From plants to coal: peat taphonomy of upper carboniferous coals

Abstract

Quantitative analyses of the botanical composition in coal-ball concretions provide a basis for assessing stratigraphic and regional patterns of peat taphonomy in relation to swamp paleoecology and, in turn, to coal origins. The categories of plant organ and tissues we discuss are lycopod outer trunk bark, lycopod cortical and twig litter, stigmarian roots, wood (all sources), fern roots, medullosan cortisal tissues, foliage and fructifications. Data are drawn from 20 coals from 34 mines, ranging from lower Westphalian A of England and Belgium to lower Stephanian (Missourian) of Illinois, U.S.A. The 53 data sets represent 8115 coal balls and 388,872 cm 2 of surface area of peels with contents identified by taxa, tissue-organs and preservational states or as an unidentified fraction of organic detritus and non-botanical components. Lycopod trees contributed 26–86% of biovolume in Westphalian coal-ball deposits with important exceptions in Iowa (cordaites = 48–60%). Relative abundances of lycopod peat generally are 75–84% (Westphalian A), 44–74% (Westphalian B—C), to 55–75% (Westphalian D). Shoot/root (S/R) ratios increase during the Westphalian from 0.5–1.0 in Westphalian A, 0.5–1.4 in Westphalian B—C to mostly > 1.4 in Westphalian D. S/R values reflect, in part, changes in abundance of outer decay-resistant bark, 10–29% in Westphalian A—C and up to 45% in Westphalian D. Lycopod litter patterns differ in the main tree genera, so that all litter abundances vary according to the mixes of taxa and diversity of other plants occurring in the Westphalian D (Desmoinesian). Wood from all sources is usually < 10% of deposits, reaching 13–32% in cordaitean-rich deposits, mostly occurring in the Westphalian C—D transition. Evidence of detritivore destruction of wood (cordaitean and calamitean) is most extensive in Westphalian B—C (upper Pottsville) coal-ball deposits. Psaronius tree ferns (mostly roots) contributed 6–27% in the Westphalian D and 33–76% in the Stephanian; seed ferns (mostly medullosan) formed up to 22% of coal-ball contents in these deposits. Stratigraphic trends of principal taphonomic patterns reflect, in large part, changes in the taxonomic makeup of peat-swamp floras. Westphalian deposits exhibit shifts from stigmarian root-rich peats in the Westphalian A, to mixed stigmarian-cordaitean root peats in Westphalian B—C, to an increase in diversity and sizes of litter types in the Westphalian D. Abundances of lycopod bark, total foliage and fructifications reach maxima in the Westphalian D. Stephanian peats are rich in tree-fern roots with variable distribution in the tissues of other tree types (expecially wood and lycopod bark); sizes of trunk litter suggest larger trees than earlier deposits. Taphonomic patterns are closely linked both to taxonomic composition of the vegetation and prevailing environmental conditions. Stratigraphic changes in the qualitative and quantitative aspects of peat formation reflect shifts in both composition of peat-swamp floras and in climatic and physical factors attendant to peat accumulation. Dominant plant groups in the lowland habitats of the Late Carboniferous were very distinctive taxonomically (representing at least four classes of vascular plants) and thus differed enormously in the kind of tissues they produced in their rooting patterns, and in their life histories. This contrasts sharply with woody seed-plant dominance of most post-Paleozoic landscapes. Consequently, extinctions or changes in dominance-diversity patterns lead to major changes in litter accumulation patterns. The Westphalian D (Desmoinesian) exhibits the greatest diversity and shared dominance and, thus, the most diverse litter and root patterns. Average litter size markedly increased in the middle Westphalian D. Lycopod stem tissues, in particular Lepidophloios and Diaphorodendron bark, suggest an increase in physical stability of the environment (reduction in disturbance frequency) that permitted lycopod trees to attain large size. Stephanian deposits are consistent with an even more stable swamp environment with still larger trees (tree ferns, medullosans, calamites and the lycopod Sigillaria) but generally more severely degraded litter. The relative importance of subsidence regimes versus paleoclimate is discussed as a major factor that may have distinct stratigraphic and regional patterns. Origins of carbonate-to-pyritic coal balls are briefly reviewed as they pertain to taphonomic data and their biases.