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Geochemical events documented in inorganic carbon isotopes
Palaeogeography, Palaeoclimatology, Palaeoecology 132 (1997) 173–182
Review Paper
Geochemical events documented in inorganic carbon isotopes W.T. Holser Departments of Geological Sciences, Cornell University, Ithaca, NY 14853 and University of Oregon, Eugene, OR 97403, USA Received 17 February 1995; accepted 19 March 1997
Abstract Screening of sampling and analytical data is crucial for the detection of real global shifts in marine d13C, against the noise of diagenesis. The real global shifts of 13C are of two types: (1) long-term secular shifts caused by changes in the fractional burial of organic carbon, and (2) transient shifts caused by abrupt changes of biological productivity in the surface photic zone. Both are applicable to stratigraphic correlation, but differ in their application to modelling of changes in oceanographic or atmospheric composition. A recent compilation found 34 carbon isotope events in the Phanerozoic and Neoproterozoic, equally distributed among positive and negative shifts. This in itself attests to a variety of proximate causes of the shifts. © 1997 Elsevier Science B.V. Keywords: carbon isotopes; carbon rocks; sedimentary rocks; correlation; diagenesis; secular variation
1. Introduction An appreciation of the possibilities of variations in carbon isotopes in ancient carbonates (d13C =d13C for this chapter) was first deduced carb by Garrels and Perry (1974) as theoretically necessary given the previously known inverse variations in the d34S of sedimentary sulfate (Holser and Kaplan, 1966), and assuming constant atmospheric composition. The model was tested by Veizer et al. (1980) and found statistically significant, but with a remaining residue of truly independent variations of d13C and d34S: variations both on the scale of geological periods (~107 yr), and in the sharper scale then known for d34S (Holser, 1977). During the past decade evidence has accumulated from many independent sources for variations in d13C, and a very recent review lists 34 events well documented on a global scale through 0031-0182/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 00 3 1 -0 1 8 2 ( 9 7 ) 0 0 0 70 - 9
the Phanerozoic and Neoproterozoic ( Holser et al., 1995).
2. Sample selection The overriding concern in methodology is to minimize diagenetic alteration, which in many localities is pervasive but not unavoidable. Selection begins with collection that emphasizes stable mineralogy and components (Fig. 1) (Marshall, 1992), petrographic selection for primary character, and chemical screening for low Mn–high Sr: these and other practical aspects have been recently reviewed (Grossman, 1994; Holser et al., 1995). When the data set is assembled, screening continues by eliminating samples whose analyses of other isotopes are suspect: d18O carb much lower than expected, or 87Sr/86Sr much
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W.T. Holser / Palaeogeography, Palaeoclimatology, Palaeoecology 132 (1997) 173–182
Fig. 1. Classification of sample types in terms of potential for preservation of the marine carbon isotope signal. After Marshall (1992).
higher than expected, or shifts of d13C differing org from those for d13C on the same samples. A carb final test is whether the pattern of variation in d13C can be recognized in separate sections or distant basins, within the time frame of the available biostratigraphy. Applying the above and related criteria, both whole-rock samples and microdrilled samples have been found useful. As long as samples of altered rock are either initially avoided or eventually discarded, by even-handed criteria, many more useful data points can be obtained by whole-rock analysis than by analyzing a number of subsamples microdrilled from single shells. Good shells for analysis may be difficult to find, and in some instances (brachiopods in shale or marl ) their isotopic ‘‘milieu’’ may not be as homogenous as in a uniform micritic limestone. But even the most careful selection of samples and data can still give anomalous results (Rush and Chafetz, 1990).
3. Analysis Methods of phosphoric acid evolution and subsequent mass spectrometry of the evolved CO are 2 standard procedure, with modification as necessary
to deal with microsamples. The carbonate analysis yields both d13C and d18O . carb carb 4. Steady state isotopic carbon cycle The details of the exogenic cycle of carbon and its isotopes, are discussed in many review papers (e.g., Holser et al., 1988). The dominant mechanism that determines the general level of d13C is photosynthetic reduction of oxidized carbon to organic carbon, either in the photic zone of the ocean, or (post-Ordovician) on land. Explicitly, the d13C of the remaining marine bicarbonate depends on the (net) fraction of carbon reduced to organic carbon and buried in sediments, and on how much this buried carbon differs in its isotopic composition from the oxidized carbon — the isotopic fractionation, D13C=d13C − org d13C , of the photosynthetic reduction process. carb The isotopic fractionation depends on the photosynthetic ‘‘pathway’’, on whether photosynthesis takes place in a marine (sea water) or non-marine (air) milieu, on the concentration of dissolved inorganic carbon (or the partial pressure of CO ), and on temperature (e.g., Rau et al., 1991). 2 Because of these factors, global d13C can shift with
W.T. Holser / Palaeogeography, Palaeoclimatology, Palaeoecology 132 (1997) 173–182
time. The marine bicarbonate is ‘‘sampled’’ by CaCO crystallization in marine fossils and lime3 stones, with a fractionation of only 1 to 2.7‰, virtually independent of temperature and rate of precipitation (Romanek et al., 1992). Most photosynthesized organic carbon is low in d13C (by −20 to −30‰); a nominal mean value is d13C =−25‰. Most of the photosynthesis takes org place in the surface waters of the ocean (<100 m), and to the extent that the generated organic carbon is buried in the sediments (in either the deep or shallow ocean) the surface zone becomes enriched in 13C — that is, d13C increases. The d13C recorded in shallow-water carbonates under long-term steady-state conditions is thus a measure of (net) worldwide carbon burial. Worldwide d13C will differ at different geological times, depending on whether oceanic conditions favor burial (anoxic bottom conditions, fast sedimentation, high productivity) or re-oxidation by dissolved O . Organic 2 carbon is buried in the sediments and completes its cycle only after uplift and erosion or re-mobilization by metamorphism or volcanism (including re-oxidation). Secular change in this cycle is a consequence of shifts in the proportions of net fluxes of C and C : inputs by rivers org carb minus outputs to burial.
5. Transients in the carbon-isotope cycle Transient fluctuations of d13C caused by prompt changes in the carbon cycle have been discussed by Kump (1991) and Holser et al. (1995), among others. A high level of net biological productivity in the surface zone raises its d13C level relative to the deep main ocean, typically by 1 or 2‰. This ‘‘biological pump’’ establishes a surface-to-deep gradient of d13C, as illustrated in the inner circuit of Fig. 2. At steady state, the d13C of surface waters is established by river input and burial of organic carbon, the level of d13C in deep waters is depressed below that of the much smaller surface reservoir by the amount of the surface-to-deep gradient. This can be detected by comparing the d13C of pelagic and benthic fossils picked from the same deep-water samples. Sharp shifts in productivity will generate corresponding signals in sediments orginating in both the surface and deep
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Fig. 2. Two competing circuits that are parts of the exogenic carbon cycle. Common to both circuits is the surface layer (photic zone) of the ocean, the site of primary photosynthetic production of organic carbon. In the outer circuit, a fraction of this organic carbon is buried in sediments for a substantial geologic interval (‘‘biological dump’’). The sequestration of this d13C-depleted carbon raises the d13C of the residual bicarbonate of the surface ocean in proportion to the fraction of organic carbon buried. In the inner circuit, a ‘‘biological pump’’ takes organic carbon sinking below the photic zone and reoxidizes it to carbonate, which is then circulated back to the surface. No carbon is lost in this circuit, but a surface-to-deep gradient of d13C is maintained (d13C >>d13C ). After Kump (1991) surface deep and Holser et al. (1995).
water levels, whereas the long term level of d13C is essentially determined by carbon burial. The relations between these two circuits of the carbon cycle are demonstrated in Fig. 3 by qualitative model carbon isotope profiles (Holser et al., 1995). In Fig. 3a we see the negative shift of both surface and deep isotope ratio consequent to a sharp decrease of organic carbon burial, during which d13C of the benthic record (dashed line) mimics that of the pelagic record (solid line), because the productivity has been held constant. In contrast, in Fig. 3d, sharply decreased productivity imposes an abrupt negative d13C excursion
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Fig. 3. Sketch diagrams of hypothetical carbon isotope profiles consequent to a sharp decrease of (a) burial in the biological dump, (d) sequestration in the deep by the biological pump, and (b) and (c) combinations of both activities. Solid line: surface waters; dashed line: deep waters. After Holser et al. (1995).
on the smaller surface reservoir, while the longterm level of d∞13C remains constant, owing to a constant fraction of organic carbon burial. Fig. 3b and c illustrate the signatures of mixes of decreased productivity and burial. Sharp increases in productivity or carbon burial will be recorded as analogous positive excursions of short and long time constant, respectively (not illustrated).
6. Examples of carbon-isotope events Carbon isotope events can be applied in stratigraphic correlation on a more or less empirical basis, analogous to correlation of downhole electric logs ( Williams et al., 1988). More weight is given to the form and shape of variations than
to the exact level of d13C, as the general level measured in today’s ocean is found to differ by 1‰ or more depending on mineralogy, on ‘‘vital effect’’ (differences among species) and on the ‘‘age’’ of the water mass. Fig. 4 is a classic example of such a correlation, based on d13C values in Miocene deep-sea cores (Loutit et al., 1983; see also Woodruff and Savin, 1989). Fig. 5 shows that the dramatic drop in d13C across the Precambrian/Cambrian boundary, in Morocco, India, Siberia and Canada ( Kaufman and Knoll, 1995) can be used for chemostratigraphic correlation. An isotopic event that has been studied in detail is the one associated with the Cretaceous–Tertiary ( K–T ) boundary (Zachos et al., 1989; Kump, 1991). As illustrated in Fig. 6, it was possible to
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Fig. 4. Correlations of carbon isotope events among five deep-sea cores of Miocene age from the southwest Pacific. After Loutit et al. (1983).
Fig. 5. Correlation of secular variations of d13C vs. stratigraphic depth for carbonates deposited in the Precambrian–Cambrian interval, from Morocco, India, Siberia and Canada (after Kaufman and Knoll, 1995). Only six of ten correlated sections are shown here.
measure carbon isotopes in both the surface (by pelagic forms) and deep-water (by benthic forms) reservoirs. The convergence of these two traces, shown in more detail in Fig. 7 by plotting different surface-to-deep gradients, signals a major decline
in productivity, which remained suppressed for nearly a million years. This negative d13C excursion is found worldwide (Perch-Nielsen et al., 1982). The corresponding failure of productivity is ascribed to decreased sunlight under a global dust
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Fig. 6. The K–T event expressed in carbon isotope shifts. Pelagic values drop almost to zero, cancelling the previous surface-to-deep gradient, and recover only slowly ( Zachos et al., 1989).
cloud consequent to a bolide impact (Alvarez et al., 1980). The decline of productivity was not only caused by a sharp reduction of marine photosynthesis but also by a large-scale decline of terrestrial organic carbon (by burning: Ivaney and Salawitch, 1993). In contrast, only a minor shift of long-term carbon burial is evident. A major anomaly associated with the Permian–Triassic (P–Tr) boundary, which is probably best developed in the Gartnerkofel (Austria) core (Holser et al., 1989; see also Baud et al., 1989; Grossman, 1994), may be re-interpreted in terms of the described model. In this carbon
isotope profile, d13C remains very high throughout the Late Permian, as can be seen at the bottom of the core in Fig. 8. During the last part of the Late Permian d13C drops at an accelerating rate through the boundary, to a transient minimum in the Early Triassic, from which it slowly recovers, interrupted by one or two subsequent lesser minima. Note that d13C never recovers to Permian values. As indicated on the figure, this difference between Permian and Triassic d13C represents a decrease in long-term burial of organic carbon. The transient minimum of −3.4‰, from which the system eventually recovers, is interpreted as a drop in
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Fig. 7. Surface-to-deep d13C gradient across the K/T boundary, the sharp drop signifies the demise of primary productivity ( Zachos et al., 1989).
productivity. The later subsidiary minima probably represent further drops in productivity, but these have not yet been verified as global effects. The mass of carbon involved in the transient shift is amplified in its effect (about 30×) in inverse relation to the mass of the deep water carbon reservoir. Thus in the Gartnerkofel a transient shift of −3.4‰ involves an amount of carbon that is only 3.4/30=0.l when compared with the amount of carbon involved in the long-term shift of 1.6‰. Another carbon isotope anomaly that has been well documented recently is associated with the Frasnian–Famennian ( Fr–Fa: Late Devonian) boundary. Joachimski et al. (1994) illustrate half a dozen sections from the Alps to the Harz Mountains in northern Germany, which show two
(the Lower Kellwasser and the Fr–Fa events, respectively) sharp rises in d13C, each followed by a slower but nearly full recovery (see Fig. 9). Comparison with the above described model suggests that the transient positive excursion represents a rapid onset of productivity, with only minimum participation of burial transfer. Such a fast rise of productivity might have been caused by a general algal bloom (‘‘red tide’’), fed by upwelling nutrients driven by a climatic shift.
7. Causes of carbon isotope events As explained above, the surface zone of the oceans, which is a reservoir only 1/30 as large as
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excursion of d13C in the surface record (Malkowski et al., 1989; Wignall and Hallam, 1992). Perturbations of sub-crustal metamorphic/igneous storage of carbon may release a large injection of light carbon into the exogenic cycle, especially via continental flood basalt eruptions (Caldeira et al., 1990; Erwin and Vogel, 1992). A bolide impact in carbonate terrane would inject a large dose of CO into the atmosphere (O’Keefe and Ahrens, 2 1982), but with little contrast in the isotopic ratio. Global forest fires produced by bolide impact would release substantial volumes of CO as well 2 as soot C , both represented by light d13C org ( Wolbach et al., 1990). Our census of carbon-isotope events in the Phanerozoic and Neoproterozoic (Holser et al., 1995) lists 34 events: 12 positive excursions, 11 negative excursions, 6 simple rises and 5 simple falls — although in some instances the distinctions are arbitrary. The degrees of association with the forcings of productivity, anoxia, overturn, burial, volcanicity, impact and fire are various and without any emergent pattern. Magaritz (1989) proposed that negative excursions of d13C would follow major extinction events, but even this correlation is not without its exceptions. 8. Conclusions
Fig. 8. Isotope profile through the P–Tr boundary in the Gartnerkofel-1 core, Carnic Alps of Austria, after Holser et al. (1989). The curve may be decomposed into two components: a long-term shift of −1.5‰ owing to release of organic carbon from burial, and a transient shift of −2.5‰ recording a failure of productivity.
the deep sea, consequently magnifies the response of its d13C, to interruptions of the carbon cycle, whatever their origin, so long as they are initiated in the smaller surface reservoir. This condition applies in varying degrees in the cases described above. Some additional examples follow. Overturn of an anoxic ocean may introduce a negative
Despite widespread effects of late diagenesis on the carbon isotope record, an important fraction of isotope events can be verified on a global scale. Examples include a range of short-term transient incidents involving productivity changes and longterm shifts of organic carbon burial. Both types of shift may be applied empirically to long-distance correlations, but are better understood if the factors of productivity and burial are separately applied to pelagic and benthic carbonates, or by distinguishing the transient vs. secular trends, as described above. The shapes of the events are variable, so must their causes be multiple. Acknowledgements I thank Ethan Grossman, Lee Kump and Jan Veizer for helpful comments on an earlier draft of the manuscript.
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Fig. 9. Carbon isotope profiles through the Upper Devonian in Germany (Joachimski et al., 1994); only three of the seven correlated sections are shown here. The two sharp positive excursions, followed by slower recovery, may be interpreted as transient onsets of productivity, with minimal participation of long-term release of carbon from burial. The black zones represent black laminated micritic horizons, which however are not present in another section in the Carnic Alps. The high productivity/carbon isotope event may be global, expressed only locally as black micrites.
References Alvarez, L.W., Alvarez, W., Asaro, F., Michel, H.V., 1980. Extraterrestrial cause for the Cretaceous–Tertiary extinction. Science 208, 1095–1108. Baud, A., Magaritz, M., Holser, W.T., 1989. Permian–Triassic of the Tethys: Carbon-isotope studies. Geol. Rundsch. 78, 649–687. Caldeira, K., Rampino, M.R., Volk, T., Zachos, J.D., 1990. Biogeochemical modeling at mass extinction boundaries: Atmospheric carbon dioxide and ocean alkalinity at the K/ T boundary. In: E.G. Kauffman and O.H. Walliser ( Editors), Extinction Events in Earth History. Springer, Berlin, pp. 333–345.
Erwin, D.H., Vogel, T.A., 1992. Testing for causal relationships between large pyroclastic volcanic eruptions and mass extinctions. Geophys. Res. Lett. 19, 893–896. Garrels, R.M., Perry, E.A. Jr., 1974. Cycling of carbon, sulfur and oxygen through geologic time. In: E.D. Goldberg ( Editor), The Sea, vol. 5. Wiley-Interscience, New York, pp. 303–336. Grossman, E.L., 1994. The carbon and oxygen isotope record during the evolution of Pangea: Carboniferous to Triassic. Geol. Soc. Am. Spec. Pap. 288, 207–228. Holser, W.T., 1977. Catastrophic chemical events in the history of the ocean. Nature 267, 399–403. Holser, W.T., Kaplan, I.R., 1966. Isotope geochemistry of sedimentary sulfates. Chem. Geol. 1, 93–135. Holser, W.T., Schidlowski, M., Mackenzie, F.R., Maynard,
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J.B., 1988. Biogeochemical cyles of carbon and sulfur. In: C.B. Gregor, R.M. Garrels, F.T. Mackenzie and J.B., Maynard (Editors), Chemical Cycles in the Evolution of the Earth. Wiley, New York, pp. 105–173. Holser, W.T., Magaritz, M., Ripperdan, R.L., 1995. Global isotopic events. In: O.H. Walliser (Editor), Global Events and Event Stratigraphy in the Phanerozoic. Springer, Berlin, pp. 63–88 [1996 ]. Holser, W.T., Scho¨nlaub, H.-P., Attrep Jr., M., Boeckelmann, K., Klein, P., Magaritz, M., Orth, C.J., Fenninger, A., Jenny, C., Kralik, M., Mauritsch, M., Pak, E., Schramm, M.-M., Stattegger, K., Schmo¨ller, R., 1989. A unique geochemical record at the Permian/Triassic boundary. Nature 337, 39–44. Ivaney, L.D., Salawitch, R.J., 1993. Carbon isotopic evidence for biomass burning at the K–T boundary. Geology 21, 487–490. Joachimski, M.M., Buggisch, W., Anders, T., 1994. Mikrofazies, Conodontenstratigraphie und Isotopengeochemie des Frasne/Famenne-Grenzprofils Wolayer Gletscher ( Karnischen Alpen). Abh. Geol. Bundesanst. [ Wien] 50, 183–195. Kaufman, A.J., Knoll, A.H., 1995. Neoproterozoic variations in the C-isotopic composition of seawater: Stratigraphic and biogeochemical implicatons. Precambrian Res. 73, 27–49. Kump, L.R., 1991. Interpreting carbon-isotope excursions: Strangelove oceans. Geology 19, 299–302. Loutit, T.S., Pisias, N.G., Kennett, J.P., 1983. Pacific Miocene carbon isotope stratigraphy using benthic foraminifera. Earth Planet. Sci. Lett. 66, 48–62. Magaritz, M., 1989. 13C minima follow extinction events: A clue to faunal radiation. Geology 17, 337–340. Malkowski, K., Gruszczynski, M., Hoffman, A., Halas, S., 1989. Oceanic stable isotope composition and a scenario for the Permo-Triassic crisis. Hist. Biol. 2, 289–309. Marshall, J.D., 1992. Climatic and oceanographic isotopic signals from the carbonate rock record and their preservation. Geol. Mag. 129, 143–160.
O’Keefe, J.D., Ahrens, T.J., 1982. The interaction of the Cretaceous/Tertiary bolide with the atmosphere, ocean and solid earth. Geol. Soc. Am. Spec. Pap. 190, 103–120. Perch-Nielsen, K., McKenzie, J., He, Q., 1982. Biostratigraphy and isotope stratigraphy and the ‘‘catastrophic’’ extinction of calcareous nannoplankton at the Cretaceous/Tertiary boundary. Geol. Soc. Am. Spec. Pap. 190, 353–371. Rau, G.H., Froelich, P.N., Takahashi, T., Des Marais, D.J., 1991. Does sedimentary organic d13C record variations in Quaternary ocean [CO (aq)]? Paleoceanography 6, 335–347. 2 Romanek, C.S., Grossman, E.L., Morse, J.W., 1992. Carbon isotope fractionation in synthetic aragonite and calcite: Effects of temperature and precipitation rate. Geochim. Cosmochim. Acta 56, 4319–4320. Rush, P.F., Chafetz, H.S., 1990. Fabric-retentive, non-luminescent brachiopods as indicators of original d13C and d18O composition: A test. J. Sediment. Petrol. 60, 968–981. Veizer, J., Holser, W.T., Wilgus, C.K., 1980. Correlation of 13C/12C and 34S/32S secular variations. Geochim. Cosmochim. Acta 44, 579–587. Wignall, P.B., Hallam, A., 1992. Anoxia as a cause of the Permian/Triassic extinction: Facies evidence from northern Italy and the western United States. Palaeogeogr. Palaeoclimatol. Palaeoecol. 93, 21–46. Williams, D.F., Lerche, I., Full, W.F., 1988. Isotope Chronostratigraphy: Theory and Methods. Academic Press, San Diego, 345 pp. Wolbach, W.S., Gilmour, I., Anders, E., 1990. Major wildfires at the Cretaceous/Tertiary boundary. Geol. Soc. Am. Spec. Pap. 247, 391–400. Woodruff, F., Savin, S.M., 1989. Miocene deepwater stratigraphy. Paleoceanography 4, 87–140. Zachos, J.C., Arthur, M.A., Dean, W.E., 1989. Geochemical evidence for suppression of pelagic marine productivity at the Cretaceous/Tertiary boundary. Nature 337, 61–64.
Review Paper
Geochemical events documented in inorganic carbon isotopes W.T. Holser Departments of Geological Sciences, Cornell University, Ithaca, NY 14853 and University of Oregon, Eugene, OR 97403, USA Received 17 February 1995; accepted 19 March 1997
Abstract Screening of sampling and analytical data is crucial for the detection of real global shifts in marine d13C, against the noise of diagenesis. The real global shifts of 13C are of two types: (1) long-term secular shifts caused by changes in the fractional burial of organic carbon, and (2) transient shifts caused by abrupt changes of biological productivity in the surface photic zone. Both are applicable to stratigraphic correlation, but differ in their application to modelling of changes in oceanographic or atmospheric composition. A recent compilation found 34 carbon isotope events in the Phanerozoic and Neoproterozoic, equally distributed among positive and negative shifts. This in itself attests to a variety of proximate causes of the shifts. © 1997 Elsevier Science B.V. Keywords: carbon isotopes; carbon rocks; sedimentary rocks; correlation; diagenesis; secular variation
1. Introduction An appreciation of the possibilities of variations in carbon isotopes in ancient carbonates (d13C =d13C for this chapter) was first deduced carb by Garrels and Perry (1974) as theoretically necessary given the previously known inverse variations in the d34S of sedimentary sulfate (Holser and Kaplan, 1966), and assuming constant atmospheric composition. The model was tested by Veizer et al. (1980) and found statistically significant, but with a remaining residue of truly independent variations of d13C and d34S: variations both on the scale of geological periods (~107 yr), and in the sharper scale then known for d34S (Holser, 1977). During the past decade evidence has accumulated from many independent sources for variations in d13C, and a very recent review lists 34 events well documented on a global scale through 0031-0182/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S 00 3 1 -0 1 8 2 ( 9 7 ) 0 0 0 70 - 9
the Phanerozoic and Neoproterozoic ( Holser et al., 1995).
2. Sample selection The overriding concern in methodology is to minimize diagenetic alteration, which in many localities is pervasive but not unavoidable. Selection begins with collection that emphasizes stable mineralogy and components (Fig. 1) (Marshall, 1992), petrographic selection for primary character, and chemical screening for low Mn–high Sr: these and other practical aspects have been recently reviewed (Grossman, 1994; Holser et al., 1995). When the data set is assembled, screening continues by eliminating samples whose analyses of other isotopes are suspect: d18O carb much lower than expected, or 87Sr/86Sr much
174
W.T. Holser / Palaeogeography, Palaeoclimatology, Palaeoecology 132 (1997) 173–182
Fig. 1. Classification of sample types in terms of potential for preservation of the marine carbon isotope signal. After Marshall (1992).
higher than expected, or shifts of d13C differing org from those for d13C on the same samples. A carb final test is whether the pattern of variation in d13C can be recognized in separate sections or distant basins, within the time frame of the available biostratigraphy. Applying the above and related criteria, both whole-rock samples and microdrilled samples have been found useful. As long as samples of altered rock are either initially avoided or eventually discarded, by even-handed criteria, many more useful data points can be obtained by whole-rock analysis than by analyzing a number of subsamples microdrilled from single shells. Good shells for analysis may be difficult to find, and in some instances (brachiopods in shale or marl ) their isotopic ‘‘milieu’’ may not be as homogenous as in a uniform micritic limestone. But even the most careful selection of samples and data can still give anomalous results (Rush and Chafetz, 1990).
3. Analysis Methods of phosphoric acid evolution and subsequent mass spectrometry of the evolved CO are 2 standard procedure, with modification as necessary
to deal with microsamples. The carbonate analysis yields both d13C and d18O . carb carb 4. Steady state isotopic carbon cycle The details of the exogenic cycle of carbon and its isotopes, are discussed in many review papers (e.g., Holser et al., 1988). The dominant mechanism that determines the general level of d13C is photosynthetic reduction of oxidized carbon to organic carbon, either in the photic zone of the ocean, or (post-Ordovician) on land. Explicitly, the d13C of the remaining marine bicarbonate depends on the (net) fraction of carbon reduced to organic carbon and buried in sediments, and on how much this buried carbon differs in its isotopic composition from the oxidized carbon — the isotopic fractionation, D13C=d13C − org d13C , of the photosynthetic reduction process. carb The isotopic fractionation depends on the photosynthetic ‘‘pathway’’, on whether photosynthesis takes place in a marine (sea water) or non-marine (air) milieu, on the concentration of dissolved inorganic carbon (or the partial pressure of CO ), and on temperature (e.g., Rau et al., 1991). 2 Because of these factors, global d13C can shift with
W.T. Holser / Palaeogeography, Palaeoclimatology, Palaeoecology 132 (1997) 173–182
time. The marine bicarbonate is ‘‘sampled’’ by CaCO crystallization in marine fossils and lime3 stones, with a fractionation of only 1 to 2.7‰, virtually independent of temperature and rate of precipitation (Romanek et al., 1992). Most photosynthesized organic carbon is low in d13C (by −20 to −30‰); a nominal mean value is d13C =−25‰. Most of the photosynthesis takes org place in the surface waters of the ocean (<100 m), and to the extent that the generated organic carbon is buried in the sediments (in either the deep or shallow ocean) the surface zone becomes enriched in 13C — that is, d13C increases. The d13C recorded in shallow-water carbonates under long-term steady-state conditions is thus a measure of (net) worldwide carbon burial. Worldwide d13C will differ at different geological times, depending on whether oceanic conditions favor burial (anoxic bottom conditions, fast sedimentation, high productivity) or re-oxidation by dissolved O . Organic 2 carbon is buried in the sediments and completes its cycle only after uplift and erosion or re-mobilization by metamorphism or volcanism (including re-oxidation). Secular change in this cycle is a consequence of shifts in the proportions of net fluxes of C and C : inputs by rivers org carb minus outputs to burial.
5. Transients in the carbon-isotope cycle Transient fluctuations of d13C caused by prompt changes in the carbon cycle have been discussed by Kump (1991) and Holser et al. (1995), among others. A high level of net biological productivity in the surface zone raises its d13C level relative to the deep main ocean, typically by 1 or 2‰. This ‘‘biological pump’’ establishes a surface-to-deep gradient of d13C, as illustrated in the inner circuit of Fig. 2. At steady state, the d13C of surface waters is established by river input and burial of organic carbon, the level of d13C in deep waters is depressed below that of the much smaller surface reservoir by the amount of the surface-to-deep gradient. This can be detected by comparing the d13C of pelagic and benthic fossils picked from the same deep-water samples. Sharp shifts in productivity will generate corresponding signals in sediments orginating in both the surface and deep
175
Fig. 2. Two competing circuits that are parts of the exogenic carbon cycle. Common to both circuits is the surface layer (photic zone) of the ocean, the site of primary photosynthetic production of organic carbon. In the outer circuit, a fraction of this organic carbon is buried in sediments for a substantial geologic interval (‘‘biological dump’’). The sequestration of this d13C-depleted carbon raises the d13C of the residual bicarbonate of the surface ocean in proportion to the fraction of organic carbon buried. In the inner circuit, a ‘‘biological pump’’ takes organic carbon sinking below the photic zone and reoxidizes it to carbonate, which is then circulated back to the surface. No carbon is lost in this circuit, but a surface-to-deep gradient of d13C is maintained (d13C >>d13C ). After Kump (1991) surface deep and Holser et al. (1995).
water levels, whereas the long term level of d13C is essentially determined by carbon burial. The relations between these two circuits of the carbon cycle are demonstrated in Fig. 3 by qualitative model carbon isotope profiles (Holser et al., 1995). In Fig. 3a we see the negative shift of both surface and deep isotope ratio consequent to a sharp decrease of organic carbon burial, during which d13C of the benthic record (dashed line) mimics that of the pelagic record (solid line), because the productivity has been held constant. In contrast, in Fig. 3d, sharply decreased productivity imposes an abrupt negative d13C excursion
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Fig. 3. Sketch diagrams of hypothetical carbon isotope profiles consequent to a sharp decrease of (a) burial in the biological dump, (d) sequestration in the deep by the biological pump, and (b) and (c) combinations of both activities. Solid line: surface waters; dashed line: deep waters. After Holser et al. (1995).
on the smaller surface reservoir, while the longterm level of d∞13C remains constant, owing to a constant fraction of organic carbon burial. Fig. 3b and c illustrate the signatures of mixes of decreased productivity and burial. Sharp increases in productivity or carbon burial will be recorded as analogous positive excursions of short and long time constant, respectively (not illustrated).
6. Examples of carbon-isotope events Carbon isotope events can be applied in stratigraphic correlation on a more or less empirical basis, analogous to correlation of downhole electric logs ( Williams et al., 1988). More weight is given to the form and shape of variations than
to the exact level of d13C, as the general level measured in today’s ocean is found to differ by 1‰ or more depending on mineralogy, on ‘‘vital effect’’ (differences among species) and on the ‘‘age’’ of the water mass. Fig. 4 is a classic example of such a correlation, based on d13C values in Miocene deep-sea cores (Loutit et al., 1983; see also Woodruff and Savin, 1989). Fig. 5 shows that the dramatic drop in d13C across the Precambrian/Cambrian boundary, in Morocco, India, Siberia and Canada ( Kaufman and Knoll, 1995) can be used for chemostratigraphic correlation. An isotopic event that has been studied in detail is the one associated with the Cretaceous–Tertiary ( K–T ) boundary (Zachos et al., 1989; Kump, 1991). As illustrated in Fig. 6, it was possible to
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Fig. 4. Correlations of carbon isotope events among five deep-sea cores of Miocene age from the southwest Pacific. After Loutit et al. (1983).
Fig. 5. Correlation of secular variations of d13C vs. stratigraphic depth for carbonates deposited in the Precambrian–Cambrian interval, from Morocco, India, Siberia and Canada (after Kaufman and Knoll, 1995). Only six of ten correlated sections are shown here.
measure carbon isotopes in both the surface (by pelagic forms) and deep-water (by benthic forms) reservoirs. The convergence of these two traces, shown in more detail in Fig. 7 by plotting different surface-to-deep gradients, signals a major decline
in productivity, which remained suppressed for nearly a million years. This negative d13C excursion is found worldwide (Perch-Nielsen et al., 1982). The corresponding failure of productivity is ascribed to decreased sunlight under a global dust
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Fig. 6. The K–T event expressed in carbon isotope shifts. Pelagic values drop almost to zero, cancelling the previous surface-to-deep gradient, and recover only slowly ( Zachos et al., 1989).
cloud consequent to a bolide impact (Alvarez et al., 1980). The decline of productivity was not only caused by a sharp reduction of marine photosynthesis but also by a large-scale decline of terrestrial organic carbon (by burning: Ivaney and Salawitch, 1993). In contrast, only a minor shift of long-term carbon burial is evident. A major anomaly associated with the Permian–Triassic (P–Tr) boundary, which is probably best developed in the Gartnerkofel (Austria) core (Holser et al., 1989; see also Baud et al., 1989; Grossman, 1994), may be re-interpreted in terms of the described model. In this carbon
isotope profile, d13C remains very high throughout the Late Permian, as can be seen at the bottom of the core in Fig. 8. During the last part of the Late Permian d13C drops at an accelerating rate through the boundary, to a transient minimum in the Early Triassic, from which it slowly recovers, interrupted by one or two subsequent lesser minima. Note that d13C never recovers to Permian values. As indicated on the figure, this difference between Permian and Triassic d13C represents a decrease in long-term burial of organic carbon. The transient minimum of −3.4‰, from which the system eventually recovers, is interpreted as a drop in
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Fig. 7. Surface-to-deep d13C gradient across the K/T boundary, the sharp drop signifies the demise of primary productivity ( Zachos et al., 1989).
productivity. The later subsidiary minima probably represent further drops in productivity, but these have not yet been verified as global effects. The mass of carbon involved in the transient shift is amplified in its effect (about 30×) in inverse relation to the mass of the deep water carbon reservoir. Thus in the Gartnerkofel a transient shift of −3.4‰ involves an amount of carbon that is only 3.4/30=0.l when compared with the amount of carbon involved in the long-term shift of 1.6‰. Another carbon isotope anomaly that has been well documented recently is associated with the Frasnian–Famennian ( Fr–Fa: Late Devonian) boundary. Joachimski et al. (1994) illustrate half a dozen sections from the Alps to the Harz Mountains in northern Germany, which show two
(the Lower Kellwasser and the Fr–Fa events, respectively) sharp rises in d13C, each followed by a slower but nearly full recovery (see Fig. 9). Comparison with the above described model suggests that the transient positive excursion represents a rapid onset of productivity, with only minimum participation of burial transfer. Such a fast rise of productivity might have been caused by a general algal bloom (‘‘red tide’’), fed by upwelling nutrients driven by a climatic shift.
7. Causes of carbon isotope events As explained above, the surface zone of the oceans, which is a reservoir only 1/30 as large as
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excursion of d13C in the surface record (Malkowski et al., 1989; Wignall and Hallam, 1992). Perturbations of sub-crustal metamorphic/igneous storage of carbon may release a large injection of light carbon into the exogenic cycle, especially via continental flood basalt eruptions (Caldeira et al., 1990; Erwin and Vogel, 1992). A bolide impact in carbonate terrane would inject a large dose of CO into the atmosphere (O’Keefe and Ahrens, 2 1982), but with little contrast in the isotopic ratio. Global forest fires produced by bolide impact would release substantial volumes of CO as well 2 as soot C , both represented by light d13C org ( Wolbach et al., 1990). Our census of carbon-isotope events in the Phanerozoic and Neoproterozoic (Holser et al., 1995) lists 34 events: 12 positive excursions, 11 negative excursions, 6 simple rises and 5 simple falls — although in some instances the distinctions are arbitrary. The degrees of association with the forcings of productivity, anoxia, overturn, burial, volcanicity, impact and fire are various and without any emergent pattern. Magaritz (1989) proposed that negative excursions of d13C would follow major extinction events, but even this correlation is not without its exceptions. 8. Conclusions
Fig. 8. Isotope profile through the P–Tr boundary in the Gartnerkofel-1 core, Carnic Alps of Austria, after Holser et al. (1989). The curve may be decomposed into two components: a long-term shift of −1.5‰ owing to release of organic carbon from burial, and a transient shift of −2.5‰ recording a failure of productivity.
the deep sea, consequently magnifies the response of its d13C, to interruptions of the carbon cycle, whatever their origin, so long as they are initiated in the smaller surface reservoir. This condition applies in varying degrees in the cases described above. Some additional examples follow. Overturn of an anoxic ocean may introduce a negative
Despite widespread effects of late diagenesis on the carbon isotope record, an important fraction of isotope events can be verified on a global scale. Examples include a range of short-term transient incidents involving productivity changes and longterm shifts of organic carbon burial. Both types of shift may be applied empirically to long-distance correlations, but are better understood if the factors of productivity and burial are separately applied to pelagic and benthic carbonates, or by distinguishing the transient vs. secular trends, as described above. The shapes of the events are variable, so must their causes be multiple. Acknowledgements I thank Ethan Grossman, Lee Kump and Jan Veizer for helpful comments on an earlier draft of the manuscript.
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Fig. 9. Carbon isotope profiles through the Upper Devonian in Germany (Joachimski et al., 1994); only three of the seven correlated sections are shown here. The two sharp positive excursions, followed by slower recovery, may be interpreted as transient onsets of productivity, with minimal participation of long-term release of carbon from burial. The black zones represent black laminated micritic horizons, which however are not present in another section in the Carnic Alps. The high productivity/carbon isotope event may be global, expressed only locally as black micrites.
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