Abstract Chemical gradients between fresh, brackish and saline waters shape biogeochemical reactions and organic matter transformation within subterranean estuaries. In the Yucatan Peninsula’s karst subterranean estuary (KSE), methane and dissolved organic matter generated during the anaerobic decomposition of tropical forest vegetation are transported into flooded cave networks where microbial consumption greatly reduces their concentrations in the groundwater. To test the hypothesis that chemoclines associated with salinity gradients of the KSE are sites of methane oxidation, we obtained methane concentration and δ13C profiles of unprecedented vertical resolution from within a fully-submerged cave system located 6.6 km inland from the coastline using the ‘OctoPiPi’ (OPP) water sampler. Along a 12–24 cm thick low-salinity-halocline at ∼4.5 m water depth, salinity increased from fresh to brackish (0.2–1.8 psu), methane concentrations decreased, and δ13C values increased, as expected for microbial methane oxidation. The underlying brackish water had elevated oxygen concentrations compared to the always anoxic freshwater, suggesting that aerobic methane oxidation is the dominant process facilitating methane consumption. By contrast, as salinity increased from 1.8 to 36 psu through a 24–36 cm thick high-salinity-halocline between the meteoric lens and the saline groundwater at ∼20 m water depth, methane concentrations and δ13C values were constant. Conservative mixing and kinetic isotope models incorporating the methane data confirm a hotspot for microbial methane oxidation at the low-salinity-halocline. At least 98% of methane originating in the anoxic freshwaters was removed before its transport via channelized flow towards the coastline. These findings provide novel insight into the spatial constraints of methane dynamics within a karst subterranean estuary.

中文翻译：

---

岩溶地下河口甲烷氧化动力学

摘要 淡水、微咸水和咸水之间的化学梯度塑造了地下河口内的生物地球化学反应和有机物质转化。在尤卡坦半岛的岩溶地下河口 (KSE)，热带森林植被厌氧分解过程中产生的甲烷和溶解的有机物被输送到淹没的洞穴网络中，在那里微生物的消耗大大降低了它们在地下水中的浓度。为了验证与 KSE 盐度梯度相关的化学跃层是甲烷氧化位点的假设，我们使用“OctoPiPi”从距离海岸线内陆 6.6 公里的完全淹没的洞穴系统中获得了前所未有的垂直分辨率的甲烷浓度和 δ13C 剖面图(OPP) 水采样器。沿着 12-24 厘米厚的低盐度盐斜线在~4。5 m 水深，盐度从新鲜增加到微咸 (0.2–1.8 psu)，甲烷浓度降低，δ13C 值增加，正如微生物甲烷氧化所预期的那样。与始终缺氧的淡水相比，下面的微咸水具有更高的氧浓度，这表明有氧甲烷氧化是促进甲烷消耗的主要过程。相比之下，随着盐度从 1.8 psu 增加到 36 psu，通过流星透镜和约 20 m 水深处的咸水地下水之间的 24-36 cm 厚的高盐度卤化层，甲烷浓度和 δ13C 值是恒定的。结合甲烷数据的保守混合和动力学同位素模型证实了低盐度卤化层微生物甲烷氧化的热点。至少 98% 源自缺氧淡水的甲烷在通过渠道流向海岸线运输之前被去除。这些发现提供了对岩溶地下河口内甲烷动力学空间限制的新见解。