Carbon isotopic record of latest Proterozoic from Oman

Abstract

Samples were taken from carbonate units within the Huqf Group of Oman, which was deposited during the interval from approximately 560 to 540 Ma. The data show an initial period of positive δ13C values, around +4‰ PDB, followed by a sharp decrease of about 8‰ in carbon isotopic values, to inferred oceanic carbon values of approximately -4‰ PDB. The δ13C values then more slowly increase to between 0 and +2‰. Similar changes have been observed in Namibia, Greenland and Siberia. The data indicate a sudden significant reduction in organic carbon burial rates in the late Vendian, followed by a slow return to more normal conditions by the end of the Proterozoic. These large changes in the carbon cycle shortly before the Precambrian/Cambrian boundary may have triggered or favored the radiation of metazoan life. -from Authors

Full text

Carbon isotopic record of the latest Proterozoic
from Oman
Autor(en): Burns, Stephen J. / Matter, Albert
Objekttyp: Article
Zeitschrift: Eclogae Geologicae Helvetiae
Band (Jahr): 86 (1993)
Heft 2
Persistenter Link: http://dx.doi.org/10.5169/seals-167254
PDF erstellt am: 01.12.2014
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Eclogae
geol.
Helv.
86/2:
595-607
(1993)
0012-9402/93/020595-13
$
1.50
+
0.20/0
Birkhäuser
Verlag,
Basel
Carbon
isotopie
record
of
the
latest
Proterozoic
from
Oman
By
Stephen
J.
Burns
and
Albert
Matter
l)
ABSTRACT
The
carbon
isotopie
ratios
of
marine
carbonate
rocks
from
the
latest
Precambrian
to
possibly
earliest
Cambrian
were
studied
to
evaluate
possible
changes
in
organic
carbon
burial
and
hence
in
the
carbon
cycle
associated
with
the
evolution
of
higher
life
forms.
Samples
were
taken
from
carbonate
units
within
the
Huqf
Group
of
Oman,
which
was
deposited
during
the
interval
from
approximately
560
to
540
Ma.
The
data
show
an
initial
period
of
positive
5'3C
values,
around
+
4%o
PDB,
followed
by
a
sharp
decrease,
over
a
few
meters
of
section,
of
about
8%o
in
carbon
isotopie
values,
to
inferred
oceanic
carbon
values
of
appoximately
—
4%o
PDB.
The
5
'
3C
values
then
more
slowly
increase
to
between
0
and
+
2
%o.
This
isotopie
pattern
may
be
correlated
across
Oman
over
a
distance
of
800
km.
Similar
changes
have
been
observed
in
Namibia,
Greenland
and
Siberia.
The
changes
are
thought
to
be
primarily
an
original
oceanic
signal,
and
not
a
diagenetic
one.
The
data indicate
a
sudden
significant
reduction
in
organic
carbon
burial
rates
in
the
late
Vendian,
followed
by
a
slow
return
to
more
normal
conditions
by
the
end
of
the
Proterozoic.
These
large
changes
in
the
carbon
cycle
shortly
before
the
Precambrian/Cambrian
boundary
may
have
triggered
or
favored
the
radiation
of
metazoan
life.
ZUSAMMENFASSUNG
Die
Kohlenstoff-Isotopenverhältnisse
mariner
Kalke
und
Dolomite
des
späten
Präkambriums
bis
frühesten
Kambriums
wurden
untersucht
um
abzuklären,
inwieweit
Veränderungen
der
globalen
Sedimentationsrate
von
organischem
Material
und
somit
des
Kohlenstoffkreislaufes
mit
der
Entwicklung
höheren
Lebens
in
Zusammen¬
hang
stehen
könnten.
Die
Karbonatproben
stammen
aus
Profilen
der
Huqf
Gruppe
im
Sultanat
Oman,
welche
den
Zeitabschnitt
von
ca.
560-540
Ma
umfasst.
Die
Messresultate
zeigen
im
unteren
Abschnitt
der
Profile
zunächst
relativ
konstante
positive
813C
Werte
um
+
4%o
PDB,
gefolgt
von
einer
abrupten
Abnahme
auf
—
8
bis
—
12%o
PDB.
Dies
entspricht
nach
Berücksichtigung
des
Diageneseeffekts
einem
8'3C
Wert
des
gelösten
anorganischen
Kohlenstoffs
im
damaligen
Ozean
um
-
4%o.
Gegen
das
Hangende
verändert
sich
das
Kohlenstoff-Isotopenverhältnis
sodann
langsam
zu
schwereren
Werten
auf
0
bis
+
2%o.
Dieses
isotopische
Muster
kann
in
Oman
über
annähernd
800
km
korreliert
werden;
ähnliche
Trends
sind auch
in
spätpräkambrischen
Serien
weitentfernter
Gebiete
wie
etwa
Namibia,
Grönland
und
Sibirien
festgestellt
worden.
Die
beobachteten
Veränderungen
des
Kohlenstoff-Isotopen-
verhältnisses
stellen
primär
ein
ozeanisches
und
nicht
ein
diagenetisches
Signal
dar.
Sie
widerspiegeln
eine
plötzliche
bedeutende
Abnahme
der
globalen
Sedimentationsrate
des
organischen
Materials
im
späten
Vendian,
möglicherweise
infolge
stark
reduzierter
Produktivität,
gefolgt
von
einem
langsamen
Anstieg
zu
normalen
Be-
digungen
am
Ende
des
Präkambriums.
Dieses
einschneidende
Ereignis
im
Kohlenstoffkreislauf
kurz
vor
der
Wende
vom
Präkambrium
zum
Kambrium
könnte
die
Entwicklung
der
Metazoen
ausgelöst
oder
begünstigt
haben.
Introduction
Temporal
variation
in
the
carbon
isotopie
composition
of
marine
carbonates
gener¬
ally
reflect
isotopie
changes
in
the
dissolved
inorganic
carbon
in
the
oceans
(Veizer
et
al.
')
Geologisches
Institut,
Universität
Bern,
Baltzerstrasse
1,
CH-3012
Bern,
Schwitzerland

596
St.
J.
Burns
and
A.
Matter
1980;
Lindh
et
al.
1981).
Observed
changes
have
been
used
to
infer
changes
in
the
global
carbon
cycle
(Garrels
and
Lerman
1984;
Berner
1987)
and
to
identify
changes
in
oceanic
productivity
associated
with
mass
extinction
events
(Magaritz
1987;
Holser
et
al.
1991).
A
number
of
recent
studies
have
focused
on
geochemical
signals
in
Late
Precambrian
to
Early
Cambrian
carbonates
(Tucker
1986;
Magaritz
etal.
1986;
Knoll
etal.
1986;
Aharon
et
al.
1987),
during
the
time
in
which
macroscopic
metazoan
life
evolved
(Stanley
1976).
These
studies
have
generally
had
two
goals
in
mind,
to
use
the
pattern
of
isotopie
changes
as
stratigraphie
markers,
and
to
discover
the
geochemical
environment
in
which
complex
metazoan
life
evolved.
In
studies
which
have
focused
on
isotopie
events
below
the
Precambrian/Cambrian
boundary,
rocks
from
several
different
late
Precambrian
basins
show
the
similar
types
of
carbon
isotope
changes.
Generally
positive
late
Precambrian
carbon
isotopie
values
are
punctuated
by
one
or
more
relatively
large,
sharp
negative
excursions
in
ô13C
(Knoll
et
al.
1986;
Knoll
1990;
Kaufman
et
al.
1990;
Derry
et
al.
1992).
The
precise
nature
and
timing
ofthe
latter
event
or
possibly
events
is
still
not
certain.
It
is
important,
therefore,
to
develop
further
detailed
records
of
carbon
isotope
changes
in
the
late
Precambrian.
Such
records
will
help
determine
whether
isotope
events
are
corrélable.
They
will
also
serve
to
further
refine
the
timing
and
magnitude
of
these
events,
and
associated
changes
in
the
carbon
cycle,
with
respect
to
evolutionary
changes.
We
report
here
on
a
study
of
carbon
isotopes
of
a
relatively
well-dated,
continuous
sequence
of
Late
Proterozoic
(c.
560-540
Ma)
sedimentary
rocks
from
Oman,
the
Huqf
Group.
The rocks
are
the
oldest
sedimentary
unit
in
Oman,
overlie
well-dated
basement
rocks,
and
are
generally
not
significantly
affected
by
diagenesis.
The
Huqf
thus
provides
an
excellent
opportunity
to
investigate
oceanic
carbon
isotope
changes
during
the
latest
Precambrian.
Stratigraphy
As
presently
defined,
the
Huqf
Group
consists
of
five
formations
resting
directly
on
volcanic
extrusives
and
igneous
basement
(Gorin
et
al.
1982;
Hughes
Clarke
1988).
It
is
a
total
of
approximately
1500
m
thick
in
central
and
southern
Oman,
and
is
little
tectonized
and
unmetamorphosed
(Gorin
et
al.
1982).
For
the
present
study,
samples
were
taken
from
carbonate
units
in
the
upper
four
formations
of
the
Huqf
Group
(Figs.
1
and
2)
from
central
and
southern
Oman,
and
from
equivalent
rocks
in
the
Oman
Mountains
(Rabu
1988).
The
first
of
these,
the
Khufai
Formation,
consists
of
dark
bituminous
dolomites
overlain
by
dolomitized
per¬
itidal
carbonates.
The
latter
include
shallow
water
grainstones
and
algal-laminated
mi¬
crites.
The
transition
from
the
Khufai
to
the
shallow
water
siltstones
and
shales
of
the
overlying
Shuram
Formation
is
very
sharp
in
the
Huqf
area,
with
a
hardground
surface
formed
on
top
of
the
Khufai,
but
is
transitional
over
a
few
meters
in
the
subsurface
(Hughes
Clarke
1988).
The
lower
portions
of
the
Shuram
are
shallow
water
marine
siltstones
with
a
few
interbedded
oolitic
limestones.
The
siltstones
grade
upward
into
deeper
water
shales
and
shaley
limestones,
and
gradually
pass
into
thickly
bedded
shallow
water
cabonates
of
the
Buah
Formation.
The
Buah
consists
largely
of
stroma¬
tolitic
dolomicrites,
with
dolomitized
grainstones.
It
is
overlain
by
an
evaporite
unit,
the
Ara
Salt,
which
includes
a
number
of
interbedded dolomite
layers.

Proterozoic
Carbon
isotopes
from
Oman
597
\
\
X
22
V
20
Indian
Ocean
100
km
58°E
Fig.
1.
Location
map
of
studied
sections
and
lithologie
columns
for
central
Oman
area
and
the
Oman
Mountains.
Closed
circles
mark
the
locations
ofthe
four
sections
studied:
1.
Outcrops
from
the
Jebel
Akhdar
in
the
Oman
Mountains,
2.
Well
MQ
from
the
flank
ofthe
Ghaba
Salt
Basin,
3.
Outcrops
from
the
Huqf
Area
in
central
Oman
and
4.
Well
T
from
the
flank
of
the
South
Oman
Salt
Basin.
The
correlative
sequence
of
rocks
in
the
Oman
Mountains
is
similar
to
the
Huqf
Group
in
lithology
and
depositional
environments
(Rabu
1988),
with
two
major
differ¬
ences.
First,
the
lower
carbonate
unit
is
undolomitized.
Second,
all
the
formations
in
the
Oman
Mountains
sustained
low
grade
metamorphism
(Hughes
Clarke
1988;
Rabu
1988)
during
overthrusting
of
the
Semail
ophiolite.
The
depositional
ages
ofthe
Huqf
Group
are
relatively
well
constrained
as
compared
to
most
sections
of
Late
Proterozoic
rocks.
New
dates
on
volcanic
and
intrusive
rocks
underlying
the
Huqf
Group
in
central
and
southern
Oman
provide
a
good
lower
absolute
age
limit
on
deposition.
Ignimbrites
and
granites,
both
dated
by
87Rb/87Sr,
from
below
the
Abu
Mahara
Formation
were
emplaced
at
554
±
10
Ma
(Dubreuilh
etal.,
1992).
Thus,
Huqf
deposition
could
not
have
begun
much
earlier than
560
Ma.
The
upper
age
limit
ofthe
Huqf
Group
is
approximately
the
age
ofthe
Precambrian/
Cambrian
boundary.
The
Ara
Salt
contains
two
ofthe
few useful
biostratigraphic
fossils
in
the
Huqf,
the
tubular
calcareous
organism
Cloudina
spp.
and
Angulocellularia
spp.
Our
own
work
and
that
of
Conway
Morris et
al.
(1990)
have
found
Cloudina
in
carbonate

598
St.
J.
Burns
and
A. Matter
PQ
X
m
X
Cloudina
Z
yy
//
zz
AZI
1
1
Limestone
/
/
Dolomite
7^4
Shale
m
Evaporite
iii
Ignimbrite
<.<
Granite
100
m
}
554
±
10
Ma
Fig.
2.
Stratigraphy
of
the
Huqf
Group
for
central
and
southern
Oman.

Proterozoic
Carbon
isotopes
from
Oman
599
beds
recovered
from
boreholes
in
southern
Oman.
Cloudina,
or
closely
related
organisms,
have
also
been
reported
from
a
number
of
other
locations
worldwide
(Grant
1990).
It
is
often
associated
with
an
Ediacaran
fauna
(Grant
1990;
Germs
1982)
and
is
found
only
below
unequivocally
Cambrian
fossils
(Grant
1990).
Grant
(1990)
has
suggested
that
Cloudina
is
a
useful
marker
for
the
terminal
Proterozoic.
Its
presence
in
the
Ara
suggest
a
latest
Proterozoic
to
possibly
earliest
Cambrian
age
for
that
formation.
This
interpre¬
tation
is
supported
by
the
presence
in
the
Ara
of
the
late
Precambrian
to
early
Cambrian
calcified
alga
Angulocellularia
spp.,
and
the
very
high
sulphur
isotope
ratios
of
Ara
evaporites
(Mattes
&
Conway
Morris
1990),
which
are typical
of
late
Precambrian
to
early
Cambrian
seawater
(Holser
1977).
The
uppermost
age
limit
for
the
Huqf
is
there¬
fore
likely
to
be
close
to
the
age
of
the
Precambrian/Cambrian
boundary.
An
absolute
age
for
the
boundary,
however,
has
not
yet
been
agreed
upon.
Estimates
range
from
600
to
530
Ma,
with
groups
of
age
data
at
570
and
540
Ma
(Cowie
&
Harland
1990).
Recent
dating
of
early
Cambrian
rocks
at
around
520-525
Ma
(Compston
et
al.
1992;
Cooper
et
al.
1992)
and
the
age
of
basement
in
Oman
would
seem
to
support
a
boundary
age
of
around
540
Ma.
Thus,
we
estimate
an
age
of
around
540
Ma
for
the
top
of
the
Huqf
Group,
recognizing
that
this
estimate
may
be
refined
as
more
age
data
for
this
time
period
appear.
Methods
A
total
of
435
samples
from
two
outcrop
sections
and
two
wells
were
used
in
the
study.
All
outcrop
samples,
148,
were
studied
petrographically,
and
were
analyzed
by
XRD
or
staining
to
determine
mineralogy.
Pétrographie analysis
of
all
samples
included
study
in
a
standard
light
microscope,
and
under
blue-fluorescent
light.
Approximately
40
samples
were
also
studied
under
cathodoluminescence.
409
samples
were
analyzed
for
carbon
and
oxygen
isotope
ratios.
For
the
outcrop
samples,
subsamples
free
of
surface
alteration
and
free
of
secondary
calcite
in
the
form
of
vein
or
cavity
filling
cement
were
used.
The well
samples
were
primarily
well-cuttings,
with additional
side-wall
samples
to
provide
stratigraphie
control.
For
all
samples,
approximately
10mg
of
powdered
samples
was
reacted
in
"100%"
H3P04
at
90°C
(McCrea
1950)
in
an
on-line
automated
preparation
system.
The
resulting
C02
was
analyzed
on
a
VG
Prism
II
ratio
mass
spectrometer.
Repeated
analyses
of
standard
material
show
a
reproducibility
of
better
than
0.1
%o
for
5180
and
less
than
0.05
%o
for
513C.
All
results
are
presented
relative
to
the
PDB
standard.
Fortyfive
outcrop
samples
from
the
Huqf
area
were
analyzed
for
strontium
and
manganese
contents.
The
samples
were
leached
for
15
minutes
in
1
N
HCl,
then
filtered
through
0.45
urn
filter
paper.
The
leaches
were
then
analyzed
by
flame
atomic
absorption
spectroscopy
for
strontium
and
manganese.
Results
The
carbon
isotopie
stratigraphy
of
the
four
locations
studied
from
Oman
show
similar
major
features
(Fig.
3).
Values
for
the
Khufai
Formation
are
relatively
constant
positive
values
of
around
+
4
+
2%o
PDB.
At
the
top
ofthe
Khufai
there
is
a
very
sharp
drop
in
carbon
isotopie
ratios
to
values
from
—
8
to
—
11
%o
PDB.
This
change
occurs
over
as
little
as
20
to
30
m
of
section.
Carbon
isotopie
ratios
remain
low
through
most

CO
a.
ca
<
m
e
CO
11
I
ca
ri«
513C
-10
-5
0
5
1800
2000
2200
2400
2600
2800
1
1
1
1
1
1
i
'
513C
-10
-5
0
5
¦*
1000
i
i
<
-¦
-
1
'
<
"¦*
800
600
400
.200
Well
"T
"
0
i
i
_i
i—i—
,i
**
2800
3000
-
3200
3400
-
3600
3800
4000
Ô13C
-10
-5
0 5
->—r-
513C
-10
-5
0
5
1000
l
'
L
—i—I—<-n
800
600
400
200
i
i
i
i
i
i
i
Oman
Mountains
Huqf
Area
Well
"MQ"
Fig.
3.
Carbon
isotope
data
for
the
four
locations
in
Figure
1.
Squares
represent
individual
samples,
scales
to
the
left
of
each
section
are
in
meters.
All
outcrop
analyses
are
from
whole
rock
samples
free
of
obvious
late
stage
cements.
Samples
from
the
wells
are
primarily
from
cuttings,
with
a
few
side
wall
samples.
The
well
depths
are
in
meters
below
the
surface.
The
baselines
for
the
outcrop
samples
from
the
Huqf
area
and
Oman
Mountains
are
the
base
of
the
Khufai
Formation.
cc
£
3
i/j
ë
CL
>

Proterozoic
Carbon
isotopes
from
Oman
601
or
all
of
the
Shuram
Formation,
increasing
slightly
to
values
of
around
—
4
to
—
5
%o
PDB
in
the
upper
parts
ofthe
Shuram,
where
the
formation
becomes
primarily
carbonate
with
fewer
interbedded
shales.
Carbon
isotopie
ratios
continue
to
increase
in
the
Buah
Formation
reaching
values
around
0%o
PDB
or
slightly
above.
Values
for
dolomite
units
within
the
Area
Salt
are
generally
slightly
positive,
between
0
and
+
2%o,
with
a
few
samples
having
more
negative
values.
Oxygen
isotope
ratios
do
not
show
any
consistent
pattern
when
plotted
versus
depth
(Fig.
4).
The
values
are
generally
between
0
and
—
6%o
except
for
the
Oman
Mountains
section
where
the
values
may
be
as
low
as
-
13%o.
No
consistent
relationship
between
Ô13C
and
5180
is
observed.
Strontium
concentrations
of
the
Huqf
area
samples
are
strongly
dependent
on
min¬
eralogy
(Fig.
5).
Dolomite
samples
from
the
Khufai
and
Buah
Formations
generally
have
strontium
contents
between
30
and
70
ppm,
with
an
average
of
45
ppm.
The
limestone
samples
from
the
Shuram
and
lowermost
few
meters
of
the
Buah
have
more
variable
strontium
contents,
from
167
to
474
ppm.
In
the
limestones,
strontium
and
ô13C
values
are
positively
correlated
(Fig.
5).
No
correlation
between
strontium
and
ô13C
is
observed
in
the
dolomites.
Manganese
contents
of
both
limestones
and
dolomites
are
quite
vari¬
able.
The
limestones
contain
from
1509
to
365
ppm
Mn
with
an
average
of
950
ppm.
The
dolomites
contain
from
51
to
2590
ppm
with
an
average
of
687
ppm.
No
covariation
of
manganese
and
513C
was
observed
for
either
limestones
or
dolomites.
Discussion
The
Carbon
Isotopie
Record
of
Oman
In
estimating
the
seawater
carbon
isotope
variation
it
is
important
to
assess
to
what
degree
observed
variations
reflect secular
changes
in
the
oceanic
carbon
reservoir
versus
postdepositional diagenetic
alteration.
In
general,
the
Huqf
Group
of
central
and
south¬
ern
Oman
are
good
candidates
for
preservation
of
the
original
isotopie
values.
In
the
Huqf
area
the
rocks
remained
a
structural
high
through
most
of
their
history
and
were
not
subject
to
major
structural
deformation
(Gorin
et
al.
1982).
The
large
majority
ofthe
samples
are
non-porous
micritic
carbonates
from
nearly
pure
carbonate
units.
It
is
very
difficult
to
alter
the
carbon
isotopie
composition
of
such
rocks
both
of
the
low
porosity
and
because
of
the
large
mass
of
carbon
in
the
rock
itself
versus
that
in
any
potential
diagenetic
fluids.
Thin
sections
of
samples
from
the
Huqf
area
often
have
very
well
preserved
primary
features,
including
stromatolitic
laminae,
pelloidal
textures,
radial
fabrics
in
ooids,
and
intraclastic
textures
in
lime
mudstones.
Under
cathodoluminescence
the
micritic
samples
were
without
exception
nonluminescent,
which
implies
a
lack
of
secondary
alteration.
The
fact
that
a
similar
isotopie
pattern
may
be
correlated
800
km
across
Oman
also
suggests
that
the
original
pattern
of
isotopie
variations
is
largely
preserved.
The
isotopie
pattern
is
even
present
in
the
Oman
Mountains
section,
where
the
rocks
have
a
much
different
diagenetic
history,
including
8-10
km
of
burial,
low
grade
metamorphic
temperatures
and
imbricate
thrusting
and
folding.
Other
potential
indicators
of
diagenetic
alteration
are
the
oxygen
and
strontium
isotopie
ratios
of
the
carbonates.
In
general,
the
oxygen
isotopes
show
no
coherent
relationship
to
the
carbon
isotope
values.
The
sharp
decrease
in
613C
at
the
top
of
the

-l->
cd
CO
ca
3
S
ca
(-.
i
CXI
ri?
<3
ô18o
-8
-6
-4-2
0
2
1600
j
J
' '
'
'
'
1800
S
2000
2200
2400
2600
2800
3000
8180
18
5
0
-8
-6-4-2024
2800
-10
-5 0
5
1000
5180
3000
6-4-2
0 2
1000
800
3200-
800
600
ST
34GÔ
600
400
3600
400
200
3800
200
4000-
L-B-l
Well
"T"
Huqf
Area
Fig.
4.
Oxygen
isotope
data
for
the
same
samples
as
in
Figure
3.
Oman
Mountains
Well
"MQ"
2

Proterozoic
Carbon
isotopes
from
Oman
603
600
500
Sm
400
m
Ö
300
Çl,
200
100
0
¦&
03
o
Calcite
¦
Dolomite
o
-
o'
O-
o
1.
¦
¦¦
¦
12
-10
-8
-6
-4
-2
13/
5iòC
Fig.
5.
Crossplot
of
8'3C
and
strontium
concentration
for
outcrop
samples
from
the
Huqf
area.
Correlation
coefficient
for
linear
regression
of
limestone
samples,
r2
0.74.
Khufai
Formation
is
not
accompanied
by
a
similar
decrease
in
8180.
Also,
in
contrast
to
the
carbon
isotopie
ratios,
the
isotopie
pattern
for
5180
versus
depth
varies
greatly
from
one
section
to
another
(Fig.
5).
The
strontium
isotopie
ratios
of
outcrop
samples
from
the
Huqf
area
are
reported
on
in
Burns
et
al.
(in
press).
The
ratios
show
a
fairly
regular
increase
from
about
0.7084
at
the
bottom
of
the
Khufai
Formation
to
0.7091
in
the
Ara
Formation.
The
values
are
all
within
the
range
found
for
Phanerozoic
marine
carbonates.
At
the
very
least,
the
values
indicate
a
rock-buffered
diagenetic
system
as
opposed
to
an
open
system.
In
a
rock-
buffered
system
the
carbon
isotopie
ratios
of
carbonate
rocks
are
very
likely
to
be
preserved.
The
strontium
contents
of
the
carbonates
are
also
a
means
of
assessing
diagenetic
alteration.
The
dolomites
have
strontium
contents
between
30
and
70
ppm
(Fig.
5),
averaging
48
ppm,
values
typical
of
lower
Palaeozoic
and
Precambrian
massive
dolomites.
Vahrenkamp
&
Swart
(1990)
recently
proposed
a
strontium
distribution
coefficient
of
0.012
for
ordered
stoichiometric
dolomites.
Based
on
this
distribution
coefficient,
an
ordered
dolomite
in
equilibrium
with
a
fluid
with
the
Sr/Ca
ratio
of
modern
seawater
would
contain
approximately
50
ppm
strontium.
The
strontium
con¬
tents
of
the
Huqf
dolomites,
which
are
well
ordered
and
near
stoichiometric,
thus
indicate
equilibrium
with
fluids
with
Sr/Ca
ratios
close
to
modern
seawater.
The
stron¬
tium
contents
are
thus
probably
close
to
their
initial
values,
suggesting
preservation
of
the
original
dolomite
chemistry.
The
strontium
contents
of
the
limestones
are
more
variable
(Fig.
5).
The
limestones
with
the
lightest
carbon
isotope
values,
less
than
—
4%o
PDB,
also
have
relatively
low
strontium
contents,
173
to
300
ppm.
Whereas
the
limestones
with
513C
values
of
—
4%o
or
higher
which
contain
400
to
500
ppm
strontium.
The
latter
samples,
using
a
strontium
distribution
coefficient
of
0.05
(Lorens
1979;
Baker
etal.
1982),
are
approximately
in
equilibrium
with
a
fluid with
a
Sr/Ca
ratio
similar
to
modern
seawater,
similar
to
the
dolomites.
Again,
this
suggests
a
high
degree
of
preservation
ofthe
original
chemistry
of

604
St.
J.
Burns
and
A.
Matter
the
samples.
The
limestone
samples
from
the
lower
half
of
the
Shuram
Formation,
those
with
lower
strontium
contents
and
the
lowest
513C
values,
seem
to
show
evidence
of
diagenetic
alteration.
These
samples
are
from
thin
limestone
beds
within
siltstones
and
shales,
and
in
some
cases
contain
sparry
calcite
cements.
The
linear
relationship
between
strontium
and
513C
for
the
limestones
(Fig.
5)
indi¬
cates
varying
degrees
of
mixing
between
diagenetic
limestone
and
original
carbonate,
and
allows
an
estimation
of
how
much
the
813C
values
were
altered.
The
linear
covariation
suggests
that
all
the
limestones
initially
had
513C
values
around
—
4%o.
Thus,
the
magnitude
of
the
negative
carbon
isotope
shift
in
the
world
ocean
is
certainly
less
than
that
observed
in
the
Oman
sections
in
which
the
lowest
values,
—
10
to
—
12%o
PDB,
are
probably
outside
of
the
possible
range
for
dissolved
inorganic
carbon
in
the
oceans.
The
ocean
carbon
isotope
record
then
probably
varied
as
follows:
values
of
around
+
4%o
were
maintained
during
deposition
of
the
Khufai
formation,
the
values
dropped
sharply
to
approximately
—
4%o
during
deposition
of
the
Shuram
Formation,
the
values
then
increased
more
slowly
during
deposition
ofthe
upper
portions
ofthe
Shuram
and
lower
portions
ofthe
Buah
formation
to
values
of
around
0
to
+
2%o,
values
which
persisted
through
deposition
of
the
upper
portions
of
the
Buah
and
the
Ara
Formations.
Other
Late
Precambrian
Carbon
Isotope
Records
Carbon
isotope
shifts
similar
to
that
observed
in
Oman
may
be
seen
in
data
from
other
areas.
Both
carbonate
carbon
and
organic
carbon
in
Vendian
age
rocks
from
Greenland
and
Norway
also
show
an
abrupt
shift
from
heavy
to
light
carbon
isotopie
ratios
(Knoll
et
al.
1986).
Upper
Vendian
rocks
from
Siberia
also
have
primarily
negative
carbon
isotopie
ratios
(Magaritz
et
al.
1986).
The
upper
portions
of
the
curve
seen
in
Figure
2,
the
relatively
constant
values
in
the
Buah
and
Ara,
may
not
be
represented
by
the
Greenland
and
Norway,
or
Siberian
sections
because
uppermost
Vendian
age
marine
rocks
are
missing
or
greatly
condensed
in
those
sections
(Knoll
&
Swett
1987;
Kaufman
etal.
1991).
Features
similar
to
the
curve
from
Oman
may
also
be
observed
in
Late
Proterozoic
rocks
from
Namibia,
including
relatively
positive
values
(Zaris
Fm.;
Schid¬
lowski
1975;
Kaufman
etal.
1991)
followed
by
a
sharp
shift
to
negative
values
in
the
overlying
rocks
(lower
Schwarzrand
Subgroup;
Kaufman
et
al.
1991),
and
followed
by
a
return
to
relatively
constant, positive
carbon
isotopie
ratios
before
the
Cambrian/Pre-
cambrian
boundary
(upper
Schwarzrand
Subgroup
ofthe
Nama
Group;
Kaufman
et
al.
1991)
in
rocks
which
also
contain
Cloudina
(Germs
1972).
Precisely
how
the
carbon
isotope
shifts
seen
in
other
basins
correlate
to
the
Oman
section
is
problematic.
Few
late
Precambrian
sequences
are
sufficiently
well
dated
to
confidently
say
that
only
one
carbon
isotope
shift
is
present,
nor
to
state
that
a
particular
carbon
isotope
shift
in
one
basin
correlates
to
that
in
another.
The
absolute
age
dates
of
the
basement
rocks
in
the
Huqf
area
indicate
that
the
carbon
isotope
shift
in
Oman
occurred
very
late
in
the
Precambrian.
It
appears
to
be
younger
than
the
similar
shift
observed
by
Knoll
et
al.
(1986)
in
Greenland
and
by
Kaufman
et
al.
(1991)
in
Namibia.
The
lack
of
good
absolute
ages
for
the
shifts
from
those
areas,
however,
makes
any
firm
statement
about
possible
correlations
premature.
Knoll
(1991)
speculated
that
the late
Precambrian
oceans
generally
had
quite
positive
carbon
isotope
values
with
several
intervals
of
much
lighter
values.
This
remains
to
be
proven,
however,
and
better
corre-

Proterozoic
Carbon
isotopes
from Oman
605
lations
ofthe
carbon
isotope
records
of
various
basins
will
be
required
before
a
complete
picture
of
the
late
Precambrian
carbon
isotope
changes
can
be
drawn.
Causes
and
Significance
of
Carbon
Isotopie
Variation
The
cause
of
the
carbon
isotopie
shifts
in
the
late
Vendian
ocean are
not
yet
clear.
On
million
year
time
scales,
models
of
the
global
carbon
system
predict
a
close
correspon¬
dence
between
the
carbon
isotopie
composition
of
inorganic
carbon
in
the
oceans
(and
thus
of
carbon
removed
from
the
ocean
as
carbonate
sediments)
and
the
burial
rate
of
organic
carbon
(Kump
&
Garrels
1986;
Berner
1987).
More
precisely,
assuming
that
the
isotopie
composition
of
carbon
entering
the
oceans
remains
relatively
constant,
the
carbon
isotopie
composition
of
the
marine
carbonates
will
be
determined
by
the
relative
amounts
of
carbon
ultimately
removed
to
sediments
as
inorganic
versus
organic
material,
with
positive
values
indicating
a
high
percentage
of
carbon
buried
as
organic
material,
and
vice
versa.
The
very
positive
values
in
the
Khufai
Formation
then
correspond
to
a
relatively
high
percentage
of
organic
carbon
burial,
and
the
negative
values
in
the
Shuram
Formation
to
a
low
percentage
of
organic
carbon
burial.
In
fact,
mass
balance
models
ofthe
carbon
cycle
indicate
that
to
lower
the
isotopie
composition
of
dissolved
inorganic
carbon
in
the
oceans
to
values
of
—
4%o
PDB
would
require
a
nearly
complete
halt
to
the
removal
of
organic
carbon
from
the
ocean
(Kump
and
Garrels
1986;
Berner
1987).
The
relative
amount
of
carbon
removed
from
the
ocean
as
organic
carbon
may
be
in
turn
affected
by
several
factors,
with,
for
example,
high
organic
carbon burial
rates
indicating
high
overall
oceanic
productivity,
widespread
anoxia
in
the
oceans,
high
global
sedimentation
rates
(Berner
&
Canfield
1989;
Derry
et
al.
1992),
or
some
combi¬
nations
of
these.
Derry
et
al.
(1992)
suggest
that
the
high
513C
values
observed
for
post-Varangian
marine
carbonates
were
primarily
due
to
high
global
sedimantation
rates
and
increased
productivity
which
promoted
deposition
and
preservation
of
deposited
organic
matter.
They
based
their
conclusion
mainly
on
the
increasing
seawater
87Sr/86Sr
ratios
from
which
they
inferred
increasing
continental
erosion
rates
and
increased
sedi¬
mentation
rates
(Derry
etal.
1992).
The
increasing
87Sr/86Sr
ratios
for
the
Khufai
Formation
(Burns
et
al.,
in
press)
suggest
that
similar
factors
may
explain
the
high
513C
values
during
this
time
period.
The
sharp
negative
excursion
at
the
base
of
the
Shuram
Formation
is
more
difficult
to
interpret.
Similar
negative
carbon
isotope
excursions
have
been
explained
as
the
result
of
a
change
form
a
poorly-ventilated
to
a
well-ventilated
ocean
associated
with
the
onset
of
glaciations
(Kaufman
et
al.
1991
;
Derry
et
al.
1992).
Renewed
more
vigorous
circula¬
tion
would
result
in
decreased
preservation
of
organic
matter
in
sediments.
No
known
glacial
sediments
or
period
of
glacial
activity,
however,
are
known
to
coincide
with
deposition
ofthe
Shuram
Formation.
The
shift
to
low
ôl3C
values
could
also
indicate
dramatically
decreased
ocean
productivity
or
sedimentation
rates,
though
there
is
no
direct
evidence
for
either.
In
any
case,
the
negative
values
indicate
a
significant
decrease
in
organic
carbon
burial
rates
during
this
time
period.
The
timing
of
the
late
Precambrian
carbon
isotope
changes
with
respect
to
the
evolution
of
specific
groups
of
organisms
has
yet
to
be
worked
out
in
detail.
If
Cloudina
is
associated
with
the
Ediacaran
fauna
and
thus
approximately
equivalent
in
time
to
the

606
St.
J.
Burns
and
A.
Matter
arrival
of
the
earliest
metazoans,
then
the
sharp
negative
carbon
isotope
shift,
which
occurs
below
the
occurrence
of
Cloudina
in
Oman,
may
record
a
global
environmental
change
closely
associated
with
the
metazoan
radiation.
Knoll
(1991)
suggested
that
the
generally
positive
carbon
isotope
values
observed
in
late
Precambrian
carbonates
are
evidence
of
high
oceanic
productivity
and
increasing
atmospheric
oxygen
levels.
An
increase
in
atmospheric
oxygen
has
been
proposed
as
a
necessary
precursor
to
the
late
Precambrian/early
Cambrian
radiation
of
animals
(Towe
1981;
Knoll
1991).
An
alternative
is
that
late
Precambrian
atmospheric
oxygen
levels
were
already high
(see
discussion
in
McMenamin
&
McMenamin
1990),
and
that
the
later
Precambrian
radiation
was
caused
by
environmental
changes
which
upset
the
biological
balance
of
the
period.
The
sharp
negative
carbon
isotope
excursion
observed
in
Oman
may
be
evidence
of
such
an
environmental
change.
That
the
fossil
record
is
occasionally
punctuated
by
rapid
extinction
of
some
groups
of
organisms
and
the
rise
of
others
(Eldridge
&
Gould
1972;
Gould &
Eldridge
1977)
suggests
that
environmental
changes,
recorded
by
the
carbon
isotopes,
may
have
triggered
the
evolution
of
new
organisms
seen
thereafter
in
the
latest
Precambrian.
Acknowledgements
We
thank
Dr.
T
Peters
and
Petroleum
Development
Oman
(PDO)
for
logistical
support;
Drs.
W.
Bollinger
and
G.
Gorin for
discussions;
and
the
Ministry
of
Petroleum
&
Minerals
of
the
Sultanate
of
Oman
and
PDO
for
permission
to
publish.
P.
Aharon,
A.
J.
Kaufman,
R.
Berner,
H.
Weissert
and
an
anonymous
reviewer
are
thanked
for
comments
on
earlier
versions
of
the
manuscript.
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Revision
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31
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