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 (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 ( , −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.

Biological Control of Chromium Redox and Stable Isotope Composition in the Surface Ocean

“…Dissolved [Cr] and δ 53 Cr data correspond well with the linear global δ 53 Cr‐ln[Cr] relationship (Scheiderich et al, ), with an ε = −0.82 ± 0.05. No influence on [Cr] or δ 53 Cr is seen in low oxygen waters in our data, in support of other recent studies indicating that oceanic OMZs may not exert large‐scale control on oceanic [Cr] and δ 53 Cr distributions (Bruggmann, Klaebe, et al, ; Goring‐Harford et al, ; Moos & Boyle, ; Wang et al, ). A strong linear relationship is observed between δ 53 Cr and [Cr (III)], which suggests that removal of Cr (III) results in increasing δ 53 Cr.…”

“…The absence of a perturbation to [Cr] and δ 53 Cr in the deepest samples (800–1,000 m), where oxygen concentrations are 10–20 μmol/kg, supports previous findings in the Pacific (Bruggmann, Klaebe, et al, ; Moos & Boyle, ; Wang et al, ) and the Atlantic (Goring‐Harford et al, ) that these O 2 levels are not sufficiently low to drive large‐scale Cr removal. A slight depletion in [Cr] observed around 400 m may be related to North Pacific Intermediate Water (NPIW), which is identified at these depths by the salinity minimum around σ θ = 26.6–27.0 (Talley, ).…”

“…Clear and strong Cr redox changes, including removal of dissolved Cr, are observed in isolated local anoxic environments (Cranston & Murray, ; Emerson et al, ). However, anoxic environments in the modern ocean are mostly restricted to marginal seas and inlets (Breitburg et al, ), and available data from low‐O 2 regions in the open and coastal ocean, including at or below O 2 detection limits, do not support a large‐scale removal of Cr (Bruggmann et al, ; Cranston, ; Goring‐Harford et al, ; Moos & Boyle, ; Rue et al, ; Wang et al, ).…”

“…The first multibasin study of dissolved δ 53 Cr found a strong linear relationship between δ 53 Cr and ln[Cr] (Scheiderich et al, ), with an isotope enrichment factor (ε) of ≈ −0.80‰. Nearly all subsequent δ 53 Cr data from the open ocean fall on this trendline (Bruggmann, Klaebe, et al, ;Goring‐Harford et al, ; Moos & Boyle, ; Rickli et al, ), though coastal environments and basins with restricted circulation may be more susceptible to localized influences and may deviate from this relationship (Paulukat et al, ; Scheiderich et al, ). The global open ocean correlation suggests that one single process or a limited range of processes with similar fractionation factors act to control global ocean δ 53 Cr (Scheiderich et al, ).…”

“…However, data were not available to test either hypothesis at the time. Subsequent investigations of the role of OMZs in Cr removal and δ 53 Cr modification have indicated that these environments likely do not have a large impact (Bruggmann, Klaebe, et al, ; Goring‐Harford et al, ; Moos & Boyle, ; Wang et al, ). Similarly, while some recent δ 53 Cr data support the potential for biological removal of light Cr from surface waters (Moos & Boyle, ) and Cr export in high productivity shelf environments (Goring‐Harford et al, ), these and other studies have also highlighted that strong and clear support for the large‐scale impact of biological cycling on δ 53 Cr distributions is lacking (Goring‐Harford et al, ; Moos & Boyle, ; Rickli et al, ).…”