CHEMOSTRATIGRAPHIC APPROACH TO TRACKING HIGH-PALEOLATITUDE NUTRIENT INFLUX IN AN EARLY TO MIDDLE DEVONIAN COASTAL SEQUENCE, WESTERN FALKLAND ISLANDS, SOUTH AMERICA

The Devonian land plant expansion, characterized by the development of advanced root systems and early soil development, resulted in extensive nutrient mobilization as plants spread into continental interiors. Upon delivery to the ocean, the enhanced nutrient flux promoted anoxic conditions leading to marine extinctions hypothesized as global events. However, most evidence of these anoxic events are based on sediments from paleotropical locations as records from polar paleolatitudes lack common correlation indicators (e.g., conodonts, goniatites), making it difficult to determine if anoxia was truly global. Spores have been previously used to correlate several Devonian near-polar regions, allowing potential correlation with anoxic events and equatorial regions; however, a chemostratigraphic approach may be a more rapid and widely applied method to track the source and timing of increased nutrient load in the higher latitudes, particularly with respect to global extinction events. Here we present isotopic and geochemical evidence from an Early to Middle Devonian paleo-coastal site from the western Falkland Islands in the Cape Fold Belt which has been previously correlated using spores, allowing potential correlation with anoxic events characterized in sediment from equatorial regions. Strata from this near-polar paleolatitude site coincide with both the Choteč and Kačák marine anoxic events and the inception and dominance of the progymnosperm Archaeopteris. Samples consisting of thick units of predominantly cross-bedded sandstones interbedded with thinner layers of shales from the Port Stephens, Fox Bay, and Port Philomel formations span approximately 27 million years (late Lochkovian to Mid Givetian) and have ∂¹³C_org values ranging from -28.3 to -21.9‰ with distinct alternations between positive and negative carbon isotope excursions. Notably, we observe carbon isotope excursions during the Kačák (+2‰) and Choteč (-1‰ ) events, that are similar to those seen in previous studies. These carbon isotope excursions coincide with increased phosphorus/metal ratios and redox-sensitive elements suggesting periods of anoxia similar to those seen in paleotropical regions.