{"refrec":{"BRefID":337213,"RR":"<b>Christensen, J.N.; Watkins, J.M.; Devriendt, L.S.; DePaolo, D.J.; Conrad, M.E.; Voltolini, M.; Yang, W.; Dong, W.</b> (2021). Isotopic fractionation accompanying CO<sub>2</sub> hydroxylation and carbonate precipitation from high pH waters at The Cedars, California, USA. <i>Geochim. Cosmochim. Acta 301</i>: 91-115. <a href=\"https://doi.org/10.1016/j.gca.2021.01.003\" target=\"_blank\">https://doi.org/10.1016/j.gca.2021.01.003</a>","BEntID":333835,"PublicFlag":1,"CheckedFlag":0,"wosflag":1,"vabbflag":1,"RefStringPartII":". <i>Geochim. Cosmochim. Acta 301</i>: 91-115. <a href=\"https://doi.org/10.1016/j.gca.2021.01.003\" target=\"_blank\">https://doi.org/10.1016/j.gca.2021.01.003</a>","DocTypID":8,"DocType":"Journal article","MarineFlag":0,"FreshFlag":0,"BrackishFlag":0,"TerrestrialFlag":0,"Authorstring":"Christensen, J.N.; Watkins, J.M.; Devriendt, L.S.; DePaolo, D.J.; Conrad, M.E.; Voltolini, M.; Yang, W.; Dong, W.","OrigTitleTranslFlag":0,"Authorstringtrunc":"Christensen, J.N. <i>et al.</i>","Englishabstract":"The Cedars ultramafic block hosts alkaline springs (pH > 11) in which    calcium carbonate forms upon uptake of atmospheric CO<sub>2</sub> and at    times via mixing with surface water. These processes lead to distinct    carbonate morphologies with “floes” forming at the atmosphere-water    interface, “snow” of fine particles accumulating at the bottom of pools and    terraced constructions of travertine. Floe material is mainly composed of    aragonite needles despite CaCO<sub>3</sub> precipitation occurring in    waters with low Mg/Ca (<0.01). Precipitation of aragonite is likely    promoted by the high pH (11.5–12.0) of pool waters, in agreement with    published experiments illustrating the effect of pH on calcium carbonate    polymorph selection.    The calcium carbonates exhibit an extreme range and approximately 1:1    covariation in δ<sup>13</sup>C (−9 to −28‰ VPDB) and δ<sup>18</sup>O (0    to −20‰ VPDB) that is characteristic of travertine formed in high pH    waters. The large isotopic fractionations have previously been attributed    to kinetic isotope effects accompanying CO<sub>2</sub> hydroxylation but    the controls on the δ<sup>13</sup>C-δ<sup>18</sup>O endmembers and slope    have not been fully resolved, limiting the use of travertine as a    paleoenvironmental archive. The limited areal extent of the springs    (∼0.5 km<sup>2</sup>) and the limited range of water sources and    temperatures, combined with our sampling strategy, allow us to place tight    constraints on the processes involved in generating the systematic C and O    isotope variations.    We develop an isotopic reaction–diffusion model and an isotopic box model    for a CO<sub>2</sub>-fed solution that tracks the isotopic composition of    each dissolved inorganic carbon (DIC) species and CaCO<sub>3</sub>. The box    model includes four sources or sinks of DIC (atmospheric CO<sub>2</sub>,    high pH spring water, fresh creek water, and CaCO<sub>3</sub>    precipitation). Model parameters are informed by new floe Δ<sup>44</sup>Ca    data (−0.75 ± 0.07‰), direct mineral growth rate measurements (4.8 to    8 × 10<sup>−7</sup> mol/m<sup>2</sup>/s) and by previously published    elemental and isotopic data of local water and DIC sources. Model results    suggest two processes control the extremes of the array: (1) the    isotopically light end member is controlled by the isotopic composition of    atmospheric CO<sub>2</sub> and the kinetic isotope fractionation factor    (KFF (‰) = (α − 1) × 1000) accompanying CO<sub>2</sub> hydroxylation,    estimated here to be −17.1 ± 0.8‰ (vs. CO<sub>2(aq)</sub>) for carbon    and −7.1 ± 1.1‰ (vs. ‘CO<sub>2(aq)</sub> + H<sub>2</sub>O’) for oxygen at    17.4 ± 1.0 °C. Combining our results with revised CO<sub>2</sub>    hydroxylation KFF values based on previous work suggests consistent KFF    values of −17.0 ± 0.3‰ (vs. CO<sub>2(aq)</sub>) for carbon and −6.8 ± 0.8‰    for oxygen (vs. ‘CO<sub>2(aq)</sub> + H<sub>2</sub>O’) over the 17–28 °C    temperature range. (2) The isotopically heavy endmember of calcium    carbonates at The Cedars reflects the composition of isotopically    equilibrated DIC from creek or surface water (mostly HCO<sub>3</sub>,    pH = 7.8–8.7) that occasionally mixes with the high-pH spring water. The    bulk carbonate δ<sup>13</sup>C and δ<sup>18</sup>O values of modern and    ancient travertines therefore reflect the proportion of calcium carbonate    formed by processes (1) and (2), with process (2) dominating the carbonate    precipitation budget at The Cedars. These results show that recent advances    in understanding kinetic isotope effects allow us to model complicated but    common natural processes, and suggest ancient travertine may be used to    retrieve past meteoric water δ<sup>18</sup>O and atmospheric δ<sup>13</sup>    C values. There is evidence that older travertine at The Cedars recorded    atmospheric δ<sup>13</sup>C that predates large-scale combustion of fossil    fuels.","AbstractOtherLang":null,"BibLvlCode":"AS","StandardTitle":"Isotopic fractionation accompanying CO<sub>2</sub> hydroxylation and carbonate precipitation from high pH waters at The Cedars, California, USA","OrigTitleLangCode":"en","OrigTitleLangCodeExtended":"eng","OrigTitleLangID":15,"DateLastModified":{"date":"2026-06-11 01:32:44.978381","timezone_type":1,"timezone":"+02:00"},"UserAccessRight":null,"UserAccID":null,"AuthorKeywords":"Calcium isotopes; Carbon isotopes; Oxygen isotopes; Calcite; Aragonite; Alkaline springs; CO<sub>2</sub> hydroxylation; Kinetic isotope effects","OtherDescriptors":null,"Notes":null,"AnaPub":2021,"MonPub":null,"DateUpdate":"2021-05-18","DateCreate":"2021-05-17","SecASFANote":null,"ConfID":null,"PeerRev":1,"VlizCoreFlag":1,"WoScode":"WOS:000640782500005","VABBcode":null,"OpenAcc":0,"DOI":"10.1016/j.gca.2021.01.003"},"refs":null,"anarec":{"AnaID":337213,"PubliDate":2021,"Pagination":"91-115","XtraPublOfAnaID":null,"ISBN":null,"Volume":"301","Issue":null,"BRefMon":null,"BRefMonRR":null,"BRefXtra":null,"BRefXtraRR":null,"SerBRefID":42901,"SerRR":"Geochimica et Cosmochimica Acta. 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