The evolution of plants able to colonize land and expand across its various geomorphic features is one of the critical events in Earth History. There is no doubt that the planet’s response to their appearance was influenced significantly. Terrestrial plants are responsible for: changing the concentration of atmospheric gasses; altering the geochemical properties of sediments and converting these into a wide variety of soils; altering the patterns of wind, rainfall, distribution of sunlight, and climate across the globe; sequestering Carbon in living biomass and “dead” peat, as a precursor to our economic coals; and affecting not only the production of sediment, but also the timing of its transport and deposition from continents. This is not a comprehensive list, but there are many questions about not only the role of terrestrial plants in the way we envision the world today, but also about the timing of their influence in the past.
- Dr. Martin Gibling, Dr. Ulrike Werner-Zwanziger, and Ms. Kirsten Kennedy, Dalhousie University, Nova Scotia
- Dr. Reginald Wilson, New Brunswick Department of Natural Resources
- Dr. Patricia Gensel, University of North Carolina–Chapel Hill
- Dr. Cortland Eble, Kentucky Geological Survey, Lexington.
These rocks are not true coals, because of a high amount of clastic sediment in the deposits. But, the rock composition comes closes to coal, known from slightly younger rocks in the Middle and Late Devonian. Coal macerals (an elementary microscopic praticle recognised by its shape, morphology, reflectance and fluorescence) are present, and consist of a high proportion of vitrinite (remains of cell walls), woody tissue of stems, branches, leaves and roots) and liptinite (spores and lipid-rich plant parts). Vitrinite reflectance in organic-rich samples (21.37-35.63 wt % total organic carbon) ranged from 0.48-1.00, but two carbon-poor samples were thermally altered to anthracite rank next to an intrusion. Atomic C/N ratios vary from 44.3-82.1 in organic-rich samples. Samples from the Val d’Amour Formation are more mature than those of the Campbellton Formation. This is indicated by higher vitrinite reflectance values and a weaker aliphatic signal in 13C nuclear magnetic resonance (NMR) cross-polarization spectra.
The plants that comprise these Lower Devonian organic-rich rocks are tracheophytes which belong to the rhyniacean, lycophyte, and euphyllophyte groups. These plants were generally herbaceous, slender, of fairly short stature, and had only shallowly penetrating root-bearing rhizomes. Due to their low growth stature and low biomass production, they rarely accumulated sufficient aerial biomass to form peat. Macerations of organic-rich rock unearthed cuticles of five different plants including forms reminiscent of Bitelaria, Taeniocrada, Zosterophyllum, and Spongiophyton.
Although plants at this time were still primitive, the presence at both sites of specimens with recognizable lignified cellular structures in vitrinite and particularly thick and resistant cuticle may represent an important step in peat development.