Constraining the subsoil carbon source to cave-air CO 2 and speleothem calcite in central Texas
Constraining the subsoil carbon source to cave-air CO 2 and speleothem calcite in central Texas
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DOI：
10.1016/j.gca.2017.08.017
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发表时间：
2017-11
影响因子：
5
通讯作者：
S. Bergel;P. Carlson;T. Larson;C. Wood;K. Johnson;J. Banner;D. Breecker
中科院分区：
文献类型：
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作者：
S. Bergel;P. Carlson;T. Larson;C. Wood;K. Johnson;J. Banner;D. Breecker
洞穴形成和地下碳循环的典型模型将土壤呼吸作为主要碳源。然而，来自一些喀斯特地区的证据表明，地下二氧化碳来自更深、更古老的来源。因此，我们调查了德克萨斯州中部洞穴的碳源。德克萨斯州中部两个洞穴的滴水化学表明，在高于上覆土壤中观察到的最大 CO2 浓度的 CO2 浓度 (PCO2_sat) 下，方解石达到平衡。这一观察结果表明，二氧化碳通过土壤渗透后被添加到水中，这需要底土碳源。我们使用洞穴空气 CO2 的 δ13C 测量直接评估地下碳源的碳同位素组成，我们独立证明了其与宿主岩石碳几乎没有贡献。我们使用氧化比 OR 来计算，OR 定义为呼吸过程中每产生一摩尔 CO2 所消耗的 O2 摩尔数。然而，影响 O2 和 CO2 浓度的其他地下过程（例如气水交换和/或扩散）也可能影响测量的氧化比，从而产生表观 OR (OR 表观)。天然桥南洞穴中的洞穴空气的 OR 表观值 (1.09 ± 0.06) 与仅呼吸作用的预期值 (1.08 ± 0.06) 无法区分。来自洞穴上方土壤的孔隙空间气体具有较低的值（ORapparent= 0.67 ± 0.05），这与呼吸作用和扩散气体传输一致。对这些观察结果的最简单解释是，新布伦特南部的洞穴空气受到开放系统基岩裂缝呼吸的影响，因此扩散或与水的交换都不会影响洞穴空气的成分。 NB South洞穴空气CO2的放射性碳活动表明地下碳源已有数百年历史。计算出的底土碳源 δ13C 值与树木来源的碳（可能是分解的根物质）一致，由于 δ13C 值和大气 CO2 浓度的变化，其 δ13C 值在工业化过程中发生了变化。大多数滴水的 PCO2_satin 的季节变化表明这些水与表层岩溶中的通风基岩裂缝进行交换，这意味着地下 CO2 源为洞穴岩提供了碳。
Canonical models for speleothem formation and the subsurface carbon cycle invoke soil respiration as the dominant carbon source. However, evidence from some karst regions suggests that belowground CO2originates from a deeper, older source. We therefore investigated the carbon sources to central Texas caves. Drip-water chemistry of two caves in central Texas implies equilibration with calcite at CO2concentrations (PCO2_sat) higher than the maximum CO2concentrations observed in overlying soils. This observation suggests that CO2is added to waters after they percolate through the soils, which requires a subsoil carbon source. We directly evaluate the carbon isotope composition of the subsoil carbon source using δ13C measurements on cave-air CO2, which we independently demonstrate has little to no contribution from host rock carbon. We do so using the oxidative ratio, OR, defined as the number of moles of O2consumed per mole of CO2produced during respiration. However, additional belowground processes that affect O2and CO2concentrations, such as gas-water exchange and/or diffusion, may also influence the measured oxidative ratio, yielding an apparent OR (ORapparent). Cave air in Natural Bridge South Cavern has ORapparentvalues (1.09 ± 0.06) indistinguishable from those expected for respiration alone (1.08 ± 0.06). Pore space gases from soils above the cave have lower values (ORapparent= 0.67 ± 0.05) consistent with respiration and gas transport by diffusion. The simplest explanation for these observations is that cave air in NB South is influenced by respiration in open-system bedrock fractures such that neither diffusion nor exchange with water influence the composition of the cave air. The radiocarbon activities of NB South cave-air CO2suggest the subsoil carbon source is hundreds of years old. The calculated δ13C values of the subsoil carbon source are consistent with tree-sourced carbon (perhaps decomposing root matter), the δ13C values of which have shifted during industrialization due to changes in the δ13C values and concentrations of atmospheric CO2. Seasonal variations in PCO2_satin most of the drip waters suggest that these waters exchange with ventilated bedrock fractures in the epikarst, implying that the subsoil CO2source contributes carbon to speleothems.