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Earth and Planetary Science Letters 128 (1994) 671-681
Sr and C isotopes in Lower Cambrian carbonates from the
Siberian craton: A paleoenvironmental record during the
‘Cambrian explosion’
L.A. Derry a,1, M.D. Brasier b, R.M. Corfield b, A.Yu. Rozanov c, A.Yu Zhuravlev c
a CNRS, Centre de Recherches Petrographiques et Geochemiques, 54501 Vandoeuore-les-Nancy, France
b Department of Earth Sciences, Oxford Uniuersity, Parks Road, Oxford OX1 3PR, UK
c Palaeontological Institute, 113 Profsoyuznaya, Moscow 117647, Russia
Received 4 May 1994; accepted 22 October 1994
Abstract
We report 87Sr/86Sr measurements on a suite of well preserved sedimentary carbonates from Lower Cambrian
strata of the Lena River region of Siberia. Stable isotopes and major and trace element chemistry have been used to
identify potentially unaltered samples for Sr isotopic measurements. The Sr data define a smooth curve of
paleoseawater 87Sr/86Sr values from the Tommotian through to the early Middle Cambrian. During the Tommotian-Atdabanian interval, 87Sr/86Sr rose rapidly from 0.7081 to 0.7085. The rate of change in Sr ratios decreased
during the Botomian but rose to 0.7088 in the late Toyonian to early- Middle Cambrian. The rate of 87Sr/86Sr
increase during the Tommotian-Atdabanian was ca. 0.0001/m.y., comparable to the late Miocene change in
seawater Sr. We infer that an interval of enhanced erosion during the ‘Cambrian explosion’ was responsible for this
increase. An important source for radiogenic Sr to the oceans may have been erosion of the Pan-African orogenic
belt of southern Africa. The rapid change in paleoseawater Sr corresponds with an interval of highly variable marine
613C values. Model results for the Sr and C isotopic records suggest that the quasi-periodicity in the 6’“C record is
not a consequence of direct erosional forcing. However, our inference of high erosion rates during the TommotianAtdabanian implies enhanced fluxes of nutrient elements such as P to the oceans. Phosphorite deposits and black
shale deposition in coeval strata suggest that periods of high marine productivity and anoxia may be in part related
to enhanced river dissolved fluxes. Our results thus provide some insight into environmental conditions during the
‘Cambrian explosion.’
1. Introduction
Measurements of strontium isotopic compositions of marine carbonates have made significant
1 Present address: Department of Geological Sciences, Cornell University, Ithaca, New York, NY 14853-1504, USA,
email: [email protected].
contributions to our understanding of Late Proterozoic and Cambrian stratigraphy and paleoenvironments. Beginning with the work of Veizer et
al. [l], several studies have shown that 87Sr/86Sr
values in seawater rose rapidly in the Vendian,
from values below 0.707 to values near 0.709 by
the Cambrian [2-6]. This rapid change appears
be comparable to the Cenozoic increase in seawater Sr isotopic ratio in both overall rate and
0012-821X/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved
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0012-821X(94)00228-2
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L.A. Derry et al. / Earth and Planetary Science Letters 128 (1994) 671-681
magnitude. The changes in the Sr isotopic composition in both Cenozoic and Neoproterozoic
seawater appear to reflect increased erosion rates,
resulting from major continental collisions and
orogeny during both eras [2,6,7]. The Neoproterozoic increase probably also reflects changes in
the rate of seafloor hydrothermal input to the
oceans [1,2]. However, the relationship between
orogenesis, erosion rate and seawater Sr isotopic
change is not completely understood. The Neoproterozoic-Cambrian transition offers a potential analog to the Cretaceous-Cenozoic interval
of increasing seawater 87Sr/86Sr ratio and, thus,
may be used to test competing hypotheses for
major Sr isotopic change in seawater [6].
The Neoproterozoic Sr record has also been
shown to be of great interest for interpreting
paleoenvironments during this key interval of
Earth history [8]. Variations in globally averaged
erosion/sedimentation rates are an important
control in the cycling of nutrient elements and
sedimentary carbon and, thus, appear to play a
key role in controlling variations in both primary
productivity and organic carbon burial [9]. Carbon isotope studies and the occurrence of sedimentary phosphorite deposits suggest that organic carbon burial and phosphorous fluxes may
have varied widely during the ‘Cambrian explosion’ of the Lower to Middle Cambrian [10-16].
Quantitative models of biogeochemical cycling in
this remarkable interval require better constraints
on erosion rates and weathering fluxes.
In this study we present Sr isotopic measurements from a suite of carbonate samples from the
Lower and Middle Cambrian of the Siberian platform. Sediments of the Siberian platform have
provided data for several recent studies of Precambrian-Cambrian
biostratigraphy,
chemostratigraphy and geochronology [12,16-18].
Our data help fill an important gap in the emerging geochemical record of paleoenvironments
from the Neoproterozoic and Cambrian interval.
2. Samples and methods
Very gently dipping and little deformed strata
along the Aldan and Lena Rivers of Siberia [19,20]
Zone
Schistccephalus
\Anabaraspis
splendens
Dominant
lithologies
Lomontovia
grandis
6 ketemensis
Thin to medium
beddedlimestone
B. ornata
Bergeromellus
asIaticus
Bergeroniellus
Q”WXll
3
micmacciformis
Judomla/
UMaspis
limestones
Pagetiellus
anabarus
Fallotasois
\ “Purella”
Medium to thick
bedded dolomite
Anabarltes
trwulatus
Fig. 1. Composite stratigraphic column with biozones of sedimentary sections along the Lena River, Siberia [16,18]. Composite stratigraphic height (in meters) marked on right.
have yielded a suite of sedimentary carbonate
samples suitable for isotope chemostratigraphy
[16,18]. Carbonate samples were selected for
analysis so as to provide good stratigraphic coverage through the Tommotian, Atdabanian, Botomian and’ Toyonian type sections (Fig. 1). Stable
isotopic measurements made on a larger sample
set [16] were used to avoid the most obviously
a l t e r e d m a t e r i a l ( i . e . s a m p l e s w i t h 6”O <
- 10%~ were not chosen). Samples thus selected
were gently crushed, and clean fragments hand
picked. These fragments were crushed to powder
in a stainless steel mortar and ca. 20 mg was
dissolved in 10% ultrapure acetic acid. Insoluble
residues were separated by centrifugation, dried
and weighed. Sr was separated by standard ion
exchange techniques and isotopic analyses were
L.A. Derry et al. / Earth and Planetary Science Letters 128 (1994) 671-681
673
made on the Finnigan 262 mass spectrometer at
the CRPG, Nancy. A further 200 mg of powder
was dissolved as above, and Ca, Mg, Mn, Fe and
Sr concentrations determined by atomic absorption.
3. Results
+
No one geochemical or textural indicator has
been shown to be an infallible test for the degree
of preservation of primary Sr isotopic signatures
in ancient carbonates. In addition to petrographic
and stable isotopic screening, we use the relative
abundances of Mn, Fe and Sr as indicators of
post-depositional alteration of the carbonates
[1,8,21]. Samples with al80 < - 10%0 were not
further analyzed; 87Sr/86Sr values are not correlated with 6”O in our sample subset (Fig. 2).
Mn/Sr
Fig. 2. (a) s’s0 versus 87Sr/86Sr values for Lena River
samples. (b) Fe/Sr
versus Mn/Sr
for carbonate samples.
Hatchured area outlines the region of apparently best preserved samples for Sr isotopic analysis. High values of Fe/Sr
(> 5) and Mn/Sr (> 2) are primarily found in dolomitized
samples (open symbols), but can also indicate alteration in
limestones.
Age, Ma
Fig. 3. 87Sr/86Sr variations in Lower Cambrian carbonates of
the Siberian platform, plotted on the Lower Cambrian timescale of Bowring et al. [17]. Vertical dashed lines mark ages of
stage boundaries as estimated by [17]. w = samples that meet
geochemical selection criteria described in text; 0 = samples
with elevated Fe/Sr and/or Mn/Sr values, which are considered as altered. Values > 0.709 not shown.
CaO/MgO ratios permit the identification of
dolomitized samples. All samples which show evidence of partial or complete dolomitization
(CaO/MgO weight ratio < 8) have high Fe/Sr
and Mn/Sr ratios and yield 87Sr/86Sr values
consistently higher than coeval limestones (Table
1). Mn/Sr and Fe/Sr are well correlated in the
limestones, consistent with previous results which
suggest that these parameters are sensitive indicators of alteration of Sr isotopic values (Fig. 2).
The percent dissolution is not correlated with
87Sr/86Sr ratios (Table 1). Based on the evaluation of these and other samples of similar age and
environment [5,6,8] we have greatest confidence
in 87Sr/86Sr values from samples that: (1) are not
dolomitized; (2) have Mn/Sr I 0.6; and (3) have
Fe/Sr < 3. However, we caution that this choice
of parameters is largely empirical and somewhat
arbitrary and the extent of alteration in each
suite of samples must be evaluated independently.
The Sr isotopic data are plotted on a time axis
(Fig. 3), using the Lower Cambrian time-scale
proposed by Bowring et al. [17]. It should be
noted that, while the basal Nemakit Daldyn, basal
Tommotian and Atdabanian-Botomian boundaries are reasonably well dated, the age of the
Lower-Middle Cambrian boundary is at present
not well known. An empirical subsidence curve
Table 1
Analytical data for Lower Cambrian Lena River carbonates
Sample
No.
Stratigraphic
height, m
E22
E20
El7
EllA
W55/32g
W56/11A
W56/1Z
W43/1A
LAB15
LAB9
LAB1
AT20-141
AT10-50
AT5-37
AT4-36
AKT20-87
AKT20-77
AKT19-71
AKT14-53
AKTlO-45
AKTAl
17E
16A
BB-2
10A
US12A
1A
US3A
AK63
AK651
AK747
916.3
905.0
895.0
879.9
819.8
794.7
769.6
653.1
644.3
589.1
560.3
551.5
462.5
451.2
441.2
436.2
431.2
426.2
408.6
401.1
351.0
283.3
270.7
228.1
194.3
188.0
183.0
175.5
155.4
122.8
97.8
Location
Elanka
Elanka
Elanka
Elanka
Titary
Titary
Titary
Titary
Labaya
Labaya
Labaya
AKT
AKT
AKT
AKT
AKT
AKT
AKT
AKT
AKT
AKT
lsit
lsit
Zhurinsky
lsit
US
lsit
US
Mt. Konus
Mt. Konus
Mt. Konus
87Sr/86Sr
0.708680
0.708753
0.708648
0.708647
0.708531
0.708556
0.708503
0.708455
0.708461
0.708440
0.708447
0.708462
0.708703
0.708650
0.708646
0.708617
0.708493
0.708841
0.708436
0.708504
0.708287
0.708191
0.708573
0.708059
0.708333
0.708550
0.708123
0.708673
0.709471
0.710127
0.708438
dissol.
%
98
100
99
98
97
100
100
97
100
96
100
97
92
87
85
92
100
87
98
75
97
88
73
91
71
95
92
88
86
77
81
CaO
wt. %
53.77
56.17
50.83
34.19
55.25
54.77
55.68
53.90
53.55
51.21
52.61
44.24
31.95
39.18
26.78
45.57
59.28
28.22
53.54
43.56
54.94
47.88
27.38
50.56
39.74
30.58
47.19
27.61
26.21
16.39
23.56
MgO
wt. %
1.91
0.32
4.34
17.54
0.44
0.55
0.43
1.23
1.74
4.3
2.94
9.37
16.43
6.6
15.39
3.18
2.46
15.99
0.95
1.92
0.51
1.75
11.81
0.91
0.68
20.41
5.04
18.79
17.24
10.09
15.69
Mn
ppm
54
32
67
137
23
23
21
21
25
32
29
51
219
195
232
202
59
189
119
304
97
82
160
82
116
157
90
217
153
85
236
Fe
ppm
Sr
ppm
150
91
235
770
65
62
84
74
120
115
135
100
1440
1515
6070
1130
320
2380
675
410
600
720
2890
270
460
685
795
1055
1840
1150
1710
165
190
130
210
160
300
230
550
355
295
305
250
150
205
150
240
1170
135
280
235
300
305
180
480
155
58
250
58
94
115
81
Mn/Sr Fe/Sr
0.33
0.17
0.52
0.65
0.14
0.08
0.09
0.04
0.07
0.11
0.10
0.20
1.46
0.95
1.55
0.84
0.05
1.40
0.43
1.29
0.32
0.27
0.89
0.17
0.75
2.71
0.36
3.74
1.63
0.74
2.91
0.91
0.48
1.81
3.67
0.41
0.21
0.37
0.13
0.34
0.39
0.44
0.40
9.60
7.39
40.47
4.71
0.27
17.63
2.41
1.74
2.00
2.36
16.06
0.56
2.97
11.81
3.18
18.19
19.57
10.00
21.11
CaO/
MgO
28.2
175
11.7
1.95
125
99.6
129
43.8
30.8
11.9
17.9
4.72
1.94
5.94
1.74
14.3
24.1
1.76
56.4
22.7
107
27.4
2.32
55.6
45.2
1.50
9.36
1.47
1.52
1.62
1.50
6’3C
6’80
0.86
1.24
0.51
0.89
-0.57
-0.33
-0.67
-1.75
-1.06
-1.24
-1.06
0.51
2.16
2.39
-1.07
-0.26
-0.94
0.81
-0.22
1.27
-0.01
-1.54
-1.62
-8.75
-5.44
-6.01
-5.54
-1.12
-6.57
-6.71
-6.50
-6.58
-6.61
-7.46
-7.10
-5.92
-4.71
-6.87
-5.46
-6.53
-5.51
-5.44
-5.98
-5.89
-7.15
-7.45
-2.00
1.42
0.58
2.34
1.87
1.05
-0.07
-6.31
-6.17
-6.29
-4.77
-4.96
-4.40
-4.26
AKT = Archchagy Kyyry Taas.
NBS-987 run during the period of analysis under similar conditions (Finnigan 262, static multicollection, I(88) = 6V) yielded a mean of 0.710171, 2~ error = 9 ppm,
n = 11.
US = Ulakhan Sulugur.
L.A. Derry et al. / Earth and Planetary Science Letters 128 (1994) 671-681
was used to interpolate ages of samples between
‘known’ tie points. After exclusion of samples
which failed to pass the geochemical ‘tests’ for
alteration listed above, the Sr data define a relatively smooth curve. Relatively low 87Sr/86Sr values ( = 0.7081) appear to characterize Tommotian
samples reliably. From these low Tommotian values, 87Sr/86Sr rises gradually to ca. 0.7085 in
Botomian strata. The climb in Sr isotopic ratio
appears to pause during the Botomian, while
Toyonian and lower Middle Cambrian samples
indicate a renewed rise toward 0.7088. Values
near 0.7088 are consistent with published data
from Middle to Upper Cambrian carbonates of
0.7090-0.7093 [22,23]. The combination of geochemical screening, agreement between stratigraphically adjacent samples and smooth variation suggests that the curve presented in Fig. 3 is
a reasonable representation of Lower Cambrian
seawater Sr isotopic variations. The stratigraphic
interval below the Tommotian low is not constrained by our data because the samples that we
measured in Nemakit-Daldynian age strata appear to be altered. Further work will be necessary
to define Sr isotopic variations closer to the Precambrian-Cambrian boundary as presently defined. Ultimately, our proposed curve should be
tested with measurements from a different, but
coeval (as established by independent methods),
stratigraphic section.
4. Discussion
A growing body of evidence supports the view
that seawater 87Sr/86Sr was < 0.707 during the
Neoproterozoic Varanger glaciation (ca. 60.5 Ma),
and began to rise rapidly only afterwards [2,4,6].
Sr isotopic data from upper Vendian carbonates
show values rising to ca. 0.7085 in samples from
the Nama and Witvlei Groups of southern Africa
and the Windermere Supergroup of northwest
Canada [6], while Burns et al. [4] argue for values
as high as 0.7092 in latest Vendian strata of the
Huqf Group, Oman. We are concerned that the
data from the Huqf Group [4] may overestimate
actual Vendian seawater 87Sr/86Sr values. The
majority of these samples have very high Mn/Sr
675
Cretaceous-Tertiary age, Ma
Fig. 4. Sr isotopic evolution of Vendian to mid-Cambrian
seawater. Heavy line (KJK) = Vendian ‘best estimate’ from
Kaufman et al. [6]. 0 = this work; A from [22]; question marks
indicate estimated Sr values for intervals without reliable
data. The periods of most rapid change are from ca. 600 to
590 Ma and again from 530 to 525 Ma. For comparison, the Sr
isotopic curve for the period 100 Ma-present (thin line,
RRD) [7] is plotted on the same time-scale (upper axis gives
actual ages).
ratios (z+ 1) and very low Sr contents (< 80 ppm).
Such numbers are not typical of primary marine
micrites and suggest that the Huqf Group carbonates have not remained a closed system for
Sr. The qualitative agreement between the curves
presented by Burns et al. [4] and Kaufman et al.
[6] is, however, encouraging. At present, no reliable data have been published from lowest Cambrian strata. Our results imply that, by the earliest Tommotian, 87Sr/86Sr in seawater had fallen
from its Vendian high to 0.7081. This low is
followed by the relatively rapid rise during the
Lower Cambrian. Thus, the overall increase of
the marine Sr isotopic ratio from Varanger lows
of 0.7066 to Upper Cambrian highs of 0.7091
appears to have taken place in two stages, separated by a decrease near the PrecambrianCambrian boundary, which was completed by the
Tommotian (Fig. 4). In this respect the
Vendian-Cambrian rise is unlike the nearly
monotonic Cretaceous-Tertiary increase.
Uncertainties remain in the absolute age calibration of Cambrian strata but a first-order calculation of the rate of the Lower Cambrian Sr rise
is possible. Accepting the Lower Cambrian time-
676
L.A. Derry et al. / Earth and Planetary Science Letters 128 (1994) 671-681
scale of [17], we estimate the rate of increase of
87Sr/86Sr in Tommotian-Atdabanian seawater to
have been about 0.0001/m.y. For comparison,
other known periods of rapid change include a
brief late Miocene step of 0.0001/m.y. [24],
0.0001/m.y. in the early Miocene [7] and ca.
0.00013/m.y. in the lower Vendian [6]. Thus, the
Lower Cambrian rise we observe is rapid but not
unprecedented.
4.1. Causes of Lower Cambrian Sr isotope change
Several workers have cited uplift and erosion
related to the Pan-African orogeny as a principal
cause of the overall Vendian-Cambrian seawater
87Sr/86Sr increase, [2,5,6,25]. The cause of the
apparent decline in seawater 87Sr/86Sr of 0.0004
to 0.0009 near the Precambrian-Cambrian
boundary is less clear. At least three alternative
hypotheses exist:
(1) Reduced rates of tectonically driven uplift or
climate change may have resulted in a temporary decline in global silicate weathering rates.
(2) A change in the type of eroding crust could
have driven a significant drop in the mean
87Sr/86Sr of river water.
(3) Rift-associated volcanic activity as well as
worldwide marine transgression might have
been sufficient to reverse temporarily the upward trend of 87Sr/86Sr
At present, the data from lowest Cambrian and
latest Vendian strata are insufficient to address
this question further.
Models using combined Nd and Sr systematics
from Neoproterozoic and Cambrian sediments
have suggested that global erosion rates were
highest in the lower to mid-Vendian and fell
gradually to moderate levels by the Upper Cambrian [5,6]. However, the new data presented in
this paper imply a second period of rapid erosion
during the Lower Cambrian that has not previously been recognized. Because of the degrees of
freedom inherent in interpreting the marine Sr
record, the Tommotian-Botomian Sr increase
could represent an interval of unroofing of old,
radiogenic crust or one of increased global erosion rates. An independent estimate of the evolution of crustal sources to the oceans (such as Nd
isotopes) through this interval is necessary to
resolve this ambiguity quantitatively but, unfortunately, Nd data of sufficient resolution are not
presently available. However, the rate of increase
of observed seawater 87Sr/86Sr during the Lower
Cambrian provides some constraints on plausible
forcing mechanisms. A calculation using the
model of [24] suggests that the 87Sr/86Sr of river
water would have had to increase by = 0.001 in 5
m.y. if the sole cause of the Tommotian-Atdabanian seawater increase was change in the isotopic ratio of continental runoff. This is greater
than the shift in river 87Sr/86Sr for the interval
40-0 Ma, estimated as resulting from Himalayan
erosion [7], and greater than the net impact of
the Ganges-Brahmaputra system on the oceanic
Sr budget today [26]. Plausible ranges and rates
of riverine 87Sr/86Sr inputs to the oceans have
been discussed in detail elsewhere [24,26,27].
From these considerations it seems unlikely that
the Tommotian-Atdabanian seawater 87Sr/86Sr
increase could have been caused by increasing
riverine 87Sr/86Sr alone. The same conclusion
applies to models of groundwater flux of Sr from
continents to the oceans [28]. Thus it appears that
the Tommotian-Atdabanian seawater 87Sr/86Sr
rise was caused at least in part by increasing river
fluxes of Sr and indicates a period of enhanced
chemical erosion. The geochemical evidence for a
period of increased erosion during an interval
well known for the development of transgressive
sequences in North America and elsewhere may
seem contradictory, but it should be pointed out
that passive margin sedimentation currently characterizes almost all of the Atlantic shorelines of
four continents, the east coast of Africa and most
of Australia, at the same as time as very rapid
erosion takes place in Asia.
The dramatic rise in the Vendian-Cambrian
seawater Sr isotopic ratio may be compared to
that during the Late Cretaceous-Tertiary (Fig.
4), the only comparable shift known from the
geologic record [6,25]. We note that the rate and
pattern of seawater Sr change during the Neogene was similar to that during most of the
Lower-Middle Cambrian. This observation hints
that the mechanisms of Sr isotopic change could
have been similar in both cases. Major unroofing
L.A. Derry et al. / Earth and Planetary Science Letters 128 (1994) 671-681
and erosion of the very radiogenic Himalayan
metamorphic core occurred in the early Miocene
[29-31], apparently driving already rising seawater 87Sr/86Sr to high values [7]. An analogous
tectonic history appears to describe the PanAfrican Damara-Gariep belt of southern Africa
in the Cambrian. The Damara belt underwent a
major episode of metamorphism, crustal melting,
thrusting and erosion beginning about 540 Ma,
exposing radiogenic metamorphic basement to
rapid erosion [32-34]. The Lower Cambrian molassic sediments of the upper Nama Group were
derived primarily from a Damara crystalline
source area and have radiogenic Sr signatures
[35,36]. These sediments are analogous to the
highly radiogenic molasse and flysch derived from
the Himalayan orogen and deposited in the Siwalik foreland and Bengal Fan [31]. Thus, we suggest that an important source of radiogenic Sr to
the oceans during the Cambrian could have been
the erosion of the Damara-Gariep belt, just as
the Himalaya have provided radiogenic Sr to the
Neogene ocean. Avalonian events may also have
contributed to rising seawater 87Sr/86Sr values.
cNd values from Avalonian clastic sediments of
Great Britain drop rapidly during the Lower
Cambrian, implying the exposure and erosion of
mature basement with radiogenic Sr [37].
4.2. Comparison with the carbon isotope record
Increased erosion implies increased sedimentation, and increased sedimentation implies increased carbon burial [9]. The use of Sr (and Nd)
isotopic records as proxies for sedimentary carbon flux has proven to be a powerful tool for
understanding the history of biogeochemical cycling. The rate of carbon cycling thus obtained
can be combined with the fractional organic carbon burial flux, obtained from carbon isotope
measurements, to estimate the absolute organic
carbon burial rate [8]. During the Tommotian to
early Botomian, the 613C value of marine carbonates was highly variable but generally increasing
[12,16]. The 613C data imply that the fractional
organic carbon burial flux reached as high as 30%
in the Atdabanian-Botomian. The Sr isotope evi-
677
Age, Ma
Fig. 5. Carbon isotope data from Lower Cambrian Siberian
carbonates [11,12,14,16] p l o t t e d o n t i m e a x i s [17]. Stage
boundaries marked as in Fig. 3.
dence for an interval of increased erosion/sedimentation and the C isotope evidence for a mean
increase in the fractional burial of organic carbon
imply that, overall, this interval was one of enhanced but episodic organic carbon burial.
The 613C data show variations of several per
mil on a < 1 Ma time-scale during the latest
Nemakit-Daldynian through to the Botomian
(Fig. 5). The rate of variation of 613C provides
some hints as to possible mechanisms. The most
reasonable explanation for the marked variations
observed in the Lower Cambrian 6i3C record is
change in the burial fraction of organic carbon in
marine sediments (c.f. [38]). Such changes might
plausibly result from two kinds of phenomena.
Rapid changes in oceanic productivity could lead
to changes in organic carbon burial. This explanation may apply to the 3%0 fall in 613C values
during the Botomian, which appears both to coincide with a significant extinction event and to
mark the end of the period of extreme variability
in 613C [16,39]. Can the Sr data provide an explanation for the apparent cyclic variability of the
613C record during the Lower Cambrian? It has
been proposed that the seawater Sr and C isotope records might be coupled through the erosion/sedimentation process [8]. High erosion
rates should lead to elevated seawater 87Sr/86Sr
ratios and at the same time to increased mean
sedimentation rates in the oceans. Global organic
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L.A. Derry et al. / Earth and Planetary Science Letters 128 (1994) 671-681
carbon burial rates should be, to a first approximation, related to clastic sedimentation rates, so
increased erosion should result in increased organic carbon burial [8,9,40]. Furthermore, if marine productivity is nutrient limited, increased
erosional fluxes of phosphorous (for example)
might result in increased organic carbon production and consequent increased burial. Thus, periodic variations in globally averaged erosion rates
might provide a mechanism to drive the oscillations in the S13C signal observed in the Lower
Cambrian.
In order to illustrate the possible relationships
between the C and Sr isotope records, we modeled the response of the 87Sr/86Sr and 613C
records to a common periodic forcing factor (e.g.,
variations in erosion rate). Our model of amplitude response and phase shift of a component in
a first-order reservoir is equivalent to those of
Lasaga [41] and Richter and Turekian [26,41]. We
chose a residence time (ri) for C of 40 kyr, for Sr
r2 = 3 myr, and a sinusoidal forcing with a period
(27~) of 750 kyr. It can be shown that, in general,
the greatest phase shift between two coupled
signals in a simple first-order system driven by a
common forcing will result when 72 > 27~ YP pi.
Thus, the Sr and C isotopic records in seawater
should show significantly different phase responses to a periodic forcing near 1 myr. The
model results (Fig. 6) show that the response of
Sr isotopes in the ocean should lag behind that of
carbon isotopes by about 180 kyr (e.g. = r/2)
and that, for a net 3%0 6i3C shift, there should
be a shift of 0.00014 in 87Sr/86Sr. Such Sr shifts
are not evident in our data but the resolution of
the data is not yet sufficient to exclude this possibility firmly. It should be noted, however, that
seawater isotopic shifts of this magnitude, if
driven by erosion, require large and rapid variations in either river Sr fluxes or isotope ratios. In
order to produce variations consistent with the
6i3C shifts, the global Sr river flux would have to
vary by ca. 30% (or its ratio by ca. 0.001) with the
same frequency (i.e., 750 kyr). Cyclic shifts in
global river Sr fluxes of this magnitude and frequency are certainly very large and may not be
plausible [26], particularly in the absence of any
evidence for continental glaciation during the
750
Time, kyr
Fig. 6. Response curves for 613C (dashed line) and 87Sr/86Sr
(solid line) in seawater as a function of an arbitrary common
periodic forcing of a 750 kyr period (fine dashed line). The
left axis shows deviations in 613C, while the right axis gives
deviations in Sr isotopic composition in units of A”Sr (A8’Sr
= [(87Sr / 86Srmeas - 87Sr / 86Srstd) / (87Sr / 86Srstd)] X 105,
where 87Sr/86Srstd = 0.70197). No vertical scale is implied for
the forcing function. Because of the short response time of C
in the oceans, the 613C variations are nearly in phase with the
forcing function. The long response time of Sr, however,
results in a response significantly out of phase with both the
forcing function and the 613C response.
Lower Cambrian. We conclude that direct forcing
by enhanced erosion and burial of organic carbon
is an unlikely explanation for the quasi-periodic
nature of the Lower Cambrian 6i3C record.
Episodic marine anoxia (possibly related to
productivity variations [10]) may have played a
key role in controlling organic carbon burial. The
rate of carbon isotope variation during the Lower
Cambrian appears similar to that found near the
Cenomanian-Turonian and Albian-Aptian
boundaries during the Cretaceous, associated with
ocean anoxic events (OAEs) [42,43]. For example,
Bralower et al. [44] have argued that three separate anoxic episodes, each of l-2 m.y. duration,
occurred near the Albian-Aptian boundary. Extensive black shale deposition is known from the
Siberian platform and China during the Lower
Cambrian, implying that OAEs also occurred during this interval [45,46]. Repeated OAEs may
have driven changes in Lower Cambrian organic
carbon burial. Relatively narrow post-rift ocean
basins (similar to the Cretaceous Atlantic and
Tethys) could have contributed to OAEs in the
L.A. Derry et al. / Earth and Planetary Science Letters 128 (1994) 671-681
Lower Cambrian. The presence of low to midlatitude evaporite deposits also suggests that conditions for forming oxygen-poor, warm, saline,
bottom waters were present [47]. Alternatively,
evaporite basins, themselves, may have been sites
of significant organic matter accumulation. However, the mechanism by which deposition in evaporite basins could produce cyclic changes in the
global 6’“C record of such a large magnitude is
not clear. Detailed stratigraphic work to establish
correlations between 613C variations, anoxic deposits, evaporites and phosphorites is necessary
to test these hypotheses.
The Sr isotopic evidence for an interval of
rapid erosion during the Lower Cambrian may
have some implications for understanding the
‘Cambrian explosion’ of rapid marine invertebrate radiation. A consequence of increased erosion rates should be increased phosphorous flux
to the oceans, although recent work has suggested that P accumulation in Cenozoic marine
sediments is not closely coupled to the Cenozioc
87Sr/86Sr record [48]. Major sedimentary phosphorite deposits are known from the Lower Cambrian and phosphatic skeletal fossils are unusually abundant in the Tommotian-Atdabanian,
suggesting that the Early Cambrian oceans could
have had relatively high P availability [15,45,49].
Enhanced oceanic nutrient levels could have led
to episodes of increased productivity, which have
been proposed as a mechanism for positive C
isotope shifts in the Precambrian-Cambrian transition [13]. However, any nutrient-driven productivity episodes should have been self-limiting on a
ca. 100 kyr time-scale as the surface ocean supply
of P was drawn down. It may be that the highly
episodic nature of the 613C record in part reflects
nutrient limitations. In any case, the apparent
availability of dissolved P in the Lower Cambrian
ocean could have played a role in providing an
environment conducive to the rapid diversification and expansion of marine invertebrates by
enhancing primary productivity at the base of the
food chain. A link between erosion, primary productivity and carbon burial is plausible for the
Lower Cambrian, which may have influenced the
environment of evolution of early invertebrates
during the ‘Cambrian explosion.’
679
5. Conclusions
Sr isotopic variations in Lower to Middle Cambrian carbonates show a rapid increase in seawater 87Sr/86Sr values from 0.7081 in Tommotian
strata to 0.7085 in Botomian strata. Values remain near 0.7085 in Toyonian strata, rising to
0.7088 in lower Middle Cambrian strata.
The low values of Tommotian carbonates imply a decrease of 2 0.0004 in the 87Sr/86Sr ratio
of seawater near the Precambrian-Cambrian
boundary, given values of 0.7085-0.7092 reported
from mid-late Vendian strata. Thus, the overall
rise in seawater Sr isotopic values beginning in
the lower Vendian was interrupted, possibly by a
decrease in the Sr isotopic ratio of the global
river flux, by decreased silicate weathering rates
and/or rift-related volcanic activity and subsidence.
Rates of change of 87Sr/86Sr in Lower Cambrian rocks are = 0.0001/m.y., comparable to
the most rapid Neogene or lower Vendian variations. The Lower Cambrian seawater Sr isotopic
increase may be related to the rapid erosion of
radiogenic crystalline rocks of the Pan-African
Damara-Gariep belt of southern Africa, and possibly of the Avalonian terrane.
Our data imply a previously unrecognized period of enhanced erosion in the Lower Cambrian.
This interval of rapid erosion coincides closely
with the ‘Cambrian explosion’ of marine invertebrates. Consideration of newly available carbon
isotope data suggests that episodic marine anoxia
could have been responsible for rapid variation in
fractional organic carbon burial during the
‘Cambrian explosion.’ High fractional rates of
organic carbon burial and episodes of marine
anoxia may also be related to enhanced nutrient
fluxes resulting from high erosion rates.
Acknowledgements
The authors wish to thank A.J. Kaufman, R.
Berner and M. Kennedy for careful reading and
helpful criticism. L. Derry wishes to thank L.
Marin, D. Dautelle and A. Moore for expert
laboratory assistance. [FA]
680
L.A. Derry et al. /Earth and Planetary Science Letters 128 (1994) 671-681
References
[l] J. Veizer, W. Compston, N. Clauer and M. Schidlowski,
87Sr/86Sr in Late Proterozoic carbonates: Evidence for a
‘mantle event’ at 900 Ma ago, Geochim. Cosmochim.
Acta 47, 295-302, 1983.
[2] L.A. Derry L.S. Keto, S.B. Jacobsen, A.H. Knoll and K.
Swett, Sr isotopic variations in Late Proterozoic carbonates from Svalbard and East Greenland, Geochim. Cosmochim. Acta 54, 2331-2339, 1989.
[3] L.A. Derry and S.B. Jacobsen, The chemical evolution of
Precambrian seawater: Evidence from REE’s in banded
iron formations, Geochim. Cosmochim. Acta 54, 29652977, 1990.
[4] S.J. Burns, U. Haudernschild and A. Matter, The strontium isotopic composition of carbonates from the late
Precambrian (ca. 560-540 Ma) Huqf Group of Oman,
Chem. Geol. (Isot. Geosci. Sect.) 111, 269-282, 1994.
[5] Y.A. Asmerom, S.B. Jacobsen, N.J. Butterfield and A.H.
Knoll, Sr isotope variations in Late Proterozoic seawater:
Implications for crustal evolution, Geochim. Cosmochim.
Acta 55, 2883-2894, 1991.
[6] A.J. Kaufman, S.B. Jacobsen and A.H. Knoll, The Vendian record of Sr- and C-isotopic variations in seawater:
Implications for tectonics and paleoclimate, Earth Planet.
Sci. Lett. 120, 409-430, 1993.
[7] F.M. Richter, D.B. Rowley and D.J. DePaolo, Sr isotope
evolution of seawater: the role of tectonics, Earth Planet.
Sci. Lett. 109, 11-23, 1992.
[8] L.A. Derry, A.J. Kaufman and S.B. Jacobsen, Sedimentary cycling and environmental change in the Late Proterozic: evidence from stable and radiogenic isotopes,
Geochim. Cosmochim. Acta 56, 1317-1329, 1992.
[9] R.A. Berner and D. Canfield, A model for atmospheric
oxygen over Phanerozoic time, Am. J. Sci. 289, 333-361,
1989.
[10] M.E. Tucker, Carbon isotope excursions in Precambrian/Cambrian boundary beds, Morocco, Nature 319,
48-50, 1986.
[ll] M. Magaritz, W.T. Holser and J.L. Kirschvink, Carbon
isotope events across the Precambrian-Cambrian boundary on the Siberian platform, Nature 320, 258-259, 1986.
[12] M. Magaritz, J.L. Kirschvink, A.J. Latham, A.Y. Zhuravlev and A.Y. Rozanov, Precambrian/Cambrian
boundary problem: carbon isotope correlations for Vendian and Tommotian time between Siberia and Morocco,
Geology 19, 847-850, 1991.
[13] P. Aharon, M. Schidlowski and I.B. Singh, Chronostratigraphic markers in the end-Precambrian carbon isotope
record of the Lesser Himalaya, Nature 327, 699-702,
1987.
[14] J. Kirschvink, M. Magaritz, R.L. Ripperdan, A.Y. Zhuravlev and A.Y. Rozanov, The Precambrian/Cambrian
boundary: magnetostratigraphy and carbon isotopes resolve correlation problems between Siberia, Morrocco
and south China, GSA Today 1, 69-71, 1991.
[15] M.D. Brasier, Global ocean-atmosphere change across
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
the Precambrian-Cambrian transition, Geol. Mag. 129,
161-168, 1992.
M.D. Brasier, R.M. Corfield, L.A. Derry, A.Y. Rozanov
and A.Y. Zhuravlev, Multiple S13C excursions spanning
the Cambrian explosion to the Botomian crisis in Siberia,
Geology 22, 455-458, 1994.
S.A. Bowring, J.P. Grotzinger, C.E. Isachsen, A.H. Knoll,
S. Pelechaty and P. Kolosov, Calibrating rates of Early
Cambrian evolution, Science 261, 1293-1298, 1993.
M.D. Brasier, A.Y. Rozanov, A.Y. Zhuravlev, R.M. Corfield and L.A. Derry, A carbon isotope reference scale
for the Lower Cambrian Series in Siberia and its significance, Geol. Mag., in press, 1994.
A.Y. Rozanov and B.S. Sokolov, Lower Cambrian Stage
Subdivision Stratigraphy, 184 pp., Akad. Nauk SSSR,
Izdat. Nauka, Moscow, 1984.
A.Y. Rozanov and A.Y. Zhuravlev, The Lower Cambrian
fossil record of the Soviet Union, in: Origin and Early
Evolution of the Metazoa, J.E. Lipps and P.W. Signor,
eds., pp. 205-282, Plenum, New York, 1992.
U. Brand and J. Veizer, Chemical diagenesis of a multicomponent carbonate system-l: Trace elements, J. Sediment. Petrol. 50, 1219-1236, 1980.
W.H. Burke, R.E. Denison, E.A. Hetherington, R.B.
Keopnick, H.F. Nelson and J.B. Otto, Variation of seawater 87Sr/86Sr throughout Phanerozoic time, Geology
10, 516-519, 1982.
L.S. Keto and S.B. Jacobsen, Nd and Sr isotopic variations of Early Paleozoic oceans, Earth Planet. Sci. Lett.
84, 27-41, 1987.
D.A. Hodell, P.A. Mueller, J.A. McKenzie and G.A.
Mead, Strontium isotope stratigraphy and geochemistry
of the late Neogene ocean, Earth Planet. Sci. Lett. 92,
165-178, 1989.
J.M. Edmond, Himalayan tectonics, weathering processes, and the strontium isotope record in marine limestones, Science 258, 1594-1597, 1992.
F.M. Richter and K.K. Turekian, Simple models for the
geochemical response of the ocean to climatic and tectonic forcing, Earth Planet. Sci. Lett. 119, 121-131, 1993.
L.R. Kump, Alternative modeling approaches to the geochemical cycles of carbon, sulfur, and strontium isotopes,
Am. J. Sci. 289, 390-410, 1989.
S. Chaudhiri and N. Clauer, Fluctuations of isotopic
composition of strontium in seawater during the
Phanerozoic eon, Chem. Geol. (Isot. Geosci. Sect.) 59,
293-303, 1986.
J.R. Cochran, Himalayan uplift, sea level, and the record
of Bengal fan sedimentation at the ODP Leg 116 Sites,
Proc. Ocean Drill. Prog. Sci. Res. 116, 397-414, 1990.
K.V. Hodges, R.R. Parrish, T.B. Housh, D.R. Lux, B.C.
Burchfield, L.H. Royden and Z. Chen, Simultaneous
Miocene extension and shortening in the Himalayan orogen, Science 258, 1466-1470, 1992.
C. France-Lanord, L. Derry and A. Michard, Evolution
of the Himalaya since Miocene time: isotopic and sedimentologic evidence from the Bengal Fan, in: Himalayan
L.A. Derry et al. / Earth and Planetary Science Letters 128 (1994) 671-681
[32]
[33]
[34]
[35]
[36]
[37]
[38]
[39]
[40]
[41]
Tectonics, P.J. Treloar and M. Searle, eds., pp. 603-621,
Geol. Soc. London, London, 1993.
R.M. Miller, The Pan-African orogen of south west
Africa/Namibia, Spec. Publ. Geol. Soc. S. Afr. 11, 431515, 1983.
LG. Stanistreet, P.A. Kukla and G. Henry, Sedimentary
basinal responses to a Late Precambrian Wilson cycle:
the Damara Orogen and Nama foreland, Namibia, J. Afr.
Earth Sci. 13, 141-156, 1991.
U. Haak and H. Martin, Geochronology of the Damara
orogen - a review, in: Intracontinental Fold Belts, H.
Martin and F.W. Eder, eds., pp. 839-846, Springer, New
York, 1983.
F. McDermott and C.J. Hawkesworth, Intracrustal recycling and upper-crustal evolution: A case study from the
Pan-African Damara mobile belt, central Namibia, Chem.
Geol. 83, 263-280, 1990.
P.G. Gresse and G.J.B. Germs, The Nama foreland
basin: sedimentation, major unconformity bounded sequences and multisided active margin advance, Precambrian Res. 63, 241-272, 1993.
E.J. Thorogood, Provenance of the pre-Devonian sediments of England and Wales: Sm-Nd isotopic evidence,
J. Geol. Soc. London 147, 591-594, 1990.
L. Kump, Interpreting carbon-isotope excursions:
Strangelove oceans, Geology 19, 299-302, 1991.
F. Debrenne, Extinction of the Archaeocyatha, Hist.
Biol. 5, 95-106, 1991.
D.J. DesMarais, H. Strauss, R.E. Summons and J.M.
Hayes, Carbon isotope evidence for the stepwise oxidation of the Proterozoic environment, Nature 359, 605609, 1992.
A.C. Lasaga, Dynamic treatment of geochemical cycles:
Global kinetics, in: Kinetics of Geochemical Processes,
[42]
[43]
[44]
[45]
[46]
[47]
[48]
[49]
681
A.C. Lasaga and R.J. Kirkpatrick, eds., pp. 69-110, Mineral. Soc. Am., 1983.
P.A. Scholle and M.A. Arthur, Carbon isotope fluctuations in Cretaceous pelagic limestones: potential stratigraphic and petroleum exploration tool, Am. Assoc. Pet.
Geol. Bull. 64, 67-87, 1980.
M.A. Arthur, W.E. Dean and L.M. Pratt, Geochemical
and climatic effects of increased marine organic carbon
burial at the Cenomanian/Turonian
boundary, Nature
335, 714-717, 1988.
T.J. Bralower, W.V. Sliter, M.A. Arthur, R.M. Leckie, D.
Allard and S.O. Schlanger, Dysoxic/anoxic episodes in
the Aptian-Albian
(Early Cretaceous), in: The Mesozoic
Pacific: Geology, Tectonics and Volcanism, pp. 5-37,
Am. Geophys. Union, 1993.
M.D. Brasier, Nutrient-enriched waters and the early
skeletal fossil record, Geol. Soc. London 149, 621-629,
1992.
A.V. Ilyin, Proterozoic supercontinent, its latest Precambrian rifting, breakup, dispersal into smaller continents,
and subsidence of their margins: Evidence from Asia,
Geology 18, 1231-1234, 1990.
W.S. McKerrow, C.R. Scotese and M.D. Brasier, Early
Cambrian continental reconstructions, J. Geol. Soc. London 149, 599-606, 1992.
M.L. Delaney and G.M. Filipelli, An apparent contradiction in the role of phosphorous in Cenozoic chemical
mass balances for the world ocean, Paleoceanography 9,
513-527, 1994.
P.J. Cook and J.H. Shergold, Proterozoic and Cambrian
phosphorites - nature and origin, in: Proterozoic and
Cambrian phosphorites, P.J. Cook and J.H. Shergold,
eds., pp. 369-386, Cambridge Univ. Press, London, 1986.