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 (δ(53)Cr) in recent mollusc shells (Mytilus) from open and restricted marine environments and compared these to ambient sea water δ(53)Cr values. We found a large range in mollusc [Cr] (12–309 ppb) and δ(53)Cr values (−0.30 to +1.25‰) and in the offset between δ(53)Cr values of mollusc shells and ambient sea water (Δ53Crsea waterbulk Mytilus, −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 δ(53)Cr values, and thus, we suggest that Cr isotope fractionation may likely take place prior to rather than during biomineralisation of Mytilus shells. Heterogeneity of δ(53)Cr values in mollusc shells depends on sea water chemistry (e.g., salinity, food availability, faeces). The main control for δ(53)Cr 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 δ(53)Cr 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 δ(53)Cr values in mollusc shells for palaeo‐reconstructions.