210Pb-derived sediment accumulation and mixing rates for the greater Puget Sound region

Abstract

Recent sediment accumulation rates of 46–1200 mg/cm 2/yr are calculated from excess 210Pb activity profiles in 44 sediment cores from 27 sites in the greater Puget Sound region. Rates calculated for nine stations in the deeper waters of central Puget Sound average 480 mg/cm 2/yr and are greatest between Seattle and Tacoma near the southern end of this region. The standard error of this mean is ±113 mg/ cm 2/ yr . Magnitudes and geographic trends of 210Pb-derived accumulation rates agree with limited data on sediment inputs and transport paths. Their accuracy is further supported by several independent approaches, including comparison with particle fluxes collected in year-long series of sediment trap deployments at two sites. Accumulation rates are calculated assuming no mixing beneath surface mixed layers of 2–22 cm. Sediment mixing coefficients in these surface layers average at least 43 cm 2/yr, four times greater than those reported in open Washington coast sediments. The difference is attributed to the greater benthic biomass in Puget Sound sediments. Particle residence times in surface mixed layers vary from 1.5 to 68 years and introduce corresponding uncertainties in ages of horizons beneath the mixed layers. For most of this region, these residence times will cause a 20–30 year time lag in achieving an equivalent response of surface sediment chemical concentrations to changes in concentrations in freshly depositing particles, for constituents whose stability and phase associations resemble those of 210Pb. These time lags limit the ability of sediment chemistry monitoring programs to recognize recent changes in composition and fluxes of chemicals to sediments. Recent fluxes calculated from the product of surface sediment chemical concentration and accumulation rates will be inaccurate for chemicals if their input concentrations have changed in the last 20–30 years.