LANDSCAPE EVOLUTION DURING THE MIDDLE DEVONIAN: EVIDENCE FROM AN OLD RED SANDSTONE LACUSTRINE SEQUENCE

The Devonian (419-359 Ma) represents a dynamic period in Earth’s history with significant biotic, geologic, and climatic events coincident with two major extinction events and numerous marine anoxic events. In addition, this period is characterized by a major expansion of land plants including the evolutionary innovation of advanced root systems and subsequent induction of modern soil formation processes. These processes significantly altered landscapes, enhanced nutrient mobility, and likely influenced water column anoxia. We turned to lake records to better understand landscape stability at the inception of soil formation. Here we present isotopic and geochemical evidence of Middle Devonian lacustrine samples from the Orcadian Basin of the Old Red Sandstone continent. Collected from northern Scotland/Orkney, these samples coincide with the Kačák extinction event and the inception and spread of the progymnosperm Archaeopteris. Taking place at the Eifelian-Givetian boundary, the Kačák is characterized by widespread marine anoxia and is correlative with evidence of both global eustasy and regional strengthening of orbitally induced monsoonal activity. The Devonian lacustrine sequences have ∂¹³C_org values ranging from -34 to -22‰ that exhibit systematic transitions between positive and negative carbon isotope excursions. Negative carbon isotope excursions correlate with high P, C/N, Sr/Cu, and redox-sensitive elements while positive carbon isotope excursions tend to track increased Rb/Sr coinciding with low Sr/Cu and TOC. This suggests that the lake systems underwent periods of enhanced P flux during wet periods as a result of both increased runoff and elevated P mobilization through stimulated terrestrial plant growth and subsequent shift toward terrestrial organic matter ∂¹³C_org values. Older sediments in the sequence have high amounts of iron. In comparison, younger sediments have a substantial decrease in Fe and P as well as periodicity between Fe, P, and climatic and weathering proxies. This could suggest a heavily weathered unstable landscape followed by a decrease in landscape reworking with periodic nutrient flux, potentially driven by climatically controlled enhanced monsoonal activity.