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Community Heterogeneity of Early Pennsylvanian Peat Mires

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Community Heterogeneity of Early Pennsylvanian Peat Mires Community heterogeneity of Early Pennsylvanian peat mires Robert A. Gastaldo* Department of Geology, Colby College, Waterville, Maine 04901, USA Ivana M. StevanovicÂ-Walls Department of Earth and Environmental Science, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6316, USA William N. Ware 1109 Wynterhall Land, Dunwoody, Georgia 30338, USA Stephen F. Greb Kentucky Geological Survey, University of Kentucky, Lexington, Kentucky 40506, USA ABSTRACT a high wall (or to what can be seen from be- Reconstructions of Pennsylvanian coal swamps are some of the most common images neath in an underground mine), limiting re- of late Paleozoic terrestrial ecosystems. All reconstructions to date are based on data from coverable rock volume, or from spoil piles, either time-averaged permineralized peats or single-site collections. An erect, in situ Early where samples from various horizons often Pennsylvanian forest preserved above the Blue Creek Coal, Black Warrior Basin, Ala- are mixed. Although the data may be tempo- bama, was sampled in 17 localities over an area of .0.5 km2, resulting in the ®rst tem- rally constrained in these autochthonous as- porally and spatially constrained Pennsylvanian mire data set. This three-tiered forest was semblages, the spatial relationships often can- heterogeneous. Lycopsid and calamitean trees composed the canopy, and lepidodendrids, not be discerned because of logistical Lepidophloios, and sigillarians grew together at most sites. More juvenile than mature constraints. lycopsid biomass occurs in the forest-¯oor litter, indicating a mixed-age, multicohort can- A data set from the Lower Pennsylvanian opy. Pteridophytes (tree fern) and pteridosperms (seed fern) dominated as understory (Langsettian) Mary Lee Coal zone in the shrubs, whereas sphenophyllaleans, pteridophytes, and pteridosperms composed the Black Warrior Basin provides the ®rst tem- ground-cover and liana tier. The proportion of canopy, understory, and ground-cover porally and spatially constrained perspective biomass varied across the forest. Low proportions of ground-cover and liana taxa existed on Pennsylvanian peat-mire ecosystems at a where canopy fossils accounted for .60% of the litter. There is a distinct spatial clustering community scale. Standing forests are pre- of sites with more or less understory (or ground cover) where canopy contribution was served above each of the coals (Jagger, Blue ,60%. Where canopy biomass was low (,50%), understory shrubs contributed more Creek, Mary Lee, and Newcastle, in ascending biomass, indicative of light interception and/or competition strategies. Sphenopteris potts- stratigraphic order), as well as at ®ve separate villea, a ubiquitous ground-cover plant, is abundant in all sites except one, where pteri- horizons between the Blue Creek and Mary dosperm creepers and lianas dominate the litter, interpreted to indicate total suppression Lee seams (Demko and Gastaldo, 1992). The of other ground-cover growth. Ecological wet-dry gradients identi®ed in other Pennsyl- most extensive in situ forest occurs above the vanian swamps do not exist in the Blue Creek mire, with the interpreted wettest (Lepi- Blue Creek Coal (Gastaldo et al., 1991) and dophloios), driest (Sigillaria), and intermediate (Lepidodendron sensu latu) taxa coexisting consists of erect lycopsids, sphenopsids, pte- in most assemblages. ridophytes (tree ferns), and pteridosperms (seed-bearing gymnosperms) rooted in the un- Keywords: Carboniferous, coal, paleobotany, peat mire, wetland. derlying coal. Cordaites pith casts are in the roof shale ¯ora, yet no standing trees of this INTRODUCTION semblages, consisting mainly of aerial plant gymnosperm were encountered. Lycopsid Pennsylvanian peat-accumulating forests parts (stems, branches, reproductive struc- trees occur to heights of 4.5 m, and all stand- are one of the most intensively studied and tures) with rarely preserved leaves (Gastaldo ing vegetation is preserved within and cast by often-reconstructed Phanerozoic ecosystems; and Staub, 1999); these ¯oras represent the tidalite deposits (Gastaldo, 1992; Gastaldo et models appeared soon after coal exploitation resistant biomass contribution from several al., 2004). Tidal rhythmites begin within the increased following demands that accompa- plant generations to the peat. Decay rates of uppermost 0.5 cm of the coal, and are respon- nied the Industrial Revolution. Unfortunately, aerial parts that fall to the surface of Holocene sible for burial of the forest ¯oor of the mire, most nineteenth and twentieth century recon- tropical mires are accelerated by high temper- preserving the leaf litter in exquisite detail. structions present a ``family portrait'' of these atures and rainfall as well as by fungal and Gastaldo et al. (2004) argued that the only mires and mainly depict principal trees and detritovore activity; leaf half-life is often less mechanism that can mold and cast erect for- understory plants. Although plant-growth ar- than a few months (Gastaldo, 1994; Gastaldo ests in estuarine tidal deposits, and bury a chitectures have been re®ned over the past and Staub, 1999). As competition for newly nondegraded forest-¯oor litter, is rapid coseis- century as insight was gained from both ad- opened space following the death of a plant mic base-level subsidence. Rapid coseismic pression and permineralized specimens, mire- may change the systematic composition in the subsidence buried a peat forest by estuarine ecosystem reconstructions remain coarse. The sample location, resistant plant parts that ac- sedimentation during the Great Alaska earth- most plausible coal swamp reconstructions to cumulate and are buried may represent a cen- quake of 1964. Subsurface peats in Turnagain date are based on permineralized plants from tury or more of biomass contribution in any Arm indicate recurrent coseismic subsidence coal-ball assemblages taken from discrete one coal ball. Hence, the most accurate recon- and burial through time (Combellick, 1991). stratigraphic horizons (e.g., DiMichele et al., structions must be based on assemblages that 2002) supplemented, at times, with palynolog- were buried in a geologic instant, freezing the LOCALITY AND METHODOLOGY ical data (Greb et al., 1999). However, even plant relationships in space and time. Such as- The roof shale ¯ora was collected above the coal-ball assemblages do not provide an in- semblages are found in some types of roof Blue Creek coal in the Drummond Brothers stantaneous snapshot of the mire community. shale ¯oras, where erect trees are preserved Cedrum mine, Townley, Walker County, Ala- Coal-ball ¯oras represent time-averaged as- above the coal (Gastaldo et al., 1995; Di- bama (Fig. 1) during the last phase of exca- Michele et al., 1996; Calder et al., 1996). Such vation and exploitation of the Mary Lee in- *Corresponding author. E-mail: ragastal@colby. collections commonly are restricted to what is terval in 1999. The mine was closed and edu. recoverable from one or two localities along reclaimed beginning in 2000. Mining opera- q 2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected]. Geology; August 2004; v. 32; no. 8; p. 693±696; doi: 10.1130/G20515.1; 3 ®gures; 1 table. 693 Figure 1. Locality map of Drummond Brother's Cedrum mine (Town- Figure 2. Ternary diagram on which proportions of canopy, under- ley 7.5 U.S. Geological Survey Quadrangle Map, T. 14 S., R. 8 W., story, and ground-cover and liana biomass are plotted for each sam- sec. 18, and T. 14 S., R. 9 W., sec. 13) in Warrior coal ®eld, Alabama ple quadrat. Biomass proportion is based on 32 identi®ed biological (inset). Ruled polygon indicates active exploitation area in which taxa. Sites are subdivided into three clusters according to propor- Blue Creek mire was investigated. RRÐrailroad. tion of canopy biomass: >60% (triangle), >50% but <60% (star), and <50% (circle). Clusters are identi®ed in inset map of Cedrum mine, where spatial position is indicated by global positioning system readings of latitude and longitude. Shaded areas on distribution map correlate with ternary diagram. Sample site 9/3 is where there is equal proportion of understory (US) and ground-cover (GC) biomass. tions cleared overburden down to the fossil- productive structures of lycopsids, calami- plants that develop along a rhizome or pros- iferous horizon, which ranged from ,5to teans, and pteridosperms; small-diameter axes trate stem can grow as a vine into the canopy. .15 cm in thickness depending upon locality, with attached leaves assignable to spheno- Therefore, taxa considered to be creepers can in 17 sampled sites within the mine (Fig. 1). phyllaleans (horsetails), ferns, and pterido- occupy either or both positions within the for- A10m2 bedding surface was cleaned and sperms (lyginopterids, medullosans, and est, depending upon space, light, and plant used as the sampling quadrat for each site, al- ?callistophytaleans); and unidenti®able, de- density. The systematic composition between lowing for identi®cation of lycopsid trees in corticated stems and branches (StevanovicÂ- sample locations was compared by using the the sample area. (Systematic assignment of ly- Walls, 2001; Ware, 2001). Form taxa were biological data set with x2 statistics to deter- copsids cannot be made on either juvenile or condensed within recognized biological taxa, mine forest homogeneity or heterogeneity, and mature leaves because all higher taxa pro- where applicable, allocating the dispersed
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