University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen K, Denmark 2ALS Scandinavia AB, Aurorum 10, 977 75 Luleå, Sweden
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A systematic look at chromium isotopes in modern shells - implications for paleo- environmental reconstructions Frei, Robert; Paulukat, Cora; Bruggmann, Sylvie; Klaebe, Robert M. Published in: Biogeosciences DOI: 10.5194/bg-15-4905-2018 Publication date: 2018 Document version Publisher's PDF, also known as Version of record Document license: CC BY Citation for published version (APA): Frei, R., Paulukat, C., Bruggmann, S., & Klaebe, R. M. (2018). A systematic look at chromium isotopes in modern shells - implications for paleo-environmental reconstructions. Biogeosciences, 15(16), 4905-4922. https://doi.org/10.5194/bg-15-4905-2018 Download date: 02. Oct. 2021 Biogeosciences, 15, 4905–4922, 2018 https://doi.org/10.5194/bg-15-4905-2018 © Author(s) 2018. This work is distributed under the Creative Commons Attribution 4.0 License. A systematic look at chromium isotopes in modern shells – implications for paleo-environmental reconstructions Robert Frei1, Cora Paulukat1,2, Sylvie Bruggmann1, and Robert M. Klaebe1 1Department of Geoscience and Natural Resource Management, University of Copenhagen, Øster Voldgade 10, 1350 Copenhagen K, Denmark 2ALS Scandinavia AB, Aurorum 10, 977 75 Luleå, Sweden Correspondence: Robert Frei ([email protected]) Received: 21 March 2018 – Discussion started: 10 April 2018 Revised: 3 July 2018 – Accepted: 6 August 2018 – Published: 20 August 2018 Abstract. The chromium isotope system (53Cr = 52Cr, ex- and elevated δ53Cr values, respectively, relative to the other pressed as δ53Cr relative to NIST SRM 979) in marine bio- species investigated. Intra-species heterogeneities, both in Cr genic and non-biogenic carbonates is currently being eval- concentrations and δ53Cr values, are favorably seen to re- uated as a proxy for the redox state of the ocean. Previous sult from vital effects during shell calcification rather than work has concentrated on using corals and foraminifera for from heterogeneous seawater composition. This is because this purpose, but investigations focusing on the behavior of we observe that the surface seawater composition in the par- Cr in bivalves as potential archives are lacking. Due to their ticular Playa Poniente location remained constant during the often good preservation, fossil marine biogenic carbonates month of July of the 3 years we collected bivalve samples. have the potential to serve as useful archives for the recon- Intra-shell heterogeneities – associated with growth zones re- struction of past ocean redox fluctuations and eventually link flecting one to several years of growth, both in δ53Cr and Cr those to climatic changes throughout Earth’s history. Here, concentrations – are observed in a sample of Placuna pla- we present an evaluation of the Cr isotope system in shells centa and Mimachlamys townsendi. We suspect that these of some modern bivalves. Shell species from Lucidinadae, variations are, at least partially, related to seasonal changes Cardiidae, Glycimerididae and Pectenidae, collected system- in δ53Cr of surface seawaters. Recognizing the importance atically from one Mediterranean location (Playa Poniente, of organic substances in the bivalve shells, we propose a Benidorm, Spain) over a 3-year period reveal δ53Cr values model whereby reduction of Cr(VI) originally contained in ranging from 0.15 ‰ to 0.65 ‰, values that are systemati- the seawater as chromate ion and transported to the calci- cally below the local seawater δ53Cr value of 0:83 ± 0:05 ‰. fying space, to Cr(III), is effectively adsorbed onto organic This attests to a significant reduction of dissolved seawa- macromolecules which eventually get included in the grow- ter chromium in the process leading to calcification and ing shell carbonates. This study, with its definition of statisti- thus for control of Cr isotope fractionation during biolog- cally sound offsets in δ53Cr values of certain bivalve species ical routes. A similar, constant offset in δ53Cr values rela- from ambient seawater, forms a base for future investigations tive to surface seawater is observed in shells from Mytilius aimed at using fossil shells as archives for the reconstruction edulis from an arctic location (Godhavn, Disko Bay, Green- of paleo-seawater redox fluctuations. land). Chromium concentrations in the studied shells are sig- nificantly 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 Published by Copernicus Publications on behalf of the European Geosciences Union. 4906 R. Frei et al.: A systematic look at chromium isotopes in modern shells 1 Introduction Redox processes on land lead to mobilization of Cr from weathering rocks and soils into the runoff. It is now known that oxidation of silicate-hosted and oxide-mineral-hosted Godhavn Cr(III), potentially with the catalytic help of MnO2, to Cr(VI) (Qeqertarsuaq) is accompanied by an isotopic fractionation rendering the mobilized Cr(VI) isotopically heavier (Ellis et al., 2002; Zink et al., 2010; Døssing et al., 2011). Recently, an alterna- Playa Albir tive, redox-independent pathway of Cr mobilization, through Playa Poniente Hawke´s Bay ligand-promoted dissolution of Cr-containing solids, was ad- Kakinada Bay of Bengal vocated by Saad et al. (2017). This mobilization path is based Bay on the ability of organic acids and siderophores to efficiently bind Cr(III) whereby respective ligand formation is accom- panied by isotope fractionation effects, leading to Cr(III) be- ing enriched in 53Cr very much like in redox-dependent mo- bilization paths. The fate of Cr transported to the oceans, its transfer and/or removal to marine sediments and its cycling through ma- Figure 1. Map with locations where bivalve and seawater samples rine organisms, is largely unexplored and complex. Much were collected. research focus today is on the understanding of the redox cycling of Cr in the ocean system, and on investigating ma- rine sediments and marine organisms as potential archives al. (2016) from the modern Caribbean Sea show that the for recording past redox conditions of the ocean–atmosphere δ53Cr of bulk carbonate sediments is about 0:46 ± 0:14 ‰ system through geological time (Frei et al., 2009, 2011, 2013, lower relative to local seawater. These results therefore op- 2016; Bonnand et al., 2013; Planavsky et al., 2014; Holmden pose those from inorganic calcite precipitation experiments et al., 2016; D’Arcy et al., 2017; Rodler et al., 2016a, b; Gil- (cf. Rodler et al. 2015). leaudeau et al., 2016). It is conceivable that the Cr isotope Furthermore, due to a local redox cycling and biological composition of seawater and marine chemical sediments re- uptake of Cr in the oceans (Semeniuk et al., 2016), the Cr iso- flect a complex signal of oxidation/reduction processes oper- tope signature of present-day seawater is not globally homo- ating within the oceans (Scheiderich et al., 2015; Paulukat et geneous (Scheiderich et al., 2015; Paulukat et al., 2016). This al., 2016), and it is therefore clear that one must first under- additionally complicates the application of δ53Cr measure- stand the individual processes and mechanisms that govern ments in marine carbonate archives with respect to deducing the transfer of dissolved Cr in seawater into the respective po- information regarding global ocean redox, and implications tential archives. Recent studies (Rodler et al., 2015; Pereira thereof for climatic changes on Earth through time. Consid- et al., 2015; Wang et al., 2016) have paved the way, but fur- ering the abovementioned issues and limitations, the full po- ther systematic investigations in both natural and laboratory- tential of Cr isotopes for paleo-redox studies can only be re- controlled settings are required. alized with more detailed calibration work done on modern Available results from inorganic calcite precipitation ex- seawater-carbonate systems from different oceanographic periments revealed that the incorporation of Cr from a solu- settings and locations, where δ53Cr data can be collected si- 2− tion into CaCO3 is facilitated as a chromate anion (CrO4 ), multaneously from (i) local ocean waters and (ii) precipitated 2− which replaces a carbonate anion (CO3 ) in the calcite lat- inorganic/biogenic carbonates. tice (Tang et al., 2007). This process of inorganic calcifica- This contribution is a follow-up of a recent study by Farkaš tion tends to preferentially incorporate heavy 53Cr isotopes et al. (2018) who for the first time present a comprehensive into the mineral, yielding the δ53Cr of calcite that is up to Cr isotope investigation of a coupled seawater-carbonate ∼ 0:3 ‰ more positive compared to the fluid; unless the lat- system from one of the world’s largest carbonate-producing ter is a Cr-poor solution (such as seawater) in which case the shelf ecosystems, the Great Barrier Reef (Lady Elliot Is- isotope fractionation between inorganic calcite and the fluid land, Australia). These authors present δ53Cr data from lo- is negligible (Rodler et al., 2015). cal seawaters and selected recent biogenic carbonates (i.e., In contrast, results from biologically produced CaCO3 gastropods, cephalopods, corals and calcifying algae), com- minerals, such as foraminiferal calcite (Wang et al., 2016) plemented by additional δ53Cr analyses of marine skeletal and/or coral aragonite (Pereira et