Dissertation Proposal
Scott Veirs
Draft: 3/14/01

Hydrothermal heat flux and hydrography: perspectives from theory, experiment, and Northeast Pacific observations

Committee members (7): Russ McDuff, Will Wilcock, Jeff Parsons, Susan Hautala, Bill Lavelle, Glenn Cannon, Graduate School representative (likely to remain Steve Porter)


The magnitude of convective heat flux through the seafloor at submarine volcanic ridges constrains how new oceanic crust is formed, how heat is extracted from the new crust, and how much energy is available to subsurface and seafloor ecosystems. Additionally, the flux of energy into the deep ocean drives local convection in the form of hydrothermal plumes. The plumes inject heat, salt, and metalliferous particulates into the ocean, measurably altering the local stratification, chemical concentrations, and degree of diapycnal mixing.

The measurement of convective heat flux, however, is complicated by the interaction of rising plumes with ambient currents. Understanding of plume dynamics in significant cross flow is prerequisite to comparison of flux measurements made through different field techniques at various scales: at hydrothermal sources, in the rising plumes, and in the advected effluent. Furthermore, combining field observations with numerical and laboratory models has the potential to elucidate how hydrothermal plumes and deep sea currents ultimately alter the regional, and even basin-wide hydrography.

The best resolution of hydrothermal plumes evolving in a complex velocity field was obtained during the August, 2000, Flow Mow cruise. During the cruise, plume distributions were mapped in the near-field by a CTD and Multi-Axis Velocity Sensor (MAVS) aboard the Autonomous Benthic Explorer (ABE), and in both the near- and far-field by a navigated CTD package (with backscatter, transmissometer, and chemistry samplers). Simultaneously, currents were recorded on a nearby mooring at 5 depths (50-250m above the axial valley floor).

I propose to analyze data from Flow Mow and other recent cruises to the Main Endeavour hydrothermal vent Field (MEF) on the Juan de Fuca Ridge, in pursuit of the following questions:

Central questions:

  1. How should multiple estimates of heat flux from the MEF be compared, and how do they compare?
  2. What is the effect of plumes on the hydrography of the Northeast Pacific ocean, based on appropriate tracers (including stability, q-ness, Thorpe scales, and possibly spiciness and mixing coefficient estimates)?
  3. Which factors predominantly govern how plumes rise into the ocean: source properties, currents, stratification, and/or the Earth's rotation?

Answering these three questions will likely lead to about 5 publications. In the interest of focusing my research goals and establishing a sequence of steps that will lead to the completion of my dissertation, I offer the following tentative titles and abstracts:

Potential publications:

High precision estimates of hydrothermal heat flux through the Main Endeavour vent field
Temperature, conductivity, and three components of velocity were measured by the Autonomous Bethic Explorer (ABE) underwater vehicle above a well-mapped hydrothermal vent field in the Northeast Pacific. During ~6 repeat surveys of a horizontal plane ~80m above the vent field, numerous rising plumes were traversed. Temperature anomalies (q-ness) of upto ~2oC and vertical velocities up to 20cm/s were encountered within plume cores. Integration of the temperature anomaly and vertical velocity data result in a heat flux estimate of ~600MW for the entire vent field. This estimate represents the flux through a control volume over the Main Field; it accounts for the possibility of advection or entrainment of local thermal contamination (presumably from diffuse flow) that was mapped by ABE and CTD during surveys of the near-field hydrography.

Observed and modeled plume dynamics and heat flux estimates above the Main Endeavour hydrothermal vent field
A modified atmospheric puff model is used to generate the distribution of salt and heat above a well-mapped hydrothermal field in a real current velocity field. Comparison to field observations is accomplished by simulating the field sampling in the modeled distribution. The advective- dispersive puff model accounts/cannot account for XX% of the observed variability. Consequently, the uncertainty of advected heat flux measurements is understood, and strategic measurement of instantaneous heat flux is possible. NN estimates of heat flux in the advected plume yield a flux of MM+/-PP MW, that can be confidently associated with the source vent field. Independent estimates of the same heat flux (from ABE measurements in the rising plume) compare. An added implication is that spatial extrapolation (of order 1km) of current meter measurements is at least occasionally justifiable.

Hydrothermal plume theory confirmed over known and new Endeavour segment hydrothermal fields
q-ness is a conservative quantity defined to be zero in the ambient fluid through which a buoyant plume rises. Hydrothermal sources with source temperatures and salinities that do not plot on the Theta-S curve of the ambient fluid have non-zero q-ness. Therefore, q-ness acts as an excellent tracer of hydrothermal contamination of the local ocean. Furthermore, the q-ness of a hydrothermal plume can be related through theory to the heat flux at the plume source. Theta-S trajectories derived from ABE/CTD data are combined with knowledge of source properties (T,S) in the Main Endeavour Field to test McDougall (qness) and McDuff plume theory... Relationship between Theta anom and w!? TC2-TC1 results? Koichi's chemical results? Field test of Middleton and Thomson (1986) trajectory model.

Hydrographic modification and enhanced mixing in the deep Northeast Pacific by hydrothermal plumes
A comparison of vertical CTD casts from multiple cruises over the Juan de Fuca volcanic ridge and WOCE data for the Northeast Pacific reveals that hydrothermal venting alters the background stratification and hydrography, and enhances vertical mixing. Spiciness and qness are mapped. The finestructure of CTD density profiles is described using Thorpe scales. (Mixing coefficients from Osborn relation?) [Is diapycnal mixing by hydrothermal plumes important in the deep ocean? What about in intermediate depths?]

Buoyant plumes in rotating, stratified, cross flow
A laboratory experiment was designed to examine the transition from a bent-over plume in a rotating, stratified, cross flow (Lavelle, 1994) to a baroclinic vortex in rotating, stratified, quiescent fluid (Helfrich and Battisti, 1991). With appropriate scaling ensured, the results are presented and compared with observations of hydrothermal plumes in a known cross flow above the Main Endeavour vent field. [Do ambient currents prevent baroclinic vortices from forming above the MEF?]

OTE partitioning:

Pete Jumars has suggested in his philosophy of science essays that an optimal PhD experience combines equal exposures to 3 different scientific processes: observation, theory, and experiment. I have sketched the likely partitioning of the processes in the following outline and hope to seek a balance between them as I construct my disseration around the specified topics...

Flow Mow 2000 ABE + CTD + currents; Perturbed (Lilley) 2000 CTD; Summer 2001 student cruise? Pacific T-S surveys (WOCE + ?); 1995 CTD data; 1992 current array data (Thomson, Cannon); other historical Thomson et al current data (1988+); application of physical oceanographic parameters: q-ness, spiciness, Thorpe and/or Osmidov scales (turbulence and mixing coefficients)
Puff model implementation; Boundary layer theory; Plume theory; Other numerical models
Competition between cross flow and vortex formation in rotating, stratified fluid; Plumes in oscillatory and/or sheared cross flow; Plume modification of stratification

Chapter outlines:

  1. Context
    1. Motivation for this study
      • Review evidence of plume variability
      • Review heat flux estimate techniques
      • Review observational evidence for/against baroclinic vortices and streaming plumes
    2. Background
      • Currents
        • Spatial correlation in Rick's Endeavour Segment data
        • Draw on theory of flow over ridges and rough topography
        • Analysis of 2000 mooring data (Thomson and Russians?)
        • Justify spatial extrapolation and interpolation
          • Spatial correlations between meters in 1992 current mooring array?
          • Correlation between Tivey MAVS and current meter mooring?
          • Correlation between ABE MAVS lateral velocity and current meter mooring?
          • Video analysis of Jackson/Jones Jason video?
          • Analytic fit to Jackson/Jones scintillation?
          • July 2001 Wasp RCM5x2 retrieval?
        • Sensitivity analysis?
      • Hydrography
        • McDougall theory (q-ness and spiciness)
        • Basin-scale: WOCE data (akin to Hautala/Riser observations? Reid?)
        • Regional: Mixing Zephyr ('95) and Flow Mow ('00) background casts (+ chemistry?)
        • Regional: Marv's 2000 CTD data

  2. High precision estimates of hydrothermal heat flux through the Main Endeavour vent field
    1. Integrated flux by buoyant plumes through the box top using ABE and vertical velocity
    2. Integrated flux through the box sides using ABE and lateral velocities from ABE and/or currrent meters (Thomson and/or Tivey)

  3. Observed and modeled plume dynamics and heat flux estimates above the Main Endeavour hydrothermal vent field
    1. Present Lavelle model adapted to rising plume MEF environment
    2. Importance of variable, local N, and q-ness as a tracer
    3. Instantaneous (or mean) flux by neutral plumes through the ribbon using CTDT+ and lateral velocities from currrent meters (Thomson and/or Tivey)
    4. Possibly apply same modeling approach to deeper CTD observations (MEF perimeter and/or Quebec ribbons)
        Then compare with:
      • Jones? Flux from focussed and diffuse sources within the MEF derived from estimates of vertical velocity (SM2000 nadir double-pings for entire field [Johnson/Hautala/Jones] and/or scintillation at specific structures like Grotto [Rona/Bemis/Jackson/Jones]) and measurements of source T and S from Alvin/Jason/etc (2000 and pre-2000).
      • Tim Crone Masters: Focussed source flux measurement of T, S, A, with w inference from video analysis
      • Irene: Johnson/Hautala measurement of segment scale diffuse heat flux
      • Previous chapter (ABE) heat flux estimates
      • Historical estimates from equilibration depths

  4. Hydrothermal plume theory confirmed over known and new Endeavour segment hydrothermal fields
    1. Theta-S trajectories in buoyant plumes
    2. Relationship between q-ness and w
    3. Instability and/or turbulence (Thorpe scale) as an indicator of source proximity
    4. Utility of vertically offset T/C pairs: T2-T1 => turbulence
    5. New source between High Rise and Salty Dawg
    6. New source between MEF and Mothra

  5. Hydrographic modification and enhanced mixing in the deep Northeast Pacific by hydrothermal plumes
    1. Erosion of NE Pacific stratification
    2. Regional analysis of E and/or Thorpe scales

  6. Buoyant plumes in rotating, stratified, cross flow
    1. Scale analysis for megaplumes and segment-to-field vent distributions and strengths
    2. Reproduce Helfrich and Battisti
    3. Repeat Bush-Woods with appropriate scaling
    4. Observe quiescent - cross flow transition

  7. Conclusions and implications
    1. Differentiation of crustal accretion and heat source models
    2. Subsurface and surface ecosystem energy flux
    3. Larval and sediment transport
    4. NE Pacific circulation and vertical diffusivity


Spring, 2001
  1. Take general exam
  2. Teach IT seminar
  3. Complete processing of CTD/navigational data
  4. Ch4: Assess utility of Thorpe scale and vertically offset T/C pairs in the hydrothermal environment
  5. Ch2: Draft article on new Endeavour segment vents and circulate to summer cruise participants

Summer, 2001

  1. Ch2: Assist in completion of draft article on MEF heat flux
  2. Ch3: Compare Matlab puff model plume distribution with observations
  3. Otherwise justify spatial extrapolation of current meter data
  4. Ch4: Submit article on new vents (Mickett as co-author?)
  5. Ch5: Devise method of mapping regional modifications and gather historical data
  6. Ch6: Continue GFD laboratory experiments and draft paper

Fall, 2001

  1. Ch2: Submit article on MEF heat flux
  2. Ch3: Calculate instantaneous heat fluxes using observations and puff model
  3. Ch4: Published
  4. Ch5: Draft regional modifications paper
  5. Ch6: Continue GFD laboratory experiments and draft paper

Winter, 2002

  1. Ch2: Published
  2. Ch3: Collaborate with other flux measurement scientists and submit
  3. Ch5: Submit
  4. Ch6: Submit
  5. Generate additional thesis figures

Spring, 2002

  1. Take final exam
  2. Modify papers to chapters
  3. Compose Ch1 (context) and Ch7 (synthesis)
  4. Complete written thesis