The most entirely known Permian terrestrial ecosystem on Earth – kept by explosive volcanism

Abstract

Fossil forests provide some of the most fascinating and compelling records of the geological past. If rapidly preserved in-situ, they yield multifaceted knowledge about the palaeoenvironment and habitat structure, diversity of organisms and their varied interrelationships. Further, fossil forests shed light on palaeoclimaric conditions and raphonomic pathways. Although discovered almost 300 years ago, only during the last decade, the Chemnitz Fossil Forest, central-east Germany, has been recognised as a complex fossil lagersratte represented by outstanding fossil assemblages preserved by pyroclastics. The fossil biota was buried as a T-0 thanarocoenosis resulting from instantaneous entombment by the Zeisigwald Tuff (Leukersdorf Formation, Chemnitz Basin). The eruptions constitute some of the last events of post-Variscan felsic volcanism in Central Europe and resulted in ash falls, pyroclasric surges, and flows, which ultimately covered and preserved the ecosystem. Based on magmatic zircons, the tuff yielded an age of 291 +/- 2 Ma (lace Sakmarian/early Artinskian). Fine-crystalline alpha-quartz, chalcedony, moganite, and particularly fluorite, which is restricted to plant fossils, pumice lapilli and vitreous matrix constituents, affected wood petrifaction. Organic remains in fossil wood are limited to scattered anthracite particles. Cathodoluminescence uncovered sire-specific multiphase silicification, including rapid precipitation under oxygen-deficiency in a low-temperature ( < 250 degrees C) hydrothermal environment. Attention is focused on two excavations in Chemnitz that recovered abundant new data and opened a unique window into a low-latitude 'wet spot' ecosystem. This spatially restricted habitat sheltered a dense hygrophilous arborescent vegetation of seed plants and cryptogams bur also epiphytes and climbers. The biota also includes a diverse fauna of vertebrates, arthropods and gastropods, several of them documented for the first rime from the Permian. Chemnitz reflects a relatively youthful but already well-established forest bionetwork with a remarkably well-developed trophic pyramid. Highlighted by many trees, still standing upright in their places of growth and rooted in the palaeosol, the Chemnitz Fossil Forest represents the most complete Permian forest biome/ecosystem recorded to date. To illustrate the environmental settings more precisely, palaeontological, pedological and geochemical indicators of deep-rime palaeoclimate have been rested. The ecosystem developed on a palaeosol of low maturity, even though it received annual precipitation of ca. 800-1100 mm. Although the habitat was affected by environmental disturbances and seasonality, conservative hygrophilous forms dominated the multi-aged vegetation. Growth features of woody trees provided excellent natural data archives and afforded reliable information, e.g., severe droughts and lightning strikes recorded as event rings and wound responses in affected trees. Tree-ring sequences were analysed to ascertain both annual growth rates and mean sensitivity and correlate growth increments between the trees. The tree-ring data provided insights into the fourth dimension (time), enabling appraisal of the stages of forest growth. The results yield a high-resolution palaeoenvironmental signal for eight decades during the early Permian and permit the recognition of fossil deadwood for the first time. Surprisingly, the woody trees, medullosan seed ferns, cordaitaleans, and calamitaleans revealed nearly 11-year cyclicity in tree-ring formation despite exhibiting different physiological reactions. The 11-year sunspot cycle, usually affecting atmospheric circulation patterns, was recorded in the trees and demonstrates how abiotic factors affect plant growth. Finally, multidisciplinary research achieved conclusions comparable to analyse modern ecosystems. The Chemnitz Fossil Lagerstatte significantly contributes to understanding the controls of palaeoclimate, palaeoenvironment, and palaeoecological dynamics on a confined wet-forest habitat of subtropical Northern Hemisphere Pangaea.