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

Microbial mats and microbialites are common in modern and ancient saline lacustrine environments and are highly responsive to biological and environ- mental factors. As such, they represent important sources of high-resolution environmental data across a wide range of geological time. Nonetheless, inter- pretation of fossil mats is non-trivial due to the variable combinations of intrin- sic and extrinsic controls, diagenetic modification, preservation and complex morphological characteristics. This study combines X-ray computed tomogra- phy (CT) scans with high-resolution morphological and petrographic analyses of Holocene microbial mats from several lacustrine sites in the Iberian Penin- sula 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 Zon~ar, 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 individ- ual lakes and at different times. Environmental factors (e.g. water chemistry, light, mineral precipitation) and community composition are additional con- trols acting in individual lakes. Domical and undulatory mats accreted rela- tively rapidly during periods of low detrital sedimentation rates. The generally flatter microbialite morphologies and higher abundance of grains within micro- bialites 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 lit- toral 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 pro- fundal environments. The controls over microbialite characteristics recognised in this study span a range of depositional settings and timescales and under- pin the effective use of microbialites in palaeoenvironmental reconstruction.