Oxygenation of the Ox Bel Ha karst subterranean estuary during Tropical Storm Hanna: Mechanisms and implications for methane turnover

Seasonal precipitation affects methane accumulation and turnover in tropical karst subterranean estuaries, the region of the coastal aquifer where hydrological and biogeochemical processes regulate material exchange between the terrestrial and marine environments. However, the impact of extreme weather events (e.g., tropical storms and hurricanes) on subsurface carbon cycling linked to coastal karst landscapes is unknown. In this study, we present a 5-month long hydrologic and chemical record from inland and coastal portions of the extensive Ox Bel Ha cave system in the northeastern Yucatan Peninsula. The record encompasses wet and dry seasons and includes effects from Tropical Storm Hanna during October 2014. Intense rainfall coincides with an episodic increase in water level and sudden shifts in salinity, indicating a spatially widespread hydrologic response. The most profound effect of the storm was a vanishing pulse of dissolved oxygen to ~0.7 mg l^-1 that declined to zero along with the disappearance of methane that had built up during the wet season. A positive shift in methane’s stable carbon isotope content from −62.6 ± 0.6‰ before the storm to −44.0 ± 2.4‰ after the storm indicates microbial methane oxidation was the primary mechanism for the loss of methane from the groundwater. Post-storm methane concentrations, which never reached pre-storm levels, suggest the tropical storm had long-lasting (months) effects on carbon cycling. These findings demonstrate that mixing and oxygen delivered during storm-induced hydrologic forcing has an outsized biogeochemical effect within typically stratified karst subterranean estuaries.