A freshwater analog for the production of Epiphyton‐like microfossils

Abstract

Calcified microbial microfossils—often interpreted as cyanobacteria—were important components of Precambrian and Paleozoic limestones, but their paucity in modern marine environments complicates our ability to make conclusive interpretations about their taxonomic affinity and geologic significance. Freshwater spring‐associated limestones (e.g., travertine and tufa) serve as terrestrial analogs to investigate mineralization in and around aquatic biofilms on observable timescales. We document the diagenesis of calcite fabrics associated with the freshwater algae Oocardium stratum, an epiphytic colonial green algae (desmid) known for producing stalks of extracellular polymeric substances (EPS) and passively producing a bifurcating tubular calcite monocrystal. Bifurcating EPS stalks produced by Oocardium colonies can become calcified and preserved in ancient carbonate deposits. Calcified micritic EPS stalks have a filamentous morphology, show evidence of branching, and maintain uniformity in diameter thickness throughout the mm‐scale colony, much like the enigmatic calcimicrobe Epiphyton. We provide a mechanism by which calcification associated with a colonial semispherical micro‐organism produces microfossils that deceptively resemble filamentous forms. These findings have implications for the use of morphological traits when assigning taxonomic affinities to extinct microfossil groups and highlight the utility of calcifying freshwater modern environments to investigate microbial taphonomy.

The paleoenvironmental and astrobiological significance of tufas in the Santa Cruz region

“…The ancient tufa outcrop described in this study sits along the Spring Trail in Pogonip Park and is exposed along the trail for 15 meters. Its thickness varies along the path from 2.5 meters to the east and 4 meters to the west and the buildup of carbonate displays a wedge shape (Ibarra & Sanon, 2019). Similar to the outcrop in Henry Cowell State Park, determining the full extent of this outcrop is difficult given the amount of topsoil and vegetation that covers it.…”

“…The mound's exposed facies display porous textures and weakly bedded macrostructures (Fig. 2) (Ibarra & Sanon, 2019). It is likely that the mound was altered by humans when the trail was built, therefore making it difficult to place it in a depositional model.…”

“…However, given the description from Ibarra and Sanon 2019, the mound likely represents a perched, inactive tufa model. Furthermore, while an ephemeral spring is relatively nearby (~100 meters) there is currently no evidence that the spring is related to the mound (Ibarra & Sanon, 2019).…”

“…In the Santa Cruz region, cyanobacteria and algae (e.g., diatoms) inhabit the spring water and influence the textures and formation of the modern and ancient tufa (e.g. Boelts et al, 2020;Ibarra & Sanon, 2019).…”

“…Moreover, water temperature controls the type of organisms that thrive in the aqueous environment, which can determine whether a particular set of laminae formed biotically or abiotically. For example, Oocardium stratum, a green algae, is known to thrive in ambient-temperature spring waters and secretes extracellular polymeric substances (EPS), which induces calcite formation (Ibarra & Sanon, 2019). Spring velocity and turbulence impacts the rate at which tufa can form-high flow velocities and turbulence induces CO2 degassing from the water, which favors calcite precipitation .…”