CARBON-ISOTOPE ANALYSES OF FOSSIL PLANTS IN TRIASSIC ISCHIGUALASTO FORMATION (ARGENTINA): PALEOECOLOGICAL IMPLICATIONS

The Triassic Ischigualasto Formation yields a rich and very well-preserved fossil flora, including roots, tree trunks, and leaves. The taphocoenoses comprise Corystospermales and Peltaspermales, interpreted as riparian forest, together with shrub-fern vegetation in undergrowth and on the floodplain environment. The age of the Ischigualasto Formation is very well known based on two radiometric dates (227.8±0.3 near the base and 218±1.7 near the top). Fossil leaves and tree trunks occur mostly in the upper part of the formation, in channel, floodplain, and marsh deposits. Stable carbon isotopic composition (δ13C) and carbon/nitrogen (C/N) ratios have been measured in 160 samples in order to investigate the average carbon isotopic values of these floras and the environmental, ecological, and taphonomic signals. The deposits contain two types of cuticles: exceptionally well-preserved, black, thick cuticles and fragmentary, orange, very thin cuticles. Only the thick cuticles are taxonomically identifiable, and most of the C isotope analyses were made on those. These gave an average δ13C of -25.27, varying between -26.91 and -22.65 which corresponds with the average of C3 plants. However, one thin cuticle was analyzed, producing an unexpected value of -15.69, which is consistent with the values of C4 plants. We also examined possible lateral and vertical variations in the carbon isotopic composition in a marsh deposit, and found a variation of δ13C values from the deeper area of the marsh to the margins, possibly the result of variation in water/salinity stress. In addition, C/N ratios were analyzed, resulting in two groups, one from deposits that likely formed in permanent standing water, and one from the channels, which likely experienced seasonally drying; the differences in C/N ratios could indicate aerobic versus anaerobic decomposition.