THE IMPORTANCE OF FROST CRACKING ON LITHOGENIC PHOSPHORIC INPUTS INTO THE LOCH, ROCKY MOUNTAIN NATIONAL PARK, COLORADO, USA

Despite being located in high-elevation wilderness, The Loch, a P⁵⁺-limited lake located at the outlet of the glacial Loch Vale watershed (LVW) of Colorado, USA, is experiencing increases in algal biomass. A sediment core collected from the The Loch displays relatively low adsorbed P⁵⁺ concentrations from ~15 ka to ~12.1 ka. At ~12.1 ka the adsorbed P⁵⁺ concentration abruptly increases and continues to increase to the end of the Pleistocene. Organic matter concentration trends in the sediment core mimic those of adsorbed P⁵⁺, indicating that The Loch was P⁵⁺-limited at the end of the Pleistocene.

Present-day dissolved PO₄^3- concentrations at The Loch outlet are typically below detection limits. However, for the 1991-2010 sample period, dissolved PO₄^3- concentrations peak well above detection limits during November and December 2008. The elevated PO₄^3- concentrations reflect a relatively cold 2007-2008 winter and 2008 having the coldest mean annual temperature (MAT) for the period of record. Such conditions would have enhanced frost cracking, allowing for exposure of highly soluble bedrock apatite. The PO₄^3- concentration further increased in November and December as ice formed on the lake.

Frost cracking was investigated for the LVW using a simple one-dimensional heat flow model. For 1993-2009 climate data, 2008 yielded the highest frost cracking intensity and depth. In addition to a lower MAT, frost cracking intensity may be enhanced by increasing differences between maximum summer temperature and minimum winter temperature. A decrease in winter temperatures alone may enhance frost cracking even when MAT and maximum summer temperatures increase. Therefore, it may be hypothesized that the increasing adsorbed P⁵⁺ and organic matter concentrations observed between ~12.1 and the end of the Pleistocene in The Loch reflect decreasing winter temperatures despite an overall period of warming. Recent climatic trends in the LVW reflect increases in maximum summer temperatures, decreases in minimum winter temperatures, with relatively little change in MAT. Such conditions are capable of enhancing frost cracking and increasing dissolved PO₄^3- concentrations in The Loch, exacerbating increases in algal biomass. Furthermore, apatite weathering is being controlled by frost cracking rather than glacial comminution.