Short-term variability of ~7Be atmospheric deposition and watershed response in a Pacific coastal stream, Monterey Bay, California, USA

摘要

Beryllium-7 is a powerful and commonly used tracer for environmental processes such as watershed sediment provenance, soil erosion, fluvial and nearshore sediment cycling, and atmospheric fallout. However, few studies have quantified temporal or spatial variability of ~7Be accumulation from atmospheric fallout, and parameters that would better define the uses and limitations of this geochemical tracer. We investigated the abundance and variability of ~7Be in atmospheric deposition in both rain events and dry periods, and in stream surface-water samples collected over a ten-month interval at sites near northern Monterey Bay (37°N, 122°W) on the central California coast, a region characterized by a rainy winters, dry summers, and small mountainous streams with flashy hydrology. The range of ~7Be activity in rainwater samples from the main sampling site was 1.3-4.4 Bq L ', with a mean (±standard deviation) of 2.2 ± 0.9 Bq L~(-1) and a volume-weighted average of 2.0 Bq L '. The range of wet atmospheric deposition was 18-188 Bq m~(-2) per rain event, with a mean of 72 ± 53 Bq m~2. Dry deposition fluxes of ~7Be ranged from less than 0.01 up to 0.45 Bq m~(-2) d~(-1) with an estimated dry season deposition of 7 Bq m~~(-2) month~(-1). Annualized ~7Be atmospheric deposition was approximately 1900 Bq m~(-2) yr~(-1), with most deposition via rainwater (>95%) and little via dry deposition. Overall, these activities and deposition fluxes are similar to values found in other coastal locations with comparable latitude and Mediterranean-type climate. Paniculate ~7Be values in the surface water of the San Lorenzo River in Santa Cruz, California, ranged from <0.01 Bq g~(-1) to 0.6 Bq g~(-1), with a median activity of 0.26 Bq g~~(-1). A large storm event in January 2010 characterized by prolonged flooding resulted in the entrainment of ~7Be-depleted sediment, presumably from substantial erosion in the watershed. There were too few paniculate ~7Be data over the storm to accurately model a ~7Be load, but the results suggest enhanced watershed export of ~7Be from small, mountainous river systems compared to other watershed types.