LAKE RECORDS OF SOIL DEVELOPMENT AND ECOSYSTEM STATUS IN ALPINE SETTINGS

Records of terrestrial climate change on glacial timescales are somewhat limited, especially in erosive alpine settings. We have utilized lake sediment records to provide information about changes in soil and ecosystem development via phosphorus (P) geochemistry. Phosphorus is a limiting nutrient in terrestrial ecosystems, and is made available to ecosystems mainly through in-situ soil development processes. New P is provided by the weathering of mineralized P forms in rocks, which in turn are converted to organic forms by plants and occluded (mainly iron-oxide bound) forms by soil development processes. By examining the fraction of P in mineralized, organic, and occluded forms in lake sediment records, soil development, and hence ecosystem development, in contributing watersheds can be inferred. Here we apply this approach to Lower Joffre Lake in the Lillooet and Kokwaskey Lake in the Kwoiek watersheds of British Columbia, with the aim of examining the history of landscape development over the last ~11 Ka.

Joffre Lake lies just west of the Coast Mountains divide, and Kokwaskey Lake lies east of the divide, positioned about 70 km east from Joffre Lake. Both records extend back to regional deglaciation, at ~11 Ka, and exhibit similar P records, revealing the regional history of soil and ecosystem development. Lake sediments show a dominance of mineralized P forms in the early (11-10 Ka) portion of the record, reflecting a regional landscape dominated by bedrock and unweathered glacial deposits just after glacial retreat. From 10-8.5 Ka, an increase in organic and occluded P forms in the lake sediments indicates increased soil formation and maturity. The interval from about 8.5-1 Ka is marked by relatively constant conditions. Although organic and occluded P together reached about 30%, mineralized P forms still dominated the total P content, reflecting both relatively immature regional soils (attributable to steep unstable slopes) and the continued presence of upstream alpine glacial sources of rock flour input. Both records reveal a shift coinciding with the onset of Little Ice Age conditions over the last ~1 ky, with mineralized P forms once again overwhelmingly dominating the sediment record. This shift is rapid, and indicates that the landscapes returned to a state of soil development similar to that of the earliest Holocene.