Showing papers by "Michael H. Bothner published in 2004"

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

Abstract: Profiles of 210 Pb and 239 + 240 Pu from sediment cores collected throughout Massachusetts Bay (water depths of 36–192 m) are interpreted with the aid of a numerical sediment-mixing model to infer bioturbation depths, rates and processes. The nuclide data suggest extensive bioturbation to depths of 25–35 cm. Roughly half the cores have 210 Pb and 239 + 240 Pu profiles that decrease monotonically from the surface and are consistent with biodiffusive mixing. Bioturbation rates are reasonably well constrained by these profiles and vary from ∼0.7 to ∼40 cm 2 yr −1 . As a result of this extensive reworking, however, sediment ages cannot be accurately determined from these radionuclides and only upper limits on sedimentation rates (of ∼0.3 cm yr −1 ) can be inferred. The other half of the radionuclide profiles are characterized by subsurface maxima in each nuclide, which cannot be reproduced by biodiffusive mixing models. A numerical model is used to demonstrate that mixing caused by organisms that feed at the sediment surface and defecate below the surface can cause the subsurface maxima, as suggested by previous work. The deep penetration depths of excess 210 Pb and 239 + 240 Pu suggest either that the organisms release material over a range of >15 cm depth or that biodiffusive mixing mediated by other organisms is occurring at depth. Additional constraints from surficial sediment 234 Th data suggest that in this half of the cores, the vast majority of the present-day 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 centimeters 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.

An operation manual for a time-series, storm-activated suspended sediment sampler deployed in the coastal ocean: function, maintenance, and testing procedures

Abstract: This manual describes the operation and testing procedures for two models of a multiport suspended sediment sampler that are moored in the coastal ocean and that collect samples on a programmable time schedule that can be interrupted to collect during a storm. The ability to sense and collect samples before, during, and after the height of a storm is a unique feature of these instruments because it provides samples during conditions when it is difficult or impossible to sample from a surface ship. The sensors used to trigger storm sampling are a transmissometer or a pressure sensor. The purpose of such samples is to assess composition and concentration of sediment resuspended from the seafloor during storms and subsequently transported within the coastal system. Both light transmission and the standard deviation of pressure from surface waves correlate with the passage of major storms. The instruments successfully identified the onset of storms and collected samples before, during, and after the storm maximum as programmed. The accuracy of determining suspended matter concentrations collected by the sediment sampler has not been fully evaluated. Preliminary laboratory tests using a suspension of muddy sediment collected in a nearbottom sediment trap yielded excellent results. However in laboratory tests with different sediment types, the suspended matter concentrations determined with these samplers became less accurate with increasing average grain size. Future calibration work is necessary and should be conducted in a facility that ideally has a water depth of at least 30 feet to prevent cavitation of the pump that draws sea water through the filters. The test facility should also have the capability for adding suspended matter of known composition and concentration to a fixed volume of seawater that is well mixed.