CRYPTOGAMIC GROUND COVERS AS MODERN ANALOGUES OF EARLY TERRESTRIAL ECOSYSTEMS: STRUCTURE, ORGANIC-MINERAL INTERACTIONS, AND BIOTIC WEATHERING AS CLUES TO THE DEVELOPMENT OF EARLY BIOTICALLY-INDUCED SOILS

Modern-day Cryptogamic Ground Covers (CGCs, i.e., communities of mosses, liverworts, lichens, algae, fungi and bacteria) are early colonizers of fresh, unstable, and potentially nutrient-poor ground surfaces. A growing body of data from both the early plant fossil record and molecular phylogenetics indicates that comparable associations were colonizing land during the Early Paleozoic.

Our goal is to understand how cryptogamic mineral substrates (CMS) develop, the influence of weathering on the substrate, and how they mature, with a view to using them as modern analogues of early soil ecosystems. We employed a variety of imaging and micro-analytical techniques to primarily bryophyte and lichen dominated volcanic substrates from Iceland and New Zealand.

X-ray micro-CT of substrate micro-cores reveals a complex layered structure; the near-surface trapping of fine-grained allochthonous material (particularly in lichen and moss dominated CGCs) and the burial of older parts of plants contributing to the CMS buried organic biomass. In addition to this, liverwort rhizoids preferentially attach to solid materials (e.g., large clasts, plant debris), which could have implications for biotic weathering. Numerous micro-structures are also repeated in a variety of CMS scans of mosses from different habitats (e.g. grain piling), whereas the majority of thalloid liverwort scans reveal a complete enveloping of the surface of the CMS. SEM reveals an assortment of organic-mineral attachment methods (mucilage secretion sticking, rhizoid sticking, symbiotic fungal entwining) demonstrating that the mechanisms of colonization are not the same within and between species and vary widely depending on substrate type. SEM-EDS analysis reveals distinct differences in the morphology and weathering of mineral grains, which could assist in distinguishing between biotic and chemical weathering markers in the sedimentological record. XRD data from colonized CMSs could indicate that smectite clays have a biotically weathered origin. Consequently, we may be able to recognize ancient CMSs in the fossil and sedimentological record based on numerous micro-analytical and imaging features found in modern CMSs, providing an idea of the cryptogamic contribution to biotic weathering and soil formation in the Early Paleozoic.