Paleosol Carbonates Research Articles

U‐Pb Geochronology of Paleosol Carbonate Cements by LA‐ICP‐MS: A Proof of Concept and Strategy for Dating the Terrestrial Record

AbstractThis study investigates the potential of laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS) U‐Pb dating for carbonate nodules in the Late Triassic Ischigualasto Formation of northwestern Argentina. We establish a fully characterized paragenetic sequence to guide the analysis of three pedogenic carbonates and compare the U‐Pb ages with published geochronology from volcanic ashes within the sedimentary succession. Our findings demonstrate the importance of interpreting U‐Pb data within a well‐defined paragenetic framework for accurate age interpretation of pedogenic carbonates. We observe variations in U‐Pb isotopic signatures across different generations of carbonate precipitates and identify syn‐pedogenic and early burial calcite cements as most suitable for precise dating. Respectively, these two calcite cements are interpreted as microcodium and crack‐lining calcite cements formed early in the paragenetic sequence during pedogenesis to early burial of the paleosols as they transitioned from the unsaturated vadose to saturated phreatic zone below the water table. The U‐Pb ages obtained from the carbonate nodules agree with the radioisotopic ages of volcanic ashes, supporting the validity of our dating strategy. These results contribute to advancing U‐Pb carbonate geochronology and highlight its increased potential for dating sedimentary records in the terrestrial realm. Future research should focus on replicating similar work on different carbonate nodules within the Ischigualasto Fm and expanding the application of LA‐ICP‐MS U‐Pb dating to other carbonate‐bearing formations, especially in successions with limited absolute ages or where volcanic ashes are sparse or absent.

Clumped isotope records of terrestrial temperatures during the Middle Jurassic (180–150 Ma) in East China

The Jurassic is a crucial greenhouse period that witnessed a transition from the relatively colder Triassic to the relatively warmer Cretaceous. However, previous studies based on marine sections suggest different long-term paleotemperature trends throughout the Jurassic. Oxygen isotope (ẟ18O) values of temperate belemnite and bivalve fossils suggest a general cooling followed by a general warming trend during the Jurassic, whereas TEX86H and clumped isotope sea surface temperature records available for parts of this period appear consistent with a relatively warm and stable climate. The paucity of Middle Jurassic paleotemperature estimates limits efforts to investigate the above disagreement. Here, we apply clumped isotope (Δ47) thermometry to paleosol carbonates from the central part of East China to reconstruct terrestrial paleotemperatures and paleoatmospheric CO2 (pCO2) in the Middle Jurassic. After evaluating potential burial diagenesis and seasonality of soil carbonate accumulation, we suggest that terrestrial mean annual temperatures ranged from 30 ± 4 °C to 34 ± 4 °C and the estimated mean pCO2 was 1219 ± 519 ppmv in mid-latitude East China during the Middle Jurassic. Based on the compiled marine temperatures and our results, we conclude that the paleoclimate was relatively warm and stable from the Middle Jurassic to at least the Earliest Cretaceous (ca. 180–135 Ma). Our estimated Middle Jurassic pCO2 aligns with previous records, revealing minimal variations compared to the pCO2 observed during the Early Cretaceous. These findings imply a link between the consistent temperatures and the relatively stable pCO2 levels from the Middle Jurassic through the Early Cretaceous.

Differential paleoelevation changes in North China during the late Mesozoic: Evidence from stable isotopes and clumped isotopes

The modern elevation of China decreases from the Tibetan plateau in the west to near sea level in the east across several topographic steps. A pronounced topographic step occurs in North China (NC), separating the plateaus and high mountains (elevations generally >1000 m) that dominate western NC from the plains and low mountain landscapes (elevations generally <500 m) of eastern NC. The topographic step in NC shapes modern climate and vegetation gradients and may have affected paleoenvironments and ecosystems in deep time. However, few studies have been conducted to explore paleoenvironmental and paleoelevation changes in NC, and thus the timing of its establishment is unclear. In this study, we use δ18O, δ13C and clumped isotope values of Middle Jurassic and Early Cretaceous paleosol carbonates from representative basins in western and eastern NC to constrain paleoenvironmental and paleoelevation changes during the late Mesozoic. Our δ13C and clumped isotope results suggest that the Ordos Basin of western NC maintained high paleoelevation (>1.5 km) during the Middle Jurassic-late Early Cretaceous, while the Hefei Basin of eastern NC experienced significant elevation reduction by the Early Cretaceous. Combined with paleogeographic, geophysical, and paleoecological evidence, these findings suggest that the lowering of eastern NC may have formed a topographic step across NC by Aptian time. This topographic change may have affected paleoclimate patterns and caused a climatic transition from dry to humid in eastern NC during this period. Broader topographic and environmental change may have occurred in Eastern China during the Early Cretaceous.

Mixed models elucidate local- and regional-scale drivers of paleoenvironmental change in eastern Africa during the emergence of Paranthropus and Homo

Climate and environmental change are considered important drivers of hominin speciation and diversification, including the emergence of Paranthropus and Homo. Therefore, making connections between potential drivers of hominin evolution on multiple spatial scales is a critical issue in hominin paleoecology. This is especially true in light of evidence that paleoenvironmental proxies operating at different spatial scales lead to divergent interpretations about whether environmental change in eastern Africa was primarily driven by regional-scale or local-scale factors. This study takes a novel approach to addressing issues of scale in hominin paleoecology by using linear mixed models to test the relative impact of local factors versus regional factors in driving variation in aridity and vegetation at important hominin paleontological sites across eastern Africa between 3.7 Ma and 2.3 Ma. Results indicate there are no significant trends in aridity as evidenced by stable oxygen isotopes of paleosol carbonates. However, the results for stable carbon isotopes of paleosol carbonates suggest regional-scale factors drove a uniform increase in grassy vegetation at nearly all sites. Additionally, we find that while regional-scale factors drive vegetation change, local-scale factors were important in determining the unique mix of trees/shrubs and grass that was present at each site and upon which vegetation change acted. Thus, while sites across eastern Africa experienced the same degree of vegetation change, at any given time the vegetation at each site would have differed from other sites. This result offers a possible resolution between regional-scale proxies which show an overall increase in C4 photosynthesizing grasses and local-scale proxies which indicate that paleoenvironments at different sites were varied. It also connects regional-scale drivers to local-scale environmental change which would have had an important impact on hominin evolution and adaptation. This study introduces linear mixed models as a methodology which to date has not been widely applied to questions of hominin paleoecology and has the potential to address critical issues of scale.

CO2-forced Late Miocene cooling and ecosystem reorganizations in East Asia

In parallel with pronounced cooling in the oceans, vast areas of the continents experienced enhanced aridification and restructuring of vegetation and animal communities during the Late Miocene. Debate continues over whether pCO2-induced global cooling was the primary driver of this climate and ecosystem upheaval on land. Here we present an 8 to 5 Ma land surface temperatures (LST) record from East Asia derived from paleosol carbonate clumped isotopes and integrated with climate model simulations. The LST cooled by ~7 °C between 7.5 and 5.7 Ma, followed by rapid warming across the Miocene-Pliocene transition (5.5 to 5 Ma). These changes occurred synchronously with variations in alkenone and Mg/Ca-based sea surface temperatures and with hydroclimate and ecosystem shifts in East Asia, highlighting a global climate forcing mechanism. Our modeling experiments additionally demonstrate that pCO2-forced cooling would have altered moisture transfer and pathways and driven extensive aridification in East Asia. We, thus, conclude that the East Asian hydroclimate and ecosystem shift was primarily controlled by pCO2-forced global cooling between 8 and 5 Ma.

Multiproxy Paleosol Evidence for Jurassic Paleoclimate Fluctuations in the Sichuan Basin, SW China

AbstractPaleoclimate reconstruction of continental environments has been hampered by the limited evidence. A thick sequence of Jurassic continental deposits in the Sichuan Basin, Southwest China yields abundant paleosols that may offer valuable insights regarding Jurassic climate scenarios. A succession of 169 paleosols belonging to Protosols, Calcisols, and Argillisols from 23 detailed stratigraphic sections was recognized and characterized macro‐ and micromorphologically and assessed for mineral and geochemical compositions. Quantitative paleoclimate reconstructions using bulk geochemical proxies, the depth to and the stable oxygen isotopic composition of paleosol carbonates indicated a predominant alternation of semiarid and arid cool/warm‐temperate climatic conditions punctuated by several episodes of subhumid and humid climates that generally prevailed in the Sichuan Basin during the Jurassic. The estimated paleoatmospheric CO2 concentrations (pCO2) from calcic paleosols yielded a low range of ∼104 ± 58 to ∼610 ± 152 ppmv during the Middle Jurassic. The terrestrial paleotemperature changes in the Sichuan Basin coincided with the pCO2 variations, which probably resulted from global geological events (e.g., volcanic activities, magmatic and oceanic events, and the ephemeral caps development) in the Middle Jurassic. Jurassic climatic fluctuations in the basin were likely attributed to true polar wander due to global plate motion, megamonsoon effect linked to global and regional paleogeography, and regional paleotopography.

Late Mesozoic elevation history of the north Taihang Mountains, China: Constraints from clumped isotope geochemistry

A series of tectonic events during the late Mesozoic, including the Yanshanian Orogeny and the Early Cretaceous tectonic extension, reshaped the tectonic regime and landscape of North China (NC). During this period, the Taihang Mountains (THM) became a prominent geomorphic and ecological separation zone, which separates NC into regions with different landforms, tectonic settings, and climatic regimes. The uplift history of the THM during the late Mesozoic is of great interest, since it is critical to constrain the geodynamic and regional climatic evolution of NC. In this study, we report the first carbonate clumped isotope thermometry (Δ47) data for paleosol carbonates from this region to constrain the paleoelevation of the north THM during the late Mesozoic. Our result indicates that the north THM was almost certainly 2.0 ± 0.8 km elevation at ca. 160 Ma associated with crust thickening and shortening. After the collapse of the eastern North China Craton (NCC), the north THM maintained high elevation at ca. 113 Ma (2.7 ± 0.7 km). The findings suggest that the present topographic features of NC may have been established in the Early Cretaceous. Finally, our study suggests elevation decrease at the end of the Early Cretaceous (ca. 110−100 Ma), which is possibly related to isostatic adjustment caused by asthenosphere upwelling and partial crustal thinning in the central NCC.

Terrestrial carbon isotope stratigraphy and mammal turnover during post-PETM hyperthermals in the Bighorn Basin, Wyoming, USA

Abstract. Paleogene hyperthermals, including the Paleocene–Eocene Thermal Maximum (PETM) and several other smaller events, represent global perturbations to Earth's climate system and are characterized by warmer temperatures, changes in floral and faunal communities, and hydrologic changes. These events are identified in the geologic record globally by negative carbon isotope excursions (CIEs), resulting from the input of isotopically light carbon into Earth's atmosphere. Much about the causes and effects of hyperthermals remains uncertain, including whether all hyperthermals were caused by the same underlying processes, how biotic effects scale with the magnitude of hyperthermals, and why CIEs are larger in paleosol carbonates relative to marine records. Resolving these questions is crucial for a full understanding of the causes of hyperthermals and their application to future climate scenarios. The primary purpose of this study was to identify early Eocene hyperthermals in the Fifteenmile Creek area of the south-central Bighorn Basin, Wyoming, USA. This area preserves a sequence of fluvial floodplain sedimentary rocks containing paleosol carbonates and an extensive record of fossil mammals. Previous analysis of faunal assemblages in this area revealed two pulses of mammal turnover and changes in diversity interpreted to correlate with the ETM2 and H2 hyperthermals that follow the PETM. This was, however, based on long-distance correlation of the fossil record in this area with chemostratigraphic records from elsewhere in the basin. We present new carbon isotope stratigraphies using micrite δ13C values from paleosol carbonate nodules preserved in and between richly fossiliferous mammal localities at Fifteenmile Creek to identify the stratigraphic positions of ETM2 and H2. Carbon isotope results show that the ETM2 and H2 hyperthermals, and possibly the subsequent I1 hyperthermal, are recorded at Fifteenmile Creek. ETM2 and H2 overlap with the two previously recognized pulses of mammal turnover. The CIEs for these hyperthermals are also somewhat smaller in magnitude than in more northerly Bighorn Basin records. We suggest that basin-wide differences in soil moisture and/or vegetation could contribute to variable CIE amplitudes in this and other terrestrial records.

Paleoatmospheric CO2 oscillations through a cool middle/Late Cretaceous recorded from pedogenic carbonates in Africa

This study presents the first African pedogenic carbonate estimated atmospheric CO2 concentrations for the Early to Late Cretaceous transition. Pedogenic carbonates have been used extensively as proxies to reconstruct past climates, yet few studies have focused on the climatic conditions of Africa during the Mesozoic. The paucity of paleoclimate data from the region represents a significant obstacle in interpreting past climate change, and thus our ability to predict future changes. Paleosol carbonate nodules were sampled from Bk horizons of seven particularly well-exposed and well-characterized paleosols through the Cretaceous Galula Formation, including four from the lower Mtuka Member (Aptian–Cenomanian) and three from the upper Namba Member (Campanian). The Cretaceous Galula Formation is a fossiliferous continental sedimentary succession of braided fluvial deposits that are well exposed in recently incised river drainages of the Rukwa Rift Basin, Tanzania. Oxygen isotope values averaged −5.6‰ and −7.2‰ VPDB for the Mtuka and Namba members, equating to mean annual temperatures of 14.5 °C and 11.2 °C, respectively. Using the stable isotope composition of pedogenic carbonates from the Mtuka Member, an average pCO2 of 1860 ppmv was estimated for the Aptian–Cenomanian, before declining to an average of 520 ppmv in the Namba Member, during the Campanian. Atmospheric pCO2 values fluctuated, but remained above 1000 ppmv through the middle Cretaceous, and correspond to the “cool” greenhouse period that spanned the Aptian–Albian. The gradual decline in pCO2 (550–502 ppmv) recorded in the Namba Member paleosols occurred during Late Cretaceous cooling following the Cretaceous Thermal Maximum.

Clumped isotope constraints on warming and precipitation seasonality in Mongolia following Altai uplift

The timing of surface uplift of the Altai Mountains in northern Central Asia—and the climatic consequences—remains controversial. Today, the Altai Mountains cast a substantial rain shadow, effectively separating the western Gobi Desert and steppe from the Siberian Taiga. We take advantage of this stark climatic gradient to trace the interaction of climate and topography in the lee of the Altai. First, we present new water stable isotope data that demonstrate that—along with this climatic gradient—the Altai modify the δ¹⁸O of precipitation via rainout on the leeward side of the range. Second, we present a new paleosol carbonate clumped isotope (Δ₄₇) record that spans much of the Neogene from the immediate lee of the Altai in western Mongolia to address how surface temperatures may have responded to potential uplift during the Neogene. We find that Δ₄₇-derived temperatures have, overall, declined by approximately 7 °C over the course of the Neogene, though the precise timing of this decrease remains uncertain. Third, we pair our Δ₄₇ record with previously published stable isotope data to demonstrate that the timing of decreasing temperatures corresponds with long-term stability in paleosol carbonate δ¹³C values. In contrast, increases in paleosol carbonate δ¹³C values—linked to declining vegetation productivity—are correlated with intervals of increasing temperatures. We speculate that declines in vegetation biomass and leaf area changed the partitioning of latent and sensible heat, resulting in rising surface temperatures during Altai uplift. In contrast, long-term Neogene cooling drove the overall decline in surface temperatures. Reconstructed soil water δ¹⁸O values (based on carbonate δ¹⁸O and Δ₄₇ values) remain surprisingly stable over our Neogene record, differing from our expectation of decreasing δ¹⁸O values due to progressive uplift of the Altai Mountains and Neogene cooling. We demonstrate that the shift in precipitation seasonality that likely accompanied Altai uplift obscured any change in lee-side precipitation δ¹⁸O that would be expected from surface elevation change alone.

Regional Patterns in Miocene‐Pliocene Aridity Across the Chinese Loess Plateau Revealed by High Resolution Records of Paleosol Carbonate and Occluded Organic Matter

AbstractPaleosols preserved in the Red Clay depositional sequence of the Chinese Loess Plateau record information about vegetation and regional hydrology responses to global temperature variation throughout the late Miocene and Pliocene. Reconstructing spatial and temporal patterns of environmental change across the Loess Plateau from carbon isotopes of pedogenic carbonate (δ13Ccarb) is complicated because multiple factors affect δ13Ccarb values and higher resolution records do not exist along the northern margin of the Loess Plateau. To address these needs, we present paired carbon isotope records of pedogenic carbonate and occluded organic matter (δ13Corg) from 697 discrete nodules sampled from 119 different depths at the Jiaxian section, North Central China. Between 7.6 and 2.4 Ma, δ13Ccarb values increase by nearly 5‰, while δ13Corg values increase by 2.5‰. These increases are explained by a progressive decline in moisture availability through time, and there is no definitive evidence from these δ13C data for C4 vegetation at the Jiaxian site until after 3.6 Ma. Comparison of the Jiaxian record to other Loess Plateau sections reveals a consistent spatial gradient with δ13Ccarb values becoming higher and more variable to the N‐NW. Additionally, an independent index of monsoonal precipitation from a southern site corresponds to fluctuations in δ13Ccarb values at Jiaxian, while southern δ13Ccarb records remain more stable. These spatial patterns are explained by a progressive decline in moisture availability across the Loess Plateau through the Late Miocene and Pliocene, with δ13Ccarb values being more sensitive to moisture availability under consistently more arid conditions to the NW.

Verification of regional-to-global scale environmental factors in paleosol stable carbon isotope ratios through the lower Permian succession of north-central Texas, U.S.A.

Upper Paleozoic (~300 Ma) rocks in the Midland Basin of Texas (USA) record a transition from icehouse (glaciated) to greenhouse (non-glaciated) conditions in low latitudes, western Pangea. This work revisits the stratigraphic interval that records stable carbon isotopic compositions of paleosol materials from previous studies and uses newly collected paleosol samples to test the reproducibility of paleosol stable carbon isotopic values, and assesses paleoclimate and paleoatmospheric pCO 2 changes across the global climate transition. Thirty upper Wolfcampian (Nacona Formation) to upper Leonardian (Clear Fork Group) paleosol profiles were measured and described in detail. Detailed petrographic analysis of paleosol carbonate nodules was used to identify the best representatives of pristine pedogenic microcrystalline calcite cements and separate them from later diagenetic or altered calcite cements. The best candidates for pedogenic micritic calcite cement were drilled, and organic-rich fractions of acid-treated residues co-existing within carbonate nodules were collected. The δ 13 C values of 23 paired, pristine pedogenic micritic calcite samples (δ 13 C cc ) and associated, occluded organic matter from carbonate nodules and calcareous rhizoliths (δ 13 C om ) range from −6.5‰ to −3.3‰ and −25.3‰ to −20.5‰, respectively. The stratigraphic trends in paleopedogenic micritic calcite δ 13 C cc values are essentially no different from results published in previous studies, which supports the reproducibility of paleosol carbonate as a reliable proxy for the carbon isotopic composition that reflects the regional-to-global paleoclimatic and paleoenvironmental trends. However, δ 13 C om values measured here are consistently more negative than previously reported for fossilized plant compressions preserved within strata associated with these paleosol profiles ( Montañez et al., 2007 ). We suggest that the coexisting organic matter from acid-treated residues in paleosol carbonate is a better material to assess original soil δ 13 C om values that was contributing toward soil CO 2 than the fossilized components of above-ground, photosynthesizing vascular plant parts as was utilized in previous studies. The isotopic differences between the paleosol micritic calcite δ 13 C cc values and associated organic-rich-residue δ 13 C om values were used in conjunction with a two-component CO 2 mixing model to yield atmospheric pCO 2 estimates. The estimates are different and with opposite excursions, ranging from 1.7 to 2.7 times pre-industrial atmospheric levels (PIAL) and from 2.3 to 4.2 times of PIAL, when applying the relationships of modern soil pCO 2 (S z )-mean annual precipitation (MAP) and Sz-pedotype, respectively, to assess the S z of paleosols. The different approaches result in different assumed S z of paleosols at the time of calcite crystallization and lead to lower estimates of atmospheric pCO 2 than previously reported. These results suggest that the icehouse-to-greenhouse climate transition was associated with much lower pCO 2 thresholds than previously thought. • The carbon isotope compositions of pedogenic calcite from microsamples are repeatable. • The carbon isotope compositions of pedogenic calcite can record regional processes. • Only limited increase in atmospheric pCO 2 across icehouse-to-greenhouse transition.

Aptian–Albian clumped isotopes from northwest China: cool temperatures, variable atmospheric &amp;lt;i&amp;gt;p&amp;lt;/i&amp;gt;CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and regional shifts in the hydrologic cycle

Abstract. The Early Cretaceous is characterized by warm background temperatures (i.e., greenhouse climate) and carbon cycle perturbations that are often marked by ocean anoxic events (OAEs) and associated shifts in the hydrologic cycle. Higher-resolution records of terrestrial and marine δ13C and δ18O (both carbonates and organics) suggest climate shifts during the Aptian–Albian, including a warm period associated with OAE 1a in the early Aptian and a subsequent “cold snap” near the Aptian–Albian boundary prior to the Kilian and OAE 1b. Understanding the continental system is an important factor in determining the triggers and feedbacks to these events. Here, we present new paleosol carbonate stable isotopic (δ13C, δ18O and Δ47) and CALMAG weathering parameter results from the Xiagou and Zhonggou formations (part of the Xinminpu Group in the Yujingzi Basin of NW China) spanning the Aptian–Albian. Published mean annual air temperature (MAAT) records of the Barremian–Albian from Asia are relatively cool with respect to the Early Cretaceous. However, these records are largely based on coupled δ18O measurements of dinosaur apatite phosphate (δ18Op) and carbonate (δ18Ocarb) and therefore rely on estimates of meteoric water δ18O (δ18Omw) from δ18Op. Significant shifts in the hydrologic cycle likely influenced δ18Omw in the region, complicating these MAAT estimates. Thus, temperature records independent of δ18Omw (e.g., clumped isotopes or Δ47) are desirable and required to confirm temperatures estimated with δ18Op and δ18Oc and to reliably determine regional shifts in δ18Omw. Primary carbonate material was identified using traditional petrography, cathodoluminescence inspection, and δ13C and δ18O subsampling. Our preliminary Δ47-based temperature reconstructions (record mean of 14.9 ∘C), which we interpret as likely being representative of MAAT, match prior estimates from similar paleolatitudes of Asian MAAT (average ∼ 15 ∘C) across the Aptian–Albian. This, supported by our estimated mean atmospheric paleo-pCO2 concentration of 396 ppmv, indicates relatively cooler midlatitude terrestrial climate. Additionally, our coupled δ18O and Δ47 records suggest shifts in the regional hydrologic cycle (i.e., ΔMAP, mean annual precipitation, and Δδ18Omw) that may track Aptian–Albian climate perturbations (i.e., a drying of Asian continental climate associated with the cool interval).

High‐Altitude and Cold Habitat for the Early Cretaceous Feathered Dinosaurs at Sihetun, Western Liaoning, China

AbstractDuring the Early Cretaceous, significant tectonic regime transformation occurred in East China. Meanwhile, the Jehol Biota, which contains world‐famous feathered dinosaurs, flourished. However, few studies have been conducted to investigate the influence of tectonic regime transformation on regional topography and climates and the possible climatic causes of the unique characteristics of these dinosaurs. Here, we address these issues by applying clumped isotope paleothermometry to Early Cretaceous carbonates in NE China. In the Sihetun area, the mean annual paleotemperature derived from paleosol carbonates was 5.9 ± 1.7°C, and the paleoelevation was 2.8–4.1 km during the Early Cretaceous. The topographic evolution was closely related to the subduction of the paleo‐Pacific plate beneath East Asia during this period. Our results suggest a high altitude and cold habitat with frozen winters for the Jehol Biota in the Sihetun area, which implies possible climate‐Keywords:influenced evolution of the feathered characteristic of the dinosaurs.

Warm High‐Elevation Mid‐Latitudes During the Miocene Climatic Optimum: Paleosol Clumped Isotope Temperatures From the Northern Rocky Mountains, USA

AbstractInterrupting a long‐term Cenozoic cooling trend, the Miocene Climatic Optimum (MCO; ca. 17–15 Ma) represents a time interval characterized globally by warmer than present temperatures, lower ice volume, and elevated pCO2 levels. Establishing quantitative Neogene temperature estimates is an important element in the effort to explore the long‐term changes in the carbon cycle and associated climate feedbacks, yet terrestrial temperature records are still sparse. Here, we present a clumped isotope (Δ47) temperature record of the MCO from intermontane basins in the Northern Rocky Mountain (NRM) region. Arikareean (22.7–21.5 Ma) to Barstovian (16.9–14.7 Ma) paleosol carbonates from the Hepburn's Mesa Formation (Montana), supplemented with data from fossil localities in western Idaho. These records yield Δ47‐temperatures ranging from 17°C to 24°C, which are rather warm given the high elevation sites and are further relatively stable (mean of 21 ± 2°C) leading into and during the MCO until ca. 14.7 Ma. At ca. 14.7 Ma, we observe low Δ47‐temperatures (8°C–10°C) concomitantly with elevated Δ47‐temperatures (ca. 22°C). In line with recently suggested climate stability in the NRM region leading into the MCO, our Δ47‐temperature record, combined with carbon isotope (δ13C) and reconstructed soil water oxygen isotope (δ18Osw) values, indicates rather stable climate and environmental conditions throughout the MCO. Combining available records from inland sites in the western United States (NRM, Mojave region) points to prevailing stable continental climates even during the MCO.

Triple oxygen and clumped isotopes in modern soil carbonate along an aridity gradient in the Serengeti, Tanzania

The isotopic composition of paleosol carbonates is used extensively to reconstruct past vegetation, climate, and altimetry, but poor constraints on soil evaporation and temperature have limited the utility of oxygen isotopes in the studies. Recent advances in carbonate clumped isotope thermometry (TΔ47) allow for independent controls on temperature, but the influence of evaporation remains unresolved. However, the sensitivity of 18O-17O-16O distributions to kinetic fractionation makes it possible to use triple oxygen isotopes (Δ′17O) to track evaporation in water. Recent work shows the sensitivity of Δ′17O to evaporation in lakes and lacustrine carbonates, but little is known about variation of Δ′17O in soil carbonates and their potential to track evaporation. For this study, we sampled soils across an aridity gradient in the Serengeti, Tanzania to evaluate how soil carbonate Δ′17O tracks soil water evaporation. Modern soil carbonates were collected from 11 sites across a transect of the Serengeti Ecosystem where mean annual precipitation and aridity index range from 499 to 846 mm yr−1 and 0.33 to 0.55, respectively. δ13C values range from −2.7 to 1.8‰ and reflect C4 dominated grasslands, whereas δ18O values of soil carbonates vary by ∼8‰ along a gradient in aridity. TΔ47 from these soil carbonates average 23°C (1σ± 4°C), which does not vary significantly across sites or with depth, likely due to minimal annual variation in temperature at the equator. Using these temperatures for each carbonate, reconstructed δ18O values of soil water are up to 6‰ higher than δ18O values of local precipitation and springs, indicating considerable soil water evaporation. The Δ′17O values of these soil carbonates range from −162 to −106 per meg and decrease as both aridity and δ18O values increase. Our results support the hypothesis that soil water evaporation drives the variance in δ18O and Δ′17O of soil carbonate in arid climates, demonstrating the potential for soil carbonate Δ′17O to track paleoaridity and constrain interpretations of paleosol carbonate δ18O records.

Molecules to Mountains: A Multi-Proxy Investigation Into Ancient Climate and Topography of the Pacific Northwest, USA

We characterize the topographic evolution of the Pacific Northwest, United States, during the Cenozoic. New paleosol carbonate stable isotope (δ18O) results from central Oregon are presented, along with published proxy data, including fossil teeth, smectites, and carbonate concretions. We interpret a polygenetic history of Cascade Mountain topographic uplift along-strike, characterized by: 1) Steady uplift of the Washington Cascades through the Cenozoic due long-term arc rotation and shortening against a Canadian buttress, and 2) Uplift of the Oregon Cascades to similar-to-modern elevations by the late Oligocene, followed by topographic stagnation as extension developed into the Neogene. Since the Miocene, meteoric water δ18O values have decreased in Oregon, possibly due to emergence of the Coast Range and westward migration of the coastline. Spatial variability in isotopic change throughout the Pacific Northwest suggests that secular global climate change is not the primary forcing mechanism behind isotopic trends, though Milankovitch cycles may be partly responsible for relatively short-term variation.

Clay mineralogical evidence for mid-latitude terrestrial climate change from the latest Cretaceous through the earliest Paleogene in the Songliao Basin, NE China

From the latest Cretaceous (late Campanian to Maastrichtian, ~76-66 Ma) through the earliest Paleogene, changes of greenhouse climate were linked to catastrophic geological events and massive biotic extinction and were primarily derived from marine records. Here we present a high-resolution, tightly age-constrained, clay mineralogical record from the Sifangtai and Mingshui formations of the terrestrial Songliao Basin, northeast China. Smectite and illite are the dominant clay species and are derived from the weathering of parent rocks and/or pedogenesis in paleosols. We use the percentage ratio of smectite and illite, the illite chemistry index, and the percentage ratio of phyllosilicate clay minerals and quartz in clay fractions for paleoclimatic reconstruction. Our results show that from the latest Cretaceous through the earliest Paleogene, values of all three clay mineralogical proxies in the Songliao Basin are generally higher during warming intervals than those during cooling intervals. This dataset suggests that warming caused strengthened moisture delivery from the Pacific, increasing precipitation and intensified chemical weathering, whereas cooling was accompanied by increasing dryness and physical weathering. Across the Cretaceous-Paleogene boundary, the warming likely related to Deccan volcanism, the transient cooling afterwards, and the warming in the earliest Paleogene are characterized by changes in the illite chemistry index and paleosol carbonate stable isotopic excursions as shown in previously published records, reflecting fluctuations in hydroclimate and weathering intensity. Our work demonstrates that terrestrial climate and weathering intensity in the mid-latitude Songliao Basin fluctuated during the latest Cretaceous through the earliest Paleogene and sensitively responded to global climate changes.

Late Paleogene paleotopographic evolution of the northern Cordilleran orogenic front: Implications for demise of the orogen

The paleotopographic history of the North American Cordilleran orogen holds the key to understanding mechanisms of orogenesis and subsequent orogenic collapse. It has been suggested that the orogenic front in western Montana (USA) and Alberta (Canada) was more than 4 km high during Late Cretaceous−early Eocene contractional deformation and during the initial phase of extension in the middle Eocene; however, the late Eocene−Oligocene topographic evolution during continued extensional collapse remains poorly constrained. Here we extend the paleotopographic record in the Kishenehn Basin in northwestern Montana and southeastern British Columbia (Canada) to the late Oligocene by studying δ18O values of fossil mollusks and cement and paleosol carbonates. The molluscan taxa changed from three sympatric groups with preferred habitats ranging from tropical wet, semi-arid subtropical, and temperate during the middle and late Eocene, to mainly a single group associated with temperate environment during the Oligocene, reflecting a decline in molluscan biodiversity induced by climate cooling across the Eocene−Oligocene transition. Reconstructed δ18O values of alpine snowmelt and basinal precipitation decreased by 1.4‰ and 3.8‰, respectively, from the middle to late Eocene, reflecting climate cooling and ∼1 km surface uplift of the basin floor. The reconstructed alpine snowmelt δ18O values then increased by 2.9‰ in the Oligocene suggesting a ∼0.5 km drop in elevation of the orogenic front. Collectively, the results of our new and previously published δ18O data chronicle the paleotopographic response to the change from flat-slab subduction to slab rollback over a 45 m.y. period. These data suggest that the orogenic front was characterized by high elevation (&amp;gt;4 km) in the ancestral Lewis-Clark-Livingston ranges during latest Cretaceous−early Eocene (ca. 75−52 Ma) contraction. The initial phase of extension related to the Kishenehn Basin created a lowland basin with a surface elevation of only ∼1.5 km during the early middle Eocene (ca. 46−44 Ma) whereas the ranges remained &amp;gt;4 km high. The high range elevations were sustained for at least 12 m.y. in the middle to late Eocene concurrent with extension, while the basin floor elevation was uplifted to ∼2.5 km by the latest Eocene (ca. 36−34 Ma). Basin aggradation can explain at most half of the 1 km basin floor uplift. The remaining amount (at least 0.5 km) and sustained high range elevation suggest that range denudation and crustal extension was compensated by the isostatic and thermal effects of slab rollback and/or passage of a slab window and infusion of hot asthenosphere beneath the continent. The range elevation in the orogenic front decreased ∼0.5 km by the late Oligocene (ca. 28 Ma), associated with a decrease in rock uplift rate associated with extension. A post-Oligocene elevation drop of ∼1 km resulted in both the ranges and basin floor reaching modern topography in the Kishenehn Basin drainage, likely due to the regional effect of Neogene Basin and Range extension. This study, along with the previous investigation of the Kishenehn Basin by Fan et al. (2017), are the first studies that systematically investigate paleorelief of the orogenic belt by reconstructing paleoelevations of the mountains and the basin at the same time. The results highlight that the Cordilleran orogenic front of northern Montana and southern British Columbia sustained its high elevation edifice for at least 12 m.y. after the start of extension. We suggest that initial crustal extension did not result in orogenic demise because of concurrent thermal and isostatic uplift.

Late Pennsylvanian Calcareous Paleosols from Central New Mexico: Implications for Paleoclimate

We document calcareous paleosols from Upper Pennsylvanian (lower Virgilian) strata of the Burrego Member of the Atrasado Formation in the Cerros de Amado of Socorro County, New Mexico. The Burrego paleosols are an excellent example of a scarce, climate-sensitive lithology in the Pennsylvanian strata of New Mexico. These paleosols contain mostly stage II to III carbonate horizons, and their overall morphology suggests deposition and pedogenesis under subhumid, seasonally dry conditions. This conclusion is consistent with paleobotanical and other data that indicate such climate conditions were widespread on Late Pennsylvanian Pangea. The mean value of the oxygen-isotope ratios from Burrego paleosol carbonates compares well with the values from Virgilian paleosols of the San Juan, the eastern Midland and Chama basins of New Mexico-Texas, suggesting similar conditions of temperature and paleoprecipitation. Application of the diffusion-reaction model to the mean carbon-isotope composition of the carbonate suggests a paleo-pCO2 of approximately 400 ppmV, which is also consistent with estimates from correlative carbonate deposits that formed farther east in Late Pennsylvanian Pangea.