Isotopic fractionation accompanying CO2 hydroxylation and carbonate precipitation from high pH waters at The Cedars, California, USA
Isotopic fractionation accompanying CO2 hydroxylation and carbonate precipitation from high pH waters at The Cedars, California, USA
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DOI：
10.1016/j.gca.2021.01.003
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发表时间：
2021-01
影响因子：
5
通讯作者：
J. Christensen;J. Watkins;L. Devriendt;D. DePaolo;M. Conrad;M. Voltolini;Wenbo Yang;W. Dong
中科院分区：
文献类型：
--
作者：
J. Christensen;J. Watkins;L. Devriendt;D. DePaolo;M. Conrad;M. Voltolini;Wenbo Yang;W. Dong
雪松超镁铁质岩块拥有碱性泉（pH> 11），其中碳酸钙在吸收大气CO2时形成，有时通过与地表水混合形成。这些过程导致了独特的碳酸盐形态，在大气-水界面形成“浮冰”，细颗粒“雪”积聚在水池底部，并形成钙华的阶状结构。在低Mg/Ca（<0.0 1）的沃茨中，尽管有CaCO 3沉淀，但絮体物质主要由文石针状物组成。文石的沉淀可能是由池沃茨的高pH值（11.5-12.0）促进的，这与说明pH值对碳酸钙多晶型物选择的影响的公开实验一致。碳酸钙在δ 13 C（− 9至− 28‰ VPDB）和δ 18 O（0至− 20‰ VPDB）方面表现出极端的变化范围和约1：1的协变，这是在高pH值沃茨中形成的碳酸钙的特征。大的同位素分馏以前被认为是伴随CO2羟基化的动力学同位素效应，但对δ 13 C-δ 18 O端元和斜率的控制尚未完全解决，限制了古环境档案的使用。有限的面积范围的弹簧（10.5平方公里）和有限的范围内的水源和温度，结合我们的采样策略，使我们能够把严格的限制过程中产生的系统的C和O同位素的变化。我们开发了一个同位素反应扩散模型和一个同位素箱模型的CO2进料的解决方案，跟踪每个溶解的无机碳（DIC）物种和碳酸钙的同位素组成。箱模型包括DIC的4个源或汇（大气CO2、高pH泉水、淡水河和CaCO 3降水）。模型参数由新的絮凝物Δ 44 Ca数据（− 0.75±0.07‰）、直接矿物生长速率测量（4.8至8× 10− 7 mol/m2/s）以及先前发表的当地水和DIC来源的元素和同位素数据提供。模型结果表明，两个过程控制着同位素序列的极端：（1）轻端成员受大气CO2的同位素组成和CO2羟基化反应的动力学同位素分馏因子（KFF（‰）=（α− 1）× 1000）控制，估算出碳的KFF（‰）为− 17.1±0.8‰（vs.CO2（aq）），碳的KFF（‰）为− 7.1±1.1‰（vs.CO2（aq））。CO2（aq）+H2 O '）在17.4±1.0° C下对氧的反应。将我们的结果与基于先前工作的修订的CO2羟基化KFF值相结合，表明碳的KFF值为− 17.0±0.3‰（相对于CO2（aq）），氧的KFF值为− 6.8±0.8‰（相对于CO2（aq））。CO2（aq）+H2 O '）在17-28° C的温度范围内。(2)雪松中碳酸钙的同位素重端元反映了来自小溪或地表水的同位素平衡DIC的组成（主要是HC O 3-，pH= 7.8-8.7），偶尔与高pH泉水混合。因此，现代和古代的碳酸盐δ 13 C和δ 18 O值反映了由过程（1）和（2）形成的碳酸钙的比例，其中过程（2）在雪松的碳酸盐沉淀预算中占主导地位。这些结果表明，在理解动力学同位素效应的最新进展，使我们能够模拟复杂的，但常见的自然过程，并建议古钙钛矿可以用来恢复过去的大气水δ 18 O和大气δ 13 C值。有证据表明，雪松的老石灰岩记录了大气中的δ 13 C，早于化石燃料的大规模燃烧。
The Cedars ultramafic block hosts alkaline springs (pH> 11) in which calcium carbonate forms upon uptake of atmospheric CO 2 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 3 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 13 C (− 9 to− 28‰ VPDB) and δ 18 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 2 hydroxylation but the controls on the δ 13 C-δ 18 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 2) 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 2-fed solution that tracks the isotopic composition of each dissolved inorganic carbon (DIC) species and CaCO 3. The box model includes four sources or sinks of DIC (atmospheric CO 2, high pH spring water, fresh creek water, and CaCO 3 precipitation). Model parameters are informed by new floe Δ 44 Ca data (− 0.75±0.07‰), direct mineral growth rate measurements (4.8 to 8× 10− 7 mol/m 2/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 2 and the kinetic isotope fractionation factor (KFF (‰)=(α− 1)× 1000) accompanying CO 2 hydroxylation, estimated here to be− 17.1±0.8‰(vs. CO 2 (aq)) for carbon and− 7.1±1.1‰(vs.‘CO 2 (aq)+ H 2 O’) for oxygen at 17.4±1.0° C. Combining our results with revised CO 2 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)+ H 2 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 HC O 3-, pH= 7.8–8.7) that occasionally mixes with the high-pH spring water. The bulk carbonate δ 13 C and δ 18 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 18 O and atmospheric δ 13 C values. There is evidence that older travertine at The Cedars recorded atmospheric δ 13 C that predates large-scale combustion of fossil fuels.