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 GSA Today, v. 10, no. 5, May 2000

  May Table of Contents (including full issue in pdf format)


Figure 1
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Figure 4

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-African–Brasiliano 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 ~0.703–0.705. Conversely, continental weathering, via riverine and groundwater fluxes, contributes Sr with high 87Sr/86Sr values (0.709–0.730) 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.

Seawater 87Sr/86Sr 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.


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).


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).

Seawater 87Sr/86Sr 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 87Sr/86Sr 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 0.70918–0.70927 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 ~0.70925–0.70930 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).


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-African–Brasiliano 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 87Sr/86Sr 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 87Sr/86Sr 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-African–Brasiliano 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 Sea–South 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 values downward.

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 (~497–500 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).


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 and 87Sr/86Sr 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 values.

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 (60–250 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 commun., 2000).

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).

Carbonate d13C 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 and 87Sr/86Sr 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 and 87Sr/86Sr 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 87Sr/86Sr 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 Geology Foundation of the University of Texas at Austin.


Banner, J.L., and Kaufman, J., 1994, The isotopic record of ocean chemistry and diagenesis preserved in non-luminescent brachiopods from Mississippian carbonate rocks, Illinois and Missouri: Geological Society of America Bulletin, v. 106, p. 1074–1082.

Barnett, W., Armstrong, R.A., and De Wit, M., 1997, Stratigraphy of the Upper Neoproterozoic Kango and lower Palaeozoic Table Mountain Groups of the Cape fold belt revisited: South African Journal of Geology, v. 100, p. 237–250.

Bowring, S.A., and Erwin, D.H., 1998, A new look at evolutionary rates in deep time: Uniting paleontology and high-precision geochronology: GSA Today, v. 8, no. 9, p. 1–8.

Brasier, M.D., 1992, Towards a carbon isotope stratigraphy of the Cambrian System: Potential of the Great Basin succession, in Hailwood, E.A., and Kidd, R.B., eds., High resolution stratigraphy: Geological Society of London Special Publication 70, p. 341–350.

Brasier, M.D., and Sukhov, S.S., 1998, The falling amplitude of carbon isotopic oscillations through the Lower to Middle Cambrian: Northern Siberia data: Canadian Journal of Earth Sciences, v. 35, p. 353–373.

Burke, W.H., Denison, R.E., Hetherington, E.A., Koepnick, R.B., Nelson, H.F., and Otto, J.B., 1982, Variation in seawater 87Sr/86Sr throughout Phanerozoic time: Geology, v. 10, p. 516–519.

Curtis, M.L., and Storey, B.C., 1996, A review of the geological constraints on the pre-Gondwana break-up position of the Ellsworth Mountains: Implications for Weddell Sea evolution, in Storey, B.C., et al., eds., Weddell Sea tectonics and Gondwana break-up: Geological Society of London Special Publication 108, p. 11–30.

Curtis, M.L., Leat, P.T., Riley, T.R., Storey, B.C., Millar, I.L., and Randall, D.E., 1999, Middle Cambrian rift-related volcanism in the Ellsworth Mountains, Antarctica: Tectonic implications for the palaeo-Pacific margin of Gondwana: Tectonophysics, v. 304, p. 275–299.

Davidek, K., Landing, E., Bowring, S.A., Westrop, S.R., Rushton, A.W.A., Fortey, R.A., and Adrain, J., 1998, New uppermost Cambrian U-Pb date from Avalonian Wales and the age of the Cambrian-Ordovician boundary: Geological Magazine, v. 135, p. 303–309.

Denison, R.E., Koepnick, R.B., Burke, W.H., and Hetherington, E.A., 1998, Construction of the Cambrian and Ordovician seawater 87Sr/86Sr curve: Chemical Geology, v. 152, p. 325–340.

DePaolo, D.J., and Ingram, B.L., 1985, High-resolution stratigraphy with strontium isotopes: Science, v. 227, p. 938–941.

Derry, L.A., Brasier, M.D., Corfield, R.M., Rozanov, A.Y., and Zhuravlev, A.Y., 1994, Sr and C isotopes in Lower Cambrian carbonates from the Siberian craton: A paleoenvironmental record during the 'Cambrian explosion': Earth and Planetary Science Letters, v. 128, p. 671–681.

Edmond, J.M., 1992, Himalayan tectonics, weathering processes, and the strontium isotope record in marine limestones: Science, v. 258, p. 1594–1597.

Encarnacion, J., and Grunow, A., 1996, Changing magmatic and tectonic styles along the paleo-Pacific margin of Gondwana and the onset of early Paleozoic magmatism in Antarctica: Tectonics, v. 15, p. 1325–1341.

Goodge, J.W., Walker, N.W., and Hansen, V.L., 1993, Neoproterozoic-Cambrian basement-involved orogenesis within the Antarctic margin of Gondwana: Geology, v. 21, p. 37–40.

Grunow, A., Hanson, R., and Wilson, T., 1996, Were aspects of Pan-African deformation linked to Iapetus opening?: Geology, v. 24, p. 1063–1066.

Harris, N., 1995, Significance of weathering Himalayan metasedimentary rocks and leucogranites for the Sr isotope evolution of seawater during the early Miocene: Geology, v. 23, p. 795–798.

Hayes, J.M., Straus, H., and Kaufman, A.J., 1999, The abundance of 13C in marine organic matter and isotopic fractionation in the global biogeochemical cycle of carbon during the past 800 Ma: Chemical Geology, v. 161, p. 103–125.

Hodell, D.A., Mead, G.A., and Mueller, P.A., 1990, Variation in the strontium isotopic composition of seawater (8 Ma to present): Implications for chemical weathering rates and dissolved fluxes to the oceans: Chemical Geology, v. 80, p. 291–307.

Jacobsen, S.B., and Kaufman, A.J., 1999, The Sr, C, and O isotopic evolution of Neoproterozoic seawater: Chemical Geology, v. 162, p. 37–57.

Kajiwara, Y., Yamakita, S., Ishida, K., Ishiga, H., and Imai, A., 1994, Development of a largely anoxic stratified ocean and its temporary massive mixing at the Permian-Triassic boundary supported by the sulfur isotopic record: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 111, p. 367–379.

Kaufman, A.J., Jacobsen, S.B., and Knoll, A.H., 1993, The Vendian record of Sr- and C-isotopic variations in seawater: Implications for tectonics and paleoclimate: Earth and Planetary Science Letters, v. 120, p. 409–430.

Kaufman, A.J., Knoll, A.H., Semikhatov, M.A., Grotzinger, J.P., Jacobsen, S.B., and Adams, W., 1996, Integrated chronostratigraphy of Proterozoic-Cambrian boundary beds in the western Anabar region, northern Siberia: Geological Magazine, v. 133, p. 509–533.

Kirschvink, J.L., Ripperdan, R.L., and Evans, D.A., 1997, Evidence for a large-scale reorganization of Early Cambrian continental masses by inertial interchange true polar wander: Science, v. 277, p. 541–545.

Kump, L.R., 1991, Interpreting carbon-isotope excursions: Strangelove oceans: Geology, v. 19, p. 299–302.

Kump, L.R., and Arthur, M.A., 1999, Interpreting carbon-isotope excursions: Carbonates and organic matter: Chemical Geology, v. 161, p. 181–198.

Landing, E., and Bartowski, K.E., 1996, Oldest shelly fossils from the Taconic allochthon and late Early Cambrian sea-levels in eastern Laurentia: Journal of Paleontology, v. 70, p. 741–761.

Montañez, I.P., Banner, J.L., Osleger, D.A., Borg, L.E., and Bosserman, P.J., 1996, Integrated Sr isotope stratigraphy and relative sea-level history in Middle Cambrian platform carbonates: Geology, v. 24, p. 917–920.

Palmer, A.R., 1998, A proposed nomenclature for stages and series for the Cambrian of Laurentia: Canadian Journal of Earth Sciences, v. 35, p. 323–328.

Palmer, M.R., and Edmond, J.M., 1989, The strontium isotope budget of the modern ocean: Earth and Planetary Science Letters, v. 92, p. 11–26.

Railsback, L.B., Ackerly, S.C., Anderson, T.F., and Cisne, J.L., 1990, Palaeontological and isotope evidence for warm saline deep waters in Ordovician oceans: Nature, v. 343, p. 156–159.

Richter, F.M., Rowley, D.B., and DePaolo, D.J., 1992, Sr isotope evolution of seawater: The role of tectonics: Earth and Planetary Science Letters, v. 109, p. 11–23.

Saltzman, M.R., Runnegar, B., and Lohmann, K.C, 1998, Carbon isotope stratigraphy of Upper Cambrian (Steptoean Stage) sequences of the eastern Great Basin: Record of a global oceanographic event: Geological Society of America Bulletin, v. 110, p. 285–297.

Unrug, R., 1997, Rodinia to Gondwana: The geodynamic map of Gondwana Supercontinent assembly: GSA Today, v. 7, no. 1, p. 1–6.

Veizer, J., and 14 others, 1999, 87Sr/86Sr, d13C, and d18O evolution of Phanerozoic seawater: Chemical Geology, v. 161, p. 59–88.

Wilde, P., and Berry, W.B.N., 1984, Destabilization of the oceanic density structure and its significance to marine 'extinction' events: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 48, p. 143–162.

Manuscript received February 17, 2000; accepted March 10, 2000.

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