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Characteristics of hydrothermal plumes from two vent fields on the Juan de Fuca Ridge, northeast Pacific Ocean
Earth and Planetary Science Letters, 85 (1987) 59-73
59
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands [21
Characteristics of hydrothermal plumes from two vent fields on the Juan de Fuca Ridge, northeast Pacific Ocean E d w a r d T. B a k e r a n d G a r y J. M a s s o t h Pacific Marine Enl;tronmental Laboratory, National Oceanic and Atmospheric A dministration, 7600 Sand Point War, NE, Seattle. WA 98115-0070 (U.S.A.)
Received February 2, 1987; revised version received June 5, 1987 Deep CTD/transmissometer tows and water bottle sampling were used during 1985 to map the regional distribution of the neutrally-buoyant plumes emanating from each of two major vent fields on the Southern Symmetrical Segment (SSS) and Endeavour Segment (ES) of the Juan de Fuca Ridge. At both vent fields, emissions from point and diffuse hydrothermal sources coalesced into a single 200-m-thick plume elongated in the direction of current flow and with characteristic temperature anomalies of 0.02-0.05° C and light-attenuation anomalies of 0.01-0.08 m-~ (10-80 p.g/I above background). Temperature anomalies in the core of each plume were uniform as far downcurrent as the plumes were mapped (10-15 km). Downcurrent light-attenuation trends were non-uniform and differed between plumes, apparently because different vent fluid chemistries at each field cause significant differences in the settling characteristics of the hydrothermal precipitates. Vent fluids from the SSS are metal-dominated and mostly precipitate very fine-grained hydrous Fe-oxides that remain suspended in the plume. Vent fluids from the ES are sulfur-dominated and precipitate a high proportion of coarser-grained Fe-sulfides that rapidly settle from the plume. The integrated flux of each vent field was estimated from measurements of the advective transport of each plume. Heat flux was 1700 -/- 11130 MW from the ES and 580 + 351 MW from the SSS. Particle flux varied from 546 _+312 g/s to 204 ___116 g/s at the ES depending on distance from the vent field, and was 92 + 48 g/s from the SSS.
1. Introduction H y d r o t h e r m a l circulation at oceanic spreading centers plays a principal role in the thermal a n d chemical budgets of the deep sea by discharging heat- and chemical-enriched fluids into the overlying seawater [1,2]. Plumes of diluted h y d r o t h e r m a l fluid, generally located 1 0 0 - 5 0 0 m above the depth of the vent source, have been observed at all vent fields where the water c o l u m n has been sampled [3-8]. Chemical a n d thermal signatures of especially strong plumes have been identified h u n d r e d s a n d even t h o u s a n d s of kilometers from their source [4,9]. Despite the u b i q u i t y of p l u m e observations, we know little a b o u t their relationship to the local hydrographic field, the physical a n d chemical evolution that occurs d u r i n g their dispersal, or their effectiveness in t r a n s p o r t i n g hy-
Contribution No. 915 from NOAA/Pacific Marine Environmental Laboratory. 0012-821x/87/$03.50
.~:;1987 Elsevier Science Publishers B.V.
d r o t h e r m a l emissions b e y o n d the vent field of their origin. We present in this paper a regional view of the plumes e m a n a t i n g from the major k n o w n vent field on each of two morphological segments of the Juan de Fuca Ridge in the northeast Pacific Ocean: the Southern Symmetrical Segment (SSS) at the south end of the ridge a n d the E n d e a v o u r Segment (ES) at the n o r t h end (Fig. 1). The submersible " A l v i n " has visited a n d sampled both vent fields to d o c u m e n t the presence of extensive high-temperature v e n t i n g [10-13]. We c o n c e n trated on m a p p i n g and sampling the neutrallyb u o y a n t phase of the plumes for two reasons. First, the b u o y a n t plume, defined as that p o r t i o n of the p l u m e not in density e q u i l i b r i u m with the s u r r o u n d i n g water c o l u m n , is presently impossible to sample systematically from a surface ship because of its narrow horizontal d i m e n s i o n s a n d its internal variability. A C T D profile recorded by " A l v i n " while ascending through a b u o y a n t plume, for example, showed several substantial density
60
inversions [7]. Second. the neutrally-buoyant plume is an important, if not predominant, agent for transporting hydrothermal emissions beyond the immediate area of the vent field and is thus the domain of a variety of chemical, physical, and biological processes affecting the form and fate of hydrothermal emissions in the water column. For each of the two vent fields we describe the
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formation of the neutrally-buoyant plume m the axial valley as observed during June 1985. the contrasting changes in conservative (heat) and nonconservative (particles) properties of the plume as it is advected away from its source, and the relationship of the plume to the regional hydrographic setting. Using the three-dimensional distribution of the emissions, and measurements of
A
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3 Fig. 1. A. Location of the Endeavour Segment (ES) and the Southern Symmetrical Segment (SSS) on the Juan de Fuca Ridge in the northeast Pacific Ocean. B. Ridge crest bathymetry, tow tracks, mooring locations (A), known high-temperature vent sites (O), and vent sites identified only from deep-tow photography ( O ) at the SSS vent field. From south to north. submersible-sampled vent sites (O) are Plume, Vent 1. and Vent 3 [13]; unsampled vent sites ( O ) are Vent 2 and Vent 4 [18]. C. Bathymet~, tow tracks, mooring location (A), and approximate positions of known vent sites (o) at the ES vent field ([12]; S. H a m m o n d , personal communication).
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the local advective flow, we then calculate the mean advective transport of heat and particles to estimate the total flux of those components out of each vent field. 2. Methods
Because we expected plumes to be dynamic features with sharp horizontal and vertical gradients, we focused out data-gathering efforts on a limited number of lengthy, two-dimensional, near-bottom instrument tows rather than a larger number of one-dimensional vertical casts. This procedure enabled us to minimize temporal aliasing by substantially reducing the time required to survey the plume. At each of the two sites the general location and dimensions of the plume were defined with one or more extensive tows of a hydrodynamically-shaped, high-precision Sea-Bird C T D / S e a Tech transmissometer package regularly cycled in a sawtooth pattern between the top and bottom of the plume layer. Tow speeds were 2--3 k m / h r . The vertical cycling range varied between 100 and 600 m depending on the plume thickness, while the distance between the sawteeth
correspondingly varied from about 200 m to 2 km. Subsequent shorter tows using identical sensors on a standard rosette further defined the regional plume characteristics and collected water samples in teflon-coated 30-1 Niskin bottles with silicon tubing springs. Sensor resolutions for all tows and casts were + 0.0003 ° C for temperature, ± 0.00004 S / m for conductivity, and +0.001 m-1 for light attenuation. Tow-track paths and mooring locations are plotted relative to bathymetry and principal vent locations for the SSS vent field (Fig. 1B) and the ES vent field (Fig. 1C). Table 1 lists the sequence and duration of the tows. All tows and casts within about 10 km of each vent field were continuously tracked by an acoustical navigation system. Post-cruise calibration using satellite navigation referenced the acoustic navigation to geographic coordinates. Currents at the SSS vent field were monitored by an Aanderaa current meter 100 m above bottom (mab) on mooring VI. Settling particles were captured by sediment traps [14] deployed on mooring V7 at 300 mab, 55 mab, 20 mab, and 5 mab in the SSS axial valley. Preliminary analysis of the current flow and the composition and magnitude of the vertical particle flux in the axial valley at the ES site is also available ([15-17]; S. Roth, personal communication). The SSS vent field (Fig. 1 B) occupies a roughly 10-km-long portion of the top of the southernmost morphological dome of Juan de Fuca Ridge. The TABLE 1 Sequence and duration of C T D tows Tow
Begin (UTC)
End (UTC)
Southern Symmetrical Segment S1 0345 June 4 $2 1340 June 5 $4 0851 June 9 C5 1430 June 10 C6 0440 June 11 C14 1505 June 17 C15 0340 June 18
1322 2053 0907 2240 1210 2320 1337
June June June June June June June
4 5 10 10 11 17 18
Endeavour Segment $6 0048 June 24 C19 0411 June 25 $7 1522 June 25 C20 0450 June 26 C21 1521 June 26
2206 1204 0110 0956 2124
June June June June June
24 25 26 26 26
62 axial valley, extensively dcscribed from submersible and deep-tow sonar and photographic data [13,18], is linear, 1 - 2 km wide and 8 0 - 1 0 0 m deep, with a floor remarkably flat except for a narrow central cleft. All of the six known vent sites on the SSS occur within our survey area, and five of the sites lie in the central cleft. Baker and Massoth [8] gave initial descriptions of the plume at this site. The ES vent field (Fig. 1C) is located on the symmetrical central portion of the Endeavour Segment of the Juan de Fuca Ridge [10,11]. Present hydrothermal activity has been observed at two principal l{xzations each about 100 m west of ridge axis ([12]; S. H a m m o n d , personal communication). Hydrothermal plumes were identified in real time by temperature ( A T ) and light-attenuation (Ac) anomalies. In the deep waters of the N o r t h Pacific, potential density (oa) and potential temperature (0) are linearly related except where influenced by hydrothermal or conductive heating from the seafloor. Lupton et al. [7] show that the hydrothermal AT is the deviation from this relationship along the 0 axis (Fig. 2). Plume signals were identified equally well by increases in light
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Fig. 2. Graphical representation of a hydrothermal temperalure anomaly (AT') based on a tow across the SSS vent field. Outside the plume the oo vs. 0 relationship is linear: within the plume a temperature anomal',' of - 0.025°C is centered around the 27.70 a,~ surface•
attenuation, which demonstrated a linear relationship to the concentration of suspended particulate matter. Light-attenuation anomalies were calculated as the increase over the regional light-attenuation minimum between 1000 and 1500 m, a value of 0.39 m ~ at the SSS vent field and 0.40 m ~ at the ES vent field. Although both temperature and light-attenuation anomalies originate from hydrothermal venting, their relative source strength may vary significantly between individual vent types. Furthermore, relative changes in the downcurrent intensity of each may serve to identifF' nonconservative sources and sinks of the particulate fraction of the hydrothermal emissions. Particulates for Fe and S determinations were captured on 0.4-btm-pore-diameter Nuclepore m e m b r a n e s by pressure filtration and then analyzed at sea using energy dispersive X-ray fluorescence spectrometry and thin-film calibrations [19,20]. NBS, USGS, and National Research Council of C a n a d a reference particulates were used as standards. The analytical precision was about 2 and 10~, respectively, for Fe and S. 3. Formation of the neutrally-buoyant plume Axial tows $2 at the SSS (Fig. IB) and C19 at the ES (Fig. IC) describe the distribution of hydrothcrmal emissions within a few kilometers of a source vent. At the SSS vent field, tow $2 passed about 200 m west of Vent 3, a known high-temperaturc discharge site [13]. Rapid dilution of the Vent 3 discharge created a neutrally-buoyant plume clearly defined by the vertically symmetrical distribution of Ac and AT contours confined below a sharp density gradient - 250 m above the valley floor (Fig. 3). The core of the plume, skewed by a northward advcction c o m p o n e n t in the valley, was centered on the 27.70 o~ surface - 150 mab in a nearly homogeneous b o t t o m layer. The absence of a clear above-bottom plume signal from Vents 2 or 4 suggests little or no high-temperature discharge at those sites. Hydrothermal plumes in the axial valley of the ES (Fig. 4) were similar to those at the SSS. The south-to-north C19 tow along the west side of the valley axis passed through two separate plumes - 2.5 km apart. We thus infer the presence of at least two distinct vent sites, herein called Vent 1 and Vent 2. The location of Vent 1 (map coordi-
63
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Fig. 3. Transects of temperature and light-attenuation anomalies and potential density from tow $2 (1340-1640 UTC, 5 June 1985) along the axial valley of the SSS vent field. The neutrally-buoyant plume is centered on the 27.700 ae surface. Vent 3 is - 200 m east of map coordinate X = 3.5, Y = 5.0 (see Fig. 1B). Inset shows the actual tow track for $2.
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Fig. 4. Transects of temperature and light-attenuation anomalies and potential density from a portion of tow C19 (0607-0900 UTC, 25 June 1985) along the axial valley of the ES vent field. The neutrally-buoyant plume is centered on the 27.695 o0 surface. Vent 1 is at map coordinate X = 1.2, Y = 1; Vent 2 is at X = 1.8, Y = 3 (see Fig. IC). Inset shows the actual tow track.
64 nates X = 1.1, Y = 0.8 in Fig. 4) agrces with that of the extensive, high-temperature site described and sampled by "fivey and Delaney [12]. Plumes from both vent sites, clearly skewed to the south by the local currents, were centered 100-200 m above bottom on the 27.695 a0 surface. Maximum values of both AT (0.060°C) and Ac (0.110 m i) observed on tow C19 were - 50% higher than the corresponding values over the SSS, the regional data presented below suggest that this increase reflects a difference in mass flux between the two areas, rather than just a near-field sampling bias. A plot of AT vs. kc (Fig. 5) illustrates the varying relationship between conservative and nonconservative species in the near-field plume. We hypothesize that the plume-core end member at each vent field was created principally by emissions from high-temperature "black smokers" that provide the buoyant force to lift the plume 150- 200 m above the sea floor. At the core of the plutne, the original hydrothermal fluid (at a nominal temperature of 350 ° C and density of 0.69 g/l) has been diluted by a factor of - 6000 (to 0.04 ° (" above ambient) at the SSS and - 4000 (to 0.06 ° (" above ambient) at the ES. The Ac/A'I" ratio decreases as the plume is further diluted. The group of points centered at A T = 0.035°( `, A c = 0 . 0 2 m ~. represent an anomalously reduced At~AT ratio in roughly the lower 100 m of the water
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column at each vent field. These points may represent the influence of warm springs, which are more widely distributed than black smokers along the valley floor [13]. Diffuse fluid discharge from such springs would supply additional heat. but probably few particles, to the lower water column. North of Vent 3 at the SSS, for example, the Ac distribution becomes vertically uniform in the lower 2(10 m whereas the AT distribution steadily increases approaching the seafloor (Fig. 3).
4. Advection of the plume Mixing of the hydrothermal fluid and the ambient bottom water in the axial valley creates a plume occupying a very narrow o0 range (Figs. 3 and 4). Advcction spreads the plume along isopycnal surfaces, and as it passes out of the axial valley it becomes an above-bottom feature with a vcrtical density gradient much weaker than that of the ambient water column over the same depth interval. At the SSS vent field, the plume spread on the 27.70 a, surface (Fig. 6), reducing the doo/d: gradient between the 27.695 and 27.705 a0 surfaces from a non-plume value of 9.1 × 10 s k g / m '~ to 4.4 × 10 -5 k g / m 4 within the plume. Isopycnal gradients in AT within and outside the advecting plume were ncgligible. A narrow zone with temperature gradients as high as 0 . 0 2 ° ( ' / k n l along the 27.70 an surface marked the lateral boundaries of the plume. f h c along-isopycnal distribution of particles on tow, C6 was patchier than that of temperature (and presumabl 3 other dissolved, conservative substances). Light attenuation values in the core of the plume steadily increased with distance from the center of the axial valley, reaching a maximum near the edge of the plume (Fig. 6B). Similar downstream increases in light attenuation were observed on tows (7"14 and C15. -['he lack of any corresponding increase in the A7" signal on these tows suggests that the particle variations resulted not from multiple or variable vent sources, but from precipitation of dissolved trace metals in the plume. Transmission-electron-microscope studies of particulate matter from the SSS plume [21] have revealed a high concentration of amorphous hydrous Fe-oxide precipitates+ as well as deposits of Mn and Fe on bacterial capsules. Erosion of sediments from the axial-valley walls might add
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particles to the plume, but the consistent location of the particle maximum above the sea floor and at the same depth as the AT maximum does not support an erosive source for the excess particles, Furthermore, the mean current speed measured 100 m above the crest of the west ridge was 6.3 _+ 3.3 c m / s , insufficient to significantly resuspend bottom sediments. Speeds > 10 c m / s occurred during < 5% of the record. Cross-plume tows C5 and C6 also detected a secondary thermal and particulate plume centered on the 27.707 o0 surface (Fig. 6), a density surface that nowhere intersects the axial valley. The 27.707 Oo surface on both tows intersected the west flank of the ridge at - 2300 m along the upslope border of a broad plateau; this secondary plume is circumstantial evidence for the presence of off-axis hydrothermal venting. The existence of off-axis hydrothermal plumes is consistent with recent thermal models [22] and chemical evidence [23,24] that require as much as 80% of the total hy-
drothermal circulation to occur off-axis. If partitioning on that scale does occur at the SSS, then off-axis venting must be mostly low-temperature and widespread, because we found no evidence of deep plumes with temperature and particle anomalies characteristic of high-temperature axial emissions. The physical evolution of the ES plume was mapped by a series of cross-plume tows extending as far as 7.5 km downcurrent from Vent 1, or about 4.1 days travel time based on a mean velocity of 2.1 c m / s at 205 ° during the survey period ([16]: S. Roth, personal communication). Crosssections of the AT contours describe a plume positioned initially over the western margin of the valley floor (Fig. 7A) (the location of submersibleobserved venting [10,11]) and advected southward approximately along the axial-valley strike. The plume was centered on the 27.695 o8 surface on all transects. Although the eastern boundary of the plume was everywhere sharp, the plume extended
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Fig. 7, (A) T e m p e r a t u r e a n o m a l y a n d (B) l i g h t - a t t e n u a t i o n a n o m a l y o n five c r o s s s e c t i o n s o f the ES p l u m e f r o m 0 to 7.5 k m d o w n c u r r e n t of Vent 1 (see Fig. IC). E a c h t r a n s e c t is a p p r o x i m a t e l y n o r m a l to the axial valley, a n d e a c h is a l i g n e d with respect to the crest of the west ridge. T h e c r o s s section at 0 krn crosses the Fig. 4 t r a n s e c t at m a p c o o r d i n a t e X = 1.2, Y = 1. O t h e r c r o s s s e c t i o n s are lettered to c o r r e s p o n d w i t h Fig. 8. T h e thin h o r i z o n t a l line o n e a c h t r a n s e c t m a r k s the p o s i t i o n of the 27.695 o 0 surface.
67
westward beyond our tows. Values of AT varied only slightly along the plume core. Occasional downstream AT increases could result from inhomogeneities in the plume, or from secondary vent sources.
Cross-sections of the Ac contours describe a more compact and symmetrical plume (Fig. 7B). Unlike the AT plume, Ac values at the plume core steadily decreased downcurrent from Vent 1.
5. Regional distribution of the plume The regional dispersal of the plumes can be mapped by plotting AT and Ac values on the isopycnal spreading surface. Areal maps of the ES plume (Fig. 8) on the 27.695 o0 surface clearly show the origin of the regional plume from at least two vent sites in the study area, and its advective transport to the south. The orientation of the plume almost exactly matches the direction of the net current during the survey (2.1 c m / s at 205 °).
TEMPERATURE
129°12 ' I0 -
I0'
The mean velocity for a 27-day period prior to and during the survey was 1.5 c m / s at 170° (S. Roth, personal communication). The areal distributions of AT [8, fig. 1] and Ac (Fig. 9) on the 27.70 o0 surface at the SSS site display many of the same features of the ES maps. The plume at the SSS site apparently evolves from a hydrothermally-active area larger than that at the ES and is transported northward rather than southward. The record from the V1 current meter at 2030 m ( - 2 7 . 7 0 % surface) indicates that the mean velocity for 75 days prior to the survey was 1.3 c m / s at 356 °. The slight westward displacement of the plume relative to the vent locations was evidently caused by a shift of the net flow to the northwest during the 15-day survey, including this shift changes the mean velocity to 1.2 c m / s at 352 °. The 90-day mean velocity along the plume axis (20 o ) was 1.0 c m / s . Temperature anomalies along the axes of both plumes were uniform at distances beyond about 2
ANOMALY
08'
LIGHT-ATTENUATION
06'
04'
129°12 '
;-
I0
"
8
I0'
08'
ANOMALY
06'
04'
J-
"
I
- 48o00 ,
- 48000 ,
6:
6 4
i
i
4
I
4
"
- 4 7 ° 58'
- 47o58 ,
2 I
v >.-
0
N
0 ~,
56'
-56' -2
-2
-
i
i
-4 -~
-4-I -
54'
- 54'
I
-6-
-6 J
i -84-6
I ---
-8-T--
-4
-2
0 X(km)
2
4
-6
"
q
-4
-.
~
-2
-.,
-
0
-,-
2
-
4
6
X(km)
Fig. 8. Areal m a p of (A) t e m p e r a t u r e a n o m a l y a n d (B) l i g h t - a t t e n u a t i o n a n o m a l y on the 27.695 o o surface from all tows over the ES in relation to the strike of the axi',d valley, the k n o w n vent fields ( I ) , a n d the current m e t e r / s e d i m e n t trap m o o r i n g ( - ) . C o n t o u r interval is 0.01° C for t e m p e r a t u r e a n d 0.01 m - l for light a t t e n u a t i o n .
68 LIG H T - A T T E N U A T I O N A N O M A L Y 130"32'
30
28'
26
21
24
22
I
18
~
15 -
0 --
<010 I m
I
0.01
--
(0102
m "l
0,02
--
(0+03
m 1
20 l
X
x]
16'
14
,.~"~ i a l ,,~"$trlk e
44¢50 ,
,[' .:.:"
I
X
>0.03 m
18
~'~'"
"
48
,]~
" :~ \
12
:
,:" 46"
~ :r'
,
d;' 9
" ~ "--
'
44'
"
40 A
o
iN -3
;::: -12
-9
-6
-3
0
1 3
6
9
12
38
15
X(km)
Fig. 9. Areal map of light-attenuation anomaly on the 27.700 surface from all tows over the SSS in relation to the strike of the axial valley, the known high-temperature vents (O), and moorings fit). Contour interval is 0.005 m- 1.
o0
km from the principal vent source. The value of the anomaly in the ES plume core was twice that of the SSS plume core, however, and temperature gradients at the lateral boundary of the plume were as high as 0 . 0 8 ° C / k m , or four times higher than found at the SSS site. The alignment of the plume axes with the principal flow direction, and the sharp property gradients surrounding the plumes, demonstrate that the horizontal plume transport was advectively, rather than diffusively. controlled. The uniform AT distribution within each plume suggests that temporal aliasing problems were minor. Diurnal and semidiurnal tidal excursions at each site were < 1 km, much smaller than the horizontal scale of the plumes. The areal distribution of particle anomalies, while similar in outline to the AT plumes, show considerably more internal variation. At the SSS site, the Ac values in the plume core were patchy, commonly showing isolated highs clearly separated from the principal vent field. Because of the uniformity of the AT distribution, we speculate that this patchiness is evidence of in situ precipitation of hydrothermal material rather than temporal or spatial source variability. The ES particle plume displayed a different trend. Values
of Ac. in the plume core were not patchy but decreased sharply in the first kilometer away from Vent 1 and continued a steady decrease for as far as we tracked the plume, suggesting that deposition rather than precipitation regulates the particle concentration, The particle plume at the ES was more tightly-constrained about the axis than the temperature plume, which also may be a result of particle deposition. Differences between the SSS and ES sites in the downcurrent trends of particle anomalies are quantified by a plot of At/AT vs. downplume distance at each site (Fig. 10). At the SSS, the Ac/AT ratio shows no consistent downplume change, varying between 1.25 and 0.63 before reaching a constant value of 0.82. At the ES. this ratio falls nearly monotonically from a high of 2.0 at Vent 1 to 0.7 9 km downplumc. The only interruption in the ES trend occurs 5 km from Vent 1 where the At~AT ratio decreases because of an increase in AT at the plume center ( X = - 0 . 5 , Y = - 4 in Fig, 8). This increase may indicate either the location of a small vent site or heterogeneity in the plume from Vent 1. Sediment trap measurements of the fallout rate of plume particulates at each site support the site differences expressed in Fig. 10, even though only a limited number of samples are presently available. Deposition of particles from the SSS axial valley plume was measured from June 7 to July 14, 1985, by sediment traps deployed on mooring VT, 300 m west of Vent 3 (Fig. IB). Deposition into a control trap 300 m above the valley floor, well above any plume influence, was 0.027 g / m ? d. Traps within the plume collected a flux of 0.032 g / m 2 d (55 mab), 0.029 g / m 2 d (20 mab), and 0.028 g / m 2 d (5 mab), not significantly differcnt than the control trap. Although specific hydro-
•
SSS ES
1 b* m l
0
~
4
0 0 10 ~ISTANCE IKIrl
12
14
16
Fig. 10. Plot of Ac/AT vs. distance down the axis for plumes emanating from the SSS and ES vent fields.
69
thermal minerals were found in the plume traps [25], the vertical mass flux of hydrothermal particles, even as close as 300 m to Vent 3, was low enough to have only a negligible effect on plume concentrations. (A similar conclusion was reached by Baker et al. [26], from earlier sediment trap results.) The deposition rate of Fe in the three lower traps, for example, averaged 67 + 24 m g / c m 2 ky, identical to the mean deposition rate (67 m g / c m 2 ky) in surface sediments from cores collected 15 km and 43 km from the ridge axis (G.J.M., unpublished data). In contrast, sediment traps from the ES mooring recorded sharp increases at and below plume depth in the vertical flux of hydrothermally-enriched elements Fe, S, Cu, Mn, and Zn, as well as of hydrothermal precipitates such as metal sulfides and anhydrite [17]. The vertical flux of Fe below the plume was more than 100 times higher than that determined from sediment cores collected 100 km off-axis [15]. A direct comparison between the near-field deposition rates of hydrothermal emissions at the SSS and ES vent fields must be made with caution, however, because near-field variability in plume advection and particle settling is expected to be high. The contrasting At~AT trends of the two plumes evidently result from contrasting hydrothermal fluid chemistries that create precipitates with different settling characteristics at each site. Hydrothermal fluids collected from three orifices at the SSS vent field in 1984 were metal-dominated, with a mean F e / S molar ratio of 4.3 + 1.0 [13]. Conversely, nearly undiluted fluid collected from a single orifice at the ES vent field in 1984 was sulfur-dominated, with an F e / S molar ratio of 0.23 (R. McDuff, personal communication). These chemical differences are maintained in the bulk chemical analyses of particles collected from throughout the SSS and ES neutrally-buoyant plumes. At the SSS site, the mean concentration ( + o ) of Fe in the suspended particles (18.0 + 7.0%, n = 23) overwhelms that of S (0.61 + 0.13%, n = 22), whereas at the ES site the concentrations of each are more closely matched (6.9 + 2.7%, n = 19, for Fe, compared with 1.6 + 0.5%, n = 19, for S). The particulate Fe percentage is significantly correlated with light attenuation only in the SSS, plume (Fig. llA), whereas the particulate S percentage is significantly correlated with light at-
e~-
A
•
E
e.48-
B
?
i0 4 :.
.
•
~ e44~
! _.: Fe I%1
S (%)
Fig. II. A. Particulate Fe vs. light attenuation from discrete samples collected above both vent fields. Linear regression of SSS samples ( y = 0.0013x +0.386) yields a correlation coefficient r = 0.89. B. Particulate S vs. light attenuation from discrete samples above both vent fields. Linear regression of ES samples (y = 0.035x +0.367) yields a correlation coefficient r = 0.85.
tenuation only in the ES plume (Fig. llB). We hypothesize, therefore, that the patchy but roughly constant Ac/AT ratios in the SSS plume arise from a fine-grained particle suite of amorphous hydrous Fe-oxides augmented by ongoing precipitation and reduced only slowly by deposition. The nearly steady decrease in Ac/AT ratios in the ES plume result from a two-component system: a background fine-grained particulate Fe-oxide phase that settles slowly and advects along a fixed density surface, and a fraction of larger and denser Fe-sulfides that settle rapidly and cause a continual attenuation decrease in the dispersing plume. The chemistry of the ES plume is similar to that of the 21°N vent field, where the F e / S molar ratio in the high-temperature vent fluids - 0 . 1 9 [27] and both oxide and sulfide minerals have been identified in the lower 20 m of the buoyant plume [28]. Regional plume maps are useful also for discovering vent fields, although the interpretation of non-synchronous water column data in a varying current field has some inherent uncertainty. At the ES site (Fig. 8), isolated highs in both Ac and AT were found outside of the primary plume and downcurrent of Vents 1 and 2. The strong anomaly at map coordinate X = 3, Y = 2.5, on the top of the east wall of the valley may indicate a new vent site, although an axial crest location would be unusual. No plume signals were detected farther north than Vent 2 (map coordinate X = 2, Y = 3). South of Vents 1 and 2, plumes from two possible
70 secondary sources within the axial valley were detected (at X = 1.2, Y = - 1.5, and at X = - 0 . 5 . Y = - 4 ) , although the fact that both plumes were centered on the 27.695 o0 surface could mean that these signals originated at Vent 1 or 2 and represent small perturbations resulting from changes in the vent flux or the advective field. It appears clear from this survey that the primary vent field within this 14-kin section of ridge is at Vent 1.
6. Heat and particle flux measurements Comprehensive information on plume distribution allows not only a description of the physical evolution of the plume but also an estimation of the source strength of vent field emissions. Transport of a conservative constituent in a neutrallybuoyant plume equals the total vent-field flux of that constituent, provided that advection rather than diffusion controls the plume distribution. and that the plume distribution is stationary on a time scale greater than the advective flushing time (i.e., vent field diameter/mean velocity) of the vent field. The fact that a net flow of 1-2 k m / d produced strongly asymmetric plumes at both sites makes these assumptions reasonable. The ventfield flux can be calculated as the difference between plume transport through a cross section downcurrent of the vent field and normal to the plume axis, and the transport through an identical upcurrent cross section. The heat flux Q, for example, through a cross section is:
Q= oc,[E , aT']u
(1)
where specific heat pCp = 4.2 M J / m 3 °C. A, = area of each AT, interval on the cross section, and U = component of mean velocity along the plume axis. The use of equation (1) implies that the heat transport is dominated by the mean flow and that time-dependent fluctuations are minor. Although the lack of a long-term simultaneous velocity and temperature record from within the plume at either site precludes a direct measure of the turbulent heat flux, the applicability of equation (1) is implicitly supported by the nearly uniform and steady temperature distribution within each plume and by the agreement of the plume orientation and mean current velocity axes at each site. Temperature fluctuations within the plume, attributable to
variations in source strength, alteration of the plume dimensions by tidal current fluctuations during the cross-plume transects, and additions from downcurrent vent sources, can be estimated by averaging the heat content from several crossplume transects. At the ES site, six transects across the plume (lines B - G , Fig. 8) were occupied downcurrent of the major vent sites and one transect (line A, Fig. 8) upcurrent. The net heat in each cross-section was determined by subtracting ~A, AT, of line A from Y~A, AT, of lines B - ( ; , the mean heat content determined from all six transects (79,000 MJ in a 1-m-thick cross section) was about three times the standard deviation (26,000 M J), indicating that net heat transport by correlated temperature-velocity fluctuations within the plume was small. Confidence limits on the plume flux are also a function of variability in the current flow. The rms error estimate E of the mean velocity, determined using the central limit theorem, is given by: E = o/(T/~)
lj2
where T is the record length, o is the standard deviation of the flow along the vector-mean axis. and ~" is an independent time scale estimated as the area under the autocorrelation function for each record [29]. The net heat transported in the ES plume calculated using equation (1) is thus: Q = (79,000 + 26,000 M J / m ) × (0.021 + 0.012 m / s ) -- 1700 + 1100 MW This result must be a lower bound because the western boundary of the plume extended beyond the survey area (Fig. 8A). Even though the uncertainty is high (about 60% of the mean), it represents a significant improvement over the initial survey of Crane et al. [30], which suffered from a lack of both current data and information on the orientation of the plume (only a single axial tow was made). Crane et al. [30], using equation (1) and assuming the plume was transported normally to the axial strike at U = 1 c m / s , estimated a heat flux of 12,600 MW for this 10-kin section of the ES. Baker and Massoth [8] used the above procedure to calculate the heat flux from the SSS site,
71
obtaining a value of 580 + 290 MW. Because multiple downcurrent cross sections were not available at that site, the reported uncertainty was based only on the error estimate from the current-meter measurements and did not include any uncertainty from the plume cross sections. If we assume that the uncertainty in the plume measurements is similar at both sites and use the ES data to determine the relative amplitude of the e r r o r / m e a n ratio, then a more conservative calculation of the heat flux at the SSS site is: Q = (57,000 5- 19,000 M J / m ) × (0.0105 5- 0.0053 m / s ) = 580 5- 351 MW Crane et al. [30] calculated a flux of - 3 0 , 0 0 0 MW for a 1 c m / s current across a 40-km ridge section centered on the SSS ventfield. Results from other models used by Crane et al. [30] gave lower power estimates for both sites. Other estimates of hydrothermal heat flux have been based on extrapolations from temperature and flow at individual orifices, and on velocity and hydrographic measurements in a buoyant plume. Both approaches require a submersible. Converse et al. [31] calculated the heat flux from hot and warm vents at 21 ° N on the East Pacific Rise using direct measurement of a few vents and extrapolation to the entire vent field by means of a photographic survey. They estimated the heat flux from high-temperature vents to be 220 + 80 MW, and flux from warm-water springs to be 120-9000 MW (the warm-water vent estimate being highly speculative). The total heat flux at both Juan de Fuca Ridge vent fields thus appears to be of the same order as the 2 1 ° N field, although the confidence limits on all the data are broad. Little et al. [32] collected simultaneous velocity, temperature, and conductivity data within the buoyant plume from a small ( - 4 m by 5 m) sulfide structure venting hot water at 11 ° N on the East Pacific Rise. Using non-linear plume theory, they calculated the heat flux from this multiplechinmey mound as 3.7 + 0.8 MW, about a factor of 100 lower than the vent field estimates at SSS, ES, and 21 ° N. This difference appears reasonable considering that both the SSS and ES fields are" composed of several individual vent sites each extending for hundreds of meters along strike
[10-13]. Individual vents measured by Converse et al. [31] had a typical heat flux of 0.5-10 MW, in agreement with the results of Little et al. [32]. Neither Converse et al. [31] nor Little et al. [32] were able to make a reliable estimate of the diffuse heat flux. A more tractable solution may lie in calculating the diffuse flux as the difference between total plume flux as outlined in this paper and the flux from individually-measured, hightemperature, point sources. The mass flux of a non-conservative plume constituent, such as hydrothermally-precipitated particles, can also be calculated, but the results must be cautiously interpreted. The flux of particulate components through a downcurrent cross-section may be greater or less than the vent field source depending on the balance between precipitation and deposition during advection. The non-uniform distribution of particles in each plume also means that turbulent fluxes, unevaluated here, may be important. The multiple cross-sections occupied at the ES allow us to measure downcurrent changes in the mass flux and to compare them with the heat flux. Unlike the results for heat flux, a significant downcurrent decrease occurs in mass flux: 4 2 0 + 2 4 0 g / s through transect B, 546 + 312 g / s through C, 336 + 192 g / s through D, 227 + 158 g / s through E, 2 5 2 + 144 g / s through F, and 204-t-116 g / s through G. This decrease could be explained by depositional losses from the plume, or by dispersal of the western boundary of the plume beyond the occupied cross-sections. Although the present data are not conclusive, the high deposition rates of hydrothermal particles within the axial valley [15,17], the downcurrent uniformity of the heat flux, and the chemical nature of the ES plume suggests that deposition is more important than dispersal in creating the observed trend in mass flux. Calculation of the particle flux at the SSS vent field yielded 92 5- 48 g / s . This figure represents the net mass transport through a cross-section 5 km downcurrent of Vent 3. No estimate of downplume variability is available, but the trend shown in Fig. 10 suggests it is small. 7. Conclusions
Dilute, neutrally-hydrothermal plumes can be readily identified and tracked by m a p p i n g
72
anomalies of temperature and light attenuation. Plumes from two vent fields on different segments of the southern Juan de Fuca Ridge show similar characteristics of formation and dispersal. Within the axial valley, the buoyant hydrothermal emissions rise 150-200 m above the seafloor until they reach a stable density surface. The local current field advects the neutrally-buoyant plume layer along this density surface; outside the axial valley it is readily observable as a 200-m-thick layer of anomalously weak vertical density, temperature, and light-attenuation gradients capped by a thin layer of much stronger gradients. At the Southern Symmetrical Segment, a secondary plume found deeper than the primary axial plume suggested the presence of off-axis venting. At both sites, point and diffuse hydrothermal sources emanating from hydrothermally-active regions as long as 10 km coalesced into a single elongated plume with a uniform internal temperature distribution and sharp lateral boundaries. Particle distributions within the two plumes differed because of their contrasting chemical signatures. The SSS plume is Fe-rich and primarily forms fine-grained amorphous hydrous Fe-oxides that disperse quasi-conservatively. The ES plume, with a more evenly matched Fe and S molar abundance, precipitates a significant amount of Fe-sulfide particles that settle rapidly and cause particle concentrations in the plume to sharply decrease with distance from the vent source. Advective transport of heat and particles in the plumes was used to estimate the total source strength of each vent field. Heat flux was 580 + 351 MW in the SSS plume and about three times higher, 1700 ± 1100 MW, in the ES plume. Particle flux was 92 ± 48 g / s at the SSS and varied from 546 ± 312 g / s to 204 ± 116 g / s at the ES depending on distance from the vent field. Regional plume mapping facilitates the design of an efficient sampling program around a vent field, guides the interpretation of the resulting distribution of physical, chemical, and biological parameters, and provides an effective technique for measuring the integrated flux strength of an entire vent field. Hydrothermal plumes are dynamic features with sharp gradients, and without adequate information on their distribution it may be impossible to accurately describe the physical and chemical evolution of the dispersing hydrothermal emissions.
Acknowledgements This research was supported by the N()AA VENTS program. R. Thomson constructively reviewed the manuscript. Wc thank G. Lebon and I). Tennant for help with shipboard sampling and data recovery, and H. Milburn for expertise in acoustic navigation. S. Hammond supplied the Sea Beam-derived bathymetry for each vent field. S. Roth and J. Dymond generously shared the preliminary results of their current meter deployment at the Endeavour Segment vent field.
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14
15 16
17
18
19
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23
Fuca Ridge: preliminary observations from the submersible Alvin, Geology 14, 823-827, 1986. E.T. Baker and H.B. Milburn, An instrument system for the investigation of particle fluxes, Cont. Shelf Res. 1. 425-435, 1983. S. Roth and J. Dymond, Hydrothermal particle flux: Endeavour Ridge Phase I, EOS 66, 930, 1985. S. Roth and J. Dymond, Where wafts the hydrothermal plume? A mt×~red-sensor record of lateral particle flux above Endeavour Ridge, EOS 67, 1027, 1986. J. Dymond and S. Roth, Plumefall: A record of the settling particle flux on the Endeavour Ridge using moored sensors, EOS 67, 1027-1028, 1986. W.R. Normark, J.L. Morton, R.A. Koski, D.A. Clague, and J.R. Delaney, Active hydrothermal vents and sulfide deposits on the southern Juan de Fuca Ridge, Geology 11, 158- 163, 1983. G.S. Holmes, The limitations of accurate "' thin-film" X-ray fluorescence analysis of natural particulate matter: problems and solutions, ('hem. Geol. 33, 333-353, 1981. R.A. Feely, G.J. Massoth, E.T. Baker, J.F. Gendron. A.J. Paulsoa and EA. Crecelius, Seasonal and vertical variations in the elemental composition of suspended and settling particulate matter in Puget Sound, Washington, E,stuarine Coastal Shelf S.ci. 22, 215--239, 1986. J.P. Cowen, G.J. Massoth and E.T. Baker, Bacterial scavenging of Mn and Fe in a mid- to far-field hydrotherreal particle plume, Nature 322, 169 171, 1986. J i . Morton and N.H. Sleep, A mid-ocean ridge thermal model: Constraints on the volume of axial hydrothermal heat. J. Geophys. Res. 90, 11,345-11,353, 1985. T.S. Bowers and H.P. Taylor. Jr., An integrated chemical and stable-isotope model of the origin of mid-ocean ridge hot spring systems, J. Geophys. Res. 90, 12.583-12,606, 1985.
24 R.A. Mortlock and P.N. Froelich, Hydrothermal germanium over the southern East Pacific Rise, Science 231, 43-45, 1986. 25 R.A. Feely, M. Lewison, G.J. Massoth, G. Robert-Baldo, J.W. Lavelle, R.H. Byrne, K.L. Von Datum and H.C. Curl, Jr., Composition and dissolution of black smoker particulates from active vents on the Juan de Fuca Ridge. J. Geophys. Res., in press. 26 E.T. Baker, J.W. l,avelle and G.J. Massoth, Hydrothermal particle plumes over the southern Juan de Fuca Ridge, Nature 316, 342.-344, 1985. 27 K.L. Von Damm, J.M. Edmond. B. Grant, C.I. Measures, B. Walden and R.F. Weiss, Chemistry of submarine hydrothermal solutions at 2 1 ° N , East Pacific Rise, G e o c h i m Cosmochim. Acta, 49, 2197-2220, 1985. 28 M.J. Mottl, Chemical processes in submarine hydrothermal plumes near 2 1 ° N on the East Pacific Rise, EOS 67, 1027, 1986. 29 P.K. Kundu and T.S. Allen, Some three-dimensional characteristics of low-frequency current fluctuations near the Oregon coast, J. Phys. Oceanogr. 6, 181-199, 1976. 30 K. Crane, F. Aikman I11, R. Embly, S. H a m m o n d , A. Malahoff and J. Lupton, The distribution of geothermal fields oll the Juan de Fuca Ridge. J. Geophys. Res. 90, 727--744, 1985. 31 D.R. Converse, H.D. Holland, and J.M. Edmond. Flow rates in the axial hot springs of the East Pacific Rise 121 o N): implications for the heat budget and the formation of massive sulfide deposits, Earth Planet. Sci. l,ett. 69, 159-175, 1984. 32 S.A. Little, K.D. Stolzenbach, and R.P. Von Flerzen, Measurements of plume flow from a hydrothermal vent field, J. Geophys. Res. 92, 2587.-2596, 1987.
59
Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands [21
Characteristics of hydrothermal plumes from two vent fields on the Juan de Fuca Ridge, northeast Pacific Ocean E d w a r d T. B a k e r a n d G a r y J. M a s s o t h Pacific Marine Enl;tronmental Laboratory, National Oceanic and Atmospheric A dministration, 7600 Sand Point War, NE, Seattle. WA 98115-0070 (U.S.A.)
Received February 2, 1987; revised version received June 5, 1987 Deep CTD/transmissometer tows and water bottle sampling were used during 1985 to map the regional distribution of the neutrally-buoyant plumes emanating from each of two major vent fields on the Southern Symmetrical Segment (SSS) and Endeavour Segment (ES) of the Juan de Fuca Ridge. At both vent fields, emissions from point and diffuse hydrothermal sources coalesced into a single 200-m-thick plume elongated in the direction of current flow and with characteristic temperature anomalies of 0.02-0.05° C and light-attenuation anomalies of 0.01-0.08 m-~ (10-80 p.g/I above background). Temperature anomalies in the core of each plume were uniform as far downcurrent as the plumes were mapped (10-15 km). Downcurrent light-attenuation trends were non-uniform and differed between plumes, apparently because different vent fluid chemistries at each field cause significant differences in the settling characteristics of the hydrothermal precipitates. Vent fluids from the SSS are metal-dominated and mostly precipitate very fine-grained hydrous Fe-oxides that remain suspended in the plume. Vent fluids from the ES are sulfur-dominated and precipitate a high proportion of coarser-grained Fe-sulfides that rapidly settle from the plume. The integrated flux of each vent field was estimated from measurements of the advective transport of each plume. Heat flux was 1700 -/- 11130 MW from the ES and 580 + 351 MW from the SSS. Particle flux varied from 546 _+312 g/s to 204 ___116 g/s at the ES depending on distance from the vent field, and was 92 + 48 g/s from the SSS.
1. Introduction H y d r o t h e r m a l circulation at oceanic spreading centers plays a principal role in the thermal a n d chemical budgets of the deep sea by discharging heat- and chemical-enriched fluids into the overlying seawater [1,2]. Plumes of diluted h y d r o t h e r m a l fluid, generally located 1 0 0 - 5 0 0 m above the depth of the vent source, have been observed at all vent fields where the water c o l u m n has been sampled [3-8]. Chemical a n d thermal signatures of especially strong plumes have been identified h u n d r e d s a n d even t h o u s a n d s of kilometers from their source [4,9]. Despite the u b i q u i t y of p l u m e observations, we know little a b o u t their relationship to the local hydrographic field, the physical a n d chemical evolution that occurs d u r i n g their dispersal, or their effectiveness in t r a n s p o r t i n g hy-
Contribution No. 915 from NOAA/Pacific Marine Environmental Laboratory. 0012-821x/87/$03.50
.~:;1987 Elsevier Science Publishers B.V.
d r o t h e r m a l emissions b e y o n d the vent field of their origin. We present in this paper a regional view of the plumes e m a n a t i n g from the major k n o w n vent field on each of two morphological segments of the Juan de Fuca Ridge in the northeast Pacific Ocean: the Southern Symmetrical Segment (SSS) at the south end of the ridge a n d the E n d e a v o u r Segment (ES) at the n o r t h end (Fig. 1). The submersible " A l v i n " has visited a n d sampled both vent fields to d o c u m e n t the presence of extensive high-temperature v e n t i n g [10-13]. We c o n c e n trated on m a p p i n g and sampling the neutrallyb u o y a n t phase of the plumes for two reasons. First, the b u o y a n t plume, defined as that p o r t i o n of the p l u m e not in density e q u i l i b r i u m with the s u r r o u n d i n g water c o l u m n , is presently impossible to sample systematically from a surface ship because of its narrow horizontal d i m e n s i o n s a n d its internal variability. A C T D profile recorded by " A l v i n " while ascending through a b u o y a n t plume, for example, showed several substantial density
60
inversions [7]. Second. the neutrally-buoyant plume is an important, if not predominant, agent for transporting hydrothermal emissions beyond the immediate area of the vent field and is thus the domain of a variety of chemical, physical, and biological processes affecting the form and fate of hydrothermal emissions in the water column. For each of the two vent fields we describe the
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formation of the neutrally-buoyant plume m the axial valley as observed during June 1985. the contrasting changes in conservative (heat) and nonconservative (particles) properties of the plume as it is advected away from its source, and the relationship of the plume to the regional hydrographic setting. Using the three-dimensional distribution of the emissions, and measurements of
A
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3 Fig. 1. A. Location of the Endeavour Segment (ES) and the Southern Symmetrical Segment (SSS) on the Juan de Fuca Ridge in the northeast Pacific Ocean. B. Ridge crest bathymetry, tow tracks, mooring locations (A), known high-temperature vent sites (O), and vent sites identified only from deep-tow photography ( O ) at the SSS vent field. From south to north. submersible-sampled vent sites (O) are Plume, Vent 1. and Vent 3 [13]; unsampled vent sites ( O ) are Vent 2 and Vent 4 [18]. C. Bathymet~, tow tracks, mooring location (A), and approximate positions of known vent sites (o) at the ES vent field ([12]; S. H a m m o n d , personal communication).
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the local advective flow, we then calculate the mean advective transport of heat and particles to estimate the total flux of those components out of each vent field. 2. Methods
Because we expected plumes to be dynamic features with sharp horizontal and vertical gradients, we focused out data-gathering efforts on a limited number of lengthy, two-dimensional, near-bottom instrument tows rather than a larger number of one-dimensional vertical casts. This procedure enabled us to minimize temporal aliasing by substantially reducing the time required to survey the plume. At each of the two sites the general location and dimensions of the plume were defined with one or more extensive tows of a hydrodynamically-shaped, high-precision Sea-Bird C T D / S e a Tech transmissometer package regularly cycled in a sawtooth pattern between the top and bottom of the plume layer. Tow speeds were 2--3 k m / h r . The vertical cycling range varied between 100 and 600 m depending on the plume thickness, while the distance between the sawteeth
correspondingly varied from about 200 m to 2 km. Subsequent shorter tows using identical sensors on a standard rosette further defined the regional plume characteristics and collected water samples in teflon-coated 30-1 Niskin bottles with silicon tubing springs. Sensor resolutions for all tows and casts were + 0.0003 ° C for temperature, ± 0.00004 S / m for conductivity, and +0.001 m-1 for light attenuation. Tow-track paths and mooring locations are plotted relative to bathymetry and principal vent locations for the SSS vent field (Fig. 1B) and the ES vent field (Fig. 1C). Table 1 lists the sequence and duration of the tows. All tows and casts within about 10 km of each vent field were continuously tracked by an acoustical navigation system. Post-cruise calibration using satellite navigation referenced the acoustic navigation to geographic coordinates. Currents at the SSS vent field were monitored by an Aanderaa current meter 100 m above bottom (mab) on mooring VI. Settling particles were captured by sediment traps [14] deployed on mooring V7 at 300 mab, 55 mab, 20 mab, and 5 mab in the SSS axial valley. Preliminary analysis of the current flow and the composition and magnitude of the vertical particle flux in the axial valley at the ES site is also available ([15-17]; S. Roth, personal communication). The SSS vent field (Fig. 1 B) occupies a roughly 10-km-long portion of the top of the southernmost morphological dome of Juan de Fuca Ridge. The TABLE 1 Sequence and duration of C T D tows Tow
Begin (UTC)
End (UTC)
Southern Symmetrical Segment S1 0345 June 4 $2 1340 June 5 $4 0851 June 9 C5 1430 June 10 C6 0440 June 11 C14 1505 June 17 C15 0340 June 18
1322 2053 0907 2240 1210 2320 1337
June June June June June June June
4 5 10 10 11 17 18
Endeavour Segment $6 0048 June 24 C19 0411 June 25 $7 1522 June 25 C20 0450 June 26 C21 1521 June 26
2206 1204 0110 0956 2124
June June June June June
24 25 26 26 26
62 axial valley, extensively dcscribed from submersible and deep-tow sonar and photographic data [13,18], is linear, 1 - 2 km wide and 8 0 - 1 0 0 m deep, with a floor remarkably flat except for a narrow central cleft. All of the six known vent sites on the SSS occur within our survey area, and five of the sites lie in the central cleft. Baker and Massoth [8] gave initial descriptions of the plume at this site. The ES vent field (Fig. 1C) is located on the symmetrical central portion of the Endeavour Segment of the Juan de Fuca Ridge [10,11]. Present hydrothermal activity has been observed at two principal l{xzations each about 100 m west of ridge axis ([12]; S. H a m m o n d , personal communication). Hydrothermal plumes were identified in real time by temperature ( A T ) and light-attenuation (Ac) anomalies. In the deep waters of the N o r t h Pacific, potential density (oa) and potential temperature (0) are linearly related except where influenced by hydrothermal or conductive heating from the seafloor. Lupton et al. [7] show that the hydrothermal AT is the deviation from this relationship along the 0 axis (Fig. 2). Plume signals were identified equally well by increases in light
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Fig. 2. Graphical representation of a hydrothermal temperalure anomaly (AT') based on a tow across the SSS vent field. Outside the plume the oo vs. 0 relationship is linear: within the plume a temperature anomal',' of - 0.025°C is centered around the 27.70 a,~ surface•
attenuation, which demonstrated a linear relationship to the concentration of suspended particulate matter. Light-attenuation anomalies were calculated as the increase over the regional light-attenuation minimum between 1000 and 1500 m, a value of 0.39 m ~ at the SSS vent field and 0.40 m ~ at the ES vent field. Although both temperature and light-attenuation anomalies originate from hydrothermal venting, their relative source strength may vary significantly between individual vent types. Furthermore, relative changes in the downcurrent intensity of each may serve to identifF' nonconservative sources and sinks of the particulate fraction of the hydrothermal emissions. Particulates for Fe and S determinations were captured on 0.4-btm-pore-diameter Nuclepore m e m b r a n e s by pressure filtration and then analyzed at sea using energy dispersive X-ray fluorescence spectrometry and thin-film calibrations [19,20]. NBS, USGS, and National Research Council of C a n a d a reference particulates were used as standards. The analytical precision was about 2 and 10~, respectively, for Fe and S. 3. Formation of the neutrally-buoyant plume Axial tows $2 at the SSS (Fig. IB) and C19 at the ES (Fig. IC) describe the distribution of hydrothcrmal emissions within a few kilometers of a source vent. At the SSS vent field, tow $2 passed about 200 m west of Vent 3, a known high-temperaturc discharge site [13]. Rapid dilution of the Vent 3 discharge created a neutrally-buoyant plume clearly defined by the vertically symmetrical distribution of Ac and AT contours confined below a sharp density gradient - 250 m above the valley floor (Fig. 3). The core of the plume, skewed by a northward advcction c o m p o n e n t in the valley, was centered on the 27.70 o~ surface - 150 mab in a nearly homogeneous b o t t o m layer. The absence of a clear above-bottom plume signal from Vents 2 or 4 suggests little or no high-temperature discharge at those sites. Hydrothermal plumes in the axial valley of the ES (Fig. 4) were similar to those at the SSS. The south-to-north C19 tow along the west side of the valley axis passed through two separate plumes - 2.5 km apart. We thus infer the presence of at least two distinct vent sites, herein called Vent 1 and Vent 2. The location of Vent 1 (map coordi-
63
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Fig. 4. Transects of temperature and light-attenuation anomalies and potential density from a portion of tow C19 (0607-0900 UTC, 25 June 1985) along the axial valley of the ES vent field. The neutrally-buoyant plume is centered on the 27.695 o0 surface. Vent 1 is at map coordinate X = 1.2, Y = 1; Vent 2 is at X = 1.8, Y = 3 (see Fig. IC). Inset shows the actual tow track.
64 nates X = 1.1, Y = 0.8 in Fig. 4) agrces with that of the extensive, high-temperature site described and sampled by "fivey and Delaney [12]. Plumes from both vent sites, clearly skewed to the south by the local currents, were centered 100-200 m above bottom on the 27.695 a0 surface. Maximum values of both AT (0.060°C) and Ac (0.110 m i) observed on tow C19 were - 50% higher than the corresponding values over the SSS, the regional data presented below suggest that this increase reflects a difference in mass flux between the two areas, rather than just a near-field sampling bias. A plot of AT vs. kc (Fig. 5) illustrates the varying relationship between conservative and nonconservative species in the near-field plume. We hypothesize that the plume-core end member at each vent field was created principally by emissions from high-temperature "black smokers" that provide the buoyant force to lift the plume 150- 200 m above the sea floor. At the core of the plutne, the original hydrothermal fluid (at a nominal temperature of 350 ° C and density of 0.69 g/l) has been diluted by a factor of - 6000 (to 0.04 ° (" above ambient) at the SSS and - 4000 (to 0.06 ° (" above ambient) at the ES. The Ac/A'I" ratio decreases as the plume is further diluted. The group of points centered at A T = 0.035°( `, A c = 0 . 0 2 m ~. represent an anomalously reduced At~AT ratio in roughly the lower 100 m of the water
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column at each vent field. These points may represent the influence of warm springs, which are more widely distributed than black smokers along the valley floor [13]. Diffuse fluid discharge from such springs would supply additional heat. but probably few particles, to the lower water column. North of Vent 3 at the SSS, for example, the Ac distribution becomes vertically uniform in the lower 2(10 m whereas the AT distribution steadily increases approaching the seafloor (Fig. 3).
4. Advection of the plume Mixing of the hydrothermal fluid and the ambient bottom water in the axial valley creates a plume occupying a very narrow o0 range (Figs. 3 and 4). Advcction spreads the plume along isopycnal surfaces, and as it passes out of the axial valley it becomes an above-bottom feature with a vcrtical density gradient much weaker than that of the ambient water column over the same depth interval. At the SSS vent field, the plume spread on the 27.70 a, surface (Fig. 6), reducing the doo/d: gradient between the 27.695 and 27.705 a0 surfaces from a non-plume value of 9.1 × 10 s k g / m '~ to 4.4 × 10 -5 k g / m 4 within the plume. Isopycnal gradients in AT within and outside the advecting plume were ncgligible. A narrow zone with temperature gradients as high as 0 . 0 2 ° ( ' / k n l along the 27.70 an surface marked the lateral boundaries of the plume. f h c along-isopycnal distribution of particles on tow, C6 was patchier than that of temperature (and presumabl 3 other dissolved, conservative substances). Light attenuation values in the core of the plume steadily increased with distance from the center of the axial valley, reaching a maximum near the edge of the plume (Fig. 6B). Similar downstream increases in light attenuation were observed on tows (7"14 and C15. -['he lack of any corresponding increase in the A7" signal on these tows suggests that the particle variations resulted not from multiple or variable vent sources, but from precipitation of dissolved trace metals in the plume. Transmission-electron-microscope studies of particulate matter from the SSS plume [21] have revealed a high concentration of amorphous hydrous Fe-oxide precipitates+ as well as deposits of Mn and Fe on bacterial capsules. Erosion of sediments from the axial-valley walls might add
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particles to the plume, but the consistent location of the particle maximum above the sea floor and at the same depth as the AT maximum does not support an erosive source for the excess particles, Furthermore, the mean current speed measured 100 m above the crest of the west ridge was 6.3 _+ 3.3 c m / s , insufficient to significantly resuspend bottom sediments. Speeds > 10 c m / s occurred during < 5% of the record. Cross-plume tows C5 and C6 also detected a secondary thermal and particulate plume centered on the 27.707 o0 surface (Fig. 6), a density surface that nowhere intersects the axial valley. The 27.707 Oo surface on both tows intersected the west flank of the ridge at - 2300 m along the upslope border of a broad plateau; this secondary plume is circumstantial evidence for the presence of off-axis hydrothermal venting. The existence of off-axis hydrothermal plumes is consistent with recent thermal models [22] and chemical evidence [23,24] that require as much as 80% of the total hy-
drothermal circulation to occur off-axis. If partitioning on that scale does occur at the SSS, then off-axis venting must be mostly low-temperature and widespread, because we found no evidence of deep plumes with temperature and particle anomalies characteristic of high-temperature axial emissions. The physical evolution of the ES plume was mapped by a series of cross-plume tows extending as far as 7.5 km downcurrent from Vent 1, or about 4.1 days travel time based on a mean velocity of 2.1 c m / s at 205 ° during the survey period ([16]: S. Roth, personal communication). Crosssections of the AT contours describe a plume positioned initially over the western margin of the valley floor (Fig. 7A) (the location of submersibleobserved venting [10,11]) and advected southward approximately along the axial-valley strike. The plume was centered on the 27.695 o8 surface on all transects. Although the eastern boundary of the plume was everywhere sharp, the plume extended
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Fig. 7, (A) T e m p e r a t u r e a n o m a l y a n d (B) l i g h t - a t t e n u a t i o n a n o m a l y o n five c r o s s s e c t i o n s o f the ES p l u m e f r o m 0 to 7.5 k m d o w n c u r r e n t of Vent 1 (see Fig. IC). E a c h t r a n s e c t is a p p r o x i m a t e l y n o r m a l to the axial valley, a n d e a c h is a l i g n e d with respect to the crest of the west ridge. T h e c r o s s section at 0 krn crosses the Fig. 4 t r a n s e c t at m a p c o o r d i n a t e X = 1.2, Y = 1. O t h e r c r o s s s e c t i o n s are lettered to c o r r e s p o n d w i t h Fig. 8. T h e thin h o r i z o n t a l line o n e a c h t r a n s e c t m a r k s the p o s i t i o n of the 27.695 o 0 surface.
67
westward beyond our tows. Values of AT varied only slightly along the plume core. Occasional downstream AT increases could result from inhomogeneities in the plume, or from secondary vent sources.
Cross-sections of the Ac contours describe a more compact and symmetrical plume (Fig. 7B). Unlike the AT plume, Ac values at the plume core steadily decreased downcurrent from Vent 1.
5. Regional distribution of the plume The regional dispersal of the plumes can be mapped by plotting AT and Ac values on the isopycnal spreading surface. Areal maps of the ES plume (Fig. 8) on the 27.695 o0 surface clearly show the origin of the regional plume from at least two vent sites in the study area, and its advective transport to the south. The orientation of the plume almost exactly matches the direction of the net current during the survey (2.1 c m / s at 205 °).
TEMPERATURE
129°12 ' I0 -
I0'
The mean velocity for a 27-day period prior to and during the survey was 1.5 c m / s at 170° (S. Roth, personal communication). The areal distributions of AT [8, fig. 1] and Ac (Fig. 9) on the 27.70 o0 surface at the SSS site display many of the same features of the ES maps. The plume at the SSS site apparently evolves from a hydrothermally-active area larger than that at the ES and is transported northward rather than southward. The record from the V1 current meter at 2030 m ( - 2 7 . 7 0 % surface) indicates that the mean velocity for 75 days prior to the survey was 1.3 c m / s at 356 °. The slight westward displacement of the plume relative to the vent locations was evidently caused by a shift of the net flow to the northwest during the 15-day survey, including this shift changes the mean velocity to 1.2 c m / s at 352 °. The 90-day mean velocity along the plume axis (20 o ) was 1.0 c m / s . Temperature anomalies along the axes of both plumes were uniform at distances beyond about 2
ANOMALY
08'
LIGHT-ATTENUATION
06'
04'
129°12 '
;-
I0
"
8
I0'
08'
ANOMALY
06'
04'
J-
"
I
- 48o00 ,
- 48000 ,
6:
6 4
i
i
4
I
4
"
- 4 7 ° 58'
- 47o58 ,
2 I
v >.-
0
N
0 ~,
56'
-56' -2
-2
-
i
i
-4 -~
-4-I -
54'
- 54'
I
-6-
-6 J
i -84-6
I ---
-8-T--
-4
-2
0 X(km)
2
4
-6
"
q
-4
-.
~
-2
-.,
-
0
-,-
2
-
4
6
X(km)
Fig. 8. Areal m a p of (A) t e m p e r a t u r e a n o m a l y a n d (B) l i g h t - a t t e n u a t i o n a n o m a l y on the 27.695 o o surface from all tows over the ES in relation to the strike of the axi',d valley, the k n o w n vent fields ( I ) , a n d the current m e t e r / s e d i m e n t trap m o o r i n g ( - ) . C o n t o u r interval is 0.01° C for t e m p e r a t u r e a n d 0.01 m - l for light a t t e n u a t i o n .
68 LIG H T - A T T E N U A T I O N A N O M A L Y 130"32'
30
28'
26
21
24
22
I
18
~
15 -
0 --
<010 I m
I
0.01
--
(0102
m "l
0,02
--
(0+03
m 1
20 l
X
x]
16'
14
,.~"~ i a l ,,~"$trlk e
44¢50 ,
,[' .:.:"
I
X
>0.03 m
18
~'~'"
"
48
,]~
" :~ \
12
:
,:" 46"
~ :r'
,
d;' 9
" ~ "--
'
44'
"
40 A
o
iN -3
;::: -12
-9
-6
-3
0
1 3
6
9
12
38
15
X(km)
Fig. 9. Areal map of light-attenuation anomaly on the 27.700 surface from all tows over the SSS in relation to the strike of the axial valley, the known high-temperature vents (O), and moorings fit). Contour interval is 0.005 m- 1.
o0
km from the principal vent source. The value of the anomaly in the ES plume core was twice that of the SSS plume core, however, and temperature gradients at the lateral boundary of the plume were as high as 0 . 0 8 ° C / k m , or four times higher than found at the SSS site. The alignment of the plume axes with the principal flow direction, and the sharp property gradients surrounding the plumes, demonstrate that the horizontal plume transport was advectively, rather than diffusively. controlled. The uniform AT distribution within each plume suggests that temporal aliasing problems were minor. Diurnal and semidiurnal tidal excursions at each site were < 1 km, much smaller than the horizontal scale of the plumes. The areal distribution of particle anomalies, while similar in outline to the AT plumes, show considerably more internal variation. At the SSS site, the Ac values in the plume core were patchy, commonly showing isolated highs clearly separated from the principal vent field. Because of the uniformity of the AT distribution, we speculate that this patchiness is evidence of in situ precipitation of hydrothermal material rather than temporal or spatial source variability. The ES particle plume displayed a different trend. Values
of Ac. in the plume core were not patchy but decreased sharply in the first kilometer away from Vent 1 and continued a steady decrease for as far as we tracked the plume, suggesting that deposition rather than precipitation regulates the particle concentration, The particle plume at the ES was more tightly-constrained about the axis than the temperature plume, which also may be a result of particle deposition. Differences between the SSS and ES sites in the downcurrent trends of particle anomalies are quantified by a plot of At/AT vs. downplume distance at each site (Fig. 10). At the SSS, the Ac/AT ratio shows no consistent downplume change, varying between 1.25 and 0.63 before reaching a constant value of 0.82. At the ES. this ratio falls nearly monotonically from a high of 2.0 at Vent 1 to 0.7 9 km downplumc. The only interruption in the ES trend occurs 5 km from Vent 1 where the At~AT ratio decreases because of an increase in AT at the plume center ( X = - 0 . 5 , Y = - 4 in Fig, 8). This increase may indicate either the location of a small vent site or heterogeneity in the plume from Vent 1. Sediment trap measurements of the fallout rate of plume particulates at each site support the site differences expressed in Fig. 10, even though only a limited number of samples are presently available. Deposition of particles from the SSS axial valley plume was measured from June 7 to July 14, 1985, by sediment traps deployed on mooring VT, 300 m west of Vent 3 (Fig. IB). Deposition into a control trap 300 m above the valley floor, well above any plume influence, was 0.027 g / m ? d. Traps within the plume collected a flux of 0.032 g / m 2 d (55 mab), 0.029 g / m 2 d (20 mab), and 0.028 g / m 2 d (5 mab), not significantly differcnt than the control trap. Although specific hydro-
•
SSS ES
1 b* m l
0
~
4
0 0 10 ~ISTANCE IKIrl
12
14
16
Fig. 10. Plot of Ac/AT vs. distance down the axis for plumes emanating from the SSS and ES vent fields.
69
thermal minerals were found in the plume traps [25], the vertical mass flux of hydrothermal particles, even as close as 300 m to Vent 3, was low enough to have only a negligible effect on plume concentrations. (A similar conclusion was reached by Baker et al. [26], from earlier sediment trap results.) The deposition rate of Fe in the three lower traps, for example, averaged 67 + 24 m g / c m 2 ky, identical to the mean deposition rate (67 m g / c m 2 ky) in surface sediments from cores collected 15 km and 43 km from the ridge axis (G.J.M., unpublished data). In contrast, sediment traps from the ES mooring recorded sharp increases at and below plume depth in the vertical flux of hydrothermally-enriched elements Fe, S, Cu, Mn, and Zn, as well as of hydrothermal precipitates such as metal sulfides and anhydrite [17]. The vertical flux of Fe below the plume was more than 100 times higher than that determined from sediment cores collected 100 km off-axis [15]. A direct comparison between the near-field deposition rates of hydrothermal emissions at the SSS and ES vent fields must be made with caution, however, because near-field variability in plume advection and particle settling is expected to be high. The contrasting At~AT trends of the two plumes evidently result from contrasting hydrothermal fluid chemistries that create precipitates with different settling characteristics at each site. Hydrothermal fluids collected from three orifices at the SSS vent field in 1984 were metal-dominated, with a mean F e / S molar ratio of 4.3 + 1.0 [13]. Conversely, nearly undiluted fluid collected from a single orifice at the ES vent field in 1984 was sulfur-dominated, with an F e / S molar ratio of 0.23 (R. McDuff, personal communication). These chemical differences are maintained in the bulk chemical analyses of particles collected from throughout the SSS and ES neutrally-buoyant plumes. At the SSS site, the mean concentration ( + o ) of Fe in the suspended particles (18.0 + 7.0%, n = 23) overwhelms that of S (0.61 + 0.13%, n = 22), whereas at the ES site the concentrations of each are more closely matched (6.9 + 2.7%, n = 19, for Fe, compared with 1.6 + 0.5%, n = 19, for S). The particulate Fe percentage is significantly correlated with light attenuation only in the SSS, plume (Fig. llA), whereas the particulate S percentage is significantly correlated with light at-
e~-
A
•
E
e.48-
B
?
i0 4 :.
.
•
~ e44~
! _.: Fe I%1
S (%)
Fig. II. A. Particulate Fe vs. light attenuation from discrete samples collected above both vent fields. Linear regression of SSS samples ( y = 0.0013x +0.386) yields a correlation coefficient r = 0.89. B. Particulate S vs. light attenuation from discrete samples above both vent fields. Linear regression of ES samples (y = 0.035x +0.367) yields a correlation coefficient r = 0.85.
tenuation only in the ES plume (Fig. llB). We hypothesize, therefore, that the patchy but roughly constant Ac/AT ratios in the SSS plume arise from a fine-grained particle suite of amorphous hydrous Fe-oxides augmented by ongoing precipitation and reduced only slowly by deposition. The nearly steady decrease in Ac/AT ratios in the ES plume result from a two-component system: a background fine-grained particulate Fe-oxide phase that settles slowly and advects along a fixed density surface, and a fraction of larger and denser Fe-sulfides that settle rapidly and cause a continual attenuation decrease in the dispersing plume. The chemistry of the ES plume is similar to that of the 21°N vent field, where the F e / S molar ratio in the high-temperature vent fluids - 0 . 1 9 [27] and both oxide and sulfide minerals have been identified in the lower 20 m of the buoyant plume [28]. Regional plume maps are useful also for discovering vent fields, although the interpretation of non-synchronous water column data in a varying current field has some inherent uncertainty. At the ES site (Fig. 8), isolated highs in both Ac and AT were found outside of the primary plume and downcurrent of Vents 1 and 2. The strong anomaly at map coordinate X = 3, Y = 2.5, on the top of the east wall of the valley may indicate a new vent site, although an axial crest location would be unusual. No plume signals were detected farther north than Vent 2 (map coordinate X = 2, Y = 3). South of Vents 1 and 2, plumes from two possible
70 secondary sources within the axial valley were detected (at X = 1.2, Y = - 1.5, and at X = - 0 . 5 . Y = - 4 ) , although the fact that both plumes were centered on the 27.695 o0 surface could mean that these signals originated at Vent 1 or 2 and represent small perturbations resulting from changes in the vent flux or the advective field. It appears clear from this survey that the primary vent field within this 14-kin section of ridge is at Vent 1.
6. Heat and particle flux measurements Comprehensive information on plume distribution allows not only a description of the physical evolution of the plume but also an estimation of the source strength of vent field emissions. Transport of a conservative constituent in a neutrallybuoyant plume equals the total vent-field flux of that constituent, provided that advection rather than diffusion controls the plume distribution. and that the plume distribution is stationary on a time scale greater than the advective flushing time (i.e., vent field diameter/mean velocity) of the vent field. The fact that a net flow of 1-2 k m / d produced strongly asymmetric plumes at both sites makes these assumptions reasonable. The ventfield flux can be calculated as the difference between plume transport through a cross section downcurrent of the vent field and normal to the plume axis, and the transport through an identical upcurrent cross section. The heat flux Q, for example, through a cross section is:
Q= oc,[E , aT']u
(1)
where specific heat pCp = 4.2 M J / m 3 °C. A, = area of each AT, interval on the cross section, and U = component of mean velocity along the plume axis. The use of equation (1) implies that the heat transport is dominated by the mean flow and that time-dependent fluctuations are minor. Although the lack of a long-term simultaneous velocity and temperature record from within the plume at either site precludes a direct measure of the turbulent heat flux, the applicability of equation (1) is implicitly supported by the nearly uniform and steady temperature distribution within each plume and by the agreement of the plume orientation and mean current velocity axes at each site. Temperature fluctuations within the plume, attributable to
variations in source strength, alteration of the plume dimensions by tidal current fluctuations during the cross-plume transects, and additions from downcurrent vent sources, can be estimated by averaging the heat content from several crossplume transects. At the ES site, six transects across the plume (lines B - G , Fig. 8) were occupied downcurrent of the major vent sites and one transect (line A, Fig. 8) upcurrent. The net heat in each cross-section was determined by subtracting ~A, AT, of line A from Y~A, AT, of lines B - ( ; , the mean heat content determined from all six transects (79,000 MJ in a 1-m-thick cross section) was about three times the standard deviation (26,000 M J), indicating that net heat transport by correlated temperature-velocity fluctuations within the plume was small. Confidence limits on the plume flux are also a function of variability in the current flow. The rms error estimate E of the mean velocity, determined using the central limit theorem, is given by: E = o/(T/~)
lj2
where T is the record length, o is the standard deviation of the flow along the vector-mean axis. and ~" is an independent time scale estimated as the area under the autocorrelation function for each record [29]. The net heat transported in the ES plume calculated using equation (1) is thus: Q = (79,000 + 26,000 M J / m ) × (0.021 + 0.012 m / s ) -- 1700 + 1100 MW This result must be a lower bound because the western boundary of the plume extended beyond the survey area (Fig. 8A). Even though the uncertainty is high (about 60% of the mean), it represents a significant improvement over the initial survey of Crane et al. [30], which suffered from a lack of both current data and information on the orientation of the plume (only a single axial tow was made). Crane et al. [30], using equation (1) and assuming the plume was transported normally to the axial strike at U = 1 c m / s , estimated a heat flux of 12,600 MW for this 10-kin section of the ES. Baker and Massoth [8] used the above procedure to calculate the heat flux from the SSS site,
71
obtaining a value of 580 + 290 MW. Because multiple downcurrent cross sections were not available at that site, the reported uncertainty was based only on the error estimate from the current-meter measurements and did not include any uncertainty from the plume cross sections. If we assume that the uncertainty in the plume measurements is similar at both sites and use the ES data to determine the relative amplitude of the e r r o r / m e a n ratio, then a more conservative calculation of the heat flux at the SSS site is: Q = (57,000 5- 19,000 M J / m ) × (0.0105 5- 0.0053 m / s ) = 580 5- 351 MW Crane et al. [30] calculated a flux of - 3 0 , 0 0 0 MW for a 1 c m / s current across a 40-km ridge section centered on the SSS ventfield. Results from other models used by Crane et al. [30] gave lower power estimates for both sites. Other estimates of hydrothermal heat flux have been based on extrapolations from temperature and flow at individual orifices, and on velocity and hydrographic measurements in a buoyant plume. Both approaches require a submersible. Converse et al. [31] calculated the heat flux from hot and warm vents at 21 ° N on the East Pacific Rise using direct measurement of a few vents and extrapolation to the entire vent field by means of a photographic survey. They estimated the heat flux from high-temperature vents to be 220 + 80 MW, and flux from warm-water springs to be 120-9000 MW (the warm-water vent estimate being highly speculative). The total heat flux at both Juan de Fuca Ridge vent fields thus appears to be of the same order as the 2 1 ° N field, although the confidence limits on all the data are broad. Little et al. [32] collected simultaneous velocity, temperature, and conductivity data within the buoyant plume from a small ( - 4 m by 5 m) sulfide structure venting hot water at 11 ° N on the East Pacific Rise. Using non-linear plume theory, they calculated the heat flux from this multiplechinmey mound as 3.7 + 0.8 MW, about a factor of 100 lower than the vent field estimates at SSS, ES, and 21 ° N. This difference appears reasonable considering that both the SSS and ES fields are" composed of several individual vent sites each extending for hundreds of meters along strike
[10-13]. Individual vents measured by Converse et al. [31] had a typical heat flux of 0.5-10 MW, in agreement with the results of Little et al. [32]. Neither Converse et al. [31] nor Little et al. [32] were able to make a reliable estimate of the diffuse heat flux. A more tractable solution may lie in calculating the diffuse flux as the difference between total plume flux as outlined in this paper and the flux from individually-measured, hightemperature, point sources. The mass flux of a non-conservative plume constituent, such as hydrothermally-precipitated particles, can also be calculated, but the results must be cautiously interpreted. The flux of particulate components through a downcurrent cross-section may be greater or less than the vent field source depending on the balance between precipitation and deposition during advection. The non-uniform distribution of particles in each plume also means that turbulent fluxes, unevaluated here, may be important. The multiple cross-sections occupied at the ES allow us to measure downcurrent changes in the mass flux and to compare them with the heat flux. Unlike the results for heat flux, a significant downcurrent decrease occurs in mass flux: 4 2 0 + 2 4 0 g / s through transect B, 546 + 312 g / s through C, 336 + 192 g / s through D, 227 + 158 g / s through E, 2 5 2 + 144 g / s through F, and 204-t-116 g / s through G. This decrease could be explained by depositional losses from the plume, or by dispersal of the western boundary of the plume beyond the occupied cross-sections. Although the present data are not conclusive, the high deposition rates of hydrothermal particles within the axial valley [15,17], the downcurrent uniformity of the heat flux, and the chemical nature of the ES plume suggests that deposition is more important than dispersal in creating the observed trend in mass flux. Calculation of the particle flux at the SSS vent field yielded 92 5- 48 g / s . This figure represents the net mass transport through a cross-section 5 km downcurrent of Vent 3. No estimate of downplume variability is available, but the trend shown in Fig. 10 suggests it is small. 7. Conclusions
Dilute, neutrally-hydrothermal plumes can be readily identified and tracked by m a p p i n g
72
anomalies of temperature and light attenuation. Plumes from two vent fields on different segments of the southern Juan de Fuca Ridge show similar characteristics of formation and dispersal. Within the axial valley, the buoyant hydrothermal emissions rise 150-200 m above the seafloor until they reach a stable density surface. The local current field advects the neutrally-buoyant plume layer along this density surface; outside the axial valley it is readily observable as a 200-m-thick layer of anomalously weak vertical density, temperature, and light-attenuation gradients capped by a thin layer of much stronger gradients. At the Southern Symmetrical Segment, a secondary plume found deeper than the primary axial plume suggested the presence of off-axis venting. At both sites, point and diffuse hydrothermal sources emanating from hydrothermally-active regions as long as 10 km coalesced into a single elongated plume with a uniform internal temperature distribution and sharp lateral boundaries. Particle distributions within the two plumes differed because of their contrasting chemical signatures. The SSS plume is Fe-rich and primarily forms fine-grained amorphous hydrous Fe-oxides that disperse quasi-conservatively. The ES plume, with a more evenly matched Fe and S molar abundance, precipitates a significant amount of Fe-sulfide particles that settle rapidly and cause particle concentrations in the plume to sharply decrease with distance from the vent source. Advective transport of heat and particles in the plumes was used to estimate the total source strength of each vent field. Heat flux was 580 + 351 MW in the SSS plume and about three times higher, 1700 ± 1100 MW, in the ES plume. Particle flux was 92 ± 48 g / s at the SSS and varied from 546 ± 312 g / s to 204 ± 116 g / s at the ES depending on distance from the vent field. Regional plume mapping facilitates the design of an efficient sampling program around a vent field, guides the interpretation of the resulting distribution of physical, chemical, and biological parameters, and provides an effective technique for measuring the integrated flux strength of an entire vent field. Hydrothermal plumes are dynamic features with sharp gradients, and without adequate information on their distribution it may be impossible to accurately describe the physical and chemical evolution of the dispersing hydrothermal emissions.
Acknowledgements This research was supported by the N()AA VENTS program. R. Thomson constructively reviewed the manuscript. Wc thank G. Lebon and I). Tennant for help with shipboard sampling and data recovery, and H. Milburn for expertise in acoustic navigation. S. Hammond supplied the Sea Beam-derived bathymetry for each vent field. S. Roth and J. Dymond generously shared the preliminary results of their current meter deployment at the Endeavour Segment vent field.
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Fuca Ridge: preliminary observations from the submersible Alvin, Geology 14, 823-827, 1986. E.T. Baker and H.B. Milburn, An instrument system for the investigation of particle fluxes, Cont. Shelf Res. 1. 425-435, 1983. S. Roth and J. Dymond, Hydrothermal particle flux: Endeavour Ridge Phase I, EOS 66, 930, 1985. S. Roth and J. Dymond, Where wafts the hydrothermal plume? A mt×~red-sensor record of lateral particle flux above Endeavour Ridge, EOS 67, 1027, 1986. J. Dymond and S. Roth, Plumefall: A record of the settling particle flux on the Endeavour Ridge using moored sensors, EOS 67, 1027-1028, 1986. W.R. Normark, J.L. Morton, R.A. Koski, D.A. Clague, and J.R. Delaney, Active hydrothermal vents and sulfide deposits on the southern Juan de Fuca Ridge, Geology 11, 158- 163, 1983. G.S. Holmes, The limitations of accurate "' thin-film" X-ray fluorescence analysis of natural particulate matter: problems and solutions, ('hem. Geol. 33, 333-353, 1981. R.A. Feely, G.J. Massoth, E.T. Baker, J.F. Gendron. A.J. Paulsoa and EA. Crecelius, Seasonal and vertical variations in the elemental composition of suspended and settling particulate matter in Puget Sound, Washington, E,stuarine Coastal Shelf S.ci. 22, 215--239, 1986. J.P. Cowen, G.J. Massoth and E.T. Baker, Bacterial scavenging of Mn and Fe in a mid- to far-field hydrotherreal particle plume, Nature 322, 169 171, 1986. J i . Morton and N.H. Sleep, A mid-ocean ridge thermal model: Constraints on the volume of axial hydrothermal heat. J. Geophys. Res. 90, 11,345-11,353, 1985. T.S. Bowers and H.P. Taylor. Jr., An integrated chemical and stable-isotope model of the origin of mid-ocean ridge hot spring systems, J. Geophys. Res. 90, 12.583-12,606, 1985.
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