Methane cycling in the carbonate critical zone
Methane cycling in the carbonate critical zone
复制标题
DOI：
10.1016/j.scitotenv.2023.165645
复制
发表时间：
2023-07-22
影响因子：
9.8
通讯作者：
Flint，Madison K.
中科院分区：
文献类型：
--
作者：
Oberhelman，Andrew;Martin，Jonathan B.;Flint，Madison K.
碳酸盐临界区（CZ）的特点是广泛的地下水与地表水交换，导致地下水氧化还原状态的高度变化。氧化还原条件的变化可能会导致甲烷 (CH4) 的产生或消耗，从而为这种重要的温室气体提供大气源或汇。为了评估地下水-地表水交换如何影响碳酸盐 CZ 中的氧化还原状态和 CH4 循环，我们测量了佛罗里达州中北部溪流、泉水系统和水井水中的 CH4 浓度和 13 C 同位素。采样的地下水的地下停留时间范围从在溪流下沉-上升系统中的数小时到洪水补给事件进入泉水口后的数月，再到在有限点补给的泉水中数十年。 CH4 浓度范围为 0.002 至 89 µM，泉水中地下停留时间与 CH4 浓度之间存在反比关系。在停留时间较短的情况下，与溶解氧 (DO) 浓度升高相关的甲烷氧化作用会导致低 CH4 浓度。洪水过后，补给后不久就会发生甲烷氧化作用，随后随着地下水逐渐减少而发生甲烷生成。从井中提取的地下水的 CH4 浓度高于泉水，表明 CH4 在流向泉水喷口时损失了。 CH4 浓度与 δ13C-CH4 值共变化，随着停留时间的变化，它支持甲烷生成和甲烷氧化。泉水喷口处 CH4 的平均通量范围为 -0.05 至 1.0 mg m−2d−1，负值是由于水中 CH4 吸收相对于大气浓度不饱和造成的。大多数泉水以甲烷氧化为主，限制了碳酸盐 CZ 中产生的 CH4 的大气逃逸。我们估计佛罗里达州所有泉水的 CH4 排放量为 12.6 × 10−6Tg a−1，比佛罗里达含水层地下水抽取的排放量 (3041 × 10−6Tg a−1) 低大约两个数量级。尽管CH4是在碳酸盐CZ中产生的，但自然衰减限制了其对全球碳循环的影响。
The carbonate critical zone (CZ) is characterized by extensive groundwater-surface water exchange that leads to highly variable redox states of groundwater. Changes in redox condition may cause either production or consumption of methane (CH4), thereby providing an atmospheric source or sink of this important greenhouse gas. To assess how groundwater-surface water exchange affects redox state and CH4cycling in the carbonate CZ, we measured CH4concentrations and13C isotopes in water from streams, spring systems, and wells in north-central Florida. Sampled groundwater has subsurface residence times ranging from hours at a stream sink-rise system, to months following a flood recharge event into a spring vent, to decades at springs with limited point recharge. Concentrations of CH4ranged from 0.002 to 89 μM, with an inverse relationship in springs between subsurface residence time and CH4concentration. Where residence time is short, low CH4concentrations result from methanotrophy linked to elevated dissolved oxygen (DO) concentrations. Following flooding, methanotrophy occurs soon after recharge and is followed by methanogenesis as groundwater becomes increasingly reducing. Groundwater extracted from wells had CH4concentrations greater than spring water indicating CH4is lost during flow to spring vents. CH4concentrations covary with δ13C-CH4values, which supports both methanogenesis and methanotrophy with changing residence times. Mean fluxes of CH4ranged from −0.05 to 1.0 mg m−2d−1at spring vents, with negative values caused by CH4uptake in water undersaturated with respect to atmospheric concentration. Most springs are dominated by methanotrophy, limiting atmospheric evasion of CH4produced in the carbonate CZ. We estimate CH4emissions to be 12.6 × 10−6Tg a−1across all Florida springs or about two orders of magnitude less than emissions from Floridan aquifer groundwater abstraction (3041 × 10−6Tg a−1). Although CH4is produced in the carbonate CZ, natural attenuation limits its effects on the global carbon cycle.