Tidal pumping and biogeochemical processes: Dissolution within the tidal capillary fringe of eogenetic coastal carbonates

Abstract

Growing evidence suggests microbial respiration of dissolved organic carbon (DOC) may be a principal driver of subsurface dissolution and cave formation in eogenetic carbonate rock. Analyses of samples of vadose zone gasses, and geochemical and hydrological data collected from shallow, uncased wells on San Salvador Island, Bahamas, suggest tidally varying water tables may help fuel microbial respiration and dissolution through oxygenation. Respiration of soil organic carbon transported to water tables generates dysaerobic to anaerobic groundwater, limiting aerobic microbial processes. Positive correlations of carbon dioxide (CO2), radon‐222 (222Rn) and water table elevation indicate, however, that tidal pumping of water tables pulls atmospheric air that is rich in oxygen, and low in CO2 and 222Rn, into contact with the tidal capillary fringe during falling tides. Ratios of CO2 and O2 in vadose gas relative to the atmosphere indicate this atmospheric oxygen fuels respiration within newly‐exposed, wetted bedrock. Deficits of expected CO2 relative to O2 concentrations indicate some respired CO2 is likely removed by carbonate mineral dissolution. Tidal pumping also appears capable of transferring oxygen to the freshwater lens, where it could also contribute to respiration and dissolution; dissolved oxygen concentrations at the water table are at least 5% saturated and decline to anaerobic conditions 1–2 m below. Our results demonstrate how tidal pumping of air to vadose zones can drive mineral dissolution reactions that are focused near water tables and may contribute to the formation of laterally continuous vuggy horizons and potentially caves. © 2020 John Wiley & Sons, Ltd.

Groundwater‐surface water interaction, dissolved organic carbon oxidation and dissolution in carbonate aquifers

“…The loss of recharged water would not alter individual solute concentrations and would thus change all Δ[x] values equally, with no impact on mixing model estimates. The elevated DOC of the aquifer matrix likely promotes greater localized dissolution as DOC is oxidized (Gulley et al, 2014(Gulley et al, , 2016(Gulley et al, , 2020.…”

“…Oxidation of organic carbon (OC) is recognized as an important source of acidity driving dissolution in the subsurface of carbonate landscapes, particularly in eogenetic carbonates, which retain high primary porosity and permeability because of the lack of burial diagenesis (Choquette & Pray, 1970; Gulley et al, 2016, 2020; Vacher & Mylroie, 2002). In contrast, dissolution of telogenetic limestone, with low porosity and permeability because of recrystallization during burial, occurs as flow is focused along fractures, faults and bedding planes (Palmer, 1991).…”

“…Another cause of undersaturation and concentrated dissolution occurs at the water table, where heterogenous distributions of CO 2 develop along flow paths through variations in root respiration and the accumulation and microbial oxidation of OC (Cooper et al, 2016;Baldini et al, 2006;Gulley et al, 2020Gulley et al, , 2016Gulley et al, , 2015Gulley et al, , 2014Mattey et al, 2013;Whitaker & Smart, 2007;Wood, 1985). Dissolution occurs as water flows from regions of low to high CO 2 concentrations in the vadose zone, leading to CO 2 dissolution and hydration to carbonic acid (H 2 CO 3 ).…”

A transient, perched aquifer model for banana hole formation: Evidence from San Salvador Island, Bahamas

“…We calculated dissolution timescales required to generate banana hole volumes in MIS 5e strandplains for three conceptual models: vadose formation (Whitaker & Smart, 1989), dissolution at water tables (Gulley et al , 2020, 2015; Harris et al, 1995) and prograding strandplains (Mylroie et al , 2016). While some banana holes also occur in MIS 9/11 bedrock, we do not estimate timescales for their formation due to the lack of constraints on the timing of bedrock deposition.…”

“…Soil‐derived organic matter transported to the water table during recharge events fuels microbial oxidation reduction reactions that drive dissolution (Bottrell et al, 1993; Cooper et al , 2016; Gulley et al , 2015). In coastal aquifers, such oxidation–reduction processes could be further promoted by tidal fluctuation of water tables that oxygenate the vadose zone and lens top (Gulley et al , 2020).…”