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

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

Stable-isotope changes in tufa stromatolites of the Quaternary Añamaza fluvial system (Iberian Ranges, Spain)

The stable isotope composition (d13C and d18O) of the laminae in three Quaternary, calcitic, tufa stromatolites of different ages (MIS6, MIS5 and MIS1) in the Añamaza valley are studied and compared with the modern tufa in the Añamaza river. The cyclic textural variations represent thick cyanobacterial growth in the light laminae and thin or absent cyanobacterial growth in the dark laminae. The textural cyclicity is parallel to d18O changes: Each light-dark couplet corresponds to one year in which the light lamina (lower d18O values) represents warmer water temperatures (Tw) than the dark lamina (higher d18O values). This is consistent with the fact that the large crystals composing the dark laminae correspond to precipitation in the absence of microbial films and likely represent the coldest conditions. The d18Ocalcite-derived Tw from MIS5 stromatolite is higher than the MIS6 and MIS1 samples, which agrees with the commonly admitted climatic conditions during MIS5 in NE Iberia. Moreover, d18Oderived Tw from MIS6 suggests a wider yearly Tw range than the two other samples. The higher and more disperse d13C values of the MIS1 stromatolite are consistent with the peculiarities of the vegetal cover and the decreased water availability in the Holocene. Se estudia la composición isotópica ( d13C y d18O) de las láminas de tres estromatolitos calcíticos de diferente edad (MIS6, MIS5 y MIS1), en el valle del río Añamaza, y se compara con tobas actuales de este río. La variación textural cíclica representa un crecimiento cianobacteriano potente en las láminas claras y uno débil o ausente en las oscuras. Esta ciclicidad es paralela a los cambios del d18O: Cada pareja clara-oscura corresponde a un año, donde la lámina clara (menor d18O) representa temperatura de agua (Tw) más cálida que la lámina oscura (mayor d18O). Los cristales grandes que forman las láminas oscuras precipitarían en ausencia de biofilms y posiblemente representan condiciones más frías. La Tw derivada de d18O calcita en la muestra MIS5 es mayor que la Tw en las muestras MIS6 y MIS1, en consonancia con las condiciones climáticas durante el MIS5 en Iberia. Ade- más, la Tw derivada de d18O calcita en la muestra MIS6 sugiere un rango de Tw anual más amplio que en las otras dos muestras. La mayor dispersión y mayores valores de d13C en el estromatolito MIS1 son compatibles con las peculiaridades de la cobertera vegetal y la menor disponibilidad hídrica en el Holoceno

Tracking the oxygen isotopic signature from the rainfall to the speleothems in Ortigosa de Cameros caves (La Rioja, Spain)

A one-year monitoring survey has been carried out in La Paz and La Vin~a Caves in the Ortigosa de Cameros Cave System (NE Iberian Peninsula), in order to track the oxygen isotope signal from rainfall to speleothem calcite, assessing the ability of this signal to retain environmental information. Oxygen isotope signals of rainfall events, drip water —sampled every three months—, and speleothem calcite, precipitated over three-months, are com- pared. Water dripping follows precipitation events in winter, spring and summer, more closely in the near-surface drip points than in the deeper ones. In autumn, dripping is delayed with respect to rainfall, suggesting that water stays in the epikarst before dripping resumes after summer. This delay causes a deviation of the total drip water sig- nal (average d18O=-8.39‰ V-SMOW) from the rainfall signal (average d18O=-7.41‰ V-SMOW). On the contrary, in winter the isotopic signal of drip water keeps the rainfall signal. Calcite isotopic signal (total average d18O=-6.83‰ V-PDB) shows a small offset (0.62–0.75%) with respect to the signal predicted by drip water oxygen composition; this points to a limited kinetic effect in calcite precipitation, therefore calcite retains the signal of rainfall, especially in winter

d13C and Mg/Ca dripwater response to environmental conditions in the Ortigosa caves (La Rioja, Spain)

Durante un año hidrológico se han monitorizado, en Ortigosa de Cameros, la lluvia, la tasa de goteo y sus valores de d13C y Mg/Ca de dos cuevas, junto con los de un río y un manantial próximos. La tasa de goteo concuerda con la lluvia, excepto por un retraso en otoño, debido a que el epikarst se seca en verano. En general, la cueva de La Viña responde más lentamente, debido al mayor espesor de roca que la cubre. Las evoluciones de d13C y Mg/Ca del agua de goteo son paralelas a la de la tasa de goteo, observándose el mismo retraso en otoño, relacionado con la pérdida de agua del epikarst en verano. El mínimo de d13C y Mg/Ca en primavera se corresponde con el máximo de lluvia y la reanudación del goteo tras el invierno, así como al aumento de la actividad biológica en primavera. Los valores de d13C, semejantes en las cuevas, el río y el manantial, reflejan claramente la mayor humedad en primavera, junto con la actividad biológica asociada, mientras que la relación Mg/Ca es más variable y más sensible a la sequía de verano, diferida al otoño. Rainfall on Ortigosa de Cameros (La Rioja), dripping rate and drip water d13C, and Mg/Ca from the caves, as well as d13C and Mg/Ca from a close river and spring, were monitored during a hydrological year. Dripping rate follows local rainfall, excepting for an autumn delay, due to the drying of the epikarst after summer. The general more slowly response of La Viña Cave is likely due to the thicker vadose zone above the cave. d13C and Mg/Ca patterns are consistent with the dripping rate, showing the dripping autumn delay, related to the water decrease in the epikarst during summer and the subsequent decline of the biological activity. The spring d13C and Mg/Ca minimum values correspond to the rainfall maximum and dripping resume after winter, and are linked to the spring biological activity surge. d13C, similar in dripping caves, river and spring, reflects clearly the general spring humidity and the accompanying biological activity, whereas Mg/Ca values are more variable and more sensitive to the summer drought, delayed to autumn

Environmental response of a fragile, semiarid landscape (Bardenas Reales Natural Park, NE Spain) to Early Holocene climate variability: A paleo- and environmental-magnetic approach

The Bardenas Reales Natural Park (western Ebro Basin, NE Spain) constitutes a fragile landscape where the prevailing semiarid climate and erodability of the bedrock have resulted in high sensitivity to erosion/sedimentation oscillations during the Holocene. In this paper, we present new chronologic, sedimentologic, paleomagnetic and environmental magnetic results from the oldest unit (Qah1) of a system of nested cut-and-fill alluvial sequences, which is 18. m thick and crops out extensively in the central part of the natural park at the Bardena Blanca Depression. Radiocarbon dating indicates that Early Holocene unit Qah1 is younger than 9.38-8.89. cal. kyr. BP and gives ages of 8.92-8.42 and 7.59-7.37. cal. kyr. BP in the middle and uppermost parts of the unit, respectively. Sedimentologic data indicate that distal alluvial sediments of unit Qah1 accumulated in sandy mud flats and ephemeral playa lakes under arid conditions. The coincidence of enhanced magnetite content with moderate to intense root bioturbation in the lowermost and likely also uppermost sediments of unit Qah1 suggests that magnetite formed authigenically in response to relatively wetter conditions at the beginning and end of this dry period. These results indicate that onset and cessation of alluvial aggradation mark the passing of a geomorphologic threshold, most likely plant cover in the watershed, during a progressive drying-wetting event. The radiocarbon-based age model of unit Qah1 indicates that alluvial aggradation under arid conditions began at 9.13. cal. kyr. BP and ended at 7.16. cal. kyr. BP. This period is strikingly centered at around the 8.2. kyr cold event but spans a longer time period, supporting the notion of a global climatic deterioration as the underlying cause for this event. Paleomagnetic directions do not clearly display the characteristic patterns recognized in paleosecular variation curves derived from other European lacustrine sediments and cannot be used to refine the radiocarbon-based age model for unit Qah1. Our results demonstrate a quick and strong response of alluvial systems to Early Holocene climate variability in fragile Mediterranean landscapes such as those at the semiarid Bardenas Reales Natural Park. © 2011 Elsevier B.V.This work was funded by projects CGL2006-08973/BTE and CGL2009-10455/BTE (Spanish Government and European Regional Development Fund) and by a CSIC JAE-Doc post-doctoral research contract (MGP).Peer Reviewe

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

This contribution focusses on stromatolites and oncolites as tools to seek diverse environmental and climate information at different temporal scales. The scales are: (a) Low frequency, dealing with macroscopic and megascopic scales, and (b) high frequency, involving calendar and solar frequency bands. Two depositional environments are used for this purpose: (a) Fluvial and fluvial–lacustrine, which can develop under high to moderate gradients, and in low-gradient conditions, and (b) lacustrine, subject to low-gradient, hydrologically closed lake conditions. Several current and ancient examples in the Iberian Peninsula allow high-frequency and low-frequency analyses. Within the wedge-shaped depositional units that fill the high- to moderate-gradient, stepped fluvial systems, stromatolites form half domes and lenticular bodies, commonly at the wedge front. Oncolites are uncommon. These stromatolites developed in moderate to fast-flowing water in stepped cascades and rapids. Their geometry and extent reflect the topography of the bedrock and later ongoing growth. In low-gradient fluvial and fluvial-(open) lacustrine systems the depositional units are tabular, low-angle wedge-shaped and lenticular and have great spatial facies variability. The dominant oncoid and coated-stem limestones form gently lenticular stacked bodies, developed in wide, low to high-sinuosity channels within wide tufaceous palustrine areas and small lakes. In the Ebro Basin saline carbonate lacustrine systems, stromatolites form thin planar to domed and stratiform bodies and are associated with muddy-grainy laminated carbonates and very rare oncolites, together forming ramp-shaped units that represent the inner fringes of high lake-level deposits. This geometry reflects low-gradient lake surface and shallow water conditions. Textural and structural features allow different ranks of laminae and types of lamination to be distinguished. Texture, together with the d13C and d18O values of consecutive laminae, are useful in distinguishing environmental and climate changes operating over different time spans. Periodicity analysis of lamination can help to discern any temporal significance in the lamination