A systematic look at chromium isotopes in modern shells – implications for paleo-environmental reconstructions

Abstract

Abstract. The chromium isotope system ( 53 Cr / 52 Cr, expressed as δ 53 Cr relative to NIST SRM 979) in marine biogenic and non-biogenic carbonates is currently being evaluated as a proxy for the redox state of the ocean. Previous work has concentrated on using corals and foraminifera for this purpose, but investigations focusing on the behavior of Cr in bivalves as potential archives are lacking. Due to their often good preservation, fossil marine biogenic carbonates have the potential to serve as useful archives for the reconstruction of past ocean redox fluctuations and eventually link those to climatic changes throughout Earth's history. Here, we present an evaluation of the Cr isotope system in shells of some modern bivalves. Shell species from Lucidinadae, Cardiidae, Glycimerididae and Pectenidae, collected systematically from one Mediterranean location (Playa Poniente, Benidorm, Spain) over a 3-year period reveal δ 53 Cr values ranging from 0.15 ‰ to 0.65 ‰, values that are systematically below the local seawater δ 53 Cr value of 0.83 ± 0.05 ‰. This attests to a significant reduction of dissolved seawater chromium in the process leading to calcification and thus for control of Cr isotope fractionation during biological routes. A similar, constant offset in δ 53 Cr values relative to surface seawater is observed in shells from Mytilius edulis from an arctic location (Godhavn, Disko Bay, Greenland). Chromium concentrations in the studied shells are significantly controlled by organic matter and typically range from 0.020 to 0.100 ppm, with some higher concentrations of up to 0.163 ppm recorded in Pectenidae. We also observe subtle, species-dependent differences in average Cr isotope signatures in the samples from Playa Poniente, particularly of Lucidinadae and Cardiidae, with considerably depressed and elevated δ 53 Cr values, respectively, relative to the other species investigated. Intra-species heterogeneities, both in Cr concentrations and δ 53 Cr values, are favorably seen to result from vital effects during shell calcification rather than from heterogeneous seawater composition. This is because we observe that the surface seawater composition in the particular Playa Poniente location remained constant during the month of July of the 3 years we collected bivalve samples. Intra-shell heterogeneities -associated with growth zones reflecting one to several years of growth, both in δ 53 Cr and Cr concentrations -are observed in a sample of Placuna placenta and Mimachlamys townsendi. We suspect that these variations are, at least partially, related to seasonal changes in δ 53 Cr of surface seawaters. Recognizing the importance of organic substances in the bivalve shells, we propose a model whereby reduction of Cr(VI) originally contained in the seawater as chromate ion and transported to the calcifying space, to Cr(III), is effectively adsorbed onto organic macromolecules which eventually get included in the growing shell carbonates. This study, with its definition of statistically sound offsets in δ 53 Cr...

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

“…Therefore, the isotopic signature of Cr exported with organic carbon would reflect isotopically light Cr (III), a record which may be preserved in marine sediments. Considerable work has been conducted to assess if δ 53 Cr in carbonates, which incorporate Cr (VI) (Tang et al, ), reflects ambient seawater δ 53 Cr (Bonnand et al, ; Bruggmann et al, ; Farkaš et al, ; Frei et al, ; Holmden et al, ; Pereira et al, ; Remmelzwaal et al, ; Rodler et al, ; Wang et al, ). Similar work is needed to determine how δ 53 Cr records in oxic marine sediments relate to δ 53 Cr in the overlying water column and how faithfully these records are preserved (Figure d).…”

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

“…This renders (biogenic) carbonates as potential archives to record δ 53 Cr variations of their precipitating fluid. The offset between biogenic carbonate and ambient sea water δ 53 Cr ( was reported in the range of 0.0–0.9‰ (Bonnand et al., ; Frei, Paulukat, Bruggmann, & Klaebe, ; Holmden, Jacobson, Sageman, & Hurtgen, ; Pereira, Voegelin, Paulukat, Sial, & Ferreira, ). Importantly, multiple analysis of a coral ( Porites sp.)…”

“…Frei et al. () and Farkaš et al. () recently reported an accumulation of Cr in the carbonate shells of molluscs of the genus Mytilus and proposed these shells as another suitable record of sea water δ 53 Cr values.…”

“…(), Frei et al. () report no or only little intra‐species variations in mollusc shell samples and attribute the resolvable variations to vital effects. This highlights that despite extensive investigations on biomineralisation processes of marine calcifying organisms (e.g., Bentov et al., ; Carré et al., ; Weiner & Addadi, ; Weiss, Tuross, & Addadi, ), only little is known about the transport of ions through membranes and incorporation into shells.…”

“…Recently, Frei et al. () emphasised the importance of organic substances onto which Cr(III) is adsorbed and incorporated into mollusc shells. Here, it is important to note that complexation with organic matter (OM) may stabilise Cr(III) in solution (Sander & Koschinsky, ; Sander and Koschinsky, ) and can contribute up to 90% of total dissolved Cr in sea water (Connelly, Statham, & Knap, ).…”