Richard E. Thomson

Estimating Mixed Layer Depth from Oceanic Profile Data

Abstract Estimates of mixed layer depth are important to a wide variety of oceanic investigations including upper-ocean productivity, air–sea exchange processes, and long-term climate change. In the absence of direct turbulent dissipation measurements, mixed layer depth is commonly derived from oceanic profile data using threshold, integral, least squares regression, or other proxy variables. The different methodologies often yield different values for mixed layer depth. In this paper, a new method—the split-and-merge (SM) method—is introduced for determining the depth of the surface mixed layer and associated upper-ocean structure from digital conductivity–temperature–depth (CTD) profiles. Two decades of CTD observations for the continental margin of British Columbia are used to validate the SM method and to examine differences in mixed layer depth estimates for the various computational techniques. On a profile-by-profile basis, close agreement is found between the SM and de facto standard threshold met...

Currents along the pacific coast of Canada

Abstract A series of oceanographic studies along the west coast of British Columbia in 1979–1982 produced several realizations of the seasonal cycle of currents. At one site, occupied for 3–1/4 years, the monthly mean currents are stable in speed and direction indicating that a single year of data reliably describes the pattern. Based upon three sets of observations at 15‐m depth (summer only) and 17 at 50‐m depth, the flow along Vancouver Island in winter is towards the northwest, parallel to shore, with some evidence that speeds are greater 15 km from shore (30–40 cm s−1) than farther offshore. In April the flow becomes more variable over the outer shelf a southeastward current develops near the shelf break in May, becoming strongest in August with speeds of 20 cm s−1. This southeastward flow appears to be a response to a shift to northerly winds in summer. Near shore during summer a flow tentatively labelled the Vancouver Island Coastal Current flows towards the northwest with speeds of 10–15 cms−1. By...

An episode of seafloor spreading and associated plate deformation inferred from crustal fluid pressure transients

Three-year records of crustal fluid pressures and temperatures at four Ocean Drilling Program (ODP) sites on the northern Juan de Fuca Ridge and eastern ridge flank reveal a broad range of variations that include hydrologic transients that are contemporaneous with earthquakes along the ridge axis, the Nootka transform fault, and within the Juan de Fuca plate. One example of such a transient is the response to what is inferred to be a seafloor spreading event that triggered a swarm of earthquakes near the Endeavour ridge segment, beginning with a MW = 4.6 event on June 8, 1999. The largest transients were observed at ODP Sites 1024 and 1025 located 25.6 and 33.5 km east of the Endeavour axis. Pressures rose coseismically with the first earthquake of the swarm by roughly 1.6 and 1 kPa, continued to rise to maxima of 3.2 and 2.0 kPa within hours, then decayed to half the peak levels in 1 and 2 days at the two respective sites. A small (∼0.2 kPa) response of the same sign followed by a decay over 100 days was observed at Site 1027 situated 101 km east of the axis, and a similarly small response but of opposite sign was observed at Site 857 in Middle Valley, located along strike of the Endeavour segment roughly 50 km to the north of the earthquake swarm. The pressure transients are inferred to reflect a combination of the instantaneous internal plate deformation associated with extension at the ridge axis and lateral water flow in the crust following strain-induced fluid pressure gradients. The rate at which the transients dissipate constrains the regional-scale permeability of the upper igneous crust to be of the order of 10−10–10−9 m2. Instantaneous strain calculated from the initial amplitude of the transients ranges from 8×10−9 at the most distal site to 1.7×10−7 at the most proximal. The magnitude of regional strain is much larger than that which would result from the simultaneous earthquake, and we conclude that the first and all subsequent earthquakes of the swarm, and the crustal strain responsible for the hydrologic transients, are the consequence of a dominantly aseismic spreading “event” involving ∼12 cm of dilatation at the ridge axis. There were no clear indications of associated magmatic activity; hence episodic spreading may take place outside the influence of either dike injection or seismic rupture. Given the excellent sensitivity of pore pressure to strain, we anticipate that this simple observational technique can be applied to the investigation of seismic and aseismic deformation in a variety of tectonically active settings.

Developing a Mechanistic Understanding of Fish Migrations by Linking Telemetry with Physiology, Behavior, Genomics and Experimental Biology: An Interdisciplinary Case Study on Adult Fraser River Sockeye Salmon

Abstract Fish migration represents one of the most complex and intriguing biological phenomena in the animal kingdom. How do fish migrate such vast distances? What are the costs and benefits of migration? Some of these fundamental questions have been addressed through the use of telemetry. However, telemetry alone has not and will not yield a complete understanding of the migration biology of fish a or provide solutions to problems such as identifying physical barriers to migration or understanding potential impacts of climate change. Telemetry can be coupled with other tools and techniques to yield new insights into animal biology. Using Fraser River sockeye salmon (Oncorhynchus nerka) as a model, we summarize the advances that we have made in understanding salmonid migration biology through the integration of disciplines (i.e., interdisciplinary research) including physiology, behavior, functional genomics, and experimental biology. We also discuss opportunities for using large-scale telemetry arrays an...

A basin‐scale oceanic instability event in the Gulf of Alaska

A composite NOAA 14 satellite advanced very high resolution radiometer image of the Gulf of Alaska for early March 1995 reveals a sequence of six anticyclonic oceanic eddies spanning more than 1500 km of the continental margin of North America from central British Columbia to central Alaska. Although isolated mesoscale eddies have previously been documented for this eastern boundary region, the composite image provides the first evidence that trains of such eddies can develop “simultaneously” along the entire coast. The 80 km radius and 250 km spacing of these warm-core eddies are consistent with a baroclinic instability event in the poleward coastal current that prevails along the northwest continental margin of North America in “winter” (roughly October through March). Wind data from offshore meteorological buoys, combined with partial cloud-free thermal images for January and February, suggest that the event was triggered in mid January by an abrupt, coast-wide reversal in the prevailing poleward wind. The instability event disrupted the alongshore transport of relatively warm, high-salinity water and presumably resulted in a greatly enhanced cross-shore flux of brackish, nutrient-rich coastal water into the open ocean. On the basis of its basin-scale extent and more than 2 month duration it is feasible that the event had a major impact on the early spring recruitment and survival of zooplankton and pelagic fish within the highly productive fishery zones of coastal British Columbia and Alaska. Less extensive eddy sequences found in several earlier satellite images for the region suggest that these instability events are fairly common. If so, this indicates that the poleward current that forms over the eastern Pacific continental slope in late fall (herein, the “Northeast Pacific Coastal Current”) is marginally stable and likely to deform into a series of mesoscale eddies following abrupt, basin-scale reversals in the seasonally dominant wind. Provided these wind events occur early enough in the season, the coastal current could reform with the onset of poleward winter winds.

Constrained circulation at Endeavour ridge facilitates colonization by vent larvae

Understanding how larvae from extant hydrothermal vent fields colonize neighbouring regions of the mid-ocean ridge system remains a major challenge in oceanic research. Among the factors considered important in the recruitment of deep-sea larvae are metabolic lifespan, the connectivity of the seafloor topography, and the characteristics of the currents. Here we use current velocity measurements from Endeavour ridge to examine the role of topographically constrained circulation on larval transport along-ridge. We show that the dominant tidal and wind-generated currents in the region are strongly attenuated within the rift valley that splits the ridge crest, and that hydrothermal plumes rising from vent fields in the valley drive a steady near-bottom inflow within the valley. Extrapolation of these findings suggests that the suppression of oscillatory currents within rift valleys of mid-ocean ridges shields larvae from cross-axis dispersal into the inhospitable deep ocean. This effect, augmented by plume-driven circulation within rift valleys having active hydrothermal venting, helps retain larvae near their source. Larvae are then exported preferentially down-ridge during regional flow events that intermittently over-ride the currents within the valley.

The landslide tsunami of November 3, 1994, Skagway Harbor, Alaska

We show that the tsunami of November 3, 1994 in Skagway, Alaska was generated by an underwater landslide formed during the collapse of a cruise ship wharf undergoing construction at the head of Taiya Inlet. This event occurred at a time of extreme low tide and was not associated with a regional seismic event or incoming oceanic tsunami. Persistent wave motions with an amplitude of 1 m and a period of 3 min recorded by a tide gauge in Skagway Harbor following the landslide are linked to the formation of a cross-inlet seiche and quarter-wave resonance within the harbor. The high Q factor for the harbor (Q ≈ 20) indicates weak dissipation and strong resonance within the harbor.

Hydrothermal event plumes from the coaxial seafloor eruption site, Juan de Fuca Ridge

The episodic magmatic intrusions that accompany seafloor spreading can profoundly affect the discharge of hydrothermal heat and fluid. A rapid field response to the acoustic detection of seismic activity on the CoAxial segment of the Juan de Fuca Ridge, June/July 1993, provided the opportunity to conduct a unique series of observations of hydrothermal event plumes. Between July 3 and 26 we detected at least three event plumes ranging in plume volume from 1.3 to 4 × 1010 m³, and in excess heat from 2.2 to 12.4 × 1015 J. Two were discovered directly above a new lava extrusion. One of these was first sampled within days of its release, as evidenced by its asymmetric shape, complex structure, and a steadily increasing light attenuation anomaly apparently generated by ongoing precipitation of hydrothermal Fe. We hypothesize that event plumes were produced intermittently in response to successive volcano-tectonic events that produced pulses in seismic activity between June 28 and July 14.

Formation‐scale hydraulic and mechanical properties of oceanic crust inferred from pore pressure response to periodic seafloor loading

Observations of fluid pressure variations in young igneous oceanic crust have been made in five sealed and instrumented Ocean Drilling Program boreholes on the flanks of the Mid-Atlantic and Juan de Fuca Ridges. The holes penetrate locally well sedimented, and hence hydrologically well-sealed crust, and are situated 1 to 85 km from areas where water can flow freely through the seafloor at extensive basement exposures. Amplitudes and phases of formation pressure variations have been determined relative to tidal and nontidal pressure variations measured simultaneously at the seafloor for periods ranging from 4.8 hours to 14 days. Formation pressure variations are reduced to amplitudes between 98% and 28% relative to those at the seafloor and shifted in phase by up to 20°. Simple theory for porous media response to periodic loading predicts that the scale of diffusive signal propagation from locations of basement outcrop through buried parts of the igneous crust should he proportional to basement permeability and the inverse square root of the period of the variation. This behavior is consistent with the observations, and the characteristic half wavelength of the diffusive signal defined by the data from the sites near basement exposures is 14 km at diurnal periods. If signals propagate in a simple one-dimensional manner, this requires a formation-scale permeability of 1.7 X 10 10 m 2 . No constraints are provided on the thickness of material characterized by this permeability, hut the high-permeability pathway must be laterally continuous. At two sites near basement exposures the hulk modulus of the rock matrix estimated from the elastic component of the pore pressure response is 3 GPa. Where the igneous crust is regionally well sealed by sediment, the elastic response yields a hulk modulus of 16 GPa. The increase in hulk modulus with age and distance from basement outcrop is consistent with an observed increase in crustal alteration, an increase in seismic velocity, and a decrease in permeability, Observed lateral gradients of pressure, coupled with the estimated permeability, suggest that the amplitude of semidiurnal tidal volumetric flux (Darcy velocity) exceeds 10 -6 m s -1 ; semidiurnal fluid particle excursions would reach 0.25 m if the full volume of water contained in 10% porosity of the rock matrix were involved. If flow is channelized along discrete pathways, tidally modulated fluid flow velocities and particle excursions would he locally greater. The amplitude of tidal velocity is similar to that estimated for buoyancy-driven hydrothermal convection, hut the direction is generally different. Thus tidal flow may enhance water-rock interactions significantly. Energy dissipated in this manner would approach 0.3 μW m 3 .

Physical characteristics of the Endeavour Ridge hydrothermal plume during July 1988

Abstract We conducted CTD-transmissometer tows from 8 to 26 July, 1988 within 15 km of the central hydrothermal vent site ( ≈ 47°57′N, 129°06′W) on the Endeavour segment of Juan de Fuca Ridge. Anomalies of temperature, salinity and light attenuation reveal possible new vent sites 4 and 8 km northeast and 6 km south of the central vent site. As a result of widespread plume dispersion, background values of potential temperature, salinity and light attenuation below the 1900 m depth exceeded those for “pristine” ambient waters by 0.05°C, 0.05 psu and 0.03 m −1 , respectively. Maximum plume anomalies relative to the background waters were of the order of 0.10°C, 0.010 psu and 0.10 m −1 at core depths of 2000–2050 m. Heat and salt anomalies were detectable more than 5 km from the central vent site whereas light attenuation (particle) anomalies were confined to within 2.5 km of the vent site. Based on the background water property anomalies and moored current meter records, the mean (time-averaged) heat fluxes for the survey region were+2.3(±1.5) × 10 8 W in the along-ridge direction (20°T) and−7.7(±4.7) × 10 8 W in the cross-ridge direction (110°T). Mean along- and cross-ridge salt fluxes were+7(±5)and−25(±15)kg s −1 ; mean particle fluxes were+0.09(±0.06)and−0.29(±0.18)kg s −1 . Estimates of the instantaneous fluxes derived from coincident current and plume measurements indicate that heat fluxes from the central vent field may have been as high as1.2(±0.6)×10 10 W and corresponding particulate fluxes as high as6(±3)kg s −1 .