Fluctuation in leaf wax D/H ratio from a southern California lake records significant variability in isotopes in precipitation during the late Holocene

Abstract

The hydrogen isotopic composition of plant molecular markers in modern vegetation and marine sediments in southern California have been intensively studied. Here we report a late Holocene reconstruction from Zaca Lake in coastal southern California, together with modern hydrological and vegetation studies in the catchment. The small catchment and decadal sampling resolution throughout the 9m, 3000yr sedimentary record provided a high resolution terrigenous counterpart of nearby marine records from the Santa Barbara Basin. Today, δD values of precipitation average −51.5‰±18 (1σ, n=7). Modern plant leaf wax δD values for the C28n-alkanoic acid averaged −141‰±11 (1σ, n=10) for Quercus agrifolia, with a calculated fractionation relative to precipitation (εwax/precip) of −94‰±22 (1σ, n=10); in contrast, there was negligible production of the C28 acid by co-dominant Pinus coulteri. Downcore, the C28 acid δD values ranged between −101‰ and −177‰ (mean −150‰±8, 1σ, n=490). Abundance distributions suggested that the sedimentary C28 acid was dominated by Quercus, implying that paleoprecipitation varied between extremes of −8‰ and −92‰ (mean −63‰±14, compound 1σ, n=490). The 3000yr leaf wax D/H record from Zaca Lake revealed substantial temporal variability, greater than observed in a speleothem reconstruction of similar resolution. We suggest that the plant-based proxy may magnify the variability by sampling spring precipitation preferentially. Centennial-duration positive isotopic excursions were associated with more sub-tropical moisture sources and drier conditions including during the Medieval Climatic Anomaly, and negative excursions were associated with N Pacific sources and wetter conditions including during the Little Ice Age.