PHOSPHORUS DYNAMICS OF THE LOCH VALE WATERSHED, ROCKY MOUNTAIN NATIONAL PARK, FROM THE LAST GLACIAL MAXIMUM TO PRESENT: GEOCHEMICAL MASS BALANCE, POTENTIAL ECOLOGICAL IMPACTS, AND PALEOCLIMATIC INFLUENCES
Recent work in the LVW found that the dissolution of the bedrock mineral apatite (Ca5(PO4)3(F,Cl,OH)) released P to surface waters at a rate of 13-47 mol/ha/year from 1984 to 2008. However, only 1.1 mol/ha/year of dissolved P was measured leaving the LVW. It was hypothesized that P was being sequestered in lake-bottom sediments of The Loch, a lake located at the watershed outlet. A core of sediment was collected from the The Loch with P-concentration and radiocarbon-age data of the sediment yielding a P flux into the sediment of 0.0070 mol/ha/year between 6.4-0.93 ka. Thus, P sequestration in lake-bottom sediments is unable to explain the apparent dissolved-P deficit at the outlet. However, the top 5 cm of the sediment contains approximately 640 kg of bioavailable P which may contribute to algal blooms during periods of high atmospheric N deposition.
Macrofossil ages in the The Loch sediment core reveal that sedimentation began 18.1 ka at the end of the last glacial maximum. Organic P increases systematically up-profile with an abrupt increase approximately coinciding with the advance of alpine glaciers at higher elevations 13.2-11.1 ka. This glacial advance was associated with the Younger Dryas Chronozone. The increase in organic P may reflect mechanical weathering associated with the glacial advance that facilitated apatite dissolution upstream from The Loch. Detrital apatite abundances in the core decrease up-profile during this period. The alpine glacial advance was followed by large fluxes of macrofossil-rich gyttja into The Loch. Such sedimentation patterns reflect climatic warming, higher lacustrine productivity, and increased vegetation within the LVW. Relatively high sedimentation rates ended approximately 7-6 ka with the onset of the greater climatic stability.