Seasonal variability of 7Be in suspended sediments from the Copper River, Alaska: implications for quantifying recent flood deposits in coastal environments

Over the past three decades, particle-reactive cosmogenic 7 Be (t 1/2 = 53.3 d) has frequently been used to quantify recent fluvial sediment inputs to estuaries and coastal marine environments, providing an important tool to understand the timing and pathways of sediment dispersal. This study utilizes a 2-year high-resolution time series collected from the Copper River, Alaska, to examine the seasonal fluctuation in 7 Be activity of suspended sediments near the river mouth. We show that there is considerable variability (particulate 7 Be = ≤ 3.3–47 Bq kg −1 ) throughout the hydrograph, with a significant negative correlation to discharge and suspended sediment concentration. This variability is interpreted to be driven by a combination of fluctuations in atmospheric deposition, water aging, and dilution by 7 Be-depleted sediments, reflecting erosion within the streambed/watershed and/or differing freshwater/sediment source. Particularly within this high-latitude setting, seasonal differences in provenance (snowpack/glacial melting vs. rainfall) likely influence 7 Be activities. Importantly, 7 Be was below detectable limits in river suspended sediments during peak discharge periods in both years, indicating that the flood signal in adjacent coastal environments will be negligible at this time. These results suggest that 7 Be will not be an effective tracer of fluvial sediment deposition in some coastal environments.