Hari Mix - Academia.edu (original) (raw)

Papers by Hari Mix

Goldschmidt Abstracts, 2020

&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;p&amp;amp;amp;amp;amp;amp;amp... more &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;p&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;We use a global array of ~120 sea-surface temperature (SST) records based on Mg/Ca, alkenone, and faunal proxies to reconstruct global and regional temperature change over the last 5 Myr. All records are placed on the LR04 age model. Here we report the reconstructions and discuss their implications for characterizing global climate evolution (frequency, variance, transitions) over this interval and its relationship to changes in CO2, orbital forcing, and mean ocean temperature. Average global temperature has cooled by ~6.5oC since 5 Ma, with significant breakpoints tentatively identified at ~3.38 Ma, 1.34 Ma, and 0.88 Ma. We also invert the global reconstruction to reconstruct global sea level for the last 5 Myr.&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;/p&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;

Earth and Planetary Science Letters, 2019

stable isotopes Basin and Range Given the intimate links between topography, tectonics, climate, ... more stable isotopes Basin and Range Given the intimate links between topography, tectonics, climate, and biodiversity, considerable effort has been devoted to developing robust climate and elevation histories of orogens. In particular, quantitative geochemical reconstructions using stable oxygen and hydrogen isotopes have been applied to many of the world's mountain belts. Recent advances in atmospheric modeling have suggested that such stable isotope records from leeward sites can be affected by the complicating role that sufficiently elevated topography such as the southern (High) Sierra plays in diverting atmospheric circulation. While such "terrain blocking" effects are a hallmark feature of modern atmospheric circulation in the Sierra, their evolution remains poorly constrained. In order to examine the history of these terrain blocking effects, we developed stable isotope records from three late Cenozoic sedimentary basins in the Eastern Sierra and Basin and Range: 1) Authigenic clay minerals in the Mio-Pliocene Verdi Basin (VB) near present-day Reno, Nevada, 2) Fluvial and lacustrine carbonates from the Plio-Pleistocene Coso Basin (CB) in the southern Owens Valley, and 3) Miocene to Holocene pedogenic, fluvial and lacustrine carbonates of Fish Lake Valley (FLV). Whereas both the VB and CB are proximal to the Sierra crest, FLV is a distal leeward site east of the White and Inyo Mountains in the Basin and Range. The CB oxygen isotope record exhibits an increase of 1-2h over the last 6 Myr while VB and FLV show no significant change. These results suggest that terrain blocking around the southern Sierra initiated prior to the late Cenozoic, though it may have been modestly enhanced during the last 6 Ma.

Atmosphere, 2019

Atmospheric rivers (ARs), and frontal systems more broadly, tend to exhibit prominent “V” shapes ... more 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-...

Atmospheric Chemistry and Physics Discussions, 2018

Ice nucleating particles (INP) have been found to influence the amount, phase, and efficiency of ... more Ice nucleating particles (INP) have been found to influence the amount, phase, and efficiency of precipitation from winter storms, including atmospheric rivers. Warm INP, 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 INP during atmospheric rivers, the role of meteorology in modulating transport and injection of warm INP into atmospheric river clouds and the impact of warm INP on mixed-phase cloud properties are not well-understood. Time-resolved precipitation samples were collected during an atmospheric river in Northern California, USA during winter 2016. Precipitation was collected at two sites, one coastal and one inland, that are separated by less than 35 km. The sites are sufficiently close that airmass sources during this storm were almost identical, but the inland site was exposed to terrestrial sources of warm INP while the coastal site was not. Warm INP were more numerous in precipitation at the inland site by an order of magnitude. Using FLEXPART dispersion modelling 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 INP at either site. We episodically detected warm INP from long-range transported sources at both sites. By extending the FLEXPART modelling using a meteorological reanalysis, we demonstrate that long-range transported warm INP are 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 • C and-10 • C. We thus conclude that terrestrial and long-range transported aerosol were important sources of warm INP during this atmospheric river. Meteorological details such as transport mechanism and cloud structure were important in determining warm INP source strength and injection temperature, and ultimately the impact of warm INP on mixed phase cloud properties.

Earth and Planetary Science Letters, 2018

Although the timing of an acceleration in late-Cenozoic exhumation of southern Alaska is reasonab... more Although the timing of an acceleration in late-Cenozoic exhumation of southern Alaska is reasonably well constrained as beginning ∼5-∼6 Ma, the surface uplift history of this region remains poorly understood. To assess the extent of surface uplift relative to rapid exhumation, we developed a stable isotope record using the hydrogen isotope composition (δD) of paleo-meteoric water over the last ∼7 Ma from interior basins of Alaska and Yukon Territory. Our record, which is derived from authigenic clays (δD clay) in silicic tephras, documents a ∼50-60h increase in δD values from the late Miocene (∼6-∼7 Ma) through the Plio-Pleistocene transition (∼2-∼3 Ma), followed by near-constant values over at least the last ∼2 Ma. Although this enrichment trend is opposite that of a Rayleigh distillation model typically associated with surface uplift, we suggest that it is consistent with indirect effects of surface uplift on interior Alaska, including changes in aridity, moisture source, and seasonality of moisture. We conclude that the δD clay record documents the creation of a topographic barrier and the associated changes to the climate of interior Alaska and Yukon Territory.

GSA Today, 2017

The timing of high surface topography and the corresponding climatic impacts of the many high ran... more 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 δ 18 O values of pedogenic carbonate exhibit a robust 4‰ decrease in the late Neogene-a trend that sharply contrasts with nearly all downwind records of δ 18 O 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 δ 18 O 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

Scientific Reports, 2017

A fast-growing stalagmite from the central California coast provides a high-resolution record of ... more 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...

Global Biogeochemical Cycles, 2014

The late Cenozoic was a time of global cooling, increased aridity, and expansion of grasslands. I... more The late Cenozoic was a time of global cooling, increased aridity, and expansion of grasslands. In the last two decades numerous records of oxygen isotopes have been collected to assess plant ecological changes, understand terrestrial paleoclimate, and to determine the surface history of mountain belts. The δ 18 O values of these records, in general, increase from the mid-Miocene to the Recent. We suggest that these records record an increase in aridity and expansion of grasslands in midlatitude continental regions. We use a nondimensional isotopic vapor transport model coupled with a soil water isotope model to evaluate the role of vapor recycling and transpiration by different plant functional types. This analysis shows that increased vapor recycling associated with grassland expansion along with biomechanistic changes in transpiration by grasses themselves conspires to lower the horizontal gradient in the δ 18 O of atmospheric vapor as an air mass moves into continental interiors. The resulting signal at a given inland site is an increase in δ 18 O of precipitation with the expansion of grasslands and increasing aridity, matching the general observed trend in terrestrial Cenozoic δ 18 O records. There are limits to the isotopic effect that are induced by vapor recycling, which we refer to here as a "hydrostat." In the modern climate, this hydrostatic limit occurs at approximately the boundary between forest and grassland ecosystems.

Geochimica et Cosmochimica Acta, 2014

ABSTRACT The oxygen and hydrogen isotopic composition of soil water (δ18Ow and δDw hereafter) ref... more ABSTRACT 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.

Geological Society of America Bulletin, 2015

Despite broad interest in determining the topographic and climatic histories of mountain ranges, ... more Despite broad interest in determining the topographic and climatic histories of mountain ranges, the evolution of California’s Sierra Nevada remains actively debated. Prior stable isotope–based studies of the Sierra Nevada have relied primarily on hydrogen isotopes in kaolinite, hydrated volcanic glass, and leaf n -alkanes. Here, we reconstruct the temperature and elevation of the early Eocene Sierra Nevada using the oxygen isotope composition of kaolinitized granite clasts from the ancestral Yuba and American Rivers that drained the windward (Pacific) flank of the Sierra Nevada. First, we evaluated the possible contributions of hydrogen isotope exchange in kaolinite by direct comparison with oxygen isotope measurements. Next, we utilized differences in the hydrogen and oxygen isotope fractionation in kaolinite to constrain early Eocene midlatitude weathering temperatures. Oxygen isotope geochemistry of in situ kaolinites indicates upstream (eastward) depletion of 18O in the northern Sierra Nevada. The δ18O values, ranging from 11.4‰ to 14.4‰ at the easternmost localities, correspond to paleoelevations as high as 2400 m when simulating the orographic precipitation of moisture from a Pacific source using Eocene boundary conditions. This result is consistent with prior hydrogen isotope studies of the northern Sierra, but oxygen isotope–based paleoelevation estimates are systematically ~500–1000 m higher than those from hydrogen-based estimates from the same samples. Kaolinite geothermometry from 16 samples produced early Eocene weathering temperatures of 13.0–36.7 °C, with an average of 23.2 ± 6.4 °C (1σ). These kaolinite temperature reconstructions are in general agreement with paleofloral and geochemical constraints from Eocene California localities and climate model simulations. Our results confirm prior hydrogen isotope–based paleoelevation estimates and further substantiate the existence of a hot and high Eocene Sierra Nevada.

American Journal of Science, 2014

Central Asia has become increasingly arid during the Cenozoic, though the mechanisms behind this ... more 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 backtrajectory modeling to understand regional moisture transport pathways at each basin. In all basins, ␦ 13 C increases, with the largest increase in western Mongolia. The first ␦ 13 C increase occurs in central and southwestern Mongolia in the Oligocene. ␦ 13 C 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 ␦ 13 C 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.

The Early Eocene Climatic Optimum (EECO), occurring roughly 52 million years ago, represents an E... more The Early Eocene Climatic Optimum (EECO), occurring roughly 52 million years ago, represents an Earth system response to elevated atmospheric CO2 levels and potentially serves as an analog for the Earth system response to anthropogenic CO2 emissions over the next several centuries. During EECO, global temperatures were 12-15 ºC warmer than modern, with the majority of warming occurring at high latitudes, effectively reducing the Earth’s latitudinal temperature gradient. Despite a wealth of studies, there remain discrepancies between proxy records and global climate models as to the magnitude of this reduction in temperature gradients. In particular, proxy records of isotopes in precipitation appear to indicate shallower temperature gradients than models are able to simulate; however, many of these studies utilize empirical relationships between precipitation isotopes and temperature gradients that likely do not hold under different climate regimes. Here we present 5 new proxy record...

Despite the broad interest in determining the topographic and climatic histories of mountain rang... more Despite the broad interest in determining the topographic and climatic histories of mountain ranges, the evolution of California’s Sierra Nevada remains actively debated. Prior stable isotope-based studies of Sierra Nevada have relied exclusively on hydrogen isotopes in kaolinite, hydrated volcanic glass and leaf n-alkanes. Additional constraints from the oxygen isotope composition of phyllosilicates increase the robustness of findings from a single isotope system and allow for the reconstruction of paleotemperatures. Here, we reconstruct the temperature and elevation of the Early Eocene Sierra Nevada using the oxygen isotope composition of kaolinitized granite clasts from the ancestral Yuba and American Rivers. We evaluate the possible contributions of hydrogen isotope exchange by direct comparison with more robust oxygen isotope measurements. Next, we utilize differences in the hydrogen and oxygen isotope fractionation in kaolinite to constrain paleotemperature. Oxygen isotope geo...

The complex interplay between air mass trajectories and precipitation in mountainous regions resu... more The complex interplay between air mass trajectories and precipitation in mountainous regions results in spatially variable oxygen and hydrogen isotope patterns. This spatial variability is observed in modern precipitation in the western United States where three distinct air masses intersect in the region between the Rocky Mountains and Sierra Nevada. Because of efforts to reconstruct past topographic and climatic histories