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Fig. 8 | Journal of Palaeogeography

Fig. 8

From: Coupling textural and stable-isotope variations in fluvial stromatolites: Comparison of Pleistocene and recent records in NE Spain

Fig. 8

Carbon isotopic values (δ13C) for the Pleistocene tufas during cold and warm periods compared with those in the present-day dissolved inorganic carbon (DIC, from -8.1‰ to -10.5‰; Osácar et al. 2016), in the modern precipitates recorded in the River Piedra (data from P-14 record are separately shown) and all of them related with the most probable sources of carbon. Only C3-metabolism plants and atmospheric CO2 have been considered (C4- and crassulacean acid-metabolism plants are not present in the area). The C3 range (shaded in gray) is the most frequent range for C3 plants (Deines 1980; Cerling 1991; Killops and Killops 2005). The range for atmospheric CO2 is defined by the pre-industrial and the present atmospheric value (δ13C = -6.5‰ and -8‰, respectively; Friedli et al. 1986). The soil-CO2 is about 4.5‰ heavier than the plant biomass (Cerling 1984, 1991). The isotopic difference between soil-CO2 and DIC depends on the pH and temperature. At pH values close to 5 this difference is near 0‰; but at pH values between 7.5 and 8, it is relatively independent of pH (Romanek et al. 1992). The theoretical isotopic DIC and calcite values have been obtained considering a calcite-bicarbonate enrichment of 1‰ (independent of temperature) and the calcite-CO2 equation proposed by Romanek et al. (1992) for temperatures of 0, 15 °C and 30 °C. A δ13C value (+3.2‰) for the carbonate aquifer (Upper Cretaceous limestones) feeding the Piedra River is available from IGME (1982). Modified from Ortiz et al. (2009)

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