Abstract
Abstract. The chromium isotope system (53Cr ∕ 52Cr, expressed as δ53Cr 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 δ53Cr values ranging from 0.15 ‰ to 0.65 ‰, values that are systematically below the local seawater δ53Cr 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 δ53Cr 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 δ53Cr values, respectively, relative to the other species investigated. Intra-species heterogeneities, both in Cr concentrations and δ53Cr 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 δ53Cr 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 δ53Cr 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 δ53Cr values of certain bivalve species from ambient seawater, forms a base for future investigations aimed at using fossil shells as archives for the reconstruction of paleo-seawater redox fluctuations.
Highlights
Redox processes on land lead to mobilization of Cr from weathering rocks and soils into the runoff
Waters collected during 4 subsequent years in July from Playa Poniente yield surprisingly homogeneous Cr isotope compositions and dissolved [Cr] that range from δ53Cr = 0.81– 0.85 ‰, and from 222–280 ng kg−1, respectively
These are comparable with surface seawater data (δ53Cr = 0.81– 0.96 ‰, [Cr] = 239–306 ng kg−1) collected from Playa Albir, a beach situated ca. 9.5 km to the east-northeast of Playa Poniente (Fig. 1) in the years 2013 through 2015
Summary
Redox processes on land lead to mobilization of Cr from weathering rocks and soils into the runoff. An alternative, redox-independent pathway of Cr mobilization, through ligand-promoted dissolution of Cr-containing solids, was advocated by Saad et al (2017). This mobilization path is based on the ability of organic acids and siderophores to efficiently bind Cr(III) whereby respective ligand formation is accompanied by isotope fractionation effects, leading to Cr(III) being enriched in 53Cr very much like in redox-dependent mobilization paths. Recent studies (Rodler et al, 2015; Pereira et al, 2015; Wang et al, 2016) have paved the way, but further systematic investigations in both natural and laboratorycontrolled settings are required
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