CLIMATIC CONTROLS ON PHOSPHORUS CONCENTRATIONS IN THE LOCH, LOCH VALE WATERSHED, ROCKY MOUNTAIN NATIONAL PARK, COLORADO, USA: APATITE WEATHERING AND PALEOCLIMATE FROM THE LATE PLEISTOCENE TO EARLY HOLOCENE

Investigating controls on nutrient phosphorus concentrations ([P⁵⁺]) in present-day surface waters is often obscured by adverse anthropogenically influenced increases in algal biomass. The alpine-subalpine Loch Vale watershed (LVW) of Colorado, USA, is a location recognized for relatively high natural P⁵⁺ fluxes to surface and ground waters attributable to bedrock apatite (Ca₅(PO₄)₃(F,Cl,OH)) dissolution. However, for the 1982 to 2018 period of LVW outlet stream sampling, <1% of samples have [P⁵⁺] above the maximum detection limit. The low [P⁵⁺] are at least partly the result of increases in algal biomass occurring in response to anthropogenically elevated atmospheric nitrogen inputs. Therefore, in order to investigate controls on [P⁵⁺] in The Loch, a lake at the outlet of the LVW, a sediment core was collected for radiocarbon dating and high-resolution adsorbed [P⁵⁺] analyses.

Of the 11 outlet stream samples that are above maximum detection limits between 1992 and 2018, nine were in 2008, with 2008 also exhibiting the highest modeled frost-cracking intensity. Therefore, cold temperatures at least partly influence stream [P⁵⁺]. R-Bacon-modeled ages of the sediment core ranged from 15,378±708 cal years BP at the base to 9,778±215 cal years BP at the top. The initial increase in adsorbed [P⁵⁺] in the core was abrupt and occurred at the onset of the climatically cold and dry Older Dryas Stadial. The highest adsorbed [P⁵⁺] in the core occurred during the climatically cold and dry late Allerød Interstadial, likely during the Intra-Allerød Cold Period ~13.6 to ~13.1 ka. The lowest adsorbed [P⁵⁺] in the core occurred during the late Oldest Dryas Stadial and Bølling Interstadial, both climatically wet periods. These findings indicate that [P⁵⁺] in The Loch are highest when the climate is cold and dry. Current climate trends in the LVW are showing warming and melting of the permanent cryosphere with the potential to reduce surface water [P⁵⁺] and consequent algal biomass.