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FLORAL CARBON ISOTOPIC RECORD, ISCHIGUALASTO FORMATION (UPPER TRIASSIC), NORTHWEST OF ARGENTINA: PALEOATMOSPHERIC IMPLICATIONS. The Upper Triassic Ischigualasto Formation carries a well-preserved flora in the mid-upper portion (~225 Ma Carnian). This contribution is the first detailed study of the isotopic ratios (i.e. 13 Carbon vs. Carbon 12 δ 13 C) and the ratio of carbon and nitrogen (C/N) applied to Argentinean Triassic flora. The aim of this study is to chemically characterize this Triassic flora and provide data for this portion of the Pangea to be used in chemostratigraphic correlation as well as to analyze changes in the global carbon cycle. A total of 97 samples were analyzed, mainly of cuticle. The isotopic values of the samples indicate minimal diagenetic or taphonomic alteration, validating the interpretations reached. A detailed analysis of the isotopic values of this paleoflora documents minor variation in δ 13 C characteristic of differences in levels of local water-stress and of floral composition variations. Discarding minor variations, a mean value of the carbon isotope ratio of paleoflora Ischigualasto Formation of - 25.4‰ was obtained within a range from -27.4 to -23.7‰, according to the average values of C3 plants. This value is consistent with the δ 13 C values reported for terrestrial plant fossils from other Upper Triassic successions worldwide, however is enriched by about 3 ‰ with respect to the Middle Triassic plants. This migration towards more positive values of δ 13 C coincide with those observed in marine carbonates and paleosols during the same time-interval, indicating a possible disturbance in the carbon cycle during Early Mesozoic. Finally, a paleo-atmosphere isotopic composition of -6.1‰ was estimated, which differs by ~ 0.5 ‰ from that inferred using the marine carbonate proxy for the ocean-reservoir. This difference can be the response of the oxygen to carbon dioxide ratio in the paleo-atmosphere, which produces differences in the plant isotopic fractionation, diagenesis in the marine carbonates or hydric-stress alteration of the Ischigualasto flora.

revista brasileira de paleontologia

Registro de la relación isotópica de carbono en la paleoflora de la Formación Ischigualasto (Triásico Superior), noroeste Argentino: implicaciones paleoatmosféricas