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The Pamir range, located in Central Asia, mainly receives moisture from the mid-latitude westerlies, but its western side (i.e., Tajikistan Pamir) receives much of its precipitation in the winter and spring and its eastern side (i.e., Chinese Pamir) in the summer. Thus, the Pamir provides a natural laboratory to study the distribution of surface water stable isotopes across a large mountain range that ultimately receives moisture from one single source but has different precipitation seasonality regimes between its two sides. In this study, we present stable oxygen (δ18O) and hydrogen (δ2H) isotope data for 113 surface water samples from the Chinese Pamir. Our new data, along with previously published stable isotope data, show that the slope of the Chinese Pamir local meteoric water line is higher than that of the Global Meteoric Water Line (GMWL), and almost all of the data plot above the GMWL, implying that the Chinese Pamir surface waters have not experienced significant isotopic modification by evaporation. The Chinese Pamir surface waters have substantially higher δ18O and d-excess values and a steeper apparent δ18O lapse rate than surface water samples collected from the Tajikistan Pamir. We suggest that this contrast results from the shift in precipitation seasonality across the Pamir, with dominantly winter and springtime precipitation on the Tajikistan side and summertime precipitation on the Chinese side of the Pamir. This predominant summertime precipitation regime results in surface waters with high δ18O values in the Chinese Pamir. Further, this summertime moisture is dominantly convectively recycled moisture, resulting in high d-excess values in surface waters. The percentage of summertime moisture, which has high δ18O values, decreases west and with elevation in the Chinese Pamir, resulting in a steep apparent δ18O lapse rate of − 3.2 ‰/km. The importance of precipitation seasonality in modulating δ18O values across the Pamir suggests that proxy-derived records of past environments in the region must consider the mechanisms that today cause the seasonality contrast. 

Abstract Predictions for the southwestern US with warming often suggest increased aridity. We investigate the sedimentary record of the Miocene Climate Optimum and Transition (MCO and MCT; ∼17–14 Ma) in northern New Mexico to understand the impact of warmer global temperatures and higherpCO₂on southwestern US hydroclimate. The MCO and MCT comprised a globally warmer period with elevatedpCO₂similar to end‐of‐the‐century (∼400–800 ppm) projections. We present new stable isotope (δ¹⁸O and δ¹³C) records of vadose‐zone and groundwater terrestrial carbonates and of modern precipitation, stream, and groundwater from the Española basin in northern New Mexico and establish a high‐resolution age model using new⁴⁰Ar/³⁹Ar ages. We interpret δ¹⁸O as reflecting the balance between summertime monsoonal and wintertime precipitation and δ¹³C as a reflection of plant productivity. Terrestrial carbonate δ¹⁸O is lowest during the MCO and MCT and is correlated with terrestrial carbonate δ¹³C and anti‐correlated with the benthic δ¹⁸O record. We interpret these data as recording an overall winter‐wet climate during the MCO and MCT, but that precipitation seasonality varied in response to changes in global climate during this period. The further correlation with carbonate δ¹³C suggests that plant productivity was driven by the amount of wintertime precipitation. Comparison with middle Miocene climate model simulations reveals that higher CO₂drives a shift toward wintertime precipitation. Though paleogeographic changes may obscure a direct comparison to modern warming, overall, our findings suggest that prolonged global warmth may be associated with increased wintertime precipitation and greater primary productivity in northern New Mexico. 

Abstract. The Great Plains of North America host a stark climatic gradient, separating the humid and well-watered eastern US from the semi-arid and arid western US, and this gradient shapes the region's water availability, its ecosystems, and its economies. This climatic boundary is largely set by the influence of two competing atmospheric circulation systems that meet over the Great Plains – the wintertime westerlies bring dominantly dry air that gives way to moist, southerly air transported by the Great Plains low-level jet in the warmer months. Climate model simulations suggest that, as CO2 rises, this low-level jet will strengthen, leading to greater precipitation in the spring but less in the summer and, thus, no change in mean annual precipitation. Combined with rising temperatures that will increase potential evapotranspiration, semi-arid conditions will shift eastward, with potentially large consequences for the ecosystems and inhabitants of the Great Plains. We examine how hydroclimate in the Great Plains varied in the past in response to warmer global climate by studying the paleoclimate record within the Ogallala Formation, which underlies nearly the entire Great Plains and provides a spatially resolved record of hydroclimate during the globally warmer late Miocene. We use the stable isotopes of oxygen (δ18O) as preserved in authigenic carbonates hosted within the abundant paleosol and fluvial successions that comprise the Ogallala Formation as a record of past hydroclimate. Today, and coincident with the modern aridity gradient, there is a sharp meteoric water δ18O gradient with high (−6 ‰ to 0 ‰) δ18O in the southern Great Plains and low (−12 ‰ to −18 ‰) δ18O in the northern plains. We find that the spatial pattern of reconstructed late Miocene precipitation δ18O is indistinguishable from the spatial pattern of modern meteoric water δ18O. We use a recently developed vapor transport model to demonstrate that this δ18O spatial pattern requires air mass mixing over the Great Plains between dry westerly and moist southerly air masses in the late Miocene – consistent with today. Our results suggest that the spatial extents of these two atmospheric circulation systems have been largely unchanged since the late Miocene and any strengthening of the Great Plains low-level jet in response to warming has been isotopically masked by proportional increases in westerly moisture delivery. Our results hold implications for the sensitivity of Great Plains climate to changes in global temperature and CO2 and also for our understanding of the processes that drove Ogallala Formation deposition in the late Miocene. 

The US Southwest is projected to get warmer and drier due to increasing atmospheric CO2, which threatens the region’s ability to support its current ecosystem. However, there is high uncertainty in this projection as precipitation and evapotranspiration remain poorly constrained. We use paleoclimate proxy data from the Miocene to gain insights into Southwest climate during periods of higher atmospheric CO2. Today, the southwest US is characterized by two wet seasons: in winter, the mid-latitude westerlies deliver Pacific-derived moisture, whereas summer moisture is predominantly delivered by the North American Monsoon. We present a new high-resolution sedimentary archive of carbon and oxygen stable isotope (d18O, d13Ccarbonate, and d13Corganic) data to constrain the hydroclimate and ecosystem productivity response to higher atmospheric CO2, derived from authigenic carbonates within the Miocene-aged Santa Fe Group of the Rio Grande Rift from the Española and Albuquerque basins. We find substantial spatial and temporal variability in d18O, likely reflecting variability in the strength of the two circulation systems that deliver moisture to the southwest US. Overall, reconstructed precipitation d18O is lower than today throughout much of the Miocene, suggesting potentially a greater influence of the wintertime westerlies in the moisture budget of the southwest US during the Miocene. Sedimentary organic d13C is < -20‰ throughout the Miocene, indicative of little C4 plant influence during this time. Sedimentary carbonate d13C is generally always less than -5‰, and is positively correlated to carbonate d18O. Such coupling may reflect the influence of evaporation on these samples or a strong link between moisture delivery and primary productivity in this arid climate. 

The stable isotopic composition of soil-formed carbonate, and bulk geochemistry of preserved soil matrix, can provide regionally constrained records of hydroclimatic change throughout deep-time. The SK cores, spanning over 10 km of sediment drilled from the Songliao Basin in Northeast China, represent near continuous terrestrial deposition across the late Jurassic to early Paleogene. In this study we analyze SK-1n paleosol core samples spanning late Maastrichtian to early Danian to interpret the regional hydroclimate response to global climate change, concurrent with Deccan Traps volcanism and the Chicxulub impact. Building on numerous paleosol carbonate datasets from the Sifangtai and Mingshui formations, we present ~40 new carbonate clumped isotope measurements at ca. 10 – 20 kyr resolution between 66.3 to 65.5 Ma. We produce a new kernel-smoothed temperature record and estimate the δ18O of soil porewater (δ18Opw), and δ13C of soil CO2 (δ13Cs) from new and previously published datasets. Molecular weathering ratios, derived from bulk geochemistry, are used to reconstruct weathering (CIA-K), clay formation (Al/Si), soil drainage (Ba/Sr), and calculate mean annual precipitation (MAP) via established transfer functions. Preliminary results suggest elevated K-Pg boundary temperatures, averaging ~30 °C, that decline by ~10 °C over the following 500 kyr. Post-impact cooling may contribute to a negative δ18Ocarb excursion (-2.5‰) at ~65.8 Ma. Further, stable subhumid MAP (~1100 mm/yr) across the dataset suggests negligible amount effect influence. Mean δ18Opw (-6.9‰) is largely stable, and does not reflect regional monsoon seasonality. Instead, stable δ18Opw indicates a consistent moisture source, a potential persistent seasonal bias in carbonate formation. Binning all compiled δ18Opw by soil profile depth reveals statistically significant enrichment in the upper 60 cm of soil profiles, and accounts for variability in the δ18Opw (σ = 1.16‰). Soil respiration, modeled from δ13Cs, increases from ca. 700 to 2000 gC/m2/year across the K-Pg boundary, indicating increased productivity despite declining pCO2 and available phosphorus. Future work will expand the temporal range of isotopic measurements (~72 to 65 Ma) and contextualize our latest Cretaceous results within a spatial framework across Asia. 

The geological record encodes the relationship between climate and atmospheric carbon dioxide (CO₂) over long and short timescales, as well as potential drivers of evolutionary transitions. However, reconstructing CO₂beyond direct measurements requires the use of paleoproxies and herein lies the challenge, as proxies differ in their assumptions, degree of understanding, and even reconstructed values. In this study, we critically evaluated, categorized, and integrated available proxies to create a high-fidelity and transparently constructed atmospheric CO₂record spanning the past 66 million years. This newly constructed record provides clearer evidence for higher Earth system sensitivity in the past and for the role of CO₂thresholds in biological and cryosphere evolution.