POLAR PRESERVATION: EXCEPTIONAL BUT SELECTIVE PRESERVATION OF FOSSILS IN UPPER CRETACEOUS MARINE SEDIMENTS FROM DEVON ISLAND IN THE CANADIAN HIGH ARCTIC

Unusual taphonomic features of an Upper Cretaceous Arctic fossil-bearing deposit appear to result from both the contemporaneous environmental setting and more recent climatic conditions. This fossil suite has great taxonomic breadth even though the sediments do not preserve calcareous fossils; dinoflagellates, marine diatoms, radiolarians, sponges, brachiopods, molluscs, crustaceans, fish, plesiosaurs, birds, fungi, and vascular plants were members of a temperate-Arctic, marine coastal community. Microfossils are abundant and body fossils of metazoans occur sporadically; the most common macrofossils are coprolites. Many fossils show exceptional preservation; diatoms retain intricate ultrastructure with sub-micron sculpturing, dinoflagellate cysts preserve fine processes, and paired valves of inarticulate brachiopods retain glossy surfaces.

Fossils are preserved within a regressive sequence, chiefly biogenic mudstones and fine-grained glauconitic sandstones, downfaulted in linear grabens that protected these Cretaceous strata from Pleistocene glacial erosion. The upper sequence is interbedded with terrestrial sediments and terrigenous debris, indicating deposition in a shallowing, near-shore setting. Thick deposits of glaucony (generally formed at the interface of oxygenated seawater and sediment) and highly bioturbated mudstone beds argue against anoxic conditions often associated with fossil Lagerstätten.

This assemblage sheds light on the taphonomic history of the deposit. Low rates of sedimentation (indicated by glaucony) may have contributed to unusually high numbers of coprolites. Siliceous fossil preservation was enhanced by the addition of volcanogenic material from local sources. Acidic conditions that prevented preservation of calcium carbonate may have been caused by abundant terrigenous organic debris, or dissolution of calcium carbonate may have been secondary, due to increased acidity and/or colder temperatures. Perhaps most interestingly, exceptional preservation of the non-calcareous fossils indicates little diagenetic alteration over time, and probably reflects the generally lower temperatures and levels of solar radiation that influence polar sediments. Diagenetic processes should have been markedly slower during Late Cenozoic cooling.