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

Abstract

-FLORAL CARBON ISOTOPIC RECORD, ISCHIGUALASTO FORMATION (UPPER TRIASSIC), NORTHWEST OF ARGENTINA: PALEOATMOSPHERIC IMPLICATIONS. The Upper Triassic IschigualastoFormation 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.Key words: carbon, isotopes, Ischigualasto Formation, paleoatmosphere, paleoflora, Triassic.RESUMO -A Formação Ischigualasto (Triássico Superior) é portadora de uma paleoflora muito bem preservada na sua porção médio-superior (~225 Ma.). No presente trabalho são detalhados os primeiros estudos isotópicos (i.e. carbono 13 vs. Carbono 12 = δ 13 C) e a relação entre carbono e nitrogênio (C/N) aplicados à flora triássica argentina. Estes estudos foram direcionados para caracterizar quimicamente esta flora e fornecer dados para esta área do Pangea, a fim de permitir a correlação quimioestratigráfica e a análise das variações no ciclo do carbono. Foram estudadas 97 amostras, principalmente de cutículas. Os valores de δ 13 C dessas amostras não apresentam sinais de alteração diagenética nem tafonômica, fatos que validam as interpretações alcançadas. A análise detalhada dos valores das relações isotópicas desta paleoflora permiti...

Vertebrate taphonomy of the Ischigualasto Formation

“…Studies of fluvial architecture (Currie et al, 2009), paleosols (Tabor et al, 2006), and plant taphonomy (Colombi and Parrish, 2008) indicate that the vertebrates that roamed the rift valley of the Ischigualasto-Villa Unión Basin did so under variable tectonic and climatic conditions (Tabor et al, 2006;Colombi and Parrish, 2008;Currie et al, 2009;Colombi et al, 2011;Martínez et al, 2011). The large-scale variations in taphonomic attributes detailed in this study are coincident with the variability in the alluvial system produced by tectonism and climate, which control accommodation space (i.e., aggradation and erosion) and the degree of confinement of the fluvial system.…”

“…The climate during the deposition of the Ischigualasto Formation has been reconstructed based on a variety of observations (i.e., fluvial architecture, paleosols, plant taphonomy), all of which imply an overall seasonal paleoclimate with respect to rainfall regime (Stipanicic and Bonaparte, 1979;Tabor et al, 2006;Colombi and Parrish, 2008;Currie et al, 2009;Colombi et al, 2011). In the basal La Peña Member, paleoclimatic inference is complicated due to the transition from lacustrine facies of the Los Rastros Formation to fluvial depositional systems of the Ischigualasto Formation.…”

“…Perhaps most notably, the basin includes extensive exposures of the Upper Triassic Ischigualasto Formation, which has yielded a diverse and well-preserved assemblage that includes the earliest-known dinosaurs (see Rogers et al, 1993;Martínez et al, 2011Martínez et al, , 2013, basal archosauromorphs, crurotarsal archosaurs, therapsids, and amphibians. The fossil record of the Ischigualasto Formation is complemented by a wealth of associated data that relate to the sedimentology, paleoclimate, and geochronology Milana and Alcober, 1994;Shipman, 2004;Tabor et al, 2006;Colombi, 2007;Colombi and Parrish, 2008;Currie et al, 2009;Colombi et al, 2011;Martínez et al, 2011). The Ischigualasto Formation offers one of our best opportunities to study terrestrial vertebrate evolution during the Late Triassic, during which important faunal turnovers occurred, including the emergence of * Corresponding author.…”

Large-Diameter Burrows of the Triassic Ischigualasto Basin, NW Argentina: Paleoecological and Paleoenvironmental Implications

“…Even though digging burrows may also be a response to increasing predatory pressure [5], [13], the Upper Triassic of the Ischigualasto-Villa Union Basin was deposited under a seasonal climate as evidenced by paleopedological [21], taphonomic [27], [28] and sedimentological studies [23], [24], [74], [75], indicating that climate had a strong influence on the appearance of burrows in this portion of Pangea. Moreover, the studied burrows appear to exclusively occur in isolated horizons of the Cancha de Bochas Member of the Ischigualasto Formation and the upper portion of the Los Colorados Formation, where in both cases a dry and highly seasonal climate is clearly evident (i.e., calcic soils, desiccation cracks, abundance of paleovertebrates, shrubby plants restricted to temporary rivers, etc.).…”

A new Late Triasssic phytogeographical scenario in westernmost Gondwana

“…2b). Semi-humid conditions are reflected in the third member (Valle de la Luna) and a return to semi-arid palaeoclimates is expressed at the top of the formation and the transition to the overlying Los Colorados Formation 11,[15][16][17][18] . These conditions prevailed during the Late Triassic, a time during which earliest-known dinosaurs 13,19 together with archosauromorphs, crurotarsal archosaurs, therapsids and amphibians lived in southwest Gondwana.…”