Radiocarbon preservation in carbonate shells of aquatic Gastropoda: Development of a geochemical proxy to measure groundwater transience

Abstract:

Direct measurement of the dynamic response of groundwater to climate changes would provide important constraints to transient hydrologic models, especially in arid regions such as the Great Basin region of western North America. Groundwater ¹⁴C activities are often used to model residence times but only represent residence times at the time of sample collection. We develop a geochemical proxy for ¹⁴C in carbonate shells, which can serve as a paleoenvironmental record of groundwater residence times and would be a useful addition to numerical groundwater models to evaluate groundwater transience.

This study develops a modern proxy of aqueous, gill-breathing gastropod shells as proxies for water ¹⁴C at mesothermal springs in western Utah, in the Great Basin province. Permanent wetlands such as Blue Lake springs display a constrained water budget and near-constant temperature and salinities. This aridland spring system receives very little precipitation, and groundwater discharge is effectively the only input of water. Among the organisms that live in this spring ecorefuge are carbonate shell producing, benthic, gill-breathing gastropods (endemic Hydrobiidae and invasive Melanoides). Samples of groundwater discharging at three sites at Blue Lake springs have a mean ¹⁴C activity of 8.47 percent modern carbon-14 (pmC) (SD = 0.16 pmC). The average ¹⁴C activity of 12 modern Melanoides shells from Blue Lake springs is 8.54 pmC (SD = 0.38 pmC). A small degree of biomediated fractionation between HCO₃^- in water and CaCO₃ shell may be occurring but is constrained by the measured δ¹³C fractionation of less than 1‰. We show that water and shell ¹⁴C activities are equal or positively correlated, and have the potential to be a proxy for groundwater residence times in paleospring deposits or sediment cores with well-constrained depositional ages.