IMPACT OF MID HOLOCENE MOUNT MAZAMA ERUPTION ON LAKE PALEOPRODUCTIVITY AND SEDIMENTOLOGY: A CASE STUDY FROM A CLOSED BASIN LAKE IN SOUTHWESTERN BRITISH COLUMBIA

The eruption of Mount Mazama, (~7,700 cal yr BP), deposited volcanic ash that carpeted much of the North American continent. We employ principal component analysis (PCA) of diffuse spectral reflectance measured on sediment from Cleland Lake, British Columbia, to explore the paleolimnologic impact of the eruption. PCA derived five components explains 97% of the variability in the data. Pre-Mazama Cleland Lake was dominated by illite, derived from glacial drift deposits and bedrock weathering. Illite content rapidly declined around 7,760 cal yr BP and was replaced by smectite and chlorite; likely derived from weathering of the iron-rich volcanic ash. Smectite levels remain high for several thousand years (7,700 to 4,400 cal yr BP) but gradually decrease, thereafter. Three phytoplankton communities, each of which included cyanobacteria exhibited a pattern of community succession over the past 14,000 years. Late glacial primary productivity (14,000 to 11,500 cal yr BP) was limited to cyanobacteria community 1. Dinoflagellate algae and cyanobacteria community 2 was prominent during the early-Holocene (11,500 to 8,500 cal yr BP), when the lake level was much lower and regional climate was warm and arid. The abundance of diatom and cyanobacteria community 3 gradually increased after 9,000 cal yr BP, as a response to an increase in regional moisture. A rapid decline in dinoflagellates, diatoms and cyanobacteria occurred from 8,400 to 7,700 cal yr BP, potentially in response to the global cooling event centered on 8,200 cal yr BP and subsequent deposition of Mazama tephra. A rapid recovery in all three phytoplankton groups occurred within several decades following the Mazama eruption. This rapid increase – particularly for diatom taxa – could be associated with an increase in micro-nutrients and silica derived from ash weathering. The abundance of primary producers gradually declined within a millennium of the volcanic eruption, probably in response to the decline in the micro-nutrients and silica released by the weathering of volcanic ash. Comparison of results from three cores obtained from shallow, intermediate and deep basin of the lake suggests that shallow lake experienced immediate long lasting productivity changes compared to the other parts of the lake, possibly due to the heterogeneity in nutrient distribution.