Controls on dissolved inorganic carbon and δ13C in cave waters from DeSoto Caverns: Implications for speleothem δ13C assessments
Controls on dissolved inorganic carbon and δ13C in cave waters from DeSoto Caverns: Implications for speleothem δ13C assessments
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DOI：
10.1016/j.gca.2010.11.006
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发表时间：
2011-02
影响因子：
5
通讯作者：
W. Lambert;P. Aharon
中科院分区：
文献类型：
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作者：
W. Lambert;P. Aharon
揭示现存岩溶系统中控制碳化学和碳迁移的因素对于评估洞穴沉积物的时间序列δ 13 C记录具有重要意义。在这里，我们报告了一个为期3年的研究结果，总溶解无机碳[DIC]和δ 13 CDIC从洞穴沃茨水在德索托洞穴（美国东南部），提供了有价值的见解碳运输和伴随的同位素分馏从端员源的洞穴沉积物。[DIC]洞穴沃茨的δ 13 CDIC值分别为0.2 ~ 6.0mM和2.7 ~-12.9（‰ VPDB）。[DIC]和“季节性滴水”的δ 13 CDIC显示出季节性（尽管有噪声）变化，并且呈负相关（δ 13 CDIC =−2.49[DIC]+0.64，r2=0.84）。一个由多滴水补给的浅水池显示出双峰δ 13 CDIC分布，冬季（−4‰至−5‰ VPDB）相对于夏季（−9‰至−10‰ VPDB）具有较重的同位素模式。在研究期间，水的可用性下降的多年趋势没有反映在洞穴水碳化学的响应表明，降雨量可能不是一个显着的控制因素的碳化学。耦合洞穴空气冬季通风/夏季停滞和不同的CO2通量通过土壤层和表层岩溶的季节[DIC]和δ 13 CDIC变化的影响最大。夏季/初秋采集的洞穴沃茨中高[DIC]和低δ 13 CDIC的测量值接近同位素平衡条件。相反，低[DIC]和高δ 13 CDIC值在冬季/月初表明动力学增强的同位素分馏内的洞穴沃茨。在脱气CO2和沉淀CaCO 3之间分配的HCO 3-的动力学增强同位素分馏[分子式：见正文]，相对于平衡条件下的相同同位素分馏（-3.1 ‰），大约大两倍（-6.7 ±0.3‰）。根据14 C质量平衡和14 C-U/Th配对测量结果，我们估计每年滴注到文石石笋中的C平均约有1.23%来自14 C-死白云岩帽，其余约有1.77%来自14 C-活生物量。在夏季几个月内从活动洞穴沉积物顶端取样的文石（n=12）的δ 13 C测量值与利用浅池夏季/初秋数据计算的理论文石δ 13 C值一致，从而证实了滴水和同时代文石中δ 13 C的季节性。在过去200年中，一个活动石笋剖面的δ 13 C值呈正态分布，平均值为−7.1±1.2‰（n=81），众数为−7‰ ~ −8‰，与所有滴水的年平均值和众数在统计上没有区别。因此，德索托洞穴石笋的长期时间序列δ 13 C记录（分辨率>10− 1年）可能带有滴水年平均值的印记，记录气候驱动的δ 13 C季节性偏差。
Unraveling the factors controlling the carbon chemistry and transport of carbon within extant karst systems has important implications concerning the assessment of time-series δ13C records of speleothems. Here we report the results of a 3-year study of total dissolved inorganic carbon [DIC] and δ13CDICfrom cave waters at DeSoto Caverns (Southeastern USA) that offer valuable insight on carbon transport and the accompanied isotope fractionations from end-member sources to speleothems. [DIC] and δ13CDICvalues of cave waters range from 0.2 to 6.0mM and 2.7 to −12.9 (‰ VPDB), respectively. [DIC] and δ13CDICof “seasonal drips” show seasonal, albeit noisy, variability and are inversely related (δ13CDIC=−2.49[DIC]+0.64, r2=0.84). A shallow pool fed by multiple drips shows a bimodal δ13CDICdistribution with an isotopically heavier mode during winter (−4‰ to −5‰ VPDB) relative to summer months (−9‰ to −10‰ VPDB). A multi-year trend of decreasing water availability during the study period is not reflected in a response of cave water carbon chemistry suggesting that rainfall amount may not be a significant controlling factor of the carbon chemistry. Coupled cave air winter ventilation/summer stagnation and varying CO2fluxes through the soil horizon and epikarst exert the strongest influence on seasonal [DIC] and δ13CDICvariability. Measured values of high [DIC] and low δ13CDICfrom cave waters collected during the summer/early fall closely approximate isotopic equilibrium conditions. Conversely, low [DIC] and high δ13CDICvalues during winter/early months indicate kinetically enhanced isotopic fractionations within the cave waters. The kinetically enhanced isotopic fractionation of HCO3-partitioned between degassed CO2and precipitated CaCO3[Formula: see text] is greater by about a factor of two (−6.7±0.3‰) relative to the same isotopic fractionation under equilibrium conditions (−3.1‰). On the basis of14C mass balance and paired14C–U/Th measurements we estimate that on average about ∼23% of C delivered annually by the drips to the aragonite stalagmites is derived from14C-dead dolomite cap while the remainder of ∼77% is derived from14C-live biomass. δ13C measurements of aragonite (n=12) sampled from the tips of active speleothems during the summer months are consistent with theoretical aragonite δ13C values calculated using the shallow pool summer/early fall data thus confirming the δ13C seasonality in both drips and coeval aragonite. δ13C values of an active stalagmite section spanning the last 200years show a normal distribution with a mean of −7.1±1.2‰ (n=81) and a mode of −7‰ to −8‰ that are statistically indistinguishable from the annual mean and mode of all dripwaters. Thus secular time-series δ13C records of stalagmites at DeSoto Caverns with resolving power >10−1year will likely carry the imprints of drip annual means that record climate-driven δ13C seasonal biases.