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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 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 Tommo- tian-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 Tommotian- Atdabanian 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 contributions to our understanding of Late Pro- terozoic and Cambrian stratigraphy and paleoen- Measurements of strontium isotopic composi- vironments. Beginning with the work of Veizer et tions of marine carbonates have made significant 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 1 Present address: Department of Geological Sciences, Cor- nell University, Ithaca, New York, NY 14853-1504, USA, be comparable to the Cenozoic increase in seawa- email: [email protected]. ter Sr isotopic ratio in both overall rate and
0012-821X/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0012-821X(94)00228-2 672 L.A. Derry et al. / Earth and Planetary Science Letters 128 (1994) 671-681 magnitude. The changes in the Sr isotopic com- position in both Cenozoic and Neoproterozoic Zone seawater appear to reflect increased erosion rates, Schistccephalus Dominant resulting from major continental collisions and \Anabaraspis lithologies orogeny during both eras [2,6,7]. The Neopro- splendens terozoic increase probably also reflects changes in Lomontovia the rate of seafloor hydrothermal input to the grandis oceans [1,2]. However, the relationship between Thin to medium orogenesis, erosion rate and seawater Sr isotopic 6 ketemensis beddedlimestone change is not completely understood. The Neo- B. ornata proterozoic-Cambrian transition offers a poten- Bergeromellus tial analog to the Cretaceous-Cenozoic interval asIaticus of increasing seawater 87Sr/86Sr ratio and, thus, Bergeroniellus may be used to test competing hypotheses for Q”WXll major Sr isotopic change in seawater [6]. 3 micmacciformis Judomla/ The Neoproterozoic Sr record has also been UMaspis limestones shown to be of great interest for interpreting Pagetiellus paleoenvironments during this key interval of anabarus Earth history [8]. Variations in globally averaged Fallotasois erosion/sedimentation rates are an important control in the cycling of nutrient elements and sedimentary carbon and, thus, appear to play a Medium to thick key role in controlling variations in both primary \ “Purella” bedded dolomite Anabarltes productivity and organic carbon burial [9]. Car- trwulatus bon isotope studies and the occurrence of sedi- mentary phosphorite deposits suggest that or- ganic carbon burial and phosphorous fluxes may have varied widely during the ‘Cambrian explo- Fig. 1. Composite stratigraphic column with biozones of sedi- mentary sections along the Lena River, Siberia [16,18]. Com- sion’ of the Lower to Middle Cambrian [10-16]. posite stratigraphic height (in meters) marked on right. Quantitative models of biogeochemical cycling in this remarkable interval require better constraints on erosion rates and weathering fluxes. have yielded a suite of sedimentary carbonate In this study we present Sr isotopic measure- samples suitable for isotope chemostratigraphy ments from a suite of carbonate samples from the [16,18]. Carbonate samples were selected for Lower and Middle Cambrian of the Siberian plat- analysis so as to provide good stratigraphic cover- form. Sediments of the Siberian platform have age through the Tommotian, Atdabanian, Boto- provided data for several recent studies of Pre- mian and’ Toyonian type sections (Fig. 1). Stable cambrian-Cambrian biostratigraphy, isotopic measurements made on a larger sample chemostratigraphy and geochronology [12,16-18]. set [16] were used to avoid the most obviously Our data help fill an important gap in the emerg- altered material (i.e. samples with 6”O < ing geochemical record of paleoenvironments - 10%~ were not chosen). Samples thus selected from the Neoproterozoic and Cambrian interval. were gently crushed, and clean fragments hand picked. These fragments were crushed to powder in a stainless steel mortar and ca. 20 mg was 2. Samples and methods dissolved in 10% ultrapure acetic acid. Insoluble residues were separated by centrifugation, dried Very gently dipping and little deformed strata and weighed. Sr was separated by standard ion along the Aldan and Lena Rivers of Siberia [19,20] 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 absorp- tion.
3. Results + No one geochemical or textural indicator has been shown to be an infallible test for the degree Age, Ma of preservation of primary Sr isotopic signatures Fig. 3. 87Sr/86Sr variations in Lower Cambrian carbonates of in ancient carbonates. In addition to petrographic the Siberian platform, plotted on the Lower Cambrian time- and stable isotopic screening, we use the relative scale of Bowring et al. [17]. Vertical dashed lines mark ages of abundances of Mn, Fe and Sr as indicators of stage boundaries as estimated by [17]. w = samples that meet geochemical selection criteria described in text; 0 = samples post-depositional alteration of the carbonates with elevated Fe/Sr and/or Mn/Sr values, which are consid- [1,8,21]. Samples with al80 < - 10%0 were not ered as altered. Values > 0.709 not shown. further analyzed; 87Sr/86Sr values are not corre- lated with 6”O in our sample subset (Fig. 2). CaO/MgO ratios permit the identification of dolomitized samples. All samples which show evi- dence 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 indi- cators of alteration of Sr isotopic values (Fig. 2). The percent dissolution is not correlated with 87Sr/86Sr ratios (Table 1). Based on the evalua- tion 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 indepen- dently. The Sr isotopic data are plotted on a time axis Mn/Sr (Fig. 3), using the Lower Cambrian time-scale Fig. 2. (a) s’s0 versus 87Sr/86Sr values for Lena River proposed by Bowring et al. [17]. It should be samples. (b) Fe/Sr versus Mn/Sr for carbonate samples. noted that, while the basal Nemakit Daldyn, basal Hatchured area outlines the region of apparently best pre- Tommotian and Atdabanian-Botomian bound- served samples for Sr isotopic analysis. High values of Fe/Sr aries are reasonably well dated, the age of the (> 5) and Mn/Sr (> 2) are primarily found in dolomitized samples (open symbols), but can also indicate alteration in Lower-Middle Cambrian boundary is at present limestones. not well known. An empirical subsidence curve Table 1 Analytical data for Lower Cambrian Lena River carbonates
Sample Stratigraphic Location 87Sr/86Sr dissol. CaO MgO Mn Fe Sr Mn/Sr Fe/Sr CaO/ 6’3C 6’80 No. height, m % wt. % wt. % ppm ppm ppm MgO
E22 916.3 Elanka 0.708680 98 53.77 1.91 54 150 165 0.33 0.91 28.2 0.86 -8.75 E20 905.0 Elanka 0.708753 100 56.17 0.32 32 91 190 0.17 0.48 175 1.24 -5.44 El7 895.0 Elanka 0.708648 99 50.83 4.34 67 235 130 0.52 1.81 11.7 0.51 -6.01 EllA 879.9 Elanka 0.708647 98 34.19 17.54 137 770 210 0.65 3.67 1.95 0.89 -5.54 W55/32g 819.8 Titary 0.708531 97 55.25 0.44 23 65 160 0.14 0.41 125 -0.57 -1.12 W56/11A 794.7 Titary 0.708556 100 54.77 0.55 23 62 300 0.08 0.21 99.6 -0.33 -6.57 W56/1Z 769.6 Titary 0.708503 100 55.68 0.43 21 84 230 0.09 0.37 129 -0.67 -6.71 W43/1A 653.1 Titary 0.708455 97 53.90 1.23 21 74 550 0.04 0.13 43.8 -1.75 -6.50 LAB15 644.3 Labaya 0.708461 100 53.55 1.74 25 120 355 0.07 0.34 30.8 -1.06 -6.58 LAB9 589.1 Labaya 0.708440 96 51.21 4.3 32 115 295 0.11 0.39 11.9 -1.24 -6.61 LAB1 560.3 Labaya 0.708447 100 52.61 2.94 29 135 305 0.10 0.44 17.9 -1.06 -7.46 AT20-141 551.5 AKT 0.708462 97 44.24 9.37 51 100 250 0.20 0.40 4.72 0.51 -7.10 AT10-50 462.5 AKT 0.708703 92 31.95 16.43 219 1440 150 1.46 9.60 1.94 2.16 -5.92 AT5-37 451.2 AKT 0.708650 87 39.18 6.6 195 1515 205 0.95 7.39 5.94 2.39 -4.71 AT4-36 441.2 AKT 0.708646 85 26.78 15.39 232 6070 150 1.55 40.47 1.74 -1.07 -6.87 AKT20-87 436.2 AKT 0.708617 92 45.57 3.18 202 1130 240 0.84 4.71 14.3 -0.26 -5.46 AKT20-77 431.2 AKT 0.708493 100 59.28 2.46 59 320 1170 0.05 0.27 24.1 -0.94 -6.53 AKT19-71 426.2 AKT 0.708841 87 28.22 15.99 189 2380 135 1.40 17.63 1.76 0.81 -5.51 AKT14-53 408.6 AKT 0.708436 98 53.54 0.95 119 675 280 0.43 2.41 56.4 -0.22 -5.44 AKTlO-45 401.1 AKT 0.708504 75 43.56 1.92 304 410 235 1.29 1.74 22.7 1.27 -5.98 AKTAl 351.0 AKT 0.708287 97 54.94 0.51 97 600 300 0.32 2.00 107 -0.01 -5.89 17E 283.3 lsit 0.708191 88 47.88 1.75 82 720 305 0.27 2.36 27.4 -1.54 -7.15 16A 270.7 lsit 0.708573 73 27.38 11.81 160 2890 180 0.89 16.06 2.32 -1.62 -7.45 BB-2 228.1 Zhurinsky 0.708059 91 50.56 0.91 82 270 480 0.17 0.56 55.6 10A 194.3 lsit 0.708333 71 39.74 0.68 116 460 155 0.75 2.97 45.2 -2.00 -6.31 US12A 188.0 US 0.708550 95 30.58 20.41 157 685 58 2.71 11.81 1.50 1.42 -6.17 1A 183.0 lsit 0.708123 92 47.19 5.04 90 795 250 0.36 3.18 9.36 0.58 -6.29 US3A 175.5 US 0.708673 88 27.61 18.79 217 1055 58 3.74 18.19 1.47 2.34 -4.77 AK63 155.4 Mt. Konus 0.709471 86 26.21 17.24 153 1840 94 1.63 19.57 1.52 1.87 -4.96 AK651 122.8 Mt. Konus 0.710127 77 16.39 10.09 85 1150 115 0.74 10.00 1.62 1.05 -4.40 AK747 97.8 Mt. Konus 0.708438 81 23.56 15.69 236 1710 81 2.91 21.11 1.50 -0.07 -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) 671-681 675 was used to interpolate ages of samples between Cretaceous-Tertiary age, Ma ‘known’ tie points. After exclusion of samples which failed to pass the geochemical ‘tests’ for alteration listed above, the Sr data define a rela- tively smooth curve. Relatively low 87Sr/86Sr val- ues ( = 0.7081) appear to characterize Tommotian samples reliably. From these low Tommotian val- ues, 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 geo- Fig. 4. Sr isotopic evolution of Vendian to mid-Cambrian chemical screening, agreement between strati- seawater. Heavy line (KJK) = Vendian ‘best estimate’ from graphically adjacent samples and smooth varia- Kaufman et al. [6]. 0 = this work; A from [22]; question marks indicate estimated Sr values for intervals without reliable tion suggests that the curve presented in Fig. 3 is data. The periods of most rapid change are from ca. 600 to a reasonable representation of Lower Cambrian 590 Ma and again from 530 to 525 Ma. For comparison, the Sr seawater Sr isotopic variations. The stratigraphic isotopic curve for the period 100 Ma-present (thin line, interval below the Tommotian low is not con- RRD) [7] is plotted on the same time-scale (upper axis gives strained by our data because the samples that we actual ages). measured in Nemakit-Daldynian age strata ap- pear to be altered. Further work will be necessary ratios (z+ 1) and very low Sr contents (< 80 ppm). to define Sr isotopic variations closer to the Pre- Such numbers are not typical of primary marine cambrian-Cambrian boundary as presently de- micrites and suggest that the Huqf Group car- fined. Ultimately, our proposed curve should be bonates have not remained a closed system for tested with measurements from a different, but Sr. The qualitative agreement between the curves coeval (as established by independent methods), presented by Burns et al. [4] and Kaufman et al. stratigraphic section. [6] is, however, encouraging. At present, no reli- able data have been published from lowest Cam- brian strata. Our results imply that, by the earli- 4. Discussion est Tommotian, 87Sr/86Sr in seawater had fallen from its Vendian high to 0.7081. This low is A growing body of evidence supports the view followed by the relatively rapid rise during the that seawater 87Sr/86Sr was < 0.707 during the Lower Cambrian. Thus, the overall increase of Neoproterozoic Varanger glaciation (ca. 60.5 Ma), the marine Sr isotopic ratio from Varanger lows and began to rise rapidly only afterwards [2,4,6]. of 0.7066 to Upper Cambrian highs of 0.7091 Sr isotopic data from upper Vendian carbonates appears to have taken place in two stages, sepa- show values rising to ca. 0.7085 in samples from rated by a decrease near the Precambrian- the Nama and Witvlei Groups of southern Africa Cambrian boundary, which was completed by the and the Windermere Supergroup of northwest Tommotian (Fig. 4). In this respect the Canada [6], while Burns et al. [4] argue for values Vendian-Cambrian rise is unlike the nearly as high as 0.7092 in latest Vendian strata of the monotonic Cretaceous-Tertiary increase. Huqf Group, Oman. We are concerned that the Uncertainties remain in the absolute age cali- data from the Huqf Group [4] may overestimate bration of Cambrian strata but a first-order calcu- actual Vendian seawater 87Sr/86Sr values. The lation of the rate of the Lower Cambrian Sr rise majority of these samples have very high Mn/Sr 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 isotopes) through this interval is necessary to 87Sr/86Sr in Tommotian-Atdabanian seawater to resolve this ambiguity quantitatively but, unfortu- have been about 0.0001/m.y. For comparison, nately, Nd data of sufficient resolution are not other known periods of rapid change include a presently available. However, the rate of increase brief late Miocene step of 0.0001/m.y. [24], of observed seawater 87Sr/86Sr during the Lower 0.0001/m.y. in the early Miocene [7] and ca. Cambrian provides some constraints on plausible 0.00013/m.y. in the lower Vendian [6]. Thus, the forcing mechanisms. A calculation using the Lower Cambrian rise we observe is rapid but not model of [24] suggests that the 87Sr/86Sr of river unprecedented. water would have had to increase by = 0.001 in 5 m.y. if the sole cause of the Tommotian-Atda- 4.1. Causes of Lower Cambrian Sr isotope change banian seawater increase was change in the iso- topic ratio of continental runoff. This is greater Several workers have cited uplift and erosion than the shift in river 87Sr/86Sr for the interval related to the Pan-African orogeny as a principal 40-0 Ma, estimated as resulting from Himalayan cause of the overall Vendian-Cambrian seawater erosion [7], and greater than the net impact of 87Sr/86Sr increase, [2,5,6,25]. The cause of the the Ganges-Brahmaputra system on the oceanic apparent decline in seawater 87Sr/86Sr of 0.0004 Sr budget today [26]. Plausible ranges and rates to 0.0009 near the Precambrian-Cambrian of riverine 87Sr/86Sr inputs to the oceans have boundary is less clear. At least three alternative been discussed in detail elsewhere [24,26,27]. hypotheses exist: From these considerations it seems unlikely that (1) Reduced rates of tectonically driven uplift or the Tommotian-Atdabanian seawater 87Sr/86Sr climate change may have resulted in a tempo- increase could have been caused by increasing rary decline in global silicate weathering rates. riverine 87Sr/86Sr alone. The same conclusion (2) A change in the type of eroding crust could applies to models of groundwater flux of Sr from have driven a significant drop in the mean continents to the oceans [28]. Thus it appears that 87Sr/86Sr of river water. the Tommotian-Atdabanian seawater 87Sr/86Sr (3) Rift-associated volcanic activity as well as rise was caused at least in part by increasing river worldwide marine transgression might have fluxes of Sr and indicates a period of enhanced been sufficient to reverse temporarily the up- chemical erosion. The geochemical evidence for a ward trend of 87Sr/86Sr period of increased erosion during an interval At present, the data from lowest Cambrian and well known for the development of transgressive latest Vendian strata are insufficient to address sequences in North America and elsewhere may this question further. seem contradictory, but it should be pointed out Models using combined Nd and Sr systematics that passive margin sedimentation currently char- from Neoproterozoic and Cambrian sediments acterizes almost all of the Atlantic shorelines of have suggested that global erosion rates were four continents, the east coast of Africa and most highest in the lower to mid-Vendian and fell of Australia, at the same as time as very rapid gradually to moderate levels by the Upper Cam- erosion takes place in Asia. brian [5,6]. However, the new data presented in The dramatic rise in the Vendian-Cambrian this paper imply a second period of rapid erosion seawater Sr isotopic ratio may be compared to during the Lower Cambrian that has not previ- that during the Late Cretaceous-Tertiary (Fig. ously been recognized. Because of the degrees of 4), the only comparable shift known from the freedom inherent in interpreting the marine Sr geologic record [6,25]. We note that the rate and record, the Tommotian-Botomian Sr increase pattern of seawater Sr change during the Neo- could represent an interval of unroofing of old, gene was similar to that during most of the radiogenic crust or one of increased global ero- Lower-Middle Cambrian. This observation hints sion rates. An independent estimate of the evolu- that the mechanisms of Sr isotopic change could tion of crustal sources to the oceans (such as Nd have been similar in both cases. Major unroofing L.A. Derry et al. / Earth and Planetary Science Letters 128 (1994) 671-681 677 and erosion of the very radiogenic Himalayan metamorphic core occurred in the early Miocene [29-31], apparently driving already rising seawa- ter 87Sr/86Sr to high values [7]. An analogous tectonic history appears to describe the Pan- African 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 mo- lassic sediments of the upper Nama Group were derived primarily from a Damara crystalline Age, Ma source area and have radiogenic Sr signatures Fig. 5. Carbon isotope data from Lower Cambrian Siberian carbonates [11,12,14,16] plotted on time axis [17]. Stage [35,36]. These sediments are analogous to the boundaries marked as in Fig. 3. highly radiogenic molasse and flysch derived from the Himalayan orogen and deposited in the Siwa- lik foreland and Bengal Fan [31]. Thus, we sug- gest that an important source of radiogenic Sr to dence for an interval of increased erosion/sedi- the oceans during the Cambrian could have been mentation and the C isotope evidence for a mean the erosion of the Damara-Gariep belt, just as increase in the fractional burial of organic carbon the Himalaya have provided radiogenic Sr to the imply that, overall, this interval was one of en- Neogene ocean. Avalonian events may also have hanced but episodic organic carbon burial. contributed to rising seawater 87Sr/86Sr values. The 613C data show variations of several per cNd values from Avalonian clastic sediments of mil on a < 1 Ma time-scale during the latest Great Britain drop rapidly during the Lower Nemakit-Daldynian through to the Botomian Cambrian, implying the exposure and erosion of (Fig. 5). The rate of variation of 613C provides mature basement with radiogenic Sr [37]. some hints as to possible mechanisms. The most reasonable explanation for the marked variations observed in the Lower Cambrian 6i3C record is 4.2. Comparison with the carbon isotope record change in the burial fraction of organic carbon in marine sediments (c.f. [38]). Such changes might Increased erosion implies increased sedimen- plausibly result from two kinds of phenomena. tation, and increased sedimentation implies in- Rapid changes in oceanic productivity could lead creased carbon burial [9]. The use of Sr (and Nd) to changes in organic carbon burial. This explana- isotopic records as proxies for sedimentary car- tion may apply to the 3%0 fall in 613C values bon flux has proven to be a powerful tool for during the Botomian, which appears both to coin- understanding the history of biogeochemical cy- cide with a significant extinction event and to cling. The rate of carbon cycling thus obtained mark the end of the period of extreme variability can be combined with the fractional organic car- in 613C [16,39]. Can the Sr data provide an expla- bon burial flux, obtained from carbon isotope nation for the apparent cyclic variability of the measurements, to estimate the absolute organic 613C record during the Lower Cambrian? It has carbon burial rate [8]. During the Tommotian to been proposed that the seawater Sr and C iso- early Botomian, the 613C value of marine carbon- tope records might be coupled through the ero- ates was highly variable but generally increasing sion/sedimentation process [8]. High erosion [12,16]. The 613C data imply that the fractional rates should lead to elevated seawater 87Sr/86Sr organic carbon burial flux reached as high as 30% ratios and at the same time to increased mean in the Atdabanian-Botomian. The Sr isotope evi- sedimentation rates in the oceans. Global organic 678 L.A. Derry et al. / Earth and Planetary Science Letters 128 (1994) 671-681
carbon burial rates should be, to a first approxi- mation, related to clastic sedimentation rates, so increased erosion should result in increased or- ganic carbon burial [8,9,40]. Furthermore, if ma- rine productivity is nutrient limited, increased erosional fluxes of phosphorous (for example) might result in increased organic carbon produc- tion and consequent increased burial. Thus, peri- odic variations in globally averaged erosion rates 750 might provide a mechanism to drive the oscilla- tions in the S13C signal observed in the Lower Time, kyr 87 86 Cambrian. Fig. 6. Response curves for 613C (dashed line) and Sr/ Sr (solid line) in seawater as a function of an arbitrary common In order to illustrate the possible relationships periodic forcing of a 750 kyr period (fine dashed line). The between the C and Sr isotope records, we mod- left axis shows deviations in 613C, while the right axis gives eled the response of the 87Sr/86Sr and 613C deviations in Sr isotopic composition in units of A”Sr (A8’Sr 87 86 87 86 87 86 5 records to a common periodic forcing factor (e.g., = [( Sr / Srmeas - Sr / Srstd) / ( Sr / Srstd)] X 10 , 87 86 variations in erosion rate). Our model of ampli- where Sr/ Srstd = 0.70197). No vertical scale is implied for the forcing function. Because of the short response time of C tude response and phase shift of a component in in the oceans, the 613C variations are nearly in phase with the a first-order reservoir is equivalent to those of forcing function. The long response time of Sr, however, Lasaga [41] and Richter and Turekian [26,41]. We results in a response significantly out of phase with both the chose a residence time (ri) for C of 40 kyr, for Sr forcing function and the 613C response. 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. Lower Cambrian. We conclude that direct forcing Thus, the Sr and C isotopic records in seawater by enhanced erosion and burial of organic carbon should show significantly different phase re- is an unlikely explanation for the quasi-periodic sponses to a periodic forcing near 1 myr. The nature of the Lower Cambrian 6i3C record. model results (Fig. 6) show that the response of Episodic marine anoxia (possibly related to Sr isotopes in the ocean should lag behind that of productivity variations [10]) may have played a carbon isotopes by about 180 kyr (e.g. = r/2) key role in controlling organic carbon burial. The and that, for a net 3%0 6i3C shift, there should rate of carbon isotope variation during the Lower be a shift of 0.00014 in 87Sr/86Sr. Such Sr shifts Cambrian appears similar to that found near the are not evident in our data but the resolution of Cenomanian-Turonian and Albian-Aptian the data is not yet sufficient to exclude this possi- boundaries during the Cretaceous, associated with bility firmly. It should be noted, however, that ocean anoxic events (OAEs) [42,43]. For example, seawater isotopic shifts of this magnitude, if Bralower et al. [44] have argued that three sepa- driven by erosion, require large and rapid varia- rate anoxic episodes, each of l-2 m.y. duration, tions in either river Sr fluxes or isotope ratios. In occurred near the Albian-Aptian boundary. Ex- order to produce variations consistent with the tensive black shale deposition is known from the 6i3C shifts, the global Sr river flux would have to Siberian platform and China during the Lower vary by ca. 30% (or its ratio by ca. 0.001) with the Cambrian, implying that OAEs also occurred dur- same frequency (i.e., 750 kyr). Cyclic shifts in ing this interval [45,46]. Repeated OAEs may global river Sr fluxes of this magnitude and fre- have driven changes in Lower Cambrian organic quency are certainly very large and may not be carbon burial. Relatively narrow post-rift ocean plausible [26], particularly in the absence of any basins (similar to the Cretaceous Atlantic and evidence for continental glaciation during the Tethys) could have contributed to OAEs in the L.A. Derry et al. / Earth and Planetary Science Letters 128 (1994) 671-681 679
Lower Cambrian. The presence of low to mid- 5. Conclusions latitude evaporite deposits also suggests that con- ditions for forming oxygen-poor, warm, saline, Sr isotopic variations in Lower to Middle Cam- bottom waters were present [47]. Alternatively, brian carbonates show a rapid increase in seawa- evaporite basins, themselves, may have been sites ter 87Sr/86Sr values from 0.7081 in Tommotian of significant organic matter accumulation. How- strata to 0.7085 in Botomian strata. Values re- ever, the mechanism by which deposition in evap- main near 0.7085 in Toyonian strata, rising to orite basins could produce cyclic changes in the 0.7088 in lower Middle Cambrian strata. global 6’“C record of such a large magnitude is The low values of Tommotian carbonates im- not clear. Detailed stratigraphic work to establish ply a decrease of 2 0.0004 in the 87Sr/86Sr ratio correlations between 613C variations, anoxic de- of seawater near the Precambrian-Cambrian posits, evaporites and phosphorites is necessary boundary, given values of 0.7085-0.7092 reported to test these hypotheses. from mid-late Vendian strata. Thus, the overall The Sr isotopic evidence for an interval of rise in seawater Sr isotopic values beginning in rapid erosion during the Lower Cambrian may the lower Vendian was interrupted, possibly by a have some implications for understanding the decrease in the Sr isotopic ratio of the global ‘Cambrian explosion’ of rapid marine inverte- river flux, by decreased silicate weathering rates brate radiation. A consequence of increased ero- and/or rift-related volcanic activity and subsi- sion rates should be increased phosphorous flux dence. to the oceans, although recent work has sug- Rates of change of 87Sr/86Sr in Lower Cam- gested that P accumulation in Cenozoic marine brian rocks are = 0.0001/m.y., comparable to sediments is not closely coupled to the Cenozioc the most rapid Neogene or lower Vendian varia- 87Sr/86Sr record [48]. Major sedimentary phos- tions. The Lower Cambrian seawater Sr isotopic phorite deposits are known from the Lower Cam- increase may be related to the rapid erosion of brian and phosphatic skeletal fossils are unusu- radiogenic crystalline rocks of the Pan-African ally abundant in the Tommotian-Atdabanian, Damara-Gariep belt of southern Africa, and pos- suggesting that the Early Cambrian oceans could sibly of the Avalonian terrane. have had relatively high P availability [15,45,49]. Our data imply a previously unrecognized pe- Enhanced oceanic nutrient levels could have led riod of enhanced erosion in the Lower Cambrian. to episodes of increased productivity, which have This interval of rapid erosion coincides closely been proposed as a mechanism for positive C with the ‘Cambrian explosion’ of marine inverte- isotope shifts in the Precambrian-Cambrian tran- brates. Consideration of newly available carbon sition [13]. However, any nutrient-driven produc- isotope data suggests that episodic marine anoxia tivity episodes should have been self-limiting on a could have been responsible for rapid variation in ca. 100 kyr time-scale as the surface ocean supply fractional organic carbon burial during the of P was drawn down. It may be that the highly ‘Cambrian explosion.’ High fractional rates of episodic nature of the 613C record in part reflects organic carbon burial and episodes of marine nutrient limitations. In any case, the apparent anoxia may also be related to enhanced nutrient availability of dissolved P in the Lower Cambrian fluxes resulting from high erosion rates. ocean could have played a role in providing an environment conducive to the rapid diversifica- tion and expansion of marine invertebrates by Acknowledgements enhancing primary productivity at the base of the food chain. A link between erosion, primary pro- The authors wish to thank A.J. Kaufman, R. ductivity and carbon burial is plausible for the Berner and M. Kennedy for careful reading and Lower Cambrian, which may have influenced the helpful criticism. L. Derry wishes to thank L. environment of evolution of early invertebrates Marin, D. Dautelle and A. Moore for expert during the ‘Cambrian explosion.’ laboratory assistance. [FA] 680 L.A. Derry et al. /Earth and Planetary Science Letters 128 (1994) 671-681
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