Carbon isotope compositionof Permian and Triassicplants in silicified peat,

Transantarctic Mountains

GUNTER FAURE and TERESA M. MENSING

Department of Geology and Mineralogyand

Byrd Polar Research CenterOhio State UniversityColumbus, Ohio 43210

EDITH L. TAYLOR

Department of Botanyand

Byrd Polar Research CenterOhio State UniversityColumbus, Ohio 43210

The Permian and Triassic rocks of the Beacon Supergroupin the vicinity of the Beardmore Glacier contain deposits ofsilicified peat first described by Schopf (1970). The originaldiscovery by John Mercer and John Gunner was on the south-eastern spur of Mount Augusta (84°47'S 163°15'E) overlookingthe Beardmore Glacier. Well-preserved glossopterid plant frag-ments from this locality were later studied by Pigg and Taylor(1985).

A second deposit of silicified peat occurs on Fremouw Peak(84°17'S 164°18'E) adjacent to the Walcott Névé in rocks of thelower Middle Triassic Fremouw Formation. Taylor and Smoot(1985) described woody tissue of Cycadales from this locality.Wood from the silicified peat deposits on both Fremouw Peak(Middle Triassic) and Mount Augusta (Middle to Late Permian)shows evidence of decay by fungi that is similar to whitepocketrot attributed to extant members of Basidiomycotina (Stubble-field and Taylor 1985).

The flora preserved at Fremouw Peak contains herbaceousferns, foliage of the seed fern Dicroidium, gymnospermous woodand seeds, fungi, and remains of cycads (Smoot, Taylor, andDelevoryas 1985; Stubblefield and Taylor 1986; Taylor, Taylor,and Collinson 1986). The silicified peat from Fremouw Peak iscontinuing to yield information on the anatomy of Triassicplants including Dicroidiuni (Pigg and Taylor 1987), reproduc-tive organs of gymnosperms (Taylor and Taylor 1987), andferns (Milay, Taylor, and Taylor 1987). These plants apparentlycoexisted in a forested swamp and accumulated to form het-erogeneous peat which was broken up by a stream. Fragmentsof this peat were redeposited in the stream, rapidly buried inmedium-grained volcaniclastic sand, and later silicified (Tay-lor, Taylor, and Collinson 1986).

The deposit at Mount Augusta, now known as the "SkaarRidge site" (Taylor et al. 1986), is located in the upper BuckleyFormation (Late Permian) and contains silicified remains ofVertebraria and Glossopteris, including plant organs, under-ground axes, and leaves. In addition, Taylor et al. (1986) re-ported finding moss leaves, axes, and attached rhizoids as wellas gymnospermous seeds containing well-preserved embryos(Smoot and Taylor 1986). Although this deposit is more than

5 million years older than the silicified peat on Fremouw Peak,it probably formed and was preserved under similar circum-stances.

The macroflora at Fremouw Peak and at Skaar Ridge is pre-served as amorphous carbon embedded in chert. Therefore, itshould be possible to determine the isotopic composition ofcarbon not only in bulk samples but also in selected plantorgans. To test the feasibility of this idea, we have measuredthe isotopic composition of carbon in the two samples of sil-icified peat from Fremouw Peak (shown in the figure) and fromSkaar Ridge.

The two specimens were sawed into four pieces (sample671, Fremouw Peak) and two pieces (sample 672, Skaar Ridge)which were crushed in a steel mortar and treated with reagent-grade hydrofluoric acid in a fume hood for several days. Theparticles of amorphous carbon released by the acid treatmentwere recovered and washed repeatedly with tap water andwith demineralized water. (A saturated solution of sodiumchloride was used in the separation of carbon from sample 671but not in the case of sample 672.) Copious amounts of carbonwere recovered from both samples. Aliquots of the carbon werecombusted in pure diatomic oxygen to form carbon dioxidewhich was analyzed to determine the isotopic composition ofthe carbon, expressed as the delta carbon-13 parameter relative

SILICIFIED PEATTriassicFremouw Peak, Antarctica

Sample 671, silicified peat from the Fremouw Formation (early Mid-dle Triassic), was collected from Fremouw Peak, Walcott Névé.

26 ANTARCTIC JOURNAL

Isotopic composition of amorphous carbon from silicified peatdeposits in the Beacon Supergroup, Transantarctic Mountains

This study was supported by the National Science Foun-dation grant DPP 88-16236.

Sample Locality Delta carbon-13, permil

671A Fremouw Peak -23.8, -23.8671B Fremouw Peak -24.0, -24.1671C Fremouw Peak -24.2671D Fremouw Peak -23.6672A Skaar Ridge -24.4672B Skaar Ridge -24.7

I Krueger Enterprises, Inc., Cambridge, Massachusetts.

to the PDB standard (Faure 1986). The results are listed in thetable.

All of the carbon samples are depleted in carbon-13 by about24 permil or 2.4 percent relative to the isotope standard (PDB).Therefore, the isotopic composition of this carbon is similar tothat of modern "C3" plants that fix carbon from the atmos-phere by the Calvin cycle and whose delta carbon-13 valuesrange from about -23 to -34 permil (Deines 1980; Hoefs1987). Another group of plants ("C4," including corn and trop-ical grasses) fix carbon by a different photosynthetic processknown as the Hatch-Slack cycle. Such plants have systemati-cally different delta carbon-13 values between -6 and -23permil.

The initial results in the table indicate that the Permian andTriassic peat contains "C3" rather than "C4" plants. The smalldifferences in the delta carbon-13 values of the individual piecesof each specimen (671: -23.8 to -24.2 and 672:- 24.4 to -24.7permil) indicate that the carbon in the peat is not isotopicallyhomogeneous. Such internal variations may be caused by thewell-known isotopic diversification of carbon in different plantorgans and in different biogenic compounds. In addition, theisotopic composition of carbon may have been altered by thepreferential decomposition of certain chemical compounds(cellulose, lignin, lipids, etc.) after the plant remains were de-posited to form peat.

References

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Faure, G. 1986. Principles of isotope geology (second ed.). New York:Wiley.

Hoefs, J. 1987. Stable isotope geochemistry. Berlin: Springer-Verlag.Milay, M.A., T.N. Taylor, and E.L. Taylor. 1987. Studies of antarctic

fossil plants: An association of ferns from the Triassic FremouwPeak. Antarctic Journal of the U.S., 22(5), 31-32.

Pigg, K.B., and T.N. Taylor. 1985. Anatomically preserved Glossop-tens from the Beardmore Glacier area of Antarctica. Antarctic Journalof the U.S., 20(5), 8-10.

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Stubblefield, S.P., and T.N. Taylor. 1986. Wood decay in silicifiedgymnosperms from Antarctica. Botany Gazette, 147, 16-125.

Taylor, T.N., and E.L. Smoot. 1985. A new Triassic cycad from theBeardmore Glacier area of Antarctica. Antarctic Journal of the U.S.,20(5), 5-7.

Taylor, TN., E.L. Taylor and J.W. Collinson. 1986. Paleoenvironmentof Lower Triassic plants from the Fremouw Formation. AntarcticJournal of the U.S., 21(5), 26-27.

Taylor, E. T. Taylor, J. W. Collinson, and D. H. Elliot. 1986. Struc-turally preserved Permian plants from Skaar Ridge, Beardmore Gla-cier region. Antarctic Journal of the U.S., 21(5), 27-28.

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