Sr and C isotopes in Lower Cambrian carbonates from the Siberian craton: A paleoenvironmental record during the ‘Cambrian explosion’

EPSL ELSEVIER

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 Vandoeuvre-les-Nancy, France b Department of Earth Science6, Oxford University, 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 e a r l y M i d d l e 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 (~13Cvalues. Model results for the Sr and C isotopic records suggest that the quasi-periodicity in the ~13C 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 M e a s u r e m e n t s o f s t r o n t i u m isotopic c o m p o s i tions o f m a r i n e c a r b o n a t e s have m a d e significant

1 Present address: Department of Geological Sciences, Cornell University, Ithaca, New York, NY 14853-1504, USA, email: [email protected].

c o n t r i b u t i o n s to o u r u n d e r s t a n d i n g o f L a t e Proterozoic and Cambrian stratigraphy and paleoenv i r o n m e n t s . B e g i n n i n g with t h e w o r k of V e i z e r et al. [1], several studies have shown t h a t S7Sr/86Sr values in s e a w a t e r rose r a p i d l y in the V e n d i a n , from values b e l o w 0.707 to values n e a r 0.709 by the C a m b r i a n [2-6]. This r a p i d c h a n g e a p p e a r s b e c o m p a r a b l e to the C e n o z o i c i n c r e a s e in seawater Sr isotopic ratio in b o t h overall r a t e a n d

0012-821X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD! 0012-821X(94)00228-2

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L..d. 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/a6Sr 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]

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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 altered material (i.e. samples with 61SO < - 1 0 % o 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

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.

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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 ~180 < - 1 0 % 0 were not further analyzed; 87Sr/86Sr values are not correlated with 6180 in our sample subset (Fig. 2).

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C a O / M g O ratios permit the identification of dolomitized samples. All samples which show evidence of partial or complete dolomitization ( C a O / M g O weight ratio < 8) have high F e / S r and M n / S r ratios and yield 87Sr/86Sr values consistently higher than coeval limestones (Table 1). M n / S r and F e / S r 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/S6Sr 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 M n / S r _< 0.6; and (3) have F e / S r < 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

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L.A. Derry et al. / Earth and Planetary Science Letters 128 (1994) 671-681

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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 S7Sr/S6Sr was < 0.707 during the Neoproterozoic Varanger glaciation (ca. 605 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 878r/86Sr values. The majority of these samples have very high M n / S r

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ratios ( >> 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, 878r/868r 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 T o m m o t i a n (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-

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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 S7Sr/S6Sr increase, [2,5,6,25]. The cause of the apparent decline in seawater 87Sr/86Sr of 0.0004 to 0.0009 near the P r e c a m b r i a n - C a m b r i a n 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 SVSr/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 T o m m o t i a n - B o t o m i a n 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/S6Sr 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 T o m m o t i a n - A t d a banian 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/S6Sr 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/a6Sr increase could have been caused by increasing riverine 87Sr/S6Sr 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 878r/S6Sr to high values [7]. An analogous tectonic history appears to describe the PanAfrican D a m a r a - G a r i e p 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 D a m a r a - G a r i e p belt, just as the Himalaya have provided radiogenic Sr to the Neogene ocean. Avalonian events may also have contributed to rising seawater 878r/86Sr values. end 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 313C data imply that the fractional organic carbon burial flux reached as high as 30% in the Atdabanian-Botomian. The Sr isotope evi-

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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 6~3C 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 ~ 3 C provides some hints as to possible mechanisms. The most reasonable explanation for the marked variations observed in the Lower Cambrian ~13C 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%o fall in ~13C 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 g13C 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 SVSr/868r 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 (~13C 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 878r/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 (%) for C of 40 kyr, for Sr r 2 = 3 myr, and a sinusoidal forcing with a period (27r) 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 r 2 > 2~r >> %. 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. ---rr/2) and that, for a net 3%o 613C shift, there should be a shift of 0.00014 in STSr/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 6~3C 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 k'yr). 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

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Time, kyr Fig. 6. Response curves for 613C (dashed line) and S7Sr/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 AS7Sr (~S7Sr = [(87Sr/86Sr ~7 Sr/S6Srsta)/(S7Sr/86Srstd)] x 10 5, where S7Sr/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 ~t3C 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 613C 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 C e n o m a n i a n - T u r o n i a n and A l b i a n - A p t i a n 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 1-2 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

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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 613C record of such a large magnitude is not clear. Detailed stratigraphic work to establish correlations between 313C 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/~13C 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.'

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5. Conclusions Sr isotopic variations in Lower to Middle Cambrian carbonates show a rapid increase in seawater S7Sr/S6Sr 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 > 0.0004 in the SYSr/S6Sr 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 a n d / o r 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 D a m a r a - G a r i e p 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. [ F A J

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