Jason R. Price

The alpine–subalpine Loch Vale watershed (LVW) of Colorado, USA, has relatively high natural lithogenic P⁵⁺ fluxes to surface waters. For 1992–2018, the largest number of stream samples with P⁵⁺ concentrations ([P⁵⁺]) above detection limits occurred in 2008, corresponding with the highest frost-cracking intensity (FCI). Therefore, relatively cold winters and warm summers with a comparatively low mean annual temperature partly influence stream [P⁵⁺]. Sediment cores were collected from The Loch, an outlet lake of the LVW. Iron-, Al-, and Mn-oxide-bound phosphorus (adsorbed and authigenic phosphates; NP) serves as a proxy measurement for paleolake [P⁵⁺]. The highest NP in the core occurred during the cold and dry Allerød interstade. The lowest NP concentrations in the core occurred during climatically very wet periods in the Late Pleistocene and Early Holocene. Therefore, [P⁵⁺] are highest with relatively cold winters followed by relatively warm summers, relatively low mean annual temperatures, and relatively little rainfall and/or cryospheric melting. Currently the LVW is experiencing warming and melting of the permanent cryosphere with a rapidly declining FCI since 2008. This has the potential to dramatically decrease [P⁵⁺] in surface water ecosystems of the LVW, reducing biological productivity, enhancing P-limitation, and increasing ecosystem reliance on aeolian P⁵⁺.

Rare earth element (REE) release from weathering of accessory monazite [(REE)PO₄] since the last glacial maximum at 18.1 ka was investigated in sediment recovered from an outlet lake within the glaciated Loch Vale watershed, Colorado, USA. Labile REEs in the sediments reveal monazite weathering increased during the Younger Dryas chronozone (YDC) 13.2 to 11.1 ka when alpine glaciers advanced as climate cooled and bedrock comminution increased. Monazite dissolution peaked at approximately the Pleistocene-Holocene transition ~10.5 ka. During the Holocene, REE concentrations decline, reflecting a reduction in monazite weathering. The REE cerium (Ce) may occur as trivalent or tetravalent cations, and the Ce anomaly (Ce_n/Ce*_n) in sediment permits interpretations of paleoredox conditions. The Ce_n/Ce*_n decreases from 18.1 ka to the Pleistocene-Holocene boundary. Initially, oxidizing conditions within the watershed increased because of enlargement of the proglacial area as the ice retreated. At the onset of the YDC, oxidation further escalated in response to enhanced chemical weathering associated with glacial advance. A more stable Holocene climate and landscape resulted in relatively small changes in Ce_n/Ce*_n values. Slightly decreasing Ce_n/Ce*_n values over the last several hundred years are consistent with present-day stream water values and may reflect the onset of present-day hydrobiogeochemical conditions.