Kenneth W. Doherty

Large aperture time-series sediment traps; design objectives, construction and application

Sediment traps with 0.5 and 1.15 m2 apertures which are capable of collecting 12–25 samples at programmed intervals, typically weekly or bi-monthly, during one continuous semi- to interannual deployment have been developed. They utilize a number of new synthetic materials and stable metallic components which ensure reliable, long-lasting performance at any oceanic depth. The key component of the trap is a set of sequentially rotating samplers which is driven by a microprocessor-controlled electronic stepping motor. The electronic power controller controls sampler exchange with a high degree of flexibility and precision, as well as independently recording the executed sampling events. Each sampling bottle is sealed from ambient water during the time samples are stored before recovery. After continuous improvement and modification during 29.5 deployment-years of application in deep ocean experiments since 1982, we are convinced that these sediment traps can provide a relatively large quantity of settling particles in time-series with high experimental reliability.

Direct optical assessment of large amorphous aggregates (marine snow) in the deep ocean

Large amorphous aggregates (LAA), frequently called ‘marine snow’, may accelerate the flux of fine particles and dissolved matter in the ocean by adsorption. The aggregates are extremely fragile, and their sampling with conventional towing and casting is impossible. To assess the size and spatial distribution of such aggregates we have developed a photographic system that can be lowered to full ocean depth. The system, based on light scattering in water, uses a well-collimated beam pointed downward from the verticle at an angle of 45°. The beam is produced by placing high-powered stroboscopic lights at the back focal plane of a Fresnel lens system. Illuminated aggregates in the light are photographed from a direction perpendicular to the beam axis. The photographed volume, 0.66 m3, is 106 × 156 × 40 cm. The resolution of the instrument is about 0.2 mm. Aggregates were distributed throughout the water column at all stations along a 200-km transect off Monterey Bay. The abundance of macroscopic aggregates (>0.5 mm) ranged from several per liter near the surface to 1 or less at depth.

A Moored Profiling Instrument

Abstract The specifications and performance of a moored vertical profiling instrument, designed to acquire near-full-ocean-depth profile time series data at high vertical resolution, are described. The 0.8-m-diameter by 0.4-m-wide device utilizes a traction drive to propel itself along a standard mooring wire at a speed of ∼0.3 m s−1. The average power required to profile at this speed is 1–2 W; the present sensor suite and controller draw about 1.5 W. Based on these figures, the instrument’s battery capacity will support approximately 1 million meters of profiling. Instrument actions are regulated by an onboard microcontroller, allowing complex dive programs to be carried out. Oceanographic and engineering data are recorded internally on a hard disk interfaced to the controller. The measurement suite thus far deployed includes a CTD for deriving ocean temperature and salinity profiles, and an acoustic current meter that returns ocean velocity profile data. Addition of other oceanographic sensors is antic...

A new gas-tight isobaric sampler for hydrothermal fluids

A new gas-tight isobaric sampler for the collection of hydrothermal fluids venting at the seafloor has been designed, constructed, and tested at a ridge-crest vent site. The new device is constructed of chemically inert titanium, is gas-tight to 450 bar and can be used to sample fluids with temperatures up to 400°C. Compressed gas is used to maintain the sample at seafloor pressure before and during sample withdrawal onboard ship, allowing subsampling without degassing the fluid remaining in the sampler. This feature eliminates the need to collect separate gas-tight and major element samples since a single fluid sample can be analyzed quantitatively for major, trace, semi-volatile, and volatile components. The sampler fill rate is regulated to minimize entrainment of ambient seawater during collection of fluids from environments characterized by low fluid flow such as diffuse hydrothermal vents. In addition to deployment at the ridge-crest, the samplers can be used to collect gas-tight samples from other subseasurface environments such as hydrocarbon seeps, areas of methane-gas hydrate formation, cold seeps associated with serpentinites, regions of groundwater egress to the oceans, and the water column.

The Development of a Fine- and Microstructure Profiler

Abstract A new freely-falling ocean profiler for measuring finescale variations in temperature, salinity and horizontal velocity and microscale variations in temperature, conductivity, velocity and refractive index has been constructed. The High Resolution Profiler (HRP) features: full ocean depth capability, a microcomputer to control sampling, data storage and operations, solid state mass storage, and commercially available sensors. A special handling rig facilitates deployment and recovery at sea. Because the data are transferred electronically to a microcomputer for archiving, the instrument can be redeployed after use without opening the pressure case. Twenty to forty drops can be made before the battery needs to be replaced. Several different sensor configurations are possible, depending on the type of small-scale mixing process of interest. By observing both finestructure and microstructure with one instrument, it is possible to study the relationship between the finescale driving forces and the sm...

Ice-tethered profilers sample the upper Arctic Ocean

Studies conducted over the past decade indicate that the Arctic may be both a sensitive indicator of climate change and an active agent in climate variability. Although progress has been made in understanding the Arctics coupled atmosphere-ice-ocean system, documentation of its evolution is hindered by a sparse data archive. This observational gap represents a critical shortcoming of the ‘global’ ocean observing systems ability to quantify the complex interrelated atmospheric, oceanic, and terrestrial changes now under way throughout the Arctic and that have demonstrated repercussions for society [Symon et al., 2005]. Motivated by the Argo float program, an international effort to maintain an ensemble of approximately 3000 autonomous profiling instruments throughout the temperate oceans (see http://w3.jcommops.org), a new instrument, the ‘Ice-Tethered Profiler’ (ITP) was conceived to repeatedly sample the properties of the ice-covered Arctic Ocean at high vertical resolution over time periods of up to three years.

Submersible incubation device (SID), autonomous instrumentation for the in situ measurement of primary production and other microbial rate processes

Abstract A Submersible Incubation Device (SID) is described that will conduct three sequential time-course incubation experiments directly in situ . Each incubation involves a cleaning cycle, procurement of a 400 ml sample at depth, with simultaneous introduction of fresh tracer, and the time-course preservation of four subsamples for analysis upon retrieval of the instrument (12 subsamples total). The instrument records elapsed time, pressure (depth), light intensity, temperature and battery voltage throughout the deployment. The instrument has been used successfully for measurements of phytoplankton production in the open ocean and coastal wates. Three modes of operation have been employed: (a) a mode for obtaining daily rate information by summation of data from multiple short-term incubations, (b) a rapid profiling mode for measurement of microbial rate processes at several depths during a single deployment, and (c) an unattended time series mode for obtaining representative short-term activity measurements at several day intervals. The described instrument is a prototype for a long-term (months) time series version deployable in the open ocean or in remote locations.

Transducer mounting and rotating system for calibrating sonars in a sea well

A transducer mounting and rotating system for use in calibrating sonars in a sea well beneath a pier at the Woods Hole Oceanographic Institution is described. The transducer is attached to a plate fitted on the end of a shaft that is supported by a sleeve bearing at the lower end of a 6-m-long trussed steel tower. A rotary table for all-weather use with ordinary television antennas has been adapted for precision use, and attached to the surface end of the shaft. Rotation is controlled by software, with user-defined parameters, from a personal computer (PC) platform. The accuracy of the rotation is /spl plusmn/0.1 deg. Initial calibration trials of the transducer mounting and rotating system itself are described.

Velocity measurements from a moored profiling instrument

Velocity profile measurements from a recently-developed moored profiling instrument are discussed. The WHOI Moored Profiler uses a traction drive system to propel itself along a standard subsurface mooring cable at a nominal speed of 0.3 m s/sup -1/. The instruments onboard controller supports complex sampling scenarios, limited chiefly by the configuration of the mooring and the capacity of the battery. The vehicle has thus far been equipped with a CTD and an acoustic travel-time current meter; data from the latter are examined. As part of this assessment, comparisons are made between velocity profile data obtained with moored profilers and those obtained from an expendable current profiler, the High Resolution Profiler, and a lowered acoustic Doppler profiler. The authors conclude with a summary of possible applications for the new instrument.

Design of a multi-purpose titanium bottle for uncontaminated sampling of carbon monoxide and potentially of other analytes

The design, construction, and performance of a non-contaminating titanium sampler for carbon monoxide (CO) are described. In light of the favorable properties of titanium and the minimal contact of O-rings with samples, this multi-purpose design is expected to excel at a broad range of other uses: sampling gases, organic compounds, some trace metals, and living and dead particles.