There is growing evidence that speleothem calcite grows out of isotopic equilibrium with cave dri... more There is growing evidence that speleothem calcite grows out of isotopic equilibrium with cave drip water, with clumped isotope analysis providing a sensitive indicator for disequilibrium. This disequilibrium is primarily the result of CO 2 degassing from a thin film of water, leading to irreversible 13 C enrichment and reversible 18 O enrichment and D 47 depletion. Here we examine isotopic disequilibrium in Soreq Cave (Israel) using multiple modern-day and late Holocene speleothems. The variability observed in D 47 is small, within the analytical uncertainty, but the D 47-derived temperature is offset from the modern cave temperature by $4°C, reflecting degassing related disequilibrium that is fairly constant spatially. d 18 O is more heterogeneous, reflecting short-term variability in drip water d 18 O w combined with variability in the speleothem growth rates and related fractionation between dissolved carbonate species and the growing calcite mineral. This complexity, however, is markedly reduced by spatial or temporal averaging, enabling an interpretation of the cave paleoclimate record. We examine the Soreq Cave speleothems through a comparison with 2 types of thermometers: one is based on CaCO 3 precipitation from a bulk solution and is typically used for calibration of the D 47 and d 18 O thermometers; a second that is based on CaCO 3 precipitating at the surface of the solution thus focusing and amplifying the thin film characteristics of speleothem formation. Soreq Cave speleothems are intermediate between these thermometers, providing a cave-specific thermometer calibration.
Proceedings of the National Academy of Sciences, 2014
Significance Reconstructions of ancient high-latitude climates can help to constrain the amplific... more Significance Reconstructions of ancient high-latitude climates can help to constrain the amplification of global warming in polar environments. Climate models cannot reproduce the elevated high-latitude temperature estimates in the Eocene epoch, possibly indicating problems in simulating polar climate change. Widely divergent near-Antarctic Eocene sea surface temperature (SST) estimates, however, question the evidence for extreme warmth. Our analysis of multiple temperature proxies near the Antarctic Peninsula improves intersite comparisons and indicates a substantial zonal SST gradient between the southwest Pacific and South Atlantic. Simulations of Eocene ocean temperatures imply that the formation of deep water in the southwest Pacific partly accounts for this SST gradient, suggesting that climate models underestimate Eocene SSTs in regions where the thermohaline circulation leads to relatively high temperatures.
Proceedings of the National Academy of Sciences, 2009
The stratospheric CO 2 oxygen isotope budget is thought to be governed primarily by the O( 1 D)+C... more The stratospheric CO 2 oxygen isotope budget is thought to be governed primarily by the O( 1 D)+CO 2 isotope exchange reaction. However, there is increasing evidence that other important physical processes may be occurring that standard isotopic tools have been unable to identify. Measuring the distribution of the exceedingly rare CO 2 isotopologue 16 O 13 C 18 O, in concert with 18 O and 17 O abundances, provides sensitivities to these additional processes and, thus, is a valuable test of current models. We identify a large and unexpected meridional variation in stratospheric 16 O 13 C 18 O, observed as proportions in the polar vortex that are higher than in any naturally derived CO 2 sample to date. We show, through photochemical experiments, that lower 16 O 13 C 18 O proportions observed in the midlatitudes are determined primarily by the O( 1 D)+CO 2 isotope exchange reaction, which promotes a stochastic isotopologue distribution. In contrast, higher 16 O 13 C 18 O proportions i...
Isoprene emission from leaves is dynamically coupled to photosynthesis through the use of primary... more Isoprene emission from leaves is dynamically coupled to photosynthesis through the use of primary and recent photosynthate in the chloroplast. However, natural abundance carbon isotope composition (δ13C) measurements in myrtle (Myrtus communis), buckthorn (Rhamnus alaternus), and velvet bean (Mucuna pruriens) showed that only 72% to 91% of the variations in the δ13C values of fixed carbon were reflected in the δ13C values of concurrently emitted isoprene. The results indicated that 9% to 28% carbon was contributed from alternative, slow turnover, carbon source(s). This contribution increased when photosynthesis was inhibited by CO2-free air. The observed variations in the δ13C of isoprene under ambient and CO2-free air were consistent with contributions to isoprene synthesis in the chloroplast from pyruvate associated with cytosolic Glc metabolism. Irrespective of alternative carbon source(s), isoprene was depleted in 13C relative to mean photosynthetically fixed carbon by 4‰ to 11‰...
Isoprene (2-methyl-1,3-butadiene) protection against effects of singlet oxygen was investigated i... more Isoprene (2-methyl-1,3-butadiene) protection against effects of singlet oxygen was investigated in Myrtus communisand Rhamnus alaternus. In M. communis, singlet oxygen produced in the leaves by Rose Bengal (RB) led to a 65% decrease in net assimilation rates within 3 h, whereas isoprene emission rates showed either a 30% decrease at ambient CO2concentrations or a 70% increase under high CO2. In both cases, these changes led to an increase in calculated internal isoprene concentrations. The isoprene protection effect was directly demonstrated by fumigation of young (non-emitting) leaves, treated with RB or bromoxynil (simulating photoinhibition). There was 42% and 29% reduction in the damage to net assimilation compared with non-fumigated leaves for RB or bromoxynil, respectively. In R. alaternus, similar effects of RB on net assimilation were observed, and additional fluorescence measurements showed a significantly smaller decrease inF v/F m in isoprene-fumigated young leaves trea...
Variation in the C 18 OO content of atmospheric CO 2 ( δ 18 O a ) can be used to distinguish phot... more Variation in the C 18 OO content of atmospheric CO 2 ( δ 18 O a ) can be used to distinguish photosynthesis from soil respiration, which is based on carbonic anhydrase (CA)-catalyzed 18 O exchange between CO 2 and 18 O-enriched leaf water ( δ 18 O w ). • Here we tested the hypothesis that mean leaf δ 18 O w and assimilation rates can be used to estimate whole-leaf C 18 OO flux (isoflux), ignoring intraleaf variations in CA activity and gas exchange parameters. • We observed variations in CA activity along the leaf (> 30% decline from the leaf center toward the leaf ends), which were only partially correlated to those in δ 18 O w (7 to 21‰), δ 18 O and δ 13 C of leaf organic matter (25 to 30‰ and -12.8 to -13.2‰, respectively), and substomatal CO 2 concentrations (intercellular CO 2 concentrations, c i , at the leaf center were ∼ 40% of those at the leaf tip). • The combined effect of these variations produced a leaf-integrated isoflux that was different from that predicted based on bulk leaf values. However, because of canceling effects among the influencing parameters, isoflux overestimations were only ∼ 10%. Conversely, use of measured parameters from a leaf segment could produce large errors in predicting leaf-integrated C 18 OO fluxes.
The geochemistry of multiply substituted isotopologues ('clumped-isotope' geochemistry) examines ... more The geochemistry of multiply substituted isotopologues ('clumped-isotope' geochemistry) examines the abundances in natural materials of molecules, formula units or moieties that contain more than one rare isotope (e.g. 13 C 18 O 16 O, 18 O 18 O, 15 N 2 , 13 C 18 O 16 O 2 2−). Such species form the basis of carbonate clumped-isotope thermometry and undergo distinctive fractionations during a variety of natural processes, but initial reports have provided few details of their analysis. In this study, we present detailed data and arguments regarding the theoretical and practical limits of precision, methods of standardization, instrument linearity and related issues for clumped-isotope analysis by dual-inlet gas-source isotope ratio mass spectrometry (IRMS). We demonstrate long-term stability and subtenth per mil precision in 47/44 ratios for counting systems consisting of a Faraday cup registered through a 10 12 resistor on three Thermo-Finnigan 253 IRMS systems. Based on the analyses of heated CO 2 gases, which have a stochastic distribution of isotopes among possible isotopologues, we document and correct for (1) isotopic exchange among analyte CO 2 molecules and (2) subtle nonlinearity in the relationship between actual and measured 47/44 ratios. External precisions of ∼0.01‰ are routinely achieved for measurements of the mass-47 anomaly (a measure mostly of the abundance anomaly of 13 C-18 O bonds) and follow counting statistics. The present technical limit to precision intrinsic to our methods and instrumentation is ∼5 parts per million (ppm), whereas precisions of measurements of heterogeneous natural materials are more typically ∼10 ppm (both 1 s.e.). These correspond to errors in carbonate clumped-isotope thermometry of ±1.2 • C and ±2.4 • C, respectively.
compared to a 47 ppm change in background air CO 2 , yet the ranges of the carbon and oxygen isot... more compared to a 47 ppm change in background air CO 2 , yet the ranges of the carbon and oxygen isotopic compositions remained essentially constant. Because the 1998-2003 data show a significant progression through time, analysis was done on data from 2002 to 2003 complete calendar years (CO 2 mixing ratios increased 41 ppm between
The early Eocene (ca. 55-48 Ma) encompasses one of the warmest intervals of the past 65 m.y. and ... more The early Eocene (ca. 55-48 Ma) encompasses one of the warmest intervals of the past 65 m.y. and is characterized by an unusually low equator-to-pole thermal gradient. Recent proxy studies suggest temperatures well in excess of 30 °C even at high latitudes, but confl icting interpretations derived from different types of data leave considerable uncertainty about actual early Eocene temperatures. A robust comparison among new paleotemperature proxies may provide insight into possible biases in their temperature estimates, and additional detail on the spatial distribution of temperatures will further resolve the early Eocene meridional temperature gradient. We use a suite of paleotemperature proxies based on the chemistry of bivalve shell carbonate and associated sedimentary organic matter from the United States Gulf Coastal Plain to constrain climate at a subtropical site during this key interval of Earth history. Oxygen isotope and clumped isotope analyses of shell carbonate and two tetraether lipid analyses of sedimentary organic carbon all yield temperatures of ~27 °C. High-resolution, intraannual oxygen isotope data reveal a consistent, large range of seasonal variation, but clumped isotope data suggest that seasonality is due primarily to precipitation, not to temperature. These paleotemperature estimates are 2-3 °C warmer than the northern Gulf of Mexico today, and generally consistent with early Eocene temperature estimates from other low and mid-latitude locations, but are signifi cantly cooler than contemporaneous estimates from high southern latitudes.
We present a revised approach for standardizing and reporting analyses of multiply substituted is... more We present a revised approach for standardizing and reporting analyses of multiply substituted isotopologues of CO 2 (i.e., 'clumped' isotopic species, especially the mass-47 isotopologues). Our approach standardizes such data to an absolute reference frame based on theoretical predictions of the abundances of multiply-substituted isotopologues in gaseous CO 2 at thermodynamic equilibrium. This reference frame is preferred over an inter-laboratory calibration of carbonates because it enables all laboratories measuring mass 47 CO 2 to use a common scale that is tied directly to theoretical predictions of clumping in CO 2 , regardless of the laboratory's primary research field (carbonate thermometry or CO 2 biogeochemistry); it explicitly accounts for mass spectrometric artifacts rather than convolving (and potentially confusing) them with chemical fractionations associated with sample preparation; and it is based on a thermodynamic equilibrium that can be experimentally established in any suitably equipped laboratory using commonly available materials. By analyzing CO 2 gases that have been subjected to established laboratory procedures known to promote isotopic equilibrium (i.e., heated gases and water-equilibrated CO 2), and by reference to thermodynamic predictions of equilibrium isotopic distributions, it is possible to construct an empirical transfer function that is applicable to data with unknown clumped isotope signatures. This transfer function empirically accounts for the fragmentation and recombination reactions that occur in electron impact ionization sources and other mass spectrometric artifacts. We describe the protocol necessary to construct such a reference frame, the method for converting gases with unknown clumped isotope compositions to this reference frame, and suggest a protocol for ensuring that all reported isotopic compositions (e.g., D 47 values; Eiler and Schauble, 2004; Eiler, 2007) can be compared among different laboratories and instruments, independent of laboratory-specific analytical or methodological differences. We then discuss the use of intra-laboratory secondary reference frames (e.g., based on carbonate standards) that can be more easily used to track the evolution of each laboratory's empirical transfer function. Finally, we show inter-laboratory reproducibility on the order of ±0.010 (1r) for four carbonate standards, and present revised paleotemperature scales that should be used to convert carbonate clumped isotope signatures to temperature when using the absolute reference frame described here. Even when using the reference frame, small discrepancies remain between two previously published synthetic carbonate calibrations. We discuss possible reasons for these discrepancies, and highlight the need for additional low temperature (<15°C) synthetic carbonate experiments.
Atmospheric carbon dioxide is widely studied using records of CO 2 mixing ratio, d 13 C and d 18 ... more Atmospheric carbon dioxide is widely studied using records of CO 2 mixing ratio, d 13 C and d 18 O. However, the number and variability of sources and sinks prevents these alone from uniquely defining the budget. Carbon dioxide having a mass of 47 u (principally 13 C 18 O 16 O) provides an additional constraint. In particular, the mass 47 anomaly (D 47) can distinguish between CO 2 produced by high temperature combustion processes vs. low temperature respiratory processes. D 47 is defined as the abundance of mass 47 isotopologues in excess of that expected for a random distribution of isotopes, where random distribution means that the abundance of an isotopologue is the product of abundances of the isotopes it is composed of and is calculated based on the measured 13 C and 18 O values. In this study, we estimate the d 13 C (vs. VPDB), d 18 O (vs. VSMOW), d47, and D 47 values of CO 2 from car exhaust and from human breath, by constructing ÔKeeling plotsÕ using samples that are mixtures of ambient air and CO 2 from these sources. d47 is defined as ðR 47 =R 47 std À 1Þ Â 1000, where R 47 std is the R 47 value for a hypothetical CO 2 whose d 13 C VPDB = 0, d 18 O VSMOW = 0, and D 47 = 0. Ambient air in Pasadena, CA, where this study was conducted, varied in [CO 2 ] from 383 to 404 lmol mol À1 , in d 13 C and d 18 O from À9.2 to À10.2& and from 40.6 to 41.9&, respectively, in d47 from 32.5 to 33.9&, and in D 47 from 0.73 to 0.96&. Air sampled at varying distances from a car exhaust pipe was enriched in a combustion source having a composition, as determined by a ÔKeeling plotÕ intercept, of À24.4 ± 0.2& for d 13 C (similar to the d 13 C of local gasoline), d 18 O of 29.9 ± 0.4&, d47 of 6.6 ± 0.6&, and D 47 of 0.41 ± 0.03&. Both d 18 O and D 47 values of the car exhaust end-member are consistent with that expected for thermodynamic equilibrium at$200°C between CO 2 and water generated by combustion of gasoline-air mixtures. Samples of CO 2 from human breath were found to have d 13 C and d 18 O values broadly similar to those of car exhaust-air mixtures, À22.3 ± 0.2 and 34.3 ± 0.3&, respectively, and d47 of 13.4 ± 0.4&. D 47 in human breath was 0.76 ± 0.03&, similar to that of ambient Pasadena air and higher than that of the car exhaust signature.
A growing number of materials and environmental settings are studied using the carbonate clumped ... more A growing number of materials and environmental settings are studied using the carbonate clumped isotope (47) thermometer. The method has been applied in both biogenic and non-biogenic carbonate systems, in marine and terrestrial settings, over a wide range of geological timescales. The current 47 temperature calibration gives good temperature estimates for most biogenic materials, however, systematic biases are commonly observed at low temperatures. In this study we report additional calibration data, that covers a wider temperature range, at more evenly distributed temperatures, and are measured at higher analytical precision than the original calibration. Combining these data with the original calibration yields a 47-T relationship that is similar to the original calibration, though slightly less steep: 47 = (0.0526 ± 0.0025) × 10 6 /T 2 + (0.0520 ± 0.0284). This revised calibration is in better agreement with biogenic carbonates, including those grown at low temperatures. The difference between the original and revised calibrations is significant for carbonates forming below 16 • C or above 49 • C (47 values of 0.68h and 0.56h). Additionally, we include a comprehensive analysis of the sources of error associated with 47 measurements and estimated temperatures and recommend measurement strategies for obtaining the desired precision. As an illustration, we apply the revised calibration and uncertainty analysis to 3 previously published studies. At low temperatures, the revised calibration results in significant differences from the original calibration and hence affects the interpretation of the environmental signal recorded. In light of our 47 errors analysis, in cases where the temperature signals are small, we find that replicate analyses are critical to identify a temperature signal.
The geochemical signature of many speleothems used for reconstruction of past continental climate... more The geochemical signature of many speleothems used for reconstruction of past continental climates is affected by kinetic isotope fractionation. This limits quantitative paleoclimate reconstruction and, in cases where the kinetic fractionation varies with time, also affects relative paleoclimate interpretations. In carbonate archive research, clumped isotope thermometry is typically used as proxy for absolute temperatures. In the case of speleothems, however, clumped isotopes provide a sensitive indicator for disequilibrium effects. The extent of kinetic fractionation co-varies in 47 and δ 18 O so that it can be used to account for disequilibrium in δ 18 O and to extract the past drip-water composition. Here we apply this approach to stalagmites from Bunker Cave (Germany) and calculate drip-water δ 18 O w values for the Eemian, MIS3, and the Holocene, relying on independent temperature estimates and accounting for disequilibrium. Applying the co-variation method to modern calcite precipitates yields drip-water δ 18 O w values in agreement with modern cave drip-water δ 18 O w of −7.9 ± 0.3 ‰, despite large and variable disequilibrium effects in both calcite δ 18 O c and 47. Reconstructed paleo-drip-water δ 18 O w values are lower during colder periods (e.g., MIS3: −8.6 ± 0.4 ‰ and the early Holocene at 11 ka: −9.7 ± 0.2 ‰) and show higher values during warmer climatic periods (e.g., the Eemian: −7.6 ± 0.2 ‰ and the Holocene Climatic Optimum: −7.2 ± 0.3 ‰). This new approach offers a unique possibility for quantitative climate reconstruction including the assessment of past hydrological conditions while accounting for disequilibrium effects.
We present a temperature to Δ 47 relationship for naturally occurring modern brachiopods, which w... more We present a temperature to Δ 47 relationship for naturally occurring modern brachiopods, which was established using a suite of thirteen brachiopod specimens from twelve sampling localities that span a mean annual seawater temperature range of −0.8 to 29.0°C. Sample locations and depths were selected to maximize the range of temperatures covered by the calibration while minimizing the seasonal temperature variability impacting the samples. Linear least-squares regression analysis yields the following relationship expressed using the Ghosh et al. (2006) reference frame: Δ 47 ¼ 0:0478 AE 0:0099 ð Þ Á 10 6 =T 2 þ 0:1164 AE 0:1203 ð Þ n ¼ 13; R 2 ¼ 0:91; 95%CI : This equation lies within the 95% confidence interval of the original relationship determined using synthetic calcite (Ghosh et al., 2006), but is in better agreement with the newly revised calibration of Zaarur et al. (2013). However, despite this broad agreement, our equation has a shallower slope than the equations for synthetic calcites, and yields temperatures at the warm end of the calibration temperature range (Δ 47 = 0.640‰) that are as much as 3.1°C offset from those predicted using the equation of Zaarur et al. (2013). On the other hand, our new equation has a steeper slope than the Henkes et al. (2013) equation for modern brachiopods and mollusks. This finding is consistent with the hypothesis that differences in the protocols employed by laboratories that use different temperatures of phosphoric acid digestion cause the observed discrepancies between the Δ 47 values reported by various laboratories.
The end of the Permian was a time of crisis that culminated in the Earth's greatest mass extincti... more The end of the Permian was a time of crisis that culminated in the Earth's greatest mass extinction. There is much speculation as to the cause for this catastrophe. Here we provide a full suite of high-resolution and coeval geochemical results (trace and rare earth elements, carbon, oxygen, strontium and clumped isotopes) reflecting ambient seawater chemistry and water quality parameters leading up to the end-Permian crisis. Preserved brachiopod low-Mg calcite-based seawater chemistry, supplemented by data from various localities, documents a sequence of interrelated primary events such as coeval flows of Siberian Trap continental flood basalts and emission of carbon dioxide leading to warm and extreme Greenhouse conditions with sea surface temperatures (SST) of~36°C for the Late Permian. Although anoxia has been advanced as a cause for the mass extinction, most biotic and geochemical evidence suggest that it was briefly relevant during the early phase of the event and only in areas of upwelling, but not a general cause. Instead, we suggest that renewed and increased end-Permian Siberian Trap volcanic activity, about 2000 years prior to the extinction event, released massive amounts of carbon dioxide and coupled with thermogenic methane emissions triggered extreme global warming and increased continental weathering. Eventually, these rapidly discharged greenhouse gas emissions, less than 1000 years before the event, ushered in a global Hothouse period leading to extreme tropical SSTs of~39°C and higher. Based on these sea surface temperatures, atmospheric CO 2 concentrations were about 1400 ppmv and 3000 ppmv for the Late and end-Permian, respectively. Leading up to the mass extinction, there was a brief interruption of the global warming trend when SST dropped, concurrent with a slight, but significant recovery in biodiversity in the western Tethys. It is possible that emission of volcanic sulfate aerosols resulted in brief cooling just after the onset of intensified warming during the end of the Permian. After aerosol deposition, global warming resumed and the biotic decline proceeded, and was accompanied by suboxia, in places of the surface ocean which culminated in the greatest mass extinction in Earth history.
The abundance of the doubly substituted CO 2 isotopologue, 13 C 18 O 16 O, in CO 2 produced by ph... more The abundance of the doubly substituted CO 2 isotopologue, 13 C 18 O 16 O, in CO 2 produced by phosphoric acid digestion of synthetic, inorganic calcite and natural, biogenic aragonite is proportional to the concentration of 13 C-18 O bonds in reactant carbonate, and the concentration of these bonds is a function of the temperature of carbonate growth. This proportionality can be described between 1 and 50 °C by the function: D 47 = 0.0592 AE 10 6 AE T À2 À 0.02, where D 47 is the enrichment, in per mil, of 13 C 18 O 16 O in CO 2 relative to the amount expected for a stochastic (random) distribution of isotopes among all CO 2 isotopologues, and T is the temperature in Kelvin. This relationship can be used for a new kind of carbonate paleothermometry, where the temperature-dependent property of interest is the state of ordering of 13 C and 18 O in the carbonate lattice (i.e., bound together vs. separated into different CO 3 2À units), and not the bulk d 18 O or d 13 C values. Current analytical methods limit precision of this thermometer to ca. ± 2 °C, 1r. A key feature of this thermometer is that it is thermodynamically based, like the traditional carbonate-water paleothermometer, and so is suitable for interpolation and even modest extrapolation, yet is rigorously independent of the d 18 O of water and d 13 C of DIC from which carbonate grew. Thus, this technique can be applied to parts of the geological record where the stable isotope compositions of waters are unknown. Moreover, simultaneous determinations of D 47 and d 18 O for carbonates will constrain the d 18 O of water from which they grew.
This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright
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