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

• Main Authors: R. Frei, C. Paulukat, S. Bruggmann, R. M. Klaebe
• Format: Article
• Language: English
• Published: Copernicus Publications 2018-08-01
• Series: Biogeosciences
• Online Access: https://www.biogeosciences.net/15/4905/2018/bg-15-4905-2018.pdf

<p>The chromium isotope system (⁵³Cr&thinsp;∕&thinsp;⁵²Cr, expressed as <i>δ</i>⁵³Cr 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 <i>δ</i>⁵³Cr values ranging from 0.15&thinsp;‰ to 0.65&thinsp;‰, values that are systematically below the local seawater <i>δ</i>⁵³Cr value of 0.83±0.05&thinsp;‰. 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 <i>δ</i>⁵³Cr values relative to surface seawater is observed in shells from <i>Mytilius edulis</i> 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&thinsp;ppm, with some higher concentrations of up to 0.163&thinsp;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 <i>δ</i>⁵³Cr values, respectively, relative to the other species investigated. Intra-species heterogeneities, both in Cr concentrations and <i>δ</i>⁵³Cr 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 <i>δ</i>⁵³Cr and Cr concentrations – are observed in a sample of <i>Placuna placenta</i> and <i>Mimachlamys townsendi</i>. We suspect that these variations are, at least partially, related to seasonal changes in <i>δ</i>⁵³Cr 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 <i>δ</i>⁵³Cr 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.</p>