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

Abstract

Abstract Textural and stable isotopic features of two middle Pleistocene fluvial stromatolite profiles are compared to a recent stromatolite, both formed in the River Piedra system (NE Spain), to test the reliability of climatic, hydrologic and depositional information derived from ancient records. The Pleistocene stromatolites formed in a multi-domed, highly-inclined cascade-barrage. The recent stromatolite also formed in a highly-inclined cascade of the River Piedra, the sedimentary conditions of which were periodically examined between the years 2000 and 2012. The Pleistocene stromatolites are formed of an alternation of 1) thin large-crystal laminae (type A), with elongated crystals up to 1 mm long, and 2) thick small-crystal laminae (type B), consisting of cyanobacterial fan- and bush-shaped bodies. The textural and isotopic comparison with the recent stromatolite shows that each A–B couplet corresponds to one year. The type-A laminae are comparable to the macrocrystalline laminae that occur in the cool-period deposits of the recent stromatolite, and the type-B laminae are comparable to the warm-period deposits of the recent stromatolite. Water temperatures (Tw), calculated from δ18Ocalcite and present measures of δ18Owater, were similar in the Pleistocene and recent specimens, and close to the measured river Tw. Thus, the Pleistocene stromatolites formed not far from isotopic equilibrium, as did the recent stromatolite. The Pleistocene δ18Ocalcite biannual oscillation is wider in amplitude than in the recent stromatolite, which suggests larger differences in Tw through the year in the Pleistocene than at present. The Pleistocene δ13Ccalcite does not show any pattern; and the values are slightly higher than the recent ones. The co-evolution of δ18O and δ13C is parallel in the Pleistocene stromatolites, matching the recent stromatolite behavior. These results and their comparison with other ancient examples prove that textural and isotopic features in ancient stromatolites are useful tools to infer past depositional, climatic and hydrological conditions. Moreover, interpretations from recent fluvial stromatolites can be extrapolated to past environments to help decipher patterns of past processes, in cases where both recent and ancient stromatolites can be compared within one environmental setting. Such comparisons may be used to help interpretations of ancient stromatolites where the modern ones are not available to study.

Keywords

• Tufa
• Recent and ancient stromatolites
• Stable isotopes
• Biotic and abiotic laminae
• Intra-annual and inter-annual water temperature variations