Abstract
The mollusc genus Mytilus is abundant in various modern marine environments and is an important substrate for palaeo‐proxy work. The redox‐sensitive chromium (Cr) isotope system is emerging as a proxy for changes in the oxidation state of the Earth's atmosphere and oceans. However, potential isotopic offsets between ambient sea water and modern biogenic carbonates have yet to be constrained. We measured Cr concentrations ([Cr]) and isotope variations (δ53Cr) in recent mollusc shells (Mytilus) from open and restricted marine environments and compared these to ambient sea water δ53Cr values. We found a large range in mollusc [Cr] (12–309 ppb) and δ53Cr values (−0.30 to +1.25‰) and in the offset between δ53Cr values of mollusc shells and ambient sea water (Δ53CrseawaterbulkMytilus, −0.17 to −0.91‰). Step digestions of cultivated Mytilus edulis specimens indicate that Cr is mainly concentrated in organic components of the shell (periostracum: 407 ppb, n = 2), whereas the mollusc carbonate minerals contain ≤3 ppb Cr. Analyses of individual Cr‐hosting phases (i.e., carbonate minerals and organic matrix) did not reveal significant differences in δ53Cr values, and thus, we suggest that Cr isotope fractionation may likely take place prior to rather than during biomineralisation of Mytilus shells. Heterogeneity of δ53Cr values in mollusc shells depends on sea water chemistry (e.g., salinity, food availability, faeces). The main control for δ53Cr values incorporated into shells, however, is likely vital effects (in particular shell valve closure time) since Cr can be partially or quantitatively reduced in sea water trapped between closed shell valves. The δ53Cr values recorded in Mytilus shells may thus be de‐coupled from the redox conditions of ambient sea water, introducing additional heterogeneity that needs to be better constrained before using δ53Cr values in mollusc shells for palaeo‐reconstructions.
Highlights
Mollusc shells are extensively used to reconstruct past sea water chemistry as they are known to record environmental parameters in their carbonate shell layers (e.g., Klein, Lohmann, & Thayer, 1996; Vander Putten, Dehairs, Keppens, & Baeyens, 2000)
Analyses of individual Cr-hosting phases did not reveal significant differences in δ53Cr values, and we suggest that Cr isotope fractionation may likely take place prior to rather than during biomineralisation of Mytilus shells
We propose that increased organic matter (OM) concentrations stabilising Cr(III) in solution in restricted dense parts of the Mytilus colonies may induce Cr fractionation within sea water, potentially leading to heterogeneous [Cr] and isotopic compositions within a Mytilus colony (e.g., M. edulis from Kiel Fjord)
Summary
Mollusc shells are extensively used to reconstruct past sea water chemistry as they are known to record environmental parameters in their carbonate shell layers (e.g., Klein, Lohmann, & Thayer, 1996; Vander Putten, Dehairs, Keppens, & Baeyens, 2000). Nutrients and trace elements (e.g., HCO−3 , Ca2+, Mg2+, Ba2+, Sr2+) used for shell formation are derived from ingested particles or directly from sea water These molecules are transported through two epithelial cell layers and the mantle to the extrapallial space (EPS). The extrapallial fluid (EPF) of M. edulis consists of inorganic ions, insoluble sulphated carbohydrates, free amino acids and proteins, which can bind divalent cations (e.g., Ca2+, Cd2+, Mn2+) and potentially detoxify heavy metals (Yin, Huang, Paine, Reinhold, & Chasteen, 2015). This composition is different to the composition of the shell proteins
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