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EVALUATION OF AQUEOUS GASTROPOD SHELLS AS GROUNDWATER RADIOCARBON PROXIES ACROSS SPECIES AND SITES

Published online by Cambridge University Press:  24 January 2023

J C Lerback Open the ORCID record for J C Lerback [Opens in a new window] ,
S Bagge  and
B B Bowen
J C Lerback*
Affiliation:
Department of Geology and Geophysics, University of Utah, 115 South 1460 East, Room 383, Salt Lake City, UT 84112, USA Department of Earth, Planetary and Space Sciences, University of California, Los Angeles, 595 Charles Young Drive East, Box 951567, Los Angeles, CA 90095, USA
S Bagge
Affiliation:
Department of Geology and Geophysics, University of Utah, 115 South 1460 East, Room 383, Salt Lake City, UT 84112, USA
B B Bowen
Affiliation:
Department of Geology and Geophysics, University of Utah, 115 South 1460 East, Room 383, Salt Lake City, UT 84112, USA Global Change and Sustainability Center, University of Utah, 115 South 1460 East, Room 234, Salt Lake City, UT 84112, USA
*
*Corresponding author. Email: jlerback@ucla.edu
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Abstract

We evaluate carbonate gastropod shells as 14C proxies for groundwater discharge at springs. Groundwater 14C is commonly used to estimate groundwater transit times, and a carbonate shell proxy would present a different way of collecting groundwater 14C data. Specifically, we test the hypothesis that in exclusively groundwater-fed spring systems, water 14C is preserved in carbonate shells at multiple sites, species, and water 14C. We first present isotopic and water temperature variability over several years at three spring sites in Utah. We then compare the 14C of contemporaneously collected water, sediment, and shells of benthic gastropods (Melanoides tuberculata, Pyrgulopsis pilsbryana, and Physella gyrina). We show that water and shell 14C activities at each site are correlated (slope = 1.00, R2 = 0.999, n = 22). These results support the hypothesis that 14C from groundwater is preserved in carbonate shells, and that aqueous gastropods a viable groundwater 14C proxy. Finally, we describe the utility and limitations of using gastropod shells as a groundwater 14C proxy.

Keywords

gastropodsgroundwaterhydrologyisotopes
Type
Research Article
Information
Radiocarbon , Volume 65 , Issue 2 , April 2023 , pp. 443 - 462
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Creative Commons
Creative Common License - CCCreative Common License - BY
This is an Open Access article, distributed under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution and reproduction, provided the original article is properly cited.
Copyright
© The Author(s), 2023. Published by Cambridge University Press for the Arizona Board of Regents on behalf of the University of Arizona

INTRODUCTION

Radiocarbon (14C) can be used to estimate apparent transit times in groundwater aquifers (also referred to as residence times and groundwater “ages” in the literature) (Ingerson and Pearson Reference Ingerson and Pearson1964; Mook et al. Reference Mook, Bommerson and Staverman1974; Tamers Reference Tamers1975; Fontes and Garnier Reference Fontes and Garnier1979; Fontes Reference Fontes, Taylor, Long and Kra1992; Kalin Reference Kalin2000; Geyh Reference Geyh2000; Plummer and Glynn Reference Plummer and Glynn2013; Han and Plummer Reference Han and Plummer2016; Cartwright et al. Reference Cartwright, Currell, Cendón and Meredith2020; and others). Modeling 14C depletion in groundwater must account for the variation of atmospheric or initial 14C activities and for soil-gas exchange and C dissolution in both open and closed systems. These processes are commonly accounted for using stable carbon isotopes (δ13C), dissolved inorganic carbon (DIC) concentration, and knowledge of the aquifer system mineralogy and flow geometry (with a variety of methods reviewed in Clark and Fritz Reference Clark and Fritz2013; Han and Plummer Reference Han and Plummer2016; Cartwright et al. Reference Cartwright, Currell, Cendón and Meredith2020 and references therein). Because of these considerations, 14C is useful to estimate groundwater mean transit times of 100–30,000 years and are particularly useful (if imprecise) in arid and semi-arid regional aquifers (Cartwright et al. Reference Cartwright, Currell, Cendón and Meredith2020). Calculation of these averaged transit times are critical for understanding groundwater flow rates, which in turn inform groundwater recharge rates and management strategies for sustainable resource use (Cook and Böhlke Reference Cook and Böhlke2000).

Gastropods and other mollusks build carbonate shells, the chemistry of which have been used as a proxy for a variety of paleoenvironmental conditions. Gastropods are excellent proxies for aquatic geochemical environments because they are globally distributed and live in a wide variety of environments from hypersaline and marine, to freshwater environments (Fortunado Reference Fortunato2016). Many studies show that aqueous δ13C signatures are retained in carbonate shells are preserved in the sedimentary record (Keith et al. Reference Keith, Anderson and Eichler1964; Mook and Vogel Reference Mook and Vogel1968; Fritz and Poplawski Reference Fritz and Poplawski1974; von Grafenstein et al. Reference von Grafenstein, Erlenkeuser, Müller and Kleinmann-Eisenmann1992; Tevesz et al. Reference Tevesz, Smith, Coakley and Risk1997; Balakrishnan et al. Reference Balakrishnan, Yapp, Theler, Carter and Wyckoff2005; Shanahan et al. Reference Shanahan, Pigati, Dettman and Quade2005; Salvador Reference Salvador, Tütken, Tomotani, Berthold and Rasser2018; and others). This paper addresses the idea that gastropod shells may provide material which records 14C of groundwater, a proxy for groundwater transit times. The collection of spring-dwelling gastropod shells requires little equipment (a shovel and a container) and the 14C analysis is a simple and cost-effective laboratory procedure.

Previous studies have suggested this direct relationship between the 14C activity of springwaters and shells. Riggs (Reference Riggs1984) described the springwater at two sites with very depleted water 14C activities (11.6 percent modern carbon, or pMC, at Crystal Pool and 3.9 pMC Big Springs in Ash Meadows, Nevada) measured in 1973 (Pearson and Bodden Reference Pearson and Bodden1975; Winograd and Pearson Reference Winograd and Pearson1976) and two shells collected the same sites at an unspecified later date (10.6 pMC and 3.3 pMC for Big Spring and Crystal Pool, respectively). Riggs reports a third shell and water at King Spring with a 4.3 pMC difference (where water is 1.6 pMC and shell is 5.9 pMC), which they attribute to partial water 14C reequilibration with the atmosphere due to the shallow flow from the discharge are to the shell collection pool. Notably, Brennan and Quade (Reference Brennan and Quade1997) use Riggs’ (Reference Riggs1984) correlation to apply this 14C depletion in sedimentary records as a for past groundwater transit times. However, they suggest but do not model the potential effect of springwater reequilibration with air as it flows away from the discharge points, nor do they investigate a numerical relationship between modern waters and modern shells at the study sites (although these sites are in the same general region of southern Nevada, U.S.A. as the Riggs Reference Riggs1984 study). Lastly, Copeland et al. (Reference Copeland, Quade, Watson, McLaurin and Villalpando2012) suggest that the 14C depletion due to groundwater chemistry described by Riggs (Reference Riggs1984) and Brennan and Quade (Reference Brennan and Quade1997) may explain their observed 14C depletion of aquatic snails relative to stratigraphic age markers in Late Pleistocene archaeological deposits, but do not argue that the shells are directly representing groundwater transit times.

To add to these previous studies, this study includes (1) a seasonal evaluation of water isotopic variation through time, (2) contemporaneously collected water and shell samples, (3) shells collected directly at spring discharge sites, (4) with a range of groundwater transit times (modern and Holocene) to derive a robust correlation at different spring types, (5) shells of several genera (including endemic genera) to constrain inter-species variation of Δ14Cs-gw, (6) a comparison of 14C of organic C in detritus to shell and water 14C, and (7) a transect of 14C in shells with increasing distance from the subsurface discharge point at one site.

We measure the relationship between modern water and modern shells, testing the viability of gastropod shells as a proxy for groundwater 14C. Specifically, we test the hypothesis that groundwater and shell 14C activities (C gw and C s , respectively) are equal across gastropod genera, sites, and at different 14C activities. We propose the notation of Δ 14 C s-gw to represent this comparison calculated by the following equation:

(1) $${\Delta ^{14}}{{{C}}_{{{s - gw}}}}{ = ^{14}}{{{C}}_{{s}}}{ - ^{14}}{{{C}}_{{{gw}}}}$$

where Δ 14 C s-gw is 0 if the hypothesis is supported.

In comparison, stable carbon isotopes (δ13C) have been shown to be generally retained in carbonate shells but also exhibit measurable variations with shell mineralogy, species, growth season, temperature, salinity, or other environmental parameters often described as “vital effects” (Balakrishnan et al. Reference Balakrishnan, Yapp, Theler, Carter and Wyckoff2005; Shanahan et al. Reference Shanahan, Pigati, Dettman and Quade2005; Salvador Reference Salvador, Tütken, Tomotani, Berthold and Rasser2018). We consider the following three C sources that contribute to carbonate shell formation: (1) Carbon dioxide in the atmosphere equilibrates with dissolved gasses in the water at the air-water interface, where 14Cair (or “modern carbon”) is approximately 100 pMC. (2) Inorganic carbon dissolved in groundwater seeps into the spring through the bottom sediments or fractured bedrock. This course of DIC is likely depleted in 14C due to the transit time in the aquifer saturated zone, with contributions from other DIC sources such as dissolution of carbonate aquifer material, among others (for more information, please see a more thorough examination of these considerations in the discussion). (3) Organic carbon in detritus from plant, algal, or microbial material either falls into the spring or grows in the water column. The detritus is consumed by gastropods and used to form shell carbonate. We measure the organic 14C in sediment which we treat as a relative proxy for metabolic 14C. These C fluxes are presented in Figure 1A and described further by McConnaughey and Gillikin (Reference McConnaughey and Gillikin2008).

Figure 1 Site information. (A) A schematic cross-section (not to scale) of spring with potential carbon sources that contribute to the formation of carbonate shells. (B) Sample sites are shown as dots on a map of Utah. The purple cross represents a weather station. (C) Sampling sites within the Blue Lake (BL) spring system. (D) Sampling site from Horseshoe Spring (HRS). (E) Field photo from Red Butte Spring.

Overall, this study examines the relationship between groundwater and aquatic gastropod shells as an alternative sampling material for researchers investigating groundwater transit times.

Site Description

We sampled three spring sites in northwestern Utah, on traditional and ancestral lands of the Newe/Western Shoshone, Goshute, and Ute peoples (Figure 1B). Blue Lake (BL) and Horseshoe (HRS) springs are large wetland spring systems in Utah’s West Desert. The third site is Red Butte Spring (RBS): an urban spring in Salt Lake City that runs down a hillside into Red Butte Creek. All three spring sites support perennial wetland ecosystems and were selected as they represent a range of groundwater transit times, spanning the Holocene (BL) to modern (RBS) (Fitzgerald Reference Fitzgerald2019; Lerback et al. Reference Lerback, Hynek, Bowen, Bradbury, Solomon and Fernandez2019).

BL is in the Salt Spring Waterfowl Management Area, near the Utah-Nevada border and HRS is just south of the Great Salt Lake (Figure 1C). Neither BL nor HRS are fed by surface water runoff as there is no visual evidence of streams directed to the springs, and both springs occur near fractured limestone outcrops and flow along a slight topographic slope towards the basinal playa deposits, remnants of glacial Lake Bonneville (Louderback and Rhode Reference Louderback and Rhode2009; Lerback et al. Reference Lerback, Hynek, Bowen, Bradbury, Solomon and Fernandez2019). BL and HRS are fed by groundwater discharge from the bottom of the springs. Prior investigation at BL-spring modeled 14C apparent groundwater transit times of Holocene ages (Lerback et al. Reference Lerback, Hynek, Bowen, Bradbury, Solomon and Fernandez2019). BL is the largest of the spring systems sampled and was sampled from three sites: (1) a shallow (0.5 m depth) spring (called “BL-spring”), which seeps from fractured limestone a meter above the other pools, (2) a pond (“BL-pond”) 2–3 m in depth just below the BL-Spring, and (3) the main lake bottom, about 17 m in depth (“BL-lake”).

HRS has two primary ponds with perennial active discharge that converge as the water runs west towards the playa sediments (Figure 1D).

RBS discharge had a tritium activity over 5 TU, representing modern groundwater recharge, and a modeled apparent transit time between 0 and 5 years using 3H-3He dating methods (Solomon et al. Reference Solomon, Schiff, Poreda and Clarke1993; Fitzgerald Reference Fitzgerald2019). This site is on a slope such that water pools only to about 5 cm depth. The springwater discharges from a piped spring box, which is where aquatic gastropods were found (Figure 1E).

Gastropod Physiology

Gastropods build their shells in a series of conical spirals, or whorls. Calcium and CO2-bicarbonate ions are primarily supplied by direct uptake from the surrounding environment, although in some species metabolic carbonate can supply up to 10% of the carbon (Wilbur and Saleuddin Reference Wilbur and Saleuddin1983; McConnaughey Reference McConnaughey2003; Shanahan et al. Reference Shanahan, Pigati, Dettman and Quade2005). Shell carbonate is precipitated in the extrapallial fluid or extracellular calcifying fluid between the mantle (soft inner tissues) and the solid shell. Calcareous material (either calcite or aragonite) is fixed in layers with an organic matrix to extend or expand the shell whorls (Wilbur Reference Wilbur1964). We note that lung-breathing gastropods may use more metabolic C in their shells, making them less ideal representatives of water chemistry (Goodfriend Reference Goodfriend1992; Goodfriend and Ellis Reference Goodfriend and Ellis2002; Stott Reference Stott2002; Copeland et al. Reference Copeland, Quade, Watson, McLaurin and Villalpando2012; Hill et al. Reference Hill, Reimer, Hunt, Prendergast and Barker2017; Padgett et al. Reference Padgett, Yanes, Lubell and Faber2019).

MATERIALS AND METHODS

Water and sediments (containing gastropod shells) were collected at BL, HRS, and RBS spring sites in Utah’s West Desert to understand spatial and temporal variation in spring chemistry. Exact locations are provided in the data hosted under the doi:10.5281/zenodo.5587202. Gastropods were collected under a research agreement with Utah Division of Wildlife Resources (4COLL10642).

Spring Environmental Characteristics

Spring environmental characteristics were sampled through multiple seasons from 2018–2020. Sample types and locations are summarized in Table 1. A Hydrolab® MS5 multiparameter Mini Sonde (OTT Hydromet, Loveland, Colorado) or a YSI® multiparameter probe (YSI Inc, Yellow Springs, OH) was used for in-situ measurements of water temperature and specific conductance with a measurement error of less than 0.5°C and 10 μs/cm, respectively. Water was collected in high-density polyethylene (HDPE) bottles that were washed with 5% HCl stable hydrogen and oxygen isotope composition analyses (δ2H and δ18O, respectively). Measurements of δ2H and δ18O were performed with a Laser Water Isotope Analyzer Picarro L2130i at the SIRFER laboratory at the University of Utah, and the reported measurement error for these is less than 0.1‰. Weather station data from station DPG25 from 2016 to 2021 (located at latitude 40.52 and longitude –113.75) was accessed through MesoWest. This data provides air temperatures measured from January 2016 to April 2021. Measured and modeled isotopes of precipitation from the University of Utah were collected from 2016 to 2022 from the Online Isotopes in Precipitation Calculator (OIPC; Bowen et al. Reference Bowen, Wassenaar and Hobson2005; West et al. Reference West, Bowen, Dawson and Tu2009; Bowen et al. Reference Bowen2021).

Table 1 Summary of samples and analyses in study.

a National Ocean Sciences Accelerator Mass Spectrometer is abbreviated as NOSAMS.

Carbon Isotope Measurements

Water samples to be analyzed for C isotopes were collected from several points where temperature and flow rates indicated spring discharge. Samples were collected in 250-mL glass bottles with solid glass stoppers and cleaned according to Woods Hole Oceanographic Institution specifications. Sample water collected was pumped into the bottle, which was held underwater and overflowed to prevent air equilibration and bubbles within the container. The stopper was then coated with Apiezon-M high vacuum grease and twisted into the bottleneck to ensure the seal.

The top 5 cm of sediment at the spring bottom was collected in clean plastic containers. Sediment for organic carbon analysis was transferred to a clean glass vial and freeze-dried within 48 hours of sample collection. Stored, dried sediments were disaggregated using Calgon and deionized (DI) water following Forester (1999).

Bulk sediment was processed at the University of Utah to disaggregate and sort gastropod shells. Shells collected for isotopic analysis were culled and stored in 90% ethanol. Gastropods analyzed for 14C in this study include Melanoides tuberculata and Pyrgulopsis pilsbryana, and Physella gyrina (Hovingh Reference Hovingh2018). All are aquatic, benthic, bottom-feeding genera, which we chose to study aquatic, gill-breathing taxa because it is likely the C used to build shells is from the aqueous environment (Fritz and Poplawski Reference Fritz and Poplawski1974; Rhoads and Lutz Reference Rhoads and Lutz1980; Wilbur and Saleuddin Reference Wilbur and Saleuddin1983). Melanoides are an invasive species to Utah and distributed globally (Facon et al. Reference Facon, Pointier, Glaubrecht, Poux, Jarne and David2003). This species on average lives from 1 to 5 years (Dudgeon et al. Reference Dudgeon and Lam1985; Pointier et al. Reference Pointier, Delay, Toffart, Lefèvre and Romero-Alvarez1992). This species lives both in brackish and freshwater, as well as at a wide range of water temperatures (Raw et al. Reference Raw, Perissinotto, Miranda and Peer2016). Pyrgulopsis species is found in the Idaho, Utah, and Nevada (Hershler Reference Hershler1994; Liu et al. Reference Liu, Hershler and Hovingh2017). Pyrgulopsis are of the subclass Hydrobiidae and are mostly found in water temperatures from 22–35°C (Hershler Reference Hershler1998). Finally, Physella is a widely distributed aquatic species found throughout the United States (Newman et al. Reference Newman, Kerfoot and Hanscom1996). The Physella subspecies sampled in this study (utahensis) is endemic to Utah, Nevada, Colorado, and Wyoming. They are found in vegetated shallow spring-fed pools. This species is categorized as a generalist with a diet that consists of detritus, diatoms, fungi, and microscopic living organisms.

Sieves were used to separate size fractions of sediments, from which gastropods were collected and sorted by genus and photographed. Carbonate shells were then cleaned for chemical analysis using the following practices, also detailed in Caporaletti (Reference Caporaletti2011) and described here. To remove organic material, we soaked samples for 10 minutes with 3% hydrogen peroxide (Xia and Engstrom Reference Xia, Engstrom and Ito1997; Janz and Vennemann Reference Janz and Vennemann2005). Shells were then rinsed with deionized water and stored in glass vials. Vials with shells and deionized water were sonicated in 10-min increments and rinsed until water remained clear after sonication (Lawrence Reference Lawrence, Hyeong, Maddocks and Lee2008). Shells were then rinsed with ethanol and dried in a dust-free environment (Mischke Reference Mischke, Zhang and Börner2007). Bulk shells were homogenized using a clean mortar and pestle, and smaller species were combined to acquire an adequate sample size.

Dissolved inorganic carbon in water, inorganic carbonate, and organic carbon in sediment were analyzed for 14C and δ13C at the National Ocean Sciences Accelerator Mass Spectrometer facility at Woods Hole Oceanographic Institution (NOSAMS). 14C to 12C in an unknown sample relative to the known ratio in appropriate measured standards. 14C was reported as fraction modern C, which we discuss in the text as percent modern C (pMC), as specified by Stuiver and Polach (Reference Stuiver and Polach1977). Modern 14C is defined at NOSAMS as 95% of the 14C concentration of NBS Oxalic Acid I, normalized to δ13CVPDB = –19‰ (Olsson Reference Olsson and Olsson1970). The average reported error is 0.12 pMC. δ13C was reported as per mil (‰) relative to Vienna Pee Dee Belemnite (subscript VPDB), where error was not reported but NOSAMS lab accession number is provided. Data processing and statistical analyses were performed using R v3.6 in RStudio.

RESULTS

Spring Environmental Characteristics

BL, HRS, and RBS exhibit relatively constant characteristics through seasons as sampled from 2016 to 2021 (Table 2 and Figure 2; Lerback et al. Reference Lerback, Hynek, Bowen, Bradbury, Solomon and Fernandez2019). BL and HRS are mesothermal spring systems, whose average temperatures are 10 above the mean annual air temperature of 12°C. BL-Lake, BL-Spring, and HRS have average water temperatures of 23°C (max = 24°C, min = 22°C, n = 6), 28°C (max = 31°C, min = 26°C, n = 9), and 21°C (max = 28°C, min = 17°C, n = 10): respectively. RBS has an average temperature of 11°C (max = 12°C, min = 10°C, n = 5), which is lower than the mean annual air temperature because it is approximately 100 m higher in elevation (1372 m MSL) than the weather station (1286 m MSL). The air temperature monthly averages range from 28°C in July to –3°C in December (see boxplots in Figure 2A).

Table 2 Spring environmental data. N.D. means no data.

Figure 2 Seasonal variation in spring characteristics. (A) Water temperature from spring sites and air temperature from weather station DPG25 are shown by month. Daily average air temperature data are represented by monthly boxplots representing four quartiles, mean, and outliers. Temperature Monthly averages of precipitation from 2016-2021 are shown as pink bars, with the axis labeled on the left of the figure. (B) The δ2H isotope composition of springwaters and regional precipitation is shown by month. The isotopic composition of precipitation is represented by measured and modeled data from OIPC. The dashed horizontal lines represent the annual average of precipitation weighted by precipitation amount. (C) The δ18O isotope composition of springwaters and regional precipitation is shown by month. The isotopic composition of precipitation is represented by measured and modeled data from OIPC. The dashed horizontal lines represent the annual average of precipitation weighted by precipitation amount.

As shown in Figure 2B and Table 2, δ2H isotope composition of springwaters varied by a maximum of 6.4‰ (at RBS) and less than 2‰ at BL and HRS. BL-Lake and BL-Spring yielded an average δ2H isotope composition of –123.8‰ (max = –123.7‰, min = –124.0‰, n = 6), and –124.7‰ (max = –123.9‰, min = –125.0‰, n = 9). The δ2H isotope compositions of HRS and RBS had averages of –122.1‰ (max = –121.5‰, min = 123.2‰, n = 11), and –118.1‰ (max = –114.4‰, min = –120.8‰, n = 4). The measured δ2H isotope composition of precipitation varied from –17.0‰ in May to –193.3‰ in January whereas the modeled δ2H isotope composition of precipitation varied from –63.0‰ in August to –193.0‰ in January. The annual weighted average (by average monthly precipitation) is –107.3‰ and –142.5‰ for measured and modeled δ2H isotope composition of precipitation, respectively.

As shown in Figure 2C and Table 2, δ18O isotope composition of springwaters varied by a maximum of 2.2‰ at RBS and less than 0.5‰ at BL and HRS. BL-Lake and BL-Spring yielded an average δ18O isotope composition of –15.8‰ (max = –15.7‰, min = –15.9‰, n = 6), and –16.0‰ (max = –15.9‰, min = –16.1‰, n = 9). The δ18O isotope compositions of HRS and RBS had averages of –16.0‰ (max = –15.9‰, min = –16.2‰, n = 11), and –15.2‰ (max = –13.8‰, min = –16.0‰, n = 4). The measured δ18O isotope composition of precipitation varied from –1.0‰ in May to –25.0‰ in December. The modeled δ18O isotope composition of precipitation varied from –9.2‰ in July to –25.2 in January. The annual weighted average (by average monthly precipitation) is –14.5‰ and –19.3‰ for measured and modeled δ2H isotope composition of precipitation, respectively.

In addition, the specific conductance (SpC) of BL-Lake is 9070 μS/cm (max = 9620 μS/cm, min = 8700μS/cm, n = 6), BL-Spring is 873 μS/cm, (max = 9310 μS/cm, min = 7560 μS/cm, n = 9), and HRS is 10310 μS/cm (max = 12120 μS/cm, min = 9640 μS/cm, n = 11). RBS had an average of 1540 μS/cm (max = 2110 μS/cm, min = 1080 μS/cm, n = 5). We did not observe a seasonal variation in SpC at these sites despite rainfall events, likely because there is less than 30 mm/year on average, and we did not observe evidence of surface water runoff entering the spring systems. Runoff from snowmelt and rain events is not channeled directly to these spring systems.

Carbon Isotope Measurements

The measured radiocarbon activity of water, shell, and sediment are reported in Table 3 and discussed in the following subsections.

Table 3 Measured carbon isotope data. N.D. means no data.

14C values from shells and water, range from 7.9 pMC at BL-Spring to 81.8 pMC at RBS, and the 14C of bulk organic C in sediment was 29.5 pMC at BL-Pond and 42.0 pMC at HRS. The average Δ 14 C s-gw across gastropod genera and site is 0.27 pMC (SD = 0.77, n = 22). A paired t-test for all shell and water samples resulted in a small, statistically insignificant difference where p > 0.05 (mean difference = 0.27 pMC, t = 1.65, df = 21, p = 0.11). This close relationship does not appear to vary by 14 C gw ; a linear model between 14 C s and 14 C gw yielded a slope of 1.002 with an R2 of 0.999 and an RMSE of 0.751 (Figure 3A). The difference between 14 C s and 14 C metabolic is more than 20 pMC at BL-Pond and 5 pMC at HRS.

Figure 3 C isotope analyses. (A) Water 14C activity is plotted against the 14C activity of solid (shell and sediment) samples collected at the same place and same date. The red solid line represents a linear regression between water and shell samples, and regression statistics are annotated on the plot. Analytical error is smaller than the points shown. (B) Water δ13C isotope composition is plotted against the δ13C isotope composition of solid (shell and sediment) samples collected at the same place. The red solid line represents a linear regression between water and shell samples, and regression statistics are annotated on the plot. Analytical error is smaller than the points shown. (C) A schematic diagram of the bathymetry at HRS. Shells and water were collected at the four marked HRS subsites with increasing distance from the primary HRS discharge point. Δ 14 C s-gw and ϵ are annotated for each sample.

Table 4 provides summarize this studies’ sample Δ 14 C s-gw by mean, sd, and n, and divided by species and site. An analysis of variance test was run to understand how Δ 14 C s-gw varies as a function of species and as a function of site. We found slight differences in average Δ 14 C s-gw by both genera (f(2) = 25.35, p < 0.001) and by site (f(4) = 18.5, p < 0.001). Separated by genera, the mean Δ 14 C s-gw for Melanoides and Physella shells is below 0.25 pMC, whereas the Pyrgulopsis shells had a Δ 14 C s-gw greater than 1. Excluding the data from Pyrgulopsis shells, a paired t-test for shell Δ 14 C s-gw (Melanoides and Physella) resulted in a small difference at p < 0.05 (mean difference = −0.21 pMC, t = −2.53, df = 13, p = 0.02). Separated by site, HRS had Δ 14 C s-gw greater than 1, whereas the other sites yielded Δ 14 C s-gw less than 0.3 pMC.

Table 4 Summary of 14C isotopic differences measured between water and shells (Δ14Cs-gw). Analyses do not include data from HRS subsites. N.A. indicates that the statistic is not applicable.

Sediment, shells, and water were analyzed for δ13C to understand potential effects of environment-related and/or biomediated carbon isotope fractionation. δ13CVPDB values range between –11.3‰ to –1.8‰ for water and shell samples whereas the sediment δ13CVPDB is –26.6‰ and 20.6‰ at BL-pond and HRS, respectively, which are within the range of values expected of plant material in the region (Hart et al. Reference Hart, Nelson and Eggett2010). A linear model between δ13Cshells and δ13Cwater yielded a slope of 0.897 and an R2 of 0.926 (Figure 3B).

To directly compare shell δ13C and water δ13C, we calculated the difference between shell and water, and plotted the average and SD by species represented by ϵ (Table 4). The ϵ of all samples was –1.88‰ (n = 21, sd = 0.77‰). A paired t-test resulted in a low p-value, where we see there is a small difference in means (t = 11.14, df = 20, p < 0.01). An analysis of variance test was run to understand how ϵ varies as a function of species and as a function of site. We found a statistically-significant difference in ϵ by both genera (f(2) = 7.7, p < 0.001) and by site (f(4) = 18.2, p < 0.001).

While the 14C activity of modern shells is similar to the 14C activity of water, it is apparent that the 14C of sediment is higher in both locations; 29.50 pMC at BL-Pond and 42.04 pMC at HRS. Due to the small sample size, we do not share statistical observation.s. The observed difference could be due to a fraction of the organic carbon in the sediment deriving from vascular plant material, which would reflect an atmospheric 14C signal (Pigati Reference Pigati2002; Shanahan et al. Reference Shanahan, Pigati, Dettman and Quade2005).

We note that half (4) of the Pyrgulopsis shells were found at the HRS site. At BL-Spring, where Pyrgulopsis and Melanoides shells were collected from the same site, we see a difference in average Δ14Cs-gw by genera (f(1)=11.96, p = 0.014). At this site, Pyrgulopsis shells had an Δ14Cs-gw of 0.67 pMC (SD = 0.15, n = 4) and the Melanoides shells had an Δ14Cs-gw of –0.10 pMC (SD = 0.42, n = 4). This Δ14Cs-gw difference of 0.76 pMC between species indicates that there may be genera-dependent biomediated fractionation or differences from gastropod shell deposition (if the shell grew in a more shallow environment than where it was deposited).

Potential explanations for the observed Δ 14 C s-gw include incorporation of metabolic carbon, live gastropod mobility, or movement after initial shell deposition by the pond bathymetry, circulation, or other sediment disturbance. Previous studies show that it is possible for shells to incorporate up to 10% of their 14C from metabolic C from consumed detritus (Wilbur and Saleuddin Reference Wilbur and Saleuddin1983; Raw et al. Reference Raw, Perissinotto, Miranda and Peer2016). The δ13C of a shell sample may be used to estimate how much of the 14C is derived from atmospheric and metabolic C sources. However, this may not be appropriate in all cases, as we show that the δ13C composition of shells is less negative than both measured water and sediment sources and that other spring-dwelling gastropod shells been reported to demonstrate some vital effect offsets (Shanahan et al. Reference Shanahan, Pigati, Dettman and Quade2005). It is also possible the shells formed at a shallower area of the pond, where modern 14C from the air (around 100 pMC) was equilibrating with the water and raising the average 14C activity in the water. The shells would have grown in an aqueous environment with a slightly elevated 14C activity, and moved, fell, or were moved by fish or circulation to the deeper sampling point.

To address potential groundwater reequilibration with air as springwater moves away from the discharge point, we provide 14C data from four subsites at HRS and a contemporaneously collected water sample from the discharge point. These samples have increasing the distance from the bottom discharge site and decreasing depth, which are shown shown schematically in Figure 3C. The shell and water data are provided in Table 3 and annotated Figure 3C, where the water at HRS-subsite0 had 33.7 pMC, and Pyrgulopsis shells ranges from 36.5 pMC at HRS-subsite4 (within a meter of the discharge) to a high 41.7 pMC at HRS-subsite2 (approximately 10 meters from the discharge area). Although there is only one sample at each subsite, these data indicate the distance from the discharge source, pond circulation patterns, and potential for gastropod movement after shell formation should be considered when using this proxy, as has been suggested by Riggs (Reference Riggs1984) and Copeland et al. (Reference Copeland, Quade, Watson, McLaurin and Villalpando2012).

DISCUSSION

The data presented in this study support the hypothesis that the 14 C s represents 14 C gw across three species and three spring wetlands (five sites). The data presented in this study demonstrate that modern shells do not reflect modern air 14C activities nor bulk sediment organic 14C (as a proxy for metabolic 14C). We note that only one site had 14C data for more than one species.

The 14 C s proxy described in this study can be used to model groundwater transit times rather than direct measurement of 14 C gw . Measurements of 14 C gw are commonly used to estimate aquifer transit times, and many methods have been developed to do this by not only considering the radioactive decay of 14C along the flowpath, but also corrections including the distiubution of flowpath lengths sampled (e.g., Jurgens et al. Reference Jurgens, Böhlke, Kauffman, Belitz and Esser2016; Broers et al. Reference Broers, Sültenfuß, Aeschbach, Kersting, Menkovich, de Weert and Castelijns2021) and the input of 14C in pre-recharge atmosphere, DIC from the dissolution of aquifer materials, or soil CO2 (Fontes and Garnier Reference Fontes and Garnier1979; Kalin Reference Kalin2000; Clark and Fritz Reference Clark and Fritz2013; Clark Reference Clark2015; Han and Plummer Reference Han and Plummer2016; Cartwright et al. Reference Cartwright, Currell, Cendón and Meredith2020; Wang et al. Reference Wang, Zhang and Chen2020 and references therein). Models of groundwater transit time demonstrate relatively small sensitivity to errors in 14C input values compared with flowpath geometries, model type, and other associated uncertainties (e.g., initial δ13C, heterogeneity in the stable isotope ratios of matrix calcite, open-system carbonate dissolution, methanogenesis; Cartwright et al. Reference Cartwright, Currell, Cendón and Meredith2020). For example, the apparent 14C transit time at BL-Spring was modelled by Lerback et al. Reference Lerback, Hynek, Bowen, Bradbury, Solomon and Fernandez2019) to be on average Holocene recharge. They used the measured 14C of 8.5 pMC, δ13C of –4.5‰, and alkalinity of 226 mg/L as CaCO3 and pH of 7.3, with an initial 14C of 100 pMC and δ13C of –22‰. DIC from limestone was modelled to have a 14C activity of 0 pMC and δ13C of 0‰, following Gardner and Heilweil (Reference Gardner and Heilweil2014) transit times models of waters in the same greater aquifer system (Kennedy and Genereaux Reference Kennedy and Genereux2007). The model developed by Han and Plummer (Reference Han and Plummer2013) as an update to the Fontes and Garnier (Reference Fontes and Garnier1979) model uses these inputs to estimate the contribution of carbonate rock dissolution, leading Lerback et al. Reference Lerback, Hynek, Bowen, Bradbury, Solomon and Fernandez2019) to calculate an apparent transit time of 5600 years. In comparison, the International Atomic Energy Agency model (Gonfiantini Reference Gonfiantini1972) focuses on the potential mixing of different flowpaths, which Lerback et al. Reference Lerback, Hynek, Bowen, Bradbury, Solomon and Fernandez2019) used to calculate an apparent transit time of 10,800 years, which aligns with their modelled transit times of 11,000 in a particle simulation using the regional steady-state numerical groundwater model (Brooks et al. Reference Brooks, Masbruch, Sweetkind and Buto2014). The difference between models is consistent with the observed model difference reviewed by Cartwright et al. (Reference Cartwright, Currell, Cendón and Meredith2020). Using these BL-Spring parameters and the Han and Plummer (Reference Han and Plummer2016) model through NETPATH, a 0.27 pMC (the average Δ 14 C s-gw ) change to measured 14C the modeled transit time by approximately 250 years, whereas the difference between models was several thousand years (Plummer et al. Reference Plummer, Prestemon and Parkhurst1994; Parkhurst et al. Reference Parkhurst and Charlton2008; El-Kadi et al. Reference El-Kadi, Plummer and Aggarwal2010; Lerback et al. Reference Lerback, Hynek, Bowen, Bradbury, Solomon and Fernandez2019).

This study builds upon three studies which touch upon this idea of shells as groundwater transit time proxies: Riggs’ (Reference Riggs1984), Brennan and Quade (Reference Brennan and Quade1997), and Copeland et al. (Reference Copeland, Quade, Watson, McLaurin and Villalpando2012). To directly compare our data with that of Riggs (Reference Riggs1984), we calculate that their three sites reported Δ 14 C s-gw of 0.6, 1.0, and 4.2 pMC, where the latter (higher) Δ 14 C s-gw is associated with some potential water reequilibration with atmosphere. Building upon these initial measurements, this study reports shell and water samples collected at the same time, adds an understanding of seasonal springwater variability, and investigate the change in Δ 14 C s-gw as distance increases from the discharge source.

In addition, Brennan and Quade (Reference Brennan and Quade1997) apply the Δ 14 C s-gw proxy to the fossil record in Nevada, USA. This paper reports the 14C-ages in years B.P. rather than 14C activities as pMC or fraction modern C, and as such, they report aquatic shell 14C-age deficiencies (Δt) of 0–10,000 years (recognize that these ages would need other corrections for groundwater transit times as is described previously). Visual interpretation from their Figure 3 shows that this is a 14C activity depletion of 0–4 pMC. While Brennan and Quade (Reference Brennan and Quade1997) discuss the impact of springwater reequilibration with air as a function of springwater circulation or morphology, they do not assess this impact or validate this Δ 14 C s-gw proxy in modern settings. They note that to apply this proxy to the fossil record it is important to select shells without evidence of overgrowth and diagenesis to account for potential post-depositional C-exchange, and they also emphasize the importance of reliable depositional age controls.

Copeland et al. (Reference Copeland, Quade, Watson, McLaurin and Villalpando2012) suggest the Δ 14C s-gw proxy to explain the apparent 14C depletion of shells in the archeological record at a cultural site in Sonora, Mexico. They identify an approximately 2000 14C-year lag for the aquatic shells (Planorbella sp.). Their data provided for Stratigraphic Unit B3 has an average charcoal 14C activity of 67.7 pMC (n = 3) and two aquatic shells to have 51.2 pMC and 55.8 pMC. Stratigraphic Unit B5 has an average charcoal 14C activity of 79.1 pMC (n = 8) and the co-occurring aquatic shell to have 58.1 pMC. Copeland et al. (Reference Copeland, Quade, Watson, McLaurin and Villalpando2012) explain that the depletion is likely due to surface water chemistry and suggest that it may in part be due to groundwater depletion (citing Brennan and Quade Reference Brennan and Quade1997), but do not describe the depositional environment as directly groundwater-fed, recognizing that it might be an anthropogenic, managed wetland environment.

In the application of this proxy, researchers should consider the following site characteristics: (1) Morphology: the spring should have a pool directly fed by groundwater discharge. The pool shape and circulation should contain carbonate shells that live in springs whose flow rate is such that the advective transport of C is greater than the downward diffusion of atmospheric C, or flowing water such that the water that has not re-equilibrated with air. (2) Temporality: these springs should be persistent over several months (hillside springs that go dry seasonally may not harbor aquatic snails). (3) C inputs: the pond recognizably spring-fed without significant water inputs from rain, ephemeral streams, rivers or the ocean. This is most likely in arid regions without storm runoff or river flooding that might carry modern carbon into the pool.

In sum, we test the hypothesis that in exclusively groundwater-fed spring systems, water 14C is preserved in carbonate shells. Water, sediment, and shells of benthic gastropods (Melanoides, Pyrgulopsis, and Physella) from three sites in Utah were analyzed for 14C. We show that water (n = 6) and shell (n = 22) 14C activities at each site are correlated (R2 = 0.999). These results support the hypothesis that 14C from groundwater is preserved in bicarbonate shells, and future studies should expand upon potential species- or site- specific vital effects.

ACKNOWLEDGMENTS

We thank Scott Hynek, Justin Bruce, Jeremiah Bernau, Thure Cerling, Gabe Bowen, and the NOSAMS team for help and guidance with this project. Thanks to Kate Holcomb, Lisbeth Louderback, Don Sada, and Saxon Sharp for assistance with gastropod identification. This work is partially supported by the Global Changes and Sustainability Center at the University of Utah, the NSF/GSA Graduate Student Geoscience Grant #12745-20, which is funded by NSF Award #1949901, and by the UC Presidential Postdoctoral Fellowship Program. Research Data associated with this article can be accessed at doi:10.5281/zenodo.5587202.

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Figure 1 Site information. (A) A schematic cross-section (not to scale) of spring with potential carbon sources that contribute to the formation of carbonate shells. (B) Sample sites are shown as dots on a map of Utah. The purple cross represents a weather station. (C) Sampling sites within the Blue Lake (BL) spring system. (D) Sampling site from Horseshoe Spring (HRS). (E) Field photo from Red Butte Spring.

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Table 1 Summary of samples and analyses in study.

Figure 2

Table 2 Spring environmental data. N.D. means no data.

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Figure 2 Seasonal variation in spring characteristics. (A) Water temperature from spring sites and air temperature from weather station DPG25 are shown by month. Daily average air temperature data are represented by monthly boxplots representing four quartiles, mean, and outliers. Temperature Monthly averages of precipitation from 2016-2021 are shown as pink bars, with the axis labeled on the left of the figure. (B) The δ2H isotope composition of springwaters and regional precipitation is shown by month. The isotopic composition of precipitation is represented by measured and modeled data from OIPC. The dashed horizontal lines represent the annual average of precipitation weighted by precipitation amount. (C) The δ18O isotope composition of springwaters and regional precipitation is shown by month. The isotopic composition of precipitation is represented by measured and modeled data from OIPC. The dashed horizontal lines represent the annual average of precipitation weighted by precipitation amount.

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Table 3 Measured carbon isotope data. N.D. means no data.

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Figure 3 C isotope analyses. (A) Water 14C activity is plotted against the 14C activity of solid (shell and sediment) samples collected at the same place and same date. The red solid line represents a linear regression between water and shell samples, and regression statistics are annotated on the plot. Analytical error is smaller than the points shown. (B) Water δ13C isotope composition is plotted against the δ13C isotope composition of solid (shell and sediment) samples collected at the same place. The red solid line represents a linear regression between water and shell samples, and regression statistics are annotated on the plot. Analytical error is smaller than the points shown. (C) A schematic diagram of the bathymetry at HRS. Shells and water were collected at the four marked HRS subsites with increasing distance from the primary HRS discharge point. Δ14Cs-gw and ϵ are annotated for each sample.

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Table 4 Summary of 14C isotopic differences measured between water and shells (Δ14Cs-gw). Analyses do not include data from HRS subsites. N.A. indicates that the statistic is not applicable.