Deep-Sea Research l Vol 40, No 7, pp 1447-1457 1993 0967~d637/93 $6 00 + ('l t~] Printed m Great Bntam Pergamon Press Ltd

Hydrothermal effects west of the Juan de Fuca Ridge

GLENN A . CANNON,* DAVID J . PASHINSKI* a n d MICHAEL R . LEMON*

(Recewed 3 October 1991, m revised form 4 September 1992; accepted 9 September 1992)

Abstract--The Juan de Fuca Ridge m the northeast Pacific Ocean is a region of extensive hydrothermal acUvlty, and plumes emanating from the ridge can be dispersed by the oceamc orculatmn at about 2000 m Off-axis temperature-sahnlty characteristics and water-property maps suggest a history of vanability m hydrothermal actwity on the ndge. Distmctwe bulges m 0-S curves from stations several hundred kilometers westward have characteristics very similar to those near and over the ridge, possibly representing plume remnants from prior venting which detached from the source and drifted with the regional circulation. If they were advected from the ndge at about 1 cm s -1 , they would represent venting events which occurred 2-3 years earher. Although not concluswe, currents observed on the ridge in 1984-1985 showed a 6-month winter interval of continuous off-ax~s flow to the west, but, unfortunately, it occurred between annual shipboard observations Year-to-year variatmns are shown by relatively large increases m the near-ndge potential temperatures from 1987 to 1988, possibly suggesting another event that may propagate westward. The thermal observations here are consistent with simultaneous &stributions of sihca, suspended pamculates and helium described by others. The vanatlons m temperature, as well as flow variability near the ndge, are sufficiently large that they need to be considered when interpreting other water column observations, not only near hydrothermally actwe ridges but also at least several hundred kdometers away from the ventmg s~tes

I N T R O D U C T I O N

THE Juan de Fuca Ridge (hereafter JDFR) is a hydrothermally active spreading center off the west coast of North America extending about 450 km from 44 to 48°N (Fig. 1). The Pacific Marine Environmental Laboratory has been studying plumes emanating from vents on the ridge since the early 1980s with the overall goal to determine plume characteristics and their effects on the northeast Pacific Ocean. Near the southern end of the ridge (called Cleft Segment), neutrally buoyant plumes have been regularly observed just above the ridge crest, with characteristics of increased temperature and light attenuation (BAKER and MnssoTn, 1987). Twice, much larger plumes in size and tempera- ture anomalies (megaplumes) have been found at shoaler depths above the ridge (and above the more regular plumes), which apparently are the result of episodic venting events (BAKER et al. , 1989). Although most of the plumes in Baker et al. 's studies appear to extend westward off the ridge, the ultimate fate is unclear because little has been known of the flow at these depths. The upper ocean is in the West Wind Drift, but flow at depth appears to be weakly westward.

CANNON et al. (1991) described long-term moored current measurements from the

*Pacific Marine Environmental Laboratory/NOAA, 7600 Sand Point Way NE, Seattle, WA 98115, U.S.A.

1447

1448 G A CANNON et al

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Fig. 1. Potential temperature in 1987 along the lsopycnal o8 = 27.67, which occurs at the approximate depth of hydrothermal plumes on the Juan de Fuca Ridge Long-term average current vectors greater than 1 cm s -1 (scale shown by 4 em s -1 vector) at about 2000 m from 1986-1987, 1987-1988, and 1988-1989, are superposed (from CANNON et al., 1991), and the bold vectors represent an hypothesized regional flow (partly based on REID, 1981). The average depth of the lsopycnal surface is about 2120 m Circled stations are locations of 0-S curves in Fig 3, and squares indicate off-axis mooring locations with weak currents less than 1 cm s-1. The ndge and seamount chain locations are shown schematically by the solid lines between 44--48"N and 129-133°W. The ridge orientation is along 20°T, and the seamount chains are nearly perpendicular to the northwest

(insert shows Axaal-Brown Bear---Cobb--Eickelberg in the north; Vance and unnamed, south).

sou the rn J D F R , which s h o w e d r e l a t i ve ly i n t e nse flow pa ra l l e l to the r idge go ing no r th - w a r d on the wes t s ide and s o u t h w a r d on the eas t . T h e y sugges t ed p l u m e s e m a n a t i n g on the wes t s ide o f the r idge p r o b a b l y a re a d v e c t e d first a long the f lanks and then t u r n e d wes tward by s e a m o u n t cha ins tha t a r e n e a r l y p e r p e n d i c u l a r to the r idge . T h e fa te of p l u m e s on the eas t s ide is less ce r t a in . A b o v e the r idge cres t the cu r ren t s we re m o r e va r i ab le , inc lud ing t idal , ine r t i a l , and a b r o a d b a n d 4-day osc i l l a t ion tha t is t hough t to be w ind - induced . V a r i a t i o n s in f low on l o n g e r t ime scales , h o w e v e r , s e e m e d i m p o r t a n t in d e t e r m i n i n g h o w p l u m e s ge t of f axis. F l o w va r i a t i ons across the r idge las t ing f rom one to a few m o n t h s we re c a p a b l e of t r a n s p o r t i n g p l u m e s off axis b o t h to the wes t and eas t . T h e p roces ses r e s p o n s i b l e for these v a r i a t i o n s we re no t obv ious . O n c e away f rom the r idge , the specu l a t ed fa te o f p l u m e s was b a s e d on w a t e r p r o p e r t y o b s e r v a t i o n s , mos t of which we re f a r the r o f f shore (REID, 1981). F r o m R e i d ' s m a p o f g e o p o t e n t i a l at 2000m re la t ive to 3500

Hydrothermal effects west of the Juan de Fuca Ridge 1449

m, we hypothesized that flow near the ridge might converge from north and south, possibly topographically caused, and then go westward (bold arrows in Fig. 1). In order to test this idea, a CTD cruise was made in 1987 covering the area westward from the ridge along 45°N to 145°W, north to 50°N (Ocean Station Papa), and back along the Papa line to the ridge (Fig. 1). Current meter moorings were deployed both on the ridge and westward for 1987- 1988. Others were deployed from 1986-1987 and in subsequent years from 1988-1989 and 1989-1990.

This paper presents a description of CTD observations, along with some moored current meter results, which suggest that hydrothermal temperature signatures can be seen at least several hundred kilometers to the west of JDFR, and that variations in venting may show up as relatively large off-axis variations in temperature from year to year. Far off-axis temperature anomalies in 1987 correspond to concurrent observations of high silica (FEELY et al., 1992) and high helium (J. LuvroN, personal communication). Elevated tempera- tures in 1988 correspond to concurrent observations of high concentrations of suspended particulate matter (FEELY et al., in press). Although the temperature characteristics here are not as pronounced as those of the large tongue extending from the East Pacific Rise (hereafter EPR) (REIn, 1982), they may represent the onset of such a feature that is forming and will grow.

OBSERVATIONS

The primary data for this report come from about 85 CTD stations made in July-August 1987 with a Sea-Bird CTD from the NOAA ship Discoverer. A smaller area near and west of the ridge was sampled in more detail in summer 1988. Observations were made to 5 m off the bottom using an acoustic altimeter in depths ranging from 2100 m on the ridge to 4500 m in the far west. Water samples were taken on the upcasts using a 12-bottle rosette sampler, and the final calibrated temperature and salinity accuracies were about 0.002°C and 0.003%o, respectively. Silica and suspended particulate observations (Feely) and helium observations (Lupton) were made at most of these stations. Current-meter observations were from year-long deployments of Aanderaa instruments (CAmqON et al., 1991). In this paper both year-long averages (Fig. 1) and monthly averages (Fig. 5) have been used.

Plume observations of BAKER and MASSOTH (1987) near the southern JDFR have shown that maximum temperature anomalies of about 0.05°C occur as slight bulges in potential temperature-salinity (0-S) curves and that plumes spread horizontally on an isopycnal surface near o0 = 27.67. Plumes sometimes sink as they move off axis, but they remain approximately on a o0 surface. Our data also show vertical spreading of isotherms within the ridge-crest plumes which decreases off axis. Jogce et al. (1986) have shown that temperature could be used as a passive tracer in a study of geothermal heating of abyssal water west of this region. This is partly possible because salinity increases with depth in the North Pacific. Thus, the focus of this study is to examine potential temperature distri- butions on the o0 = 27.67 surface corresponding to Baker's plumes to determine any westward effects from heating at the ridge.

RESULTS

The distribution of potential temperature on the isopycnal surface intersecting the neutrally buoyant plumes on the southern JDFR shows a temperature change of almost

1450 G A CAN~ON et al

0.07°C, with the coldest water occurring in the northwest corner (Fig. 1). The depth of this surface (not shown) varies from 2200 m in the northwest to less than 2100 m in the southeast. Salinity (also not shown) is very uniform, with a range of about 0.004%0 from south to north. The warmest temperatures, 1.76°C, occur at two locations, over and just west of the crest and along 45°N some 700 km west of the ridge between 135 and 140°W. The question to be addressed here is whether these warmer temperatures in the west could have originated at the ridge.

Long-term mean currents near the ridge, which are statistically significant and greater than 1 cm s -1 (CANNON et al., 1991), are superposed on the potential temperature distribution in Fig. 1. Although these data are from several years, they emphasize that the flow is relatively strong, and that it could be steered by the topography. The largest vector (4 cm s-l), which appears to cross topography, actually flows through the gap between Brown Bear and Axial seamounts. The one westerly vector (1.5 cm s -1) is along the southern side of the Vance seamount chain. Along the southern JDFR, the flow is northerly (3 cm s -1 ) on the west side and southerly (2 cm s -1) on the east. Thus, water from near the ridge on the west side appears to flow first northward and eventually westward. Moored observations from farther west and away from topographic features have shown some shorter intervals of weaker westward flow, but the long-term means are not significant. The bolder vectors are our hypothesized regional flow based on these observations and on extrapolating REID'S (1981) map of geopotential into this region. It also should be noted that the currents near the JDFR are about an order of magnitude greater than those so far observed on the EPR (S. RISER, Univ. Washington, personal communication).

Potential temperature-salinity diagrams in the high temperature region to the west show a distinctive bulge centered approximately at the same isopycnal as plumes at the ridge crest (Fig. 2). These bulges are characteristic of plumes over the ridge but extend over a larger depth range. Water in these depths would be expected to be a mixture of shallower Intermediate Water and deeper Common Water. In fact, other stations here exhibit nearly a straight line between the two, and straight-line 0-S curves have been observed in the Pacific (MuNK, 1966). The bulge indicates a warming along an isopycnal of 0.05°C relative to the straight line mixture of known water types. The only occurrences of water with similar 0-S characteristics occur below about 1800 m over the ridge crest and immediately to the west in the region of highest temperature m Fig. 1. Water in the southeast corner of our study area, a possible source from the south, is colder than the above regions at depths greater than 1800-1900 m. The picture may be further complicated in that the Intermediate Water (o 0 = 27.28) at depths of 800-900 m appears to be part of the West Wind Drift over most of the region, while near the ridge it is warmer, possibly coming from the south near the coast as shown in REID (1965). Common Water in this region occurs on an isopycnal (o0 = 27.76) with the coldest water along the northern boundary. Water at the depth of the potential temperature bulge seems to be meandering westward along 45°N (2000 m geopotental relative to 3500 m). Thus, it may occur in a region between waters flowing in different directions both above and below it. ROEMMICn and MCCALLISXER (1989) show opposing gyres in the northeast Pacific with the deeper one at mid-depths corresponding to our southwest flow. Deep water is suggested to flow from the north, but weakly, also as implied here.

In order to describe the spatial distribution of the 0-S bulge, a potential temperature anomaly section was created along 45°N using the western most station at 145°W as the

Hydrothermal effects west of the Juan de Fuca Ridge 1451

2.5 I I

700 Km West 45N, 139W ~ ' ~ ' ~

• 30 Km West" 27

! |

1.0 I I

34.4 34.5 34.6 34.7 Sahnity ( " / ,o )

Fig. 2 Potential temperature-salimty diagrams for three stations from 1500 m to the bottom within the two regions of highest temperatures shown in Fig. 1. The curvature of the westernmost station from a straight hne between shallower Intermediate Water and deeper Common Water

lmphes another source of water which may be heating at the Juan de Fuca Ridge.

reference and calculating anomalies along o0 surfaces (Fig. 3). The highest positive anomalies near 2000 m are 0.02°C and occur at 137-139°W and at the ridge, the same as indicated by 0-S curves. The fact that they are not as high as the 0.05°C implied by the 0-S curves indicates that there is possibly still an influence from the ridge as far as 145°W. All 0-S curves in the southern part of this region indicate at least a slight bulge. Using station Papa as a reference increases the anomaly by 0.01-0.02°C. Anomalies perpendicular to these along 134°W are positive only south of 45.5°1'4. Along 145°W positive anomalies are only south of 45°N. TALLEY and JOYCE (1992) show a similar section along 47°N, with anomalies of 0.01°C about as far west as our 0.02°C. FEELY et al. (in press) show the highest silica of hydrothermal origin occurring along 45°N, consistent with the thermal anomalies.

Of added interest are the much higher anomalies centered over the ridge at about 900 m. These are located in the Intermediate Water and probably are related to interactions of the West Wind Drift and the JDFR. The distribution of temperature on the shallower isopycnal surface (not shown) is relatively uniform and cooler to west; the warmer water over the ridge may come from the south along the coast (REID, 1965). RODEN (1987) has described topographic induced variability above the Emperor Seamount chain extending almost to the surface. His results support the shallower variability here, but further analysis would require a separate study. The cause of the off-axis intermediate anomalies between the two isopycnals on Fig. 3 is not clear. An interesting speculation might be that they represent old megaplumes. However, the depths are somewhat shallower than those described by BAKER et al. (1989), and J. LorroN (personal communication) and FEELY et al. (in press) indicate there is no helium or elevated silica, respectively, at these shallower

1452 G A CANNON et al

5OO

LOHGI'TUI)K 145"1/ 143°W 141 "V 13~W 137'11 131Y Dll 133"W 131 "11

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Fig. 3. Potential temperature anomaly on o0 surfaces along 45"N referenced to the westernmost staUon at 145°W The two isopycnals shown approximately are in plume water at the ridge crest (o0 = 27 67) and m shallower Intermediate Water (or o = 27.30). The hatched areas of high anomahes near tra = 27.67 correspond to the baghest temperatures on F~g. 1 and to the 0--S bulges in

Fig. 2

depths. It is curious, however, that one of these higher anomaly regions occurs above the highest anomalies at ridge-crest depths.

The warmer temperatures 700 km west of the J D F R perhaps indicate a prior event that has been advected from the ridge. I f it moved at a speed of about 1 cm s -1, it would take about 2-3 years to transit the approximate 10 ° of longitude, and the hypothesized venting event would have taken place in early 1985. During our early studies of the J D F R , there was an unexplained change in flow observed in late 1984 to early 1985 at a mooring on the west side of the ridge crest (Fig. 4; f rom CANNON et a l . , 1991). The two moorings shown as V1 and V5 in Fig. 4 were deployed during early summer 1984 shipboard plume obser- vations. Late in the summer a 6-7 month period of steady off-axis flow of about 1 cm s -1 commenced at V1 on the ridge crest. It was first thought to be quest ionable because it did not occur at V5 about 5 km to the west. We now believe that this off-axis flow could have been real, and once off the crest it became incorporated into the fairly regular, and more intense northward flow (2-3 cm s -1) along the ridge as described in CANNON et al. (1991). Also, the 200 km horizontal extent of the PVD is about the same as the distance between the four stations in the t empera tu re max imum along 45°N. Al though not conclusive, when combined with other current me te r information below, a case can be made for eventual west or southwest flow.

The nor thward flow along the J D F R is most likely turned westward by the Vance

Hydrothermal effects west of the Juan de Fuca Ridge 1453

t

I~s VS s ~/

I I

45"

44*30'

Fig 4. Progressive vector diagrams from about 2000 m near the west side of the Juan de Fuca Ridge (from CAN~ON et al., 1991). Moorings V1 and V5 are from 1984-1985; V8, 1985-1986; and V9, 1986-1987 V1 is on the west wall of the rift valley, and V5 is on the west flank about 5 km from

the center of the ridge. Note the PVD scale is &fferent from the chart scale.

Seamount chain. A more recent mooring (1988-1989) on the south side of that chain shows a long-term mean westward flow at 2000 m of about 1.5 cm s -1 (Fig. 1), which is slightly larger than speeds of 1.1 and 0.8 cm s -1 observed 500 m shallower and deeper , respectively. During winter the westward flow increases, and speeds of 3-5 cm s -1 were observed for about 4 months. West of this location, moorings near 45.25°N, 132°W and 45.75°N, 134°W (Fig. l ) have shown weaker long- term flows at 2000 m. However , at 45.75°N intervals of southwestward flow of 0.3-0.4 cm s -1 occurred for up to 3 months. Flow at 45.25°N was weak and nor thward, and it may have merged with the westward flow along the south side of the Vance seamounts . Figure 5 shows weak southerly flow at these sites for 1-month averages during 1987 CTDs , and it shows westerly flow during 1988 CTDs. Thus it seems possible that vented fluid, once separa ted f rom the source, first flows north along the ridge, is turned west by the Vance Seamount chain, and then continues west or southwest in meanders or eddies. Speeds are initially large near the ridge and seamounts , and up to 1 cm -1 may be a reasonable net westward flow over the region.

Al though the potential tempera ture-sa l in i ty curves are basically the same at the widely separated sites in Fig. 2, we would expect the far western tempera tures to decrease with time due to mixing. CTDs made near the ridge in early and late summer 1988 showed considerable change f rom the preceding year (Fig. 5). The water was everywhere warmer than in 1987, ranging f rom 1.77 to 1.83°C. Thus, perhaps a new surge of warmer water had

1454 G A CANNON et al

136"W 1330W 130°W 127°w

, F t I , / , I , I ~ I , I , I, , I L ~

Seomoun ts

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Fig. 5. Comparison of potential temperature on the isopycnal surface ao = 27.67 for 1987 and 1988. One-month average currents at 2000 m from just before or just after the CTD observations

- are superposed (scale is shown by the 2 cm s -1 vector) The Juan de Fuca Ridge and seamount chains are shown schematically by the solid lines (see Fig. 1 for names). The 1987 temperature data

are repeated from Fig. 1 for comparison with 1988.

commenced flowing westward f rom the ridge. One-mon th average currents at about 2000 m from year-long moorings ei ther recovered just before or deployed just after the CTD observations have been superposed on the potent ial t empera tu re distributions. There are off-axis components of flow over the ridge crest both to the west and east. Along the west side of the J D F R , the flow is nor thward parallel to the ridge, and then, particularly in 1988, it turns to the northwest. The highest t empera tu re region to the north and west in 1988 corresponds to high suspended particulates of hydro thermal origin described by FEELY et

al. (1992). In 1987 the weak southwestward flow may indicate the route of the 1.76°C water just south of that mooring. Again, the general ly west-southwest flow may take place as meanders or propagating eddies, and it is var iable in time. Station spacing is insufficient for further resolution.

The fate of plume water discharging to the east side of the J D F R is uncertain. This eastward discharge occurs particularly just south of the Vance seamounts where there is no wall on the east side of the rift valley. Wate r on the east side flows south along the ridge and possibly passes around the southern end through one of two gaps where the ridge turns eastward. Such a flow path could possibly explain the high tempera ture water off the southwest end of the ridge.

DISCUSSION

CTD observations west of the J D F R show that the effect of heating at the ridge may be seen at least as far as 1000 km. These studies, as far as we know, are the first to show the far

Hydrothermal effects west of the Juan de Fuca Ridge 1455

off-axis thermal characteristics which appear as a distinctive bulge in 0-S curves. Its vertical extent is through a depth range of about 1000 m, and a maximum temperature anomaly of 0.05°C occurs on about the same density surface as plumes over and near the JDFR. These anomalies are comparable with those observed in neutrally buoyant plumes over the southern JDFR, which extended vertically only 200-300 m (BAKER and MASSOa'H, 1987). Assuming the excess heat at the far western maximum was mixed during its 2-3 year transit from the ridge, the hydrothermal event that produced it would have been larger than any thus far observed. Current meter data, although not conclusive, suggest such an event may have occurred in 1984--1985 at a time between yearly cruises to the ridge. Unfortunately, the temperature sensors on the current meters at that time had insufficient resolution to detect significant increases of temperature in the westward flowing water. More recent observations have shown occurrences of increased temperature in month- long intervals of off-axis flow (CANNON et al. , 1991).

Although the observations here are insufficient to spatially define a plume, they can be used to estimate an order of magnitude heat flux corresponding to the western thermal anomalies. The flux of excess heat (Q) through a vertical cross-section normal to a plume axis is the product of the heat anomaly (H) and the plume velocity (V)

Q = H V = p C p A A T V ,

where pCp = 4 x 106 J m -3 (°C) -1, A is the cross-section area, and AT is the average temperature anomaly. The 0-S curve in Fig. 2 has an average anomaly of about AT = 0.03°C over a depth of 1000 m. The velocity is least well known, but based on an average flow from the JDFR of I cm s- i and that even higher speeds occurred near the ridge, Vwas assumed to be about 0.5 cm s -1. The excess heat flux is then

Q = 6 x 108 W km-width -1.

This flux is the same as that calculated by BAKER and MASSOTrI (1987) for a three- dimensionally defined neutrally buoyant plume over the southern JDFR. There are numerous uncertainties here, however, particularly the width and speed of the water with the temperature anomalies. But, the fact that it is the same order of magnitude suggests its probable origin at the JDFR. As the plumes move westward from the ridge, the new plume water could mix upwards with Intermediate Water and downwards with Common Water creating the larger vertical range of its effect in the west.

Studies of other water properties sampled simultaneously on these cruises also support the westward extension of effects of hydrothermal venting from the JDFR. FEELY et al. (in press) plotted silica for 1987 on the same density surface in Fig. 1 that showed the highest values over the ridge and westward along 45°N. They further showed a silica maximum at depths of 2000-3500 m with some closed-contoured maxima in the far west corresponding to high temperatures shown here. Combining the silica data with helium data, they estimated the hydrothermically derived silica in the section. Significant values extended to the western limit of the observations at 145°W. This fact supports our statement that the potential temperature anomaly section (Fig. 3) was low compared to the 0-S curve anomaly (Fig. 2) because some thermal effect still existed at 145°W. Helium distributions are similar to silica (J. LuvroN, personal communication). Maxima in helium occurred in the vertical at ridge crest depth and horizontally along 450N, decreasing westward. Earlier observations in 1985 of silica along 47°N (TALLEY et al. , 1992) showed a similar mid-depth maximum at 2000-2500 m and determined that a significant amount of silica was

1456 G A CANNON et al

of hydrothermal origin. FEELY et al. (1992) also showed north-south sections of silica with a second maximum north of 45°N. They speculated its source might be the Endeavour vent field at the north end of the JDFR at 48°N, with the southern maximum originating on Cleft Segment.

Nearer the JDFR in 1988, FEELY et al. (1992) showed the westward turning of suspended particulate plumes by the Vance Seamount chain. Their observations were coincident with the potential temperature distribution and current meter observations in Fig. 5. The highest temperatures occurred with the highest particulates in a closed region to the northwest and not as a continuous plume from the ridge. Combined, these data nicely show the topographic steering by the local topography. These observations, plus those in CANNON et al. (1991) and a few more recent current measurements, clearly show intensified northward flow along the west flank of the JDFR. Thus, plume waters emanating on that side, after separating from their source, first flows north before turning westward at the Vance Seamont chain. This along-ridge, intensified flow pattern with shallow water to the right looking downstream may be a general feature of importance at other ridges (HoLLOWAY, 1987).

The off-axis patch-like distributions both near the ridge in 1988 (Fig. 5) and far to the west in 1987 (Fig. 1) may be isolated plume eddies that detached from their source. SPEER (1989) showed that hydrothermal venting could generate an anticyclonic eddy near the equilibrium density level of the plume and trap anomalous properties. HELFRICrI and BATTISTI (1992) in laboratory experiments showed the eddy could become unstable, detach itself from the source, and thus be able to propagate in the background flow. They further speculated that isolated plume eddies might be similar to Meddies formed from high salinity water exiting the Strait of Gibraltar (ARm et a l . , 1989). Meddies have durations exceeding 2 years and propagate thousands of kilometers while retaining their anomalous properties. Hydrothermal plume eddies might similarly contribute to basin-scale signa- tures.

Finally, although observations in the western region of this study are limited, they show a plausible effect from hydrothermal venting on the JDFR 700-1000 km westward. The general circulation at these depths is southwestward (REID, 1981), and the probable extent of the features described here in that direction is unknown. Preliminary data from recent CO2 cruises along 135°W and 150°W (R. FE~.LY, personal communication) show several isolated CTDs farther south and west than ours with 0-S anomalies similar to, but smaller than, those reported here. REio's map (1982) of potential temperature showing the warm tongue extending from the EPR is too deep to show any similar feature extending from the JDFR. However, the westward and equatorward directions of flow are the same. HAUTALA and RISER (1989) showed that geothermal heating in combination with other forcing could produce flow in that direction for the EPR. Perhaps flow from the JDFR in that direction indicates similar processes. This is a topic for future investigation.

Further study is needed to determine the extent of these apparent hydrothermal venting effects to the west and south of the region shown here. Continuous monitoring of currents and water properties on the JDFR (now ongoing) will provide new information on the variability of venting, and repeated off-axis CTD surveys could determine if the JDFR is in the formative stage of a thermal plume as seen emanating from the EPR. Lastly, potential temperature anomalies are not the only indicators of off-axis plumes, but thermal signatures, not previously reported, also exist. They show year-to-year variability that would not be apparent in data combined from many years. This time variability of water

Hydrothermal effects west of the Juan de Fuca Ridge 1457

temperatures, not only near the ridge, but at least several hundreds of kilometers off axis is sufficiently large to require careful consideration when interpreting other water column observations

Acknowledgement--This is contnbutlon no 1329 from NOAA's Paofic Manne Environmental Laboratory

R E F E R E N C E S

ARMI L , D HERBERT, N. OAKLEY, J F PRICE, P L. RICHARDSON, H. T ROSSRY and B RUDDICK (1989) Two years in the life of a Me&terranean salt lens. Journal of Physlcal Oceanography, 19,354--370.

BAKER E T. and G. J. MASSOTH (1987) Characteristics of hydrothermal plumes from two vent fields on the Juan de Fuca Ridge, northeast Pacific Ocean. Earth and Planetary Science Letters, 85, 59-73.

BAKER, E T., J W LAVELLE, R. A. FEELY, G J. I~4ASSOTH and S. A. WALKER (1989) Episodic venting of hydrothermal fluids from the Juan de Fuca Radge. Journal of Geophys,cal Research, 94, 9237-9250.

CANNON G A , D J. PASmr~sRI and M. R. LEMON (1991) Mlddepth flow near hydrothermal venting sites on the southern Juan de Fuca Ridge. Journal of Geophyszcal Research, 96, 12,815-12,831.

FEELY R A , J. E LUPTON, G. J MASSOTH, D. A BUITERFIELD, E. T BAKER, R. A. MORTLOCK, P. N. FROEL1CH and C. A NITCHMAN (in press) The effect of Juan de Fuca hydrothermal venting processes on the slhclc acid dlstributtons m the northeast Pacific. Journal of Geophysical Research

FEELY R A., G. J. MASSOTH, E. T. BAKER, T. L. GEISELMAN and G. T. LEBON (1992) Tracking the dispersal of hydrothermal plumes from the Juan de Fuca Ridge using suspended matter composition Journal of Geophystcal Research, 97, 3457-3468

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