Controls on the Barium and Strontium Isotopic Records of Water Chemistry Preserved in Freshwater Bivalve Shells

Abstract

Biogenic carbonates, including bivalve shells, record past environmental conditions, but their interpretation requires understanding environmental and biological factors that affect trace metal uptake. We examined stable barium (δ¹³⁸Ba) and radiogenic strontium (⁸⁷Sr/⁸⁶Sr) isotope ratios in the aragonite shells of four native freshwater mussel species and two invasive species in five streams and assessed the effects of species identity, growth rate, and river water chemistry on shell isotopic composition. Shells were robust proxies for Sr, accurately reflecting ⁸⁷Sr/⁸⁶Sr ratios of river water, regardless of species or growth rate. In contrast, shell δ¹³⁸Ba values, apart from invasive Corbicula fluminea, departed widely from those of river water and varied according to species and growth rate. Apparent fractionation between river water and the shell (Δ¹³⁸Ba_shell-water) reached -0.86‰, the greatest offset observed for carbonate minerals. The shell deposited during slow growth periods was more enriched in lighter Ba isotopes than the rapidly deposited shell; thus, this phenomenon cannot be explained by aragonite precipitation kinetics. Instead, biological ion transport processes linked to growth rate may be largely responsible for Ba isotope variation. Our results provide information necessary to interpret water chemistry records preserved in shells and provide insights into biomineralization processes and bivalve biochemistry.

Keywords: aragonite; biomineral; fractionation; invasive species; mussel; trace metals; unionid; zebra mussel.

Figure 1

Map of the study area in western Pennsylvania (USA) showing sites of water and shell sample collection. River water samples and bivalve shells collected at each site identified by points. AR = Allegheny River; the number corresponds to the navigational pool.

Figure 2

(A) F. flava shell illustrating where samples were extracted from ventral margin and umbo. Axis of maximum growth shown, where each shell was cut and thin-sectioned to count and measure annual growth lines. (B) Illustration of shell thin-section with annual growth lines identified, and locations of ventral margin and umbo. (C) Photograph of F. flava shell in thin-section, 7-years old.

Figure 3

Sr isotope ratios for each sample collected, with stream samples (open circles) plotted with corresponding shells. ⁸⁷Sr/⁸⁶Sr values for all shell samples fall within the range of ⁸⁷Sr/⁸⁶Sr values for streamwater from which it was collected. Analytical error for each sample is smaller than symbol. AR = Allegheny River, with numbers corresponding to navigation pool.

Figure 4

Ba isotopic offset (δ¹³⁸Ba_shell – δ¹³⁸Ba_water) in shells of six freshwater bivalve species compared with the streamwater from which they were collected. The blue shaded region represents the maximum range of δ¹³⁸Ba values measured from any individual stream in the study area. The average δ¹³⁸Ba of each stream was normalized to Δ¹³⁸Ba_shell-water = 0 so that the shell offsets could be directly compared.

Figure 5

Ba isotopic offset from streamwater for all shell samples, plotted with respective instantaneous growth rates. Larger Ba isotopic offset exhibited by shells of slower growth. Linear regression shown with 95% confidence interval; δ¹³⁸Ba offset = 0.145 × growth −0.576, N = 33, P < 0.0001. The blue shaded region represents the maximum range of δ¹³⁸Ba values measured from any individual stream in the study area. The average δ ¹³⁸Ba of each stream was normalized to Δ ¹³⁸Ba_shell-water = 0 so that the shell offsets could be directly compared.

Figure 6

Schematic illustration showing ion movement from the external river water through the animal and into the shell. Three general interfaces across which Ba isotopes could undergo fractionation as they move through the animal: the gills and digestive tract, the mantle epithelium, and shell mineralization from the extrapallial fluid.