Seasonal cycles of particle and solute transport processes in nearshore sediments: [formula omitted] and [formula omitted] disequilibrium at a site in Buzzards Bay, MA

Abstract

Radiochemical measurements leading to the estimation of particle mixing rates and irrigation rates in the sediments of a site in Buzzards Bay, Massachusetts, have been made over two nearly full seasonal cycles. Both particle mixing and sediment irrigation show significant seasonal variability at this site; the variability is consistent with biologically driven transport mechanisms. 234Th 238U disequilibrium measurements were used to estimate particle mixing rates: based on three cores taken from December through March and four taken from June through October, a cold-season average mixing rate of 5.6 cm 2/yr and a warm-season average rate of 17 cm 2/yr were found. Excess 234Th inventories in the upper 5 cm of the sediments also varied seasonally, with a cold-season average value of 1.8 dpm/cm 2, significantly less than both the warm-season value of 3.4 dpm/cm 2 and the predicted average value for Buzzards Bay (≈3 dpm/cm 2). 222Rn 226Ra disequilibrium in sediment pore waters was used to estimate sediment irrigation. Based on a total of seven cores, three seasonal groupings of results were made: (1) December through March, when 222Rn deficits could be explained by vertical molecular diffusion alone, (2) early summer (June), when irrigation was important to at least 20 cm, the maximum depth of the sampled region, and (3) early fall, when irrigation was important to about 10 cm. A nonlocal exchange/vertical molecular diffusion model ( Emerson et al., 1984) was used to obtain quantitative estimates of irrigation rates; when the exchange parameter was allowed to vary exponentially with depth below the sediment-water interface, model Rn deficit profiles fit measured profiles quite well.