Evolution of the Sr and C Isotope Composition of Cambrian Oceans
Isabel P. Montañez,
David A. Osleger, Department
of Geology, University of California, Davis, CA 95616, USA
Jay L Banner, Larry
E. Mack, MaryLynn Musgrove, Department
of Geological Sciences, University of Texas, Austin, TX 78712, USA
The recent proliferation of chemostratigraphic studies has clearly documented
that systematic fluctuations in the strontium and carbon isotope composition of
seawater have occurred throughout Earth history across a range of temporal scales.
In particular, significant isotopic variation during key intervals of geologic
time has provided unprecedented quantitative constraints on crustal and surficial
processes, and enhanced chronostratigraphic resolution for intrabasinal and interbasinal
correlations. We present the first set of high-resolution, seawater Sr and C isotope
curves for the late Early through early Late Cambrian. These curves are defined
in continuous exposures of marine carbonates in the Great Basin and southern Canadian
Rockies, and they are used to better constrain primary variations in ocean chemistry
during this time period. The Sr curve documents a rapid rate of increase through
this period that is comparable to that recorded by the late Cenozoic seawater
Sr proxy record of uplift and attendant weathering of the Himalaya-Tibetan Plateau.
The Cambrian rise in Sr values is interpreted to record Pan-AfricanBrasiliano
orogenesis, and reaches 87Sr/86Sr
values that are higher than at any other time in Earth history. Numerous superimposed,
smaller-scale oscillations may constrain the timing of individual short-term tectonic
events. The C isotope curve for the same time interval reveals several previously
unrecognized short-term fluctuations of up to 4. A sharp shift in d13C
values near the Early-Middle Cambrian boundary indicates major paleoceanographic
and climatic change associated with a trilobite mass extinction event. Used in
concert, this set of high-resolution 87Sr/86Sr
and d13C records
provides complementary quantitative constraints for the chronology of Cambrian
tectonic, paleoceanographic, paleoecologic, and biogeochemical events.
Systematic variations in the Sr (87Sr/86Sr)
and C (d13C) isotopic
composition of seawater have been documented by numerous chemostratigraphic studies
across a range of temporal scales (e.g., Burke et al., 1982; Jacobsen and Kaufman,
1999; Hayes et al., 1999; Veizer et al., 1999). These "secular" isotopic variations
are valuable proxies of paleoenvironmental change and paleotectonic events. They
also provide valuable means of chronostratigraphic correlation in intervals plagued
by a paucity of age-diagnostic biostratigraphic markers. The utility of 87Sr/86Sr
and d13C values in
well-preserved carbonate rocks as proxies resides in the processes that they record.
Temporal variation in seawater 87Sr/86Sr
is governed mainly by changes in the balance between Sr fluxes to the ocean from
continental weathering and hydrothermal fluid-rock interaction at mid-ocean ridges
(Palmer and Edmond, 1989). The 87Sr/86Sr
composition of the volumetrically smaller seafloor hydrothermal flux is buffered
at low values of ~0.7030.705. Conversely, continental weathering, via riverine
and groundwater fluxes, contributes Sr with high 87Sr/86Sr
values (0.7090.730) to the ocean. It is primarily the variation in the larger
continental flux and its isotopic composition that has affected secular change
in seawater 87Sr/86Sr
values. The 87Sr/86Sr
values of unaltered marine carbonate minerals directly record seawater 87Sr/86Sr
values, given the negligible fractionation of Sr isotopes (Banner and Kaufman,
1994) and the homogeneity of Sr in seawater (DePaolo and Ingram, 1985).
Ocean-scale variation in the d13C
composition of surface seawater on time scales of >105 yr primarily arises
from changes in the long-term throughput of carbon in the ocean and the isotopic
fractionation associated with partitioning of carbon into reduced and oxidized
reservoirs (Kump, 1991; Kump and Arthur, 1999). These mechanisms are driven by
continental weathering, global sedimentation rates, primary productivity, organic
carbon burial, and ocean circulation mode, all of which in turn modulate atmospheric
pCO2 and hence climate. Thus, 87Sr/86Sr
and d13C records
are highly complementary proxies of surficial and crustal cycling as well as paleoclimate
and paleoecologic change.
and d13C records
for a number of important intervals of pre-Cretaceous time are of relatively low
resolution, thus compromising their utility as paleoenvironmental proxies. The
low resolution of these curves primarily reflects significant age uncertainties
and the difficulty of obtaining a reliable marine signal due to diagenetic effects.
In particular, the existing isotope curves for the late Early through early Late
Cambrian need to be significantly refined, given that this was a time of dramatic
change in Earth systems. Large-scale continental reorganization associated with
the amalgamation of Gondwana (Unrug, 1997) and anomalously fast rotation (up to
90°) and latitudinal drift of continents (>30 cm/yr), driven by an inertial interchange
true polar wander event (Kirschvink et al., 1997), characterize this time interval.
This tectonic forcing likely generated major changes in oceanic circulation, geochemistry,
and primary productivity, as well as enhanced rates of continental weathering
and organic carbon burial. Thus, accurate isotope curves are imperative for interpreting
such significant environmental change.
To that end, we present detailed 87Sr/86Sr
and d13C curves derived
from continuous exposures of marine carbonate strata in the Great Basin and southern
Canadian Rockies that significantly refine the resolution of existing Cambrian
trends (Fig. 1). These newly defined seawater curves document previously unrecognized
fluctuations, revealing a more dynamic evolution of Cambrian ocean chemistry than
defined to date. Used in concert, these high-resolution 87Sr/86Sr
and d13C records
offer an unprecedented level of chronostratigraphic resolution for intrabasinal
and interbasinal correlation and refined paleogeographic reconstructions, as well
as provide quantitative geochemical constraints on paleoenvironment and paleoclimate
change during the Cambrian.
CONSTRUCTION OF SEAWATER ISOTOPE CURVES
Cambrian carbonates in the Great Basin and southern Canadian Rockies have a
complex burial history and hence potential for postdepositional chemical alteration.
Thus, all samples in this study were petrographically and geochemically evaluated
using previously defined criteria (Banner and Kaufman, 1994; Montañez et al.,
1996) believed to be most effective in identifying components with the highest
potential for yielding primary marine values. All samples were pretreated using
ultrapure ammonium acetate, a cation exchange solution that preferentially removes
readily leachable Sr from the surfaces, lattice, or fluid inclusions of Rb-rich
clays and oxides, which are commonly associated in trace amounts with marine carbonates
(description of methods available from Montañez et al. on request). Penecontemporaneous
marine cements in algal bioherms and grainstones, and secondarily, very finely
crystalline micrites were found to yield a best estimate of seawater 87Sr/86Sr
and d13C values.
Relative age assignment of data in this study is based on the stratigraphic
position of samples relative to one another within thick continuous sections.
Constrained by all available biostratigraphy, samples were merged into a composite
stratigraphic section constructed from ten sections in the southern Canadian Rockies
and five sections in the Great Basin. The composite section was proportioned along
a linear time axis by stratigraphic position of the chronometrically defined Lower
to Middle Cambrian (509 Ma) and Middle to Upper Cambrian (500 Ma) boundaries (Bowring
and Erwin, 1998; Davidek et al., 1998).
SECULAR Sr ISOTOPE CURVE
The temporal variation in the Sr isotope composition of late Early through
early Late Cambrian oceans is defined by the 87Sr/86Sr
values of best-preserved calcite marine components plotted on the most recent
Cambrian time scale (Fig. 2). Our interpretation of the isotope trend as "secular"
(i.e., recording temporal variation in the isotopic composition of the global
oceans) is supported by the consistency of isotope trends between the two passive-margin-setting
study areas separated by ~1600 km. The temporal resolution of the curves is optimized
by the extraordinary stratigraphic continuity of the sampled sections (hundreds
to thousands of meters; Fig. 1). Hence, this is the first continental-scale correlation
of such temporally extensive and continuous Paleozoic Sr isotope trends and, as
such, provides the most rigorous assessment of the global nature of the seawater
Sr isotope curve. Further confidence for the veracity of the curve is provided
by multiple sets of contemporaneous samples that have overlapping 87Sr/86Sr
values (Fig. 2).
values through the late Early to early Late Cambrian define a non-monotonic rise,
the values varying between a minimum of ~0.7089 and a maximum of ~0.7094 (Fig.
2). In this study, minimum 87Sr/86Sr
values (0.70886-0.70895) characterize the latest Early Cambrian, and overlap with
values previously defined from latest Early Cambrian carbonates of the Siberian
platform (Derry et al., 1994). Seawater 87Sr/86Sr
values may have risen rapidly to peak values of 0.709180.70927 coincident with
the Early-Middle Cambrian transition. This short-term rise is defined tenuously,
given the paucity of reliable samples available in this interval. This proposed
rise in 87Sr/86Sr
values warrants further evaluation, given that elevated 87Sr/86Sr
values define this interval in several sections throughout the Cordilleran margin
(this study; E. Fermann, 1999, personal commun.).
In the early Middle Cambrian, 87Sr/86Sr
values decrease to a low point of ~0.7089 (mid-Glossopleura biozone),
and then progressively increase through the latter half of the Middle Cambrian
to a maximum of ~0.709250.70930 in the Late Cambrian Cedaria
biozone. These maximum 87Sr/86Sr
values are followed by a rapid decline to ~0.70920 before rising again to
the highest recorded seawater values (ca. 0.70940) around 498 Ma. Notably, this
maximum value is 0.00023 higher than that of present-day seawater (87Sr/86Sr
= 0.709174, adjusted to a NBS-SRM 987 value of 0.710250; DePaolo and Ingram, 1985).
TECTONIC AND CLIMATIC IMPLICATIONS OF CAMBRIAN SEAWATER 87Sr/86Sr
The latest Early to early Late Cambrian seawater Sr isotope curve presented
here is the continuation of a previously defined long-term rise in seawater 87Sr/86Sr
values that began around 0.7063 in the late Neoproterozoic and continued through
the Early Cambrian (Kaufman et al., 1993, 1996; Derry et al., 1994; Jacobsen and
Kaufman, 1999). This rise (rate of 0.00002/m.y.) is interpreted to record uplift
and attendant increased weathering associated with the Pan-AfricanBrasiliano
orogeny (Edmond, 1992; Richter et al., 1992; Kaufman et al., 1993; Derry et al.,
1994). The overall rate of rise in the newly defined part of the Cambrian seawater
curve (0.00004/m.y.; Fig. 2) exceeds that of the preceding interval and is comparable
to the Cenozoic rate over the past 23 m.y. (Fig. 3). The rapid rise in seawater
values over the late Cenozoic is interpreted to record the cumulative effects
of uplift of the Himalaya-Tibetan Plateau on erosion rate, climate, and continental
weathering (Hodell et al., 1990; Richter et al., 1992). By analogy, the rapid
Cambrian trend suggests that tectonic control on riverine Sr flux and its isotopic
composition was the dominant driving mechanism. Increased erosion and silicate
weathering associated with the Himalayan-scale, Pan-African-Brasiliano orogeny
would have significantly increased the flux of radiogenic 87Sr to the oceans.
Our quantitative modeling suggests that an increase in the 87Sr/86Sr
composition, rather than the magnitude of the riverine Sr flux, would have been
required to maintain the estimated rate of rise and to attain the very high 87Sr/86Sr
values observed for the early Late Cambrian. The Pan-AfricanBrasiliano orogeny,
of late Neoproterozoic through Cambrian age, resulted in convergence and metamorphism
of previously rifted Archean-Mesoproterozoic craton margins (Unrug, 1997). Uplift
would have resulted in deep exhumation and erosion of these strongly metamorphosed
mobile belts. The Sr released during weathering of these highly metamorphosed
cratonal rocks would have been significantly more radiogenic than Sr released
by weathering of average global continental rocks (cf. Edmond, 1992; Harris, 1995),
and could have rapidly increased the 87Sr/86Sr
composition of the riverine flux.
The evolutionary trend in Cambrian seawater 87Sr/86Sr
values differs from the late Cenozoic in that the Cambrian curve exhibits considerable
variability (Fig. 3). It is possible that recognizable Sr isotope fluctuations
superimposed on the longer-term trend may provide constraints for the timing of
discrete tectonic phases of the Pan-African-Brasiliano orogeny, for which considerable
uncertainty exists. A tentatively defined short period of increased rate of rise
(greater than or equal to 0.0002 in ~1 m.y.) across the Early-Middle Cambrian transition is followed by
a progressive fall in 87Sr/86Sr
values over ~4 m.y. of early Middle Cambrian time. This decrease in seawater 87Sr/86Sr
values is possibly coincident with an episode of widespread rifting along the
Weddell SeaSouth African sector of the paleo-Pacific margin of Gondwana and the
western margin of Laurentia (Grunow et al., 1996; Barnett et al., 1997; Curtis
et al., 1999). Continental rifting and associated mafic to alkaline magmatism
began during the latest Early Cambrian and extended into the Middle Cambrian.
During this period, we infer a decrease in the 87Sr/86Sr
composition of the continental Sr flux to the ocean, due to weathering of these
young mantle-derived mafic rocks, coupled with an increase in the hydrothermal
Sr flux, due to seawater interaction with MORB-like basalts at the subaqueous
rift axes. Both processes would have driven seawater 87Sr/86Sr
The subsequent rapid rise during the latter half of the Middle Cambrian to
peak values in the early Late Cambrian is interpreted to record the large magnitude
of coeval orogenic events in Antarctica and Australia (Goodge et al., 1993; Curtis
and Storey, 1996; Encarnacion and Grunow, 1996). If these proposed relationships
can be further documented, then the shift from low seawater 87Sr/86Sr
values to increasing values during the middle Middle Cambrian could place constraints
on the timing of changing tectonic styles along the margins of Gondwana. Finally,
the relatively short time span of the Cambrian radiogenic extreme (~497500 Ma)
may constrain the timing of the terminal phase of Pan-African orogenesis to the
latest Cambrian (cf. Encarnacion and Grunow, 1996; Barnett et al., 1997).
INTEGRATION OF Sr AND C ISOTOPE CURVES
The d13C values
of least-altered carbonate components define the first high-resolution, seawater
secular C isotope curve for Middle Cambrian time (Fig.
4). Seawater d13C
values through this time interval exhibit high-frequency fluctuations (shifts
of up to >4) around a mean value of 0.5. Although the precise
form of the long-term d13C
trends differ, both curves define a broadly similar progressive fall in values
during the early Middle Cambrian that reach minima at different times in the middle
Middle Cambrian (Glossopleura) (Fig.
4). A rapid increase in d13C
values at the youngest part of the curve (Crepicephalus-Aphelaspis biozones)
marks the initiation of a globally recognized positive C isotope excursion (inset,
Fig. 4) (Brasier, 1992;
Saltzman et al., 1998). This rapid rise in seawater d13C
values is matched by a coincident rise in 87Sr/86Sr
Our secular C isotope curve defines a previously undocumented, rapid (~100
k.y.), large-magnitude shift (greater than or equal to 4) to negative d13C
values in the terminal Early Cambrian. This negative C isotope excursion begins
just prior to the oldest known mass extinction of trilobites and other less common
community elements recorded at the Lower-Middle Cambrian boundary (Palmer, 1998).
It is possible that the negative C isotope excursion is of even higher magnitude
than currently defined, given that the Lower-Middle Cambrian boundary in our sampled
sections is underlain by several meters of shale with few intercalated carbonates.
This interval is being further evaluated by C isotope analyses of organic matter
in shales and carbonates. The temporal relationship of the negative C isotope
excursion to the tentatively defined rapid rise in seawater 87Sr/86Sr
values during the terminal Early Cambrian cannot be clearly resolved, given that
the C isotope trend was defined, in part, by marine components that were not suitable
for Sr isotope analysis.
In the Great Basin sections (Split Mountain and Echo Canyon) where the C isotope
anomaly is best defined, the Early to Middle Cambrian transition is considered
relatively conformable, on the basis of preservation of all trilobite zones and
lack of evidence of erosion. Erosion of time-equivalent successions worldwide
during an early Middle Cambrian regression explains why this negative C isotope
excursion has not been previously recognized despite it being among the largest
magnitude Phanerozoic excursions. This isotope excursion is recorded in the Great
Basin by the topmost few meters of thick (60250 m) carbonate successions and
sharply overlying, highly condensed shales with thin intercalated carbonate beds
that extend across the Early to Middle Cambrian boundary. The carbonate-shale
stratigraphic relationship records a rapid rise in relative sea level, which was
previously recognized in other Laurentian and Siberian sections (Brasier and Sukhov,
1998; Landing and Bartowski, 1996). Evidence of prolonged environmental stress
during the negative C isotope excursion is indicated by the lack of bioturbation
in shales and the occurrence of carbonate shell beds that are interpreted to record
episodic deposition of trilobite death assemblages (L. and M. McCollum, personal
The negative C isotope excursion indicates that major paleoceanographic changes,
and probably climatic changes, preceded the mass extinction at the end of the
Early Cambrian. The negative C isotope excursion is interpreted as recording (1)
the introduction of 13C-depleted, anoxic waters onto shallow-water carbonate platforms
during the latest Early Cambrian transgression, and (2) the associated decrease
in organic C burial due to major biomass reduction (cf. Wilde and Berry, 1984;
Kajiwara et al., 1994). An anoxic water column below the surface mixed layer may
have developed in Early Cambrian oceans during the transgression, accompanied
by sluggish circulation and strong stratification. These oceanic conditions would
be favored by the low-latitude continentality and depressed meridional temperature
gradients that likely characterized Early Cambrian greenhouse time (Railsback
et al., 1990). In order to sustain the negative isotope excursion and prolong
exposure of shallow-marine organisms to environmental stress, transgression may
have occurred in a pattern of stepwise onlap, thus episodically introducing pulses
of toxic waters from the anoxic layer onto normally ventilated parts of the platforms
(cf. Wilde and Berry, 1984).
values at the peak of the negative isotope excursion (< 4) suggest that
the ocean's isotopic composition ultimately approached that of the weathering
input to the oceans (d13C
of ~5; Kump, 1991). This proposed increased influence of the riverine flux
on the ocean's isotopic composition may reflect greatly reduced primary productivity
and organic C burial rates in response to building environmental stress prior
to the mass extinction. The subsequent shift to more positive d13C
values in the earliest Middle Cambrian likely records the effects of (1) contraction
of the oxygen-minimum zone during the subsequent early Middle Cambrian sea-level
fall and attendant enhanced oceanic circulation, and (2) recovery of surface water
productivity levels after environmental conditions improved sufficiently.
Superimposed on the long-term C and Sr trends are other short-term (±1
m.y.) fluctuations that are interpreted to record high-frequency changes in seawater
d13C and 87Sr/86Sr
values, given that they are defined in several sections throughout the Cordilleran
passive margin (Fig. 4).
Some of the short-term d13C
fluctuations (e.g., Bolaspidella and Cedaria biozones) exhibit similar
but out-of-phase trends. The short-term increases in seawater 87Sr/86Sr
values during this interval could reflect the influence of increased continental
flux to the ocean. In turn, the associated short-term increases in d13C
values may record increased oceanic nutrient levels, primary productivity, and
organic C burial driven by increased continental flux and associated oceanic sedimentation.
Conversely, periods of dampened surface water fluxes to the ocean and lowered
global oceanic sedimentation rates could result in decreased seawater d13C
values. The mechanism linking these processes could have been short-term Cambrian
sea-level oscillations (Montañez et al., 1996) or short-lived tectonic events
and their effect on paleoceanographic conditions and organic carbon burial.
The high-resolution Sr and C isotope curves presented in this paper significantly
refine our understanding of the isotopic evolution of Cambrian seawater. These
isotope curves document previously unrecognized fluctuations that strongly suggest
periods of significant perturbation to the global Sr and C cycles during the late
Early through early Late Cambrian. We suggest that future studies of Cambrian
successions focus on certain biostratigraphically constrained intervals in order
to (1) better define these short-term events in other basins as a test of their
validity, (2) test the proposed relationships between observed changes in seawater
and d13C values and
crustal and surficial processes, and (3) refine the structure of less well-defined
portions of the curves. Future radiometric studies are likely to elucidate the
timing of discrete tectonic phases and the temporal relationships between these
events and variations in seawater 87Sr/86Sr
and d13C values.
In turn, the new Sr and C isotope curves may clarify the mechanistic links between
tectonic events, oceanic processes, and paleoclimatic conditions, and allow for
determination of their rates of change.
We thank Eric Mountjoy, Linda McCollum, Mike McCollum, and Pete Palmer for
their contributions to our understanding of Cambrian field relations. C. Lehmann,
N. Tabor, and K. Tambo assisted with field sampling and data collection. J. Fong
helped with illustration design. Detailed reviews by A.J. Kaufman, K. Bice, and
M. Miller helped us to improve the manuscript. This research was supported by
National Science Foundation grants to Montañez, Osleger, and Banner, and by the
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Manuscript received February 17, 2000;
accepted March 10, 2000.
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