The effect of suspended sediment concentration on the settling velocity of cohesive sediment in quiescent water

A laboratory study was carried to qualitatively investigate the effect of suspended sediment concentration C on the settling velocity w_s of cohesive sediment in quiescent water. A bay mud sample was mixed with water in a cylindrical container, and three optical back scatterance sensors were then used to measure suspended sediment concentrations of the mud–water mixture at three levels every 15 s for 5 h while sediments were settling in the quiescent water. Based on the measured sediment concentrations, the settling velocities at different concentrations were derived from the depth-integrated mass balance equation. This study has found that the settling velocity w_s is independent of C in the free settling regime of C<0.3 g/l, and then increases nonlinearly with C in the enhanced settling regime of 0.3<C<4.3 g/l, and finally decreases sharply with C in the hindered settling regime of C>4.3 g/l. The maximum settling velocity occurs at C≈4.3 g/l and is about nine times faster than the settling velocity in the free settling regime. A single empirical formula is also proposed to calculate the settling velocities at different sediment concentrations.     

An improved “benchmark” method for estimating particle settling velocities from time-series sediment trap fluxes

A new method, based on fitting Fourier series to time-series (TS) data from sediment traps, was developed to estimate the settling velocities (SVs) of sinking particles in the open ocean. This new method was applied to data from MedFlux, as well as from the US JGOFS NABE, EqPac, and ASPS studies. Fluxes of mass and of four chemical tracers, as well as the molar ratios of the latter, were plotted on logarithmic scales; Fourier series were then fit to these data. In each case we determined the most likely settling velocity using a likelihood-based nonlinear fitting algorithm. Variation among estimates using single tracers was significantly less than variation using tracer ratios; we therefore concluded that estimates based on single tracers are to be preferred to estimates based on tracer ratios. Our results also showed no obvious differences among SVs estimated using different single tracers. The best estimate of settling velocity using single-tracer fluxes with good temporal resolution (i.e. for sites with cup rotation times 8.5 days) is 205 m/d, with standard deviation 74 m/d. For MedFlux data alone (which have a resolution of 4–6 days), the estimate is 220±65 m/d. This latter value is within 10% of the estimate of average settling velocity (242±31 m/d) made using MedFlux IRS traps in “settling velocity” mode.     

Investigating the effect of ballasting by CaCO3 in Emiliania huxleyi: I. Formation, settling velocities and physical properties of aggregates

To investigate the role of ballasting by biogenic minerals in the export of organic matter in the ocean, a laboratory experiment was conducted comparing aggregate formation and settling velocity of non-calcifying and calcifying strains of the coccolithophore Emiliania huxleyi. Experiments were conducted by making aggregates using a roller table and following aggregate properties during incubation for a period of 40 days. Size, shape, and settling velocities of aggregates were described by image analysis of video pictures recorded during the roller tank incubation. Our results show that biogenic calcite has a strong effect on the formation rate and abundance of aggregates and on aggregate properties such as size, excess density, porosity, and settling velocity. Aggregates of calcifying cells (AGG_CAL) formed faster, were smaller and had higher settling velocities, excess densities, and mass than those of non-calcifying cells (AGG_NCAL). AGG_CAL showed no loss during the duration of the experiment, whereas AGG_NCAL decreased in size after 1 month of incubation. Potential mechanisms that can explain the different patterns in aggregate formation are discussed. Comparison of settling velocities of AGG_CAL and AGG_NCAL with aggregates formed by diatoms furthermore indicated that the ballast effect of calcite is greater than that of opal. Together these results help to better understand why calcite is of major importance for organic matter fluxes to the deep ocean.     

Measuring N–NH4 in marine, estuarine and fresh waters: An adaptation of the ammonia diffusion method for samples with low ammonium concentrations

We present a method for measuring ¹⁵N–NH₄⁺ in marine, estuarine and fresh waters. The advantage of this method is that it is broadly applicable to all types of water and it allows measurements in samples with lower ammonium concentrations than has previously been possible. The procedure is a modification of the ammonia diffusion method and uses large sample volumes (often 4 l) to obtain sufficient N for isotope ratio mass spectrometric analysis. Large volume samples have not previously been used with the diffusion procedure because isotopic fractionation occurs due to incomplete recovery of ammonium. However, the method we present accounts for this fractionation and allows precise correction of measured δ¹⁵N values.