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A steep, mid- to late Paleozoic decline in atmospheric CO2: evidence from the soil carbonate CO2 paleobarometer

A steep, mid- to late Paleozoic decline in atmospheric CO2: evidence from the soil carbonate CO2 paleobarometer
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  Chemical Geology 107 ( 1993 ) 217-219 217 Elsevier Science Publishers B.V., Amsterdam A steep, mid- to late Paleozoic decline in atmospheric C02: evidence from the soil carbonate CO2 paleobarometer Claudia I Mora and Steven G. Driese Department of Geological Sciences University of Tennessee Knoxville TN 37996 USA (Received March 9, 1993; revised and accepted March 30, 1993 ) Long-term fluctuations in the CO2 content of the paleoatmosphere may have played a critical role in determining ancient paleocli- mates (cf. Crowley and North, 1991 ). Deter- mining the magnitude of these fluctuations is therefore an important problem, but a difficult one, as direct measurements of the paleoat- mosphere are only possible for the immediate past. One promising CO2 paleobarometer uses the carbon isotope composition of pedo- genic (soil-formed) carbonate to infer atmo- spheric levels of CO2 (Ceding, 1991 ). We have applied this paleobarometer to pedogenic car- bonate formed in Paleozoic, vertic paleosols from the central and southern Appalachian basin (Mora et al., 1991; Driese et al., 1992; Driese and Mora, 1993). Theoretical models point to the Paleozoic Era as a period of ex- treme fluctuations in atmospheric Pco2, char- acterized by a steep drop in CO2 levels be- tween the early Paleozoic (Ordovician) and the onset of the extensive Permo-Carbonifer- ous glaciation (Berner, 1991 ). Our results in- dicate a decrease in atmospheric Pco2 from 3800-5500 ppmV in the Late Silurian to 200- 350 ppmV in the Early Permian, in excellent qualitative agreement with the steep decline in CO2 suggested by the long-term carbon cycle model. Indirect measurements of paleoatmospheric CO2 contents involve a substantial amount of assumptions and reliable, reproducible, and self-consistent results depend on a clear under- standing of these assumptions and a disci- plined sampling approach. Cerling ( 1991 ) carefully described the assumptions inherent in the soil carbonate paleobarometer, the most obvious being the true pedogenic character of the carbonate sampled. We have applied this paleobarometer to paleosols of a highly restric- tive srcin and can place additional con- straints on its application, based on our under- standing of the pedogenic processes and the environment of pedogenic carbonate precipi- tation. Carbonate-bearing paleosols come in many varieties, each with distinctive physical and chemical characteristics that may poten- tially affect the ~3C value of the pedogenic carbonate. Our focus on clay-rich, vertic pa- leosols, which contain little or no pre-existing carbonate greatly reduces the likelihood of car- bon isotope inheritance. Pedogenic carbonate in these paleosols as- sume two general morphologies that occur in different and identifiable zones within the pa- leosol: spherical micrite nodules and micritic rhizoliths. Nodules form higher in the soil pro- file, within the zone of soil shrink-swell. They are invariably enriched in ~3C relative to rhi- zoliths in the same profile. This enrichment most likely reflects an increased concentration of isotopically-heavy, atmospheric CO2 in the upper portions of the soil; vertic soils may crack open to depths of >~ 1 m during the dry season 0009-2541/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.  218 C.l. MORA AND S.G. DRIESE (Dudal and Eswaran, 1988; Wilding and Tes- sier, 1988). Rhizoliths formed at slightly deeper levels, below the lowermost occurrence of pedogenic slickensides, and have a more distinctly biogenic micromorphology. Regard- less of the size and geological age, rhizoliths share a distinct sequence of carbonate cemen- tation, identified using cathodoluminescence petrography and trace element analysis. The earliest-precipitated phase is a dull red-brown luminescent micrite, followed by one or more generations of calcite spar cements which oc- clude the centers of the rhizoliths, as well as circumgranular cracks and septarian voids in nodules. Microsamples of the early-formed micrite, which resemble micrite root-sheaths formed around geologically-young plant roots (cf. Mount and Cohen, 1984), are used exclu- sively for CO2 estimation. The carbon isotope composition of micritic rhizoliths are isotopi- cally-lighter and less variable than composi- tions in coexisting nodules. Isotopic data on rhizoliths from time-equivalent paleosol sec- tions are remarkably consistent: data from five geographically-distinct, time-equivalent pa- leosols from the Late Devonian Catskill For- mation yield ~3C values of 9.81_+0.31°/0o (PDB), regardless of their development in proximal-alluvial or coastal-marine soils. In contrast, ~13C values of nodules in these same paleosols scatter over a range of almost 4%0. The distinct carbon isotope signatures of nod- ules vs. rhizoliths have clear implications for the soil carbonate paleobarometer: composi- tions in nodules yield pco2 estimates up to 30% larger than estimates made from rhizoliths in the same paleosol. Other key assumptions that must be made in order to estimate Pco2 using the soil carbonate paleobarometer are more difficult to quantify. These variables include the ~3C of atmo- spheric CO> the ~13C of soil-respired CO2 and the amount of soil-derived CO2 [Cerling's S(z) function, where S z)=Pco2~,soio- Pco~ ..... pho~o~ , which is in part a function of the soil respiration rate. To a first approximation, we can fix the atmospheric c~ ~ C to the isotopic composition of marine carbonate, using a frac- tionation of ~ 7%0. The model is not particu- larly sensitive to small changes in atmospheric 13C, but significant differences in Pco2 esti- mations (25-50%) may result during periods of significant positive excursion in the marine carbonate record (e.g., during the Late Missis- sippian and Early Permian; Popp et al., 1986 ). The extent to which changes in the terrestrial organic C composition will be affected by al- teration of the atmospheric ~3C is poorly understood and, unfortunately, organic C has very poor preservation potential in these clay- rich, oxidized, red-bed paleosols. Lacking any direct measurements, we have fixed the terres- trial organic ~13C at -26%o for these calcula- tions (Popp et al., 1989). Presently, it is not possible to estimate the soil productivity of a fossil soil and our estimates are given for S z) in the broad range of 5000-10,000 ppmV, typ- ical for modern, well-drained, non-desert soils (Cerling, 1991 ). We note, however, that a de- crease in this function may result in a decrease in estimated atmospheric Pco2 by > 100%. Using these assumptions, estimates of Pa- leozoic paleoatmospheric C02 are: Late Silurian 408-414 Ma) Late Devonian 360-367 Ma) Late Mississippian 320-333 Ma ) Early Permian 268-286 Ma) 3800-5500 ppmV 1000-1500 ppmV 1800-3700 ppmV) 200-300 ppmV The Mississippian estimates are based on isotopic data that are of much poorer quality than data for other time periods. They are in- cluded for completeness, and as an example of the complications that can arise in this type of study. Mississippian paleosols developed in the coastal marine environment exhibit cryptic dolomitization of pedogenic calcite; most of the other Mississippian data represent nodule compositions. We persist in our attempts to lo- cate and sample micritic rhizoliths in vertic paleosols of Mississippian age that remain un- affected by significant post-pedogenic altera-  A STEEP, MID- TO LATE PALEOZOIC DECLINE IN ATMOSPHERIC CO2 2 1 9 tion. Except for the Mississippian results, these paleo-Pco2 estimates are based on more than 200 analyses from more than a dozen paleosol profiles, consistently applying the same ma- cro-and micromorphological criteria for sam- ple selection. Their consistency within a given time period and the significant differences be- tween the time periods strongly suggest the soil carbonate isotope record is indeed recording changes in paleoatmospheric CO2 levels. Clearly, all of the isotopic systematics of pe- dogenic carbonate formed in ancient soils have yet to be worked out. For example, a keener understanding of the paleoecology of the Pa- leozoic terrestrial ecosystems may help to quantify the soil productivity. However, our results suggest that, judiciously applied Cer- ling s soil carbonate model is a very promising CO2 paleobarometer which, coupled with the wide spatial and temporal distribution of ver- tic paleosols in the geologic record, may ulti- mately yield estimates of paleoatmospheric Pco,_ spanning most of the Phanerozoic. References Berner, R.A., 1991. A model for atmospheric CO2 over Phanerozoic time. Am. J. Sci., 291: 339-376. Cerling, T.E., 1991. Carbon dioxide in the atmosphere: evidence from Cenozoic and Mesozoic paleosols. Am. J. Sci,, 291: 377-400. Crowley, T.J. and North, G.R., 1991. Paleoclimatology. Oxford University Press, New York, N.Y., 339 pp. Driese, S.G. and Mora, C.I., 1993. Physico-chemical en- vironment of pedogenic carbonate formation in De- vonian vertic paleosols, central Appalachians, USA. Sedimentology (in press). Driese, S.G., Mora, C.I., Cotter, E. and Foreman, J.L., 1992. Paleopedology and stable isotope geochemistry of Late Silurian vertic paleosols, Bloomsburg Forma- tion, central Pennsylvania. J. Sediment. Petrol., 62: 825-841. Dudal, R. and Eswaran, H., 1988. Distribution, proper- ties and classification of Vertisols. In: L.P. Wilding and R. Puentes (Editors), Vertisols: Their Distribution, Properties, Classification and Management. Texas A&M University Printing Center, pp. 1-22. Mora, C.I., Driese, S.G. and Seager, P.G., 1991. Carbon dioxide in the Paleozoic atmosphere: Evidence from C-isotopic compositions of pedogenic carbonate. Ge- ology, 9: 1017-1020. Mount, J.F. and Cohen, A.S., 1984. Petrology and geo- chemistry of rhizoliths from Plio-Pleistocene fluvial and marginal-lacustrine deposits, East Lake Turkana, Kenya. J. Sediment, Petrol., 54: 263-275. Popp, B.N., Anderson, T.F. and Sandberg, P.A., 1986. Brachiopods as indicators of srcinal isotopic compo- sitions in some Paleozoic limestones. Bull. Geol. Soc. Am., 97: 1262-1269. Popp, B.N., Takigiku, R., Hayes, J.M., Louda, J.W. and Baker, E.W., 1989. The post-Paleozoic chronology and mechanism of I3C depletion in primary marine or- ganic matter. Am. J. Sci., 289: 436-454. Wilding, L.P. and Tessier, D., 1988. Genesis of Vertisols: shrink-swell phenomena. In: L.P. Wilding and R. Puentes (Editors), Vertisols: Their Distribution, Properties, Classification and Management. Texas A&M University Printing Center, pp. 55-81.
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