Evaluating the Roles of Rainout and Post-Condensation Processes in a Landfalling Atmospheric River with Stable Isotopes in Precipitation and Water Vapor

Atmospheric rivers (ARs), and frontal systems more broadly, tend to exhibit prominent “V” shapes in time series of stable isotopes in precipitation. Despite the magnitude and widespread nature of these “V” shapes, debate persists as to whether these shifts are driven by changes in the degree of rainout, which we determine using the Rayleigh distillation of stable isotopes, or by post-condensation processes such as below-cloud evaporation and equilibrium isotope exchange between hydrometeors and surrounding vapor. Here, we present paired precipitation and water vapor isotope time series records from the 5–7 March 2016, AR in Bodega Bay, CA. The stable isotope composition of surface vapor along with independent meteorological constraints such as temperature and relative humidity reveal that rainout and post-condensation processes dominate during different portions of the event. We find that Rayleigh distillation controls during peak AR conditions (with peak rainout of 55%) while post-condensation processes have their greatest effect during periods of decreased precipitation on the margins of the event. These results and analyses inform critical questions regarding the temporal evolution of AR events and the physical processes that control them at local scales

Synorogenic evolution of large-scale drainage patterns: Isotope paleohydrology of sequential Laramide basins

In the past decade, we and others have compiled an extensive dataset of O, C and Sr isotope stratigraphies from sedimentary basins throughout the Paleogene North American Cordillera. In this study, we present new results from the Piceance Creek Basin of northwest Colorado, which record the evolving hydrology of the Eocene Green River Lake system. We then place the new data in the context of the broader Cordilleran dataset and summarize implications for understanding the synorogenic evolution of large-scale drainage patterns. The combined data reflect (1) a period of throughgoing foreland rivers heading in the Sevier fold-and-thrust belt and flowing east, (2) ponding of freshwater lakes in the foredeep as Laramide uplifts blocked drainage, (3) hydrologic closure that led to both intensive evaporation in the terminal sink of the Piceance Creek Basin and integration of catchments over length-scales \u3e1000 km, (4) infilling of basin accommodation by southward migrating magmatism in distal catchments, leading to the freshening and demise of intraforeland lakes that also stepped south over time

Aridification of Central Asia and uplift of the Altai and Hangay mountains, Mongolia: stable isotope evidence

Central Asia has become increasingly arid during the Cenozoic, though the mechanisms behind this aridification remain unresolved. Much attention has focused on the influence and uplift history of the Tibetan Plateau. However, the role of ranges linked to India-Asia convergence but well north of the Plateau—including the Altai, Sayan, and Hangay—in creating the arid climate of Central Asia is poorly understood. Today, these ranges create a prominent rain shadow, effectively separating the boreal forest to the north from the deserts of Central Asia. To explore the role of these mountains in modifying climate since the late Eocene, we measured carbon and oxygen stable isotopes in paleosol carbonates from three basins along a 650 km long transect at the northern edge of the Gobi Desert in Mongolia and in the lee of the Altai and Hangay mountains. We combine these data with modern air-parcel back-trajectory modeling to understand regional moisture transport pathways at each basin. In all basins, δ¹³C increases, with the largest increase in western Mongolia. The first δ¹³C increase occurs in central and southwestern Mongolia in the Oligocene. δ¹³C again increases from the upper Miocene to the Quaternary in western and southwestern Mongolia. We use a 1-D soil diffusion model to demonstrate that these δ¹³C increases are linked to declines in soil respiration driven by dramatic increases in aridity. Using modern-day empirical relations between mean annual precipitation and soil respiration, we estimate that precipitation has likely more than halved over the Neogene. Given the importance of the Hangay and Altai in steering moisture in Mongolia, we attribute these changes to differential surface uplift of the Hangay and Altai. Surface uplift in the Hangay began by the early Oligocene, blocking Siberian moisture and aridifying the northern Gobi. In contrast, surface uplift of the Altai began in the late Miocene, blocking moisture from reaching western Mongolia. Thus, the northern Gobi became increasingly arid east to west since the late Eocene, likely driven by orographic development in the Hangay during the Oligocene and the Altai in the late Miocene through Pliocene

Climate response to the 8.2 ka event in coastal California

A fast-growing stalagmite from the central California coast provides a high-resolution record of climatic changes synchronous with global perturbations resulting from the catastrophic drainage of proglacial Lake Agassiz at ca. 8.2 ka. High frequency, large amplitude variations in carbon isotopes during the 8.2 ka event, coupled with pulsed increases in phosphorus concentrations, indicate more frequent or intense winter storms on the California coast. Decreased magnesium-calcium ratios point toward a sustained increase in effective moisture during the event, however the magnitude of change in Mg/Ca suggests this event was not as pronounced on the western North American coast as anomalies seen in the high northern latitudes and monsoon-influenced areas. Nevertheless, shifts in the White Moon Cave record that are synchronous within age uncertainties with cooling of Greenland, and changes in global monsoon systems, suggest rapid changes in atmospheric circulation occurred in response to freshwater input and associated cooling in the North Atlantic region. Our record is consistent with intensification of the Pacific winter storm track in response to North Atlantic freshwater forcing, a mechanism suggested by simulations of the last deglaciation, and indicates this intensification led to increases in precipitation and infiltration along the California coast during the Holocene

Contrasting local and long-range-transported warm ice-nucleating particles during an atmospheric river in coastal California, USA

Ice-nucleating particles (INPs) have been found to influence the amount, phase and efficiency of precipitation from winter storms, including atmospheric rivers.Warm INPs, those that initiate freezing at temperatures warmer than -10°C, are thought to be particularly impactful because they can create primary ice in mixed-phase clouds, enhancing precipitation efficiency. The dominant sources of warm INPs during atmospheric rivers, the role of meteorology in modulating transport and injection of warm INPs into atmospheric river clouds, and the impact of warm INPs on mixed-phase cloud properties are not well-understood. In this case study, time-resolved precipitation samples were collected during an atmospheric river in northern California, USA, during winter 2016. Precipitation samples were collected at two sites, one coastal and one inland, which are separated by about 35 km. The sites are sufficiently close that air mass sources during this storm were almost identical, but the inland site was exposed to terrestrial sources of warm INPs while the coastal site was not. Warm INPs were more numerous in precipitation at the inland site by an order of magnitude. Using FLEXPART (FLEXible PARTicle dispersion model) dispersion modeling and radar-derived cloud vertical structure, we detected influence from terrestrial INP sources at the inland site but did not find clear evidence of marine warm INPs at either site.We episodically detected warm INPs from long-range-transported sources at both sites. By extending the FLEXPART modeling using a meteorological reanalysis, we demonstrate that long-range-transported warm INPs were observed only when the upper tropospheric jet provided transport to cloud tops. Using radar-derived hydrometeor classifications, we demonstrate that hydrometeors over the terrestrially influenced inland site were more likely to be in the ice phase for cloud temperatures between 0 and -10°C. We thus conclude that terrestrial and long-rangetransported aerosol were important sources of warm INPs during this atmospheric river. Meteorological details such as transport mechanism and cloud structure were important in determining (i) warm INP source and injection temperature and (ii) ultimately the impact of warm INPs on mixed-phase cloud properties

The Cenozoic Climatic and Topographic Evolution of the Western North American Cordillera

Herein we present oxygen isotope records from Cretaceous to Recent terrestrial sediments in the western North American Cordillera. The purpose of this analysis is to use oxygen isotope records to understand the coupled surface elevation and climate histories of this region through the Cenozoic. To do this we constructed δ¹⁸(O) maps of surface waters for time intervals that trace the development of topography of western North America. These maps are based on 4861 oxygen isotope analyses from both published (4478) and new (383) data. We determined the δ¹⁸(O) values of surface waters using paleotemperatures derived previously from floral assemblages and the appropriate isotope fractionation factors. These data suggest that in the late Cretaceous to early Eocene the Sevier hinterland formed a plateau of unknown height. Around 50 Ma, a topographic wave developed in British Columbia and eastern Washington that swept southward reaching northeastern Nevada at ~40 to 38 Ma, and southern Nevada ~23 Ma. This southward encroachment of an Eocene Plateau (SWEEP) caused reorganization of drainage patterns such that the intraforeland basins of Wyoming and Utah drainages extended deep within the Sevier hinterland as the wave swept southward. The landscape within the Sevier hinterland developed into a rugged and high mountain range with the hypsometric mean elevation of ~4 km and relief of ~1.5 km. This Eocene highland was bordered on the west by a high Sierra Nevada ramp and on the east by the intraforeland basins that captured water draining these growing highlands. The spatial and temporal evolution of this highland correlates with the timing of volcanism and extension. These observations support tectonic models that call for north to south removal of the Farallon slab or piecemeal removal of mantle lithosphere. The isotopic data show that prior to growth of this highland the North American Monsoon (NAM) penetrated much farther north in the Paleocene/Eocene than today. The combined effects of global cooling, increasing latitudinal temperature gradients, and the generation of the orographic barrier created by the growing north to south highland produced a southward migration of the NAM front. By the Oligocene the hydrologic regime that we observe today was in place. It has been modified since then as a result of Basin and Range extension and collapse of the highlands in the mid-Miocene. This collapse allowed the NAM to penetrate farther north into the Great Basin of Nevada and Utah

Late Miocene uplift of the Tian Shan and Altai and reorganization of Central Asia climate

The timing of high surface topography and the corresponding climatic impacts of the many high ranges north of the Tibetan Plateau, such as the Altai and Tian Shan, remain poorly constrained. Most Neogene reconstructions of Central Asia climate come from interior China, where the influences of Altai and Tian Shan uplift are difficult to deconvolve from effects due to Tibetan Plateau uplift and changes in global climate. We present a new pedogenic carbonate oxygen and carbon isotope record from terrestrial Neogene sediments of the Zaysan Basin in eastern Kazakhstan, which lies upwind of the Altai and Tian Shan, in contrast to the numerous paleoclimate records from interior China. The δ18O values of pedogenic carbonate exhibit a robust 4‰ decrease in the late Neogene—a trend that sharply contrasts with nearly all downwind records of δ18O from Central Asia. We attribute this decrease to the establishment of the modern seasonal precipitation regime whereby Kazakhstan receives the majority of its moisture in the spring and fall, which lowers the δ18O of pedogenic carbonates. The dominance of spring and fall precipitation in Kazakhstan results from the interaction of the mid-latitude jet with the high topography of the Altai and Tian Shan during its movement northward in spring and southward in fall. The late Miocene interaction of the jet with these actively uplifting northern Central Asia ranges reorganized Central Asia climate, establishing starkly different seasonal precipitation regimes, further drying interior China, and increasing the incidence of the lee cyclones that deposit dust on the Loess Plateau. We conclude that paleoclimatic changes in Central Asia in the Neogene are more tightly controlled by the interaction of the mid-latitude westerlies with the bounding ranges of northern Central Asia than by changes in the height or extent of the Tibetan Plateau

Stable isotope records of hydrologic change and paleotemperature from smectite in Cenozoic western North America

The oxygen and hydrogen isotopic composition of soil water (δ18Ow and δDw hereafter) reflect the history of water through processes such as source evaporation, precipitation and vapor recycling. Temperature, humidity, evaporation, and post-condensation processes can affect δ18Ow and δDw. As such, isotope proxy records are often limited in their ability to constrain paleoclimate, paleoecology or paleoelevation without independently corroborating data. Smectite preserves both the hydrogen and oxygen isotope signature of parent water, and therefore provides critical insight into meteoric water line relationships and paleotemperature. Here, we use in situ pedogenic smectite δ18O and δD records to characterize the evolution of the hydrologic cycle in Cenozoic western North America. We incorporate 192 samples, 119 of which are previously unpublished, from 11 Cenozoic basins representing a range of environments in the Basin and Range, Rocky Mountains and Great Plains. Our results indicate that the processes controlling smectite isotopic compositions vary both regionally and temporally. In some localities such as Oligocene to Pleistocene western Nebraska, change in temperature is the primary control on smectite isotopic composition. In other basins such as in Miocene Trapper Creek, ID, isotope values lie along the meteoric water line, suggesting change in meteoric water composition is responsible for the variation. In most basins, especially those in the Neogene Basin and Range, smectite line slope suggests either evaporation of previously meteoric water or a combination of change in paleotemperature and meteoric water composition. Smectite geothermometry suggests mineral formation temperatures of 30–40 °C in the Middle Miocene in the Rocky Mountains, Great Plains and Basin and Range, and a decrease of 10–15 °C since the Middle Miocene Climatic Optimum, consistent with clumped isotope and paleofloral temperature estimates

Evaluating the Roles of Rainout and Post-Condensation Processes in a Landfalling Atmospheric River with Stable Isotopes in Precipitation and Water Vapor

Atmospheric rivers (ARs), and frontal systems more broadly, tend to exhibit prominent “V„ shapes in time series of stable isotopes in precipitation. Despite the magnitude and widespread nature of these “V„ shapes, debate persists as to whether these shifts are driven by changes in the degree of rainout, which we determine using the Rayleigh distillation of stable isotopes, or by post-condensation processes such as below-cloud evaporation and equilibrium isotope exchange between hydrometeors and surrounding vapor. Here, we present paired precipitation and water vapor isotope time series records from the 5⁻7 March 2016, AR in Bodega Bay, CA. The stable isotope composition of surface vapor along with independent meteorological constraints such as temperature and relative humidity reveal that rainout and post-condensation processes dominate during different portions of the event. We find that Rayleigh distillation controls during peak AR conditions (with peak rainout of 55%) while post-condensation processes have their greatest effect during periods of decreased precipitation on the margins of the event. These results and analyses inform critical questions regarding the temporal evolution of AR events and the physical processes that control them at local scales

Grassland expansion as an instrument of hydrologic change in Neogene western North America

The evapotranspiration (ET) flux accounts for approximately two thirds of terrestrial precipitation worldwide, and in grassland regions ET is equivalent in magnitude to precipitation. Regional contributions to the terrestrial hydrologic budget, however, have been far from constant in the past as distribution of vegetation changed dramatically. The rise of grass-dominated ecosystems is one of the most profound paleoecological changes in the Cenozoic. Why then, would grassland expansion not feature prominently in the record of Neogene hydrologic change? Despite numerous stable isotope paleoenvironmental studies in Neogene North America, the contributions of land cover change have been largely ignored. We present a compilation of 16 oxygen isotope studies of pedogenic carbonate and smectite from western North America, including 4 new records. Nearly all records from California, the Basin and Range, the Rocky Mountains and the Great Plains show increases in View the MathML source on the order of 2–6‰. In order to assess the role of ET in the hydrologic cycle, we developed an isotopic water vapor transport model wherein we manipulated ET parameters along a specified air mass trajectory. Grasslands lead to View the MathML source of precipitation (View the MathML source) values that are up to 5‰ greater than broadleaf and needleleaf vegetation at inland study sites. These results demonstrate that changes in vegetation played a critical role in establishing the modern hydrologic regime in western North America. We suggest that this isotopic increase is due to three primary reasons: 1) Increased evaporation and transpiration fluxes in grassland regions affect water balance, 2) Shallower rooting depths of grasses lead to the transpiration of soil water enriched in View the MathML source due to evaporation, and 3) Grasslands transpire View the MathML source-rich waters during a shorter, more punctuated growing season. We argue that the observed isotope signals are indicative of a feedback mechanism wherein grasslands not only respond to regional and global climatic trends, but also act as a driver of hydrologic change. By enhancing seasonality and aridity, grasslands transmit hydrologic disturbances downstream, engineering climatic conditions favorable for their expansion