Temperature Dependence of Clumped Isotopes (∆47) in Aragonite.

Clumped isotope thermometry can independently constrain the formation temperatures of carbonates, but a lack of precisely temperature-controlled calibration samples limits its application on aragonites. To address this issue, we present clumped isotope compositions of aragonitic bivalve shells grown under highly controlled temperatures (1-18°C), which we combine with clumped isotope data from natural and synthetic aragonites from a wide range of temperatures (1-850°C). We observe no discernible offset in clumped isotope values between aragonitic foraminifera, mollusks, and abiogenic aragonites or between aragonites and calcites, eliminating the need for a mineral-specific calibration or acid fractionation factor. However, due to non-linear behavior of the clumped isotope thermometer, including high-temperature (>100°C) datapoints in linear clumped isotope calibrations causes them to underestimate temperatures of cold (1-18°C) carbonates by 2.7 ± 2.0°C (95% confidence level). Therefore, clumped isotope-based paleoclimate reconstructions should be calibrated using samples with well constrained formation temperatures close to those of the samples.

Carbonate clumped isotope analyses with the long-integration dual-inlet (LIDI) workflow: scratching at the lower sample weight boundaries.

RATIONALE: Clumped isotope analyses (Δ47 ) of carbonates by dual inlet (DI) mass spectrometry require long integration times to reach the necessary high precision due to the low abundance of the rare isotopologue 13 C18 O16 O. Traditional DI protocols reach this only with large amounts of sample and/or a large number of replicates as a large portion of the analyte gas is wasted. We tested an improved analytical workflow that significantly reduces the sample sizes and total analysis time per sample while preserving precision and accuracy. METHODS: We implemented the LIDI (long-integration dual-inlet) protocol to measure carbonates in micro-volume mode using a Kiel IV carbonate device coupled to a Thermo Scientific 253 Plus isotope ratio mass spectrometer without the new 1013 ohm amplifier technology. The LIDI protocol includes a single measurement of the sample gas (600 s integration) followed by a single measurement of the working gas (WG) with the same integration time. RESULTS: The Δ47 measurements of four calcite standards over a period of 5 weeks demonstrate excellent long-term stability with a standard deviation of ±0.021 to ±0.025 ‰ for the final values of the individual aliquots. The Δ47 analyses of a coral, four foraminifera and a calcite precipitated in the laboratory demonstrate that 14 replicates of 90 to 120 µg are sufficient to achieve an external precision of ±0.007 ‰ (1SE) or of ±0.013 ‰ at the 95% confidence level. CONCLUSIONS: This study demonstrates that by using a Kiel IV-253 Plus system with LIDI it is possible to achieve the same analytical precision as conventional DI measurements with at least a factor of 40 less sample material. With the new 1013 ohm resistor technology there is the potential to reduce the required sample material even more. This opens new avenues of research in paleoceanography, paleoclimatology, low-temperature diagenesis and other currently sample size limited applications. Copyright © 2017 John Wiley & Sons, Ltd.

Modern applications for a total sulfur reduction distillation method - what's old is new again.

BACKGROUND: The use of a boiling mixture of hydriodic acid, hypophosphorous acid, and hydrochloric acid to reduce any variety of sulfur compounds has been in use in various applications since the first appearance of this method in the literature in the 1920's. In the realm of sulfur geochemistry, this method remains a useful, but under-utilized technique. Presented here is a detailed description of the distillation set-up and procedure, as well as an overview of potential applications of this method for marine sulfur biogeochemistry/isotope studies. The presented applications include the sulfur isotope analysis of extremely low amounts of sulfate from saline water, the conversion of radiolabeled sulfate into sulfide, the extraction of refractory sulfur from marine sediments, and the use of this method to assess sulfur cycling in Aarhus Bay sediments. RESULTS: The STrongly Reducing hydrIodic/hypoPhosphorous/hydrochloric acid (STRIP) reagent is capable of rapidly reducing a wide range of sulfur compounds, including the most oxidized form, sulfate, to hydrogen sulfide. Conversion of as little as approximately 5 micromole sulfate is possible, with a sulfur isotope composition reproducibility of 0.3 permil. CONCLUSIONS: Although developed many decades ago, this distillation method remains relevant for many modern applications. The STRIP distillation quickly and quantitatively converts sulfur compounds to hydrogen sulfide which can be readily collected in a silver nitrate trap for further use. An application of this method to a study of sulfur cycling in Aarhus Bay demonstrates that we account for all of the sulfur compounds in pore-water, effectively closing the mass balance of sulfur cycling.

North Atlantic surface ocean warming and salinization in response to middle Eocene greenhouse warming.

Quantitative reconstructions of hydrological change during ancient greenhouse warming events provide valuable insight into warmer-than-modern hydrological cycles but are limited by paleoclimate proxy uncertainties. We present sea surface temperature (SST) records and seawater oxygen isotope (δ18Osw) estimates for the Middle Eocene Climatic Optimum (MECO), using coupled carbonate clumped isotope (Δ47) and oxygen isotope (δ18Oc) data of well-preserved planktonic foraminifera from the North Atlantic Newfoundland Drifts. These indicate a transient ~3°C warming across the MECO, with absolute temperatures generally in accordance with trace element (Mg/Ca)-based SSTs but lower than biomarker-based SSTs for the same interval. We find a transient ~0.5‰ shift toward higher δ18Osw, which implies increased salinity in the North Atlantic subtropical gyre and potentially a poleward expansion of its northern boundary in response to greenhouse warming. These observations provide constraints on dynamic ocean response to warming events, which are consistent with theory and model simulations predicting an enhanced hydrological cycle under global warming.

Reducing Uncertainties in Carbonate Clumped Isotope Analysis Through Consistent Carbonate-Based Standardization.

About a decade after its introduction, the field of carbonate clumped isotope thermometry is rapidly expanding because of the large number of possible applications and its potential to solve long-standing questions in Earth Sciences. Major factors limiting the application of this method are the very high analytical precision required for meaningful interpretations, the relatively complex sample preparation procedures, and the mass spectrometric corrections needed. In this paper we first briefly review the evolution of the analytical and standardization procedures and discuss the major remaining sources of uncertainty. We propose that the use of carbonate standards to project the results to the carbon dioxide equilibrium scale can improve interlaboratory data comparability and help to solve long-standing discrepancies between laboratories and temperature calibrations. The use of carbonates reduces uncertainties related to gas preparation and cleaning procedures and ensures equal treatment of samples and standards. We present a set of carbonate standards of diverse composition, discuss how they can be used to correct for mass spectrometric biases, and demonstrate that their use significantly improves the comparability among four laboratories. We propose that the use of these standards or of a similar set of carbonate standards will improve the comparability of data across laboratories.