HR: 08:30h
AN: B41A-03    [PDF]
TI: Parallel $\delta ^{13}$C and Conifer Physiognomic Trends Across the Triassic-Jurassic Boundary
AU: * Whiteside, J H
EM: jhw@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Olsen, P E
EM: polsen@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Sambrotto, R N
EM: sambrott@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, 61 Route 9W, Palisades, NY 10964 United States
AU: Cornet, B
EM: wcornet@raritanval.edu
AF: Raritan Valley Community College, Route 28 and Lamington Road, Somerville, NJ 08876 United States
AB: The Triassic-Jurassic mass extinction event ($\sim$200 Ma) had a profound effect on biotic evolution, and herein we describe trends in cheirolepidaceous conifer leaf physiognomy from the Pangean tropics (present northeastern USA) that at least broadly parallel a negative $\delta ^{13}$C excursion recorded in the same strata. The physiognomic changes appear at an abrupt ($<10$ ky) negative carbon isotope excursion (1) synchronous with a previously described palynological extinction level, fern spike, and Ir anomaly (2), and continue through a prolonged negative excursion, lasting 900 ky (through all three CAMP basaltic extrusive events), encompassing most of the Hettangian age. The physiognomic changes seen in the cheirolepidaceous conifer leafy shoot forms {\it Brachyphyllum} and {\it Pagiophyllum} through the $\delta ^{13}$C excursions include primarily the development of microphyllous leaves with thickened cuticle and sunken papillate stomata (3). These floral modifications are consistent with intense thermal stress plausibly due to very high atmospheric CO$_{2}$ concentrations and corroborate McElwain's (4) thermal damage hypothesis for the Triassic-Jurassic transition that was originally based on different plant taxa from the higher Pangean latitudes in present Greenland and Sweden. Subsequently, a 2- to 5-fold increase in the area of leafy shoots in strata of latest Hettangian age suggest a return to lower thermal stress levels perhaps due to lower CO$_{2}$, despite the fact that eastern North America continued to drift into more arid latitudes. The floral physiognomic changes associated with the negative $\delta ^{13}$C excursion and likely very elevated CO$_{2}$ levels is in many ways a microcosm of the Mesozoic in which the dominance of cheiroleps apparently overlaps with the highest CO$_{2}$ levels of the Mesozoic (5). References. (1) Whiteside JH, Olsen PE, Sambrotto RN. 2003. Geol. Soc. Amer. Abst. Prog. (in press). (2) Olsen PE et al., Science 296:1305-1307 (3) Cornet B. 1989. in Olsen PE, Schlische RW, Gore PJW, Internat. Geol. Cong., Guidebooks T351, p. 115. (4) McElwain JC, Beerling, DJ, Woodward FI. 1999. Science 285:1386. (5) Ekart D, Cerling TR, Montanez IP, Tabor NJ. 1999. Am. J. Sci. 299:805.
DE: 1055 Organic geochemistry
DE: 1851 Plant ecology
DE: 4806 Carbon cycling
DE: 4815 Ecosystems, structure and dynamics
DE: 9609 Mesozoic
SC: Biogeosciences [B]
MN: 2003 Fall Meeting