Proposal

1. Executive Summary / Abstract

This project aims to develop an LA-ICP-MS method to analyze cations in freshwater mussel shells. These shells belong to the order Unionida from the Wichita River watershed. The goal is to test whether proxies in shell chemistry can reflect salinity gradients, using strontium cations as a primary tracer. Expected outcomes include exploring how watershed hydrology, and mussel ecology are represented through changing environment salinity.
Freshwater mussel shells, including those from the order Unionida used in this study, grow by accretion, biologically building layered structures in their shells that can serve as chemical archives. Through the study of these sequential layers, sclerochronology can be used to resolve environmental change at seasonal level. Because shell carbonate incorporates trace elements and isotopes from the waters in which bivalves grow through biomineralization, growth bands can be used to observe environmental variations in salinity, temperature conditions across different locations across the Wichita and Little Wichita River watersheds. This project develops an LA-ICP-MS method to quantify elemental ratios and isotopes along the growth bands of Unionida shells to observe the varying conditions of salinity in relation to seasonality across the Wichita and Little Wichita River watersheds.

2. Research Objective / Hypothesis & Method Objectives

This research aims to develop and test an LA-ICP-MS method for quantifying elemental and isotopic variations in freshwater mussel shells belonging to Unionida. This will allow us to reconstruct salinity gradients in relation to seasonality within the Wichita River Watershed and the Little Wichita River Watershed of north central Texas. The WRW crosses Permian evaporites causing raised total dissolved solids, exceeding 3000 ppm, and unradiogenic strontium isotope signatures. In contrast, the LWRW drains siliciclastic and carbonate strata rather than evaporites, producing waters with lower TDS falling around 430 ppm with more radiogenic strontium ratios. Isotopic data provided by Dr. Nathaniel Miller showed that mussel shells record an East to West decreasing TDS gradient. 

We hypothesize that shell Sr/Ca and ^87Sr/^86Sr ratios will mirror river water salinity and geological sources of solvents in a gradient across the WRW and LWRW. Mg/Ca and Sr/Ca variations within growth increments will reflect seasonal hydrologic cycles between evaporative concentration and dilution, and Fe/Ca ratios will capture redox fluctuations that influence trace metal incorporation. The method’s objectives are to optimize 193 nm LA-ICP-MS conditions for aragonitic biocarbonates, and to quantify Mg, Sr, and Fe relative to Ca along growth layers in Unionida across the WRW and LWRW. We also aim to confirm isotopic alignment between shells and historic TDS readings from water samples, and produce a reproducible workflow for freshwater sclerochronology and salinity reconstruction.

3. Significance / Justification

Unionida mussels are filter feeders with long lifespans that integrate elemental proxies from their environment as they precipitate sequential carbonate growth layers, offering natural archives of water chemistry across long durations of time. Because Texas hosts several Unionida species, understanding how salinity is recorded in their shells carries both ecological and geochemical importance. This study of the Wichita and Little Wichita system allows us to study their east to west salinity gradient controlled by evaporite dissolution, and how it is affected by meeting low salinity inflows that supply water to Wichita Falls. This also allows us to observe how these dynamics are subject to seasonal variations. Developing LA-ICP-MS methods for freshwater systems expanding sclerochronology beyond marine focus can expand its application to hydrology and paleoenvironmental reconstruction. 

4. Review of Relevant Work

Sclerochronology studies the growth layering in carbonate shells of living organisms including bivalves, and the way environmental change is archived between these layers over time. Early studies including Dodd (1982) showed that elemental substitution in shell aragonite reflects environmental conditions. This paper describes how magnesium and strontium incorporation in Mytilus shells varies with salinity and temperature in a way that is useful for environmental reconstructions. Additionally, Schöne (2005) showed that the marine bivalve Arctica preserves seasonal elemental and isotopic signals that record seasonal temperature change, a concept later extended to freshwater taxa by Stringer (2023).

Later research helped to develop the understanding of how to distinguish environmental and biological influences on how elements are preferentially incorporated into shells. Gillikin (2006) proved that Sr/Ca ratios in aragonitic shells are affected both by water chemistry where an organism grew as well as by organism variation, showing that studies using multiple genera need calibrations across species. Geeza (2019) and Lyubas (2023) confirmed that unionid mussel shells can record dissolved salinity and redox fluctuations with high accuracy, supporting their usefulness as environmental proxies in freshwater systems. Banner (2004) and Peucker-Ehrenbrink (2019) described how strontium isotopes, through ^87Sr/^86Sr ratios, are controlled by lithologic mixing in river systems, providing a method for distinguishing between unradiogenic evaporitic and radiogenic siliciclastic sources, archiving changes in water sources over time. Recent research in LA-ICP-MS has also improved the resolution we can use when sampling carbonates. Hathorne (2007) described how the wavelength of laser used in ablation influences aerosol particle size and fractionation, with 193 nm excimer lasers producing finer, more homogeneous particles than higher wavelengths used, improving accuracy for Mg/Ca and Sr/Ca ratios.

5. Materials & Methods

Four shells from the Wichita River Watershed and Little Wichita River Watershed dataset were selected to span a complete salinity and strontium isotope range for the area of study, dataset courtesy of Dr. Nathaniel Miller. Sample WR01.1006 from the upper Wichita River was recorded as having the highest TDS (3196 ppm) and the most unradiogenic ^87Sr/^86Sr (0.70894). On the inverse, LWR02.1012 shows the lowest TDS (430 ppm) and the most radiogenic ratio (0.71174), representing the south flowing Little Wichita River. LK01.1022 samples Lake Kemp, and HC02.1009 samples Head Camp. These two shells are used as tracers of intermediate salinity between the two extremes, representing transitional hydrologic and lithologic conditions.

Each shell will be sectioned perpendicular to the growth axis through the umbo using an isometric diamond saw. This will provide a visible section of the periostracum, for a cross sectional view of the growth layers of the shells. Shell sections will be embedded in epoxy resin spiked with ^111In as a means of clearly distinguishing ablated epoxy from the ablated samples. A polished section approximately 100 µm thick will be examined under reflected light and scanning electron microscopy to identify annual and subannual growth increments (Stringer 2023). Laser ablation will be carried out using a 193 nm ArF excimer laser coupled to an Agilent 8900 Triple Quadrupole ICP-MS housed in the Molecular Biology Building of the University of Texas at Austin. The 193 nm wavelength was chosen for its efficiency with carbonate matrices, producing fine aerosols and reducing elemental fractionation (Hathorne, 2007). Ablations will be performed in a He atmosphere with Ar gas added to improve aerosol transport and ionization efficiency (Lores-Padín, 2025). Line scans from the umbo to the distal margin of the shell will record elemental variations along the shell growth axis, showing seasonal and annual changes.

The analytes to be examined in this study include ^43Ca, ^24Mg, ^88Sr, ^56Fe, and the radiogenic isotope ratio ^87Sr/^86Sr. Calcium ^43Ca serves as both an analyte and an internal standard (Jochum, 2012). Magnesium ^24Mg will be analyzed because it substitutes for Ca in aragonite, and is influenced by both temperature and ionic strength. This means we can use Mg/Ca as a proxy for seasonality and evaporative concentration (Dodd, 1982; Geeza, 2019). Strontium ^88Sr behaves conservatively in solution, so Sr/Ca ratios act as an indicator of dissolved Sr concentration and salinity. Strontium, in the form of ^87Sr/^86Sr provides an independent tracer of water provenance, distinguishing low ^87Sr/^86Sr Permian evaporitic inputs from higher ^87Sr/^86Sr siliciclastic and carbonate sources (Peucker-Ehrenbrink, 2019). Iron ^56Fe is included as an indicator of detrital input at the sediment and water interface during shell growth (Lyubas, 2023). Collectively, this set of isotopic and elemental systems provide multiple useful proxies for reconstructing location-based and seasonal variations in salinity across the Wichita River Watershed.

6. Possible Outcomes

From this project, we expect to show that Unionida mussel shells record measurable seasonal variations in TDS across the Wichita and Little Wichita River watersheds. This research could validate LA-ICP-MS for freshwater shells, and provide support for or against shell isotope and TDS link. If this experiment yields expected results, Sr/Ca ratios will increase in correlation with increasing TDS, confirming Unionida’s reliability as a salinity proxy in freshwater systems. ^87Sr/^86Sr values should reflect lithologic controls with lower ratios in the evaporite rich upper Wichita River and higher, more radiogenic values in the non-evaporitic Little Wichita River (Peucker-Ehrenbrink, 2019). Mg/Ca ratios may represent variability in seasonal hydrologic cycles or temperature fluctuations (Dodd, 1982; Geeza, 2019). Fe/Ca is expected to reveal redox variations or detrital influence during shell formation, offering contextual information about water chemistry (Lyubas, 2023). Results could reflect the degree to which Unionida shell chemistry captures environmental conditions versus physiological effects, given the associated hydrological dataset for sampling locations. This guides interpretation of biomineralization processes in variable salinity environments. Even if relationships between certain proxies and hydrologic conditions prove weak, the findings will still help develop best practices for freshwater shells for geochemical reconstructions (Hathorne, 2007).

7. Timeframe & Budget

Budget Estimate: LA-ICP-MS: 8 hrs × $60 = $480

Nov 6, 2025: Sample Preparation Completed

Dec 11, 2025: Final Project Presentation