Chemical Geology 578 (2021) 120304
Available online 5 May 2021
0009-2541/© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (
http://creativecommons.org/licenses/by/4.0/
).
Invited
Research
Article
Cold-water
corals
as archives
of seawater
Zn and Cu isotopes
Susan
H. Little
a
,
b
,
*
, David
J. Wilson
a
, Mark Rehk
̈
amper
b
, Jess F. Adkins
c
, Laura
F. Robinson
d
,
Tina van de Flierdt
b
a
Department
of Earth
Sciences,
University
College
London,
Gower
Place,
London
WC1E
6BS,
UK
b
Department
of Earth
Science
and Engineering,
Royal
School
of Mines,
Imperial
College
London,
London
SW7
2BP,
UK
c
Division
of Geological
and Planetary
Sciences,
California
Institute
of Technology,
Pasadena,
CA 91125,
USA
d
School
of Earth
Sciences,
University
of Bristol,
Bristol
BS8 1RJ,
UK
ARTICLE
INFO
Editor:
Michael
E. Boettcher
Keywords:
Cold-water
coral
Zinc
Copper
Isotopes
Aragonites
Palaeoceanography
ABSTRACT
Traditional
carbonate
sedimentary
archives
have proven
challenging
to exploit
for Zn and Cu isotopes,
due to the
high concentrations
of trace metals
in potential
contaminants
(e.g., Fe-Mn
coatings)
and their low concentrations
in carbonate.
Here, we present
the first dataset
of
δ
66
Zn
JMC-Lyon
and
δ
65
Cu
SRM 976
values
for cold-water
corals
and
address
their potential
as a seawater
archive.
Extensive
cleaning
experiments
carried
out on two corals
with well-
developed
Fe-Mn
rich coatings
demonstrate
that thorough
physical
and chemical
cleaning
can effectively
remove
detrital
and authigenic
contaminants.
Next,
we present
metal/Ca
ratios
and
δ
66
Zn and
δ
65
Cu values
for a
geographically
diverse
sample
set of Holocene
age cold-water
corals.
Comparing
cold-water
coral
δ
66
Zn values
to
estimated
ambient
seawater
δ
66
Zn values
(where
Δ
66
Zn
coral-sw
=
δ
66
Zn
coral
– δ
66
Zn
seawater
), we find
Δ
66
Zn
coral-sw
= +
0.03
±
0.17
‰
(1SD,
n
=
20). Hence,
to a first order,
cold-water
corals
record
seawater
Zn isotope
com
-
positions
without
fractionation.
The average
Holocene
coral Cu isotope
composition
is
+
0.59
±
0.23
‰
(1SD,
n
=
15), similar
to the mean
of published
deep seawater
δ
65
Cu values
at
+
0.66
±
0.09
‰
, but with considerable
variability.
Finally,
δ
66
Zn and
δ
65
Cu data are presented
for a small subset
of four glacial-age
corals.
These
values
overlap
with the respective
Holocene
coral datasets,
hinting
at limited
glacial-interglacial
changes
in oceanic
Zn
and Cu cycling.
1. Introduction
Zinc (Zn) and copper
(Cu) are bioessential
trace metals
with isotopic
systems
that are emerging
as promising
tracers
of past ocean
nutrient
and redox cycling.
To date, reliable
archives
for past seawater
Zn and Cu
isotopes
have been lacking,
because
both metals
are present
at low
concentrations
in carbonate
and opal, but at high concentrations
in
potential
contaminating
material,
such as detrital
or authigenic
(e.g., Fe-
Mn oxide)
phases
(Boyle,
1981; Shen and Boyle,
1988; Pichat
et al.,
2003; Andersen
et al., 2011; Hendry
and Andersen,
2013). Zinc isotopes
have previously
been applied
in Pleistocene
to ancient
marine
sedi
-
ments,
typically
using bulk carbonate
leachates
in an attempt
to side-
step the contamination
problem
(Pichat
et al., 2003; Kunzmann
et al.,
2013; John et al., 2017; Liu et al., 2017; Sweere
et al., 2018). We pro
-
pose that cold-water
coral skeletons
provide
an exciting
new possibility
for a seawater
Zn and Cu isotope
archive.
Their
global
distribution,
combined
with an ability
to obtain
precise
ages for individual
specimens,
gives
corals
distinct
advantages
over more
traditional
palaeoclimate
archives
(e.g., Robinson
et al., 2014), potentially
enabling
reconstructions
of ocean
chemistry
on centennial
or even shorter
time
-
scales
(e.g., Wilson
et al., 2014; Chen et al., 2015). In addition,
their
large size confers
a particular
advantage
for analysing
trace metal
iso
-
topes,
because
it should
enable
rigorous
cleaning
to remove
surficial
contaminant
phases,
while still providing
sufficient
quantities
of Zn and
Cu for isotope
analysis.
Zinc has a classic
nutrient-type
distribution
in the modern
ocean,
reflecting
a combination
of biological
cycling
and the physical
ocean
circulation
(Bruland,
1980; Vance
et al., 2017; Middag
et al., 2019).
Away from local sedimentary
sources
and hydrothermal
vents,
the deep
ocean
is isotopically
homogeneous,
at about
+
0.45
‰
(
δ
66
Zn relative
to
JMC-Lyon),
while
the upper
ocean
exhibits
considerable
variability,
ranging
from