Microstructure and mineral composition of Holocene stromatolites from Lagoa Vermelha, a hypersaline lagoon in Brazil: Insights into laminae genesis

Abstract

Stromatolites are domes, columns, or nearly flat crusts of laminated sedimentary rocks, usually consisting of Ca-Mg carbonates. Stromatolites result from lithification of microbial mats, which are benthic microbial ecosystems where microorganisms arrange themselves in layers according to their physiology. Despite a century of research, the hypothesis of stromatolite genesis by lithification of microbial mats remains controversial, and a convincing explanation for how stromatolites arise from microbial mats is still lacking. In this work, we analyze in detail a stromatolite from Lagoa Vermelha, a coastal hypersaline lagoon in Rio de Janeiro State, Brazil. The stromatolite presents a laminated core and thrombolitic regions at the periphery. Both thrombolitic and laminated facies consist of fine-grained authigenic minerals with minor contributions of bioclasts and quartz grains. X-ray diffraction shows aragonite, high-magnesium calcite (HMC) containing about 17% MgCO3, a very-high-Mg calcite (VHMC) containing 29–46% MgCO3, and small amounts of quartz and pyrite. Scanning electron microscopy of polished samples coupled to energy-dispersive X-ray analysis (EDS) showed that each lamina was composed of 1–4 distinct mineral phases embedded within each other, indicating sequential steps of precipitation of Ca-Mg carbonates under distinct biogeochemical conditions. The coexistence of different phases in a single lamina suggests that several processes contribute to mineral deposition as the incipient stromatolite laminae are left behind by microorganisms from the lower layers of the microbial mat when they grow and/or move upwards.

Mapping mineralogical heterogeneities at the nm-scale by scanning electron microscopy in modern Sardinian stromatolites: Deciphering the origin of their laminations

“…In particular, SEM-EDXS analyses show the presence of low-Mg calcite grains with a Mg/Ca lower than 0.05 on one hand, and on the other hand a solid solution of high-Mg calcite with a Mg/Ca ratio varying continuously from 0.05 to 0.43. Mg-Ca carbonates with varying Mg substitutions were also reported in single stromatolites from the hypersaline Lagoa Vermelha (Keim et al, 2020) and the soda lake Petukhovskoe (Samylina and Zaytseva, 2019). Lagoa Vermelha shares some environmental similarities with Mari Ermi, which may explain the variations in the Mg content of calcite as further discussed later: they are both shallow coastal ponds with seasonal salinity fluctuations between brackish and hypersaline conditions ( Bahniuk et al, 2015a;Moreira et al, 2004;Spadafora et al, 2010;Van Lith et al, 2002;Vasconcelos and McKenzie, 1997).…”

Lamina structure in fine-grained sedimentary rocks: scale, mineral assemblage and well log characterization

“…Fluorescent-light images will show more details than normal light (Keim et al, 2020). Core and thin section observation show that the mineral particles are homogeneously distributed in massive rocks (Fig.…”

Stromatolites and calcitized evaporite in a hypersaline playa lake: Rossport Formation (Mesoproterozoic, Ontario)

“…Proterozoic lacustrine stromatolites have been associated with restricted alkaline or hypersaline conditions (Cheadle, 1986a; Fedorchuk et al, 2016; Rogala et al, 2005, 2007); similar conditions of hypersalinity in Phanerozoic lacustrine (Awramik & Buchheim, 2015; Gallois et al, 2018; Martin‐Bello et al, 2019), marine (Mercedes‐Martín et al, 2014; Oliveri et al, 2010), and Recent (Gomez et al, 2014; Keim et al, 2020) lacustrine settings are associated with intervals of stromatolite growth and with bedded sulphate deposition (Gomez et al, 2014; Mlewski et al, 2018). The Mesoproterozoic Sibley Group, although dominantly clastic (Franklin et al, 1980) contains a thin horizon of stromatolites and microbial laminites interpreted as occurring under restricted, hypersaline lake conditions (Cheadle, 1986a; Rogala et al, 2005).…”

“…In contrast stromatolites occur in a wide variety of modern and Phanerozoic lakes (Chidsey et al, 2015; Gomez et al, 2014; Martin‐Bello et al, 2019; Zeng et al, 2019) and may present opportunities as analogues for ancient stromatolites. For example, modern lacustrine stromatolites share textural features with Proterozoic stromatolites, including clotted micritic to microsparitic fabric, botryoidal texture and radial‐fibrous fabric (Gomez et al, 2014; Keim et al, 2020), each of which are common in Proterozoic marine stromatolites (Bartley et al, 2000; Knoll et al, 2013) across a wide range of macroscopic forms. As both chemical sedimentary rocks and macroscopic fossils, stromatolite‐bearing carbonate successions have the capacity to record palaeoenvironmental conditions and inform understanding of the conditions under which mineralizing microbial communities thrived (Grotzinger & Knoll, 1999); however, a general model describing the relationship between form and environment has proved to be elusive.…”

“…Recent work on modern microbialites from a hypersaline lake in Argentina suggests that carbonate nucleated in extracellular polymeric substances exhibits a distinct microsparitic microstructure (Gomez et al, 2018; Mlewski et al, 2018). Work in a hypersaline lagoon in Brazil found that peloidal microsparitic texture can result from the intimate intergrowth of calcite crystals with different Mg content and grain size (Keim et al, 2020). Taken together, these observations of modern microbialites suggest that microbialite microspar and peloids are primary features resulting from carbonate growth in living and decomposing microbial consortia and may represent a robust microbialite textural signature (Mlewski et al, 2018).…”