79-11 Burned: What can char debris tell us about the paleoecology and paleoclimate of Pennsylvanian coal-ball swamps?

Abstract:

Coal balls preserve permineralized peat from Pennsylvanian swamps, providing a detailed record of ancient swamp ecology with implications for Pennsylvanian paleoclimate. We use patterns of fossil preservation to investigate the nature of wildfire in Pennsylvanian peat swamps. Medullosans are early seed plants that contributed 4 – 20% of the plant debris in most Pennsylvanian coal-ball swamps and  40% in some. The most common medullosan fossils are the branches of their compound leaves known as Myeloxylon, which are hypothesized to have produced large amounts of plant resin, making medullosans vulnerable to fire. To test this, we compared the percentage of charred Myeloxylon to the percentage of charred cordaitean wood or lycopsid periderm in coal balls collected between 1930 and 1980 from three stratigraphic intervals, now in the Phillips collection (IGS) or Thompson-Darrah collection (Harvard). In all three, significantly more Myeloxylon debris was charred than cordaitean wood or lycopsid periderm (Chi-squared test of independence, p  0.02). 

The three stratigraphic intervals fell on a rainfall gradient from seasonal in the early mid-Pennsylvanian Kalo Formation, wetter in the mid-Pennsylvanian Fleming Coal, to very wet in the late mid-Pennsylvanian Herrin Coal.  However, all three localities had similar char percentages (Chi-squared test of independence, p  0.37). The percentage of charred debris in Pennsylvanian coal balls is not a reliable proxy for relative precipitation or length of the low-rain season.

The organ composition and taxonomic affiliation of charred debris in Kalo Formation coal balls suggests that this debris resulted from canopy, rather than surface or ground fire. We censused 1186 pieces of charred debris in 78 randomly selected coal balls from the Kalo Formation.  Whereas roots are 49% of Kalo Formation peat, all identifiable char belonged to aerial organs). 

In 90% of coal balls with char, the taxonomic composition of identifiable char matched that of the unburned debris in the coal ball, suggesting that char derived from local vegetation, consistent with canopy fire. Our sample contained 31 layers of charred debris, each interpreted as resulting from one fire.  In all cases, the organ composition of peat above and below the charred layer matched, suggesting that charred layers resulted from small, local fires, and that fire did not drive ecological succession in Kalo Formation swamps.

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