Field assessment of sediment traps

Sediment traps whose particle collection abilities had been calibrated in a laboratory flume at velocities of 0, 4, and 9 cm/sec were deployed in natural bodies of water to intercalibrate larger traps under current conditions ranging from tranquil to over 20 cm/sec. For cylinders, the height to width ratio is the controlling factor of the mass of sediment collected. Traps can be scaled up in size and maintain a similar (though not necessarily correct) collection rate...

Fluxes, dynamics and chemistry of particulates in the ocean

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution October, 1977.Sediment traps designed to yield quantitative data of particulate fluxes have been deployed and successfully recovered on four moorings in the deep sea. The traps were designed after extensive calibration of different shapes of containers. Further intercalibration of trap design was made in field experiments over a range of current velocities. Experiments with Niskin bottles showed that concentrations of suspended particulate matter obtained with standard filtration methods were low and had to be increased by an average factor of 1.5 to correct for particles settling below the sampling spigot. The trap arrays were designed to sample the particulate fluxes both immediately above and within the nepheloid layer. The data derived from the traps have been used to estimate vertical fluxes of particles including, for the first time, an attempt to distinguish between the flux of material settling from the upper water column (the "primary flux") and material which has been resuspended from some region of the sea floor (resuspension flux). From these data and measurements of the net nepheloid standing crop of particles one can also estimate a residence time for particles resuspended in the nepheloid layer. This residence time appears to be on the order of days to weeks in the bottom 15 m of the water column and weeks to months in the bottom 100 m. Between 80% and 90% of the particles collected in the six traps where particle size was measured were less than 63 μm. The mean size of particles collected in the nepheloid layer was about 20 μm, and above the nepheloid layer the mean was 11 μm. Less than 3% of the organic carbon produced in the photic zone at the trap sites was collected as primary flux 500 m above the sea floor. The primary flux measured at two sites was enough to supply 75% on the upper Rise and 160% on the mid Rise of the organic carbon needed for respiration and for burial in the accumulating sediments. From an intercomparison of the composition of particles falling rapidly (collected in traps), falling slowly or not at all (collected in water bottles), and resting on the sea floor (from a core top), it was determined that elements associated with biogenic matter, such as Ca, Sr, Cu, and I, were carried preferentially by the particles falling rapidly. Once the particles reached the bottom, the concentration of those elements was decreased through decomposition, respiration, or dissolution. Dissolution appears rapid in the vicinity of the sea floor, because despite an abundance of radiolarians, diatoms, and juvenile foraminifera collected in all traps, these forms were rare in core samples. The dynamic nature of thenepheloid layer makes it possible for particles to be resuspended many times before they are finally buried. This enables sediment to be carried long distances from its origin. The recycling of particles near the sea floor may increase dissolution of silicious and carbonate matter.Financial aid was provided in the form of a research assistantship from the Office of Naval Research through MIT and WHOI

Sediment trap dynamics and calibration: A laboratory evaluation

The flow dynamics and particle trapping characteristics of several designs of sediment traps were investigated using dye, sea-water, and deep-sea !utite in a recirculating flume and fish tank at velocities of 0, 4, and 9 cm/sec. Particles are collected through a process of fluid exchange rather than falling freely into a trap. The efficiency of a trap is therefore a function of the residence time and circulation pattern of fluid within the trap, processes which are controlled primarily by trap geometry and secondarily by current velocity...

The effect of brine on the collection efficiency of cylindrical sediment traps

Cylindrical sediment traps are frequently used to measure the downward flux of particles in the upper ocean. Some protocols recommend filling these traps with brine (50 psu in excess of ambient) before deployment to better retain the collected sample. However, this also changes the aspect ratio of the traps—a critical parameter affecting the trapping efficiency. We conducted controlled experiments in a flume that demonstrate filling traps with a 5 psu brine decreases their trapping efficiency to 54% at a velocity of 5 cm sec−1, and to 75% at 15 cm sec−1 relative to the same trap with no brine. This suggests that a revision of the protocols and further experiments are needed