Chapter 2
Peat
Its Origins, Characteristics, and Geological Transformations
Abstract
Peat is a naturally existing sedimentary material that is both common and unusual; its origins are due to botanical and geological processes, and significant contributions to any peat deposit are attributable to animals, plants, and diverse groups of microbial taxa. Peat deposits develop where the accumulated remains of biological communities exceed the capacity of the environment to destroy or recycle those components; otherwise individual peats have little in common. Many peats represent sediment accumulation under the cold conditions of the Arctic, though temperate peats are abundant in such places as Ireland, and subtropical peats have developed for thousands of years in the Okefenokee Swamp and the Everglades in the southeastern US.
As a sediment composed of discrete particles, peat could only have accumulated after plants, particularly, had developed the capacity to remain intact after plant parts had been deposited as sedimentary debris. The development of woody tissue, waxy cuticle, and, eventually, spores, pollen, and resins eventually lead to the formation of vast amounts of chiefly botanical debris after the initiation of the Carboniferous Period. The steady evolution of plant groups from mosslike organisms, to tree-sized ferns, conifers, and, eventually angiosperms such as Sassafras allowed for a continuous and highly variable accumulation of sediments that we know as coal.
Diagenetic changes in peats, attributable to microbial activity, geochemical changes in the environments of deposition, and the intervention of geological processes including increased heat, lithostatic pressure, and metamorphism have lead to a wide variety of coals being available for human use. These might be known as lignite, bituminous coal, or anthracite, but they all owe their origins to the presence of microbes and plants that could produce geologically durable remains.
Keywords
Coal; Everglades; Hydrosere; Okefenokee; Palynology; Peat; Petrography; Wetlands
Chapter Contents
Humified tissues from the Trail Ridge brown coal, recovered from an exploratory hole drilled beneath the Trail Ridge heavy mineral sand deposit, Starke, Florida. Trail Ridge is estimated to be of early Pleistocene age. The brown elliptical object at the right and just below the center is a Taxodium (cypress) needle, and it is 140µm in width. The core sample for this thin section is courtesy of Dr Fredric Pirkle, formerly of E.I. DuPont de Nemours and Co., Lawtey, Florida.
2.1
Peat: Its Origins, Characteristics, and Transformations
Fredrick J. Rich
A fire ignited by a lightning strike and a downed power line at the Okefenokee Swamp, Waycross, southeast Georgia. More than 2400km2 of the swamp burned before the fire was extinguished with the aid of firefighters.
From Stracher et al. (2013). Introduction
Peat is a material that is both commonplace and unusual; it has origins that are both botanical and geological, and there are potentially significant contributions from animals as well as diverse groups of microbial taxa. The generic bag of horticultural peat that one might purchase to spread over the garden is but a small representative of the naturally occurring materials that contribute to what we call peat.
The number of published studies that focus in some respect on peat is very large. In order to focus on the subject of peat, its characteristics and its origins, I have relied upon a few key sources of printed information, as well as considerable personal experience.
Stout and Spackman (2002) defined peat as being “...decayed plant organic materials that accumulate in environments where conditions preclude organisms’ ability to completely recycle the plant substances.” This depiction is simple and to the point, but more detail might be useful to understanding how and why peat accumulates, and how and why coal beds that result from the metamorphism of peats both differ and vary in time and space.
Moore and Belamy (1974) offer a fairly pragmatic definition of peat, and peat-bearing landscapes when they describe peat-producing landscapes (peatlands) as “…unbalanced systems in which the rate of production of organic material by living organisms exceeds the rate at which these compounds are respired and degraded.” They state further that peat represents “… an energy surplus stored within the ecosystem, resulting mainly from the relatively low levels of decomposer activity...”
Early characterizations of peat deposits date back many years. According to
Moore and Bellamy (1974), “...coal swamps should be regarded as enormous ‘peat bogs’ (mires) in which the active peat formers were giant Pteridophytes…” Pteridophytes are ferns, and it was generally believed in Grand’Eury’s time that
Carboniferous swamps were filled with nothing but ferns. We now know that the landscape was much more complex than that. A much later reference to peat and peat bogs as geological components of Earth’s sedimentary record is presented by
Moore (1940) and, in his discussion of the
Varieties and Ranks of Coal, Moore characterizes peat as “…an accumulation of vegetal matter which has suffered varying degrees of disintegration and decomposition.” Thereafter, he categorizes peat according to the ranges of composition one might expect in proximate (i.e., gross constituent) and ultimate (i.e., elemental) composition. Some compositional ranges were listed as, for example, carbon 37.15–66.55%, hydrogen 4.08–10.39%, and volatile matter 43.38–73.00%. For the purposes of this volume, these numbers might be too broad or ill-defined to be of great value. Still, they reflect one quality of peat that remains oddly consistent regardless of how one views it—peat is a poorly defined material.
The Earliest Plants: Peat and Coal Beds
Peats, in the sense of the partially decomposed remains of plants such as are found in bogs, fens, swamps, moors, etc. (
Mitsch and Gosselink, 1993) are geological materials. Their physical characterization will be undertaken later in this chapter, but we will first focus on the geologic history of peats, as represented by their appearance as organic sedimentary deposits of potentially great age.
Stach et al. (1975) provide one of the best accounts of the relationship of peat deposits to coals. In the discussion of the origin of peat forming environments, those authors recount how the earliest coals are actually of
Precambrian age (e.g., the Michigame
Anthracite unit of the Middle Huronian Nonesuch Shale, Michigan, ~2.1–2.5billionyears BP). Such coals were undoubtedly derived from the innumerable accumulated remains of
prokaryotes (microbes with no distinguishable cell walls or organelles, including bacteria) that accumulated in quiet marine waters of the distant past. Relating prokaryotes, and the accumulation of their remains to those of
eukaryotes (i.e., organisms with cell walls, organelles and, commonly tissue systems), and the sediments that their remains might represent would, in the context of this volume, be similar to comparing a bacterium to an earth worm; there is neither space nor need to distinguish among the characteristics of the various Kingdoms of organisms. However, the sizes of the cells or the organisms that they were a part of, and the composition of the cell walls are important and, consequently, their molecular and structural complexities deserve some discussion.
Little is known of the earliest coals or the living things that lead to their accumulation. There is, however, the likelihood that the earliest detrital organic sediments were produced by microorganisms such as those from the Gunflint Chert of North America and the Bitter Springs Chert of Australia (
Barghoorn, 1971, in
Folsome, 1979;
Figures 2.1.1 and
2.1.2). The sizes of the cells of the presumed microbes are such that astronomical numbers of them had to have accumulated in order to produce something of the density and composition of the Michigame Anthracite. However, the metamorphism of those Precambrian deposits to the rank of anthracite has all but obscured the microscopic evidence of the coals origins.
Between the time of accumulation of the Huronian deposits and the preservation of the first thin layers of
Paleozoic coals there was a vast gulf of time. Plant and animal evolution seem to have taken place in irregular pulses, reflecting, perhaps, the model of punctuated equilibrium as an evolutionary process (
Eldridge and Gould, 1972). Plant evolution appears to have...
