ABSTRACT. Stable isotopes of carbonate and organic carbon from two Up- per Devonian sections in Alberta, Canada, indicate a ''heavy carbon event'' across ...
Carbon and sulfur isotope anomalies across the FrasnianFamennian extinction boundary, Alberta, Canada K. Wang Department of Geology, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada H. H. J. Geldsetzer Geological Survey of Canada, 3303-33rd Street N.W., Calgary, Alberta T2L 2A7, Canada W. D. Goodfellow Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada H. R. Krouse Department of Physics and Astronomy, University of Calgary, Calgary, Alberta T2N 1N4, Canada
ABSTRACT Stable isotopes of carbonate and organic carbon from two Upper Devonian sections in Alberta, Canada, indicate a ‘‘heavy carbon event’’ across the Frasnian-Famennian (F-F) boundary and a negative d13C shift at the boundary. At Cinquefoil Mountain, d13C values in carbonate are anomalously high (3.5‰ to 4.5‰ PDB [Peedee belemnite]) within an interval (10 m below to 23 m above the F-F boundary) representing about 1 m.y. (two standard conodont zones), compared to background values of ;1‰ above and below this interval. Superimposed on this positive excursion is a marked d13C decrease of about 1‰ at precisely the F-F boundary. At Medicine Lake, a similar d13C excursion in organic carbon, from a background of 229‰ to a high of 226.5‰, is recorded across the F-F boundary. The d34S data for iron sulfide in these sections show unusually high values (as much as 15‰ to 20‰ CDT [Can ˜ on Diablo troilite]) in the F-F boundary interval. These data, together with those from other areas, indicate a period of major global changes (e.g., greenhouse and marine anoxia) that took place well before and after the sudden biomass killing event at the F-F boundary. Existing evidence suggests that the F-F mass extinction was probably caused by a large extraterrestrial impact on Earth at a time when the global ecosystem was already severely stressed.
INTRODUCTION The transition from Frasnian to Famennian (;367 Ma) in the Late Devonian was a time of one of the five largest mass extinction events in the Phanerozoic (see McGhee, 1994). From the important recent developments toward understanding this global biotic crisis, two major hypotheses have emerged. The Frasnian-Famennian (F-F) extinction has long been attributed to a large bolide impact (McLaren, 1970; McLaren and Goodfellow, 1990), a theory that has been greatly strengthened recently. However, on the basis of high carbonate d13C values measured in European F-F sections, Joachimski and Buggisch (1993) proposed that marine anoxia and extreme temperatures were likely the cause of the biotic crisis. Many excellent stratigraphic sections are known to span the F-F boundary in Europe, North America, Australia, and south China, but only a few have been subject to detailed isotope-stratigraphic studies. Previous analyses were carried out on highly condensed sequences, especially in Europe, where each conodont zone (average 0.5 m.y.; Ziegler and Sandberg, 1990) near the F-F boundary is represented by only a few tens of centimetres (e.g., Joachimski and Buggisch, 1993). In this study we have analyzed stratigraphically expanded F-F sections with high sedimentation rates, in the hope that such sections will allow us to establish a well-defined, high-resolution isotope-stratigraphic record across the F-F extinction boundary, and to gain further insights into extinction processes. In this paper we present new analytical data for two F-F sections in Alberta, Canada, and discuss a model for the F-F extinction mechanism. Geology; February 1996; v. 24; no. 2; p. 187–191; 7 figures; 1 table.
GEOLOGIC SETTING The two sections we studied are located in Jasper National Park, Alberta, Canada (Fig. 1). Paleogeographically, both sections were in the Jasper basin, a broad embayment on the west side of an extensive Frasnian reef domain (Morrow and Geldsetzer, 1988). The Cinquefoil Mountain section is characterized by upper-slope carbonate sediments with a calculated paleo-water depth of about 50 m, whereas the Medicine Lake section is represented by basinal sediments deposited in about 150 m of water (Geldsetzer et al., 1987). During the early Famennian, the Jasper basin was infilled from the west by siliciclastic material of the Sassenach Formation, gradually onlapping the eastern slopes of the basin and mixing with
Figure 1. Late Devonian paleogeography of Alberta, Canada, with locations of studied sections at Cinquefoil Mountain and Medicine Lake in Jasper National Park (after Morrow and Geldsetzer, 1988). 187
carbonate detritus derived from carbonate banks (Ronde Formation) overlying the former reef domain. The Sassenach Formation is therefore time transgressive; the first siliciclastic material arrived in basinal areas such as Medicine Lake before reaching the upper slopes of the basin at Cinquefoil Mountain.
setzer et al. (1987) placed the F-F boundary in the Medicine Lake section at the base of the Sassenach Formation (Fig. 3), whereas the accurately determined F-F boundary at Cinquefoil Mountain lies in the middle of the Ronde Formation (Fig. 2), demonstrating the time-transgressive nature of the Sassenach Formation.
STRATIGRAPHY Systematic sedimentology and conodont biostratigraphy were carried out in the Cinquefoil Mountain section (Fig. 2). On the basis of conodont Palmatolepis species, the F-F boundary is precisely located at the base of a tempestite bed (20 cm thick) that is directly overlain by a prominent oncolite bed (65 cm thick) in the Ronde Formation (Fig. 2). The standard Late Devonian conodont zones (Ziegler and Sandberg, 1990) upper rhenana-linguiformis (or Montagne Noire Zone 13 of Klapper, 1988), Lower triangularis, and Middle triangularis have been recognized in this section (Fig. 2). Important conodonts recovered from this section include Ancyrodella curvata, Palmatolepis subrecta, Palmatolepis rotunda, Palmatolepis rhenana nasuta, Palmatolepis eureka, Icriodus alternatus alternatus, Icriodus alternatus helmsi, Palmatolepis praetriangularis, Palmatolepis triangularis, Icriodus iowaensis iowaensis, Icriodus iowaensis ancylus, Palmatolepis delicatula platys, and Palmatolepis protorhomboidea (see Wang and Geldsetzer, 1995). The two conodont zones recognized in the Medicine Lake section (M. Orchard, in Geldsetzer et al., 1987) are indicated in Figure 3. The upper Mount Hawk to the basal Ronde interval most likely belongs in the Upper rhenana (gigas) Zone and the upper Sassenach Formation is in the Upper triangularis Zone. The Uppermost gigas (linguiformis) and Lower and Middle triangularis zones are probably represented by the Ronde to lower Sassenach interval, where no age-diagnostic conodonts were recovered. Geld-
GEOCHEMICAL ANALYSES The F-F boundary section at Cinquefoil Mountain is represented by a clean carbonate sequence; samples we analyzed consist of almost pure carbonate (CaCO3 .90%) and contain very little organic carbon (Corg). The Medicine Lake section, however, is a more argillaceous, silty sequence, and most samples analyzed contain ,40% carbonate (as low as 20% CaCO3 in some samples) but moderately high Corg (up to 2.5%). Therefore, we decided to perform C-isotope analysis on carbonate for the Cinquefoil Mountain section and on Corg for the Medicine Lake section. Sulfur isotope analysis was carried out for both sections on fine sulfide fractions extracted from selected samples near the F-F boundary. In addition, total organic carbon (TOC) contents were determined for all samples analyzed isotopically. The C and S isotopic data are reported in the conventional d notation as deviation (in parts per thousand; ‰) from the PDB (Peedee belemnite) and CDT (Can ˜on Diablo troilite) standards, respectively. The analytical uncertainties were ,0.1‰ for d13C, ,0.2‰ for d34S, and ;0.05 wt% for TOC.
Figure 2. Biostratigraphic, lithostratigraphic, and chemostratigraphic profiles of Cinquefoil Mountain section, Alberta, Canada. Lithofacies: 1—pelletal carbonate, 2—silty carbonate, 3—pelletal grainstone, 4 — oncolite horizon with 20-cm-thick tempestite bed at base, 5—mudstone-packstone, 6 —silty carbonate, 7— coral biostrome, 8 —nodular carbonate. PDB is Peedee belemnite; TOC is total organic carbon.
Figure 3. Biostratigraphic, lithostratigraphic, and chemostratigraphic profiles of Medicine Lake section, Alberta, Canada. Lithofacies: 1—laminated calcareous shaly siltstone, 2— dolomitic siltstone, 3— calcareous siltstone, 4 —shaly mud-wackestone, 5—nodular mudstone-wackestone. PDB is Peedee belemnite; TOC is total organic carbon.
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RESULTS We analyzed 80 samples for carbonate d13C spanning a 140 m stratigraphic interval (80 m above and 60 m below the F-F boundary) at Cinquefoil Mountain (Fig. 2). The normal background d13C values for this section are about 1‰, as shown in the lower and upper 50 m of the section (Fig. 2). A sudden upward increase of d13C
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Figure 4. Plot of d13C in Frasnian-Famennian boundary interval showing negative d13C shift at precisely Frasnian-Famennian boundary at Cinquefoil Mountain, Alberta, Canada. PDB is Peedee belemnite.
Figure 5. Plot of sulfide d34S in selected whole-rock samples from Frasnian-Famennian boundary interval. See Figures 2 and 3 for stratigraphic information. CDT is Can˜on Diablo troilite.
values to .3‰ occurs about 10 m below the F-F boundary. The d13C values remain highly positive (average ;4‰) upsection through the F-F boundary to ;23 m above the boundary, where d13C values start to decrease gradually over the next 10 m to background values (Fig. 2). The TOC contents are very low (near 0%) over most of the section; high TOC values (up to 1.6%) are only recorded in samples within a few metres below the F-F boundary and within 1 m above the boundary (Fig. 2). Superimposed on this broad positive d13C excursion, which lasted for about two conodont zones (;1 m.y.), is a negative d13C shift of about 1‰ at precisely the F-F boundary (Fig. 4), a shift that would be expected as a result of a significant reduction of productivity and biomass in the upper water column associated with the F-F boundary extinction (Wang et al., 1991; Goodfellow et al., 1988). The broad positive d13C excursion across the F-F boundary seen at Cinquefoil Mountain is also recorded by Corg isotopes in the Medicine Lake section (Fig. 3), where 42 samples from a 30 m interval (10 m above and 20 m below the F-F boundary) were analyzed for d13Corg. The d13Corg values in this section show a positive excursion from 229‰ in the Frasnian to 226.5‰ in the F-F boundary interval (Fig. 3). The TOC contents in this section (Fig. 3) are much higher than those in the Cinquefoil Mountain section. There is also a drop in d13Corg of about 0.5‰ at the F-F boundary (Fig. 3). Data for whole-rock sulfide S isotopes for these two sections display similar d34S profiles (Fig. 5). Most of the d34S values are unusually high (.0‰), and show maximum values at the F-F boundary level (Fig. 5). Using petrographic and trace element abundance data (unpublished), we have evaluated known diagenetic effects that may affect the carbon isotopic compositions of the samples in our studied sections, and we concluded that the d13C data reported here are predominantly primary. No correlation (diagenetic trend) is observed between d13C and d18O and between d13C and TOC (Fig. 6) in the Cinquefoil Mountain section, or in the Medicine Lake section between d13Corg and TOC (Fig. 7). DISCUSSION Our data for the studied sections clearly show a first-order, positive d13C excursion (‘‘heavy-C event’’) across the F-F boundary. GEOLOGY, February 1996
This heavy-C event is recorded in both carbonate and organic carbon reservoirs, and is estimated to have lasted for about 1 m.y., on the basis of the high-resolution conodont biostratigraphy in the Cinquefoil Mountain section (Fig. 2) and when a duration of 0.5 m.y. is used for each conodont zone (Ziegler and Sandberg, 1990). The d13Corg excursion in the Medicine Lake section is consistent with this estimated duration, although conodont zones are not as well defined there. The previously reported carbonate d13C values in the Medicine Lake section (Geldsetzer et al., 1987), however, are based on samples with rather low CaCO3 contents (,40%), whereas samples analyzed from the Cinquefoil Mountain section contain .90% CaCO3. Given the high resolution of the Cinquefoil Mountain section as a result of stratigraphic expansion due to a high sedimentation rate (see Table 1 for comparison with other sections), it may serve as a standard section for C-isotope stratigraphy across the F-F boundary. Following McGhee et al. (1986), Joachimski and Buggisch (1993) analyzed several F-F carbonate sections from Germany, France, and Austria, and found two discrete positive d13C excursions that are stratigraphically associated with the Corg-enriched upper and lower Kellwasser horizons. It is apparent that the upper d13C excursion spanning the F-F boundary is widespread. Carbon isotope analyses performed on brachiopod shells collected from a F-F section in Poland also indicate a positive d13C excursion spanning the F-F boundary (Halas et al., 1992). Other previously studied sections that may record a similar positive d13C excursion across the F-F boundary include those from McWhae Ridge in the Canning basin, Australia (Playford et al., 1984), and Trout River in the Northwest Territories, Canada (Geldsetzer et al., 1993; Goodfellow et al., 1988). A few studied sections from south China do not define a positive d13C excursion across the F-F boundary, but record a rapid, short-lived negative d13C shift at the F-F boundary (Wang et al., 1991; Bai et al., 1995), interpreted to be the result of a temporary reduction of surface-water productivity and biomass associated with the F-F boundary extinction event. All these studies are based on carbonate d13C data. The broad positive d13C excursion spanning the F-F boundary appears to be synchronous and widespread. This study also indicates that the d13C of organic carbon (Medicine Lake) tracks changes of carbonate d13C (Cinquefoil Mountain) in the same basin. These 189
Figure 7. Cross plot of d 13C org vs. total organic carbon (TOC) for all samples analyzed from Medicine Lake section. PDB is Peedee belemnite.
Figure 6. Cross plots of d C vs. d O and total organic carbon (TOC) for all samples analyzed from Cinquefoil Mountain section show area of normal samples and area of ‘‘heavy carbon’’ samples, separated by three transitional samples with intermediate d13C values. There is no correlation between d13C and d18O, and between d13C and TOC within each of two areas, suggesting that neither d18O alteration nor Corg oxidation has significantly affected primary d13C values in these samples. PDB is Peedee belemnite; TOC is total organic carbon. 13
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data suggest that during the time of the F-F transition, there was an increase in d13C of the dissolved inorganic carbon reservoir in most of the world basins. The 13C enrichment apparently started about 0.5 m.y. before and ended long after the F-F boundary event (see Fig. 2). Because the F-F positive d13C excursion is stratigraphically associated with Corg-rich sediments (the Upper Kellwasser facies) in many areas, especially in Europe, a logical explanation is that a higher rate of Corg (12C-enriched) burial into sediments at that time resulted in the 13C enrichment in the water column. The increased rate of Corg burial most likely resulted from favorable anoxic conditions in the lower water column. Independent sedimentological and geochemical evidence— e.g., black, laminated reducing sediments, lack of bioturbation, and chalcophile enrichments—suggests widespread marine anoxia at the F-F transition. Our sulfide d34S data (Fig. 5) provide further strong evidence for reducing water conditions in the paleo-Jasper basin during the F-F transition. In anoxic marine water columns, the enhanced sedimentation of isotopically light sulfide minerals (e.g., pyrite), combined with an increase of the fraction of sulfate bacterially reduced to sulfide, would result in preferential removal of 32S from the sea water, leaving the residual sulfate increasingly enriched in 34S (Goodfellow, 1987). Sedimentary sulfide that was formed later through bacterial 190
sulfate reduction would therefore exhibit higher d34S values. Thus, the unusually high d34S values in the studied sections (Fig. 5) are indicative of reducing water conditions in the Jasper basin during F-F transition time. The difference (fractionation) between carbonate and organic d13C values (d13Ccar-d13Corg) across the F-F interval in the two studied sections (Figs. 2 and 3) does not support either a higher primary productivity or a lower pCO2 level for this time interval. The enhanced rate of Corg burial during the F-F transition, as a result of marine anoxia rather than of a higher productivity, may have not been sufficient to cause a significant CO2 drawdown. MODEL FOR THE F-F EXTINCTION The study of bioevent stratigraphy has resulted in the placement of the F-F boundary at a major faunal break that is readily correlatable across continents. The F-F boundary has now been formally defined to coincide with the acme of the extinction at the base of the conodont Lower triangularis Zone in the stratotype section in southern France (Klapper et al., 1993). Thorough conodont dating of the F-F extinction event in North America, Europe, and North Africa indicates that the biotic crisis was accompanied by drastic sedimentary events, including major storms, slumping, debris flows, and the formation of tsunami deposits; the F-F mass extinction took place in far less time than a few tens of thousands of years and was probably instantaneous (Sandberg et al., 1988). McLaren and Goodfellow (1990) gathered evidence from around the world for a globally synchronous F-F boundary biomass ‘‘killing’’ event that is marked in the field by the disappearance of many shallowmarine taxa at a narrowly defined stratigraphic interval. The original idea that the F-F extinction was caused by a large bolide impact (McLaren, 1970) has been greatly strengthened. Glass microtektites are reported from the F-F boundary bed in Belgium GEOLOGY, February 1996
(Claeys and Casier, 1994) and possibly in other areas (Bai et al., 1995). In southern France, a strong Ir anomaly (5.6 ppb) with a meteoritic Ru/Ir ratio has been found at the F-F boundary. In New York, Over et al. (1996) reported platinum group element anomalies and chondritic Ir/Ru ratios near the F-F boundary. In south China, Wang et al. (1991) documented a moderate Ir anomaly at the F-F boundary. At least two large impact craters are known with an age matching that of the F-F boundary: the 55 km Siljan Crater in Sweden (368 6 1.1 Ma) and the 54 km Charlevoix Crater in Quebec (357 6 15 Ma) (Grieve, 1991). High-energy or tsunami deposits are known to be widespread near the F-F boundary (Sandberg et al., 1988; Wang et al., 1991; Bai et al., 1995; Over et al., 1996). An abrupt and short-lived negative d13C shift documented at the F-F boundary in several carbonate sequences (Goodfellow et al., 1988; Wang et al., 1991; Geldsetzer et al., 1993; Bai et al., 1995) was probably associated with the marine biomass crash during the F-F extinction event (McLaren and Goodfellow, 1990). All these data strongly suggest the occurrence of a bolide impact at the F-F boundary, causing biological, geochemical, and sedimentological perturbations. The new isotopic data from our studied sections in Alberta, Canada, have provided further insights that a bolide impact at the F-F boundary was probably not the only event; some drastic global changes appear to have already taken place in the latest Frasnian linguiformis Zone (Figs. 2, 3, and 5). The latest Frasnian was a time of maximum sea-level highstand in the Devonian (Johnson and Sandberg, 1988), characterized by flooded cratons at low and middle latitudes, pole-to-equator equable temperatures, and high evaporation rates. On the basis of unaltered brachiopod shell d18O data from North America and Europe, Brand (1989) estimated that the world mean temperatures of the epeiric seas during the F-F transition may have been extremely high (37– 40 8C), well above optimal temperatures for most living organisms. Phosphate d18O data from conodonts also indicate similarly high temperatures (;40 8C) during the F-F period (Luz et al., 1984). The majority of marine organisms may have been living near their tolerance threshold in such a hot greenhouse climate (Thompson and Newton, 1988; Becker and House, 1994). The hypothesis of cold-water oceans for this time period is not supported by geochemical data. Accumulation of warm saline bottom waters in world basins and the subsequent spreading of such oxygen-poor facies into shallow-marine areas would have imposed additional stresses on the benthic communities. The combined effect of thermal heat and widespread marine anoxia (see also Thompson and Newton, 1988) probably created an overall stressed and ‘‘fragile’’ ecosystem prior to F-F boundary time; such a system would have been sensitive to large external perturbations. At the F-F boundary, a large bolide impact may have triggered the collapse of such an ecosystem, causing mass extinctions. Data suggest that the F-F mass extinction was the consequence of a large bolide impact on a severely stressed, ‘‘fragile’’ ecosystem that was already in existence for about 0.5 m.y. prior to the F-F boundary event. ACKNOWLEDGMENTS We thank Lee Kump, George McGhee, Jr., and three anonymous referees for critical reviews, and the staff in the Stable Isotope Laboratory at the University of Calgary for their help. Geological Survey of Canada contribution 32995. REFERENCES CITED Bai, S., Bai, Z., Ma, X., Wang, D., and Sun, Y., 1995, Devonian events and biostratigraphy of South China: Beijing, Beijing University Press, 280 p. Becker, R. T., and House, M. R., 1994, Kellwasser events and goniatite successions in the Devonian of the Montagne Noire with comments on possible causations: Courier Forschungsinstitut Senckenberg, v. 169, p. 45–77. Brand, U., 1989, Global climatic changes during the Devonian-Mississippian: Stable isotope biogeochemistry of brachiopods: Palaeogeography, Palaeoclimatology, Palaeoecology, v. 75, p. 311–329.
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