Taphonomy and paleoecology of the Lontras Shale Lagerstätte: Detailing the warming peak of a Late Paleozoic Ice Age temperate fjord

Abstract

Fjords are considered biodiversity hotspots and aquatic critical zones, being extremely sensitive to climate change due to close oceanic, terrestrial, and glacial interactions. These ecosystems have received a great deal of attention in research on current and future anthropic impacts. Despite this, there is no analog in the geological record that presents icehouse-greenhouse biological and climatic changes. Here we present an analog, through a detailed taphonomic survey of the Lontras Shale Lagerstätte (Itararé Group, Paraná Basin, Brazil), related to a climatic optimum of the end of the Late Paleozoic Ice Age (Late Pennsylvanian), in which a temperate outer paleofjord with a rich well-preserved biota was installed. In the monotone layers of black shale, we find subtle variations of the dominant skeletal type, rates of fragmentation and disarticulation, and other taphonomic aspects that define distinct taphofacies. Each of them is the result of distinct time-averaging related to mass mortality events, turbidity, and depositional hiatus periods at different scales and intensities, mixing the ecologic census with short-term and long-term within-habitat assemblages. In addition, the rich paleobiota was reconstructed with autochthonous and allochthonous benthic fauna, many marine nektonic organisms, and intense continental contribution of terrestrial bioclasts, that proliferated and were exceptionally preserved by the establishment of an anoxic temperate outer fjord. The taphofacies show an evolution in a high-frequency sequence within a highstand systems tract, linked to climatic improvement. Furthermore, taphonomic detailing can be used as a comparison of deep marine and deep lacustrine taphofacies, in addition to serving as an analog for the short-time scale biological, biogeochemical, climatic, and stratigraphic changes associated with the icehouse–greenhouse transition in the past, present, and future.