Heterogeneity and incorporation of chromium isotopes in recent marine molluscs (Mytilus)

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 (δ53 Cr) in recent mollusc shells (Mytilus) from open and restricted marine environments and compared these to ambient sea water δ53 Cr values. We found a large range in mollusc [Cr] (12-309 ppb) and δ53 Cr values (-0.30 to +1.25‰) and in the offset between δ53 Cr 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 δ53 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 δ53 Cr values in mollusc shells depends on sea water chemistry (e.g., salinity, food availability, faeces). The main control for δ53 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 δ53 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 δ53 Cr values in mollusc shells for palaeo-reconstructions.