Bioturbation depths, rates and processes in Massachusetts Bay sediments inferred from modeling of 210Pb and 239+240Pu profiles

Abstract

Profiles 234 Th data suggest that in this half of the cores, the vast majority of the presentday flux of recent, nuclide-bearing material to these core sites is transported over a timescale of a month or more to a depth of a few cm below the sediment surface. As a consequence of the complex mixing processes, surface sediments include material spanning a range of ages and will not accurately record recent changes in contaminant deposition.3

Ensuring confidence in radionuclide-based sediment chronologies and bioturbation rates

“…Once an ideal profile was constructed, sampling at intervals of when the core sampling interval is 2, 4 and 8cm (Figure 1, right panels). These rates of effective mixing caused by sampling at coarse resolution are comparable to the diffusive rate of bioturbation often observed in sedimentary records (Olsen et al, 1981;Cochran, 1985;Smith and Schafer, 1984;Thomson et al, 1988;Anderson et al, 1988;Crusius et al, 2004) and suggest that the sampling 9 strategy alone could have contributed to the apparent mixing observed in some previously published studies. Based on our results, the effects of sampling at coarse resolution are likely to be even more pronounced in areas where sedimentation rates are lower than 1 cm yr -1 .…”

“…In this work, we use a numerical model that uses a finite difference approximation of Eq. 1 whose origin dates to Santschi et al, 1980 but has been subsequently modified (Crusius, 1992;Crusius et al, 2004). This model is written in both the C programming language and in Matlab™ and uses the upwind differencing scheme to model sedimentation (as a process of advection) and the forward time, centered space differencing scheme to model diffusive bioturbation.…”

“…It is important to bear in mind with this type of example that bioturbation can extend to depths of ~30 cm in continental shelf sediments, so bioturbation-driven slopes of excess 210 Pb profiles can reach to these depths as well (e.g. Anderson et al, 1988;Crusius et al, 2004), far below the depth of the global average marine sediment mixed layer depth of 10 cm (Boudreau, 1994). It is also important to realize that rates inferred from tracers with different half lives may sometimes be different precisely because they may record processes on different timescales.…”

Storm-driven sediment transport in Massachusetts Bay

“…Stellwagen Basin, the deepest part of Massachusetts Bay, is 80-90 m deep and is floored with fine-grained mud; the basin is generally considered to be a tranquil long-term depositional site for sediments winnowed from the inshore areas and the shallow banks. Sediment accumulation rates are about 0.001 m year À1 (Crusius et al, 2004). An area of coarser sediments in the southern portion of Stellwagen Basin at about 70 m water depth separates the fine sediments in central Cape Cod Bay and northern Stellwagen Basin.…”

Depositional variability of estuarine intertidal sediments and implications for metal distribution: An example from Moreton Bay (Australia)

“…A profile with these characteristics may result from the combination of low 210 Pb activities, bioturbation, physical mixing or sediment loss caused by waves and tidal action, responsible for transporting radionuclides downward in the sediment column, as commonly occurs in such environments (Heijnis et al, 1987;Crusius et al, 2004;Ruiz-Fernandez et al, 2009). As discussed by Palinkas and Nittrouer (2007), variations in 210 Pb activities can also be related to episodic delivery of sediments, such as flood sediments supplied in higher concentrations that commonly preclude particles from scavenging the usual 14 amount of 210 Pb.…”