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.
Wildfire and Atmospheric Oxygenation
In collaboration with Dr. Ian Glasspool, formerly curator of paleobotany at the Chicago Field Musem and, currently, Research Scientist at Colby, we are evaluating the record of late Silurian and early Devonian wildfire as represented in the fossil record by charcoal. The project is funded by the National Science Foundation (NSF EAR 1828359).
We use a number of techniques and tests to determine if any fossil remain has been burned and turned to charcoal. When the plants are diminutive, as are the earliest land plants, it is not possible to rub the remains to return a sooty black streak to a finger. Rather, microscopic observation of the remains physical features is necessary using both Scanning Electron Microscopy and reflectance microscopy (Romax).
As cells walls in a plant are subjected to increasing temperatures associated with wildfire, the laminated structure of the wall gradually disappears. The cell wall becomes homogenized or fused, which occurs at temperatures above 325̊ C (≈620̊ F). Ground fires can attain temperatures of 200̊ C, whereas crown fires can exceed 500̊ C. But, wildfires often exceed these temperatures. Another character of carbon, reflectivity, is affected by increasing temperature and the how long the plant material is charred.
Fossil charcoal can be polished to a shine by impregnating the fossil in a synthetic resin and using very fine, micron-sized carbide and diamond grits to achieve a polish of the cell walls. When polished, the fossil cells are examined using a specialized microscope under immersion oil to determine the percentage of carbon reflectance. Reflectance values may be as low as 0.05% and range to greater than 4.0%. Under controlled experiments, such low values are found to be produced when temperatures exceed 200̊ C (≈400̊ F). With a rise in temperature and duration of burning, reflectance values around 1.0% are found to equate to burns of more than 400̊ C. When reflectance values reach 4.0% or more, burn temperatures of at least 700̊ C (≈1300̊ F) are reached. Cell-wall homogenization accompanied by reflectance values of plant fossils tell us that charcoal became more common in deep time, with an increasing frequency of wildfire occurrence in the Devonian Period.
We have recovered diminutive charcoalified remains from the Devonian rocks in Baxter State Park, Maine. All plant tissues exhibit good cell anatomy and cell-wall homogenization, indicating that fire temperatures ubiquitously exceeded 620̊ F (325° C). This minimum temperature is typical of what is found, today, at the lower limit of North American surface fires burning either grassy or shrubby vegetation. However, reflectance data from Baxter State Park fossils indicate that the fires burned at a minimum temperature of between 800–1110 ̊ F (425–600° C)! For comparison, the 2003 Simi chaparral fires of southern California burned at a median temperature of about 1200̊ F (650° C) and peak soil surface temperatures of chaparral fires in Arizona at about 800̊ F (425° C). Temperatures approaching 1300̊ F (700° C) are recorded during a intense prescribed fire. These temperatures from modern fires burning under a Mediterranean climate appear comparable with our Devonian fires. The difference, of course, is that Devonian fires burned diminutive, leafless, spindly plants and litter all growing in a coastal setting where the entire vegetation, much like ferns today, needed damp conditions to reproduce and thrive.
Early Devonian Wetlands
- 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.