RECONSTRUCTING NUTRIENT HOTSPOTS IN TIME: INSIGHTS FROM THE GRAY FOSSIL SITE, TENNESSEE

Decomposing vertebrates provide a critical source of limiting nutrients in all terrestrial and aquatic ecosystems at present and in the geologic past. Based on studies in modern terrestrial systems, carrion decay results in the rapid stimulation of carbon (C) and nitrogen (N) biogeochemical cycles at localized hotspots and perturbs C and N pools for up to several years. Carrion decay occurs in a series of stages, starting with soft tissue degradation, and culminating in the exposure of bones and teeth. During decay, there is a pulse of microbial activity, releasing CO₂, and enhancing N mineralization. Once all soft tissues are degraded, increased rates of nitrification and denitrification continue to cycle the pulse of nutrients. The fossil record preserves some information about a once living animal (i.e., bones and teeth), but decomposition processes are not preserved. Using new insights gained from modern systems, we can approximate how much C and N would be introduced by the deposition of fossil animals and begin to unravel how biogeochemical cycles would have been perturbed during mortality events. The Gray Fossil Site (Gray, TN) represents a Miocene-aged paleosinkhole and paleolake deposit that preserves thousands of animals ranging in size from small rodents to one of the largest mastodons from North America. Fossils accumulated in the paleolake over thousands of years. The introduction of each animal would have generated localized hotspots, releasing C and N to the surrounding sediment and water. The deposition of a single mastodon (~16,00 kg), for example, would have lead to the release of 553 kg of N and 3,200 kg of C. Similar to decay observed in modern systems, the carcass would have stimulated soil microbial communities and the release of significant amounts of CO₂, likely for several months. Anaerobic conditions adjacent to the carcass and within the paleolake would have persisted for months. Inorganic N (as NH₄⁺) would have accumulated and persisted in the soil until a return to oxygenated conditions. The large inorganic N pool would have stimulated nitrification and denitrification, leading to the release of NO₃^- and N₂O. The Gray Fossil Site paleolake was a long-lived and dynamic biogeochemical hotspot, and using observations from modern carrion decay can provide new insights into (paleo)nutrient cycling.