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1. A rapid high‐precision analytical method for triple oxygen isotope analysis of CO 2 gas using tunable infrared laser direct absorption spectroscopy

   https://doi.org/10.1002/rcm.9391  

   Perdue, Nathan ; Sharp, Zachary ; Nelson, David ; Wehr, Rick ; Dyroff, Christoph ( September 2022 , Rapid Communications in Mass Spectrometry)

   The simultaneous analysis of the three stable isotopes of oxygen—triple oxygen isotope analysis—has become an important analytical technique in natural sciences. Determination of the abundance of the rare¹⁷O isotope in CO₂gas using magnetic sector isotope ratio mass spectrometry is complicated by the isobaric interference of¹⁷O by¹³C (¹³C¹⁶O¹⁶O and¹²C¹⁶O¹⁷O, both have mass 45 amu). A number of analytical techniques have been used to measure the¹⁷O/¹⁶O ratio of CO₂gas. They either are time consuming and technically challenging or have limited precision. A rapid and precise alternative to the available analytical methods is desirable.

   We present the results of triple oxygen isotope analyses using an Aerodyne tunable infrared laser direct absorption spectroscopy (TILDAS) CO₂analyzer configured for¹⁶O,¹⁷O, and¹⁸O combined with a custom gas inlet system. We evaluate the sensitivity of our results to a number of parameters. CO₂samples with a wide range of δ¹⁸O values (from −9.28‰ to 39.56‰) were measured and compared to results using the well‐established fluorination‐gas source mass spectrometry method.

   The TILDAS system has a precision (standard error, 2σ) of better than ±0.03‰ for δ¹⁸O and ±10 per meg for Δ′¹⁷O values, equivalent to the precision of previous analytical methods. Samples as small as 3 μmol CO₂(equivalent to 300 μg CaCO₃) can be analyzed with a total analysis time of ~30 min.

   We have successfully developed an analytical technique for the simultaneous determination of the δ¹⁷O and δ¹⁸O values of CO₂gas. The precision is equal to or better than that of existing techniques, with no additional chemical treatments required. Analysis time is rapid, and the system is easily automated so that large numbers of samples can be analyzed with minimal effort.

    

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2. Modern Eastern Canadian Arctic Lake Water Isotopes Exhibit Latitudinal Patterns in Inflow Seasonality and Minimal Evaporative Enrichment

   https://doi.org/10.1029/2021PA004384  

   Gorbey, D. B. ; Thomas, E. K. ; Sauer, P. E. ; Raynolds, M. K. ; Miller, G. H. ; Corcoran, M. C. ; Cowling, O. C. ; Crump, S. E. ; Lovell, K. ; Raberg, J. H. ( May 2022 , Paleoceanography and Paleoclimatology)

   Lacustrine δ²H and δ¹⁸O isotope proxies are powerful tools for reconstructing past climate and precipitation changes in the Arctic. However, robust paleoclimate record interpretations depend on site‐specific lake water isotope systematics, which are poorly described in the eastern Canadian Arctic due to insufficient modern lake water isotope data. We use modern lake water isotopes (δ¹⁸O and δ²H) collected between 1994–1997 and 2017–2021 from a transect of sites spanning a Québec‐to‐Ellesmere Island gradient to evaluate the effects of inflow seasonality and evaporative enrichment on the δ²H and δ¹⁸O composition of lake water. Four lakes near Iqaluit, Nunavut sampled biweekly through three ice‐free seasons reflect mean annual precipitation isotopes with slight evaporative enrichment. In a 23° latitudinal transect of 181 lakes, through‐flowing lake water δ²H and δ¹⁸O fall along local meteoric water lines. Despite variability within each region, we observe a latitudinal pattern: southern lakes reflect mean annual precipitation isotopes, whereas northern lakes reflect summer‐biased precipitation isotopes. This pattern suggests that northern lakes are more fully flushed with summer precipitation, and we hypothesize that this occurs because the ratio of runoff to precipitation increases with latitude as vegetation cover decreases. Therefore, proxy records from through‐flowing lakes in this region should reflect precipitation isotopes with minimal influence of evaporation, but vegetation changes in lake catchments across a latitudinal transect and through geologic time may influence the seasonality of lake water isotopic compositions. Thus, we recommend that future lake water isotope proxy records are considered in context with temperature and ecological proxy records.

    

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3. Seasonal SIMS δ18O record in Astarte borealis from the Baltic Sea tracks a modern regime shift in the NAO

   https://doi.org/10.3389/fmars.2023.1293823  

   Hughes, Hunter P ; Surge, Donna ; Orland, Ian J ; Zettler, Michael L ; Moss, David K ( December 2023 , Frontiers in Marine Science)

   Astarte borealisholds great potential as an archive of seasonal paleoclimate, especially due to its long lifespan (several decades to more than a century) and ubiquitous distribution across high northern latitudes. Furthermore, recent work demonstrates that the isotope geochemistry of the aragonite shell is a faithful proxy of environmental conditions. However, the exceedingly slow growth rates ofA. borealisin some locations (<0.2mm/year) make it difficult to achieve seasonal resolution using standard micromilling techniques for conventional stable isotope analysis. Moreover, oxygen isotope (δ¹⁸O) records from species inhabiting brackish environments are notoriously difficult to use as paleoclimate archives because of the simultaneous variation in temperature and δ¹⁸O_watervalues.

   Here we use secondary ion mass spectrometry (SIMS) to microsample anA. borealisspecimen from the southern Baltic Sea, yielding 451 SIMS δ¹⁸O_shellvalues at sub-monthly resolution.

   SIMS δ¹⁸O_shellvalues exhibit a quasi-sinusoidal pattern with 24 local maxima and minima coinciding with 24 annual growth increments between March 1977 and the month before specimen collection in May 2001.

   Age-modeled SIMS δ¹⁸O_shellvalues correlate significantly with bothin situtemperature measured from shipborne CTD casts (r² = 0.52, p<0.001) and sea surface temperature from the ORAS5-SST global reanalysis product for the Baltic Sea region (r² = 0.42, p<0.001). We observe the strongest correlation between SIMS δ¹⁸O_shellvalues and salinity when both datasets are run through a 36-month LOWESS function (r² = 0.71, p < 0.001). Similarly, we find that LOWESS-smoothed SIMS δ¹⁸O_shellvalues exhibit a moderate correlation with the LOWESS-smoothed North Atlantic Oscillation (NAO) Index (r² = 0.46, p<0.001). Change point analysis supports that SIMS δ¹⁸O_shellvalues capture a well-documented regime shift in the NAO circa 1989. We hypothesize that the correlation between the SIMS δ¹⁸O_shelltime series and the NAO is enhanced by the latter’s influence on the regional covariance of water temperature and δ¹⁸O_watervalues on interannual and longer timescales in the Baltic Sea. These results showcase the potential for SIMS δ¹⁸O_shellvalues inA. borealisshells to provide robust paleoclimate information regarding hydroclimate variability from seasonal to decadal timescales.

    

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4. Influence of sample volume on nitrate N and O isotope ratio analyses with the denitrifier method

   https://doi.org/10.1002/rcm.9224  

   Zhou, Mengyang ; Granger, Julie ; Chang, Bonnie X. ( December 2021 , Rapid Communications in Mass Spectrometry)

   Analyses of the isotope ratios of nitrogen (¹⁵N/¹⁴N) and oxygen (¹⁸O/¹⁶O) in nitrate (NO₃⁻) with the denitrifier method require relatively high sample volumes at low concentrations (≤1 μM) to afford sufficient analyte for mass spectrometry, resulting in isotopic offsets compared to more concentrated samples of the same isotopic composition.

   To uncover the origins of isotopic offsets, we analyzed the N and O isotope ratios of NO₃⁻reference materials spanning concentrations of 0.5–20 μM. We substantiated the incidence of volume‐dependent isotopic offsets, then investigated whether they resulted from (a) incomplete sample recovery during N₂O sparging, (b) blanks – bacterial, atmospheric, or in reference material solutions – and (c) oxygen atom exchange with water during the bacterial conversion of NO₃⁻to N₂O.

   Larger sample volumes resulted in modest offsets in δ¹⁵N, but substantial offsets in δ¹⁸O. N₂O recovery from sparging was less complete at higher volumes, resulting in decreases in δ¹⁵N and δ¹⁸O due to associated isotope fractionation. Blanks increased detectably with volume, whereas oxygen atom exchange with water remained constant within batch analyses, being sensitive to neither sample volume nor salinity. The sizeable offsets in δ¹⁸O with volume are only partially explained by the factors considered in our analysis.

   Our observations argue for bracketing of NO₃⁻samples with reference materials that emulate sample volumes (concentrations) to achieve improved measurement accuracy and foster inter‐comparability.

    

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5. Isotopic labeling of metabolic water with 18 O 2

   https://doi.org/10.1002/rcm.9447  

   Koch, George W. ; Schwartz, Egbert ( January 2023 , Rapid Communications in Mass Spectrometry)

   Water is the medium of life, is involved in biochemical reactions, and is exchanged among internal pools and with the water in the external environment of organisms. Understanding these processes can be improved by isotopically labeling the metabolic water that is produced inside the cells of organisms during aerobic respiration.

   Here we describe a new method for isotopically labeling cellular water by incubating microbes and plant tissues in air enriched in¹⁸O₂. As oxygen gas is reduced during respiration, H₂¹⁸O is produced. The rate of H₂¹⁸O production and the synthesis of biomolecules that incorporate¹⁸O from H₂¹⁸O can be quantified using cavity ringdown spectrometry and isotope ratio mass spectrometry.

   ForEscherichia coliin solution culture, soil microbial communities, and respiring tissues of plants, the amount of H₂¹⁸O produced was strongly correlated with that of¹⁸O₂consumed during incubations. Measurements of¹⁸O in DNA, microbial biomass, and CO₂showed that metabolic water was an important substrate in biosynthesis reactions.

   Any organism with aerobic respiration is amenable to labeling with¹⁸O₂, and the method described here enables a new approach to investigate questions regarding plant and microbial physiology. In plants,¹⁸O introduced as metabolic water could be tracked as it moves between living cells and exchanges with external water. For probing soil microbial physiology, the method described here has the advantage over the application of exogenous H₂¹⁸O of not increasing the soil moisture, a disturbance that can affect microbial metabolism.

    

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