Transport of trace metals in nearshore sediments

Abstract

The focus of this thesis is on rates of transport of metals both across the sediment/water interface and within the sediment column of nearshore sediments. The early diagenesis of several first-row transition metals exhibiting a variety of behaviors in the ocean – Mn, Fe, Co, Ni, and Cu – has been studied intensively at a site in Buzzards Bay, Mass. By limiting the study to a single site, independent measurements over the seasonal cycle of the concentrations of the metals in pore water, of the pore water constituents important to metal cycling, and of particle and solute transport rates could be made at the same site. In addition, a direct, in situ study of the interaction of chemical and transport processes was undertaken using radiotracer techniques. Thus, the study emphasizes the mechanisms of metal cycling near the interface of nearshore sediments. Transport rates were estimated using excess 234Th distributions for particle transport, and pore water 222Rn deficit distributions for solute transport. Particle transport rates, modeled by analogy to Fickian diffusion, ranged from 7-80x10-8 cmz/sec, with excess 234Th reaching to 2-2.5 cm below the interface. There was a significant seasonal variation in rates, with a warm-season average of 40x10-8 cm2/sec and a cold-season average of 20x10-8 cm 2/sec. 234Th-derived mixing rates were applied to Mn distributions through a mass balance model of Mn cycling. It was found that a particulate flux due to bioturbation, from the net dissolved Mn removal layer to a net dissolved Mn production layer adjacent to the interface, was as large as 38% of net dissolved Mn production. Mixing of particulate Fe sulfides may have a similar importance for Fe cycling. Solute transport was estimated using measured 222Rn/ 226 Ra disequilibrium. The pore water 222Rn deficit could be explained using a model including vertical molecular diffusion and exchange with overlying seawater via exchange of pore water with bottom water in rapidly flushed burrows. Cores taken in all seasons could be split into three groups: (1) December through March: the 222Rn deficit was explained by vertical molecular diffusion alone; (2) early summer (June): irrigation affected the 222Rn profile to a depth of at least 20cm; (3) late summer/fall: irrigation was still important near the interface, affecting 222Rn profiles to depths of 10-12 cm. 222Rn deficits were adequately explained by an exchange parameter (a) which decreased exponentially with depth below the interface, but not by a constant-α model. Previous studies have explained irrigation using a constant exchange parameter throughout the irrigated layer. For comparative purposes, an α averaged over the upper 20 cm of the sediment column was calculated at the Buzzards Bay site: the range of depth-averaged α values found, 4-12x10-7 sec-1, is in agreement with values reported previously for a variety of nearshore sediments, using pore water Si02 as a tracer, of 1-20x10-7 sec-1. 222Rn-derived irrigation rates were…