Endothermic physiology of extinct megatooth sharks.

The evolution of the extinct megatooth shark, Otodus megalodon, and its close phylogenetic relatives remains enigmatic. A central question persists regarding the thermophysiological origins of these large predatory sharks through geologic time, including whether O. megalodon was ectothermic or endothermic (including regional endothermy), and whether its thermophysiology could help to explain the iconic shark's gigantism and eventual demise during the Pliocene. To address these uncertainties, we present unique geochemical evidence for thermoregulation in O. megalodon from both clumped isotope paleothermometry and phosphate oxygen isotopes. Our results show that O. megalodon had an overall warmer body temperature compared with its ambient environment and other coexisting shark species, providing quantitative and experimental support for recent biophysical modeling studies that suggest endothermy was one of the key drivers for gigantism in O. megalodon and other lamniform sharks. The gigantic body size with high metabolic costs of having high body temperatures may have contributed to the vulnerability of Otodus species to extinction when compared to other sympatric sharks that survived the Pliocene epoch.

Massive formation of early diagenetic dolomite in the Ediacaran ocean: Constraints on the "dolomite problem".

Paleozoic and Precambrian sedimentary successions frequently contain massive dolomicrite [CaMg(CO3)2] units despite kinetic inhibitions to nucleation and precipitation of dolomite at Earth surface temperatures (<60 °C). This paradoxical observation is known as the "dolomite problem." Accordingly, the genesis of these dolostones is usually attributed to burial-hydrothermal dolomitization of primary limestones (CaCO3) at temperatures of >100 °C, thus raising doubt about the validity of these deposits as archives of Earth surface environments. We present a high-resolution, >63-My-long clumped-isotope temperature (TΔ47) record of shallow-marine dolomicrites from two drillcores of the Ediacaran (635 to 541 Ma) Doushantuo Formation in South China. Our T∆47 record indicates that a majority (87%) of these dolostones formed at temperatures of <100 °C. When considering the regional thermal history, modeling of the influence of solid-state reordering on our TΔ47 record further suggests that most of the studied dolostones formed at temperatures of <60 °C, providing direct evidence of a low-temperature origin of these dolostones. Furthermore, calculated δ18O values of diagenetic fluids, rare earth element plus yttrium compositions, and petrographic observations of these dolostones are consistent with an early diagenetic origin in a rock-buffered environment. We thus propose that a precursor precipitate from seawater was subsequently dolomitized during early diagenesis in a near-surface setting to produce the large volume of dolostones in the Doushantuo Formation. Our findings suggest that the preponderance of dolomite in Paleozoic and Precambrian deposits likely reflects oceanic conditions specific to those eras and that dolostones can be faithful recorders of environmental conditions in the early oceans.

Carbonate clumped isotope analysis (Δ47 ) of 21 carbonate standards determined via gas-source isotope-ratio mass spectrometry on four instrumental configurations using carbonate-based standardization and multiyear data sets.

RATIONALE: Clumped isotope geochemistry examines the pairing or clumping of heavy isotopes in molecules and provides information about the thermodynamic and kinetic controls on their formation. The first clumped isotope measurements of carbonate minerals were first published 15 years ago, and since then, interlaboratory offsets have been observed, and laboratory and community practices for measurement, data analysis, and instrumentation have evolved. Here we briefly review historical and recent developments for measurements, share Tripati Lab practices for four different instrument configurations, test a recently published proposal for carbonate-based standardization on multiple instruments using multi-year data sets, and report values for 21 different carbonate standards that allow for recalculations of previously published data sets. METHODS: We examine data from 4628 standard measurements on Thermo MAT 253 and Nu Perspective IS mass spectrometers, using a common acid bath (90°C) and small-sample (70°C) individual reaction vessels. Each configuration was investigated by treating some standards as anchors (working standards) and the remainder as unknowns (consistency standards). RESULTS: We show that different acid digestion systems and mass spectrometer models yield indistinguishable results when instrument drift is well characterized. For linearity correction, mixed gas-and-carbonate standardization or carbonate-only standardization yields similar results. No difference is observed in the use of three or eight working standards for the construction of transfer functions. CONCLUSIONS: We show that all configurations yield similar results if instrument drift is robustly characterized and validate a recent proposal for carbonate-based standardization using large multiyear data sets. Δ47 values are reported for 21 carbonate standards on both the absolute reference frame (ARF; also refered to as the Carbon Dioxide Equilibrated Scale or CDES) and the new InterCarb-Carbon Dioxide Equilibrium Scale (I-CDES) reference frame, facilitating intercomparison of data from a diversity of labs and instrument configurations and restandardization of a broad range of sample sets between 2006, when the first carbonate measurements were published, and the present.

Analytical effects on clumped isotope thermometry: Comparison of a common sample set analyzed using multiple instruments, types of standards, and standardization windows.

RATIONALE: Carbonate clumped isotope geothermometry is being increasingly used in multiple disciplines in the geosciences. However, potential interlaboratory issues are arising from different standardization procedures that may contribute to the multiple Δ47 -temperature calibrations reported in the literature. We investigate this issue by comparing a common temperature calibration sample set across three different mass spectrometers, using multiple standardization methods. METHODS: The same temperature calibration sample set was analyzed on three different mass spectrometers. Several standardization methods were utilized, including the use of carbonate versus gas standards, and different types of background correction were applied to the raw data. RESULTS: All standardization types applied resulted in statistically indistinguishable Δ47 -temperature slopes, with the exception of standardization calculations that did not correct for background effects. Some instruments and standardizations showed different intercepts relative to each other. The use of carbonate standards improved comparability between different instruments relative to gas standards. CONCLUSIONS: Our results show that background effects are the largest factor potentially affecting Δ47 results, and there may be an improvement in interlaboratory precision using carbonate standards. Critically, all techniques used for standardizing Δ47 results converge on a common slope as long as background effects are properly corrected. The use of carbonate standards is recommended as a component of standardization procedures.

Effects of different constants and standards on the reproducibility of carbonate clumped isotope (Δ47 ) measurements: Insights from a long-term dataset.

RATIONALE: Carbonate clumped isotope (Δ47 ) thermometry examines the temperature-dependent excess abundance of the 13 C-18 O bond in the carbonate lattice. Inconsistent temperature calibrations and standard values have been reported among laboratories, which has led to the use of equilibrated gases and carbonate standards for standardization. Furthermore, different acid fractionation factors and isotopic parameter sets have been proposed for improving inter-laboratory data comparability. However, few long-term datasets have been generated to explore the effects of these factors on the long-term reproducibility of Δ47 data within a laboratory. METHODS: Four standards (ISTB-1, NBS-19, GBWO4416, and GB04417) were analyzed as unknowns by isotope ratio mass spectrometry from 2015 to 2019. The values of Δ47 were calibrated using the ETH standards. We investigated the Assonov, Brand, and Gonfiantini isotope parameter sets for carbon and oxygen isotopes, as well as two correction schemes of equilibrated gas and carbonate standardization, using the same sample measurements to determine which procedures enhanced reproducibility. ISTB-1 (calcite) and ZK312-346W (dolomite) were measured to determine the 90°C acid fractionation factor. RESULTS: The corrected 90°C acid fractionation factors are 0.076 ± 0.008‰ for ISTB-1 and 0.077 ± 0.009‰ for ZK312-346W. The choice of isotope parameter set had no significant influence on final Δ47 values in this study. However, using the Assonov parameters to calculate Δ47 values improved the reproducibility of the results. The use of carbonate standards improved reproducibility through time compared with the use of equilibrated gases for standardization. CONCLUSIONS: At 90°C, the acid fractionation factors of calcite and dolomite are statistically indistinguishable. We find an insignificant effect from changing the isotope parameter set, suggesting that the choice of isotope parameter set among laboratories is not a major factor affecting inter-laboratory reproducibility. We find that using carbonate standards improved the reproducibility of results, suggesting that the use of carbonate standards may help to achieve inter-laboratory comparability of results in future studies.

Antarctic ice sheet sensitivity to atmospheric CO2 variations in the early to mid-Miocene.

Geological records from the Antarctic margin offer direct evidence of environmental variability at high southern latitudes and provide insight regarding ice sheet sensitivity to past climate change. The early to mid-Miocene (23-14 Mya) is a compelling interval to study as global temperatures and atmospheric CO2 concentrations were similar to those projected for coming centuries. Importantly, this time interval includes the Miocene Climatic Optimum, a period of global warmth during which average surface temperatures were 3-4 °C higher than today. Miocene sediments in the ANDRILL-2A drill core from the Western Ross Sea, Antarctica, indicate that the Antarctic ice sheet (AIS) was highly variable through this key time interval. A multiproxy dataset derived from the core identifies four distinct environmental motifs based on changes in sedimentary facies, fossil assemblages, geochemistry, and paleotemperature. Four major disconformities in the drill core coincide with regional seismic discontinuities and reflect transient expansion of grounded ice across the Ross Sea. They correlate with major positive shifts in benthic oxygen isotope records and generally coincide with intervals when atmospheric CO2 concentrations were at or below preindustrial levels (∼280 ppm). Five intervals reflect ice sheet minima and air temperatures warm enough for substantial ice mass loss during episodes of high (∼500 ppm) atmospheric CO2 These new drill core data and associated ice sheet modeling experiments indicate that polar climate and the AIS were highly sensitive to relatively small changes in atmospheric CO2 during the early to mid-Miocene.

High regional climate sensitivity over continental China constrained by glacial-recent changes in temperature and the hydrological cycle.

The East Asian monsoon is one of Earth's most significant climatic phenomena, and numerous paleoclimate archives have revealed that it exhibits variations on orbital and suborbital time scales. Quantitative constraints on the climate changes associated with these past variations are limited, yet are needed to constrain sensitivity of the region to changes in greenhouse gas levels. Here, we show central China is a region that experienced a much larger temperature change since the Last Glacial Maximum than typically simulated by climate models. We applied clumped isotope thermometry to carbonates from the central Chinese Loess Plateau to reconstruct temperature and water isotope shifts from the Last Glacial Maximum to present. We find a summertime temperature change of 6-7 °C that is reproduced by climate model simulations presented here. Proxy data reveal evidence for a shift to lighter isotopic composition of meteoric waters in glacial times, which is also captured by our model. Analysis of model outputs suggests that glacial cooling over continental China is significantly amplified by the influence of stationary waves, which, in turn, are enhanced by continental ice sheets. These results not only support high regional climate sensitivity in Central China but highlight the fundamental role of planetary-scale atmospheric dynamics in the sensitivity of regional climates to continental glaciation, changing greenhouse gas levels, and insolation.

Oxygen rise in the tropical upper ocean during the Paleocene-Eocene Thermal Maximum.

The global ocean's oxygen inventory is declining in response to global warming, but the future of the low-oxygen tropics is uncertain. We report new evidence for tropical oxygenation during the Paleocene-Eocene Thermal Maximum (PETM), a warming event that serves as a geologic analog to anthropogenic warming. Foraminifera-bound nitrogen isotopes indicate that the tropical North Pacific oxygen-deficient zone contracted during the PETM. A concomitant increase in foraminifera size implies that oxygen availability rose in the shallow subsurface throughout the tropical North Pacific. These changes are consistent with ocean model simulations of warming, in which a decline in biological productivity allows tropical subsurface oxygen to rise even as global ocean oxygen declines. The tropical oxygen increase may have helped avoid a mass extinction during the PETM.

Centering Equity in the Nation's Weather, Water, and Climate Services.

Water, weather, and climate affect everyone. However, their impacts on various communities can be very different based on who has access to essential services and environmental knowledge. Structural discrimination, including racism and other forms of privileging and exclusion, affects people's lives and health, with ripples across all sectors of society. In the United States, the need to equitably provide weather, water, and climate services is uplifted by the Justice40 Initiative (Executive Order 14008), which mandates 40% of the benefits of certain federal climate and clean energy investments flow to disadvantaged communities. To effectively provide such services while centering equity, systemic reform is required. Reform is imperative given increasing weather-related disasters, public health impacts of climate change, and disparities in infrastructure, vulnerabilities, and outcomes. It is imperative that those with positional authority and resources manifest responsibility through (1) recognition, inclusion, and prioritization of community expertise; (2) the development of a stronger and more representative and equitable workforce; (3) communication about climate risk in equitable, relevant, timely, and culturally responsive ways; and (4) the development and implementation of new models of relationships between communities and the academic sector.

Body temperatures of modern and extinct vertebrates from (13)C-(18)O bond abundances in bioapatite.

The stable isotope compositions of biologically precipitated apatite in bone, teeth, and scales are widely used to obtain information on the diet, behavior, and physiology of extinct organisms and to reconstruct past climate. Here we report the application of a new type of geochemical measurement to bioapatite, a "clumped-isotope" paleothermometer, based on the thermodynamically driven preference for (13)C and (18)O to bond with each other within carbonate ions in the bioapatite crystal lattice. This effect is dependent on temperature but, unlike conventional stable isotope paleothermometers, is independent from the isotopic composition of water from which the mineral formed. We show that the abundance of (13)C-(18)O bonds in the carbonate component of tooth bioapatite from modern specimens decreases with increasing body temperature of the animal, following a relationship between isotope "clumping" and temperature that is statistically indistinguishable from inorganic calcite. This result is in agreement with a theoretical model of isotopic ordering in carbonate ion groups in apatite and calcite. This thermometer constrains body temperatures of bioapatite-producing organisms with an accuracy of 1-2 degrees C. Analyses of fossilized tooth enamel of both Pleistocene and Miocene age yielded temperatures within error of those derived from similar modern taxa. Clumped-isotope analysis of bioapatite represents a new approach in the study of the thermophysiology of extinct species, allowing the first direct measurement of their body temperatures. It will also open new avenues in the study of paleoclimate, as the measurement of clumped isotopes in phosphorites and fossils has the potential to reconstruct environmental temperatures.

Equilibrated Gas and Carbonate Standard-Derived Dual (Δ47 and Δ48) Clumped Isotope Values.

Carbonate clumped isotope geochemistry has primarily focused on mass spectrometric determination of m/z 47 CO2 for geothermometry, but theoretical calculations and recent experiments indicate paired analysis of the m/z 47 (13C18O16O) and m/z 48 (12C18O18O) isotopologues (referred to as Δ47 and Δ48) can be used to study non-equilibrium isotope fractionations and refine temperature estimates. We utilize 5,448 Δ47 and 3,400 Δ48 replicate measurements of carbonate samples and standards, and 183 Δ47 and 195 Δ48 replicate measurements of gas standards from 2015 to 2021 from a multi-year and multi-instrument data set to constrain Δ47 and Δ48 values for 27 samples and standards, including Devils Hole cave calcite, and study equilibrium Δ47-Δ48, Δ47-temperature, and Δ48-temperature relationships. We compare results to previously published findings and calculate equilibrium regressions based on data from multiple laboratories. We report acid digestion fractionation factors, Δ*63-47 and Δ*64-48, and account for their dependence on the initial clumped isotope values of the mineral.

Establishing a Framework for Sustainable Community Action Research.

Community-based participatory research/community-partnered participatory research (CBPR/CPRR) is viewed as a critical approach for improving health and addressing inequities found in under-resourced communities by pairing community partners and academic partners to address health and environmental concerns. This article aims to amplify the potential of the current CBPR/CPPR models through insights learned from the underserved community of Watts in south central Los Angeles. We discuss our framework that shifts the primary academic focus in the community-academia partnership from individual investigators and/or research groups to the academic institution to generate sustainable partnerships. We summarize the Community Action Research Engagement (CARE) Framework as a new set of recommended tenets to expand CBPR/CPPR. This framework can provide guidance for how universities can catalyze: 1) building trust; 2) facilitating knowledge; 3) advancing solutions; and 4) fostering mentorship in the context of leveraging a university's position to address the root causes of community inequities and thus create more sustained partnerships that achieve greater impact within their surrounding communities.

Systemic racial disparities in funding rates at the National Science Foundation.

Concerns about systemic racism at academic and research institutions have increased over the past decade. Here, we investigate data from the National Science Foundation (NSF), a major funder of research in the United States, and find evidence for pervasive racial disparities. In particular, white principal investigators (PIs) are consistently funded at higher rates than most non-white PIs. Funding rates for white PIs have also been increasing relative to annual overall rates with time. Moreover, disparities occur across all disciplinary directorates within the NSF and are greater for research proposals. The distributions of average external review scores also exhibit systematic offsets based on PI race. Similar patterns have been described in other research funding bodies, suggesting that racial disparities are widespread. The prevalence and persistence of these racial disparities in funding have cascading impacts that perpetuate a cumulative advantage to white PIs across all of science, technology, engineering, and mathematics.

Alpine permafrost could account for a quarter of thawed carbon based on Plio-Pleistocene paleoclimate analogue.

Estimates of the permafrost-climate feedback vary in magnitude and sign, partly because permafrost carbon stability in warmer-than-present conditions is not well constrained. Here we use a Plio-Pleistocene lacustrine reconstruction of mean annual air temperature (MAAT) from the Tibetan Plateau, the largest alpine permafrost region on the Earth, to constrain past and future changes in permafrost carbon storage. Clumped isotope-temperatures (Δ47-T) indicate warmer MAAT (~1.2 °C) prior to 2.7 Ma, and support a permafrost-free environment on the northern Tibetan Plateau in a warmer-than-present climate. Δ47-T indicate ~8.1 °C cooling from 2.7 Ma, coincident with Northern Hemisphere glacial intensification. Combined with climate models and global permafrost distribution, these results indicate, under conditions similar to mid-Pliocene Warm period (3.3-3.0 Ma), ~60% of alpine permafrost containing ~85 petagrams of carbon may be vulnerable to thawing compared to ~20% of circumarctic permafrost. This estimate highlights ~25% of permafrost carbon and the permafrost-climate feedback could originate in alpine areas.

Eocene bipolar glaciation associated with global carbon cycle changes.

The transition from the extreme global warmth of the early Eocene 'greenhouse' climate approximately 55 million years ago to the present glaciated state is one of the most prominent changes in Earth's climatic evolution. It is widely accepted that large ice sheets first appeared on Antarctica approximately 34 million years ago, coincident with decreasing atmospheric carbon dioxide concentrations and a deepening of the calcite compensation depth in the world's oceans, and that glaciation in the Northern Hemisphere began much later, between 10 and 6 million years ago. Here we present records of sediment and foraminiferal geochemistry covering the greenhouse-icehouse climate transition. We report evidence for synchronous deepening and subsequent oscillations in the calcite compensation depth in the tropical Pacific and South Atlantic oceans from approximately 42 million years ago, with a permanent deepening 34 million years ago. The most prominent variations in the calcite compensation depth coincide with changes in seawater oxygen isotope ratios of up to 1.5 per mil, suggesting a lowering of global sea level through significant storage of ice in both hemispheres by at least 100 to 125 metres. Variations in benthic carbon isotope ratios of up to approximately 1.4 per mil occurred at the same time, indicating large changes in carbon cycling. We suggest that the greenhouse-icehouse transition was closely coupled to the evolution of atmospheric carbon dioxide, and that negative carbon cycle feedbacks may have prevented the permanent establishment of large ice sheets earlier than 34 million years ago.

Patterns of Element Incorporation in Calcium Carbonate Biominerals Recapitulate Phylogeny for a Diverse Range of Marine Calcifiers.

Elemental ratios in biogenic marine calcium carbonates are widely used in geobiology, environmental science, and paleoenvironmental reconstructions. It is generally accepted that the elemental abundance of biogenic marine carbonates reflects a combination of the abundance of that ion in seawater, the physical properties of seawater, the mineralogy of the biomineral, and the pathways and mechanisms of biomineralization. Here we report measurements of a suite of nine elemental ratios (Li/Ca, B/Ca, Na/Ca, Mg/Ca, Zn/Ca, Sr/Ca, Cd/Ca, Ba/Ca, and U/Ca) in 18 species of benthic marine invertebrates spanning a range of biogenic carbonate polymorph mineralogies (low-Mg calcite, high-Mg calcite, aragonite, mixed mineralogy) and of phyla (including Mollusca, Echinodermata, Arthropoda, Annelida, Cnidaria, Chlorophyta, and Rhodophyta) cultured at a single temperature (25°C) and a range of pCO2 treatments (ca. 409, 606, 903, and 2856 ppm). This dataset was used to explore various controls over elemental partitioning in biogenic marine carbonates, including species-level and biomineralization-pathway-level controls, the influence of internal pH regulation compared to external pH changes, and biocalcification responses to changes in seawater carbonate chemistry. The dataset also enables exploration of broad scale phylogenetic patterns of elemental partitioning across calcifying species, exhibiting high phylogenetic signals estimated from both uni- and multivariate analyses of the elemental ratio data (univariate: λ = 0-0.889; multivariate: λ = 0.895-0.99). Comparing partial R 2 values returned from non-phylogenetic and phylogenetic regression analyses echo the importance of and show that phylogeny explains the elemental ratio data 1.4-59 times better than mineralogy in five out of nine of the elements analyzed. Therefore, the strong associations between biomineral elemental chemistry and species relatedness suggests mechanistic controls over element incorporation rooted in the evolution of biomineralization mechanisms.

Factors controlling the oxygen isotopic composition of lacustrine authigenic carbonates in Western China: implications for paleoclimate reconstructions.

In the carbonate-water system, at equilibrium, the oxygen isotopic composition of carbonate is dependent not only on the temperature but also on the isotopic composition of host water in which the carbonate is formed. In this study, lake surface sediment and water samples were collected from 33 terminal lakes in Western China to evaluate controls on the oxygen isotopic composition of lacustrine authigenic carbonates (δ18Ocarb) and its spatial distribution. Our results show that water oxygen isotopic composition (δ18Owater) rather than lake summer water temperature (Twater), is the main determinant of δ18Ocarb, irrespective of whether oxygen isotope equilibrium is achieved. There are significant linear correlations between δ18Ocarb and elevation, as well as that between δ18Ocarb and latitude for lakes located on the Tibetan Plateau. In Western China, the spatial distribution of δ18Ocarb is consistent with that of δ18Owater, and is ultimately controlled by the isotopic composition of local precipitation (δ18Oprecipitation) that depends on the source of water vapor. Therefore, changes in δ18Ocarb can be predominantly interpreted as variations of δ18Owater, which in turn represent changes in δ18Oprecipitation for paleoclimate reconstructions in this region, and may be relevant to studies of other areas.

Influence of surface ocean density on planktonic foraminifera calcification.

This study provides evidence that ambient seawater density influences calcification and may account for the observed planktonic foraminifera shell mass increase during glacial times. Volumes of weighed fossil Globigerina bulloides shells were accurately determined using X-ray Computer Tomography and were combined with water density reconstructions from Mg/Ca and δ18O measurements to estimate the buoyancy force exerted on each shell. After assessment of dissolution effects, the resulting relationship between shell mass and buoyancy suggests that heavier shells would need to be precipitated in glacial climates in order for these organisms to remain at their optimum living depth, and counterbalance the increased buoyant force of a denser, glacial ocean. Furthermore, the reanalysis of bibliographic data allowed the determination of a relationship between G. bulloides shell mass and ocean density, which introduces implications of a negative feedback mechanism for the uptake of atmospheric CO2 by the oceans.

Evidence for ephemeral middle Eocene to early Oligocene Greenland glacial ice and pan-Arctic sea ice.

Earth's modern climate is defined by the presence of ice at both poles, but that ice is now disappearing. Therefore understanding the origin and causes of polar ice stability is more critical than ever. Here we provide novel geochemical data that constrain past dynamics of glacial ice on Greenland and Arctic sea ice. Based on accurate source determinations of individual ice-rafted Fe-oxide grains, we find evidence for episodic glaciation of distinct source regions on Greenland as far-ranging as ~68°N and ~80°N synchronous with ice-rafting from circum-Arctic sources, beginning in the middle Eocene. Glacial intervals broadly coincide with reduced CO2, with a potential threshold for glacial ice stability near ~500 p.p.m.v. The middle Eocene represents the Cenozoic onset of a dynamic cryosphere, with ice in both hemispheres during transient glacials and substantial regional climate heterogeneity. A more stable cryosphere developed at the Eocene-Oligocene transition, and is now threatened by anthropogenic emissions.