Well-dated lacustrine records are essential to establish the timing and drivers of regional hydroclimate change. Searles Basin, California records the depositional history of a fluctuating saline-alkaline lake in the terminal basin of the Owens River system draining the eastern Sierra Nevada. Here we establish a U-Th chronology for the ~76-m-long SLAPP-SLRS17 core collected in 2017 based on dating of evaporite minerals. 98 dated samples comprising 9 different minerals were evaluated based on stratigraphic, mineralogic, textural, chemical and reproducibility criteria. After application of these criteria, a total of 37 dated samples remained as constraints for the age model. A lack of dateable minerals between 145-110 ka left the age model unconstrained over the penultimate glacial termination (Termination II). We thus established a tie point between plant wax δD values in the core and a nearby speleothem δ18O record at the beginning of the Last Interglacial. We construct a Bayesian age model allowing stratigraphy to inform sedimentation rate inflections. We find the >210 ka SLAPP-SRLS17 record contains five major units that correspond with prior work. The new dating is broadly consistent with previous efforts but provides more precise age estimates and a detailed evaluation of evaporite depositional history. We also offer a substantial revision of the age of the Bottom Mud-Mixed Layer contact, shifting it from ~130 ka to 178±3 ka. The new U-Th chronology documents the timing of mud and salt layers and lays the foundation for climate reconstructions.
Clumped isotope thermometry can independently constrain the formation temperatures of carbonates, but a lack of precisely temperature-controlled calibration samples limits its application on aragonites. To address this issue, we present clumped isotope compositions of aragonitic bivalve shells grown under highly controlled temperatures (1‒18°C), which we combine with clumped isotope data from natural and synthetic aragonites from a wide range of temperatures (1‒850°C). We observe no discernible offset in clumped isotope values between aragonitic foraminifera, mollusks, and abiogenic aragonites or between aragonites and calcites, eliminating the need for a mineral-specific calibration or acid fractionation factor. However, due to non-linear behavior of the clumped isotope thermometer, including high-temperature (>100°C) datapoints in linear clumped isotope calibrations causes them to underestimate temperatures of cold (1‒18°C) carbonates by 2.7 ± 2.0°C (95% confidence level). Therefore, clumped isotope-based paleoclimate reconstructions should be calibrated using samples with well constrained formation temperatures close to those of the samples.
Viscosity is of great importance in governing the dynamics of volcanoes, including their eruptive style. The viscosity of a volcanic melt is dominated by temperature and chemical composition, both oxides and water content. The changes in melt structure resulting from the interactions between the various chemical components are complex, and the construction of a physical viscosity model that depends on composition has not yet been achieved. We therefore train an Artificial Neural Networks (ANN) on a large database of measured compositions, including water, and viscosities that spans virtually the entire chemical space of terrestrial magmas, as well as some technical and extraterrestrial silicate melts. The ANN uses composition, temperature, a structural parameter reflecting melt polymerisation and the alkaline ratio as input parameters. It successfully reproduces and predicts measurements in the database with significantly higher accuracy than previous global models for volcanic melt viscosities. A calculator based on our ANN model is available at https://share.streamlit.io/domlang/visc_calc/main/final_script.py. Viscosity measurements are restricted to low and high viscosity range, which exclude typical eruptive temperatures. Without training data at such conditions, the ANN cannot reliably predict viscosities for this important temperature range. To overcome this limitation, we use the ANN to create a synthetic viscosity data in the high and low viscosity regime and fit these points using a physically motivated, temperature-dependent viscosity model. An Excel file to calculate viscosities using these parameters and the MYEGA equation is supplied in the Supporting Information.
Trees in seasonal climates may use water originating from both winter and summer precipitation. However, the seasonal origins of water used by trees have not been systematically studied. We used stable isotopes of water to compare the seasonal origins of water found in three common tree species across 24 Swiss forest sites sampled in two different years. Water from winter precipitation was observed in trees at most sites, even at the peak of summer, although the relative representation of seasonal sources differed by species. However, the representation of winter precipitation in trees decreased with site mean annual precipitation in both years; additionally, it was generally lower in the cooler and wetter year. Together, these relationships show that precipitation amount influenced the seasonal origin water taken up by trees across both time and space. These results suggest higher turnover of the plant-available soil-water pool in wetter sites and wetter years.
The mid-to-late Miocene is proposed as a key interval in the transition of the Earth’s climate state towards that of the modern-day. However, it remains a poorly understood interval in the evolution of Cenozoic climate, and the sparse proxy-based climate reconstructions are associated with large uncertainties. In particular, tropical sea surface temperature (SST) estimates largely rely on the unsaturated alkenone Uk37 proxy, which fails to record temperatures higher than 29˚C, the TEX86 proxy which has challenges around its calibration, and Mg/Ca ratios of poorly preserved foraminifera. We reconstruct robust, absolute, SSTs between 13.5 Ma and 9.5 Ma from the South West Indian Ocean (paleolatitude ~5.5˚S) using Laser-Ablation (LA-) ICP-MS microanalysis of glassy planktic foraminiferal Mg/Ca. Employing this microanalytical technique, and stringent screening criteria, permits the reconstruction of paleotemperatures using foraminifera which although glassy, are contaminated by authigenic coatings. Our absolute estimates of 24-31⁰C suggest that SST in the tropical Indian Ocean was relatively constant between 13.5 and 9.5 Ma, similar to those reconstructed from the tropics using the Uk37 alkenone proxy. This finding suggests an interval of enhanced polar amplification between 10 and 7.5 Ma, immediately prior to the global late Miocene Cooling.
We use a recently developed spectrally resolved bio-optical module to better represent the interaction between the incoming irradiance and the heat fluxes in the upper ocean within the (pre-)operational physical-biogeochemical model on the North-West European (NWE) Shelf. The module attenuates light based on the simulated biogeochemical tracer concentrations, and thus introduces a two-way coupling between the biogeochemistry and physics. We demonstrate that in the late spring-summer the two-way coupled model heats up the upper oceanic layer, shallows the mixed layer depth and influences the mixing in the upper ocean. The increased heating in the upper oceanic layer reduces the convective mixing and improves by ~5 days the timing of the late phytoplankton bloom of the ecosystem model. This improvement is relatively small compared with the existing model bias in bloom timing, but sufficient to have a visible impact on model skill. We show that the changes to the model temperature and salinity introduced by the module have mixed impact on the physical model skill, but the skill can be improved by assimilating the observations of temperature, salinity and chlorophyll concentrations into the model. However, in the situations where we improved the simulation of temperature, either via the bio-optical module, or via assimilation of temperature and salinity, we have shown that we also improved the simulated oxygen concentration as a result of the changes in the simulated air-sea gas flux. Overall, comparing different 1-year experiments showed that the best model skill is achieved with joint physical-biogeochemical assimilation into the two-way coupled model.
U-Pb dating of calcite veins allows direct dating of brittle deformation events. Here, we apply this method to hydrothermal calcite veins in a fold-and-thrust belt and a large scale strike-slip fault zone in central and western Thailand, in an attempt to shed new light on the regional upper crustal deformation history. Calcite U-Pb dates for the Khao Khwang Fold and Thrust Belt (KKFTB) of 221 ± 7 Ma and 216 ± 3 Ma demonstrate that calcite precipitated during tectonic activity associated with stage II of the Indosinian Orogeny (Late Triassic – Early Jurassic). One additional sample from the KKFTB suggests that the Indosinian calcite has locally been overprinted by a Cenozoic fluid event with a different chemistry. For the Three Pagodas Fault Zone (TPFZ), our calcite U-Pb results suggest a complex, protracted history of Cenozoic brittle deformation. Petrographic information combined with contrasting redox-sensitive trace elemental signatures suggest that the vein arrays in the TPFZ precipitated during two distinct events of brittle deformation at ∼48 and ∼23 Ma. These dates are interpreted in the context of far-field brittle deformation related to the India-Eurasia collision. The presented calcite U-Pb dates are in excellent agreement with published age constraints on the deformation history of Thailand, demonstrating the utility of the method to decipher complex brittle deformation histories. The paper further illustrates some of the complexities in relation to calcite U-Pb dating and provides suggestions for untangling complex datasets that could be applied to future studies on the deformation history of Thailand and other regions.
Tropospheric 18O18O is an emerging proxy for past tropospheric ozone and free-tropospheric temperatures. The basis of these applications is the idea that isotope-exchange reactions in the atmosphere drive 18O18O abundances toward isotopic equilibrium. However, previous work used an offline box-model framework to explain the 18O18O budget, approximating the interplay of atmospheric chemistry and transport. This approach, while convenient, has poorly characterized uncertainties. To investigate these uncertainties, and to broaden the applicability of the 18O18O proxy, we developed a scheme to simulate atmospheric 18O18O abundances (quantified as ∆36 values) online within the GEOS-Chem chemical transport model. These results are compared to both new and previously published atmospheric observations from the surface to 33 km. Simulations using a simplified O2 isotopic equilibration scheme within GEOS-Chem show quantitative agreement with measurements only in the middle stratosphere; modeled ∆36 values are too high elsewhere. Investigations using a comprehensive model of the O-O2-O3 isotopic photochemical system and proof-of-principle experiments suggest that the simple equilibration scheme omits an important pressure dependence to ∆36 values: the anomalously efficient titration of 18O18O to form ozone. Incorporating these effects into the online ∆36 calculation scheme in GEOS-Chem yields quantitative agreement for all available observations. While this previously unidentified bias affects the atmospheric budget of 18O18O in O2, the modeled change in the mean tropospheric ∆36 value since 1850 C.E. is only slightly altered; it is still quantitatively consistent with the ice-core ∆36 record, implying that the tropospheric ozone burden increased less than ~40% over the twentieth century.
Collision between the Pontides and Anatolide-Tauride Block along the İzmir-Ankara-Erzincan suture in Anatolia has been variously estimated from the Late Cretaceous to Eocene. It remains unclear whether this age range results from a protracted, multi-phase collision or differences between proxies of collision age and along strike. Here, we leverage the Cretaceous-Eocene evolution of the forearc-to-foreland Central Sakarya Basin system in western Anatolia to determine when and how collision progressed. New detrital zircon and sandstone petrography results indicate that the volcanic arc was the main source of sediment to the forearc basin in the Late Cretaceous. The first appearance of Pontide basement-aged detrital zircons, in concert with exhumation of the accretionary prism and a decrease in regional convergence rates indicates intercontinental collision initiated no later than 76 Ma. However, this first contractional phase does not produce thick-skinned deformation and basin partitioning until ca. 54 Ma, coeval to regional syn-collisional magmatism. We propose three non-exclusive and widely applicable mechanisms to reconcile the observed ~20 Myr delay between initial intercontinental collision and thick-skinned upper plate deformation: relict basin closure north and south of the İAES, gradual underthrusting of thicker lithosphere, and Paleocene slab breakoff. These mechanisms highlight the links between upper plate deformation and plate coupling during continental collision.
The sciences struggle to integrate across disciplines, coordinate across data generation and modeling activities, produce connected open data, and build strong networks to engage stakeholders within and beyond the scientific community. The American Geophysical Union (AGU) is divided into 25 sections intended to encompass the breadth of the geosciences. Here, we introduce a special collection of commentary articles spanning 19 AGU sections on challenges and opportunities associated with the use of ICON science principles. These principles focus on research intentionally designed to be Integrated, Coordinated, Open, and Networked (ICON) with the goal of maximizing mutual benefit (among stakeholders) and cross-system transferability of science outcomes. This article 1) summarizes the ICON principles; 2) discusses the crowdsourced approach to creating the collection; 3) explores insights from across the articles; and 4) proposes steps forward. There were common themes among the commentary articles, including broad agreement that the benefits of using ICON principles outweigh the costs, but that using ICON principles has important risks that need to be understood and mitigated. It was also clear that the ICON principles are not monolithic or static, but should instead be considered a heuristic tool that can and should be modified to meet changing needs. As a whole, the collection is intended as a resource for scientists pursuing ICON science and represents an important inflection point in which the geosciences community has come together to offer insights into ICON principles as a unified approach for improving how science is done across the geosciences and beyond.
Supercritical CO2 (ScCO2) invades oilwell cement under geological CO2 sequestration conditions. With the penetration of ScCO2, cement structure prone to damage when the coupled effects of capillarity and carbonation were found. Microstructural evolution of oilwell cement samples was investigated by the CT scanning and the quantitative image-based analysis and show that ScCO2 with the high humid condition would penetrate much deeper than the dry ScCO2 because of the capillarity effects. Due to the deep saline condition in the sequestration formation, the penetration of ScCO2 was retarded by the salt deposition, comparing with the ultrapure water (UP water) conditions. For further assessment of this coupled mechanism, the permeability property and contact angle changes were proposed to analyse the interface region between ScCO2, saline/UP water and oilwell cement.
We characterized the texture, composition, and seismic properties of the lithospheric mantle atop the Hawaiian plume by petrostructural analysis of 48 spinel-peridotite xenoliths from four localities in three Hawaiian islands. Coarse-porphyroclastic peridotites with variable degrees of recrystallization, recorded by growth of strain-free neoblasts onto the deformed microstructure, predominate. Full evolution of this process produced equigranular microstructures. Some peridotites have coarse-granular microstructures. Coarse-granular and coarse-porphyroclastic peridotites have strong orthorhombic or axial- olivine crystal-preferred orientations (CPO). Recrystallization produced moderate dispersion and, locally, changed the olivine CPO towards axial-. Enrichment in pyroxenes relative to model melting trends and pyroxenes with interstitial shapes and CPO uncorrelated with the olivine CPO suggest refertilization by reactive melt percolation. The unusual spatial distribution of the recrystallized fraction, Ti-enrichment, and REE-fractionation in recrystallized, equigranular, and coarse-granular peridotites support that these microstructures are produced by static recrystallization triggered by melt percolation. However, there is no simple relation between microstructure and chemical or modal composition. This, together with marked variations in mineral chemistry among samples, implies multiple spatially heterogeneous melt-rock reaction events. We interpret the coarse-porphyroclastic microstructures and CPO as representative of the original oceanic lithosphere fabric. Annealing changed the microstructure to coarse-granular, but did not modify significantly the olivine CPO. Recrystallization produced moderate dispersion of the CPO. “Normal” oceanic lithosphere seismic anisotropy patterns are therefore preserved. Yet Fe-enrichment, refertilization, and limited heating of the base of the lithosphere may reduce seismic velocities by up to 2%, partially explaining negative velocity anomalies imaged at lithospheric depths beneath Hawaii.
Carbonate clumped isotopes (∆47) have become a widely applied method for paleothermometry, with applications spanning many environmental settings over hundreds of millions of years. However, ∆47-based paleothermometry can be complicated by closure temperature-like behavior whereby C–O bonds are reset at elevated diagenetic or metamorphic temperatures, sometimes without obvious mineral alteration. Laboratory studies have constrained this phenomenon by heating well-characterized materials at various temperatures, observing temporal ∆47 evolution, and fitting results to kinetic models with prescribed C–O bond reordering mechanisms. While informative, these models are inflexible regarding the nature of isotope exchange, leading to potential uncertainties when extrapolated to geologic timescales. Here, we instead propose that observed reordering rates arise naturally from random-walk 18O diffusion through the carbonate lattice, and we develop a “disordered” kinetic framework that treats C–O bond reordering as a continuum of first-order processes occurring in parallel at different rates. We show theoretically that all previous models are specific cases of disordered kinetics; thus, our approach reconciles the transient defect/equilibrium defect and paired reaction-diffusion models. We estimate the rate coefficient distributions from published heating experiment data by finding a regularized inverse solution that best fits each ∆47 timeseries without assuming a particular functional form a priori. Resulting distributions are well-approximated as lognormal for all experiments on calcite or dolomite; aragonite experiments require more complex distributions that are consistent with a change in oxygen bonding environment during the transition to calcite. Presuming lognormal rate coefficient distributions and Arrhenius-like temperature dependence yields an underlying activation energy, E, distribution that is Gaussian with a mean value of μE = 224.3 ± 27.6 kJ mol−1 and a standard deviation of σE = 17.4 ± 0.7 kJ mol−1 (±1σ uncertainty; n = 24) for calcite and μE = 230.3 ± 47.7 kJ mol−1 and σE = 14.8 ± 2.2 kJ mol−1 (n = 4) for dolomite. These model results are adaptable to other minerals and may provide a basis for future experiments whereby the nature of carbonate C–O bonds is altered (e.g., by inducing mechanical strain or cation substitution). Finally, we apply our results to geologically relevant heating/cooling histories and suggest that previous models underestimate low-temperature alteration but overestimate ∆47 blocking temperatures.
Experiment and observation have established the centrality of oxygen fugacity (fO2) to determining the course of igneous differentiation, and so the development and application of oxybarometers have proliferated for more than half a century. The compositions of mineral, melt, and vapor phases determine the fO2 that rocks record, and the activity models that underpin calculation of fO2 from phase compositions have evolved with time. Likewise, analytical method development has made new sample categories available to oxybarometric interrogation. Here we compile published analytical data from lithologies that constrain fO2 (n=860 volcanic rocks - lavas and tephras and n=326 mantle lithologies- the majority peridotites) from ridges, back-arc basins, forearcs, arcs, and plumes. Because calculated fO2 varies with choice of activity model, we re-calculate fO2 for each dataset from compositional data, applying the same set of activity models and methodologies for each data type. Additionally, we compile trace element concentrations (e.g. vanadium) which serve as an additional fO2-proxy. The compiled data show that, on average, volcanic rocks and mantle rocks from the same tectonic setting yield similar fO2s, but mantle lithologies span a much larger range in fO2 than volcanics. Multiple Fe-based oxybarometric methods and vanadium partitioning vary with statistical significance as a function of tectonic setting, with fO2 ridges < back arcs < arcs. Plume lithologies are more nuanced to interpret, but indicate fO2s ridges. We discuss the processes that may shift fO2 after melts and mantle lithologies physically separate from one another. We show that the effects of crystal fractionation and degassing on the fO2 of volcanics are smaller than the differences in fO2 between tectonic settings and that effects of subsolidus metamorphism on the fO2 values recorded by mantle lithologies remain poorly understood. Finally, we lay out challenges and opportunities for future inquiry.
Magmas readily react with their surroundings, which may be other magmas or solid rocks. Such reactions are important in the chemical and physical evolution of magmatic systems and the crust, for example, in inducing volcanic eruptions and in the formation of ore deposits. In this contribution, we conceptually distinguish assimilation from other modes of magmatic interaction and discuss and review a range of geochemical (+/- thermodynamical) models used to model assimilation. We define assimilation in its simplest form as an end-member mode of magmatic interaction in which an initial state (t0) that includes a system of melt and solid wallrock evolves to a later state (tn) where the two entities have been homogenized. In complex natural systems, assimilation can refer more broadly to a process where a mass of magma wholly or partially homogenizes with materials derived from wallrock that initially behaves as a solid. The first geochemical models of assimilation used binary mixing equations and then evolved to incorporate mass balance between a constant-composition assimilant and magma undergoing simultaneous fractional crystallization. More recent tools incorporate energy and mass conservation in order to simulate changing magma composition as wallrock undergoes partial melting. For example, the Magma Chamber Simulator utilizes thermodynamic constraints to document the phase equilibria and major element, trace element, and isotopic evolution of an assimilating and crystallizing magma body. Such thermodynamic considerations are prerequisite for understanding the importance and thermochemical consequences of assimilation in nature, and confirm that bulk assimilation of large amounts of solid wallrock is limited by the enthalpy available from the crystallizing resident magma. Nevertheless, the geochemical signatures of magmatic systems-although dominated for some elements (particularly major elements) by crystallization processes-may be influenced by simultaneous assimilation of partial melts of compositionally distinct wallrock.
Solute transit or travel time distributions (TTDs) in catchments are relevant to both hydrochemical response and inference of hydrologic mechanisms. Long-tailed TTDs and fractal scaling behavior of stream concentration power spectra (~1/frequency, or 1/frequency to a power < 2) are widely observed in catchment studies. In several catchments, a significant fraction of streamflow is derived from groundwater in shallow fractured bedrock, where matrix diffusion significantly influences solute transport. I present frequency and time domain theoretical analyses of solute transport to quantify the influence of matrix diffusion on fractal scaling and long-tailed TTDs. The theoretical concentration power spectra exhibit fractal scaling, and the corresponding TTDs resemble a gamma distribution. The tails of the TTDs are influenced by accessible matrix width, exhibiting a sustained power-law (rather than exponential) decline for large matrix widths. Application to an experimental catchment shows that theoretical spectra match previously reported power spectral estimates derived from concentration measurements.
Drilling and multi-stage hydraulic fracturing bring a large amount of water into the formation, and clay-bearing shale reservoirs interact with water, which may lead to reduction of gas production, attenuation of fracturing effects, and even wellbore instability. Because of the complex fabric of shale, a thorough understanding of changes in shale micromechanics and corresponding mechanisms when exposed to water remains unclear. In this work, representative terrestrial and marine shale samples were selected for experiments based on clay enrichment. Then, contact resonance (CR) technique was performed to characterize micromechanics of shale after exposure to water. Visual phenomena provided by environmental scanning electron microscopy (ESEM) assisted to explain the underlying mechanisms. It was found that the hydration effect lowered both the storage modulus and stiffness of samples, but with different contributions from brittle minerals and clay, as well as variations depending on bedding plane orientation. Owing to the difference in composition, terrestrial shale exhibited stronger water sensitivity and anisotropy, with a general 15%-25% decrease in modulus, while marine shale changed relatively little (-5%-15%). Moreover, microscopic observation experiments revealed that complex interaction mechanisms may have existed that produced the mechanical changes. The reduction of capillary force and the interlaminar swelling of clay particles after water adsorption weakened the strength-related behavior of shale. However, the swelling-caused confining effect or void space closure during the water imbibition process might have offset this weakening effect, and even increased mechanical properties. At mesoscale, excessive shrinkage caused the growth of micro-cracks, which significantly attenuated overall mechanical behavior.
In the Arctic, the spatial distribution of boreal forest cover and soil profile transition characterizing the taiga-tundra ecological transition zone (TTE) is experiencing an alarming transformation. The SIBBORK-TTE model provides a unique opportunity to predict the spatiotemporal distribution patterns of vegetation heterogeneity, forest structure change, arctic-boreal forest interactions, and ecosystem transitions with high resolution scaling across broad domains. Within the TTE, evolving climatological and biogeochemical dynamics facilitate moisture signaling and nutrient cycle disruption, i.e. permafrost thaw and nutrient decomposition, thereby catalyzing land cover change and ecosystem instability. To demonstrate these trends, in situ ground measurements for active layer depth were collected to cross-validate below-ground-enhanced modeled simulations from 1996-2017. Shifting trends in permafrost variability (i.e. active layer depth) and seasonality were derived from model results and compared statistically to the in situ data. The SIBBORK-TTE model was then run to project future below-ground conditions utilizing CMIP6 scenarios. Upon visualization and curve-integrated analysis of the simulated freeze-thaw dynamics, the calculated performance metric associated with annual maximum active layer depth rate of change yielded 76.19%. Future climatic conditions indicate an increase in active layer depth and shifting seasonality across the TTE. With this novel approach, spatiotemporal variation of active layer depth provides an opportunity for identifying climate and topographic drivers and forecasting permafrost variability and earth system feedback mechanisms.