A Systematic Look at Chromium Isotopes in Modern Shells - Implications for Paleo-Environmental Reconstructions
Abstract
The chromium isotope system in marine skeletal and non-skeletal carbonates is currently being evaluated as a proxy for the redox state of the ocean. Marine biogenic carbonates, if preserved in suitable geological records as fossils, could serve as Cr isotope archives for the reconstruction of past ocean redox fluctuations and eventually link those to climatic changes throughout Earth's history. 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 over a three year period reveal δ53Cr ranging from 0.15 to 0.65 ‰, values that are systematically below the local seawater δ53Cr of 0.83 +/- 0.05 ‰. This attests for 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 relative to surface seawater is observed in shells from Mytilus edulis from an arctic location. We also observe subtle, species-dependent differences in Cr isotope signatures, particularly of Lucidinadae and Cardiidae, with significantly depressed and elevated δ53Cr values, respectively, relative to the other species investigated. Within-species heterogeneities are favorably seen to result from vital effects during shell calcification rather than from heterogeneous seawater composition. Within single shell heterogeneities associated with growth zones reflecting one to several years of growth 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. We propose a model whereby chromium, via 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.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFMPP51F1206F
- Keywords:
-
- 0419 Biomineralization;
- BIOGEOSCIENCESDE: 0473 Paleoclimatology and paleoceanography;
- BIOGEOSCIENCESDE: 4924 Geochemical tracers;
- PALEOCEANOGRAPHYDE: 4994 Instruments and techniques;
- PALEOCEANOGRAPHY