A systematic look at chromium isotopes in modern shells – implications for paleo-environmental reconstructions
<p>The chromium isotope system (<sup>53</sup>Cr ∕ <sup>52</sup>Cr, expressed as <i>δ</i><sup>53</sup>Cr relative to NIST SRM 979) in marine biogenic and non-biogenic carbonates is currently being evaluated as a proxy for...
| Main Authors: | , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-08-01
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| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/15/4905/2018/bg-15-4905-2018.pdf |
| Summary: | <p>The
chromium isotope system (<sup>53</sup>Cr ∕ <sup>52</sup>Cr, expressed as
<i>δ</i><sup>53</sup>Cr 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><sup>53</sup>Cr
values ranging from 0.15 ‰ to 0.65 ‰, values that are systematically
below the local seawater <i>δ</i><sup>53</sup>Cr 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
<i>δ</i><sup>53</sup>Cr 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 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 <i>δ</i><sup>53</sup>Cr
values, respectively, relative to the other species investigated.
Intra-species heterogeneities, both in Cr concentrations and <i>δ</i><sup>53</sup>Cr
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><sup>53</sup>Cr 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><sup>53</sup>Cr 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><sup>53</sup>Cr 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> |
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| ISSN: | 1726-4170 1726-4189 |