An estimation of Central Iberian Peninsula atmospheric δ13C and water δD in the Upper Cretaceous using pyrolysis compound specific isotopic analysis (Py-CSIA) of a fossil conifer

Comunicación oral presentada presentado en el EGU General Assembly 2015, 12-17 April 2015, Vienna, AustriaFrenelopsis is a frequently found genus of the Cretaceous floras adapted to dry, saline and in general to environmental conditions marked by severe water stress [1]. Stable isotope analysis of fossil organic materials can be used to infer palaeoenvironmental variables helpful to reconstruct plant paleohabitats [2]. In this study stable isotope analysis of organic fossil remains (FR) and humic fractions (FA, HA and humin) of Frenelopsis oligiostomata are studied in bulk (C, H, O, N IRMS) and in specific compounds released by pyrolysis (C, H, Py-CSIA). Well preserved F. oligiostomata fossils were handpicked from a limestone included in compacted marls from Upper Cretaceous (Senonian c. 72 Mya) in Guadalix de la Sierra (Madrid, Spain) [3]. The fossils were decarbonated with 6M HCl. Humic substances were extracted from finely ground fossil remains (FR) by successive treatments with 0.1M Na4P2O7 + NaOH [4]. The extract was acidified resulting into insoluble HA and soluble FA fractions. The HA and FA were purified as in [5] and [6] respectively. Bulk stable isotopic analysis (δ13C, δD, δ18O, δ15N IRMS) was done in an elemental micro-analyser coupled to a continuous flow Delta V Advantage isotope ratio mass spectrometer (IRMS). Pyrolysis compound specific isotopic analysis Py-CSIA (δ13C, δD): was done by coupling a double-shot pyrolyzer to a chromatograph connected to an IRMS. Structural features of specific peaks were inferred by comparing/matching mass spectra from conventional Py-GC/MS (data not shown) with Py-GC/IRMS chromatograms obtained using the same chromatographic conditions. Bulk C isotopic signature found for FR (-20.5±0.02 h was in accordance with previous studies [2, 7–9]. This heavy isotopic δ13C signature indicates a depleted stomatal conductance and paleoenvironmental growth conditions of water and salt stress. This is in line with the morphological and depositional characteristics [3] confirming that F. oligostomata was adapted to highly xeric and saline habitats being a component of salt-marsh vegetation. The values obtained for δD (-101.9±2.2 h, δ15N (10.7±0.2 h and δ18O (20.9±0.39 h lay within those previously reported for fossil floras [10] growing in warm environment and probably with very high evaporation rates. δ13C Py-CSIA was recorded for biogenic compound; polysaccharides, lipid series, lignin and degraded lignin compounds (alkyl benzenes and alkyl phenols) and for a S containing compounds probably with a diagenetic origin. In general δ13C Py-CSIA values were more depleted that the bulk ones and can be considered a better approach to the real plant δ13C value (c. -22 h. Considering that plant-air C fractionation in degraded lignin compounds for a C4 photosystem plant is c. ∆13C≈ 20.0 h [11] and a an extra fractionation (∆13C≈ −3.0 h due to the plant depleted stomatal conductance growing in extreme warm, saline and dry conditions, we estimate atmospheric δ13C value in the area during the Upper Cretaceous in c. δ13C = −5.3±0.2h ...Projects CGL2012-38655-C04-01 and CGL2008-04296 and fellowship BES-2013-062573 given by the Spanish Ministry for Economy and Competitiveness to N.T.J.M. Dr. J.M. R. is the recipient of a fellowship from the JAE-Doc subprogram financed by the CSIC and the European Social Fund.N