Reconstructing long-term continental temperature change provides the required counterpart to age equivalent marine records and can reveal how terrestrial and marine temperatures were related during ...times of extreme climate change such as the Miocene Climatic Optimum (MCO) and the following Middle Miocene Climatic Transition (MMCT). Carbonate clumped isotope temperatures (T(Δ
)) from 17.5 to 14.0 Ma Central European paleosols (Molasse Basin, Switzerland) display a temperature pattern during the MCO that is similar to coeval marine temperature records. Maximum temperatures in the long-term soil T(Δ
) record (at 16.5 and 14.9 Ma) lag maximum ocean bottom water temperatures, lead global ice volume, and mark the initiation of minimum global ice volume phases. The suggested onset of the MMCT, deduced by a marked and rapid decline in Molasse Basin soil temperatures is coeval with cooling reported in high-latitudinal marine records. This is best explained by a change in the seasonal timing of soil carbonate formation that was likely driven by a modification of rainfall seasonality and thus by a major reorganization of mid-latitude atmospheric circulation across Central Europe. In particular, our data suggest a strong climate coupling between the North Atlantic and Central Europe already in the middle Miocene.
Abstract
Surface temperature is a fundamental parameter of Earth’s climate. Its evolution through time is commonly reconstructed using the oxygen isotope and the clumped isotope compositions of ...carbonate archives. However, reaction kinetics involved in the precipitation of carbonates can introduce inaccuracies in the derived temperatures. Here, we show that dual clumped isotope analyses, i.e., simultaneous ∆
47
and ∆
48
measurements on the single carbonate phase, can identify the origin and quantify the extent of these kinetic biases. Our results verify theoretical predictions and evidence that the isotopic disequilibrium commonly observed in speleothems and scleractinian coral skeletons is inherited from the dissolved inorganic carbon pool of their parent solutions. Further, we show that dual clumped isotope thermometry can achieve reliable palaeotemperature reconstructions, devoid of kinetic bias. Analysis of a belemnite rostrum implies that it precipitated near isotopic equilibrium and confirms the warmer-than-present temperatures during the Early Cretaceous at southern high latitudes.
High-precision analysis of the excess abundance (relative to the stochastic distribution) of mass 48 isotopologues in CO2 evolved from acid digestion of carbonates (∆48) has not been possible until ...recently due to the relatively low natural abundance of 18O. Here we show that the 253 Plus™ gas source mass spectrometer equipped with Faraday cups and 1013 Ω resistors can perform combined ∆47 and ∆48 analyses on carbonates with external reproducibilities (1SD) of 0.010 ‰ and 0.030 ‰, respectively.
~10 mg aliquots of five carbonate reference materials (ETH 1, ETH 2, ETH 3, ETH 4, and Carrara) are digested with phosphoric acid at 90 °C using a common acid bath. The evolved CO2 is purified using an automated gas preparation system (including cryotraps and a GC) and analyzed for its ∆47 and ∆48 compositions using the dual inlet system of a 253 Plus™ gas source mass spectrometer. Raw ∆47 and ∆48 values are finally normalized to the Carbon Dioxide Equilibrium Scale (CDES).
In ∆47, CDES 90°Cvs. ∆48, CDES 90°C space, calcite reference materials Carrara, ETH 3 and ETH 4 agree with the equilibrium curve for calcite after adding semi-empirically determined 90 °C acid fractionation factors of 0.196 ‰ (for ∆47) and 0.136 ‰ (for ∆48) to theoretical ∆63 and ∆64 data. Agreement between measured and theoretically expected ∆48, CDES 90°C highlights the accuracy of our high-precision clumped isotope analytical setup. Combined analysis of the abundances of mass 47 and mass 48 isotopologues in CO2 evolved from acid digestion of natural carbonates has excellent potential for the determination of accurate and highly precise paleotemperatures as well as for the identification of rate-limiting kinetic processes involved in biomineralization. A formation temperature of 15(±2) °C is obtained on the 95 % confidence level for the Upper Cretaceous chalk sample ETH 3.
It has recently been shown that the clumped isotopic composition (Δ47) of carbonate-bearing hydroxyapatite (CHAP) from teeth and bones reveals important information about the body temperature of ...vertebrates (Eagle et al., 2010, 2011). In this study we reinvestigate the temperature dependence of Δ47 in CHAP, extending the temperature range from 2 to 59°C. In addition, the effects of chemical pre-treatment of CHAP on its bulk and clumped isotopic composition are studied.
CHAP is best reacted with phosphoric acid at 90 to 110°C minimizing the potential of evolved CO2 or reaction intermediates to re-equilibrate with traces of water in the acid environment. Reaction at 110°C ensures that digestions of CHAP samples are complete within 60min. We determined a Δ47⁎90–110 value of 0.032±0.008‰ that is - within errors - indistinguishable from a Δ47⁎90–110 value of 0.019±0.007‰ received for aragonite.
For tooth enamel pre-treated with H2O2 lower Δ47 values, and higher δ18O and δ13C values were measured than for pre-treated tooth dentine. In addition, similar trends were observed for pre-treated dentine and bone material: higher Δ47 values, and lower δ18O and δ13C values were determined compared to untreated samples.
A new tentative clumped isotope temperature calibration based on a synthetic apatite, untreated tooth enamel of an African elephant and enameloid from teeth of a Greenland shark is presented using a reaction temperature of 110°C. It follows the equation:
Δ47=0.0320(±0.0022)×106/T2+0.1977(±0.0259)(Δ47in‰andTinK).
The slope of this regression line is identical to those previously obtained from 90°C digestions of calcite and/or aragonite in several laboratories (e.g., Henkes et al., 2013; Wacker et al., 2014; Defliese and Lohmann, 2015). The Δ47 data of untreated enamel(oid) samples reacted at 90°C closely match a Δ47-1/T2 relationship for calcite that was made at the same digestion temperature (Wacker et al., 2014). These preliminary results suggest that calcite calibrations made at a reaction temperature of 90°C might be directly applicable to CHAP samples to determine their formation temperatures.
It is well known that a subtle nonlinearity can occur during clumped isotope analysis of CO ₂ that – if remaining unaddressed – limits accuracy. The nonlinearity is induced by a negative background ...on the m / z 47 ion Faraday cup, whose magnitude is correlated with the intensity of the m / z 44 ion beam. The origin of the negative background remains unclear, but is possibly due to secondary electrons. Usually, CO ₂ gases of distinct bulk isotopic compositions are equilibrated at 1000 °C and measured along with the samples in order to be able to correct for this effect. Alternatively, measured m / z 47 beam intensities can be corrected for the contribution of secondary electrons after monitoring how the negative background on m / z 47 evolves with the intensity of the m / z 44 ion beam. The latter correction procedure seems to work well if the m / z 44 cup exhibits a wider slit width than the m / z 47 cup. Here we show that the negative m / z 47 background affects precision of dual inlet-based clumped isotope measurements of CO ₂ unless raw m / z 47 intensities are directly corrected for the contribution of secondary electrons. Moreover, inaccurate results can be obtained even if the heated gas approach is used to correct for the observed nonlinearity. The impact of the negative background on accuracy and precision arises from small imbalances in m / z 44 ion beam intensities between reference and sample CO ₂ measurements. It becomes the more significant the larger the relative contribution of secondary electrons to the m / z 47 signal is and the higher the flux rate of CO ₂ into the ion source is set. These problems can be overcome by correcting the measured m / z 47 ion beam intensities of sample and reference gas for the contributions deriving from secondary electrons after scaling these contributions to the intensities of the corresponding m / z 49 ion beams. Accuracy and precision of this correction are demonstrated by clumped isotope analysis of three internal carbonate standards. The proposed correction scheme can be easily applied if the slit width of the m / z 49 Faraday cup is bigger than that of the m / z 47 cup.
Triple oxygen isotope data of tooth enamel (phosphate) from 16 modern marine vertebrates (sharks, mammals) from different marine habitats are presented. Marine mammal data show a clear signature of ...isotopically anomalous metabolic oxygen in their bioapatite. This is related to a low flux ratio of isotopically normal drinking and food water to anomalous metabolic oxygen. Traces of anomalous metabolic oxygen are also observed in shark bioapatite. Sharks and mammals have overlapping 18O/16O ratios, but distinct 17O signatures. Our new data suggest that triple oxygen isotopes are a valuable new tracer to assess the physiology of extant and extinct marine vertebrates and may provide a new proxy for past atmospheric CO2 mixing ratios and global primary biosphere production.
Subaqueous carbonates from the Devils Hole caves (southwestern USA) provide a continuous Holocene to Pleistocene North American paleoclimate record. The accuracy of this record relies on two ...assumptions: That carbonates precipitated close to isotope equilibrium and that groundwater temperature did not change significantly in the last 570 thousand years. Here, we investigate these assumptions using dual clumped isotope thermometry. This method relies on simultaneous analyses of carbonate ∆47 and ∆48 values and provides information on the existence and extent of kinetic isotope fractionation. Our results confirm the hypothesis that calcite precipitation occurred close to oxygen and clumped isotope equilibrium during the last half million years in Devils Hole. In addition, we provide evidence that aquifer temperatures varied by less than ±1°C during this interval. Thus, the Devils Hole calcite δ18O time series exclusively represents changes in groundwater δ18O values.
Plain Language Summary
The oxygen isotope composition of cave carbonates records changes in Earth's climate. However, the reliability of such records depends on how stable the carbonate precipitation environment was. Here, we use a novel method called dual clumped isotope thermometry that can provide simultaneous information on a carbonate's growth temperature and whether any additional fractionation processes affected its oxygen and clumped isotope signatures. Specifically, we investigated the Devils Hole caves, which provide a reference oxygen isotope time series for North America. We find that groundwater temperature did not change significantly in the last half‐million years. Variations in the oxygen isotope composition of the deposited carbonates solely reflect variations in the oxygen isotope composition of the groundwater.
Key Points
We use dual clumped isotope thermometry (∆47 and ∆48) to test for changes in calcite δ18O unrelated to variations in δ18O of groundwater
Calcite δ18O record represents changes solely in δ18O of groundwater without contributions from variations in temperature and kinetics
It is possible to achieve a stable carbonate precipitation environment over long timescales
Brachiopod shells are the most widely used geological archive for the reconstruction of the temperature and the oxygen isotope composition of Phanerozoic seawater. However, it is not conclusive ...whether brachiopods precipitate their shells in thermodynamic equilibrium. In this study, we investigated the potential impact of kinetic controls on the isotope composition of modern brachiopods by measuring the oxygen and clumped isotope compositions of their shells. Our results show that clumped and oxygen isotope compositions depart from thermodynamic equilibrium due to growth rate-induced kinetic effects. These departures are in line with incomplete hydration and hydroxylation of dissolved CO
. These findings imply that the determination of taxon-specific growth rates alongside clumped and bulk oxygen isotope analyses is essential to ensure accurate estimates of past ocean temperatures and seawater oxygen isotope compositions from brachiopods.
PDF
