Phosphorus Cycling in an Extreme Environment: Grain-scale Investigation of Apatite Weathering in the McMurdo Dry Valleys, Antarctica

Abstract:

Phosphorus (P) availability varies considerably throughout Taylor Valley, Antarctica. As an essential nutrient, P content and weathering have significant implications for the diversity and functioning of biota in soils, streams, and lakes. In upper Taylor Valley (Bonney Basin), soils have much lower total P content than in lower Taylor Valley (Fryxell Basin), 0.4 versus 1.5 g/kg soil. We hypothesize that this striking contrast is related to differences in glacial drift source and weathering rates.

We investigated the primary source of mineral P (apatite) and the grain-scale weathering processes resulting in the release of P into streams and adjacent soils in the Bonney and Fryxell Basins. Physical erosion, chemical weathering, and biological weathering all leave distinctive patterns on the surface of individual apatite grains. By inspecting these surfaces, we determined the relative importance of weathering processes for P cycling.

Apatite grains were separated from loose glacial drift using heavy-liquid and magnetic techniques. Individual grains were analyzed using SEM-EDS. We observe significant differences between the grains collected from the two basins. Grains from the Bonney Basin are more elongated and have a higher proportion of crystal faces. These results suggest that rates of physical weathering have been lower in the Bonney Basin than in the Fryxell Basin. In addition, we will present results on grain surface morphologies, indicative of chemical and biological weathering.

As the climate warms, more liquid water is expected to be available within the Taylor Valley system. This increase in water will likely lead to higher rates of P weathering, potentially altering nutrient limitation in this environment. Understanding how the P cycle will respond to a changing climate is contingent on understanding the current weathering processes and controls on P availability.