Using Stable Isotopes of Carbon to Investigate the Seasonal Variation of Carbon Transfer in a North-western Arkansas Cave

Abstract

Stable-isotope analyses are valuable in karst settings, where characterizing biogeochemical cycling of carbon along groundwater flow paths is critical for understanding and protecting sensitive cave and karst water resources. This study quantified the seasonal changes in concentration and isotopic composition (d 13 C) of aqueous and gaseous carbon species-dissolved inorganic carbon (DIC) and gaseous carbon dioxide (CO 2 )-to characterize sources and transfer of these species along a karst flow path, with emphasis on a cave environment. Gas and water samples were collected from the soil and a cave in northwestern Arkansas approximately once a month for one year to characterize carbon cycling along a conceptual groundwater flow path. In the soil, as the DIC concentration increased, the isotopic composition of the DIC became relatively lighter, indicating an organic carbon source for a component of the DIC and corroborating soil DIC as a proxy for soil respiration. In the cave, a positive correlation between DIC and surface temperature was due to increased soil respiration as the organic carbon signal from the soil was transferred to the cave environment via the aqueous phase. CO 2 concentration was lowest in the cave during colder months and increased exponentially with increasing surface temperature, presumably due to higher rates of soil respiration during warmer periods and changing ventilation patterns between the surface and cave atmosphere. Isotopic disequilibrium between CO 2 and DIC in the cave was greatest when CO 2 concentration was changing during November/ December and March/April, presumably due to the rapid addition or removal of gaseous CO 2 . The isotopic disequilibrium between DIC and CO 2 provided evidence that cave CO 2 was a mixture of carbon from several sources, which was mostly constrained by mixture between atmospheric CO 2 and soil CO 2 . The concentration and isotopic composition of gaseous and aqueous carbon species were controlled by month-to-month variations in temperature and precipitation and provided insight into the sources of carbon in the cave. Stable carbon isotope ratios provided an effective tool to explore carbon transfer from the soil zone and into the cave, identify carbon sources in the cave, and investigate how seasonality affected the transfer of carbon in a shallow karst system.

Seasonal δ13CDIC sourcing and geochemical flux in telogenetic epikarst of south‐central Kentucky

“…() also suggest that, based on similar findings in an investigation of Jack's Cave in Arkansas, the magnitude of different source inputs changes seasonally. For example, surface temperature is a proxy for soil respiration (Clark and Fritz, ; Knierim et al ., ), and at lower temperatures soil respiration rates are lower. For temperatures at or higher than 10°C, more microbial activity is likely to occur, thus producing increased concentrations of soil CO 2 .…”

“…The extent of dissolution at LRS is possibly governed by several influences: (1) the rate and frequency of recharge capable of infiltrating the epikarst and diffusing available CO 2 to the epikarst; (2) the residence time of infiltrating water allowing for ongoing water–rock interaction; and (3) the frequency of storm events which are capable of exceeding storage capacity to flush the epikarst of dissolved constituents while reinforcing the epikarst with freshly diffused CO 2 , especially during the summer months (Li et al, , ; Pu et al, , ). Further, these factors support the interpretation that DIC values are heavily influenced by CO 2 concentrations in groundwater, as found by other studies (e.g.Emblanch et al ., ; Shin et al ., ; White, ; Knierim et al ., ; Zhang et al ., ).…”

“…During the winter months, atmospheric and carbonate bedrock weathering contributions increased relative to soil CO 2 contributions. Further, the atmospheric CO 2 signal dominates the carbonate weathering signal, which could be from shifts in airflow into the cave during winter (Knierim et al ., ), as well as increased enrichment from longer residence time increasing dissolution (Figure ) (Lambert and Aharon, ; S.‐L. Li et al ., ; Zhao et al ., ).…”

“…At WF1, distinct seasonal trends are apparent in pH, CO 2 , and DIC which are indicative of increased summertime CO 2 diffusion to the epikarst, driving down pH values, and increased dissolution driving DIC concentration increases ( Figure 5) (Q. Li et al, 2008a;S.-L. Li et al, 2008b;Liu et al, 2010;Pu et al, 2014aPu et al, , 2014bGulley et al, 2015;Knierim et al, 2015). During the autumn transition, the inverse relationship between pH and CO 2 becomes most apparent, as values for each parameter suddenly shift; pH values increase while CO 2 values decrease.…”

Integrated Use of Geological, Geophysical, Radiocarbon and Stable Isotopes Data for Tracing the Conduit Flow Paths in a Small Karstic Aquifer: Poocher Swamp Freshwater Lens, South Australia

“…Knierim et al (2015) show that stable isotope analyses are valuable in karst settings. As groundwater flow in karst aquifers is shared between the granular porosity and karst conduits, an indirect way of identification of major flow paths are possible using salinity distributions (Somaratne, 2015a), tracer techniques such as radiocarbon activity ( 14 C) and stable isotope (δ 18 O) distribution (Somaratne et al, 2016).…”