Flow Mow -a study of hydrothermal heat flux

• Who: Russ McDuff, Fritz Stahr, Scott Veirs, Christian Sarason - (UW); Ko-ichi Nakamura - (GSJ); Al Bradley, Dana Yoerger - (WHOI)

• What: measuring heat flux from a whole vent field

• Where: Main Endeavour Vent Field, Juan de Fuca Ridge

• Why: constrain geophysical models, improve knowledge

• When: August 2000 -- among the ROBE crowd

• With: an AUV (ABE) and good sensors

• Funded by: NSF grant OCE-9872090. Thank you!

Motivations

• Constrain geophysical models– 20% of crustal heat flux is through hydrothermal systems

– heat transfer location and quantity critical to ridge models

• Improve heat flux estimates– prior results highly uncertain and disagree

– covered different spatial scales and vent types

• New technology allows new measurements– AUVs available to cover area quickly

– accurate navigation allows more repeatable surveys

– new sensors allow better plume detection

Prior Results - Segment Scale

• 1700 ± 1100 MW Baker & Massoth, ‘87

• 3000 ± 2000 MW Rosenberg et al., ‘88

• 1000 ± 620 MW Thomson et al., ‘92

• All used neutrally buoyant plume: not clear what’s included

Prior Results - Field Scale

• 6550 ± 3000 MW; Thomson et al., ‘92

• 9390 ± 74* MW; Schultz et al., ‘92 * diffuse only

• 154 ± 84* MW; Bemis et al., ‘93 * smokers only

• 364 ± 73* MW; Ginster et al., ‘94 * smokers only

• All extrapolated so highly uncertain, especially diffuse flow

100°C

380°C

Hulk

Puffer

HeatSource

300°C

Kelley and Delaney, 2000

Plan of Attack, part 1 - Measure the buoyant part of the plume

Find best height to “mow” it using a model:

• MTT plumes (point sources)

• conserves mass, momentum, heat, salt

• 200 x 400 m horizontal plane

• sampling with “real” uncertainties

… says 75-100 m is the best!

Plan of Attack, part 2 -Team up with a robust & capable AUV

The Autonomous Benthic Explorer

Plan of Attack, part 3 - put good sensors on ABE

• Temperature - Sea Bird CTDs (ducted & pumped)

• Velocity - MAVS current meter plus vehicle tracking/velocity

• Area - use new “plume-sniffers” e.g., redox potential (Ko-ichi Nakamura)

Execute, part 1 - good background data

• Microtopography (Imagenix on ABE)

• 40- CTD stations (backgrounds, time-series, curtains)

• Currents (mooring, 5 @ 50m intervals)

Execute, part 2 - fly ABE around box

• Fly all sides of a box surrounding the vent field -- obtained one N/S/E/W sides & two further N/S sides

• Because most flux is vertical, fly as many box-tops as possible -- did 12 total

Execute, part 3 - Process Data

Heat-flux (Watts) = [A Cp () w ]

• Area from actual track length x nominal spacing

• Cp from average T, S, P for whole top

from CTD & background measurements

• Vertical velocity (w) from MAVS minus ABE or from ABE dynamic model

from CTD

• Sum for each point over whole track

Contours of ABE-48, Top 1

• heat-flux = 670 MW

4700 4800 4900 5000 5100 5200 5300

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Contours of ABE-50, Top 1

• heat-flux = 528 MW

4700 4800 4900 5000 5100 5200 5300

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ABE top150 trackline

Results (to date) -Average heat-flux = 588 ± 168 MW

• Calculated from ten tops with background = 0.063 subtracted

• 0.063 from mean of ABE-50 side-walls, represents avg. background heat (more on this later)

• background heat an issue for all prior results

More of the fine print...• Currents are highly variable in time & space• Therefore flux through sides is variable (more on

this later)

0 5 10 15 20 25-4

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6Currents during ABE top 251 - 551, Aug 19, 2000

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Meter at 100 mab (2118 m depth) above, 50 mab (2168 m) below

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Time in hours from 1218 UTC (MJD 51775.12)

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Currents during ABE top 150 - 250, Aug 17, 2000

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Meter at 100 mab (2118 m depth) above, 50 mab (2168 m) below

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Time in hours from 2347 UTC (MJD 51773.991)

Summary of Flow Mow ABE work

• Measured heat-flux over the Main Endeavour Vent Field more precisely than previously

• Using this technique, variations in vent-field scale heat-flux can be tracked in space and time, repeat occupations necessary

• More results coming from– finding N/S side fluxes from combined ABE, CTD and

current meter data

– combined CTD “curtains” and ABE-51 data to look at neutrally buoyant plume flux

Endeavour Segment Currents• Means from July-Oct, 2000 (74 days, hr averages)

• ~5cm/s above ridge crests, intermittently to the SW or W

• ~2cm/s within axial valley, predominantly to the N

Hodographic histograms

• Low velocities to W

• Symmetry at 100mab

• Strong shear between valley and ridge tops

Progressive vector diagrams

• Upper flow to SW, but with NW event, ~20d

• Top 3 ~uniform

• Minimal transport at ridge crest elevation

• Northerly net flow within valley

Regional hydrography, part 1

Near-field hydrography is variable on tidal time scales.

Regional hydrography, part 2

Near-field hydrography is spatially variable on tidal time scales also.

Regional hydrography, part 3

The axial valley is generally thermally contaminated, but even in the Main Field, background water is present.

Northern side of MEF

Southern side of MEF

Calculating side heat-flux

mean mean v q (MW) n (hrs)North

ABE 0.0708 0.0072 50 5Near 0.0810 0.0055 46 6Far 0.0418 0.0027 11 19

SouthABE 0.0664 -0.0088 -62 4

Near 0.0565 -0.0053 -32 9Far 0.0399 0.0083 35 13

where q = mean(i

C * vi m/s) * 1030 kg/m3 …* 3816 J/kg C * (350 m * 75 m);

Ongoing issues...

Flux out the MEF control volume can be calculated by

1) subtracting “background”, and/or

2) integrating fluxes through all sides

Background heat may be 0.063 or less.

Resolving variations in currents is critical to more accurate side wall heat flux estimates