Development of Holocene lacustrine microbialites on the Iberian Peninsula: Insights into environmental and depositional controls using X‐ray CT and petrography

ABSTRACT Microbial mats and microbialites are common in modern and ancient saline lacustrine environments and are highly responsive to biological and environmental factors. As such, they represent important sources of high‐resolution environmental data across a wide range of geological time. Nonetheless, interpretation of fossil mats is non‐trivial due to the variable combinations of intrinsic and extrinsic controls, diagenetic modification, preservation and complex morphological characteristics. This study combines X‐ray computed tomography (CT) scans with high‐resolution morphological and petrographic analyses of Holocene microbial mats from several lacustrine sites in the Iberian Peninsula to determine relationships between microbialite morphology, texture and environmental controls. Modern and subrecent mats (<0.2 ka BP) occur in Laguna Salada de Chiprana and saline Laguna Zóñar, whereas older Holocene mats (~0.2 to 10 ka BP) were sampled from saline Lago de Arreo and brackish‐saline Lago de Estanya. All studied mats developed preferentially during periods of low lake levels, reduced sediment input and elevated water salinity, although various combinations of these factors can operate in individual lakes and at different times. Environmental factors (e.g. water chemistry, light, mineral precipitation) and community composition are additional controls acting in individual lakes. Domical and undulatory mats accreted relatively rapidly during periods of low detrital sedimentation rates. The generally flatter microbialite morphologies and higher abundance of grains within microbialites from lakes Arreo and Estanya reflect elevated detrital input. Endogenic mineral phases such as gypsum further promote domical, columnar and pseudo‐dendritic mat structures. Formation of columns and conical tufts in littoral settings of Laguna Salada de Chiprana is a response to gradients of light, oxygenation and concomitant community change between photosynthesisers, green and red sulphur bacteria in littoral settings and sulphate reducers in profundal environments. The controls over microbialite characteristics recognised in this study span a range of depositional settings and timescales and underpin the effective use of microbialites in palaeoenvironmental reconstruction.