Trabajo presentado en el 20th International Sedimentological Congress (ISC) celebrado del 13 al 17 de agosto de 2018 en Quebec (Canadá), Some of the high energy mountain rivers draining carbonate catchments in the Pyrenees show
carbonate coatings on the bedload and also as framestone patches, which can be included within
the term tufa. However, the size of coated clasts (several dm) and the high energy of the waters
make them different to the commonly described tufa or fluvial carbonates. The studied carbonates
come from Orós and Oliván streams, all tributaries from the East of the Gállego River, and draining
carbonate catchments within the Spanish Pyrenees. In many areas of the rivers water velocities are
higher than 1 m/s. Temperatures vary along the year from 4 to 20ºC and pH is around 8. At present
the coarse clastic dominated deposits of the streams are formed by pebble to boulders sourced from
the Eocene Flysch. The carbonate precipitates within these streams occur as: 1) Irregularly
laminated carbonate coatings (oncoids), 1-15m thick, around pebbles to boulders and phytoclasts;
2) Framestone patches formed by “in situ” coated plants occur in relatively protected areas and
also in cascades; 3) Rudstones of coated phytoclasts joined together by carbonate.
All the carbonate is LMC. Diatoms and vegetal debris are common in all the deposits. The
laminated coatings of oncoids, framestones and phytoclastic rudstones are composed by coarse
calcite and/or micrite-microspar laminae. Coarse calcite laminae/crystals are relatively thick (1
mm) and composed either of: a) bladed to fibrous crystals about 1 mm long and from 0.5 to 0.1
mm wide, b) irregular bands formed by fans of crystals containing micritic filaments. Micrite
laminae constitute the first coating of any nuclei, they are dark and relatively homogenous with
low porosity. Micrite to microspar laminae are very porous, the crystals are arranged following
irregular lines and leaving high porosity. Micritic filaments are common.
Carbonate precipitation in these mountains cold-high energy rivers is not uncommon, but rarely
studied because the dominant coarse detrital sedimentation and the reduced amount of carbonate
precipitates in comparisons with tufas. Carbonate sedimentation in these environments is favoured
by the suitable pH and the high concentration of calcium and carbonates sourced from the
hinterland. Although CO2 mechanical degassing due to turbulence is the main driving force for
precipitation microbial biofilms also contributed either actively (photosynthesis) or acting as
templates. The alternation of different type of laminae reflects changes in environmental conditions
either seasonally or on a longer term. The location in a mountain area and wide range of water
temperature along the year, very low in winter very low, suggests that these carbonates could be
considered as cool water continental carbonates.
Our study widens the scope of carbonate fluvial deposition and provides clues to better understand
some features of the sedimentary record, such as some carbonate coatings and matrix found in
fluvial coarse deposits, allowing so more precise palaeoenvironmental interpretations., This work is funded by MINECO projects CGL2014-54818P and CGL2013-42867-P, Peer reviewed

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.

The stable isotope composition of the laminae in two stromatolites of the Middle Miocene lacustrine record in the Sierra de Alcubierre (unit T6, Ebro Basin) reveals cyclic variations between laminae with different texture and distinct evolution of the isotopic composition through time. Light, porous (micrite-microspar) laminae (0.65 mm to 6 mm thick) and dark, dense (micrite) laminae (0.5 mm to 2 mm thick) alternate through time. Most of these laminae are composite. The significant correlation between d13C and d18O suggests that the precipitation/evaporation ratio (P/E) mainly controlled the short-term isotopic evolution of the saline carbonate closed-lake environment of unit T6 in the Ebro Basin. The dark laminae have higher d13C and d18O than the light laminae, which represents cyclic changes (seasonal to interannual) in the P/E ratio. The decrease in both d13C and d18O composition and in d13C vs. d18O correlation coefficient values between the studied stromatolites indicate a trend toward higher P/E ratio through time.

La composición de isótopos estables de las láminas de dos estromatolitos lacustres del Mioceno medio de la Sierra de Alcubierre (unidad T6, Cuenca del Ebro) revela variaciones cíclicas entre láminas con diferente textura y clara evolución de la composición isotópica a lo largo del tiempo. Consisten en una alternancia de láminas claras, porosas (micrita-microesparita, de 0.65 mm a 6 mm de espesor) y láminas oscuras, densas (micrita, 0.5 m a 2 mm de espesor). La mayoría de estas láminas son compuestas. La significativa correlación entre d13C y d18O sugiere que la relación precitación/evaporación (P/E) controló en gran medida la evolución isotópica a corto plazo del ambiente lacustre salino de la unidad T6 en la Cuenca del Ebro. Las láminas oscuras tienen valores mayores de d13C y d18O que las claras, lo cual representa cambios cíclicos (estacionales a plurianuales) en la relación P/E. El decrecimiento en d13C y d18O y en el coeficiente de correlación (d13C vs. d18O) entre los estromatolitos estudiados indica una tendencia hacia una mayor relación P/E a lo largo del tiempo.

The significance of stromatolites as depositional environmental indicators and the underlying causes of lamination in the lacustrine realm are poorly understood. Stromatolites in a ca 600 m thick Miocene succession in the Ebro Basin are good candidates to shed light on these issues because they are intimately related to other lacustrine carbonate and sulphate facies, grew under variable environmental conditions and show distinct lamination patterns. These stromatolites are associated with wave-related, clastic-carbonate laminated limestones. Both facies consist of calcite and variable amounts of dolomite. Thin planar stromatolites (up to 10 cm thick and less than 6 m long) occurred in very shallow water. These stromatolites represented first biological colonization after: (i) subaerial exposure in the palustrine environment (i.e. at the beginning of deepening cycles); or (ii) erosion due to surge action, then coating very irregular surfaces on laminated limestones (i.e. through shallowing or deepening cycles). Sometimes they are associated with evaporative pumping. Stratiform stromatolites (10 to 30 cm high and tens of metres long) and domed stromatolites (10 to 30 cm high and long) developed in deeper settings, between the surge periods that produced hummocky cross-stratification and horizontal lamination offshore. Changes in stromatolite lamina shape, and thus in the growth forms through time, can be attributed to changes in water depth, whereas variations in lamina continuity are linked to water energy and sediment supply. Growth of the stromatolites resulted from in situ calcite precipitation and capture of minor amounts of fine-grained carbonate particles. Based on texture, four types of simple laminae are distinguished. The simple micrite and microsparite laminae can be grouped into light and dark composite laminae, which represent, respectively, high and low Precipitation/Evaporation ratio periods. Different lamination patterns provide new ideas for the interpretation of microbial laminations as a function of variations in climate-dependent parameters (primarily the Precipitation/Evaporation ratio) over variable timescales.

Stratigraphic and sedimentological study of distal alluvial and lacustrine deposits in the Plana de la Negra-Sancho Abarca area (western-central Ebro Basin, NE Iberia) within the early and middle Miocene allows five main lithofacies to be characterized and mapped within two tectosedimentary units, construction of a sedimentary facies model and discussion on allogenic controls on sedimentation. In this area, the boundary between tectosedimentary units T5 and T6 appears to be conformable and is marked by the change from dominant clastics to carbonates. Correlation of the studied outcrops with nearby sections that already had magnetostratigraphic and biostratigraphic data allows the studied succession to be dated from C5Dr to C5Cn (Burdigalian-Langhian), placing the boundary T5/T6 at ca.16.1-16.05Ma. Seven vertical facies sequences document deposition of distal alluvial clastics and palustrine and lacustrine carbonates. Sandstones and mudstones represent low-sinuosity channels and lateral and terminal splays by unconfined flows runnig across the alluvial plain, associated to the Pyrenean-derived Luna fluvial system. The carbonates contain charophytes, ostracods, bivalves and gastropods, indicating deposition in 2-4m deep lakes. Laminated carbonate facies record reworking of shore carbonates and the influx fine-siliciclastic sediment offshore. Abundant bioturbation and desiccation features indicate episodic submergence and subaerial exposure. Four main episodes of alluvial and associated palustrine/lacustrine facies belt shifts are identified. Alluvial deposition in the studied T5 unit is related to low lake level conditions, rather than to a Pyrenean uplift. The maximum extent of the freshwater carbonates occur at the base of unit T6. This is consistent with conditions of increasing humidity of the Middle Miocene Climatic Optimum.

Extensive palustrine tufa deposits are not common in the geological record, in part due to their liability to erosion. This work discusses the formation and preservation factors that allow the wide development of such facies, taking the example of a Middle-Late Pleistocene dominantly palustrine area that formed downstream of a high-slope stepped tufa fluvial stretch, in the Ebron river Valley (south Iberian Range). The study case (Los Santos area) consists of coarse detrital sediments at the base, associated with pronounced incisions on the underlying deposits, followed by a variety of tufa and associated carbonate facies, with a minimum thickness of 19 m. The most abundant facies are the phytoclastic rudstones, carbonate sands and silts with gastropods and ostracods and up-growing stem boundstones. Less common are moss boundstones, down-growing stem boundstones and bioclastic limestones. Up to five simple vertical sequences of facies (facies associations) are characterized. The sedimentary facies model corresponds to a low-slope and wide stretch at the end of a stepped, cascade-barrage fluvial system, with extensive palustrine areas, shallow ponded areas, at places with stagnant conditions, and small cascades. Variations of bedrock lithology, from Mesozoic carbonate rocks upstream to Neogene alluvial rocks downstream, allowed the gentle sloping and wide low-layering surface at the distal termination of the fluvial system, with overall low-energy facies, which supports the absence of stromatolites. Indeed, the lack of vigorous erosional processes enabled a dense cover of hydrophilous plants to thrive on diverse environments and the accumulation and preservation of organic matter. The small thickness of the studied deposits, compared to that of upstream deposits, is consistent with a reduction of dissolved calcium and bicarbonate contents downstream (linked to the distal position from the carbonate aquifer sourced springs), and with the diminishing mechanical CO2-degassing in low-gradient environments. En el registro geológico, los depósitos de tobas palustres extensos no son comunes. Este trabajo discute los factores para la formación y conservación de extensos depósitos palustres, tomando como ejemplo los depósitos del Pleistoceno medio-superior que se formaron en el tramo distal del sistema de tobas fluviales del río Ebrón (Cordillera Ibérica, en Teruel y Valencia). Los depósitos estudiados (área de Los Santos) constan de sedimentos detríticos en la base, asociados a incisiones fluviales profundas sobre el sustrato, seguidos por una sucesión de carbonatos con amplia variedad de facies tobáceas, con un mínimo de 19 m de espesor. Las facies más abundantes son los rudstonesfitoclásticos, arenas y limos de carbonato con gasterópodos y ostrácodos, y los boundstones de tallos de plantas creciendo hacia arriba. Menos comunes son los boundstones de musgos y de tallos colgantes, y las calizas bioclásticas. Hasta cinco secuencias verticales de facies se han caracterizado. El modelo de facies corresponde al tramo al final de un sistema tobáceo fluvial escalonado con cascadas-barreras, en el que se formarían extensas áreas palustres, áreas encharcadas someras, algunas con estancamiento, y pequeñas cascadas. Las variaciones litológicas del sustrato pre-Cuaternario, desde rocas carbonáticas aguas arriba a rocas aluviales aguas abajo, permitió la formación de esa superficie amplia y de poca pendiente al final del sistema fluvial escalonado, en conjunto con facies de poca energía, lo cual apoya la ausencia de estromatolitos. Además, la escasez de procesos erosivos intensos permitiría que una densa cubierta de plantas hidrófilas prosperara en diversos ambientes, así como la acumulación y conservación de materia orgánica. El escaso espesor de la secuencia estudiada se relaciona con la disminución del contenido en calcio y bicarbonato disueltos en el agua (por la situación distal respecto a las fuentes alimentadas por el acuífero carbonatado) y la disminución de la intensidad de desgasificación de CO2 en ambientes de poca pendiente.

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.

This paper discusses the formation and preservation of a fluvial tufa system influenced by Atlantic climate based on stratigraphical, chronological (amino-acid racemization, AAR), sedimentological and stable-isotope analyses. On the southwestern Iberian Peninsula, the tufas and associated deposits of the Almonda River valley occur as isolated terraced bodies and reach 25 m thick. AAR dated most deposits to within the warm Marine Isotope Stage 5 (MIS-5). Two Holocene ages were reset within MIS-5 based on diverse criteria. Widely varied carbonate and minor allochthonous coarse detrital facies occur arranged in four simple vertical associations. The deposit geometry and facies association distribution correspond to a low- to moderate-sloped fluvial valley consisting of several short knickpoints and extensive flat areas between them. The latter are occupied by slow-flowing water facies (carbonate sand, lime mud, phytoclast and oncoid rudstones, and up-growing stem boundstones). Facies that formed in moderate- to high-slope substrates were stromatolite, moss and down-growing stem boundstones. The homogeneous Miocene bedrock lithology and gentle structural deformation propitiated this depositional architecture. Calcite δ13C and δ18O values suggest that the aquifer water provided the outflowing Almonda water with (1) 18O-enriched water, compared with present precipitation and groundwater δ18O values, and (2) 13C-depleted CO2 from bituminous rocks and vegetation cover in the catchment. The proximity to the Atlantic coast favoured the Mesozoic-rock aquifer recharge with 18O-enriched water precipitation, assuring water availability during the formation of the studied tufas. No evidence of frequent intense erosion phases might indicate stable precipitation regimes, which would have allowed the preservation of loose fine-grained and palustrine deposits.

The spectral decomposition of time series obtained from ancient rock records can be used to study the similarity between the dynamics of present-day and past climate systems. A high-resolution periodicity analysis of luminance and lamina thickness of lacustrine stromatolites in the Ebro Basin (northeast of Iberian Peninsula) reveals a significant signal of interannual and decadal climatic variability in the Miocene. This is one out of very few works that presents the use of spectral analysis to estimate the potential of stromatolite lamination as multiple-scale recorders of climate parameters. The effects of precipitation and evaporation variations on stromatolite lamination have been detected at three orders of cyclicity based on textural and high-resolution stable-isotope analyses (C and O) obtained in prior studies. These analyses also revealed that the light and dark simple lamina couplets are identified with annual cycles (third-order isotopic cycles). In the present study, the spectral analysis results obtained through different paths in five stromatolite specimens reveal significant periods in the power spectrum at around 2.5, 3.7, 5, 7, 10, and 22 years. These cycles can be correlated with the typical oscillation bands of different climate-related agents. The 2.5-year period corresponds to the Quasi Biennial Oscillation (QBO), or to the biennial component of the El Niño-Southern Oscillation (ENSO), or to the North Atlantic Oscillation (NAO). The 3 to 5 and 5 to 7-year bands can be linked to ENSO or NAO variability (second-order isotopic cycles). The 8 to 11-year bands fit the 11-year Schwabe (first-order isotopic cycles) and 22 to 23-year fit the 22-year Hale sunspot cycles. Thus, the stromatolite growth was controlled by ENSO, NAO, and solar activity cycles. The close relationship between these climate-related agents makes it difficult to specify the dominant agent controlling the stromatolite growth. Nevertheless, the significant periods obtained from this study, within interannual (2.5, 3.7, 5, 7 years), decadal (10, 22 years), and even multidecadal bands (37–42 years), support the existence, and concurrence, of ENSO and NAO precursors during the early and middle Miocene.

Bioturbation has long been considered an antagonist of microbialite development and preservation, because metazoan grazing and burrowing destroy benthic microbial communities. However, metazoan bioturbation, in conjunction with microbial accretion, may have had a significant role in the morphogenesis of some columnar microbialites, as suggested by the case study presented and by some Phanerozoic and Upper Proterozoic analogues discussed here. Late Miocene in age, the studied microbial biostrome developed in a western Mediterranean restricted shallow-water platform dominated by grainy sediments and with a notable influence of bioturbation. This study is focused on the complex accretionary history of the columnar microbialite biostrome and on its striking dark grey colour, which is attributed to Mn-oxyhydroxides precipitated during meteoric diagenesis linked to subaerial exposure. The characteristic columnar structure of the microbialite biostrome has features consistent with an accretionary origin of the columns, but also has features suggesting metazoan disruption. Therefore, a new morphogenetic model for columnar microbialites is presented, highlighting the concomitant roles of microbial accretion, bioturbation and grainy sediment infill of the intercolumn space. Whether this model is an exception or a rule, should be tested on other examples of Phanerozoic and Upper Proterozoic columnar microbialites. Nevertheless, this model is a step forward in understanding the complex microbe–metazoan interactions as constructive coexistence rather than just as destructive competition.

The River Piedra in the Monasterio de Piedra Natural Park (NE Spain) is a modern tufa-depositing river that encompasses various depositional environments that are inhabited by different cyanobacterial populations. Molecular (16S rDNA) and morphological analyses of the cyanobacteria from different facies showed that Phormidium incrustatum dominates in the fast-flowing water areas where the mean depositional rate is 1.6 cm/year. Stromatolites in these areas are formed of palisades of hollow calcite tubes (inner diameter of 6.0-7.5 µm, walls 2-12 µm thick) that formed through calcite encrustation around the filaments followed by decay of the trichomes. In contrast, in slow-flowing water areas with lower depositional rates (mean depositional rate of 0.3 cm/year), Phormidium aerugineo-caeruleum is the dominant species. In these areas, randomly oriented calcite tubes (inner diameter of 5-6 µm, walls 3-8 µm thick) formed by calcite encrustation, are found in thin and uneven laminae and as scattered tubes in the loose lime mud and sand-sized carbonate sediment. Although this species did not build laminated deposits, it gave cohesiveness to the loose sediment. In the stepped and low waterfalls, with intermediate deposition rates (mean depositional rate of 0.9 cm/year), both species of Phormidium are found in association with spongy moss and algal boundstones, which is consistent with the variable flow conditions in this setting.The calcite encrustations on the cyanobacteria from different environments exhibit irregular patterns that may be linked to changes in the calcite saturation index. The physicochemical conditions associated with extracellular polymeric substances may be more significant to CaCO3 precipitation in microbial mats in slow-flowing water conditions than in fast-flowing water conditions. These results show that flow conditions may influence the distribution of different cyanobacteria that, in turn, leads to the development of different sedimentary facies and structures in fluvial carbonate systems.

This work tests the relationship between quasi-periodic climate-linked parameters and cyclicity recorded through microbial lamination using spectral analysis of time series based on lamina thickness and luminance, as well as C and O stable isotope data. Three oncolites of the Ebro Basin were used: specimens C4-1 and C4-31 from a lacustrine bed dated at 13.8 My, and specimen VD-13 from a fluvial bed whose age is younger than 12 My. Lamination consists of alternating light and dark calcite laminae, which correspond to crystal size and porosity variations. Composite light and dark laminae can be seen by the naked eye. Spectral analysis based on the thickness and luminance measurements of simple light–dark lamina couplet successions revealed that the three specimens had recurrent periods of 2.5, 3 and 10–13. On the hypothesis that each simple light–dark lamina couplet represents a 1-year duration, the 2.5 and 10–13 periods can be related to the 2.5-year quasi-biennial oscillation and the 11-year Schwabe sunspot cycle, respectively. Other less frequent periods present around 5 and 7, together with period 3, could be related to the most typical modes of the North Atlantic Oscillation and/or El Niño Southern Oscillation. The periods at 19–23, present in specimen C4-31, can be correlated with the 22-year Hale sunspot cycle. The 10–13 and 19–23 periods were also detected using the δ13C and δ18O values of successive laminae in specimen C4-31. These data suggest that solar irradiance variations influenced the characteristics of the oncolite laminae, through temperature and precipitation. Although the studied oncolites do not present an optimal thickness lamination pattern for spectral analysis, their lamination is capable of registering periodic climatic variations, and these results provide further evidence that microbial laminations can serve as high-resolution records of climate and climate-linked cycles over different time scales.

Bioturbation has long been considered an antagonist of microbialite development and preservation, because metazoan grazing and burrowing destroy benthic microbial communities. However, metazoan bioturbation, in conjunction with microbial accretion, may have had a significant role in the morphogenesis of some columnar microbialites, as suggested by the case study presented and by some Phanerozoic and Upper Proterozoic analogues discussed here. Late Miocene in age, the studied microbial biostrome developed in a western Mediterranean restricted shallow-water platform dominated by grainy sediments and with a notable influence of bioturbation. This study is focused on the complex accretionary history of the columnar microbialite biostrome and on its striking dark grey colour, which is attributed to Mn-oxyhydroxides precipitated during meteoric diagenesis linked to subaerial exposure. The characteristic columnar structure of the microbialite biostrome has features consistent with an accretionary origin of the columns, but also has features suggesting metazoan disruption. Therefore, a new morphogenetic model for columnar microbialites is presented, highlighting the concomitant roles of microbial accretion, bioturbation and grainy sediment infill of the intercolumn space. Whether this model is an exception or a rule, should be tested on other examples of Phanerozoic and Upper Proterozoic columnar microbialites. Nevertheless, this model is a step forward in understanding the complex microbe–metazoan interactions as constructive coexistence rather than just as destructive competition.

Microbial buildups that predate the Messinian salinity crisis, including one of the few Phanerozoic examples of ‘giant’ microbialites, crop out upon the island of Mallorca (W Mediterranean). Sedimentological and geochemical data from microbialites and associated deposits, both essentially dolomitic, indicate that they grew in shallow marine conditions but relatively restricted from the open marine realm. Two microbialite-bearing sequences occur, both consisting of subtidal to supratidal deposits. Periods of restriction led to evaporative and hypersaline conditions that favored the development of microbialites and local precipitation of sulfates. By contrast, ephemeral periods marked by more open conditions were devoid of microbialites and allowed production of bioclastic deposits. Microbialites range from decameter- to decimeter-scale, reflecting differences in accommodation space. Despite contrasting sizes, all microbialite bodies record similar mesostructure evolution through time, from thrombolites to stromatolites, with a sharp transition between these endmembers. The change from subtidal to shallower, more restricted and saline intertidal environments triggered biotic substitution of thrombolite-generating microbial communities to stromatolite-generating ones. Furthermore, a wide variety of microstructures, from agglutinated to micritic with fossilized microbes, indicates that two main accretion processes occurred: microbially-influenced primary dolomite precipitation and grain trapping and binding, which were controlled by the interaction between microbes and changes in environmental conditions (e.g. grain supply, hydrodynamics and hydrochemistry). Therefore, the diversity of macro-, meso- and microstructures of these microbialites was caused by a complex interplay between depositional, biotic and hydrological parameters, which offers useful insights for the palaeoecological interpretation of other examples, at any scale and throughout geological time.