Abstract
Abstract. Chitons (Mollusca) are marine invertebrates that produce radulae (teeth or rasping tongues) containing high concentrations of biomineralized magnetite and other iron-bearing minerals. As Fe isotope signatures are influenced by redox processes and biological fractionation, Fe isotopes in chiton radulae might be expected to provide an effective tracer of ambient oceanic conditions and biogeochemical cycling. Here, in a pilot study to measure Fe isotopes in marine invertebrates, we examine Fe isotopes in modern marine chiton radulae collected from different locations in the Atlantic and Pacific oceans to assess the range of isotopic values, and to test whether or not the isotopic signatures reflect seawater values. Values of δ56Fe (relative to IRMM-014) in chiton teeth range from −1.90 to 0.00 ‰ (±0.05‰ (2σ) uncertainty in δ56Fe), probably reflecting a combination of geographical control and biological fractionation processes. Comparison with published local surface seawater Fe isotope data shows a consistent negative offset of chiton teeth Fe isotope compositions relative to seawater. Strikingly, two different species from the same locality in the North Pacific (Puget Sound, Washington, USA) have distinct isotopic signatures. Tonicella lineata, which feeds on red algae in the sublittoral zone, has a mean δ56Fe of −0.65 ± 0.26‰ (2σ, 3 specimens), while Mopalia muscosa, which feeds on both green and red algae in the eulittoral zone, shows lighter isotopic values with a mean δ56Fe of −1.47 ± 0.98‰ (2σ, 5 specimens). Three possible pathways are proposed to account for the different isotopic signatures: (i) physiologically controlled processes within the chitons that lead to species-dependent fractionation; (ii) diet-controlled variability due to different Fe isotope fractionation in the red and green algal food sources; and (iii) environmentally controlled fractionation that causes variation in the isotopic signatures of bioavailable Fe in the different tidal regions. Our preliminary results suggest that while chitons are not simple recorders of the ambient seawater Fe isotopic signature, Fe isotopes provide valuable information concerning Fe biogeochemical cycling in near-shore environments, and may potentially be used to probe sources of Fe recorded in different organisms.
Highlights
Iron plays a critical role in controlling biological productivity in the oceans (Martin et al, 1990; De Baar et al, 1995; Coale et al, 1996), and understanding the biogeochemical cycling of Fe is key in reconstructing the history of life on Earth
Chiton stokessi from the South Pacific (Panama) has a mean δ56Fe value of −1.09 ± 0.44 ‰ (2σ, 5 specimens), while T. lineata and M. muscosa from the North Pacific (Puget Sound, Washington) possess mean δ56Fe values of −0.65 ± 0.26 ‰ (2σ, 8 specimens) and −1.47 ± 0.98 ‰ (2σ, 5 specimens), respectively, Such large variation in isotopic signatures between the chitons in the different locations might be expected given the widely varying δ56Fe values reported for dissolved Fe in seawater in different oceans
Large variations have been reported in the Atlantic Ocean: δ56Fe values in the range of −0.14 to +0.23 ‰ have been reported for the Atlantic Section of the Southern Ocean (Lacan et al, 2008, 2010), while values of −0.13 to 0.27 ‰ have been measured in the southeastern Atlantic (Lacan et al, 2010); in the North Atlantic δ56Fe values varying between +0.30 and +0.71 ‰ have been reported in some studies (John and Adkins, 2010; John and Adkins, 2012; Lacan et al, 2010), while isotopic signatures in the range of −0.90 to +0.10 ‰ have been reported off the northeastern coast of North America (Rouxel and Auro, 2010)
Summary
Iron plays a critical role in controlling biological productivity in the oceans (Martin et al, 1990; De Baar et al, 1995; Coale et al, 1996), and understanding the biogeochemical cycling of Fe is key in reconstructing the history of life on Earth. One potentially rewarding way to reconstruct past marine conditions is to examine variations in the isotopic signature of iron. Changes to Fe isotope ratios occur due to shifts in redox state, chemical bonding environment, adsorption properties, and microbial and organic-ligand bonding processes Matthews et al, 2001; Zhu et al, 2002; Beard et al, 2003a, b; Brantley et al, 2004; Croal et al, 2004; Welch et al, 2003; Johnson et al, 2005; Teutsch et al, 2005; Crosby et al, 2007; Matthews et al, 2008), and precise measurements of these isotopes could yield vital information about geochemical and ecological conditions in both presentday and past environments. S. Emmanuel et al.: Iron isotope fractionation in marine invertebrates
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