MECHANISMS FOR INCREASED PHOSPHORUS INPUTS AND CONSEQUENT ENHANCED LACUSTRINE PRODUCTIVITY DURING THE YOUNGER DRYAS CHRONOZONE IN THE LOCH, ROCKY MOUNTAIN NATIONAL PARK, COLORADO, USA

Despite being located in remote wilderness, The Loch, a lake located at the outlet of the glacial Loch Vale watershed (LVW) of Colorado, USA, is experiencing increases in algal biomass. The LVW is also undergoing cryospheric melting as climate warms. Increased atmospheric nitrogen deposition from anthropogenic activities in the region is partially responsible for the enhanced within-lake production. Dissolved phosphorus also facilitates production of algal biomass, and has been investigated both since the last glacial maximum from a sediment core recovered from The Loch, and in present-day stream water. Maximum adsorbed P⁵⁺ concentrations in the sediment core occurred during the Younger Dryas Chronozone (YDC) 13.2 to 11.1 ka when alpine glaciers advanced at higher elevations in response to climatic cooling. Consequent bedrock comminution facilitated dissolution of the phosphate minerals apatite [Ca₅(PO₄)₃(Cl,F,OH)] and concomitantly monazite (REE(PO₄)). Despite cooling, lacustrine organic matter content in the lake increased during the YDC, a trend not observed elsewhere in the Rocky Mountains of Colorado. The increase in organic matter is believed to reflect elevated PO₄^3- concentrations in surface waters as the alpine glaciers advanced. Present-day increases in lacustrine biomass occur twice annually; during the winter when ice formation concentrates PO₄^3- in lakes, and during the summer when the PO₄^3- flux is at its maximum following seasonal freeze-thawing. Such processes would have been enhanced during the cooler YDC. This study also demonstrates that the most sensitive elemental paleoclimatic proxy data in lacustrine sediments originates from highly soluble, potentially radiation-damaged, minerals.