Cave airflow patterns control calcite dissolution rates within a cave stream: Blowing Springs Cave, Arkansas, USA

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Authors

Matthew D Covington , Katherine Knierim, Holly A. Young, Josue Rodriguez, Hannah Gnoza

Abstract

Erosion rates within streams vary dramatically over time, as differences in discharge and sediment load enhance or inhibit erosion processes. Within cave streams, and other bedrock channels incising soluble rocks, changes in water chemistry are an important factor in determining how erosion rates will vary in both time and space. Prior studies within surface streams, springs, and caves suggest that variation in dissolved ${\rm CO_2}$ is the strongest control on variation in calcite dissolution rates. However, the controls on ${\rm CO_2}$ variation remain poorly quantified. Limited data suggest that ventilation of karst systems can substantially influence dissolved ${\rm CO_2}$ within karst conduits. However, the interactions among cave ventilation, air-water ${\rm CO_2}$ exchange, and dissolution dynamics have not been studied in detail. Here we analyze three years of time series measurements of dissolved and gaseous ${\rm CO_2}$, cave airflow velocity, and specific conductance from Blowing Springs Cave, Arkansas. We use these time series to estimate continuous calcite dissolution rates and quantify the correlations between those rates and potential physical and chemical drivers. We find that chimney effect airflow creates temperature-driven switches in airflow direction, and that the resulting seasonal changes in airflow regulate both gaseous and dissolved ${\rm CO_2}$ within the cave. As in previous studies, partial pressure of ${\rm CO_2}$ (${\rm pCO_2}$) is the strongest chemical control of dissolution rate variability. However, we also show that cave airflow direction, rather than stream discharge, is the strongest physical driver of changes in dissolution rate, contrary to the typical situation in surface channel erosion where floods largely determine the timing and extent of geomorphic work. At the study site, chemical erosion is typically active in the summer, during periods of cave downdraft (airflow from upper to lower entrances), and inactive in the winter, during updraft (airflow from lower to upper entrances). Storms provide only minor perturbations to this overall pattern. We also find that airflow direction modulates dissolution rate variation during storms, with higher storm variability during updraft than during downdraft. Finally, we compare our results with the limited set of other studies that have examined dissolution rate variation within cave streams and draw an initial hypothesis that evolution of cave ventilation patterns strongly impacts how dissolution rate dynamics evolve over the lifetime of karst conduits.

DOI

https://doi.org/10.31223/osf.io/t7a4q

Subjects

Earth Sciences, Environmental Sciences, Geochemistry, Geomorphology, Hydrology, Physical Sciences and Mathematics

Keywords

karst, bedrock channel, carbon dioxide, cave, Critical Zone, dissolution, speleogenesis

Dates

Published: 2020-05-25 02:13

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License

CC BY Attribution 4.0 International

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