Both studies simulate rain events and measured resulting net N outputs from crusts to subsurface soil. Some studies give direct estimates of this export flux of N and leachates from BSCs colonizing beach dunes at the Indiana Dunes National Lakeshore along southern Lake Michigan in Indiana, USA and at the Colorado Plateau, USA.
Reported on the existence of a large reservoir of nitrate beneath desert soils, that clearly indicates long-term net export of N from topsoil to arid land subsoils. Whereas multiple studies have examined a single compartment of the modern BSCs N budgets, the lack of studies that simultaneously address N inputs, losses, and soil pools, preclude the construction of definitive N balances (see a review of estimate fluxes of these processes in ref. We show that the colonization of a minor fraction of Archean landmasses by BSCs should have sufficed to attain the modern equivalent land-to-ocean N export flux of ammonium and nitrate. Here, we address at a theoretical level the impact of early colonization of Archean continental landmasses by BSCs in terms of N species source fluxes to the ocean. This scenario echoes theoretical predictions of a long lasting, mass-independent sulfur isotopic signal due to long-term sedimentary recycling of rare sulfur isotope anomalies. Pre-GOE BSC-driven photosynthetic oxygen flux to the atmosphere and ensuing oxidative weathering efficiency was proposed as a scenario to explain the existence of transient episodes of mild environmental oxygenation (so called “whiffs of oxygen”) and oxidative continental weathering.
The impact of a pre-GOE BSCs colonization of the continents has already been explored in the framework of atmospheric oxygen concentration evolution. The evidence timeline for BSC development is consistent with results of molecular clock estimates, that place the colonization of land at the latest between 3.05 and 2.78 Ga based on shared properties of pigment synthesis and resistance to desiccation exhibited by typically terrestrial extant bacterial phyla. Modern BSC do indeed produce such chelators, and are known to mobilize metals. Chemical pieces of evidence based on element mobility patterns in several paleosols also suggest the presence of an ancient terrestrial biosphere where organic ligands chelated metals during weathering. Tufted microbial mats inhabiting coastal habitats have also been described within the Moodies Group (South Africa) at 3.2 Ga. But, indirect evidence of BSCs developing on Archean coastal plain paleosols date them back to 3.0 Ga. The oldest direct evidence for fossil BSCs comes from the 1.2 Ga mid-Proterozoic Apache Supergroup in the Dripping Springs Formation of Arizona, including sedimentological evidence for microbially induced sedimentary structures (MISS) and cyanobacteria-like organic microfossils. With fresh volcanic material to act as fertile soil, in the absence of plant root systems and grazers, one could entertain the idea that BSCs should have had the potential to rapidly develop and thrive during Archean time due to lack of competition, even beyond their restricted modern habitat, which is circumscribed to areas devoid of extensive plant cover. Modern BSCs are arguably the most extensive biofilm on the planet covering up to 12% of Earth’s continental area, and while they are composed of a wide diversity of microorganisms they are primarily built by cyanobacteria performing oxygenic photosynthesis. Therefore, despite a “faint young sun” and the absence of a UV-protective ozone layer in the Archean, a terrestrial phototrophic biosphere composed of systems similar to modern biological soil crusts (BSCs) may have existed early, before the GOE, and could have colonized the exposed land surfaces. Moreover, Cockell & Raven show that under a worst-case UV flux (no environmental UV screen) on the Archean Earth, the landmasses could have been colonized by early photosynthetizers.
Also, a high concentration of ferrous ion (Fe II) may have been present in anoxic waters to significantly screen UV radiation. However, recent findings demonstrate that in the primitive anoxic atmosphere sulfur vapor composed of sulfur molecules and hydrocarbon smog may have strongly attenuated ultraviolet radiation. Because of this peculiar environmental condition Berkner & Marshall first postulated that the colonization of the landmasses was not possible before the formation of an ozone shield.