The Meaning of Stromatolites

Abstract

Stromatolites document microbial interactions with sediments and flowing water throughout recorded Earth history and have the potential to illuminate the long-term history of life and environments. Modern stromatolites, however, provide analogs to only a small subset of the structures preserved in Archean and Proterozoic carbonates. Thus, interpretations of secular trends in the shapes and textures of ancient columnar stromatolites require nonuniformitarian, scale-dependent models of microbial responses to nutrient availability, seawater chemistry, influx of sediment grains, shear, and burial. Models that integrate stromatolite scales, macroscopic organization, and shapes could also help test the biogenicity of the oldest stromatolites and other structures whose petrographic fabrics do not preserve direct evidence of microbial activity. An improved understanding of stromatolite morphogenesis in the presence of oxygenic and anoxygenic microbial mats may illuminate the diversity of microbial metabolisms that contributed to stromatolite growth in early oceans.

Mushroom Speleothems: Stromatolites That Formed in the Absence of Phototrophs

“…δ 18 O and δ 13 C values of cap samples are not dissimilar to those characterizing the other subunits of the mushroom, suggesting that unusual evaporative regimes or unusual carbonate sources were not the key factor controlling its formation. The porous cerebroid-like morphology of the cap is similar to that of stromatolites and thrombolites that occur in the photic zone of marine embayments and lagoons (Jahnert and Collins, 2012;Bosak et al, 2013). The analogies with microbialites are even more evident at the microscopic scale.…”

“…Although these speleothems have been well-known by speleologists for decades, and Santa Catalina has been declared "National Monument" in 1996 (Watson et al, 1997;D'Angeli et al, 2015), the exact mechanism of their formation, as well as their petrography and mineralogy, remain virtually unstudied. Besides representing a yet uncharacterized type of speleothem, they are of utmost interest due to their uppermost unit (i.e., the mushroom cap), which is similar to stromatolitic and thrombolitic microbialites that form in the presence of microbial mats (Figures 1A,B; Jahnert and Collins, 2012;Bosak et al, 2013). Microbialites occur in a large variety of distinctive morphologies and are found in the geological record from the early Archean onwards (Grotzinger and Knoll, 1999).…”

“…The mechanisms controlling their genesis have been extensively studied, trying to shed light on the evolution of early metabolisms (Allwood et al, 2010;Bontognali et al, 2012). Although most microbialite studies focus on processes occurring in the photic zone of marine and lacustrine environments (Bosak et al, 2013), a growing number of works also deals with biomineralization processes that take place in subsurface environments (Jones, 2001(Jones, , 2011Melim et al, 2001;Olivier et al, 2003;Baskar et al, 2007Baskar et al, , 2011Baskar et al, , 2014Blyth and Frisia, 2008;Lisker et al, 2009;Rossi et al, 2010;Florea et al, 2011;Lundberg and McFarlane, 2011;Gradzíński et al, 2012;Lozano and Rossi, 2012;Léveillé et al, 2000;Pacton et al, 2013;Tisato et al, 2015). It has often been hypothesized that life originated in the dark, protected from the strong UV radiation that characterized the surface environments of the early Earth and other planets (Homann et al, 2016).…”

Intracellular Ca-carbonate biomineralization is widespread in cyanobacteria

“…By performing oxygenic photosynthesis-a unique capability that appeared only once in evolution-in this particular group of bacteria, cyanobacteria have contributed significantly to the primary production on the past and present Earth (4). Moreover, cyanobacteria have received great attention from geologists as major players in the formation of carbonate sedimentary deposits such as stromatolites (5,6), the oldest ones formed by cyanobacteria possibly as old as 2.98 billion years (Ga) (7). Fossils of Ca-carbonateencrusted cyanobacterial cells (calcimicrobes) have been looked for extensively.…”

Astrobiology: An astronomer's perspective

“…At these sites multiple types (morphological) of stromatolites are found [33] in potentially shallow water environment to foster formation [34,35]. As always there are some uncertainties and there are other ancient sites; summaries of the state-of-the-art can be found in Tice et al [36] and Bosak et al [21]. 3.…”