conditions around 1200 BCE, associated with directional socio-economic changes in Mongolian pastoralist cultures 8, 9 . Wetter conditions are associated with the onset of sedimentation in the western shallow lake basin previously dated to 1300 BCE 22 and a prominent lake level rise of 9.4 m indirectly dated to ca. 0 CE 22 Another lake high-stand is recorded by a prominent +4.8 m shoreline terrace, which has been directly dated to between 600 and 700 CE 22 and very likely points to a humid phase shortly before the onset of the drier MCA.

External forcing on the regional climate
Our results suggest that these long-term humidity trends are driven by changes in solar insolation (Fig.   3a). On the one hand, reduced summer insolation leads to a weakening of the monsoon system and thus to less precipitation in the areas affected by the monsoonal system and higher precipitation in the adjacent northern regions (sometimes referred to "Arid Central Asia") including Mongolia 47 ( Supplementary Fig. 1). This effect can be explained with weak subsidence and more prominent convection. On the other hand, a long-term reduced summer insolation (~6 Wm −2 from the Mid-to the Late Holocene, Fig. 3a) will inevitably lead to lower temperatures, reduced evaporation and more effective ecosystem moisture (Supplementary section S7).
However, superimposed on this long-term trend we nd a strong resemblance of our E I record with shortterm uctuations of the high-resolution TSI record (Fig. 3b, e). The solar minima at ~800 BCE, 400 BCE, as well as ~700 CE and after 1300 CE all coincide with prominent minima displayed by our E I record. We interpret this as strong empirical evidence for solar forcing being the most important factor in uencing the local hydrological conditions at our site.
TSI varies by about 1 Wm −2 , which modi es the radiative forcing at the Earth's surface and the mean global temperature by ~0.17 Wm −2 and ~0.07°C, respectively 48 . Lean 49 suggested a higher sensitivity for the mid-latitudes and regional temperature modi cations of up to 2°C. A temperature decrease at this magnitude should entail a signi cant decrease of evaporation, which directly affects the regional moisture balance (see section "Hydrological modelling" below). In addition, reduced TSI during solar minima favored northern hemispheric cooling, which strengthened latitudinal temperature gradients and the Westerly jet streams 48,50,51 . Then, the North Atlantic Oscillation (NAO) tends to be in a distinct negative mode (Fig. 3c), which forces the Westerly jets to the South, advecting more moisture to "Arid Central Asia", and thus, our research site 21,51 ( Supplementary Fig. 1). In contrast, higher TSI leads to reduced latitudinal temperature gradients, which coincides with a distinct positive NAO mode (Fig. 3b, c) as well as a northward shift of the Westerly jets 21,51 , which favors dry conditions at Lake Telmen.

Hydrological modelling
To evaluate how temperature and precipitation changes related to solar forcing could have impacted the water balance of Lake Telmen, we set up a hydrological model (Supplementary Tab. 3). The modelsimulated long-term average water balance of Lake Telmen indicates a state close to equilibrium, which coincides well with the relatively stable average lake extent of Lake Telmen over the past decades 52 . Several sensitivity tests show that the water balance is highly sensitive to changes in temperature and precipitation (Supplementary Tab. 4). A 1°C decrease in air temperature (and lake surface temperature, respectively), for example, reduces lake evaporation by already 5% or 8%, respectively.
During the lake high-stand of +4.8 m above present (600 -700 CE) 22 , the lake area was 13% larger than today ( Supplementary Fig. 1). To maintain the corresponding water balance state close to equilibrium, air temperature would need to be 0.35°C lower, or precipitation would need to be 2.5% higher (~12 mm a −1 ) compared to present-day conditions. Considering the joint effect of air and water temperatures, a decrease of only 0.15°C would already be su cient. This highlights that even small temperature changes related to solar forcing (Fig. 3b) could explain the observed hydrological changes and lake level uctuations. The +4.8 m lake high-stand could therefore be explained by the solar minimum at ~700 CE, while age uncertainties in this part of the core section do not rule out a potential correlation of this particular lake high-stand with the Late Antique Little Ice Age (LALIA; 536-660 CE) 32 , which marks the onset of prominent temperature anomalies within the common era 53 . Major volcanic eruptions may have reduced temperatures by up to 2.5°C at 536 CE and 543 CE 33,34 . Our hydrological model suggests that such a strong, short-term temperature decline would maintain a water-balance state close to equilibrium -even if precipitation decreased by 18% relative to present-day conditions. All this con rms that Lake Telmen and the hydrological water balance at the study site are extremely sensitive to solar forcing and even small temperature changes.

Climate impact on human history in Mongolia
Sustained humid conditions likely enabled the expansion of fertile grasslands and thus, increased ecosystem carrying capacity 14,17,54 -allowing to raise larger numbers of livestock and horses for both meat and dairy production 9,11 . Particularly in the dry and seasonal steppe environment, domestic livestock herds experience "economies of scale" -wherein smaller herds are more vulnerable to loss from disease, predation, or weather, and larger herds are more resilient 55 . With productive areas distributed unequally across the landscape, and some herders inevitably subject to disaster and loss, periods of environmental productivity appear to encourage the formation of larger steppe social networks 17  While the NAO weakening during the grand solar minimum is associated with a general climate and environmental crisis 51 , triggering human migrations and the collapse of cultures in large parts of northern Europe 61,62 , we nd the opposite causal link in Mongolia -between increased effective ecosystem moisture and positive socio-environmental impacts due to enhanced biomass production and an expansion of fertile grasslands 15,16,63 . During the grand solar minimum from 800 to 600 BCE (Fig. 3d, I.) 64 in key social changes, and the emergence of the rst integrated pastoral empires took place during a prolonged period of humid conditions, as indicated by our E I . As the DSK culture waned, Mongolia witnessed an expansion of the Slab Burial culture, whose sites also yield the rst direct evidence of riding tack 65 , royal equestrian burials and the earliest evidence for horsemanship appear in the archaeological record at Arzhan, in Tuva, and early mounted Scythian groups spread westward out of interior Asia 66 .
From this rst expansion of horse culture, a prolonged period of humid conditions in central Mongolia supported the convergence of Mongolia's rst united pastoral polities. The Xiongnu Empire thrived particularly between 200 BCE and 100 CE (Fig. 3f) 10 Finally, our record supports previous arguments that moisture balance also played an important role in the emergence and success of the largest pastoral empire, the Great Mongol Empire of Genghis and Khubilai Khan. Our E I shows a shift to humid conditions since 1100 CE and a positive effective moisture balance at the MCA-LIA transition around 1300 CE (Fig. 3e). This likely favored the union of nomadic tribes under Genghis Khan and the formation of the Mongol Empire, which began during the early 13th century and reached its greatest spatial extent during the late 13th through the mid-14th century (Fig.   3f) 13,14,54 .
We conclude that solar forcing played an important role in controlling regional climate at Lake Telmen over the past 4000 years. We have shown that even small changes in temperature and precipitation have a huge impact on the effective ecosystem moisture balance and thus, biomass production and the expansion of fertile grasslands. This apparent causal link between favorable climate conditions and positive socio-environmental impacts for herding cultures in the Mongolian steppe likely had tremendous impact on the broader trajectory of human history in Eurasia, as the cyclical emergence of pastoral cultural networks and empires helped to forge some of the rst pan Eurasian trade networks, spreading goods, plants, and animals, people, ideas, and even catastrophic pandemic disease [1][2][3][4] .
While these moisture uctuations seem to have exerted an important impact on the rise and fall of Mongolian steppe cultures over the past 4000 years, in light of the paleoclimate record we expect that the near-future consequences of global warming will put the ecosystems and livelihood of the pastoral population in Central Asia at great risk. Mongolia is already experiencing a 2°C temperature increase since 1963 71 , and will likely exceed TSI-induced temperature uctuations in the near-future. Previous studies have shown a rapid loss of lakes 52 , melting mountain ice 72 , persistent soil moisture de cits 73,74 , and an increased frequency of droughts 73,75,76 and heavy rainstorms 15,77,78 . Increased rainfall may not counteract the impact of rising temperatures. Instead, rainfall may exacerbate ongoing land degradation as these short-term heavy rainstorms exceed the soil's in ltration capacity and cause surface runoff, soil erosion, and even oods 77,78 . Although, modeling results show a low probability that future drought intensities will exceed those of the last two millennia 76 , present-day climate changes already cause enhanced socio-environmental consequences 15,75,77 , and it is uncertain whether and how modern pastoralists will to adapt to the future climate.

Methods
Coring:  vacuum and the isotopic composition were subsequently measured on the released and cryogenic puri ed CO 2 . The isotope ratios are given in delta notation against the Vienna Pee Dee Belemnite (VPDB) standard. Analytical precision was checked using replicate measurements of reference materials (NBS19, C1-internal standard), and yielded standard errors <0.07‰ for both, δ 13 C carb , δ 18 O carb . n-Alkane extraction and compound-speci c δ 2 H n-alkane measurements: Total lipids of 120 sediment samples (0.4 -4.5 g) were ultrasonically extracted using a mixture of dichloromethane and methanol (9:1, v/v) as a solvent, the procedure was repeated in three cycles of 15 min each 38 . Total lipid extracts were separated by solid phase extraction using aminopropyl (Supelco; 45 µm) as stationary phase, n-alkanes were eluted with hexane and additionally puri ed over coupled silvernitrate (AgNO 3 ) coated silica gel (Supelco, 60-200 mesh) and zeolite (Geokleen Ltd.) pipette columns.
Analytical measurements were performed at Friedrich Schiller University Jena. n-Alkane identi cation and quanti cation were performed on an Agilent 7890B gas chromatograph (Agilent, Santa Clara, California, USA) equipped with an Agilent HP5MS column (30 m × 320 µm × 0.25 µm lm thickness) and a ame ionization detector (GC-FID). For identi cation and quanti cation, external n-alkane standards (n-alkane mix n-C 21 -n-C 40 , Supelco) were measured with each sequence. n-Alkane concentrations are given in micrograms per gram (µg g −1 ) dry weight and were calculated as the sum of n-C 23 to n-C 35 . δ 2 H n-alkane analyses were performed on an isoprime visION isotope ratio mass spectrometer (Elementar, Manchester, UK) coupled via a GC5 pyrolysis-combustion interface (Elementar, Manchester, UK) to an Agilent 7890B gas chromatograph equipped with an Agilent HP5GC column (30m × 320 µm × 0.25 µm lm thickness). The GC5 operated in pyrolysis mode (ChromeHD reactor) at 1050°C. Samples were injected in splitless mode and measured in triplicates. n-Alkane standards (n-C 27 , n-C 29 and n-C 33 ) with known isotopic composition (Schimmelmann n-alkane standards, Indiana, USA) were measured as duplicates after every third triplicate. The standard deviation for the triplicate measurements was <3.6‰ for δ 2 H n-C23 and <6.1‰ for δ 2 H n-31 . However, the relatively high maximum standard deviation for δ 2 H n-31 concerns only one sample triplicate and the standard deviation was <2.7‰ for the remaining δ 2 H n-31 triplicates. The standard deviation of standard duplicates was <4.3‰ (n = 124). δ 2 H n-alkane measurements were drift and amount-corrected relative to the standards in each sequence. The H3+ correction factor was checked routinely after system tuning and was stable at 4.2 ± 0.63 (n = 15). The compound-speci c isotopic composition is given in delta notation versus the Vienna Standard Mean Ocean Water (VSMOW).

The evaporation index (E I ):
The E I is based on predominantly autochthonous stable isotope values (δ 13 C carb , δ 18 O carb , and δ 2 H n-C23 ), which are sensitive to lake evaporation causing a distinct enrichment in 13 C, 18 O, and 2 H, respectively. Since multiple isotope fractionation processes on each isotope can alter the isotopic signatures differently, all isotope values were z-transformed for standardization. The three isotopes show a similar down-core trend in terms of relative enrichment and depletion, respectively, and the E I was calculated as the average of the z-standardized values.
Water balance modelling, sensitivity analysis and model scenarios: The hydrological model J2000g adapted and extended according to the speci c characteristics of closedlake basins on the Tibetan Plateau 80 , was transferred to the Lake Telmen basin. A detailed description of the model components, model-parameter estimation and input data requirements are given in Biskop et al. 80 . As meteorological input we used climate station data (1990-2020) from Tosontsengel and lake surface water temperature from the ARC Lake data set (v3.0) 81 . To better understand the sensitivity of lake response to climate variability, we explored the effects of changes in climate input variables on several hydrological model-output components (lake evaporation, actual evapotranspiration, runoff). Lake-level changes were estimated by using the stage volume curve derived from the digital bathymetry and elevation (SRTM) elevation. Considering the paleo-lake extension of Lake Telmen, the hydrological model built for present-day conditions was run through several scenarios of precipitation and temperature changes in order to gain more quantitative knowledge about climatic conditions needed to maintain high lake-level stands during the Late Holocene. We calculated the paleo-lake extension for the +4.8 m and +9.4 m terrace above the present-day lake level 22 using the water-level area curve derived from digital elevation and bathymetric data (Supplementary Figure 1).

Applied statistics:
Pearson's correlation coe cients (r values) were calculated to identify correlations within the geochemical and stable isotope dataset. Signi cance of correlations were tested using a two-sided t-test (α = 0.05). For autochthonous and allochthonous endmember identi cation, we further calculated a principal component analysis for the elements Al, Fe, K, Mg, Na, Sr, and Ca. The applied statistic was performed with the statistical software Origin (version Pro 2019b).

Data availability
The dataset used for this study will be uploaded to PANGEA.

Figure 1
Photo of the sediment core from Lake Telmen, taken after oxidation, and age-depth model. The age-depth model is based on tie-points (blue dots) with the total solar irradiance (TSI)36 ( Supplementary Fig. 3).
The 14C ages are plotted for comparison -The modern hard-water effect (ΔHW) is the difference between the year of coring in 2017 and the 14C age of a modern water plant (yellow). ΔHW-corrected bulk TOC 14C ages and compound-class n-alkane 14C ages are shown in red and green, respectively (Supplementary Tab. 1).

Figure 2
Geochemical and isotope records for the sediment core from Lake Telmen.