DEVONIAN TREES OF NEW YORK AND PENNSYLVANIA: CAUSES OR CONSEQUENCES OF BLACK SHALES AND BIOTIC CRISES?

Devonian black shales and mass extinctions have been blamed by Thomas Algeo and Stephen Scheckler on the eutrifying effect of early trees. There are at least a dozen Appalachian Devonian black shale events separated by marine limestones and sandstones. Black shales can now be correlated with red-bed (Catskill Magnafacies) paleosols with much deeper carbonate (Bk) horizons (Vertisols, Alfisols) than most Catskill paleosols (Aridisols). From what is known about depth to carbonate in modern soils, these deep-calcic paleosols represent transient paleoclimatic spikes of subhumid (mean annual precipitation 730 ± 147 mm) woodlands, in contrast to persistent semiarid shrublands (mean annual precipitation 484 ± 147 mm). The first trees in New York were Middle Devonian (earliest Givetian) cladoxyls (?Duisbergia, Wattieza), with shallow-rooted manoxylic trunks. Cladoxyl trees in New York thus postdate their latest Emsian evolution in Spitsbergen. Progymnosperm trees (?Svalbardia, Callixylon-Archaeopteris) appeared in New York later (mid-Givetian) than progymnosperm trees from Spitsbergen (early Givetian). Associated paleosols are evidence that Wattieza formed intertidal to estuarine mangal and Callixylon formed dry riparian woodland. Devonian mangal and riparian woodland spread into New York from warmer and wetter regions elsewhere during transient paleoclimatic spikes of humid climate, which carbon isotopic studies of pedogenic carbonate indicate were times of very high atmospheric CO₂ (3923 ± 238 ppmv). In contrast, semiarid shrublands of New York lived under lower CO₂ (2263 ± 238 ppmv). Tree migrations into semiarid cool New York were thus consequences of transient CO₂ crises, which the geographic spread of deeply rooted soils helped to mitigate. Black shales were consequences not of trees, but of these paleoclimatic crises, which were in turn due to extrinsic perturbations such as impacts and large volcanic events.