EXTREME HEAT STRESS ASSOCIATED WITH CARBON CYCLE PERTURBATION ACROSS THE TRIASSIC-JURASSIC BOUNDARY AS INDICATED BY TERRESTRIAL PLANT BIOMARKERS AND THEIR COMPOUND SPECIFIC STABLE HYDROGEN AND CARBON ISOTOPES

Major and abrupt ecological change including 80% extinction among terrestrial plant species coincides with increased atmospheric carbon dioxide concentration (CO_2atm, based on stomatal analysis of fossil Ginkoales leaves) and a negative excursion in δ¹³C of fossil wood from a Triassic-Jurassic boundary section at Astartekløft, East Greenland [1]. Here we report abundance and compound specific stable hydrogen and carbon isotope data for terrestrial plant biomarkers from the same section.

δ¹³C of the C₂₇ n-alkane (n-C₂₇, a constituent of leaf waxes synthesized by many plants) shows a 5‰ negative shift corresponding with similar shifts in fossil wood, increasing CO_2atm and plant extinctions.

During the interval containing the rise in CO_2atm prior to peak plant extinctions, δD of n-C₂₇ shifts positively from -125‰ to -85‰, which we attribute in part to an increasing degree of evaporative fractionation in leaf water due to increasingly elevated leaf temperatures. In strata containing consistent indicators of elevated CO_2atm, δD of n-C₂₇ shifts negatively to -158‰. This may represent a temperature-driven increase in photosynthetic isotopic fractionation by surviving plants which tended to have smaller leaves less prone to heat stress and evaporative fractionation. In the uppermost part of the section, where CO_2atm declines, δD of n-C₂₇ shifts positively again to -136‰, similar to the pre-extinction value.

The rise in CO_2atm is also accompanied by the appearance of significant concentrations of a series of terpenoid compounds (e.g. dehydroabietane) derived from gymnosperm resins, similar to reports from Arctic sediments deposited during the Paleocene-Eocene thermal maximum [2]. Trends in δD and δ¹³C of terpenoids are similar, though less clearly resolved than those for n-C₂₇. In the bed containing peak plant extinctions, the dominant resin-derived terpenoid is over twenty times as abundant as the leaf wax-derived n-C₂₇. We suggest that this is due to the increased production of resins by gymnosperms under extreme environmental stress.

[1] J.C. McElwain, P.J. Wagner, S.P. Hesselbo, 2009. Science 324: 1554-1556.

[2] S. Schouten, M. Woltering, I.C. Rijpstra, A. Sluijs, H. Brinkhuis, J.S. Sinninghe Damsté, 2007. Earth Planet Sc Lett 258: 581-592.