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Reconstructing paleoenvironments of the Late Cretaceous Western Interior Seaway, USA, using paired triple oxygen and carbonate clumped isotope measurements
Abstract
Fossiliferous carbonate concretions are commonly found in sediments deposited in the Late Cretaceous Western Interior Seaway. Although concretions are diagenetic features, well-preserved fossils from within them have been instrumental in reconstructing the temperature and δ18O value of Western Interior Seaway seawater, which is essential for accurate reconstruction of Late Cretaceous climate. Here, we constrain formation conditions of Late Campanian and early Maastrichtian carbonate concretions by combining triple oxygen isotope measurements with carbonate clumped isotope paleothermometry on different carbonate phases within the concretions. We measured both fossil skeletal aragonite and sparry calcite infill from cracks and within macrofossil voids to evaluate differences between “primary” and “altered” geochemical signals. Based on the two temperature-sensitive isotope systems of the primary fossil shell aragonite, the temperature of the Western Interior Seaway was between 20 °C and 40 °C and was likely thermally stratified during the Campanian. The reconstructed δ18Oseawater values of ∼−1‰ for Campanian Western Interior Seaway waters are similar to those expected for the open ocean during greenhouse climates, while the Maastrichtian Western Interior Seaway may have been more restricted, with a δ18Oseawater value of ∼2‰, which reflects more evaporative conditions. We reconstructed the diagenetic history of the sparry infill and altered fossils using a fluid-rock mixing model. Alteration temperature, alteration fluid δ18O value, and the initial formation temperature were calculated by applying the fluid-rock mixing model to a particle swarm optimization algorithm. We found a different range of initial formation temperatures between the Campanian (25−38 °C) and Maastrichtian (9−28 °C). We also found that alteration in the presence of light meteoric fluids (δ18O ≈ −10‰) is required to explain both the sparry infill and the altered fossil isotopic values. Based on our results, both lithification and alteration of the carbonates occurred soon after burial, and light meteoric fluids support prior findings that high-topographic relief existed on the western margin of the Western Interior Seaway during the Late Cretaceous. As one of the first studies to apply these techniques in concert and across multiple mineralogical phases within samples, our results provide important constraints on paleoenvironmental conditions in an enigmatic ocean system and will improve interpretations of the overall health of ecosystems leading into the end-Cretaceous mass extinction.
High sea levels in the Late Cretaceous led to the formation of vast seaways on every continent. These shallow seaways are without modern analogs and many fundamental aspects of their oceanography are poorly understood. In the Campanian (∼83–72 Ma), the Western Interior Seaway (WIS) of North America linked the proto-Gulf of Mexico and Arctic Ocean. Given its shallow depth, freshwater inputs to the WIS could have had a greater influence on conditions in the seaway compared to a deeper ocean and would have become increasingly important as the WIS regressed through the Maastrichtian. The isotopic composition of mollusk shells in freshwater facies can help constrain the composition and temperature of these inputs, improving our understanding of surface temperature, hydrological dynamics, and paleoelevation. Here we measure Δ47 temperature, δ18Owater, and 87Sr/86Sr in late Campanian (∼75 Ma) unionid bivalve shells from fluvial and pond deposits near the western shore of the WIS (∼42°N-56°N). Sample mean surface water temperatures spanned 22–44 °C, with a mean of 30 ± 2.7 °C. The latitudinal temperature gradient across this region is reduced compared to today, at ∼7 °C across these 14° of latitude based on stream sample means. These temperatures are outside the optimal growth conditions of modern unionids in North America, indicating a shift in niche. Following this finding, we recalculate δ18Owater values from previously published δ18Ocarb values using new Δ47 temperatures instead of assumed growth temperatures. Our findings support previous observations of a bimodal distribution in freshwater δ18Owater values in this region although the absolute values shift higher. Spatial patterns of δ18Owater are consistent with a Campanian Proto-North American Monsoon and the lowest δ18Owater values we report are consistent with paleoelevation of >3500 m in the Proto-Cordillera. 87Sr/86Sr values broadly align with different facies, with more radiogenic values occurring in major trunk rivers draining the highlands and less radiogenic values in streams recharged by low-elevation precipitation. Predominance of a 87Sr/86Sr signature consistent with weathering Paleozoic carbonates could be consistent with seasonal increases in rock weathering associated with a monsoon.
The Niobrara Formation of north‐east Colorado, USA, has anomalously negative δ18O values compared to all other Cretaceous chalks. These unique δ18O values have been attributed to elevated heat flow and/or freshening of the Cretaceous Western Interior Seaway. This work utilises clumped isotopes of calcite (Δ47), peak burial temperatures estimated from pyrolysis data, and strontium and neodymium isotopes of carbonate to re‐evaluate the origin of the calcite's 18O‐depletion. Peak temperatures indicate lateral variability in geothermal gradients of ca 20°C/km at the tens of kilometre scale, and corroborate prior studies proposing locally elevated palaeotemperatures. Greater insight is provided by numerical models of calcite recrystallisation and oxygen isotope evolution that are constrained by measured Δ47‐derived temperatures, calcite δ18O values and inferences from the 87Sr/86Sr and εNd values. The models indicate that (1) seawater in the seaway had normal marine δ18O values of ‐1 (VSMOW) except on the eastern margin of the basin where some freshwater dilution yielded ‐2 to ‐3‰ (VSMOW) water, and (2) the main driver of the anonymously negative calcite δ18O values was a semi‐open hydrologic system that provided a few percent by pore volume of meteoric groundwater derived from post‐Laramide recharge into the basin. Minor contributions were a Laramide‐aged heat pulse related to the underlying Colorado Mineral Belt, the thermal insulating effects of now eroded coals, and a small flux of compaction‐driven Cretaceous seawater evolved by smectite dehydration. However, those three factors alone were insufficient drivers of the calcites’ 18O depletion. High burial temperatures are interpreted to have caused clumped isotope reordering in at least one well, but those temperatures cannot yield the observed calcite δ18O values. The study illustrates the unique attributes of the Niobrara's diagenetic system that results in its anomalous δ18O values, and reaffirms the value of clumped isotopes in unravelling the diagenetic history of chalk systems.
The question why non-avian dinosaurs went extinct 66 million years ago (Ma) remains unresolved because of the coarseness of the fossil record. A sudden extinction caused by an asteroid is the most accepted hypothesis but it is debated whether dinosaurs were in decline or not before the impact. We analyse the speciation-extinction dynamics for six key dinosaur families, and find a decline across dinosaurs, where diversification shifted to a declining-diversity pattern ~76 Ma. We investigate the influence of ecological and physical factors, and find that the decline of dinosaurs was likely driven by global climate cooling and herbivorous diversity drop. The latter is likely due to hadrosaurs outcompeting other herbivores. We also estimate that extinction risk is related to species age during the decline, suggesting a lack of evolutionary novelty or adaptation to changing environments. These results support an environmentally driven decline of non-avian dinosaurs well before the asteroid impact.
Increased use and improved methodology of carbonate clumped isotope thermometry has greatly enhanced our ability to interrogate a suite of Earth‐system processes. However, inter‐laboratory discrepancies in quantifying carbonate clumped isotope (Δ₄₇) measurements persist, and their specific sources remain unclear. To address inter‐laboratory differences, we first provide consensus values from the clumped isotope community for four carbonate standards relative to heated and equilibrated gases with 1,819 individual analyses from 10 laboratories. Then we analyzed the four carbonate standards along with three additional standards, spanning a broad range of δ⁴⁷ and Δ₄₇ values, for a total of 5,329 analyses on 25 individual mass spectrometers from 22 different laboratories. Treating three of the materials as known standards and the other four as unknowns, we find that the use of carbonate reference materials is a robust method for standardization that yields inter‐laboratory discrepancies entirely consistent with intra‐laboratory analytical uncertainties. Carbonate reference materials, along with measurement and data processing practices described herein, provide the carbonate clumped isotope community with a robust approach to achieve inter‐laboratory agreement as we continue to use and improve this powerful geochemical tool. We propose that carbonate clumped isotope data normalized to the carbonate reference materials described in this publication should be reported as Δ₄₇ (I‐CDES) values for Intercarb‐Carbon Dioxide Equilibrium Scale.
The past decade has seen a remarkable expansion of studies that use mass-dependent variations of triple oxygen isotopes (¹⁶O, ¹⁷O, ¹⁸O) in isotope hydrology and isotope geochemistry. Recent technological and analytical advances demonstrate that small deviations of δ′¹⁸O and δ′¹⁷O from a mass-dependent reference relationship are systematic and are explained by well-known equilibrium and kinetic fractionations. Measurements of δ′¹⁸O and δ′¹⁷O complement traditional metrics like deuterium-excess, constrain isotope effects of kinetic fractionation that are impossible to discern with δ¹⁸O alone, and help reconstruct past environmental conditions from geologic records. In this review, we synthesize published meteoric (derived from precipitation) water triple oxygen isotope data with a new, near-global surface water dataset of δ′¹⁸O, δ′¹⁷O, δ²H, deuterium-excess, and ∆′¹⁷O, where ∆′¹⁷O is defined as δ′¹⁷O – λref δ′¹⁸O, δ′ notation is a logarithmic definition of the common δ value (δ′=ln(δ + 1), and λref is equal to 0.528. The expanded dataset shows that meteoric water δ′¹⁸O and δ′¹⁷O fit multiple regression lines and indicates that one global meteoric water line does not adequately describe all triple oxygen isotope data. Instead, this isotope system may be sensitive to processes such as moisture transport, rainout, and evaporation that do not affect the water cycle equally across the globe. This review provides a practical guide to understand ∆′¹⁷O variation in waters, explains the utility of this isotope system in hydrologic and paleoclimate studies, and outlines directions of future work that will expand the use of ∆′¹⁷O.
We test for the presence of evolutionary stasis in a species of Late Cretaceous ammonoid cephalopod, Hoploscaphites nicolletii, from the North American Western Interior Seaway. A comprehensive dataset of morphological traits was compiled across the entire spatial and temporal range of this species. These were analysed in conjunction with sedimentologically and geochemically derived palaeoenvironmental conditions hypothesized to apply selective pressures. All changes in shell shape were observed to be ephemeral and reversable, that is, no unidirectional trend could be observed in any of the morphological traits analysed. Correlations between palaeoenvironmental conditions and morphological traits suggests ecophenotypic processes were at play; however, either environmental changes were too minor and/or provided no isolating mechanism to drive speciation. These data support mechanisms of stasis such as homogenizing gene flow or stabilizing selection under a fluctuating optimum (probably reflecting spatiotemporally heterogeneous palaeoenvironmental conditions). Finally, changes in shell size were not significantly associated with changes in shell‐specific δ18O, despite a correlation between shell size and δ18O averaged across horizons. This suggests a mismatch in scales of geochemical sampling that supports caution when making broad interpretations based on averaged geochemical data.
Raw morphological and isotopic data available on the Dryad digital repository: https://doi.org/10.5061/dryad.1ns1rn8q5
Cold methane seeps were common in the Late Cretaceous Western Interior Seaway of North America. They provided a habitat for a diverse array of fauna including ammonites. Recent research has demonstrated that ammonites lived at these sites. However, it is still unknown if they hatched at the seeps or only arrived there later in ontogeny. To answer this question, we documented the abundance and size distribution of small specimens of Baculites and Hoploscaphites at eight seep sites in the Pierre Shale of South Dakota. The specimens of Hoploscaphites range from 0.8 to 8.1 mm in shell diameter, with most of them falling between 1 and 1.5 mm. The specimens of Baculites range from 0.7 to 19.2 mm in length, with most specimens falling between 6 and 8 mm. The small size and morphology of these specimens indicate that they are neanoconchs, that is, newly hatched individuals that lived for a short time after hatching. We also analyzed the isotope composition (δ13C and δ18O) of 12 small specimens of Baculites and one specimen of Hoploscaphites with excellent shell preservation from one seep deposit. The values of δ13C and δ18O range from -16.3 to -2.5‰ and -3.0 to -0.9‰, respectively. The values of δ18O translate into temperatures of 19–28°C, which are comparable to previous estimates of the temperatures of the Western Interior Seaway. The low values of δ13C suggest that the tiny animals incorporated carbon derived from anaerobic oxidation of 12C-enriched methane into their shells. Evidently, they must have lived in close proximity to seep fluids emerging at the sediment-water interface and the associated microbial food web. However, this may have contributed to their demise if they were exposed to elevated concentrations of H2S derived from the anaerobic oxidation of methane.
The clumped isotopic composition of carbonate-derived CO2 (denoted Δ47) is a function of carbonate formation temperature and in natural samples can act as a recorder of paleoclimate, burial, or diagenetic conditions. The absolute abundance of heavy isotopes in the universal standards VPDB and VSMOW (defined by four parameters: R¹³ VPDB, R¹⁷ VSMOW, R¹⁸ VSMOW, and λ) impact calculated Δ47 values. Here, we investigate whether use of updated and more accurate values for these parameters can remove observed interlaboratory differences in the measured T-Δ47 relationship. Using the updated parameters, we reprocess 14 published calibration data sets measured in 11 different laboratories, representing many mineralogies, bulk compositions, sample types, reaction temperatures, and sample preparation and analysis methods. Exploiting this large composite data set (n = 1,253 sample replicates), we investigate the possibility for a “universal” clumped isotope calibration. We find that applying updated parameters improves the T-Δ47 relationship (reduces residuals) within most labs and improves overall agreement but does not eliminate all interlaboratory differences. We reaffirm earlier findings that different mineralogies do not require different calibration equations and that cleaning procedures, method of pressure baseline correction, and mass spectrometer type do not affect interlaboratory agreement. We also present new estimates of the temperature dependence of the acid digestion fractionation for Δ47 (Δ*25-X), based on combining reprocessed data from four studies, and new theoretical equilibrium values to be used in calculation of the empirical transfer function. Overall, we have ruled out a number of possible causes of interlaboratory disagreement in the T-Δ47 relationship, but many more remain to be investigated.
Ammonites, as well as other fauna, were common in methane seeps of the Late Cretaceous Western Interior Seaway (WIS) of North America. Biogeochemical processes at the seeps, in particular the anaerobic oxidation of methane, produced a dissolved inorganic carbon reservoir with a low 13C, manifested in the carbon isotope
composition of the inorganic calcium carbonate concretions associated with the seeps and recorded in well-preserved shells of ammonites documented at the sites. Detailed sclerochronological sampling of six well-preserved specimens of Baculites compressus
collected at seep sites in the Pierre Shale of South Dakota reveals three patterns that can be explained by reference to two specimens of the same species collected at age-equivalent non-seep sites. Three of the specimens exhibit uniformly low values of 13C that are significantly different (unpaired t-test, p < .0001) from similarly sized
specimens of the same species collected at age-equivalent non-seep sites, suggesting that these ammonites lived at the seeps during the time interval over which the shell was secreted (adult portion of the shell). Two of the specimens collected from a seep site exhibit values of 13C consistent with early ontogeny at a non-seep site followed by
later ontogeny at a seep site. The values of 18O of all the specimens reveal water temperatures of 16 to 28 °C. One small juvenile (15 mm long) collected at a seep site exhibits higher values of 13C consistent with a non-seep environment, but values of 18Othat indicate very warm or slightly brackish water, suggesting that this animal lived
in surface waters during its early ontogeny and died soon after arriving at the seep. Our results demonstrate that seep fluids affected the geochemistry of the water column above the seeps and that seeps provided habitats for ammonites in the WIS. Thus, although ammonites were mobile animals, they probably exploited a low-energy life style, remaining at the same site for extended periods of time.
Significance
The elemental and isotopic compositions of seawater have evolved throughout Earth’s history, in tandem with major climatic, tectonic and biologic events, including the emergence and diversification of life. Over geological timescales, the oxygen isotope composition of seawater reflects a global balance between mineral–rock reactions occurring at the Earth’s surface (weathering and sedimentation) and crustal (hydrothermal alteration) environments. We put constraints on the oxygen isotope composition of seawater throughout the Phanerozoic and demonstrate that this value has remained stable. This stability suggests that the fluxes of globally averaged oxygen isotope exchange, associated with weathering and hydrothermal alteration reactions, have remained proportional through time and is consistent with the hypothesis that a steady-state balance exists between seafloor hydrothermal activity and surface weathering.
High precision triple oxygen isotope analyses of terrestrial materials show distinct fields and trends in Δ′¹⁷O - δ′¹⁸O space that can be explained by well understood fractionation processes. The Δ′¹⁷O - δ′¹⁸O field for meteoric waters has almost no overlap with that of rocks. Globally, meteoric water defines a λ value of ∼0.528, although a better fit to waters with δ¹⁸O values >-20 ‰ is δ ′¹⁷O = 0.52654 (±0.00036) δ′¹⁸O + 0.014 (±0.003). Low temperature marine sediments define a unique and narrow band in Δ′¹⁷O - δ′¹⁸O space with high δ′¹⁸O and low Δ′¹⁷O values explained by equilibrium fractionation. Hydrothermal alteration shifts the rock composition to lower δ′¹⁸O values at low fluid/rock ratios, and finally higher Δ′¹⁷O when F/R ratios are greater than 1. In order to make the triple isotope data tractable to the entire geological community, consensus on a reporting scheme for Δ′¹⁷O is desirable. Adoption of λRL= 0.528 (λ RL = slope of δ′¹⁷O - δ′¹⁸O reference line, the 'Terrestrial Fractionation Line' or TFL) would bring the 'rock' community in line with well established hydrological reporting conventions.
It is well established that greenhouse conditions prevailed during the Cretaceous Period (~ 145–66 Ma). Determining the exact nature of the greenhouse-gas forcing, climatic warming and climate sensitivity remains, however, an active topic of research. Quantitative and qualitative geochemical and palaeontological proxies provide valuable observational constraints on Cretaceous climate. In particular, reconstructions of Cretaceous sea-surface temperatures (SSTs) have been revolutionised firstly by the recognition that clay-rich sequences can host exceptionally preserved planktonic foraminifera allowing for reliable oxygen-isotope analyses and, secondly by the development of the organic palaeothermometer TEX86, based on the distribution of marine archaeal membrane lipids. Here we provide a new compilation and synthesis of available planktonic foraminiferal δ¹⁸O (δ¹⁸Opl) and TEX86-SST proxy data for almost the entire Cretaceous Period. The compilation uses SSTs recalculated from published raw data, allowing examination of the sensitivity of each proxy to the calculation method (e.g., choice of calibration) and places all data on a common timescale. Overall, the compilation shows many similarities with trends present in individual records of Cretaceous climate change. For example, both SST proxies and benthic foraminiferal δ¹⁸O records indicate maximum warmth in the Cenomanian–Turonian interval. Our reconstruction of the evolution of latitudinal temperature gradients (low, <±30°, minus higher, >±48°, palaeolatitudes) reveals temporal changes. In the Valanginian–Aptian, the low-to-higher mid-latitudinal temperature gradient was weak (decreasing from ~ 10–17 °C in the Valanginian, to ~ 3–5 °C in the Aptian, based on TEX86-SSTs). In the Cenomanian–Santonian, reconstructed latitudinal temperature contrasts are also small relative to modern (< 14 °C, based on low-latitude TEX86 and δ¹⁸Opl SSTs minus higher latitude δ¹⁸Opl SSTs, compared with ~ 20 °C for the modern). In the mid-Campanian to end-Maastrichtian, latitudinal temperature gradients strengthened (~ 19–21 °C, based on low-latitude TEX86 and δ¹⁸Opl SSTs minus higher latitude δ¹⁸Opl SSTs), with cooling occurring at low-, middle- and higher palaeolatitude sites, implying global surface-ocean cooling and/or changes in ocean heat transport in the Late Cretaceous. These reconstructed long-term trends are resilient, regardless of the choice of proxy (TEX86 or δ¹⁸Opl) or calibration. This new Cretaceous SST synthesis provides an up-to-date target for modelling studies investigating the mechanics of extreme climates.
The Cretaceous Western Interior seaway extended some 6000 km through the middle of North America and it provided a wide, relatively shallow connection between the polar ocean and the subtropical ocean, both of which invaded the comparatively shallow Western Interior Seaway and left distinctive faunal and floral records. Water masses from these very different climatic regions may have had similar densities but very different salinities and temperatures, as is the case with the modern Gulf of Mexico and eastern Arctic Ocean. Mixing of such disparate water masses could have produced a third water mass with a density greater than one or both of them, and this mechanism may have caused the Western Interior Seaway to become a significant source of intermediate water in the world ocean. Intermediate water production would have been greatest during times of maximum transgression. Surface waters of the world ocean would be drawn into both ends of the seaway with a return flow at depth either to the north or south. The abrupt change in environmental conditions at the oceanic front where mixing occurred would probably have killed the plankton and introduced large amounts of organic matter into the descending third water mass , leading to the development of an intense oxygen minimum.
The Western Interior Seaway (WIS) was a shallow and expansive body of water that covered the central United States during the Late Cretaceous. Attempts to reconstruct temperatures in the seaway using the oxygen isotopic composition of biogenic carbonates have suffered from uncertainty in the oxygen isotopic composition of seawater (delta O-18(w)) in the semi-restricted basin. We present new reconstructed temperature and delta O-18(w) data from marine and estuarine environments in the WIS and freshwater environments in WIS source rivers, derived from clumped isotope analyses of bivalve and gastropod shells. We find temperatures of 5-21 degrees C, delta O-18(w) values below contemporaneous Gulf of Mexico marine sites, and a strong correlation between delta O-18(w) and environmental setting. We propose that decreasing delta O-18(w) values reflect decreasing salinity driven by an increasing contribution of continental runoff. Using a two-end-member salinity-delta O-18(w) mixing model, we estimate salinities of 29-35 psu (practical salinity units) for the deep marine, 20-32 psu for the shallow marine, and 11-26 psu for the estuarine environments of the WIS. New climate model simulations agree with reconstructed temperatures and salinities and suggest the presence of salinity driven stratification within the seaway.
The carbon and oxygen isotope compositions of calcium carbonate shells are widely accepted and applied proxies for tracking changes in paleoenvironmental conditions such as temperature, salinity and productivity. In order to accurately interpret isotopic measurements, diagenetic alteration must first be assessed. The occurrence of aragonitic shells is often taken as a first order sign of unaltered material because aragonite at the Earth's surface is metastable and converts to calcite. However, even specimens that retain an aragonitic composition and are macroscopically well preserved often exhibit subtle to considerable signs of alteration when shell microstructure is examined using scanning electron microscopy (SEM). To determine the textural and isotopic effects of progressive alteration on differing types of shell microstructure, we undertook a comparative study of aragonitic shells from two regions frequently used in paleoenvironmental reconstructions and diagenetic studies - the Upper Cretaceous of the Gulf Coastal Plain (GCP) and the Western Interior Seaway (WIS). Because the GCP is within a passive margin setting and the WIS has experienced tectonic processes, we also evaluate how diagenetic history affects isotopic composition. Visually well-preserved ammonite, bivalve, and gastropod shells were collected from one locality in the GCP and examined using SEM. To evaluate microstructural preservation, we used a previously published scale based on WIS specimens for nacreous shell material and a complementary scale we developed for crossed lamellar microstructure. The stable carbon and oxygen isotope compositions of specimens across the preservational scale from both regions were analyzed. Isotopic composition does not change systematically with degrading preservation in the GCP specimens, a result that contrasts strongly with patterns in WIS specimens where isotopic composition systematically decreases with decreasing microstructural preservation. Dissimilar tectonic regimes that led to unique post-depositional conditions for each region are invoked to explain this difference. The WIS was a foreland basin subjected to significant meteoritic groundwater flow driven by active uplift, while the GCP was a passive margin that has remained close to sea level, with low hydraulic potential, since the time of deposition. Thus, during diagenesis the WIS shells were exposed to fluids that differed greatly from the seawater in which the organisms grew, whereas diagenetic fluids in GCP settings likely were similar to marine waters. As expected, shells with the best-preserved microstructure provide the most consistently reliable isotopic data for paleoenvironmental reconstructions, but our comparisons show that similar degrees of microstructural alteration can express very different diagenetic shifts in isotopic values.
Significance
Whether dinosaurs were in decline before their final extinction 66 Mya has been debated for decades with no clear resolution. This dispute has not been resolved because of inappropriate data and methods. Here, for the first time to our knowledge, we apply a statistical approach that models changes in speciation and extinction through time. We find overwhelming support for a long-term decline across all dinosaurs and within all three major dinosaur groups. Our results highlight that dinosaurs showed a marked reduction in their ability to replace extinct species with new ones, making them vulnerable to extinction and unable to respond quickly to and recover from the final catastrophic event 66 Mya.
To address the limited temporal and spatial context of earlier paleogeographic studies of the Western Interior Seaway (WIS), we provide a broad overview of the paleogeographical history of this epeiric sea and the basin it filled by synthesizing available data from detailed analyses of lithostratigraphic sections, isopach maps, as well as biostratigraphic and biogeographic distributions. The Early Cretaceous to Paleocene WIS connected the Arctic Ocean with the Gulf of Mexico and was likely episodically linked to the Atlantic Ocean via the Hudson Seaway. The paleogeography of the WIS primarily reflects the interplay between sea level and physiography, which were controlled by eustasy, tectonics, and pre-existing topographic features. The Western Interior Foreland Basin (WIFB) initiated in the Middle Jurassic with the subduction of Panthalassan crust beneath the North American plate which resulted in a continental margin arc-trench system and the Cordilleran Orogenic Belt. The deformation front of the Cordillera migrated progressively eastward forcing a cratonward (i.e., eastward) shift in marine sedimentation throughout its history. Several punctuated Albian marine transgressions in the WIFB were restricted to the northern part of the basin due to low sea level compounded by its physiography. The first marine connection of the WIS linking the Arctic Ocean and the Gulf of Mexico occurred during the late Albian as a response to a eustatic rise, together with increased basin subsidence. The sea's north-south connectivity was lost during lower sea levels, but, once established in the middle Cenomanian, persisted at least until the early Maastrichtian and possibly into the Paleocene. The onset of the Laramide Orogeny that broke the WIFB into a series of smaller intermountane basins along with a drop in sea-level caused the final withdrawal of the WIS from North America during the Paleocene.
We utilize carbonate clumped isotope thermometry to explore the diagenetic and thermal histories of exhumed brachiopods, crinoids, cements, and host rock in the Permian Palmarito Formation, Venezuela, and the Carboniferous Bird Spring Formation, Nevada, USA. Carbonate components in the Palmarito Formation, buried to ~4 km depth, yield statistically indistinguishable clumped isotope temperatures [T(Δ47)] ranging from 86 to 122 °C. Clumped isotope temperatures of components in the more deeply buried Bird Spring Formation (>5 km) range from ~100 to 165 °C and differ by component type, with brachiopods and pore-filling cements yielding the highest T(Δ47) (mean = 153 and 141 °C, respectively) and crinoids and host rock yielding significantly cooler T(Δ47) (mean = 103 and 114 °C). New high-resolution thermal histories are coupled with kinetic models to predict the extent of solid-state C-O bond reordering during burial and exhumation for both sites. Application of these models, termed "THRMs" (thermal history reordering models), suggests that brachiopods in the Palmarito Formation experienced partial bond reordering without complete equilibration of clumped isotopes at maximum burial temperature. In contrast, clumped isotope bonds of brachiopods from the Bird Spring Formation completely equilibrated at maximum burial temperature, and now reflect blocking temperatures achieved during cooling. The 40-50-°C-cooler clumped isotope temperatures measured in Bird Spring Formation crinoids and host rock can be explained by recrystallization and cementation during shallow burial combined with a greater inherent resistance to solid-state reordering than brachiopods.
Bivalves that host sulfur-oxidizing bacterial gill-hosted endosymbionts can inhabit low-diversity, sulfidic environmental niches. However, understanding the history of this life strategy is limited by the lack of a robust method that can be applied to fossils. Measurements of carbonate-associated sulfate S isotope ratios (CAS-δ34S) in carbonate fossils could fill this void by fingerprinting symbiont-driven oxidation of environmental sulfide. We begin to evaluate this prediction using modern lucinid bivalves, a useful test case because: (1) all modern genera host symbionts and live in sulfidic sediments, and (2) morphological evidence suggests that this has been true since the earliest ancestral lucinids. We measured S speciation, abundance, and CAS-δ34S values in the shells of a suite of modern infaunal lucinids, in addition to epifaunal bivalves with and without S-oxidizing symbionts as controls. For infaunal lucinids, CAS concentrations were at most one-third of those of non-symbiotic epifaunal bivalves, and CAS-δ34S values were lower (9.2‰−18.5‰) than in modern seawater (21‰) or epifaunal bivalves (20.8‰−21‰). These observations indicate that lucinids with symbionts incorporate sulfide-derived sulfate into their shells as a direct consequence of their chemosymbiosis. We argue that both the concentration and the magnitude of 34S depletion in infaunal lucinid CAS reflect environmental sulfide concentrations and could viably reveal chemosymbiosis in fossils.
Carbonate minerals contain stable isotopes of carbon and oxygen with different masses whose abundances and bond arrangement are governed by thermodynamics. The clumped isotopic value Δ i is a measure of the temperature-dependent preference of heavy C and O isotopes to clump, or bond with or near each other, rather than with light isotopes in the carbonate phase. Carbonate clumped isotope thermometry uses Δ i values measured by mass spectrometry (Δ 47 , Δ 48 ) or laser spectroscopy (Δ 638 ) to reconstruct mineral growth temperature in surface and subsurface environments independent of parent water isotopic composition. Two decades of analytical and theoretical development have produced a mature temperature proxy that can estimate carbonate formation temperatures from 0.5 to 1,100°C, with up to 1–2°C external precision (2 standard error of the mean). Alteration of primary environmental temperatures by fluid-mediated and solid-state reactions and/or Δ i values that reflect nonequilibrium isotopic fractionations reveal diagenetic history and/or mineralization processes. Carbonate clumped isotope thermometry has contributed significantly to geological and biological sciences, and it is poised to advance understanding of Earth's climate system, crustal processes, and growth environments of carbonate minerals. ▪ Clumped heavy isotopes in carbonate minerals record robust temperatures and fluid compositions of ancient Earth surface and subsurface environments. ▪ Mature analytical methods enable carbonate clumped Δ 47 , Δ 48 , and Δ 638 measurements to address diverse questions in geological and biological sciences. ▪ These methods are poised to advance marine and terrestrial paleoenvironment and paleoclimate, tectonics, deformation, hydrothermal, and mineralization studies.
Expected final online publication date for the Annual Review of Earth and Planetary Sciences, Volume 51 is May 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
Triple oxygen isotope (δ17O and δ18O) values of high- and low-temperature altered oceanic crust and products of basalt alteration experiments were measured to better constrain ocean isotope compositions in deep time. The data define an array of δ18O and Δ′17O (Δ′17O=δ′17O – λRL × δ′18O + γ) values from mantle values toward 1‰ and –0.01‰, respectively, with a λ of ~0.523. The altered oceanic crust data were used to construct a model for estimating δ18O-Δ′17O values of the ancient oceans if the continental weathering flux (FCW) and/or hydrothermal oceanic crust alteration flux (FHT) changed through time. A maximum lowering of 7‰ and 4‰, respectively, is achieved in the most extreme cases. The δ18O value of the ocean cannot be raised by more than 1.1‰. Eclogites from the Roberts Victor kimberlite (South Africa), with a protolith age of 3.1 Ga, have δ18O-Δ′17O values that precisely overlap with those of the modern altered oceanic crust, suggesting that the Archean oceans had similar isotope values as today. Published triple isotope data for Archean cherts show that all samples have been altered to some degree and suggest an Archean ocean surface temperature of ~70–100 °C. An ocean as light as –2‰ is still consistent with our eclogite data and reduce our temperature estimates by 10 °C.
The mid-Cretaceous thermal maximum (KTM) during Cenomanian to Santonian times from ca. 100 to 83 Ma is considered among Earth’s warmest sustained intervals of the Phanerozoic. The time interval is also characterized by major paleoceanographic changes in the form of an oceanic anoxic event and the flooding of epicontinental seaways, such as the Western Interior Seaway in North America. We report carbonate clumped isotope (Δ47) paleotemperatures (TΔ47) of the KTM measured from Cenomanian oyster fossils of the Western Interior Seaway. Following screening of specimens for carbonate diagenesis and exclusion of geographic zones with evidence consistent with solid-state Δ47 reordering, a mean TΔ47 of 28–34 °C (95% confidence interval for the standard error of mean) for primary oyster calcite quantifies extreme mid-latitude warmth in North America. When combined with existing Campanian and Maastrichtian marine TΔ47 records, the new data constrain Late Cretaceous temperature trends underlying the evolution of North American faunal and stratigraphic records. These TΔ47 data from the peak KTM highlight the potential of this proxy to quantitatively resolve the upper thermal limits of Phanerozoic greenhouse climates.
The δ18O of carbonate minerals that formed at Earth’s surface is widely used to investigate paleoclimates and paleo-elevations. However, a multitude of hydrologic processes can affect δ18O values, including mixing, evaporation, distillation of parent waters, and carbonate growth temperatures. We combined traditional carbon and oxygen isotope analyses with clumped (Δ47) and triple oxygen isotopes (Δ′17O) analyses in oyster shells (Acutostrea idriaensis) of the Goler Formation in southern California (USA) to obtain insights into surface temperatures and δ18O values of meteoric waters during the early Eocene hothouse climate. The Δ47-derived temperatures ranged from 9 °C to 20 °C. We found a correlation between the δ18O of growth water (δ18Ogw) (calculated using Δ47 temperatures and δ18O of carbonate) and the δ13C values of shells. The Δ′17O values of shell growth waters (0.006‰–0.013‰ relative to Vienna standard mean ocean water–standard light Antarctic precipitation [VSMOW-SLAP]) calculated from Δ′17O of carbonate (–0.087‰ to –0.078‰ VSMOW-SLAP) were lower than typical meteoric waters. These isotopic compositions are consistent with oyster habitation in an estuary. We present a new triple oxygen isotope mixing model to estimate the δ18O value of freshwater supplying the estuary (δ18Ofw). The reconstructed δ18Ofw of –11.3‰ to –14.7‰ (VSMOW) is significantly lower than the δ18Ogw of –4.4‰ to –9.9‰ that would have been calculated using “only” Δ47 and δ18O values of carbonate. This δ18Ofw estimate supports paleogeographic reconstructions of a Paleogene river fed by high-elevation catchments of the paleo–southern Sierra Nevada. Our study highlights the potential for paired Δ47 and Δ′17O analyses to improve reconstructions of meteoric water δ18O, with implications for understanding ancient climates and elevations.
Speleothem oxygen isotope (δ¹⁸O) records provide key insight into the rate and timing of terrestrial paleoclimate changes during the late Quaternary. However, it can be difficult to deconvolve the δ¹⁸O signal into individual components, which include processes related to moisture source, moisture transport, temperature, precipitation amount, infiltration, and the cave environment. We developed a framework that uses triple oxygen isotope distributions in speleothems to refine interpretations of δ¹⁸O speleothem records. This framework identifies the influence of dominant processes on δ¹⁸O values through time by their characteristic (although not necessarily unique) trends in δ’¹⁸O vs. Δ’¹⁷O space, where Δ′¹⁷O = δ’¹⁷O – 0.528δ’¹⁸O and δ’xO = ln(δxO+1). Following Guo and Zhou (2019a), we expect that ‘cave kinetic’ processes (e.g., fast degassing at the drip site, prior calcite precipitation) will drive positive trends between δ’¹⁸O and Δ′¹⁷O. In contrast, we can identify hydrologic processes from near–horizontal trends that reflect Rayleigh–type meteoric water processes and negative trends driven by changes in evaporation processes at the moisture source region or at the cave site, mineralization temperature, and seasonality in precipitation/infiltration amount. We applied this framework to four western USA speleothems from Cave of the Bells (Arizona), Leviathan Cave (Nevada), and Lehman Caves (Nevada). The Cave of the Bells and Leviathan data have near–horizontal to negative trends indicating δ¹⁸O variability was driven largely by changes in Rayleigh distillation of atmospheric moisture and moisture source conditions, supporting prior interpretations. We analyzed two Lehman Caves records because they were likely influenced by non-equilibrium processes and the data show weak to moderate negative trends. For sample LMC–12b, chosen for its extreme 7.5 ‰ δ¹⁸O range, the trend is statistically distinct from the near–horizontal Rayleigh–process trend and most consistent with changes in local evaporation intensity and infiltration seasonality as primary drivers. None of these records displays a positive covariation slope between δ’¹⁸O and Δ′¹⁷O, suggesting limited variability in cave kinetic processes through time or unknown limitations to the kinetic model of Guo and Zhou (2019a). Additionally, reconstructed formation waters for all sites fall near the δ’¹⁸O vs. Δ′¹⁷O Local Meteoric Water Line, a correlation we suggest as a novel test of the absolute magnitude of isotopic offset due to cave kinetic processes. More broadly, our framework adds context to the only other study of carbonate speleothem triple oxygen isotope composition (Sha et al., 2020). We find that positive to negative δ’¹⁸O vs. Δ′¹⁷O trends likely exist in speleothem data that may reasonably be expected from regional climate processes and that, combined with other proxy data, triple oxygen isotope data will be useful in constraining interpretations of δ¹⁸Ospeleothem records.
The stable oxygen isotope composition (δ¹⁸O) of ammonites and belemnites is a common proxy for Jurassic and Cretaceous sea temperatures. The challenges and uncertainties associated with cephalopod δ¹⁸O and other proxy based paleotemperature reconstructions make cephalopods such as ammonites and belemnites a favourable target for carbonate clumped isotope paleothermometry. Our measurements of the clumped isotopic composition of modern cephalopods (Nautilus pompilius, Nautilus macromphalus, Nautilus belauensis and Sepia officianalis), however, confirm significant “vital effects” in clumped isotopic of cephalopod aragonite.
We present the first intra-shell measurements of clumped isotopic composition of nautilus demonstrating that clumped isotope composition (∆47) is correlated with δ¹⁸O shell carbonate. We observe a decrease in ∆47 from juvenile to adult septa, which does not correspond with changes in surface seawater temperature or temperature variation on vertical migration of the nautilus within the water column. The clumped isotope composition of juvenile septa yield temperatures that are within error of growth temperature while most recently formed septa reflect growth temperatures 3–8 °C above the upper temperature limit of survival of the organism (27 °C). Non-equilibrium effects observed in the ∆47 of modern nautilus comprise up to a 29 °C overestimate in shell formation temperature.
Combined δ¹⁸O and ∆47 measurements are used to differentiate between potential mechanisms that may generate non-equilibrium signatures observed. Non-equilibrium clumped isotope signatures may result from variations in the salinity of cameral fluid within nautilus chambers, dehydration of HCO³⁻ and/or isotopic fractionation during CO2 diffusion across cell walls. The inter-skeletally variable non-equilibrium offsets in ∆47 of nautili aragonite observed in this study suggest that care must be taken when using the clumped isotope composition of cephalopod carbonate, in particular ammonite aragonite, as a paleotemperature proxy. Belemnite calcite and ontogenetically early septa of aragonitic ammonites may be feasible targets for paleotemperature reconstruction by carbonate clumped isotope thermometry.
Carbonate clumped isotopes (Δ47) have become a widely applied method for paleothermometry, with applications spanning many environmental settings over hundreds of millions of years. However, Δ47-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 Δ47 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 O18 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 Δ47 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 Δ47 blocking temperatures.
Methane seep deposits, comprising large, carbonate-rich mounds formed from hydrocarbon seepage, were widely distributed in the Late Cretaceous Western Interior Seaway (WIS) of North America. Well-preserved, methane-derived authigenic carbonates (MDACs) from these deposits have been shown to retain petrological, paleontological, and geochemical imprints of their ancient depositional setting, all of which are important for understanding the dynamics and evolution of the shallow, epeiric WIS. To better characterize the environmental conditions of WIS seeps, we applied clumped isotope paleothermometry to magnesium calcite MDAC samples from five seep localities in the upper Campanian Pierre Shale, South Dakota, USA. We measured 21 subsamples, including 18 micritic carbonates and demonstrated apparent clumped isotope equilibrium between MDACs and WIS bottom waters. Extreme 13C depletion in most samples (δ13C ranging to −45.44‰) indicates they were precipitated with oxidized methane as a major source of dissolved inorganic carbon, which itself implies a close association with ancient methanotrophic metabolism. The average clumped isotope paleotemperature from the micritic carbonates is 23 ± 7 °C (1σ standard deviation), which agrees with bottom water paleotemperatures inferred from δ18O measurements of MDACs and well-preserved mollusk shells at similar localities in the WIS. The calculated average δ18Ow value for these samples is −0.5 ± 1.7‰ (1σ SD), which is indistinguishable from previously reported calculation on Campanian seawater δ18Ow from fossil mollusk shells, but elevated above younger fossils collected from other locations in the WIS. Our conclusions are inconsistent with previously hypothesized disequilibrium for WIS MDAC clumped isotope and therefore we propose that fossil MDAC deposits may be used as paleotemperature archives.
The future in the past
A major cause of uncertainties in climate projections is our imprecise knowledge of how much warming should occur as a result of a given increase in the amount of carbon dioxide in the atmosphere. Paleoclimate records have the potential to help us sharpen that understanding because they record such a wide variety of environmental conditions. Tierney et al. review the recent advances in data collection, statistics, and modeling that might help us better understand how rising levels of atmospheric carbon dioxide will affect future climate.
Science , this issue p. eaay3701
High precision triple oxygen isotope measurements of carbonates can better constrain temperatures and oxygen isotope compositions of seawater through geologic time than ¹⁸O/¹⁶O measurements alone, but lack of a definitive calibration has hindered progress. In this study, we fluorinated both carbonate and water samples to measure quantitatively the triple oxygen isotope composition of each phase. We compared the oxygen isotope fractionation between carbonate and water for different carbonate materials: calcite synthesized with and without carbonic anhydrase, abiogenic calcite from Devils Hole, and extant biogenic calcite and aragonite of marine origin. We found similar 1000lnα¹⁸Occ-wt values for all materials and combined the results with the high temperature experimental data of O’Neil et al. (1969), resulting in the following fractionation equation (T in Kelvins). The calcite triple oxygen isotope values yielded a θ-T relationship of θcc-wt= –1.39(±0.01)/T+0.5305 whereas the aragonite triple oxygen isotope values yielded a θ-T relationship of θara-wt= –1.53(±0.02)/T+0.5305. The calcite-water triple oxygen isotope equilibrium fractionation equation for natural samples is. The combined 1000lnα¹⁸O and 1000lnα¹⁷O relationships can be used to assess equilibrium in ancient samples and to evaluate potential secular changes in the δ¹⁸O value of seawater. Most of the Phanerozoic samples analyzed in this study, which were determined to be pristine in previous studies, have undergone some level of diagenesis. Two samples appear to preserve their original oxygen isotope compositions and suggest a cool ocean with a δ¹⁸O value similar to the modern ocean. Using a fluid-rock interaction model, we can “see through” the diagenetic process and estimate the triple oxygen isotope composition of the carbonate prior to alteration. In doing so, we show that for the time intervals and sample locations measured in this study, Phanerozoic oceans had a comparable range of oxygen isotope compositions and temperatures as modern seawater.
High precision triple oxygen isotope measurements are becoming a more common analysis in laboratories. There is a lack of calibrated standards to use for triple oxygen isotope measurements and this has led to data being presented on different scales rather than to the traditional VSMOW2-SLAP2 scale. Here we present triple oxygen isotope values of standard carbonates, CO2 liberated from carbonates, silicates and air calibrated to the VSMOW2-SLAP2 scale. We analyzed VSMOW2 and SLAP2 to calibrate our reference gas. Our measured δ¹⁸O value of SLAP2 is −55.55‰, indistinguishable from the accepted value of −55.5‰. Our Δ′¹⁷O value of SLAP2 (λ = 0.528) is not zero, but rather −0.015‰, corresponding to a δ¹⁷O value of −29.741‰. The Δ′¹⁷O values of carbonate standards NBS19, IAEA603 and NBS18 are −0.102, −0.100 and − 0.048‰, respectively (±0.010). For CO2 of calcite liberated by phosphoric acid digestion at 25 °C, the θACID value at 25 °C is 0.5230 ± 0.0003. These results can be used to correct triple oxygen isotope measurements of CO2 released by phosphoric acid digestion in other laboratories. We present triple oxygen isotope values for UW Garnet-2, NBS-28, San Carlos Olivine (NM-SCO), and our in-house quartz standard (NM-Q). Aliquots of NM-Q and NM-SCO are available from the Center for Stable Isotopes (CSI), New Mexico for interlaboratory comparison. Our δ¹⁷O and δ¹⁸O values for air are 12.178‰ (±0.066) and 24.046‰ (±0.117), respectively, with a corresponding Δ′¹⁷O value of −0.441 ± 0.012‰. With the availability of common standards, all laboratories making δ¹⁷O-δ¹⁸O measurements can calibrate their reference gas relative to the VSMOW2-SLAP2 scale. Laboratories making triple oxygen isotope measurements on CO2 released from carbonates using phosphoric acid digestion can correct to the bulk carbonate value.
Triple oxygen isotope composition of carbonate minerals reflects the isotope composition of the water from which carbonates precipitate, and is emerging as a promising proxy for constraining past changes in Earth hydroclimate and surface environment. However, quantitative interpretation of this proxy is not straightforward when carbonate minerals do not form under isotope equilibrium with water. For these carbonates, isotope composition of dissolved inorganic carbon (DIC) in the precipitating solution usually exerts the dominant control on their isotope composition. Here we examine the systematics of triple oxygen isotope fractionation in the DIC-H2O-CO2 system by deriving the fundamental equilibrium and kinetic triple oxygen isotope fractionation factors in this system and simulating the evolution of triple oxygen isotope composition of DIC during three common isotope fractionation processes, i.e., DIC-H2O isotope exchange, CO2 degassing and CO2 absorption. We show that under thermodynamic equilibrium dissolved HCO3– and CO32– both exhibit similar Δ′17O as calcite and aragonite (within 10 per meg at 25 °C), with temperature dependences around 0.61 per meg/°C. However, kinetic isotope fractionations associated with CO2 hydration and hydroxylation and their reverse reactions can produce a range of disequilibrium triple oxygen isotope effects in DIC during all three processes we simulated. The magnitudes of these disequilibrium effects vary with both time and the physicochemical conditions of the solution, e.g., temperature, pH and initial composition. Particularly, we predict correlated enrichments in Δ′17O and δ¹⁸O of DIC during CO2 degassing but depletions during CO2 absorption. The slopes of these correlations vary mainly as a function of solution pH but not temperature, DIC concentration or air pCO2, yielding values of 8.8 and 12.0 per meg/‰ at pH = 8 and 9, respectively, at 25 °C. Such disequilibrium isotope effects in DIC are expected to be inherited by carbonate minerals that form from these solutions (e.g., speleothem, coral skeleton, and high pH travertine) and, if not accounted for, could lead to inaccurate estimates of the triple oxygen isotope compositions of the parent water and carbonate formation temperatures. Our numerical model provides a quantitative framework for interpreting triple oxygen isotope composition of DIC and for correcting disequilibrium triple oxygen isotope effects in natural carbonates.
Triple oxygen isotope analyses were made on geothermal fluids and precipitates from Chile and Iceland to calibrate the silica-water isotopic fractionation for abiotic silica formation at elevated temperatures and were used to evaluate potential fractionation effects of biogenic vs. abiogenic samples and polymorphism. Coexisting water and amorphous silica precipitated inside the heat exchanger of the Hellisheiði power plant at 60 and 118 °C have triple oxygen isotope fractionations in excellent agreement with previous results from analyses of biogenic silica precipitated in cold waters. In contrast to samples from the geothermal plant, natural amorphous silica precipitates and waters formed in active hot springs (T = 63–84 °C) in the Puchuldiza geothermal area of northern Chile gave temperature estimates from the silica-water thermometer far lower (37–46 °C) than the measured water temperatures. Active silica precipitation was found to only occur at and above the air-water interface on glass slides placed in the hot spring waters for 9 days. The calculated temperatures and visual inspection suggest that precipitation occurred along channel edges when saturation was overstepped by a factor of two. In contrast to the surficial neoformed amorphous silica, subsurface silica samples (>10 cm) have recrystallized to opal-CT and quartz within a sinter mound and these samples preserve isotope temperatures of 82 °C and 89 °C, in good agreement with the ambient temperatures of the thermal spring conduit system. The δ¹⁸O values of abiogenic, low temperature silica formed in spring water far from the thermal waters with a measured temperature of 19 °C correspond to a silica-water temperature estimate of 20 °C. All samples preserved isotope data corresponding to their expected formation temperatures and appear to be in equilibrium in the triple oxygen isotope system. A best-fit θ–T relationship for silica-water using our inorganic silica-water samples is θ=0.5305-[Formula presented],R²=0.998whereθa-b=[Formula presented]. This new equation is indistinguishable from a previous empirical fit by Sharp et al. (2016) based primarily on biogenic silica samples, suggesting that the biogenic and abiogenic samples secreted silica with the same fractionation. Our results show that triple oxygen isotope measurements are robust and can be used to estimate the temperature of formation, the isotopic composition of the formation water, and discern between equilibrium and non-equilibrium processes.
The field of isotope geochemistry began with the study of oxygen isotope geothermometry, most notably for carbonates. For traditional oxygen isotope geothermometry only the relationship between one rare isotope, oxygen-18, and the common isotope, oxygen-16, is used because for most terrestrial processes the ¹⁷O-¹⁶O relationship scales with the ¹⁸O-¹⁶O relationship and is thought to not grant any new information. However, theoretical analysis predicts a small temperature-dependence of the equilibrium triple oxygen isotope relationship and instrumentation and techniques now allow for high-precision determination of the oxygen isotope composition for all three oxygen isotopes for a variety of sample types. To set the groundwork for triple oxygen isotope geothermometry, here we present new calibrations based on statistical thermodynamics and density functional theory for both the traditional two isotope and the recently introduced triple isotope thermometer for pairs of quartz, calcite, dolomite, fluorapatite, hematite, magnetite and liquid water. The results compare well with previous studies on ¹⁸O/¹⁶O fractionation where theoretical and experimental data are available. Of the models given here, pairs of quartz, calcite, dolomite and fluorapatite with water, hematite or magnetite show promising temperature sensitivities as triple isotope thermometers with acceptable uncertainties for surface and low-T hydrothermal environments.
A compilation of foraminiferal stable isotope measurements from southern high latitude (SHL) deep-sea sites provides a novel perspective important for understanding Earth's paleotemperature and paleoceanographic changes across the rise and fall of the Cretaceous Hot Greenhouse climate and the subsequent Paleogene climatic optimum. Both new and previously published results are placed within an improved chronostratigraphic framework for southern South Atlantic and southern Indian Ocean sites. Sites studied were located between 58° and 65°S paleolatitude and were deposited at middle to upper bathyal paleodepths. Oxygen isotope records suggest similar trends in both bottom and surface water temperatures in the southern sectors of the South Atlantic and in the Indian Ocean basins. Warm conditions were present throughout the Albian, extreme warmth existed during the Cretaceous Thermal Maximum (early-mid-Turonian) through late Santonian, and long-term cooling began in the Campanian and culminated in Cretaceous temperature minima during the Maastrichtian. Gradients between surface and seafloor δ 18 O and δ 13 C values were unusually high throughout the 11.5 m.y. of extreme warmth during the Turonian-early Campanian, but these vertical gradients nearly disappeared by the early Maastrichtian. In absolute terms, paleotemperature estimates that use standard assumptions for pre-glacial seawater suggest sub-Antarctic bottom waters were ≥21 °C and sub-Antarctic surface waters were ≥27 °C during the Turonian, values warmer than published climate models support. Alternatively, estimated temperatures can be reduced to the upper limits of model results through freshening of high latitude waters but only if there were enhanced precipitation of water with quite low δ 18 O values. Regardless, Turonian planktonic δ 18 O values are ~1.5‰ lower than minimum values reported for the Paleocene-Eocene Thermal Maximum (PETM) from the same region , a difference which corresponds to Turonian surface temperatures ~6 °C warmer than peak PETM temperatures if Turonian and Paleocene temperatures are estimated using the same assumptions. It is likely that warm oceans surrounding and penetrating interior Antarctica (given higher relative sea level) prevented growth of Antarctic ice sheets at all but the highest elevations from the late Aptian through late Campanian; however, Maastrichtian temperatures may have been cool enough to allow growth of small, ephemeral ice sheets. The standard explanation for the sustained warmth during Cretaceous Hot Greenhouse climate invokes higher atmospheric CO 2 levels from volcanic outgassing, but correlation among temperature estimates, proxy estimates of pCO 2 , and intervals of high fluxes of both mafic and silicic volcanism are mostly poor. This comparison demonstrates that the relative timing between events and their putative consequences need to be better constrained to test and more fully understand relationships among volcanism, pCO 2 , temperature ocean circulation, Earth's biota and the carbon cycle.
Ar/³⁹Ar dating of a bentonite associated with a cold methane seep deposit in the upper Campanian Baculites compressus Zone of the Pierre Shale in South Dakota yields an age of 73.79 ± 0.36 Ma. This is in close agreement with the previously published age of 74.05 ± 0.39 Ma for this zone (Obradovich, 1993) and nearly identical to an unpublished age of 73.70 ± 0.13 Ma for the same zone, both samples of which are from a more proximal locality near the base of the Bearpaw Shale in Montana, as recalculated relative to a Fish Canyon standard age of 28.201 Ma. We also report dates for two bentonites from the upper Campanian Exiteloceras jennyi and Didymoceras stevensoni zones.
The measurement of multiply isotopically substituted (‘clumped isotope') carbonate groups provides a way to reconstruct past mineral formation temperatures. However, dissolution-reprecipitation (i.e., recrystallization) reactions, which commonly occur during sedimentary burial, can alter a sample’s clumped-isotope composition such that it partially or wholly reflects deeper burial temperatures. Here we derive a quantitative model of diagenesis to explore how diagenesis alters carbonate clumped-isotope values. We apply the model to a new dataset from deep-sea sediments taken from Ocean Drilling Project site 807 in the equatorial Pacific. This dataset is used to ground truth the model. We demonstrate that the use of the model with accompanying carbonate clumped-isotope and carbonate δ18O values provides new constraints on both the diagenetic history of deep-sea settings as well as past equatorial sea-surface temperatures. Specifically, the combination of the diagenetic model and data support previous work that indicates equatorial sea-surface temperatures were warmer in the Paleogene as compared to today. We then explore whether the model is applicable to shallow-water settings commonly preserved in the rock record. Using a previously published dataset from the Bahamas, we demonstrate that the model captures the main trends of the data as a function of burial depth and thus appears applicable to a range of depositional settings.
Most studies examining faunal assemblages use their sedimentary context as a critical element in constraining and reconstructing their underlying environmental controls. This has resulted in the assumption that an absence of lithofacies change in a section should be reflected in a lack of environmental variation. This inference, however, has been placed into question by evidence that marine species are influenced by a broader range of environmental dynamics than just change in lithofacies. In this study, we examine the sensitivity of marine faunas to broadly defined environmental change within lithologically homogenous strata by examining concretionary fossil assemblages of the Baculites eliasi through B. clinolobatus biozones in monotonous, clay-rich strata of the Campanian-Maastrichtian Pierre Shale in Wyoming. We recognize five biofacies, which reflect different environmental conditions related to benthic oxygenation, substrate firmness, and water depth. Analyses of abundance patterns, raw species richness trends, and life-habit patterns display recurrent switching, upsection, between low- and high-diversity intervals. Our data reveal that samples with lower diversity show a strong relationship with intervals when water conditions were deepest, whereas higher diversity samples are associated with periods when shallow-water conditions prevailed in the study area. The distribution of taxa and diversity of the assemblages most likely reflect migrating oxygen- and substrate-controlled biofacies that were responding to changes in depth. This study shows that substantial changes in biofacies, diversity, and life habits can arise in response to variations in water depth with limited to no apparent change in lithofacies supporting the hypothesis that fossil taxa are much more sensitive indicators of environmental change than lithofacies.
We examined the stratigraphic distribution of ammonites at a total of 29 sites around the world in the last 0.5 myr of the Maastrichtian. We demarcated this interval using biostratigraphy, magnetostratigraphy, cyclostratigraphy, and data on fossil occurrences in relation to the K/Pg boundary in sections without any facies change between the highest ammonites and the K/Pg boundary. The ammonites at this time represent all four Mesozoic suborders comprising six superfamilies, 31 (sub)genera, and 57 species. The distribution of ammonites is dependent on the environmental setting. Recent data suggest that ammonites persisted to the boundary and some species may have survived for several tens of thousands of years into the Paleogene. The best explanation for ammonite extinction is a brief episode of ocean acidification immediately following the Chixculub impact, which caused the decimation of the calcareous plankton including the planktic post-hatching stages of ammonites. The geographic distribution of ammonites may also have played a role in the events with more broadly distributed genera being more resistant to extinction.
The widespread phenomenon of faunal clustering in concretions is examined in examples from the Late Cretaceous Bearpaw Formation of southern Alberta. Concretion-hosted shell clusters have traditionally been interpreted as biologically formed features. In contrast, sedimentologic, taphonomic and faunal evidence in the Bearpaw Formation suggests that shelf clustering is inherently related to a common physical control. The clustered nature of the Bearpaw, assemblages reflects the accumulation of shells in isolated storm scours. Differences in faunal composition resulted from variations in (1) the mode of scour initiation (2) the faunal content of sea-floor sediments, and (3) the degree of shell transport experienced during storms. The formation of the isolated scours is attributed to locally augmented erosion in, the vicinity of bed defects and objects exposed on the sea floor. The common restriction of fossils to concretions reflects preferred concretion growth around the shell-rich scour fills. Previously described examples of concretion-hosted shell clusters are re-interpreted in light of the scour-till hypothesis.
Carbonate minerals provide a rich source of geochemical information because their δ13C and δ18O values provide information about surface and subsurface Earth processes. However, a significant problem is that the same δ18O value is not reported for the identical carbonate sample when analyzed in different isotope laboratories in spite of the fact that the International Union of Pure and Applied Chemistry (IUPAC) has provided reporting guidelines for two decades. This issue arises because (1) the δ18O measurements are performed on CO2 evolved by reaction of carbonates with phosphoric acid, (2) the acid-liberated CO2 is isotopically fractionated (enriched in 18O) because it contains only two-thirds of the oxygen from the solid carbonate, (3) this oxygen isotopic fractionation factor is a function of mineralogy, temperature, concentration of the phosphoric acid, and δ18O value of water in the phosphoric acid, (4) researchers may use any one of an assortment of oxygen isotopic fractionation factors that have been published for various minerals at various reaction temperatures, and (5) it sometimes is not clear how one should calculate δ18OVPDB values on a scale normalized such that the δ18O value of SLAP reference water is –55.5 ‰ relative to VSMOW reference water.
Diagenesis commonly is a subtle, yet significant source of taphonomic bias. Failure to recognize diagenetic influences on primary depositional features can lead to spurious paleoecological and paleoenvironmental conclusions. This is exemplified by early diagenetic calcareous concretions and laminated siderite in the lower part of the Dugger Formation (Pennsylvanian: Desmoinesian) in SW Indiana. Here, paleontological evidence supports previous geochemical interpretations of laminated-siderite formation. Recognition and exploitation of early diagenetic lithotypes in other rock units may also provide enhanced insights into faunal diversity, paleoecology, and depositional setting.-from Author
Non-avian dinosaurs went extinct 66 million years ago, geologically coincident with the impact of a large bolide (comet or asteroid) during an interval of massive volcanic eruptions and changes in temperature and sea level. There has long been fervent debate about how these events affected dinosaurs. We review a wealth of new data accumulated over the past two decades, provide updated and novel analyses of long-term dinosaur diversity trends during the latest Cretaceous, and discuss an emerging consensus on the extinction’s tempo and causes. Little support exists for a global, long-term decline across non-avian dinosaur diversity prior to their extinction at the end of the Cretaceous. However, restructuring of latest Cretaceous dinosaur faunas in North America led to reduced diversity of large-bodied herbivores, perhaps making communities more susceptible to cascading extinctions. The abruptness of the dinosaur extinction suggests a key role for the bolide impact, although the coarseness of the fossil record makes testing the effects of Deccan volcanism difficult.