UC Riverside

Understanding century to millennial-scale variation in snowfall and extreme hydrologic events in the Sierra Nevada of California is hampered by short instrumental records (<120 years) and uncertainty caused by extrapolating paleoclimate data from lower elevation to the alpine, snow deposition zone. We investigated Holocene hydrologic variability and lake productivity in the alpine Sierra Nevada using a ca. 12,000 year record of sediments collected from Pear Lake (36°35'49"N, 118°40'29"W, 2904 m asl, maximum depth 24 m). High-resolution scanning revealed alternating, fine grained, light-dark bands (1 mm to 5 mm thick) for most of the core length. This pattern was interrupted at intervals by visually homogenous layers (up to 75 mm thick) containing coarse grain particles up to 10 mm in diameter. Bulk elemental and stable isotope measurements were used to investigate sources of organic matter in lake sediments and used as a proxy for aquatic productivity. Laser diffraction particle analysis was used to obtain particle size distributions of the core at 1 cm increments to quantify the frequency of periods of relative wetness (rain-on-snow flooding, large avalanches, atmospheric rivers) and dryness (decadal to centennial-scale droughts).

At Pear Lake, the frequency of pluvial episodes declined during the Holocene from 37 in the early Holocene (11250 to 7250 cal yr BP) to 22 episodes during the middle Holocene (7250 to 3000 cal yr BP) to 15 episodes during the late Holocene (3000 cal yr BP to present). High percent silt and silt unimodal distributions, which indicate drier conditions, were more frequent in the middle and late Holocene relative to the early Holocene. The highest concentration of dry periods occurred within the last 1500 years. The δ13C and δ15N of terrestrial and aquatic organic matter overlaps substantially in the Sierra Nevada, thus they do not indicate organic matter sources in Pear Lake sediments. However, correlations between elemental and stable isotope data in the middle and late Holocene may indicate the influence of climate change on lake productivity. Overall, the data suggest a wetter, more dynamic, early Holocene followed by a drier, more drought-proned middle Holocene, and a variable early Holocene with intense climate extremes.