Methane cycling in the carbonate critical zone

Andrew Oberhelman; Jonathan B Martin; Madison K Flint

doi:10.1016/j.scitotenv.2023.165645

Methane cycling in the carbonate critical zone

Sci Total Environ. 2023 Nov 15:899:165645. doi: 10.1016/j.scitotenv.2023.165645. Epub 2023 Jul 19.

Authors

Andrew Oberhelman¹, Jonathan B Martin², Madison K Flint²

Affiliations

¹ Department of Geological Sciences, 241 Williamson Hall, PO Box 112120, University of Florida, Gainesville, FL 32611-2120, USA. Electronic address: aoberhelman@ufl.edu.
² Department of Geological Sciences, 241 Williamson Hall, PO Box 112120, University of Florida, Gainesville, FL 32611-2120, USA.

PMID: 37474066
DOI: 10.1016/j.scitotenv.2023.165645

Abstract

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 CH₄ cycling in the carbonate CZ, we measured CH₄ concentrations and ¹³C 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 CH₄ ranged from 0.002 to 89 μM, with an inverse relationship in springs between subsurface residence time and CH₄ concentration. Where residence time is short, low CH₄ concentrations 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 CH₄ concentrations greater than spring water indicating CH₄ is lost during flow to spring vents. CH₄ concentrations covary with δ¹³C-CH₄ values, which supports both methanogenesis and methanotrophy with changing residence times. Mean fluxes of CH₄ ranged from -0.05 to 1.0 mg m^-2 d^-1 at spring vents, with negative values caused by CH₄ uptake in water undersaturated with respect to atmospheric concentration. Most springs are dominated by methanotrophy, limiting atmospheric evasion of CH₄ produced in the carbonate CZ. We estimate CH₄ emissions to be 12.6 × 10^-6 Tg a^-1 across all Florida springs or about two orders of magnitude less than emissions from Floridan aquifer groundwater abstraction (3041 × 10^-6 Tg a^-1). Although CH₄ is produced in the carbonate CZ, natural attenuation limits its effects on the global carbon cycle.

Keywords: Carbonate critical zone; Methane; Methanogenesis; Methanotrophy; Springs.