The geological record encodes the relationship between climate and atmospheric carbon dioxide (CO 2 ) over long and short timescales, as well as potential drivers of evolutionary transitions. However, reconstructing CO 2 beyond 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 CO 2 record 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 CO 2 thresholds in biological and cryosphere evolution.

AbstractAuthigenic magnesium‐clays have been observed and documented in the Green River Formation, specifically as ooidal stevensite grains. Magnesian clays are a valuable proxy for reconstructing shallow‐water, saline–alkaline lake palaeoenvironments due to their responses to chemical changes. Magnesium‐rich clay minerals are relatively common components in modern and ancient lake systems. A rare interaction of lacustrine magnesian clays, microbialites, carbonates and volcanoclastic deposits of the Eocene Green River Formation crop out in Sanpete Valley, Utah, USA. The characterization of this interaction, Mg‐rich clay and carbonate minerals genesis, the environmental controls on Mg‐rich clay minerals distribution and accumulation is still poorly understood. This study has identified various species of Mg‐clay minerals (stevensite, mixed‐layer kerolite–stevensite and sepiolite), along with calcite and dolomite, found in four Mg‐clay‐bearing lithofacies: (i) Mg‐clay stromatolites; (ii) Mg‐clay arenites; (iii) Mg‐claystone; and (iv) intraclastic Mg‐clay hybrid arenites and conglomerates. Magnesium‐clay bearing rocks from the Sanpete Valley area were deposited in a south‐western, shallow embayment of Eocene Lake Uinta along the margin of the Sevier fold and thrust belt. Sanpete Valley's Mg‐clay‐bearing section shows syngenetic mineralogical paragenesis with neoformation of magnesian clays accompanied by calcite and silica. Regional chronostratigraphic correlation of Sanpete Valley's Mg‐clay‐bearing section with the Uinta Basin was made by zircon U–Pb dating (laser ablation–inductively coupled plasma–mass spectrometry) of two tuff beds. Stevensite and kerolite are precipitated under very specific conditions. This study presents a depositional model, considering evidence from the authigenic mineralogy, facies, fossil evidence and basin context. All of these findings allow a comprehensive analysis of the lithostratigraphic and chemostratigraphic evolution of this isolated bay system, as well as texture classification. This study provides analogue for detailed correlation and comparison to other basins containing similar Mg‐clay and microbialite‐bearing deposits.

Abstract. The Paleocene–Eocene Thermal Maximum (PETM) represents the most pronounced hyperthermal of the Cenozoic era and is hypothesized to have resulted in an intensification of the paleohydrologic cycle, including enhanced seasonality and increased sediment discharge to the coastal ocean. Although the PETM has been widely documented, there are few records from deposits that form the distal, deepwater components of large sediment-routing systems. This study presents new constraints on the stratigraphic placement of the PETM in the deepwater Gulf of Mexico basin through analysis of geochemical, carbon isotopic, and biostratigraphic data within a ∼124 m cored interval of the Wilcox Group. Biostratigraphic and carbon isotopic data indicate that the PETM extends over ∼13 m based on acmes in the dinoflagellate Apectodinium homomorphum and calcareous nannoplankton Rhomboaster cuspis as well as a ∼-2 ‰ shift in bulk organic δ13C values. A decrease in bioturbation and benthic foraminifera suggests that a reduction in oxygen of Gulf of Mexico bottom waters and/or an increase in sedimentation rates were coincident with the onset of the PETM. A ∼2 m lag in the depositional record separates the onset of the PETM negative carbon isotope excursion (CIE) and deposition of a 5.7 m thick interval of organic-lean claystone and marlstone that reflects a shut-off of the supply of sand, silt, and terrestrial palynomorphs to the basin. We interpret deposits of the PETM in the deepwater Gulf of Mexico to reflect the combined effects of increased erosional denudation and rising sea level that resulted in sequestration of sand and silt near the coastline but that allowed delivery of terrigenous mud to the deep sea. The similarity of oceanographic changes observed in the Gulf of Mexico and Atlantic Ocean during the PETM supports the inference that these water masses were connected during latest Paleocene–earliest Eocene times. Although deposition of typical Wilcox Group facies resumed during and after the PETM recovery, an increased influx of terrestrial detritus (i.e., pollen, spores, terrestrial organic debris) relative to marine dinoflagellates is suggestive of long-lasting effects of the PETM. This study illustrates the profound and prolonged effects of climatic warming on even the most distal reaches of large (≥1×106 km2) sediment-routing systems.

AbstractThe Early Cretaceous is an important time of transition in Earth history, marked by a succession of oceanic anoxic events and carbon cycle perturbations that drove changes on land and in the ocean. The need for more precise geochronologic constraints in terrestrial sediments of Early Cretaceous age that record faunal and floral transitions is especially critical. The Cedar Mountain Formation (CMF) is a continental lithostratigraphic unit that hosts a trove of paleoclimate archives and important dinosaurian fossil localities. Determining the timing of deposition of CMF strata has been an ongoing effort for many years. Here, we present new lithostratigraphic and carbon isotope chemostratigraphic data along with high‐precision radiometric ages to further constrain the Ruby Ranch Member of the CMF at a unique locality referred to as “Lake Carpenter,” where a thick section of dominantly lacustrine strata overlies fluvial‐overbank to palustrine strata more typical of other Ruby Ranch Member outcrops. A bentonite bed near the base of the section provides one of the most precise ages yet determined within the Ruby Ranch Member of 115.92 ± 0.14 Ma via CA‐ID‐TIMS U‐Pb analysis of zircons. The age and the trends in the carbon isotope record indicate that the Lake Carpenter sediments were deposited entirely within the late Aptian Stage. These unique new data provide an important step toward improving our understanding of the timing of Early Cretaceous evolutionary and paleoclimate events.

ABSTRACT Investigation of exhumed and well-exposed crustal-scale fault zones provides a rare window into the mechanics and timing of a broad range of deformation mechanisms, strain localization, and fault zone behavior. Here, we apply and integrate geo- and thermochronology analytics to carefully described brittle-ductile structural characteristics of the Catalina detachment zone as exposed in the Rincon Mountains domain of the Catalina-Rincon metamorphic core complex. This core complex is an exhumed extensional, broad-scale-normal-slip shear zone near Tucson, Arizona, USA. The Catalina detachment zone, as formulated here, is partitioned into a brittle-ductile fault-rock stratigraphy that evolved through progressive deformation. The Catalina-Rincon Mountains metamorphic core complex is one of the original type localities of Cordilleran metamorphic core complexes in western North America and has a long history of scientific study to document its structural characteristics and decipher its evolution in the context of Mid-Cenozoic extension. In this Memoir, we seek to provide a thorough accounting of the evolution of this shear zone, through integrating and synthesizing decades of previous research with new mapping, structural data, and geochronological analyses. The Catalina detachment zone stratigraphy is made up of the Catalina detachment fault, cataclasite, chloritic protocataclasite (referred to in most core-complex literature as “chlorite breccia”), subdetachment faults, and mylonites. When it was active, this zone accommodated a minimum of ~36 km of top-to-the-SW displacement. Characterizing the progressive evolution of this metamorphic core complex fault-rock stratigraphy requires a detailed accounting of the kinematic and temporal history of the detachment zone. Consequently, we first characterize and describe each structural unit and feature of this crustal-scale fault and shear zone network through the combination of previously published mapping, structural and microfabric analyses and newly collected structural data, thin-section analysis, large-scale mapping, and reinterpretation of stratigraphic and structural relations in the adjacent Tucson Basin. To improve our broad-scale mapping efforts, we employ multispectral analysis, successfully delineating specific fault-rock stratigraphic units at the core-complex scale. We then establish kinematic and absolute timing constraints by integrating results from well-log and seismic reflection data and with new and previously published zircon U-Pb, 40Ar/39Ar, 40K/40Ar geochronological, (U/Th)/He, 4He/3He, and apatite fission track thermochronological analyses. These temporal constraints indicate a deformation sequence that progressed through mylonitization, cataclasis, mini-detachment faulting, subdetachment faulting, and detachment faulting. This multidisciplinary investigation reveals that mylonitization occurred in late Oligocene time (ca. 26–22 Ma), coeval with rapid exhumation of the lower plate, and that slip on the Catalina detachment fault ceased by early Miocene, ca. 17 Ma. This temporal framework is consistent with results of our subsurface analysis of stratigraphic and structural relations in the Tucson Basin. Onset of metamorphic core complex deformation in southern Arizona slightly preceded that in central and western Arizona and southeasternmost California. Our compiled data sets suggest a shear-zone evolution model that places special emphasis on the transformation of mylonite to chloritic protocataclasite, and strain localization onto subdetachment, minidetachment, and detachment faults over time. Our model envisions mylonites drawn upward through a fluids-sourced brittle-ductile transition zone marked by elevated fluid pressures. This emphasis draws upon seminal work by Jane Selverstone and Gary Axen in analyzing structural-mechanical evolution in the Whipple Mountains metamorphic core complex. Progressive embrittlement and strength-hardening of the lower-plate rocks are manifest in intensive fracturing and minidetachment faulting, favored by the change in rheology produced by alteration-mineral products. Subdetachment faults, localized by earlier-formed ultramylonite and calc-silicate tectonite, coalesce to produce a proto-detachment fault, which marks the interface between mylonite and chlorite protocataclasite. Linking and smoothing of minidetachment faults within chloritic protocataclasite led to emergence of the Catalina detachment fault proper. All of this, from mylonite formation to final slippage on the detachment fault, kinematically conforms to top-to-the-SW shear. The macro-form of the antiformal-synformal corrugations of the Rincon Mountains began developing while mylonites were forming, continuing to amplify during proto-detachment faulting and detachment faulting. We emphasize and describe with examples how the timing and tectonic significance of mylonitization, cataclasis, and detachment faulting within the Catalina-Rincon metamorphic core complex continues to be hotly debated. Disagreements center today, as they have in the past, on the degree to which the structures and fabrics in the Rincons are Laramide products, mid-Cenozoic products, or some combination of both. In addressing tectonic heritage with respect to the Catalina detachment zone, it is hoped that the proposed model of progressive evolution of the Catalina detachment-zone shear zone will inform other studies of active and ancient metamorphic core complexes around the globe. In this regard, some new transferable emphases and methodologies emerged from this work, above and beyond what are now standard operating procedures for understanding crustal shear zones in general, and metamorphic core complexes particularly. For example, remote multispectral image analysis combined with ground-truth field analysis permitted mapping the full extent of chloritic protocataclasite, one of the best exposures of same globally, which is perhaps the most strategic fault rock in exploring the brittle-ductile transition. The added value of complete map control for chloritic protocataclasite is exploring, at its base in other metamorphic core complexes, for the presence of subdetachment faulting, i.e., proto-detachment faulting that influenced localization of detachment zones proper. Another example is the importance of continuously searching for certain mylonite protolith that yields opportunities for closely constraining timing of mylonitization. In our case, it is the Loma Alta mylonite that, more than any other protolith unit in the Rincon Mountains, permitted ‘locking’ the age of mylonitization as late Oligocene. We hope that insights from this detailed study will inform analyses of similar crustal-scale fault zones, both ancient and modern. Given its ready accessibility compared to most metamorphic core complexes, the Rincon Mountains present opportunities for others to use this contribution as part of the basis for exploiting this natural laboratory in research, teaching, and public science.

As Great Salt Lake (GSL) elevation declined below ∼1278.6 m (4195 ft) beginning in the fall of 2019, crystalline mounds formed on the southeastern shore, near Great Salt Lake State Park, Utah. Soon after, several more mound complexes were discovered on nearby Antelope Island. Recent historic low lake levels permitted sulfate-saturated spring brines to emerge on the exposed shoreline. When the spring waters encountered cold winter air, sodium sulfate precipitated in the form of the mineral mirabilite (Na2SO4·10H2O). Mirabilite-saturated spring waters pool in shallow sediment-collapse depressions or, with sufficient hydraulic head, precipitate subaerial crystalline spring terraced mounds. The mounds are composed of clear, bladed, and tabular mirabilite crystals (1 to 10 cm in length; 1 to 5 cm in width) that form mini pools and rimstone dams due to cascading spring water flow. The waters have elavated μM concentrations of sulfide, an oxidation-reduction potential (ORP) of −250 mV, pH of 7, densities of 1.103–1.1169 g/cm3, and temperatures of 10°–15 °C, as compared to nearby lake waters that had undetectable sulfide, an ORP of 12 mV, pH of 8.2, density of 1.0872 g/cm3, and temperatures of 0°–3 °C. Green algae and probable sulfide oxidizing bacteria thrive in the spring waters and some are occluded within the mirabilite matrix and fluid inclusions. Sulfur δ34S and oxygen δ18O isotopes of brine and mirabilite sulfate are enriched +18.8–20.41‰ and + 12.2–14.5‰, respectively, indicating groundwater and mirabilite are in equilibrium for oxygen. Brine sulfide is heavily fractionated (δ 34S of ∼ − 30 ‰) relative to sulfate, indicating active sulfate reduction in subsurface aquifers that feed the spring seeps and which is consistent with higher HS− concentrations relative to adjacent lake waters. The recognition of such biosignatures retained in mirabilite provides further insights for the preservation potential of biological material in extraterrestrial sulfate minerals.Today, Gilbert Bay (south arm of GSL) is undersaturated with respect to mirabilite. However, pre-earthen causeway historical reports (prior to 1959) indicate mirabilite precipitated in the south arm during the winter and through wave action accumulated along the leeward southeastern oolitic shore (similar to current processes occurring in the hypersaline Gunnison Bay, the north arm of GSL). Upon solar irradiance in the spring, the shoreline mirabilite dissolved and reprecipitated in the shallow subsurface (<1 m) as an ooid cement. We hypothesize that this subsurface layer of mirabilite is the source for active lake margin sodium sulfate-saturated spring brines. The emergence and surface exposure of the brines and associated chemical sediments are an additional contributing solute source that may increase GSL salinity during lake level decline more than evaporation alone.

Abstract Various sulfate minerals exist on Mars; except for gypsum, they are understudied on Earth. Extremophiles have been documented in modern gypsum and halite and ancient halite, but other chemical sediments have not been evaluated for biosignatures. Here, we present the first observations and analysis of microorganisms and organic compounds in primary fluid inclusions in the Mars-analog mineral mirabilite, Na2SO4·10H2O, from Great Salt Lake, Utah, USA. Microscopy by transmitted light and ultraviolet-visible (UV-vis) light, and Raman spectroscopy, show abundant bacteria and/or Archaea, algae, fungi, diatoms, protozoa, and organic compounds such as beta-carotene. This discovery expands our current knowledge of biological materials trapped in salt and aids the search for life on Mars, both for sample selection by rover and for analyses of return samples on Earth.

AbstractGreat Salt Lake (GSL), Utah, is a hypersaline terminal lake in the Great Basin, and the remnant of the late glacial Lake Bonneville. Holocene hydroclimate variations cannot be interpreted from the shoreline record, but instead can be investigated by proxies archived in the sediments. GLAD1‐GSL00‐1B was cored in 2000 and recently dated by radiocarbon for the Holocene section with the top 11 m representing ∼7 ka to present. Sediment samples every 30 cm (∼220 years) were studied for the full suite of microbial membrane lipids, including those responsive to temperature and salinity. The Archaeol and Caldarchaeol Ecometric (ACE) index detects the increase in lipids of halophilic archaea, relative to generalists, as salinity increases. We find Holocene ACE values ranged from 81 to 98, which suggests persistent hypersalinity with &lt;50 g/L variability across 7.2 ka. The temperature proxy, MBTʹ5Me, yields values similar to modern mean annual air temperature for months above freezing (MAF = 15.7°C) over the last 5.5 ka. Several glycerol dialkyl glycerol tetraether metrics show a step shift in microbial communities and limnology at 5.5 ka. Extended archaeol detects elevated salinity during the regional mid‐Holocene drought, not readily detected in the ACE record that is often near the upper limit of the index. We infer that the mid‐Holocene GSL was shallower and saltier than the late Holocene. The current drying may be returning the lake to conditions not seen since the mid‐Holocene.

Abstract. The Chaos Canyon landslide, which collapsed on the afternoon of June 28th, 2022 in Rocky Mountain National Park presents an opportunity to evaluate instabilities within alpine regions faced with a warming and dynamic climate. Video documentation of the landslide was captured by several eyewitnesses and motivated a rapid field campaign. Initial estimates put the failure area at 66,630 m2, with an average elevation of 3,555 m above sea level. We undertook an investigation of previous movement of this landslide, measured the volume of material involved, evaluated the potential presence of interstitial ice/snow within the failed deposit, and examined potential climatological forcings at work in causing the collapse of the slope. Satellite radar and optical measurements were used to measure deformation of the landslide in the years leading up to collapse. From 2017 to 2019, the landslide moved ∼5 m yr-1, accelerating to 17 m yr-1 in 2019. Movement took place through both internal deformation and basal sliding. Climate analysis reveals the collapse took place during peak snowmelt, and 2022 followed 10 years of higher than average positive degree day sums. We also made use of slope stability modeling to test what factors controlled the stability of the area. Models indicate even a small increase in the water table reduces the Factor of Safety to &lt;1, leading to failure. Material volumes were estimated using Structure from Motion (SfM) models incorporating photographs from two field expeditions on July 8th, 2022 – 10 days after the slide. Detailed mapping and SfM models indicate ∼ 1,258,000 ± 150,000 m3 of material was deposited at the slide toe and ∼1,340,000 ± 133,000 m3 of material was evacuated from the source area. Our holistic approach to the collapse of the Chaos Canyon landslide provided an opportunity to examine a landslide that may be representative of future dynamic alpine topography, wherein failures becomes more common in a warming climate.

AbstractThe physical processes that facilitate long‐distance translation of large‐volume gravity slides remain poorly understood. To better understand these processes and the controls on runout distance, we conducted an outcrop and microstructural characterization of the Sevier gravity slide across the former land surface and summarize findings of four key sites. The Sevier gravity slide is the oldest of three mega‐scale (&gt;1,000 km2) collapse events of the Marysvale volcanic field (Utah, USA). Field observations of intense deformation, clastic dikes, pseudotachylyte, and consistency of kinematic indicators support the interpretation of rapid emplacement during a single event. Furthermore, clastic dikes and characteristics of the slip zone suggest emplacement involved mobilization and pressurized injection of basal material. Across the runout distance, we observe evidence for progressive slip delocalization along the slide base. This manifests as centimeter‐ to decimeter‐thick cataclastic basal zones and abundant clastic dikes in the north and tens of meters thick basal zones characterized by widespread deformation of both slide blocks and underlying rock near the southern distal end of the gravity slide. Superimposed on this transition are variations in basal zone characteristics and slide geometry arising from interactions between slide blocks during dynamic wear and deposition processes and pre‐existing topography of the former land surface. These observations are synthesized into a conceptual model in which the presence of highly pressurized fluids reduced the frictional resistance to sliding during the emplacement of the Sevier gravity slide, and basal zone evolution controlled the effectiveness of dynamic weakening mechanisms across the former land surface.