CARBON/PHOSPHORUS DYNAMICS OF MIDDLE DEVONIAN LACUSTRINE SEQUENCES OF THE OLD RED SANDSTONE CONTINENT

The Devonian period (419-359 Mya) was characterized by major climatic, geologic, and evolutionary changes including two major extinction events, the Kellwasser and Hangenberg events, and numerous anoxic events that coincided with these biological crises. This period is also significant for the evolutionary innovation of land plants, in which the development of advanced root structures in the Late Devonian influenced the initiation of modern soil formation processes and into continental interiors. These terrestrial dynamics have been invoked in potentially influencing marine eutrophication and subsequent water column anoxia via enhanced nutrient mobility. To test these ideas, we have turned to more proximal records of terrestrial dynamic changes stored in lake sediments. Here we present isotopic and geochemical evidence from five separate Middle Devonian sites from the Orcadian and East Greenland Devonian Basins of the Old Red Sandstone continent. Collected from northern Scotland/Orkney and Greenland, these lacustrine sequences coincide with the Kačák and Taghanic extinction events, and the inception and spread of the progymnosperm Archaeopteris. The Devonian lacustrine sequences have ∂¹³C_org values ranging from -34 to -22‰ that exhibit systematic transitions between positive and negative isotope excursions. Negative carbon isotope excursions exhibit correlation with high P, TOC, C/N, and Sr/Cu, while positive carbon isotope excursions tend to track increased Rb/Sr at the same time as low Sr/Cu and TOC. This suggests that the lake systems underwent periods of enhanced P flux as a result of increased runoff during wet periods and subsequent shift toward terrestrial organic matter ∂¹³C_org values. In this way, C/P dynamics may be used to track the source and timing of increased nutrient load into the basin and subsequent enhanced eutrophication. Further, utilizing sequential iron extractions to resolve potential anoxia may help determine the response to enhanced P fertilization.