IMPLICATING LAND PLANT EXPANSION IN THE KAČÁK EVENT: INCORPORATING NITROGEN ISOTOPES IN A DEVONIAN LACUSTRINE RECORD

The Kačák Event at the Eifelian-Givetian boundary is one of the multiple marine extinction events in the Devonian characterized by widespread marine anoxia/dysoxia and represents a significant extinction of benthic species. It is a temporal correlative with the Orcadian Basin within the Old Red Sandstone continent and is associated with the distinctive Archanarras lake horizon. It is concurrent with evidence of global eustatic sea level rise as well as regional strengthening of the monsoon system driven by an orbitally induced insolation maximum. It is also temporally coincident with the significant evolution and expansion of land plants which accelerated beginning in the Mid Devonian. Much remains unresolved regarding what impact land plants had on the global biosphere and their subsequent involvement in Devonian biotic crises. Here we present high resolution nitrogen isotope (δ¹⁵N) data combined with organic carbon (C_org) and total nitrogen (N_tot) from 1.2 million years of late Eifelian-early Givetian lake record from Easter Ross in northern Scotland, which includes the Kačák Event. Initial results suggest a sustained and significant influx of vascular land plant material (C/N ratios > 20) at critical points during this 1.2-million-year record, including leading up to and during the Kačák Event and concurrent with elevated terrestrial phosphorus (P) input previously reported for this same interval. δ¹⁵N values show significant perturbations from 4 to 12‰ concurrent with the Kačák Event, suggesting significant changes to the N cycle and further supporting the presence of elevated terrestrial plant material input during this crucial period. These results suggest that orbitally driven monsoon activity in the region likely resulted in enhanced input of terrestrial plant material into the lake. Combined with previously reported data of concurrent increases in terrestrial P delivery, evidence suggests that climatic factors may have influenced the expansion of land plants in the region, which in turn may have resulted in increased terrestrial nutrient delivery ultimately to the Rheic Ocean, promoting eutrophication and anoxia.