External controls on the distribution, fabrics and mineralization of modern microbial mats in a coastal hypersaline lagoon, Cayo Coco (Cuba)

Abstract

Active, carbonate-mineralizing microbial mats flourish in a tropical, highly evaporative, marine-fed lagoonal network to the south of Cayo Coco Island (Cuba). Hypersaline conditions support the development of a complex sedimentary microbial ecosystem with diverse morphologies, a variable intensity of mineralization and a potential for preservation. In this study, the role of intrinsic (i.e. microbial) and extrinsic (i.e. physicochemical) controls on microbial mat development, mineralization and preservation was investigated. The network consists of lagoons, forming in the interdune depressions of a Pleistocene aeolian substratum; they developed due to a progressive increase in sea-level since the Holocene. The hydrological budget in the Cayo Coco lagoonal network changes from west to east, increasing the salinity. This change progressively excludes grazers and increases the saturation index of carbonate minerals, favouring the development and mineralization of microbial mats in the easternmost lagoons. Detailed mapping of the easternmost lagoon shows four zones with different flooding regimes. The microbial activity in the mats was recorded using light-dark shifts in conjunction with microelectrode O 2 and HS À profiles. High rates of O 2 production and consumption, in addition to substantial amounts of exopolymeric substances, are indicative of a potentially strong intrinsic control on mineralization. Seasonal, climate-driven water fluctuations are key for mat development, mineralization, morphology and distribution. Microbial mats show no mineralization in the permanently submersed zone, and moderate mineralization in zones with alternating immersion and exposure. It is suggested that mineralization is also driven by water-level fluctuations and evaporation. Mineralized mats are laminated and consist of alternating trapping and binding of grains and microbially induced magnesium calcite and dolomite precipitation. The macrofabrics of the mats evolve from early colonizing Flat mats to complex Cerebroid or Terrace structures. The 972 macrofabrics are influenced by the hydrodynamic regime: wind-driven waves inducing relief terraces in windward areas and flat morphologies on the leeward side of the lagoon. Other external drivers include: (i) storm events that either promote (for example, by bioclasts covering) or prevent (for example, by causing erosion) microbial mat preservation; and (ii) subsurface degassing, through mangrove roots and desiccation cracks covered by Flat mats (i.e. forming Hemispheroids and Cerebroidal structures). These findings provide in-depth insights into understanding fossil microbialite morphologies that formed in lagoonal settings.

Formation of stromatolite lamina at the interface of oxygenic–anoxygenic photosynthesis

“…In the CCLN, most of the microbial mats show a laminated (stromatolitic) mesofabric, typically composed of three types of layers (Bouton et al., ), from bottom to top (Figures , , and ): (i) layer C is composed of buried ancient microbial mats (between 511 ± 5 and 751 ± 20 14 C yrs BP; Figure d,e) and associated sediments, mostly lime mud and thin shelly beds (Figure a); (ii) layer B is constituted of cohesive lithifying mats that are precipitating CaCO 3 and are being degraded (16 + /‐ 52 14 C yrs BP; Figures and ). Layer B is referred to as the leathery layer because of its color and cohesiveness.…”

“…Although the Mg:Ca ratio is ca . 2.9 in the overlying water (Bouton et al., ), EPS from layer A can bind ca . 8 times more Ca than Mg, implying that Mg is likely available for precipitation.…”

“…The salinity in the lagoon varied seasonally from 54 to 63‰ in July 2014 (wet season; Table ) to 67–75‰ in January 2013 (dry season). Abundant microbial mats were observed; fish, insects, gastropods, and arthropods were rather common, with a lower abundance and diversity during the dry season, when the salinity was higher (Bouton et al., ).…”

“…Our study provides for the first time a quantification of the major microbial metabolisms at play in a lithifying stromatolite along a vertical (i.e., depth) profile. We selected the hypersaline Cayo Coco Lagoonal Network (CCLN), where lithifying laminated microbial mats are abundant, actively forming and produce copious amounts of EPS (Bouton et al., ). To avoid early diagenetic transformations, we focused on the surface of actively growing CCLN microbial mats (<18 months).…”

Enhanced development of lacustrine microbialites on gravity flow deposits, Great Salt Lake, Utah, USA

“…Microbial mats are organo-sedimentary systems consisting of diverse microbial communities embedded in an organic biofilm matrix (Stal et al, 1985;Decho, 2000;Baumgartner et al, 2009;Bouton et al, 2016a). Lithified (i.e., mineralized) microbial mats form microbialites, which may be subsequently preserved in the rock record (Dupraz et al, 2009).…”