A systematic look at chromium isotopes in modern shells – implications for paleo-environmental reconstructions
Authors: Frei Robert , Paulukat Cora , Bruggmann Sylvie , Klaebe Robert M. 
Published: August 2018

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.