The Cedars ultramafic block hosts alkaline springs (pH > 11) in which calcium carbonate forms upon uptake of atmospheric CO₂ 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 CaCO₃ 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 δ¹³C (−9 to −28‰ VPDB) and δ¹⁸O (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 CO₂ hydroxylation but the controls on the δ¹³C-δ¹⁸O 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 km²) 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 CO₂-fed solution that tracks the isotopic composition of each dissolved inorganic carbon (DIC) species and CaCO₃. The box model includes four sources or sinks of DIC (atmospheric CO₂, high pH spring water, fresh creek water, and CaCO₃ precipitation). Model parameters are informed by new floe Δ⁴⁴Ca data (−0.75 ± 0.07‰), direct mineral growth rate measurements (4.8 to 8 × 10⁻⁷ mol/m²/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 CO₂ and the kinetic isotope fractionation factor (KFF (‰) = (α − 1) × 1000) accompanying CO₂ hydroxylation, estimated here to be −17.1 ± 0.8‰ (vs. CO_2(aq)) for carbon and −7.1 ± 1.1‰ (vs. ‘CO_2(aq) + H₂O’) for oxygen at 17.4 ± 1.0 °C. Combining our results with revised CO₂ hydroxylation KFF values based on previous work suggests consistent KFF values of −17.0 ± 0.3‰ (vs. CO_2(aq)) for carbon and −6.8 ± 0.8‰ for oxygen (vs. ‘CO_2(aq) + H₂O’) 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 $\text{HC}{\text{O}}_{\text{3}}^{\text{-}}$, pH = 7.8–8.7) that occasionally mixes with the high-pH spring water. The bulk carbonate δ¹³C and δ¹⁸O 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 δ¹⁸O and atmospheric δ¹³C values. There is evidence that older travertine at The Cedars recorded atmospheric δ¹³C that predates large-scale combustion of fossil fuels.

Cedars超镁铁质块体具有碱性泉水（pH> 11），吸收大气中的CO ₂并有时与地表水混合时会形成碳酸钙。这些过程导致了独特的碳酸盐形态，在大气-水界面形成了“絮状物”，在池底和梯形钙华构造中积聚了细小颗粒的“雪”。尽管CaCO ₃沉淀发生在低Mg / Ca（<0.01）的水中，但浮游物质主要由文石针组成。池水的高pH（11.5-12.0）可能促进文石的沉淀，这与已发表的实验表明pH对碳酸钙多晶型物选择的影响相一致。

的碳酸钙显示出极端的范围和近似1：在δ1个共变¹³ C（-9至-28‰VPDB）和δ ¹⁸ O（0至-20‰VPDB），其是形成在高pH水石灰的特性。大的同位素分馏先前已经归因于伴随着CO动力学同位素效应₂羟化但控制在δ ¹³ C-δ ^18个ö端元和斜率没有得到充分解决，限制使用石灰作为古环境存档。弹簧的有限面积（约0.5 km ²）以及有限的水源和温度范围，再加上我们的采样策略，使我们对产生系统的C和O同位素变化所涉及的过程施加了严格的约束。

我们开发了一种CO _2进料溶液的同位素反应扩散模型和同位素盒模型，该模型跟踪每种溶解的无机碳（DIC）物种和CaCO ₃的同位素组成。箱形模型包括DIC的四个来源或汇（大气中的CO ₂，高pH的泉水，新鲜的小溪水和CaCO ₃沉淀）。模型参数由新絮状物Δ通知⁴⁴的Ca数据（-0.75±0.07‰），直接矿物生长速率测量（4.8〜8×10 ^-7摩尔/米²/ s），以及以前发布的当地水和DIC来源的元素和同位素数据。模型结果表明，有两个过程控制阵列的极端情况：（1）同位素轻的末端成员受大气CO ₂的同位素组成和动力学同位素分馏因子（KFF（‰）=（α− 1）×1000）的控制。伴随着CO ₂羟基化，碳在17.4时估计为-17.1±0.8‰（vs. CO _2（aq））和氧为-7.1±1.1‰（vs.'CO _2（aq）  + H ₂ O'） ±1.0°C。将我们的结果与基于先前工作的修正的CO ₂羟基化KFF值结合起来，得出一致的KFF值为-17.0±0.3‰（vs。CO _2（aq）在17-28°C的温度范围内，碳含量为-，氧气的含量为-6.8±0.8‰（vs。'CO _2（aq）  + H ₂ O'）。（2）雪松的碳酸钙同位素重的末端成员反映了小溪或地表水中同位素平衡的DIC的组成（主要是$\text{HC}{\text{Ø}}_{\text{3}}^{\text{--}}$，pH = 7.8–8.7）有时会与高pH的泉水混合。散装碳酸盐δ ¹³ C和δ ^18个因此现代和古石灰华的O值反映在支配的雪松碳酸盐沉淀预算由方法（1）和（2）形成的碳酸钙的比例，与方法（2）。这些结果表明，最近在了解动力学同位素效应的进步让我们复杂，但常见的自然流程建模，并建议古洞可用于检索过去的大气降水δ ¹⁸ O和大气δ ¹³ C值。有证据表明，在雪松旧的钙华记录大气δ ¹³ C ^化石燃料的早大规模燃烧。