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

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 bushshaped 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 δ 18 O calcite and present measures of δ 18 O water , 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 δ 18 O calcite 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 δ 13 C calcite does not show any pattern; and the values are slightly higher than the recent ones. The co-evolution of δ 18 O and δ 13 C 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.

A multi‐scale approach to laminated microbial deposits in non‐marine carbonate environments through examples of the Cenozoic, north‐east Iberian Peninsula, Spain

“…However, there is not a perfect match. The 7–8.1°C difference calculated between the large‐crystal and the small‐crystal laminae of the fossil stromatolites is larger than the present difference between summer and winter water temperatures, partially because the large‐crystal laminae represent only the coldest record, while the small‐crystal laminae represent a longer time span (Arenas et al, 2019). Regardless of this difference, the results are satisfactory and significant enough to interpret similar cyclic variations in the same terms in other laminated microbial records.…”

“…The textural and isotopic comparison of Pleistocene and recent stromatolites in the River Piedra showed a consistent cyclic pattern in both cases and demonstrated that each lamina couplet corresponds to 1 year (Arenas et al, 2019). The large‐crystal laminae in the fossil stromatolites are comparable to the macrocrystalline laminae that occur in the cool‐period deposits of the recent stromatolites, and the small‐crystal laminae in the fossil specimens are comparable to the warm‐period deposits of the recent stromatolites.…”

“… b Values estimated in Pleistocene stromatolites of the Añamaza River (Osácar Soriano et al, 2017) and Piedra River (Arenas et al, 2019). …”

“…In fluvial records, stromatolite laminae present textural and geochemical variations through time that suit seasonal and sub‐seasonal changes, as in the case of recent and Pleistocene deposits in the River Piedra (Arenas et al, 2019; Osácar et al, 2016) and other recent and ancient fluvial records (Andrews & Brasier, 2005; Kano et al, 2007). The thickness and composite nature of seasonal laminae in most recent fluvial stromatolites allows for high‐resolution studies (textural, δ 13 C and δ 18 O values) and these results can be compared with measured and known environmental and climate parameters.…”

“…A present‐day stromatolite formed on an artificial substrate, installed on a small cascade and monitored every 3 and 6 months from 2000 to 2012 (Arenas et al, 2014a), was used for the comparison (Arenas et al, 2019). At that site, the mean sedimentation rate measured over 13 years was 16 mm/year.…”