Hydrologic controls of methane dynamics in Karst subterranean estuaries

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Brankovits, David
Pohlman, John W.
Ganju, Neil K.
Iliffe, Thomas
Lowell, Nick
Roth, Erich
Sylva, Sean P.
Emmert, Jake
Lapham, Laura L.
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Subterranean estuary
Coastal aquifer
Carbon cycling
Anchialine ecosystem
Karst subterranean estuaries (KSEs) extend into carbonate platforms along 12% of all coastlines. A recent study has shown that microbial methane (CH4) consumption is an important component of the carbon cycle and food web dynamics within flooded caves that permeate KSEs. In this study, we obtained high‐resolution (~2.5‐day) temporal records of dissolved methane concentrations and its stable isotopic content (δ13C) to evaluate how regional meteorology and hydrology control methane dynamics in KSEs. Our records show that less methane was present in the anoxic fresh water during the wet season (4,361 ± 89 nM) than during the dry season (5,949 ± 132 nM), suggesting that the wet season hydrologic regime enhances mixing of methane and other constituents into the underlying brackish water. The δ13C of the methane (−38.1 ± 1.7‰) in the brackish water was consistently more 13C‐enriched than fresh water methane (−65.4 ± 0.4‰), implying persistent methane oxidation in the cave. Using a hydrologically based mass balance model, we calculate that methane consumption in the KSE was 21–28 mg CH4·m−2·year−1 during the 6‐month dry period, which equates to ~1.4 t of methane consumed within the 102‐ to 138‐km2 catchment basin for the cave. Unless wet season methane consumption is much greater, the magnitude of methane oxidized within KSEs is not likely to affect the global methane budget. However, our estimates constrain the contribution of a critical resource for this widely distributed subterranean ecosystem.
Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 32(12), (2019): 1759-1775, doi:10.1029/2018GB006026.
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Brankovits, D., Pohlman, J. W., Ganju, K., Iliffe, T. M., Lowell, N., Roth, E., Sylva, S. P., Emmere, J. A., & Lapham, L. L. (2018). Hydrologic controls of methane dynamics in Karst subterranean estuaries, Global Biogeochemical Cycles, 32(12), 1759-1775.
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