Discrete and diffuse heat transfer atashes vent field, Axial Volcano, Juan de Fuca Ridge

Earth and Planetary Science Letters, 109 (1992) 57-71 Elsevier Science Publishers B.V., Amsterdam

57

[CL]

Discrete and diffuse heat transfer at ASHES vent field, Axial Volcano, Juan de Fuca Ridge Peter A. Rona

a

and D. Andrew

Trivett

b

" National Oceanic and Atmospheric Administration, Atlantic Oceanographic and Atmospheric Laboratory, 4301 Rickenbacker Causeway, Miami, FL 33149, USA h Department of Applied Ocean Physics and Engineering, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

Received June 26, 1991; revision accepted January 15, 1992

ABSTRACT

Heat fluxes of discrete and diffuse components of hydrothermal discharge are estimated from submersible measurement in the ASHESvent field located in the caldera of Axial Volcano on the central Juan de Fuca Ridge. The discrete component comprises discharge from individual vents and the diffuse component comprises seepage from large areas of the seafloor. The investigation centered on the high-temperature portion of the field, a 100 × 100 m area of fractured lobate and sheet lava flows encompassing discrete discharge (~< 326°C) from seven individual vents and diffuse flow from intervening areas. A total of 4.4 _+ 2 × 106W was estimated for the discrete heat flux based on measurements of effluent temperature, flow rate and orifice diameter at all the known individual vents by D.S.V. Ah,in in September 1987. A total of 15-75 × 106W was calculated with a standard plume model for the diffuse flux based on a grid of temperature measurements at altitudes of 1 m and 20 m above the study area using a 1 m long vertical array of temperature sensors mounted at the front of AIcin's instrument platform. The study indicates that (1) the diffuse component rises separately from the discrete component of hydrothermal discharge and is laterally advected by a prevailing current below the discrete component, and that (2) the diffuse component of convective heat flux is approximately an order of magnitude greater than the discrete component in the high-temperature portion of the ASHESvent field.

1. Introduction O b s e r v a t i o n s o f h y d r o t h e r m a l fields at s e a f l o o r spreading centers reveal that transfer of heat and mass from the seafloor into the water column o c c u r s by d i s c r e t e a n d d i f f u s e f l o w (Fig. 1) [1-3]. Discrete flow comprises the focused discharge of h y d r o t h e r m a l s o l u t i o n s t h r o u g h i n d i v i d u a l orifices as p o i n t s o u r c e s , p r i m a r i l y in m i n e r a l i z e d c h i m n e y s . H e a t flux f r o m d i s c r e t e s o u r c e s c a n b e c a l c u l a t e d f r o m d i r e c t m e a s u r e m e n t s [4,1], a n d by m o d e l i n g b a s e d o n j e t a n d p l u m e t h e o r y [5-9]. D i f f u s e f l o w c o m p r i s e s t h e d i s s e m i n a t e d discharge of hydrothermal solutions through areas

Correspondence to: Peter A. Rona, National Oceanic and Atmospheric Administration, Atlantic Oceanographic and Atmospheric Laboratory, 4301 Rickenbacker Causeway, Miami, FL 33149, USA. Elsevier Science Publishers B.V.

of the seafloor. The areas are made permeable p r i m a r i l y by n e t w o r k s o f f r a c t u r e s . H y d r o t h e r m a l sites a r e p e r m e a t e d w i t h m i c r o p l u m e s , w h i c h a r e individually insignificant but which collectively c o m p r i s e s i g n i f i c a n t t h e r m a l a n d m a s s f u x e s [10]. A r e a s o f d i f f u s e flow m a y b e s u f f i c i e n t l y e x t e n sive t h a t t h e i r t o t a l t h e r m a l o u t p u t m a y e q u a l o r e x c e e d t h a t o f a s s o c i a t e d d i s c r e t e s o u r c e s s u c h as black or white smokers. Fluxes from diffuse s o u r c e s a r e difficult to m e a s u r e b e c a u s e f l o w v e l o c i t i e s a n d fluid t e m p e r a t u r e s a r e low a n d d i s c h a r g e is u n e v e n l y d i s t r i b u t e d o v e r a r e a s m e t e r s to t e n s o f m e t e r s in d i a m e t e r [2,3]. W e d e s c r i b e t h e r e s u l t s o f a field e x p e r i m e n t to m e a s u r e t h e d i s c r e t e a n d d i f f u s e c o n v e c t i v e h e a t s o u r c e s in a k n o w n h y d r o t h e r m a l field, t h e ASHES v e n t field in t h e c a l d e r a o f A x i a l V o l c a n o o n t h e J u a n d e F u c a R i d g e (Fig. 2). W e u s e d direct measurements of temperature and flow v e l o c i t y at i n d i v i d u a l v e n t s to e s t i m a t e t h e dis-

58

P.A. RONA A N D D.A. T R I V E T T

crete c o m p o n e n t . Direct estimate of diffuse heat flux requires m e a s u r e m e n t of t e m p e r a t u r e and flow velocity over irregular areas of the seafloor tens of meters in diameter. This has not been technically feasible to date. We m e a s u r e d the t e m p e r a t u r e field at altitudes of about 1 m and 20 m in the water column at the ASHES vent field and vertical velocity of diffuse flow at points within the field. We use these m e a s u r e m e n t s and the results of laboratory tank simulations, in conjunction with a standard b u o y a n t plume model, to estimate the diffuse c o m p o n e n t of heat flux. The laboratory tank simulations show relationships between plume rise height, horizontal current, stratification, and source intensity for diffuse flow [10,11]. T h e m e a s u r e m e n t s and observations reported were m a d e from Ah,in on a dive series (dive n u m b e r s 1916-1927) in S e p t e m b e r 1987. 2. ASHES vent field

T h e ASHES (Axial S e a m o u n t t t y d r o t h e r m a l Emissions Study) vent field is situated in the caldera of Axial Volcano on the spreading axis of the central segment of the Juan de Fuca Ridge. This site is well suited for our heat flux investigation because the field has been m a p p e d in detail

(Fig. 2) [12-14], contains both discrete and diffuse flow, and is limited in area and n u m b e r of vents to facilitate its quantification (Fig. 1). The rectangular-shaped caldera of Axial Volcano is 8 km long in the direction N20°W by 5 km wide. T h e ASHES vent field occupies an area of approximately 200 × 1200 m of patchy, primarily lowt e m p e r a t u r e hydrothermal precipitates and discharge c e n t e r e d at 45056 ' N, 130°01' W adjacent to the faulted southwest wall of the caldera. T h e field is situated in a subtle 4 m depression between the depths of 1540 m and 1544 m (Fig. 2), within the shallowest region of the caldera floor, the floor increasing in depth northward to 1600 m at its northeast wall [14]. The depression is rectangular in shape and floored by fractured lobate flows, fractured sheet flows, and jumbled sheet flows [14]. Fractures in the sheet flows form an intersecting set with predominantly N20°W and N10°E orientations corresponding to a calderawide pattern of tectonic lineations subparallel and transverse to the long axis of the caldera (N20°W) and a subsidiary set of orthogonal intersections [13]. T h e location of the ASHES vent field in the caldera may be controlled by higher permeability at principal intersections of these tectonic lineations which focus upwelling hydrother-

50

g I----1¢0 LU "1"

00

50 100 DISTANCE (m) Fig. 1. Cartoon showing how plumes from discrete and diffuse sources in a seafloor hydrothermal field achieve density equilibrium with surrounding seawater at different altitudes above the seafloor in a stably stratified water column. Diffuse source areas comprising multiple microplumes actually exhibit a patchy distribution on the seafloor and coalesce to produce larger plumes that rise to different heights that are generally lower than rise heights of plumes from discrete sources [10]. The plumes are deflected by a prevailing current.

HEAT TRANSFER

AT ASHES VENT FIELD

59

mal solutions, as observed at other sites on oceanic ridges [15]. O u r investigation c e n t e r e d on an area 100 m in d i a m e t e r at the n o r t h e r n e n d of the ASHES field that contains the seven k n o w n high-temperature sources within the field (Fig. 2). F o u r of the sources are sulfide edifices (Inferno, Hell, Hillock and M u s h r o o m ) that rise 1 - 4 m above nearly sediment-free, fractured lobate basalt flows. The

sulfide edifices i n c o r p o r a t e multiple orifices discharging fluids that are either black or clear, exhibit a r a n g e of chloride c o n c e n t r a t i o n s from n o r m a l to e n r i c h e d (480-625 m m o l / k g ; a m b i e n t seawater 539 m m o l / k g ) [16], c o n t a i n relatatively high metal c o n c e n t r a t i o n s , and attain t e m p e r a tures of 326°C (Table 1). Clear, h i g h - t e m p e r a t u r e c h l o r i d e - d e p l e t e d fluids ( e l c o n c e n t r a t i o n 176258 m m o l / k g ) with low metal c o n c e n t r a t i o n s [16]

46°i

150 °

IZ6ow

Canada ~ r~#~

"..:

~Lf i i

EAST ~

,

Cobb-Eickelberg Smt. Chain

ASHES

/,~

45055 ,

4,ol

130o00 , Gorda Ridge "

40 °

Pacific Plate

~50t

I

W 0 Z

0

_m a

O O

5O DISTANCE (m) 1 INFERNO 2 MUSHROOM

3 VIRGIN 4 HELL

5 HILLOCK 6 NEW

7CRACK VENTS

Fig. 2. Bathymetric map of ASHESvent field from [14] based on Aluin pressure sensor (accuracy 1 dbar) and altimeter (0.5 m isobath interpolated). The map shows locations of the seven known high-temperature vents (dots and shaded area numbered 1-7). Index maps show the location of Axial Volcano on the axis of the central Juan de Fuca Ridge (upper left) and the location of vent fields including ASHESin the caldera of Axial Volcano (upper right) with major structural lineations (dashed) [13].

(~0

P.A. RONA

discharge from two other sources: Virgin Mound (Tm~x = 299°C), a 0.4 m tall group of several white anhydrite chimneys at the boundary between jumbled and fractured sheet flows, and Crack Vents (Tin, x = 226°C), a zone of anhydrite-filled fractures each several meters long, 7 - 1 0 cm wide, and about 1 m apart in basalt sheet flows. There is also an unsampled, unnamed, source (New) similar in appearance to Virgin Mound. The chloride-normal and enriched (high metals) and chloride-depleted (low metals) solutions are interpreted as the brine and vapor phases, respectively, of solutions phase-separated by boiling beneath the seafloor and partially mixed with ambient seawater [16,17]. Vent types, lava flow morphology and fracture patterns are closely related [14]. The four sulfide

AND

D,A, TRIVETI"

edifices with black smokers that are normal to enriched in chloride all occur in fractured lobate flows (Table 1; Inferno, Hell, Hillock, Mushroom). The two sulfate mounds and the zone of fractures that vent clear, chloride-depleted solutions occur either in jumbled sheet flows (Table 1, New), fractured sheet flows (Crack Vents), or at the boundary between jumbled and fractured sheet flows (Virgin). The areal distribution of low- and high-chlorinity venting is consistent with segregation of the separated brine and vapor phases by mechanisms based on differential buoyancy or relative permeability [17]. A qualitative model for the distribution of fluids observed at ASHES vent field suggests that brine-phase fluids are confined within flow conduits by a surrounding relative permeability barrier related to conduit diameter.

Temperature Map at 1m Elevation 180 160 ....

140

E 120

100

~, o

Z

80 60 ".-'-~-"-.~-JJ):',

40~-

00t 0

77/7,'

"

w

6. . ° -'~,

"

-

0 50

loo

150

East-West Distance (m) Fig. 3. Isotherm map (interval 0.020°C) of potential temperature measured in the water column at an altitude of about 1 m above the seafloor with the lowermost temperature sensor mounted on Alcin (dive 1919, 25 September 1987, 1700-1900 GMT) at the high-temperature portion of ASHES vent field. Isotherms show temperatures >~ 2.40°C (solid lines) and < 2.40°C (dotted lines). Tracklines (dot-dash) and positions of discrete high-temperature vents (dots and shaded area numbered as in Fig. 2) are shown. Three areas of positive temperature anomalies associated with diffuse sources are delineated by closures of the 2.40°C isotherm (A I, Bp Cl). P P' is the line of the temperature profile shown in Fig. 5.

61

HEAT TRANSFER AT ASHES VENT FIELD

V a p o r - p h a s e fluids flow diffusely into the surr o u n d i n g host rock [18]. T h e s e g r e g a t i o n m e c h a nism a p p a r e n t l y favors d i s c r e t e d i s c h a r g e o f chlor i d e - n o r m a l to - e n r i c h e d fluids t h r o u g h sulfide edifices on f r a c t u r e d l o b a t e flows a n d b o t h discrete a n d diffuse d i s c h a r g e of c h l o r i d e - d e p l e t e d fluids p a r t i a l l y m i x e d with a m b i e n t s e a w a t e r t h r o u g h a n h y d r i t e d e p o s i t s on j u m b l e d a n d fract u r e d s h e e t flows. Diffuse flow in ASHES vent field is unevenly d i s t r i b u t e d , b a s e d on direct o b s e r v a t i o n s from Alvin o f schlieren effects a n d o u r t e m p e r a t u r e survey in the n e a r - b o t t o m w a t e r c o l u m n (Figs. 3 - 5 ) . P a t c h e s of diffuse flow with m e a s u r e d temp e r a t u r e s of up to 27°C e m a n a t e from the four sulfide edifices b e t w e e n discrete orifices. Vestim e n t i f e r a n t u b e w o r m s cover the four sulfide edifices, e v i d e n c i n g the pervasive n a t u r e of the dif-

fuse flow t h r o u g h the edifices i n f e r r e d from the r e c o g n i z e d association of the t u b e w o r m s with c h e m o s y n t h e t i c b a c t e r i a that grow in the hyd r o t h e r m a l solutions [20]. A n a r e a of diffuse flow ( C l u m p vent) occurs a r o u n d the b a s e of the sulfide edifice o f Hell vent. P a t c h y a r e a s of diffuse flow o c c u r in the a r e a s b e t w e e n all of the seven c h i m n e y c o m p l e x e s a n d at C r a c k Vents. Lowt e m p e r a t u r e diffuse venting e x t e n d s intermittently to the c a l d e r a wall to the west a n d south of the h i g h - t e m p e r a t u r e p o r t i o n of the field.

3. Measurement methods W a t e r t e m p e r a t u r e , flow rates, and orifice dia m e t e r w e r e m e a s u r e d at d i s c r e t e vents in the various chimneys d e s c r i b e d ( T a b l e 1). W a t e r temp e r a t u r e was directly m e a s u r e d using a p l a t i n u m

Temperature Map at 20m Elevation 180 160 140

i

120 100

~

6o 40 20 0 0

50

t 100

150

East-West Distance (m) Fig. 4. Isotherm map (interval 0.005°C) of potential temperature measured in the water column at an altitude of about 20 m above the seafloor with the lowermost temperature sensor mounted on Alvin (dive 1919, 25 September 1987, 2022-2130 GMT) over the high-temperature portion of ASHESvent field. Isotherms show temperatures /> 2.375°C (solid lines) and < 2.375°C (dotted lines). Three areas of positive temperature anomalies are delineated by closures of the 2.39°C isotherm (A20 , B20 , C20 ). Estimated trajectories of plumes from discrete sources to the 20 m altitude are shown (arrows). Tracklines (dot-dash) and positions of discrete high-temperature vents (dots and shaded area numbered as in Fig. 2) are shown.

Loba,e ifractuled!

Lobate Ifr~ctured)

Lobate (fractured/

Boundary between fractured and

inlerno

Hillock

MushroOm

Virg,n

Sheet (fractured)

Crack

1543

1543

1543

1543

1543

1049

0

2

2

4

3

2

Mound Basal Diameter (m)

[16]

lanhydrite)

Sulfate

lanhy#rite)

Sulfate

Sulfate (anhydrite)

Sulfide

Sulfide

Sulfide

Sulfide

Mound Composition

1Chtor~de c o n c e n t r a t i o n o f a m b i e n t s e a w a t e r is 5 3 9 m m o l / k g

Jumbled sheet flow

New

flows

fumbled sheet

_a-~ F l o w T y o e

Sile

D e p t h at Base of Mound (m)

west

2

Fracture:

east -west

Anhydrite chimney

Several small chimneys

Central spire

3 small chimneys

C e n t r a l sp~re

0

0

0 1

1

0.1

0 4

2

south

2

center

TOp: two v e ~ s

Top:

Side:

2

center

Top:

0-3

2

west

Side: SlOes

3

0

4

4

Top

Base: northwest

east

1

Base: southwest

Top:

1

Top:

1

southwest

Base: s o u t h w e s t

2

Base

southwest

Side:

4

4

2

west

H e i g h t of Vent Above Seafloor {m)

Sides

east

Top:

Venl Top

ASHES v e n t field: H e a t flux f r u m discrete s o u r c e s

TABLE l

Clear

Clear

Clear

Clear

C~ear

Clear

Ctear

B)ack

Stack

Black

Clear

Black

Black

C~ear

Slack

Black

Gray

Gray

Clear

Clear

Slack

Clear

Black

Effluent Type

23

23

2 3

-=

23

23

23

23

2.3

2.3

23

2 3

23

23

23

2 3

93

2.3

23

2 3

2.3

23

2.3

Ambient Water Tamp (°C)

.

.

.

226

226

.

---

.

-

---

Assumed .

=-

---

---

---

1

3

Diffuse

2

1

2

Diffuse

2

2 5

DJftuse

5

75

1

1

15

Diffuse

3

3

4

Orifice Diameter (cm)

.

.

Diffuse

2

.

.

20

.

5-10

.

30

10

20

20

20

10

20

25

10

30

30

30

30

90

10

40

20

00

.

.

FlOw Rate [eros)

equivalent to Virgin

15:21 5 30:137 45:10.8 60;56 . . . . . .

219

.

180

240

315

27

240

15:27 30:11

--

21

108

---

15:90 60:35

301 299

---

---

15:63 458

15:70 30:40

---

15:40 60:45

Tamp Above Orifice (em~*C)

211

217

160

24

230

140

326

Temp at O r i f i c e (~C)

.

.

.

0.863

.

.

0.869

.

0.896

0847

0732

.

0847

0 960

.

0762

0 758

0676

0.970

0 910

0.862

0926

0709

.

.

.

Density (gcm 3)

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

540

.

.

.

.

.

170 2 0 6

.

175 2 5 8

.

--

480 0 4 0

.

;30

-

480 5 4 0

-

.

.

10 E

.

0 1 8 × 106

0 1 S x 106

0.04 × 106

0 0 2 x 106

0 08 ~ 106

.

0 0 0 × 106

0 05 x 106

.

.

0 0 6 x 105

.

.

10 ~ 9

68

106

154

10 ~

~0 ~

106

002

x

O 02

C10

O 26

9 1 0 ~ 10 E

1 0 0 * 10 F

025

-

Heat Flux (W)

Chloride Concenlration f nqmol kgl

Total heat flux f rom discrete vents:

4.42

.

.

4.35

.

428

4.46

4.8t

.

4 46

4 20

4 55

4 66

428

4 33

423

=--

442

424

5 00

Mean Specific Heat Capacltv ~J g
44×106

0 05 x 106

0 ~ 9 ×10 ~

0 1 8 × 101

0 14 x 10 E

---

0 1 ~ ~ 10~

2 22 > 10~

---

Tolal Heat Flu> ,~,

HEAT TRANSFER

63

AT ASHES VENT FIELD

resistance temperature detector (RTD) mounted in a probe (Yellow Springs International Model 19377) as part of an in-situ sensing and sampling system [20]. The probe was held in the centerline of flow at each vent orifice using one of the manipulator arms of the submersible. RTD specifications are in the range 0-500°C, have an accuracy of 1.0°C, a resolution of 0.01°C and a time constant of 2 s. Flow rates were measured by placing a rod marked at 10 cm intervals vertically at the orifice of each vent or in an area of diffuse flow. A 5 minute video of the flow was made with a camera mounted on an arm of the submersible and held as nearly orthogonal to the rod as feasible within about 1.5 m of the vent. Flow velocities were measured by advancing the video image frame-by-frame (0.033 s per frame) and timing the rise of eddies or particles in the initial 10 cm interval above the orifice (discrete flow) or seafloor (diffuse flow); schlieren effects revealed eddies in clear diffuse flow. Each flow rate reported (Table 1) is the average of ten determinations by this method and has an estimated error of _+ 15%. To test the accuracy of determinations, we compared flow velocities from the same video of one of the vents (Table 1, Hell, top: east) using our calibrated rod method and a method based on digital correlation of eddy images [21]. The

v

2.65 2.60 2.55

Z

p

P

A

[9 0

flow velocities determined by the two different methods agree within measurement error (30 _+ 5 c m / s , Table 1; 32 c m / s with standard deviation of 13 c m / s ) [21]. Orifice diameter was measured directly by laying the calibrated rod horizontally at each orifice using one of the manipulator arms of the submersible. We estimate a measurement error of _+15% in diameter due to deviation from circularity and boundary layer effects, which resuits in an error of _+30% in orifice area. The total estimated error in convective heat flux due to flow rate and orifice diameter is _+45%. Temperature measurements made with the same R T D at areas of diffuse flow on and at the base of chimneys (Table 1) are considered to be minima owing to dilution by rapid mixing with ambient seawater. Near-bottom water temperature at the hydrothermal field was measured using a vertical array of three thermistors (Fenwal part number K2365; accuracy 0.02°C, precision 0.004°C, time constant 2 s) spaced 0.5 m apart mounted on a 1 m long pipe secured to the front of an instrument basket at the forward end of Ah.,in. The resistance of each of the three thermistors was recorded sequentially at a 0.5 s interval every 10 s in the solid-state memory of a digital recorder (Sea Data model 4-TDR-1). The thermistors were

t

t

# ....... 0 SENSOR 0 D SENSOR

!

i,li

2.50 2.45

<

2.40

H

I 2.35

Z 2.30

O m

0

20

40 DISTANCE

60

80

i00

(m)

Fig. 5. P o t e n t i a l t e m p e r a t u r e profiles m e a s u r e d with a 1 m long vertical array of t h r e e sensors m o u n t e d on the front of Ah,in with the l o w e r m o s t s e n s o r (1) at an altitude of about 1 m above the seafloor a l o n g trackline P - P ' (Fig. 3).

()4

P.A. R O N A A N D D.A. T R I V E ' I ' I "

calibrated from - 2 ° C to + 8°C in 2 ° increments before and after the experiment. Water depth was obtained from the output of a pressure transducer (accuracy 5 dbar, precision 1 dbar) which recorded continuously in the solid-state memory of another digital recorder (Sea Data model 4TDR-1) housed in a pressure case also mounted in the instrument basket. The near-bottom water temperatures are presented as potential temperatures referred to sea level (Figs. 3-7). Submersible navigation was carried out with a Sonatrack acoustic navigation system operated from the support vessel with an estimated positional accuracy of 20 m for the submersible relative to a net of four bottom-mounted acoustic transponders with 5 km baselines. The horizontal flow velocity and direction of a current prevailing during the water t e m p e r a t u r e survey was estimated at 7_+ 3 c m / s to the north based on the lateral offset of the track of the submersible along lines steered east-west at a speed of 1 knot (1.85 k m / h ) within the acoustic navigation net. This estimate is consistent with the nearest currentmeter array moored at a depth of 1855 m on a sill on the western side of Axial Volcano about 10 km from the caldera, which recorded 7-15

c m / s to the north throughout the day of our temperature survey (September 23, 1987) [23]. 4. Discrete component of heat flux Measurements of fluid temperature, diameter and flow rate were made at each of the orifices on six of the seven chimney complexes known in the ASHES vent field. The measurements were used to calculate heat flux with the following equation [4]:

H = ~r2upCp AT

(l)

where H is the hydrothermal heat loss (W), r is the radius of the vent orifice (cm, measured), u is the flow rate ( c m / s , measured in the initial 10 cm above the orifice), p is the density of the vent water (Table 1) [24], Cp is the mean specific heat capacity from 0°C to the vent temperature (Table 1) [24], and AT is the temperature difference between the ambient water and the hydrothermal fluid (°C, temperature measured at orifice). An inventory of the measurements and heat fluxes is presented in Table 1. Ranges of values measured at individual orifices are 20-90 c m / s for flow rate, 108-326°C for fluid temperature, and 1-7.5

60

5O

4O

J

30

PLUME

"

'-I/

20

10

00

I 20

I 40

I I I 60 80 100 HORIZONTAL DISTANCE (m)

I 120

I 140

160

Fig. 6. Plot of trajectories of three plumes corresponding to areas of diffuse sources in the ASIIESvent field (A, B and C in Figs. 3 and 4; Table 2) calculated with the EPA UMER~3E plume model [26-27].

65

H E A T T R A N S F E R AT ASHES V E N T FIELD

cm for orifice diameters. These individual measurements yield a range of heat outputs between 0.02 × 106 W and 1.54 × ]06 W. The seventh chimney complex is a previously unknown source, similar in appearance to Virgin Mound, that was observed from the submersible (dive 1927) several meters southeast of Hillock, but was not measured. The heat output of this source (New) is assumed to be equivalent to that of Virgin Mound. The sum of the convective heat flux of each of the known discrete orifices is 4.4 + 2 × 10 ~' W (Table 1). A video of the ASHES vent field made in August 1986, one year prior to the measurements reported here [25], showed a visibly higher intensity of venting from the two orifices at the

top of the Hell sulfide edifice, indicating significant changes of mass flow rate in a period of one year or less. 5. Diffuse component of heat flux A near-bottom temperature survey was carried out to record the anomalies associated with diffuse discharge. T e m p e r a t u r e measurements were made along two grids with the 1 m long array of temperature sensors mounted on Ah, in. The grids were laid out as sets of 100 m long orthogonal lines spaced 20 m apart at altitudes of about 1 m and 20 m above the seafloor. The 1 m altitude was maintained visually by the submersible pilot;

2.50

2.45

UJ

DIFFUSE SOURCE AVERAGE TEMPERATURE

I--

< nr" I,LI n

2.40

UJ I-.-I

< I--

£CEGROUND

z UJ

0 2.35 (3.

TEMPERATURE

~"-SOURCE DIA METE R---~-I

2.30 0

i

I

I

l

10

20

30

40

DISTANCE ALONG TRACKLINE (ml Fig. 7. Diagram showing method for estimating the diameter and temperature of a thermal anomaly from a diffuse source. We averaged the potential temperature data recorded along a trackline intersecting each anomaly and used the horizontal dimension of the diffuse source average temperature for calculations with the UMERCE numerical model. The example shown is the diffuse source average temperature for a trackline through thermal anomaly A~ (Fig. 3).

66

the 20 m altitude was maintained using the digital readout of the pressure depth sensor (accuracy 1 dbar) on the submersible. To run the grids the submersible pilot was directed from the support ship by underwater telephone. The actual tracklines were displaced to the north by prevailing currents and shortened owing to time constraints (Figs. 3 and 4). The temperature survey conducted from Ah~in was merged with navigation data. These data were contoured to produce plots of isotherms in plume cross sections at altitudes of about 1 m and 20 m above the seafloor (Figs. 3 and 4). The 1 m survey is below the level of discharge of the major discrete sources from the sulfide edifices (Table 1). The discrete sources were observed to rise rapidly above the survey level without much lateral spreading, consistent with tank experiments [10,11] (Fig. 1) and other vent observations. The 1 m altitude map shows three principal areas of diffuse venting, primarily located around the Hell, Hillock and Mushroom sulfide edifices (Fig. 3). These correspond to visual observation from the submersible of areas of noticeable diffuse discharge. The maximum temperature anomalies associated with the diffuse sources occurred on the northwest side of these three edifices. Vertical velocities of diffuse flow measured by the calibrated rod method in five areas within ASHES vent field near the sulfide edifices and Crack vents ranged between 5 c m / s and 10 c m / s (Table 1). A profile of temperatures recorded on the three sensors of the 1 m long vertical array at the 1 m altitude shows stable temperature gradients (neutral to cooling downward) over the Crack Vents area and unstable gradients (mixed and warming downward) over the areas of diffuse venting to the northwest of Hillock and Inferno (Fig. 5). The stable temperature gradients suggest that diffuse flow traversing the Crack Vents area lacks the buoyancy flux for further ascent. In contrast, the unstable temperature gradients recorded over the diffuse venting areas around the sulfide edifices indicate a capacity for continued rise. The temperature anomalies identified in the 1 m altitude isotherm map are correlated with anomalies on the 20 m altitude map based on consideration of spatial patterns and source parameters (temperature anomaly, area of anoma-

P.A. R O N A A N D D . A . T R I V E T T

lous temperature, and vertical flow velocity). Three areas of diffuse flow are identified on the 1 m altitude map (Fig. 3; A 1, B1, Ci). These three areas form a right triangle with sides AB and BC oriented east-west and north-south, respectively. Area C is the largest area with the highest temperature anomaly; area B is the smallest area with the lowest temperature anomaly. The three areas of highest temperature anomalies on the 20 m altitude map (Fig. 4; A20, B20, C20) lie to the north-northwest of areas A~, B 1 and C~ (Fig. 3), as expected for corresponding anomalies under the prevailing current flow. The three areas at the 20 m altitude are oriented and configured with respect to one another in a pattern similar to the three areas at the 1 m altitude. However, the sides AB and BC at the 20 m altitude are not aligned east-west and north-south, but have apparently rotated clockwise relative to the 1 m altitude triangle. Different horizontal offsets are observed between the three areas at the two altitudes. This observation indicates that the warm fluid does not simply rise as a passive flow parcel, but interacts with prevailing currents in a way that relates to source conditions. The observed changes in orientation and offset may result from differences in source parameters in the three areas. The Environmental Protection Agency (EPA) UMERC,E model, designed to model buoyant plumes from sewage outfalls in the ocean [26,27], was applied to calculate the relationship between plume trajectories and source parameters. UMER~E, as described in [27], calculates plume properties in a linearly stratified cross-flow using a Lagrangian formulation of the conservation equations for mass, vertical momentum, horizontal momentum, temperature and salinity. The model deals exclusively with two-dimensional problems. We have assumed uniform velocity profiles and a linear density gradient in the application of this model to our problem. We have ignored any contribution from salinity differences between the vent fluid and the ambient seawater. A sensitivity analysis showed that the salinity difference produced by dilution with ambient seawater of an undiluted, high-temperature hydrothermal fluid to attain the state of the source fluid for diffuse venting is too small to have a major impact on our results. A stable density gradient positive downward

HEAT TRANSFER

67

AT ASHES VENT FIELD

for the water column was used in the model based on a Brunt-Vaisala frequency of 0.0050.012 r a d / s calculated from three vertical cond u c t i v i t y - t e m p e r a t u r e - d e p t h (CTD) profiles. The profiles were recorded with a Seabird CTD mounted on Ah'in during dives to the ASHES vent field in 1988 [28]. The trajectories of the three buoyant plumes were calculated using the UMERGE model to reproduce the horizontal offsets, diameters and temperature anomalies at the 1 m and 20 m altitudes observed in areas A, B and C on the isotherm maps (Figs. 3 and 4). The plot of the three plume trajectories (Fig. 6) and source parameters derived from the model calculations are in reasonable agreement with the observed offsets and measured source parameters (source diameter and temperature) for the three plumes (Table 2); the calculated source vertical velocities (5.5-14 c m / s ) are close to the range measured for diffuse flow at other points in the

hydrothermal field (Table 1, 5-10 cm/s). The source vertical velocity representative of an area of diffuse flow may be less than that of individual microplumes within the area. The calculated effective dilution of the plume by mixing during its rise from 1 m to 20 m is similar to that estimated from the measured temperature data (Table 2). The calculated trajectories (Fig. 6) and source parameters help to explain the difference in offsets of the three areas of temperature anomalies at the 20 m altitude (Table 2). The larger offset of plume A than that of B or C may result if the vertical flow velocity of A is relatively small despite its larger area limiting its upward penetration as it drifts downcurrent. Plume B originates from a smaller area with a lower temperature anomaly than A, so that its vertical velocity would have been high to penetrate to the 20 m altitude. A relatively large source area and temperature anomaly at the 1 m altitude in area C corre-

TABLE 2 ASHES vent field: H e a t flux from diffuse sources Diffuse sources (Figs. 3 and 4) Source d i a m e t e r ( ~ 1 m altitude) 1 Source t e m p e r a t u r e ( ~ 1 m altitude) 1Ambient temperature (a 1 m (a 20 m B r u n t - V a i s a l a frequency (CTD data) Offset ~: 20 m ( C T D d a t a ) I (measured) 2 (calculated) D i l u t i o n (a: 20 m 1(measured) (calculated) Source vertical velocity 2 (calculated) C u r r e n t h o r i z o n t a l velocity 3 (measured) Rise h e i g h t 2 (calculated, Fig. 6) H e a t flux 4 (calculated)

A

B

C

20 m

10 m

35 m

2.4°C

2.4°C

2.6°C

2.33°C 2.35°C

2.33°C 2.35°C

2.33°C 2.35°C

0.0012 r a d / s

0.0012 r a d / s

0.0012 r a d / s

63 m 65 m

29 m 34 m

32 m 32 m

2.3 9.2

2.3 10.2

6.4 5.5

7.5 c m / s

14 c m / s

5.5 c m / s

7 cm/s

7 cm/s

7 cm/s

18 m 7 X 106 W

21 m 3 ×106w

46 m 55 × 1 0 6 w

M e a s u r e d from D.S.V. Ah,in d a t a set (Figs. 3 and 4). 2 C a l c u l a t e d using UMERGE m o d e l to e s t i m a t e vertical flow velocity at source. "~ E s t i m a t e d from lateral offset of D.S.V. Ah'in d u r i n g m e a s u r e m e n t s . 4 C a l c u l a t e d using m e a s u r e m e n t s from D.S.V. Ah,in d a t a set (Figs. 3 and 4) and UMERGE m o d e l e s t i m a t e of vertical flow velocity at source.

6~

sponds to the largest area of warm water observed at the 20 m altitude and the highest rise calculated by the model for plume C. The temperature field is highest where anomalies A, B and C are situated in the northwest quadrant of the 20 m altitude isotherm map (Fig. 4), consistent with coalescence of diffuse flow from the three source areas (Fig. 3) at that altitude. The explanation of plume trajectories offered can only be confirmed by knowledge of the maximum rise height of the three plumes, such as might have been obtained by additional temperature surveys at higher altitudes. Convective heat flux was calculated for the three diffuse source areas with plumes at the 20 m altitude using eq. (1) (Table 2). We estimated the approximate plume diameter (cross-sectional area) and plume average temperature anomaly from the 1 m altitude isotherm map (Fig. 3), as shown in Fig. 7. We measured downcurrent displacement between the plume cross sections at 1 m and 20 m altitude anomalies on the isotherm maps (Figs. 3 and 4). Diffuse flow rate (v) in eq. (1) was estimated with the UMERGE plume model (source vertical velocity in Table 2) using the estimated source parameters and current velocity. The diffuse flow rates estimated are consistent with diffuse flow rates measured by the calibrated rod method in other areas of ASHES vent field (Table 1). Measured and calculated source characteristics are presented in Table 2. Source area C has the largest flux related to size of its area and magnitude of temperature anomaly (55 x 106 W). The heat flux from source areas A and B is proportionally smaller (7 × 106 W and 3 x l0 t' W respectively) for a total estimated convective heat flux from the three diffuse source areas of 65 x 10 6 W.

6. Discussion Several uncertainties exist in our estimate of heat flux from diffuse sources in the ASHES vent field. It was assumed that stratification was entirely due to temperature and that salinity was constant in the ambient environment. This assumption was justified by comparing the B r u n t Vaisala frequency calculated using temperature data from the measured temperature field with that calculated from CTD measurements. The

P.A. RONA AND D.A. TRIVETI"

frequency based on temperature alone is comparable to that based on CTD measurements. As previously stated, this observation implies that salinity differences play a minor role in the dynamics. The source velocity necessary to produce the observed anomalies would decrease slightly if lower salinities based on a separate chloride-depleted phase [16,17] were used for the diffuse flow. Another concern is whether the 20 m altitude temperature anomalies were indeed correlated with their corresponding sources at the 1 m altitude. Typical trackline spacing of the 20 m altitude survey in the area of interest is 30 m. The UMERGE plume model shows that the observed diffuse source parameters could have produced the temperature anomalies measured at the 20 m altitude. The UMERGE plume model further shows that offsets between discrete sources and corresponding plumes at the 20 m altitude (14 _+ 5 m) would be less than offsets for diffuse sources (Table 2), owing to the higher flow velocities (20-60 c m / s , Table 1), higher temperatures (108-326°C, Table 1), and the smaller mixing cross section of the discrete sources (Fig. 4). Gaps in coverage precluded recording of plumes from the discrete sources, which would have estimated diameters of 2-5 m at the 20 m altitude (Fig. 4). Not all the diffuse sources in the study area contributed to the temperature anomalies measured at the 20 m altitude. The presence of additional anomalies is suggested by our 20 m altitude isotherm map (Fig. 4) and by a C T D / r o s e t t e tow over the north end of the ASHES vent field which recorded and sampled four separate areas of thermal and chemical anomalies within 15 m of the seafloor in August 1987 [29]. Stable temperature gradients measured at about an altitude of 1 m over the portion of the Crack Vents area traversed (Fig. 5) indicate that a component of the diffuse flow is being advected by the prevailing current at less than 20 m above the seafloor. The presence of this weak diffuse flow, a tendency to underestimate areas of diffuse sources owing to limited trackine control for the isotherm contouring routine, and the presence of diffuse sources outside of the study area all contribute to underestimation of the diffuse component of convective heat transfer in ASHES vent field.

69

HEAT TRANSFER AT ASHES VENT FIELD

An additional uncertainty in the diffuse heat flux estimate comes from the plume model itself. Laboratory tank simulations of relationships between plume rise height, horizontal current, density stratification and source intensity for diffuse flow show that standard plume models such as UMER~E agree well with the simulations when the rise height of the plume is greater than the source area diameter [10]. As plume rise height approaches source area diameter, the standard model over-predicts the source intensity necessary to produce the rise height. Comparison between simulations and laboratory data indicates that this over-prediction is within a factor of two for the weakest diffuse plumes. For the worse case estimate that UMER~E over-predicts the necessary source intensity to produce a given plume by a factor of two, our calculated source vertical velocities of between 5.5 and 14 c m / s would reduce to 3 - 7 c m / s (Table 2). The total diffuse heat flux from the three source areas would decrease to 15-38 x 10 6 W as a lower estimate. Alternatively, if all three source areas (Fig. 3; A, B, C) exhibited the same heat f l u x / a r e a ratio as area C, an upper estimate of the total diffuse heat flux from these areas would be 75 × 106 W. Measurements of diffuse heat flux in other seafloor hydrothermal fields are not directly comparable to these results because they pertain to small portions of the fields and involve different assumptions. For example, a diffuse heat flux betweeen 11 M W and 900 MW was estimated for an 80-700 m 2 area of diffuse flow in a hydrothermal field at 21°N on the East Pacific Rise, assuming a flow rate of 10 c m / s , a temperature of 20°C, and effective porosity of between 2 and 17% [1]; the measured discrete heat flux of individual vents in that field ranges between 0.5 MW and 10 MW [1]. A diffuse heat flux of 28 MW was estimated for an individual sulfide edifice on the Endeavor Segment of the Juan de Fuca Ridge by extrapolating a m e a s u r e m e n t with an electromagnetic sensor (temperature, flow velocity) on a portion of the edifice [30]; a discrete heat flux of 2.9 MW was measured at a nearby high-temperature vent [30].

7. Conclusions The convective heat flux from discrete vents on four sulfide edifices (Inferno, Mushroom, Hell,

Hillock) and two anhydrite mounds (Virgin and New) in the high-temperature portion of the ASHES vent field is 4.4 _+ 2 × 106 W based on an inventory of direct measurements made from D.S.V. Alc'in in September 1987. Prior observations suggest that the mass flow rate of venting at individual sulfide edifices may fluctuate substantially on an annual or shorter term basis. We have applied a standard plume model to facilitate estimation of heat flux from the three principal areas of diffuse flow observed at the high-temperature portion of the ASHES vent field using measurements of the near-bottom water temperature field and prevailing current velocity. Diffuse heat flux from these three areas is shown to be between 15 × 106 W and 75 × 10 ~ W, with intervening areas of weaker or unmeasured diffuse flow unaccounted for. The diffuse flow does not merge to form a single plume that rises to one level, but separates at various levels related to a balance between buoyancy flux, ambient density stratification and prevailing current velocity. The areas of most intense diffuse flow distinctly occur around and between discrete high-temperature vents (Figs. 3 and 4). Similar observations of the distribution of diffuse venting, separation between diffuse and discrete components of hydrothermal discharge, and lateral advection at different levels in the prevailing current were made in the TAG hydrothermal field on the Mid-Atlantic Ridge [3] and are seen in tank simulations [10]. The evidence presented indicates that convective heat transfer of diffuse flow from areas of the seafloor exceeds that of discrete flow from point sources in the high-temperature portion of the ASHES vent field by an order of magnitude [31]. Similar work is needed in other seafloor hydrothermal fields to test the hypothesis that diffuse flow is the dominant process of convective heat transfer at oceanic ridges.

Acknowledgements We thank Captain R. Baker and complement of R.V. Atlantis H, Expedition Leader R. Hollis and the D.S.V. Ale,in group, and Chief Scientist S. H a m m o n d for support on the NOAA VENTS Program 1987 cruise to Axial Volcano. J. Morton of the U.S. Geological Survey directed the acqui-

70

sition and processing of acoustic navigation data. Thanks for help in making vent observations to members of the dive team: B. Applegate, D. Butterfield, A. DeBevoise, R. Embley, R. Feely, C. Fox, J. Franklin, S. Little, J. Lupton, G. Massoth, R. McDuff, K. Murphy and V. Tunnicliffe. W. Frick provided EPA software and advice on the UMERGE program. R. Feely and G. Massoth provided CTD data. G.A. Cannon provided currentmeter data. J. Bischoff and R. Rosenbauer provided information on seawater properties at high temperatures and pressures. We appreciate helpful reviews by E. Baker, J. Cann, R. Lowell and R. Von Herzen. PAR thanks the NOAA VENTS Program for support. The work of D A T is the result of research sponsored by NOAA National Sea Grant College Office, U.S. Department of Commerce under grant number NA86-AA-DSG090 and wnol Sea Grant Project number R1011-PD. The U.S. Government is authorized to produce and distribute reprints for governmental purposes, notwithstanding any copyright notation that may appear herein. References 1 D.R. Converse, H.D. Holland and J.M. Edmond, Flow rates in the axial hot springs of the East Pacific Rise 21°N: Implications for the heat budget and the formation of massive sulfide deposits, Earth Planet. Sci. Lett. 69, 159 175, 1984. 2 S.A. Little, K.D. Stolzenbach and F.J. Grassle, Tidal current effects on temperature in diffuse hydrothermal flow: G u a y m a s Basin, Geophys. Res. Lett. 15, 1491-1494, 1988. 3 P.A. Rona and K.G. Speer, An Atlantic hydrothermal plume: Trans-Atlantic Geotraverse (TAG) area, MidAtlantic Ridge crest near 26°N, J. Geophys. Res. 94, 13,879-13,893, 1989. 4 K.C. Macdonald, K. Becker, F.N. Spiess and R.D. Ballard, Hydrothermal heat flux of the "black smoker" vents on the East Pacific Rise, Earth Planet. Sci. Lett. 48, 1-7, 1980. 5 H. Rouse, C.-S. Yih and H.W. Humphreys, Gravitational convection from a boundary source, Tellus 4, 201-210, 1952. 6 B.R. Morton, G.I. Taylor and J.S. Turner, Turbulent gravitational convection from maintained and instantaneous sources, Proc. R. Soc. London, Ser. A 234, 1-23, 1956. 7 J.S. Turner, Buoyancy Effects in Fluids, 367 pp., Cambridge University Press, New York, 1973. 8 R.P Lowell and P.A. Rona, On the interpretation of near-bottom water temperature anomalies, Earth Planet. Sci. Lett. 32, 18-24, 1976.

P.A. RONA AND D.A. TRIVETT 9 S.A. Little, K.D. Stolzenbach and R.P. Von Herzen, Measurements of plume flow from a hydrothermal vent field, J. Geophys. Res. 92, 2587-2593, 1987. 10 D.A. Trivett, Diffuse flow from hydrothermal vents, M I T / W H O I Joint Program in Oceanographic Engineering, Sc.D. Thesis, 1991. 11 D.A. Trivett and J.R. Cann, A simple model of plumes from diffuse hydrothermal venting, J. Geophys. Res., submitted. 12 A. Malahoff, G. McMurtry, S. H a m m o n d and R. Embley, High-temperature hydrothermal fields Juan de Fuca Ridge--Axial Volcano, EOS, Trans. Am. Geophys. Union 65, 1112, 1984. 13 R.W. Embley, K.M. Murphy and C.G. Fox, High resolution studies of the summit of Axial Volcano, J. Geophys. Res. 95, 12,785-12,812, 1990. 14 S.R. H a m m o n d , Relationships between lava types, seafloor morphology, and the occurrence of hydrothermal venting in the A S H E S vent field of Axial Volcano, J. Geophys. Res. 95, 12,875-12,893, 1990. 15 P.A. Rona, R.P. Denlinger, M.R. Fisk, K.J. Howard, G.L. Taghon, K.D. Klitgord, J.S. McClain, G.R. McMurray and J.C. Wiltshire ( F e d e r a l / S t a t e Gorda Ridge Technical Task Force Working Group B), Major off-axis hydrothermal activity on the northern Gorda Ridge, Geology 18, 493 496, 1990. 16 G.J. Massoth, D.A. Butterfield, J.E. Lupton, R.E. McDuff, M.D. Lilley and I.R. Jonassou, Submarine venting of phase-separated hydrothermal fluids at Axial Volcano, Juan de Fuca Ridge, Nature 340, 702-705, 1989. 17 D.S. Butterfield, G.J. Massoth, R.E. McDuff, J.E. Lupton and M.D. Lilley, The geochemistry of hydrothermal fluids from ASHES vent field, Axial Seamount, Juan de Fuca Ridge: Subseafloor boiling and subsequent fluid-rock interaction, J. Geophys. Res. 95, 12,895-12,921, 1990. 18 C.G. Fox, The consequences of phase separation on the distribution of hydrothermal fluids at ASHES vent field, Axial Volcano, Juan de Fuca Ridge, J. Geophys. Res. 95, 12,923-12,926, 1990. 19 V. Tunnicliffe, Observations on the effects of sampling on hydrothermal vent habitat and fauna of Axial Seamount, Juan de Fuca Ridge, J. Geophys. Res. 95, 12961-12,966, 1990. 20 G.J. Massoth, H.B. Milburn, S.R. H a m m o n d , D.A. Butterfield, R.E. McDuff and J.E. Lupton, T h e geochemistry of submarine venting fluids at Axial Volcano, Juan de Fuca Ridge: New sampling methods and program rationale, in: Global Venting and Midwater and Benthic Ecological Processes, M.B. De Luca and I. Babb, eds., pp. 29-59, Natl. U n d e r s e a Res. Program Res. Rep. 88-4, 1988. 21 J.T. Wells, M.O. Smith, V.A. Atnipp and R.E. McDuff, Determining fluid velocity of black smoker jets from digital correlation of video images, EOS, Trans. Am. Geophys. Union 70, 1383, 1989. 22 J.T. Wells, pers. commun. 23 G.A. Cannon and D.J. Pashinski, Circulation near Axial Seamount, J. Geophys. Res. 95, 12,823-12,828, 1990. 24 J . L Bishoff and R.J. Rosenbauer, An empirical equation of state for hydrothermal seawater (3.2 percent NaCI), Am. J. Sci. 285, 725-763, 1985.

HEAT TRANSFER

AT ASHES VENT FIELD

25 R.W. Embley, pers. commun. 26 W.E. Frick, Non-empirical closure of the plume equations, Atmos. Environ. 18, 653-662, 1984. 27 W.P. Muellenhoff, A.M. Soldate, Jr., D.J. Baumgartner, M.D. Schuldt, E.R. Davis and W.E. Frick, Initial mixing characteristics of municipal ocean discharges: volume 1 procedures and applications, U.S. Environ. Protect. Agency Rep. EPA-600/385-073a, 1985. 28 G.J. Massoth, pers. commun. 29 E.T. Baker, R.E. McDuff and G.J. Massoth, Hydrotherreal venting from the summit of a ridge axis seamount:

71 Axial Volcano, Juan de Fuca Ridge, J. Geophys. Res. 95, 12,843-12,854, 1990. 30 A. Schultz, J.R. Delaney and R.E. McDuff, Extended geophysical observations of ridge crest hydrothermal systems: heat flux amd related quantities, EOS, Trans. Am. Geophys. Union 71, 1619, 1990. 31 P.A. Rona and D.A. Trivett, Discrete and diffuse heat transfer at ASHES Vent Field, Axial Volcano, Juan de Fuca Ridge, EOS, Trans. Am. Geophys. Union 71, 1570, 1990.