Permian Palsa Mires as Paleoenvironmental Proxies

Abstract

Palsas are peat mounds with permafrost cores that occur in regions of discontinuous permafrost. The distribution and stratigraphic context of clastic dikes in a mid-Permian coal seam in Antarctica, together with geochemical, paleobotanical, and carbon isotopic data, are interpreted as indicators of ancient palsas of a cool-temperate paleo-climate. Clusters of clastic dikes in a coal seam from the mid-Permian Weller Coal Measures are characterized by massive fill from overlying sandstones and irregular spacing. These features are unlike the laminated fill and regular spacing associated with ice or soil wedges of polygonal patterned ground. Above the clastic dike-bearing coal, lenses of lacustrine shales in sandstones conform directly to the extent of clastic dikes. Geochemically, the clastic dike-bearing coal seam has a significantly lower ash content than stratigraphically adjacent coals that lack dikes. The clustered assemblage of clastic dikes is analogous to the distribution of cracks observed in modern palsas. A low ash content is characteristic of (but not restricted to) raised peat structures, such as palsas. The overlying lacustrine shale lenses may represent depressions that formed after melting of the permafrost core and collapse of the palsa mound. A paleo-palsa interpretation, when added to data from coal stratigraphy, paleobotany, and isotope geochemistry, may indicate a change from frigid, glacial conditions in the Early Permian to cold temperate conditions in the mid-Permian. The basal part of the Weller Coal Measures section lacks coals and contains alluvial siltstones with striated pebbles and definite dropstones, suggestive of a glacially influenced paleoclimate. Thin, Early Permian coals occur stratigraphically higher and contain leaves of the species Gangamopteris, interpreted as a taiga-like plant. The clastic dike-bearing coal seam occurs in mid-Permian strata that contain thicker and laterally more continuous coals than found in Early Permian sequences. The mid-Permian coals are associated with abundant Vertebraria roots and Glossopteris leaves, thought to represent deciduous forests of cold-temperate conditions. Stable carbon isotopes from underclays show a trend from highly fluctuating values in the Early Permian to homogeneous values in the mid-Permian. Highly variable delta 13 C values could have been produced by frequent freeze-thaw episodes, whereas the uniform delta 13 C values are thought to indicate less frigid soil temperatures. Thus, delta 13 C chemostratigraphy supports the interpretation of paleoclimatic change from frigid to permafrost-free conditions, indicated by coal stratigraphy and paleobotany.