Hydrothermal Venting at Vailulu’u Seamount:
The Smoking End of the Samoan Chain
Geochemistry, Geophysics, Geosystems 5(2): DOI: 10.1029/2003GC000626. ISSN: 1525-2027, 2004.
H. Staudigel2, S.R. Hart1, A.A.P. Koppers2, C. Constable2, R. Workman1, M. Kurz1, and E. T. Baker3
1 Woods Hole Oceanographic Institution, Woods Hole, MA 02543
2 Scripps Institution of Oceanography, UCSD, La Jolla, CA 92093-0225
3 Pacific Marine Environmental Laboratory, NOAA, Seattle, WA 98115
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Abstract
The summit crater of Vailulu’u Seamount, the youngest volcano in the Samoan chain, hosts an active hydrothermal system with profound impact on the ocean water column inside and around its crater (2 km wide and 407 m deep at a 593 m summit depth). The turbidity of the ocean water reaches 1.4 NTU — values that are higher than in any other submarine hydrothermal system. The water is enriched in hydrothermal Mn (3.7 ppb) and 3He (1x1017 cc/g) and we measured water temperature anomalies near the crater floor up to 0.2 °C. The hydrothermal system shows complex interactions with the ocean currents around Vailulu’u that include tidally-modulated vertical motions of about 40-50 m, and replenishment of waters into the crater through breaches in the crater wall that reach 795 m deep. Inside and outside potential density gradients suggest that hydrothermal venting exports substantial amounts of water from the crater (1.3x108 m3/day), which is in good agreement with fluxes obtained from a tracer release experiment inside the crater of Vailulu’u (0.9x108 m3/day; Hart et al., 2002). This mass flux, in combination with the differences in the inside and outside crater temperature, yields a power output of around 760 megawatts, the equivalent of more than a hundred MOR black smokers and a Mn output of 300 kg/day, approximately ten times the output of a single black smoker.
Introduction
Volcanism and associated hydrothermal systems are amongst the fundamental processes that shape the planet Earth. The most abundant and volumetrically significant of these hydrothermal systems are hidden beneath the oceans, at the mid-ocean ridges. However, submarine hydrothermal systems also occur in intra-plate and island arc settings. In total, these submarine hydrothermal systems play an important role in buffering the chemical and isotopic composition of seawater (Elderfield and Schultz, 1996; Mottl and Wheat, 1994), in modifying the chemical composition of the oceanic crust (Hart and Staudigel, 1982; Staudigel et al., 1995) and in providing a habitat for life (Hessler and Kaharl, 1995; Jannasch, 1995). However, despite decades of research on submarine hydrothermal systems, some of the most important aspects of these processes are still very poorly understood. One critical aspect is the nature and relationship of the thermal and chemical fluxes, and the coupling between the hydrothermal systems and the oceans.
Many of these problems remain unresolved due to the fact that mid-ocean ridges are linear open-ended features with diffuse boundaries. Off-axis volcanoes, however, tend to be more centralized, with a focused, point-source geometry that is more suitable for flux estimates and for studying how hydrothermal systems interact with the ocean water column. We recently discovered such a volcano at the east-end of the Samoan chain, Vailulu’u seamount (Hart et al., 2000). This volcano holds great promise for improving our understanding of mass, thermal and chemical fluxes in submarine hydrothermal systems: its summit crater has an enclosed radial geometry that focuses and confines a very active hydrothermal flow.
Following the mapping of this volcano in 1999, we discovered and surveyed the active hydrothermal system of Vailulu’u with the U.S. Coast Guard Icebreakers Polar Star and Polar Sea (in March/April 2000 and 2001, respectively). In addition to the deployment and retrieval of hydrophones for a one-year seismic monitoring effort, we piggy-backed a series of measurements and experiments to study this hydrothermal system. In particular, we monitored water temperature at four of the one-year hydrophone locations, we sampled and analyzed waters inside and outside the crater, and we surveyed hydrographic water column properties. We also carried out an active dye tracer release experiment, to measure water fluxes in the crater (Hart et al., 2002).
In this paper we report on the hydrothermal aspects of these two expeditions: 180 water column profiles for temperature, salinity and particulates, year-long temperature logger records for four locations and Mn and 3He analyses of water samples from inside and outside the crater. We established the major principles of how the Vailulu‘u hydrothermal system works and the coupling between the hydrothermal system and the ocean surrounding the seamount. We also constrained Mn, 3He and thermal fluxes for the summit crater.
Data Acquisition and Methods
During our 2000 hydrophone deployment cruise aboard the U.S. Coast Guard Icebreaker Polar Star, CTD hydrocasts were utilized to survey for temperature and water particulate contents (the salinity cell was not used since it proved erratic). We used a WET labs light backscattering sensor (LBSS) calibrated with standard particulate suspensions (Hart et al., 2000; Baker et al., 2001). During our 2001 cruise aboard the Polar Sea, we carried out a large number of hydrocasts for conductivity, temperature, depth and optical water column properties, using a SEABIRD SBE 911 CTD system and the same WET Labs LBSS sensor as we used in the previous year. We note that the icebreakers, while ideal for hydrophone deployment and retrieval, were not ideal for water column surveys (lack of dynamic positioning capabilities forced us to do most of our hydrocasts in a free-drift tow-yo mode, in particular, during our 2001 cruise). The tracks of the 2001 hydrocasts are plotted in Figure 1 and all of the hydrographic data, and details on data reduction, are given in Electronic Data Supplement 1.
Figure 1. Map of the summit region of Vailulu’u crater with CTD hydrocast tracks (with cast numbers; Data Supplement 1). For index maps see Hart et al. (2000). Open squares indicate location of temperature recorders attached to ocean bottom hydrophones with their respective names (named after Polynesian gods, Data Supplement 2). Lightly shaded areas (“Northern Cross section” and “Southern Cross Section”) intersect tow-yo tracks projected on two east-west sections through the crater in Figure 7.
Vemco® temperature recorders were strapped to the exterior of four (out of five) one-year hydrophone deployments. The hydrophones were named after Polynesian gods: Mafui’e was positioned inside the crater on the crater floor, whereas Sasa’umani (Sasa), Fe’e and Lefaleilelangi (Lefa) were positioned along the crater rim (Figure 1). The hydrophones floated approximately two meters above their sea floor anchors. At each site, we recorded temperatures for a 377 day period beginning March 24, 2000 (08:00 GMT) for every hour. In addition, the Mafui’e site on the crater-floor carried a second logger, which sampled at a 5-minute rate for 38 days, beginning May 1, 2000 (00:00 GMT). All temperature data, calibrations and some basic statistics are given in Electronic Data Supplement 2.
We also analyzed 36 filtered water samples from the 2000 and 2001 cruises for Mn by ICP-MS and 14 “copper-tube” samples from the 2000 cruise for 3He concentrations. Data and details of the analyses are given in Electronic Data Supplement 3.
In addition, we carried out a dye tracer-release experiment, where 20 kg of fluorescein dye was released in a point source 30 m above the crater floor, in the north-central portion of the crater. The dispersal of the dye was tracked for four days to determine the vertical and horizontal dispersion velocities of the dye. The location of the deployment is given in Figure 1, and the experiment is described in detail in (Hart et al., 2002) .
Figure 2. Overall physical and water property characteristics as a function of depth in the Vailulu’u crater region. In the “Crater Volume & Breach Area” panel, we plotted the depth relationships of cumulative crater volume within the circumference of the crater rim, and the fraction of the crater rim (linear distance) that is open to circulation. The deepest point of the crater floor is at 1020 m and 38% of the total crater volume is located below the deepest breach at 795 m water depth. The breached crater volume extents between 594-795 meters water depth. Salinity, temperature and potential density display steeper gradients above and outside the crater than inside the crater. Salinity decreases rapidly with depth, reaches a minimum at 740 m water depth and slowly decreases with depth (note the expanded scale and substantial scatter). Individual temperature and density profiles scatter substantially, in particular, in the shallowest and steepest parts of the curves, with a significant overlap between inside and outside profiles almost through the entire breached crater depth range. Averaged temperature and density profiles, however, are indistinguishable only down to depths of 730 m below which they begin to bifurcate into the greatly reduced gradients in the isolated portion of the crater and the steeper gradients outside the crater. This bifurcation depth plays an important role in our modeling and for this reason, we distinguish the Upper and Lower Breached Crater or “UBC” and “LBC” at 730 m. Significant temperature and density gradients can be observed down to about 890 m water depth inside the crater, below which they are relatively well mixed forming the Mixed Basal Layer “MBL”. We refer to the remaining portion of the unbreached crater as Isolated Crater Interior "ICI". Particulates show a gradual increase from Open Ocean depths to the LBC, and moderately but generally high values in the ICI and MBL.
A. Oceanographic Setting
Vailulu’u is located at the eastern end of the Samoan chain, and is linked to Ta’u Island, the easternmost island in Samoa, by a deep ridge. Vailulu’u volcano rises approximately 4400 m above the surrounding ocean floor, to a water depth of only 593 m (Johnson, 1984) (then called Rockne Seamount). Vailulu’u has a 407 m deep and 2 km wide crater that is the host of a very active hydrothermal system (Hart et al., 2000). The crater rim is variable in elevation, with three summits and three breaches. The shallowest summit is in the west (593 m water depth) and the two major breaches are in the NW (795 m) and SE (764 m; Figure 1).
The water column in the crater region may be subdivided into several major depth ranges, based on the extent of crater breach area at a given depth and on some key hydrographic parameters, as summarized in Figure 2. The fraction of breach area at a given depth is shown in Figure 2a, and this allows us to distinguish between three main regions: (1) the “open ocean” (OO) above the shallowest summit at 593 m, (2) the underlying breached crater down to the deepest breach level at 795 m, and (3) the isolated crater interior down to a depth of 1000 m. We further sub-divided the breached and the isolated zones of the crater into two distinct regions, largely based on the nature of the potential temperature and density profiles. The “upper breached crater” region (UBC; 593-730 m) shows no difference between inside and outside potential density, with nearly identical means between inside and outside potential density. The “lower breached crater” region (LBC; 730-795 m) marks diverging trends in potential temperature and density, whereby the means are different, but the data distribution still shows some overlap between inside and outside but the means clearly diverge downward (Figure 2). The UBC makes up 41% of the crater volume and the LBC makes up 21%. The closed portion of the crater below breach level displays slightly increasing potential density with depth but the very bottom reaches rather constant values, with very little scatter below ~900 m depth, suggesting a rather well-mixed bottom layer. For this reason, we distinguish a Mixed Bottom Layer (“MBL”; 890-1000 m) below the otherwise Isolated Crater Interior (“ICI”; 795-890 m). The depths of these crater subdivisions are indicated in Figure 2, 3, 7 and 8 for reference purposes.
The breaches are unevenly distributed; the NW breach is the deepest and largest, opening up the crater to water circulation at 795 m. It remains the only breach to a water depth of 763 m, where the SE breach opens. The SW breach opens at the 730 m deep LBC/UBC boundary; there. the total breached area is about 5% with the NW breach contributing about 67% and the SE breach 33%.
Figure 3. Comparison of Mn and 3He concentrations with depth, for water samples collected both inside (open symbols; squares: 2000; diamonds: 2001) and outside (filled triangles) Vailulu’u crater. Mn given in parts-per-billion (1 ppb = 18.2 nm/kg); note that the Mn values given in (Hart et al., 2000) are low by a factor of ten, due to a computational error. Background values for ambient water outside the crater, and well above or below breach depths, are 0.04-0.11 ppb Mn and 5.3-6.8x10-14 cc/g 3He. The water in the deep crater is virtually pure rock He, with 3He/4He ratios of 7.7-9.0 Ra (crater rim basalts range from 7.7-10.3 Ra; Hart and Kurz, unpublished data).
B. Nature of Hydrothermal Venting
The summit region of Vailulu’u is located at relatively shallow water depth, with steep and changing gradients in hydrographic parameters (Figure 2). Most of the profiles show very noisy records; this is suggestive of substantial structure in the water column, with significant temporal and spatial variability.
Salinity around the summit of Vailulu’u decreases rapidly with water depth, reaching a minimum in the UBC depth range, and then slowly increasing again with depth. This salinity minimum is regional in extent, and indicates an important boundary between shallow and deep-water masses in this region; these can be maintained only by horizontal currents. This salinity minimum is not related to the presence of the crater, as it is apparent in profiles 50 km from the volcano.
Potential temperature outside the crater shows steep decreases with depth in the upper part of the profile and substantially smaller gradients at greater water depths. Similar to salinity, the temperature profiles within the crater are indistinguishable from outside profiles at OO and UBC depths, but their gradients become smaller and nearly linear inside the crater at LBC and ICI depths. The MBL is virtually isothermal.
Potential density gives a mirror-image of the temperature variations, inside and outside the crater. Average density inside and outside the crater are indistinguishable for OO and UBC depths, diverging at LBC and ICI and nearly constant at MBL depth. Some individual profiles show reversals of gradient or staircase-like gradients. Potential density gradients offer an important driving force for circulation of water in the crater and between the crater and surrounding ocean. To quantify these properties, we calculated averages for potential density and density gradients (buoyancy frequency) inside and outside of the crater. 22 hydrocasts penetrated deeper than 900m, yielding an average density of 27.2369±0.0010 (1σ SD). For comparison, seven profiles from outside the crater (and at distances up to 100 km) had an average potential density of 27.3290±0.0080. Therefore, the water outside is heavier at 900 m than the water inside by ~0.01%. Thirteen of the in-crater profiles have density gradients that are fairly linear over a 50-100 m interval bracketing 900 m; from these, the mean buoyancy frequency at 900 m is: N = 0.37±0.11 cycles per hour, the total range is from 0.11-0.55 cph). Outside the crater, the buoyancy frequency at 900 m is significantly higher and less variable: N = 1.25±0.07 cph.
Terminal salinities, potential densities and temperatures at the crater floor are similar to exterior values at depths of 760-800m. In this depth range the crater has only one major breach in the NW and, therefore, this breach must play a crucial role in the delivery of waters to the crater floor. It is interesting to note that none of our measurements at breach locations displays densities as high as the crater floor. This is probably due to the fact that our breach hydrocasts never went down to near breach depth.
Our surveys for particulate contents in the water column offer the most comprehensive view of the spatial and temporal variation of hydrothermal inventory in the water column. In order to discuss the overall distribution of particulates, we have plotted all of our data from inside the crater in Figure 2. These data were acquired during an one-week period in April 2001 and they are generally quite similar to the data obtained during our 2000 cruise, with the exception of some very high values (1.3 NTU) in the mixed bottom layer in the NW crater (see Figure 5 in Hart et al., 2000). Overall, the peak particulate contents at Vailulu’u reach up to 1.4 NTUs, which is substantially higher than the particulate density around hydrothermal vents at Mid-Ocean ridges that reach up to 0.3 NTU (Resing, 1999; Baker et al., 1995; Baker et al., 2001; Chin et al., 1998). Particulate density data display substantial variation with time and with location, and they show very complex relationships with each other, even though there are some systematic patterns that can be observed in nearly all depth profiles.
Outside profiles display nearly zero particulates above the summit depths (<593 m) and below the deepest breaches (>795 m) and show significantly elevated particulate contents between these depths, with a maximum typically at about 730 m.
Nephelometer data from within and above the crater show substantial variability. Based on our 2001 data, the highest particulate counts can be found 720-770 m deep with an isopycnal equilibrium at a sigma-theta between 27.19 and 27.22. The shallowest turbidity anomaly was found at 560 m depth at 40 m above the highest summit. In other hydrocasts inside the crater, we find near-background values down to depths of 660 m, well within the UBZ. Generally, NTU values show a steady increase with depth from 600 to 730 m, with maximum values above 0.90 NTUs in the LBC. Peak values from inside the crater are at 40m deeper than the peak values outside. However, particulates inside the crater are extremely variable, in particular in the peak region in the LBC. Below the lowest breach (at 795 m) maximum turbidity readings begin to decrease slightly with depth to about 0.6-0.7 NTU and stay rather constant (or slightly increase) in the bottom layer (Figure 2).
In Figure 3, we have plotted our Mn and 3He water data as a function of depth, for water samples from five casts inside and two casts outside the crater (at distances of 2.4 and 7 km). The hydrocasts from outside the crater show elevated Mn (0.12-1.1 ppb) and 3He (8-45x10-14 cc/g) at depths correlative with the crater breaches. These values are well above background values measured at shallower or deeper depths (0.04-0.11 ppb Mn; 5.3-6.7x10-14 cc/g 3He). Hydrocasts within the crater show increasing Mn and 3He with depth through the UBC and LBC; no clear trends emerge within the ICI and MBL but the values are uniformly high. There is no clear difference in Mn content between the samples collected in 2000 and 2001, suggesting fairly steady hydrothermal activity. The deep-crater Mn values are lower than values found in hydrothermal plumes from ridge-crest systems, where values are commonly in the 2-15 ppb range (Baker et al., 1995; Field and Sherrell, 2000; James et al., 1995; Mottl et al., 1995; Resing, 1999). The deep-crater 3He values average 50 times higher than ridge-crest event plumes and 30 times higher than steady-state plumes (Lupton, 2000).
Figure 4. Temperature logger data for three rim sites (Lefa, Fee and Sasa) and one crater floor site (Mafuie), each one of them measured in the water column two meters above the seamount at locations indicated in Figure 1. Figure 4a indicates the whole one year time series, while 4b gives the same data on an expanded scale for Day 60-70, to show short term variation. All sites recorded temperature hourly (colored lines), while one sensor on Mafuie measured in 5 minute intervals (blue line in Figure 4a and b). Rim sites show substantial high frequency scatter (with a strong tidal signal) black lines are 188 knot least squares spline fits evealing the low frequency variations which show remarkable similarity among the records. Black lines are 188 knot least squares spline fits revealing the low frequency variations which show remarkable similarity among the records. Tidally modulated temperature variations are caused by 40-50 m vertical water displacements of water masses with strong vertical thermal gradients. Note the prominent downward spikes of the averaged rim site data around Day 310. The relative timing of these spikes were used to infer a 2.5 cm/min, SW current at Vailulu’u summit water depths. Mafui’e shows substantially reduced high frequency variation but averaged data show an overall variation that is relatively similar to the rim sites. Mafui’e temperature spikes are often asymmetric, with rapid rises (or drops) and often slowly decaying signals. Rises are interpreted as hydrothermal events, drops indicate input of colder water cascading down from the breaches.
C. Temporal Variation of Temperature
The depth profiles discussed above display substantial scatter that could be due to spatial or temporal variability. Detailed temperature records on Vailulu’u seamount offer the opportunity to analyze temperature variations in a time-series of one-year duration. We recorded in-situ temperatures in four locations on Vailulu'u; three on the crater rim (Sasa, Fe’e and Lefa) and one on the crater floor (Mafui’e, Figure 1). These instruments are located at rather distinct water depths and exposure types. Lefa is the shallowest site at 630 m, located just SW of the SW summit, within the depth range of the UBC, where vertical temperature gradients are about 1.0 °C/100m. Fe’e and Sasa are at 695 and 705 m respectively, which is a region of substantially reduced temperature gradients (0.6 °C/100m). Both of these instruments are located at the boundary between the UBC and LBC, at a depth where inside the crater isotherms begin to diverge from the exterior isotherms. Since both Sasa and Fe’e are facing the “inside” of the crater they will (more directly) measure inside-crater isotherms that begin to diverge from the outside isotherms. Mafui’e is near the bottom of the crater (994 m), well within the MBL crater bottom water pool where thermal gradients are very low.
Temperature data are presented in Figure 4 for all four deployment sites in one year-long time series (Figure 4a) and a blow-up of these records over a 10-day period (Figure 4b). All temperature loggers ran with 1-hour sampling rates; Mafui’e includes a 38 day record at five-minute sampling part of which is displayed in the Day 60-70 expanded record in Figure 4b. All records show substantial variation from roughly bimonthly to five-minutes (in the high resolution record of Mafui’e).
Temperatures at any particular point in the water column around the summit of Vailulu’u seamount may change due to three processes: (1) vertical water transport in local thermal gradients, (2) “seasonal” long term variations from regional eddy circulation, or (3) hydrothermal activity. Vertical water transport has been shown to be an important effect at seamounts like Fieberling Guyot (e.g. Kunze and Toole, 1997; Eriksen, 1991) and it is likely to be driven and/or modulated by ocean tides. Ideally, the water column temperature gradient could be used to determine the vertical heave of a particular water mass, but errors are largely due to the highly variable gradients.
Temperature records display substantial short-term (“daily”) variations superimposed on a long-term trend that we represented here with a 188 knot least-squares spline (using the algorithm of Constable and Parker, 1988). This spline explains most of the twelve-month variance at Mafui’e (86%), but only 20-30% at the crater rim sites. Most of the variance at the crater rim sites is caused by short-term variations with typical daily ranges of about 0.2-0.75 °C (peak-to-peak), with an average of about 0.3-0.4°C. This temperature range would correspond to an average water heave of about 50-60m, and up to 120m on some days. The Mafui’e crater floor site displays much lower daily temperature variations, ranging from 0.05-0.2 °C, with individual rises in temperature up to 0.2 °C (from background). The mean daily variation at Mafui’e is about 0.1 °C.
Short-term temperature anomalies in the crater rim records have very different shapes from the ones in the crater (Figure 4). Crater rim temperatures typically vary up or down from a mean value with a total amplitude up to 0.5 °C, but the probability of repeating a mean value in sequential measurements is close to zero. The (de-trended) crater rim data show an overall symmetric Gaussian data distribution, where upward deviations from the mean are as abundant as downward variations.
Crater floor temperatures commonly stay within very narrowly defined bounds, often measuring the same temperature (0.01 °C resolution) for several hours. Most of the Mafui’e temperature anomalies are asymmetric. They show rapid initial deflections from steady-state, with a slow decay back to the same or a slightly different steady-state value. Rather calm, isothermal periods of several days (e.g. day 60-66 in Figure 4b) may give way to periods of more active temperature variation (e.g. day 66.5-68.5 in Figure 4b). However, even the more active periods display much smaller temperature variation than the crater rim sites. At Mafui’e, temperature rises are more common than drops, giving the (de-trended) cumulative Mafui’e temperature record a non-Gaussian character.
Figure 5. Power spectrum for time series rim sites and Mafuie, including an estimate of their average uncertainty. Rim sites show consistently high power in the higher frequency range, with pronounced peaks at diurnal and semidiurnal tidal frequencies. The rim site has much lower power in the high frequency spectrum with a small, but significant semidiurnal tidal signal. All instruments show significant power in the low frequency band but this range does not reveal any significant peaks even for high resolution estimates.
The five-minute record at Mafui’e shows substantially more detail than the one-hour record. Instant temperature rises in the one-hour record commonly reveal themselves as ten to twenty minute events in the five-minute record. The maximum temperature deviations at such events are typically larger in the five minute record than the one hour record (e.g. the temperature drop at day 60.94). Many upward temperature spikes are only visible in the five-minute record (e.g. a 0.06 °C peak at day 62.3 or a 0.1 °C peak at day 68.5). This shows the rather rapid circulation (and mixing) of water in the deep crater.
The rather isothermal environment at the Mafui’e location is consistent with the very low temperature gradients found for the MBL in the CTD profiles (Figure 2). The relatively rapid and short-term temperature deflections from this nearly isothermal state may be caused by two major processes. Downward deflections are most likely due to cold and dense rim-water cascading down into the crater, particularly when rim temperatures are unusually low. Upward temperature changes are most likely due to hydrothermal input and upward moving plume(let)s of warm water.
Upward temperature deflections are commonly on the order of 0.1 °C but may reach up to 0.2 °C (e.g. at Day 185). While these anomalies may be small when compared with the tidal shifts in the crater rim records, they are still an order of magnitude larger than temperature anomalies found in other hydrothermal systems, e.g. at Axial Seamount on the Juan de Fuca Ridge (Baker et al., 1990). Downward temperature excursions at Mafui’e may be as large as 0.2 °C and they are often preceded by major low temperature events at Fe’e, Sasa approximately 1-3 hours earlier. However, not all major temperature drops at Fe’e or Sasa are followed by temperature drops at Mafui’e. This apparently inconsistent behavior is probably due to the fact that the crater rim temperature loggers are not located in the crater breaches, through which the deep-water import is occurring.
Figure 5 displays multitaper power spectra (Riedel and Siderenko,1995; http://mahi.ucsd.edu/parker/Software/software.html, see also Electronic Supplement 2) for all five sites, including average one sigma error estimates, for each record. Overall, the spectral behavior in the frequency domain confirms the similarity of the rim sites and the fundamentally different behavior of Mafui’e, particularly at frequencies higher than 0.5 cpd. This clear difference between the crater time-series and those from rim sites in the high frequency domain shows they are controlled by different processes. All of the time-series have rather high power in the low-frequency domain (<0.2 cpd), but expansion of the power spectrum in this frequency range does not reveal any evidence for a truly periodic variation. The rim sites display substantial power near diurnal and semidiurnal tidal frequencies. Consistent with the expected time-series from the actual tidal variation, the lunar tide has more power than the solar tide. The spectra show that tidal frequencies exert very important controls on temperature variability, but the temperature variation is not strictly periodic reflecting a somewhat complicated (diffusive?) response of the temperature record to tidal forcing. Overall, Lefa has the most power, and is characterized in particular by a wider and split-peak for the diurnal frequencies. Fe’e has the lowest power of the rim sites but has a spectrum closely matching that of Sasa. Relative to background, Sasa displays much stronger response to semidiurnal tides than Fe’e, but rather similar diurnal tides. Mafui’e displays almost two orders of magnitude less power than the rim sites at frequencies >1 cpd, with only a minor peak at semidiurnal frequencies.
Cross-spectra of two time-series can be used to quantify their similarities (coherence) and to estimate potential leads or lags between them. High coherence between two power spectra suggests that they are controlled by the same processes, at least in the frequency range for which they are coherent. Time lags between them may indicate the overall current direction that carried a particular type of temperature signal. Due to the fundamentally different behavior of rim sites and the crater site, the results of our coherence analysis are shown separately for the rim sites (Figure 6a) and between the crater and rim sites (Figure 6b). In both diagrams, we also plotted (gray line) the level below which the coherence is expected to lie 95% of the time for uncorrelated signals. This level is less than 0.1 for all coherence estimates in the frequency range displayed.
All crater rim sites show significant coherence amongst each other for low frequencies (< 0.05 cpd). Sasa and Fe’e are almost perfectly coherent, while Lefa and Sasa display the lowest coherence at about 60% (Figure 6a). This high coherence is also obvious when comparing the splines in Figure 4a, where even records with a coherence of only 60% (Lefa and Sasa) show markedly similar splines which basically model the low frequency behavior. The crater floor site Mafui’e shows no really significant coherence with any crater rim sites at frequencies greater than 0.1 cpd (Figure 6b). Low frequency (<0.05 cpd) coherence is quite high with Sasa and Fe’e, but almost negligible with Lefa. Cross-spectra between time-series can also provide estimates for the phase relationships between the time-series and, these, may be used to determine the lead or lag of a particular time-series relative to another. We show these group delays for all frequencies with meaningful coherence (Figure 6a, 6b). The phase lags at tidal periods do not offer any consistent leads or lags due to their low coherence, but the low frequency (> 0.05 cpd) spectrum does, whereby both coherence and phase lag offer consistent results. In this frequency range, Lefa leads Sasa by about 4-10 hrs and Fe’e by 7-12 hrs. In both cases, the lead varies systematically from a systematic and short lead at the low-frequency end to a somewhat longer lead for the shorter periods (Figure 6). Sasa appears to lead Fe’e by a very small amount (0.5-1 hr), but its phase shift is so small that it cannot be quantified with reasonable accuracy.
A broadly similar phase lag between the rim sites can be seen in the time domain arrival times of a major cold pulse around Day 320 (Figure 4). This pulse of cold water arrived at Lefa around 8:00 GMT on February 8, 2001, and persisted for 6.0 days (anomaly width at half-height). This pulse showed up 12 hours later at Sasa and 29 hours later at Fe’e and persisted for similar time periods at these sites (5.4 and 5.6 days, respectively).
The phase-lags observed in the time-series and the arrival time of the cold water pulse between three locations, may be interpreted in terms of flow directions and speeds of water motion. Lefa generally leads and Sasa and Fe’e follow. We used simple trigonometry to estimate the horizontal component of the azimuth and velocity of flow, ignoring vertical motion and assuming linear and laminar flow of a planar water front that is orthogonal to its velocity. None of these assumptions is likely to be entirely correct, and for this reason we emphasize that these estimates only provide a broad constraint on flow direction and velocity.
Figure 6. Cross Spectra for rim sites (Horizontal Panel A) and for crater to rim sites (Horizontal Panel B). Coherence is insignificant for high frequencies (not shown). Low frequencies show relatively stable and high coherence for rim to rim and rim to crater coherence (to about 0.05 cpd, shaded). Highly coherent, long period signals show systematic group delays, whereby Lefa leads Fe’e and Sasa by 4-16 hours, whereas the extremely highly correlated Sasa-Fe’e shows no significant group delay. Crater rim sites Fe’e and Sasa lead Mafuie by 6-16 hours. Coherence of Mafuie and Lefa is too small to place any confidence into their group delays.
The phase-lags in the cross-spectra suggest a water flow vector of approximately 5 cm/sec (0.1 kt) from the southwest (230°). This flow direction and velocity is based on a statistical analysis of year-long records, and thus represents an averaged, long-term flow at the seamount. It is quite likely that, at any given time, local currents may deviate from this direction, in particular for periods where coherence is not demonstrated (< 2 days).
The time-lag for the arrival of the cold water pulse on February 8, 2001 may be used to check this general averaged flow vector in a particular, well-identified event. The sequence of arrival times of the cold pulse, coupled with the site locations, constrains the current flow vector to be from the western quadrant, between 210-310°. Factoring in the estimated lag-times as given above, the best fit is 260±50°. From the site-spacing and the time-lags, we calculate a horizontal current velocity along the 260° vector of order 2.5±0.5 cm/sec. This single event estimate is not much different from the averaged flow vector obtained from the time-series analysis, and thus is probably representative of the overall flow direction. We also note that the lateral dispersion of dye during our tracer release experiment (Hart et al., 2002) was most rapid to the NE, suggesting a mean flow in the deep crater from this same azimuth. At the same time, we recognize that it is likely that water flow will vary locally and with time, as is the case for summit regions of other seamounts (e.g. Eriksen, 1991; Kunze and Toole, 1997).
D. Spatial Variation of Potential Density and Temperature
Three key observations derived from the time-series analysis of crater rim temperatures offer some boundary conditions regarding the interaction between the ocean and the hydrothermal system inside the Vailulu’u crater: (1) Crater-rim temperature records suggest tidal heaves of 50-60 m per tidal cycle, and occasionally up to 120 m, allowing for a tidally modulated import of outside, dense waters into the deep crater, as it was seen in occasional sudden drops in temperature at Mafui’e; (2) The mean flow across the summit of the seamount is from the southwest; (3) sudden temperature rises in MBL waters at Mafui’e suggest hydrothermal input at the crater floor. From these boundary conditions, we can make predictions for how the crater waters should vary in terms of their density and temperature and we can test these predictions against the measured data.
Crater breaches play a crucial role for the ventilation of the crater, for the import of denser outside waters into the crater, and the flow of water around the summit, modifying the southwesterly ocean currents near Vailulu’u. The deepest and widest breaches in the NW and the SE of the crater are the most important places for the isopycnal exchange of water between the crater interior and the outside ocean (Figure 1). These two breaches make up more than 80% of the total breached area of the crater at any depth and they form a NW-SE axis of preferred isopycnal water exchange. A third, somewhat shallower breach can be found in the SW of the summit. The alignment of the deepest breaches in combination with the SW breach suggests that the SE half of the summit is likely to be the most ventilated portion of the crater summit region. However, the NW-SE main “ventilation axis” of Vailulu’u crater is roughly orthogonal to the predominant direction of water flow in the summit region (as inferred from the phase delays), whereby the breach faces are parallel to the direction of water flow. It is also interesting to note that the current direction bisects the angle of the southern and western volcanic rift zones, and it is oblique to east and west elongation of the seamount. Thus, the current direction relative to the rift zones also influences the overall resistance of the seamount to flow. These relationships between the main current directions and the physical geography of Vailulu’u suggest that, slight directions in current may make a very large difference in size and (3D) direction of current pressure on the two deep breaches. This may make a substantial differences for the circulation of water inside and out of the crater, and it can be expected that water circulation is dominated by overall flow from the SW and its breach geometry, but it is likely that water flow is complex, similar to, or maybe even more so than at other seamounts (Eriksen, 1991; Kunze and Toole, 1997).
Figure 7. Temperature and Density variation in the Northern section (see Figure 1) and the particulate contents in the Northern and Southern section. Crater bathymetry of the center line of these sections is also indicated . Temperature and density variation shows a wavy character, indicating the effects of the time – variant vertical motion of water in the crater. Visually averaging these undulations in the temperature and density variation shows that vertical gradients appear to be expanded in the eastern part of the section relative to the western part near the breach longitude. Particulate distributions show a relatively layered structure in the eastern part of both sections, that contains the highest particulate counts in the LBC levels and the lowest counts at ICI levels. The western region is much less stratified.
In Figure 7a and 7b we show a projection of our temperature and density measurements from all of our 2001 tow-yo tracks located within the broad east –west band labeled “Northern Section” in Figure 1. In the same diagram we also indicated the bathymetry of the crater at the centerline of this section. We have chosen this section because it includes the deepest breach and most of the isolated NE portion of the crater. This section is oblique to most of the tow-yo tracks and, therefore, maximizes three-dimensional data coverage throughout the period of our cruise in April 2001.
The potential temperature isotherms in Figure 7a display substantial undulations, on the order of 30-40 m (although individual troughs and peaks are often constrained by only one tow-yo leg). The vertical amplitudes of these undulations are well within to the tidal heaves inferred from the one-year temperature time-series data at the crater rim sites. As a result, they most likely reflect temporal variations, rather than significant local variations that were captured by a particular CTD section. For this reason, temporal variation in temperature is expressed here as a (pseudo-) spatial variation, and a time-integrated view can be approximated only by smoothing neighboring peaks and troughs over a length scale of 200-300 m. After applying such a “visual filter” it is interesting to note that such an averaged record shows rather flat contours above 600 m depth, suggesting that these shallow temperature variations are effectively explained by the tidally-driven temperature variations observed in the time series. However, below this depth, temperature contours vary quite systematically; the 5.9 °C contour moves upwards towards the East, while the 5.5 °C contour appears to move downward. This results in an expansion of the vertical extent of this temperature range (light blue) in the East, when compared to its extent in the NW breach region. Contours outside of the NW crater breach expand again, with the separation of the 5.5 and 5.9 °C isotherms being similar to the eastern side of the crater. The temperature contours in the west are based on only four legs of two tow-yo sections, both completed in 20 minute time periods; therefore, this part of the section reflects only a very short snapshot in the tidal cycle.
Potential density contours are plotted in Figure 7b and show a very similar picture to the temperature contours. The light-orange range in potential density (27.16-27.21) appears to be compressed near the western breach and expanded in the eastern part of the crater. As with the isotherms, the top of this density interval slopes up and the bottom slopes gently down towards the east, expanding the density gradient in the east and compressing it in the west. This is consistent with the predictions made above, whereby the flow of water from the SW has to move up as it impinges on the NE summit. Deeper waters enter the crater through the NW (and SE?) breach, effectively deepening the density contours at sill depths.
E. Distribution of Particulates in the Crater
The distribution of particulates is probably the most robust and sensitive method for mapping the distribution of the hydrothermal inventory in the crater. We carried out over 60 CTD/nephelometer casts within the crater perimeter. These data are projected onto two E-W cross-sections in Figures 7c and 7d (the “Northern Section” and the “Southern Section” in Figure 1, respectively). Patterns of particulate distribution may be correlated across these two cross-sections, to provide a three-dimensional view of the crater.
Particulate distributions shown in Figures 7c and 7d display substantial variations between neighboring tow-yo casts. As with the temperature and density data, we need to distinguish variations due to short term or local “noise” from the more systematic regional variations. Some of the local variations closely resembles the “tidal” variations identified in Figure 7a and b, and it is likely that there is some tidal component in the particulate distribution as well. However, it is also likely that hydrothermal convection in the crater may cause some part of this variability. We often observe distinctive details in the particulate distribution in individual casts that can be correlated over distance in several subsequent tow-yo casts. However, profiles taken along the same tracks on subsequent days are generally not easily correlated with the patterns observed previously. Such variation is likely be caused by rapid hydrothermal convection (like “billowing” clouds), that is also indicated by the positive short-period temperature anomalies observed in the crater floor temperature records. Unfortunately, our observations were not sufficiently systematic to trace tidal heave or hydrothermal convection, and we are limited to a more broad-brush discussion of the overall variability in the crater.
There are several prominent features in the overall distribution of particulates in the cross sections of Figures 7c and 7d:
Together, these clues further corroborate the suggestion that the SW-half of the crater is better mixed, while the NE-half of the crater is more stratified. Furthermore, most of the particular inventory accumulates near the eastern-most wall of the crater. These high particulate counts may be due to the in-situ origin at the NE crater wall, or they come from the crater and have accumulated at this level. We prefer a crater origin because there is strong evidence for hydrothermal input from our crater floor temperature record and from one hydrocast with very high particulate counts during our 2000 cruise. In the crater-origin scenario, plumes of particulates would rise to the level of their neutral buoyancy (approximately at breached crater levels) and accumulate largely in the NE region, because this is more protected from outside currents, tidal exchange and mixing than the more breached SE-half of the crater.
Overall, our particulate distribution data from 2001 are consistent with the ones from our 2000 cruise (Hart et al., 2000) but a direct comparison is almost impossible because the two data campaigns differed fundamentally in nature and strategy. During our 2000 cruise, we focused on one major circumnavigation outside the crater rim (see Figure 6 of Hart et al., 2000) but very little data inside the crater and the inverse applies for the 2001 data, with no reliable repeats of any particular casts (due to the lack of dynamic positioning). Furthermore, 2000 salinity data were flawed and we had substantial problems with the CTD. During our 2001 cruise, we had many more tracks, all with CTDNF data, but most of them were along free-drift lines; only two runs were made under power which we used to explore the crater interior (hydrocast 18 and 19; Figure 1).
The circumnavigation in 2000 showed that the lowest particulate counts were due west of the crater, in the 260°-290° segment; the highest anomalies and gradients were found between 80°-190° (Hart et al., 2000). The low particulate density in the western halo is consistent with a dominant current from the SW. The particulates outside the crater were confined to the depth range between the lowest breach and the highest summit of Vailulu’u, but the lower bound was shallower around the SE breach (Hart et al., 2000). In our 2001 survey, we confirmed that the outside particulate halo was still confined to the depth range of the breached crater; however, the particulate levels in the NE-SE sector were very variable. One track (14, Figure 1) to the East of the SE breach showed an intense (0.8-0.9 NTU) particulate peak in the 700-800 m depth range that diminished as the track approached the breach. A second track (26, Figure 1) through the same location 65 hours later yielded very low-amplitude (0.025 NTU) turbidity peaks in the 600-800 m depth range that were barely above background levels. This suggests extreme time-dependence of the halo in the NE-SE sector, probably due to a complex flow regime and potential incursion of clear outside waters into the crater. This further supports the importance of both major breaches to crater ventilation.
F. Hydrothermal Mass Fluxes
The circulation of water in the crater is clearly turbulent and highly time-varying. Despite this, there are several ways to estimate the mass flux from this hydrothermal system. One uses the difference in potential density at breach level between the crater water and the outside ambient seawater (see Figure 2) to model inward density-driven flow. The other utilizes the results from our dye tracer release experiment in the crater bottom (Hart et al., 2002) to model vertical export velocities.
The potential density profiles show a divergence or bifurcation at 730 m (Figure 2). Below this depth, outside water is heavier and will be driven into the crater through the NW and SE breaches, which have depths of 795 and 765m respectively (the SW breach sill is right at the bifurcation depth of 730 m). To first order, we estimate the water mass flux, driven by this potential density gradient, following the formulation of Whitehead (1998). We have adapted his equation for flow through a rectangular notch (without rotation effects) to flow through a triangular notch with the result for mass flux Q:
where α is the triangular notch slope (height/width), g' = g< Δρ/ρ, Δρ is the potential density difference between inside and outside profiles at sill depth, and hu = (sill depth - bifurcation depth). The NW breach has a slope of 0.24, a sill depth of 795 m and Δρ/ρ = 4.25x10-5, the SE breach has a slope of 0.11, a sill depth of 765 m and Δρ/ρ = 1.52x10-5.
For the NW breach, Q = 1.0x108 m3/day; for the SE breach, Q = 2.8x107 m3/day; the total import flux is then 1.3x108 m3/day (all calculated from the mean of 14 outside profiles; see below). Realistically, these are probably higher than the true fluxes, due to frictional and turbulent mixing effects (Whitehead, 1998). Other uncertainties arise from the time-variability of the potential density profiles both inside and outside the crater, and the consequent variability of Δρ/ρ and hu. We estimated these uncertainties by calculating the fluxes from 14 individual outside profiles relative to the mean inside profile (there is much more variability in the outside profiles than in the inside profiles). The standard error (2σ) of the 14 individual fluxes is 11% for the NW breach and 20% for the SW breach; for the total flux, the standard error is 13%. Note that 78% of the total import flux is carried by the NW breach, due largely to its greater sill depth (even though its width at the bifurcation depth is only 540 m, compared to 640 m for the SW breach). Because the 14 individual flux estimates for the NW and SE breaches are closely correlated (slope, NW/SE = 1.99±0.18, two-error York regression), this partition factor is tightly constrained to be 78.5±1.5%. While the absolute total fluxes may carry an undefined uncertainty due to the limitations of the model, we believe this flux partitioning is a robust outcome.
The dye release experiment of (Hart et al., 2002) constrained the vertical transport of water inside the crater (advection plus diffusion) to be 0.91±0.44x108 m3/day. This is in remarkably good agreement with the import estimate derived above (1.3±0.2x108 m3/day), especially considering that the import flux is likely to be somewhat higher than the true value, both because the model neglects frictional and turbulence effects, and because some of the imported water will flow laterally along isopycnals before it reaches the depth level where the dye release velocities were measured (~850 m). We will adopt a flux value of 1x108 m3/day as a best estimate for our discussions below of chemical and thermal fluxes from the deep crater. This flux corresponds to an average import velocity of 3.3 cm/sec over the breach area from 730-795 m, which is within the bounds of our estimates of the outside currents estimated earlier in this paper (2.5-5.0 cm/sec). We further note that the water volume in the crater below 795 m (the deepest breach level) is 3.43x108 m3, and that the import fluxes above suggest that the ICI+MBL waters in crater are turned over in about 3.5 days.
Figure 8. Mn/NTU and Mn/3He versus depth (panels A and B) for water samples collected both inside (open symbols; squares: 2000; diamonds: 2001) and outside (filled triangles) Vailulu’u crater. Mn given in parts-per-billion; 3He is given as 10-14 cc/gram. Mn/3He ratios are Mn in ppb/3He in 10-12 cc/gram. Mn/NTU ratios are given as Mn in ppb/NTU in nephelometric turbidity units. The solid curves are eye-ball fits to the data.
G. Mn and 3He in Hydrothermal fluids
Earlier in this paper, we showed that the particulate-rich waters, both inside and outside the crater, are also enriched in Mn and 3He, supporting their hydrothermal origin. Particulates, Mn and 3He are all positively correlated (Figure 9a and 9b). For Mn and particulates, there is no difference between the water samples collected in 2000 and 2001, supporting a generally steady hydrothermal system. Most of the data in Figure 9a falls on a positive correlation with a Mn/NTU slope of 4-5 ppb/NTU. As shown in Figure 8a, Mn/NTU ratios appear to decrease with depth from values >4 above 800 m to values <4 in the deep crater. We suggest decrease of Mn/NTU with depth reflects a concentration of particulates in the deep water due either to settling of the larger particles, or to mild entrainment or re-suspension of bottom particles in rising hydrothermal plumes. Alternatively, these data could also define a mixing line between a hydrothermal end-member and ambient seawater; however, the particulates in ambient seawater are so low that the Mn/NTU ratio is indeterminable. Ignoring the depth variation, the average Mn/NTU ratio is 4.7±0.6 (2σ standard error). The Mn/NTU ratios of ~3-4 in the deepest crater water are somewhat lower than those observed in plumes on most ridges (where ratios of 10-80 are typical; (Resing et al., 1999; Mottl et al., 1995; Chin et al., 1998). The low ratios at Vailulu’u are caused by the much higher NTU values (0.6-1.4) and lower Mn values (3 ppb) in the deep crater, compared to maximum Mn concentrations in ridge crest plumes commonly in the 2-15 ppb range; (James et al., 1995; Field and Sherrell, 2000 ; Baker et al., 1995; Mottl et al., 1995; Resing, 1999).
Figure 9. Mn versus NTU and 3He (panels A and B), for water samples collected both inside and outside Vailulu’u crater (units, symbols as in Figure 8).
As with the particulates, 3He also shows a broad positive correlation with Mn (Figure 9b) with a wide range in Mn/3He ratio that broadly correlates with depth (Figure 8b). The Mn-He correlation is not a simple two-component mixing between a hydrothermal end-member and ambient seawater, as the 3He in Figure 9b is plotted on a log scale. While the Mn-He data below 2 ppb Mn is closely linear, the higher concentration data is not, suggesting a hydrothermal end-member with Mn/3He ratio varying in the range 0.4-0.7 (ppb/10-12cc/g).
The observed hydrothermal Mn/3He ratio of ~0.5 is several orders of magnitude lower than ratios observed on ridge-crest vent fields, which are typically in the range 30-100 (Lupton et al., 1980; Massoth et al., 1994; Rudnicki and Elderfield, 1992). This could be a result of phase-separation in the hot subsurface waters, because Vailulu’u is much shallower than typical ridge-crest settings (note from Figure 2 that the deep crater waters are in fact slightly “fresher” than the ambient seawater, by about 0.01 permil). Here phase-separation will lead to retention of metal-rich brines in sub-surface waters, but to ejection of volatile and 3He-rich, metal-poor fluids in the vent-plumes (Butterfield et al., 1990). At the depth of the crater floor, the temperature of the two-phase curve for hydrothermal fluids is ~ 320 °C; if the low Mn/3He ratios are indeed evidence for phase-separation, then circulating fluid temperatures in excess of 320 °C are expected for the Vailulu’u hydrothermal system. It is interesting to note that Loihi, with a hydrothermal system at the same depth as Vailulu’u, also has a very low Mn/3He ratio (0.8; Sakai et al., 1987).
The calculated Mn/heat ratio of the crater water is ~2 ng/joule based on the temperature difference of 0.34 °C between inside and outside water at 795m). This is quite low compared to ridge-crest hydrothermal settings; for example, hotsmokers have Mn/Q ratios of 50-60 ng/j (Von Damm, 1990); low-temperature diffuse flow fluids have Mn/Q ratios of 10-40 ng/j (Wheat and Mottl, 1994); and steady-state plumes average 32ng/j (Baker et al., 1993); however, event plumes tend to be lower, with Mn/Q ratios of 3-8 ng/j (Lupton, 2000).
In contrast, the He/heat ratio for the Vailulu’u hydrothermal system (15x10-17 moles/joule) is 20 times higher than typical black smokers and 5 times higher than steady-state ridge crest plumes (Hart et al., 2002; Lupton et al., 1995). The low Mn/heat ratio and high He/heat ratio observed at Vailulu’u is additional evidence for a phase-separation process, as discussed above.
H. Mn Export Budget
With an average Mn concentration in the deep crater water of ~3 ppb and a mass flux of 1x108 m3/day adopted above, the hydrothermal system is exporting ~300 kg/day of manganese. This is about 10 times the typical Mn flux from a ridge crest black smoker (~30 kg/day) and also 10 times the Mn export from the Broken Spur vent field (Murton et al., 1999). From this flux, we can estimate the replenishment time of Mn in the smog halo surrounding the crater rim. From the many nephelometry profiles (collected in 2000 and 2001) an average NTU value of 0.25 is reasonable over a depth interval of 100 meters and within a 3 km radius from the center of the crater (Figure 2). With the average Mn/NTU value of 4.7 described above, the Mn concentration in this halo volume is 1.2 ppb; with a total halo volume of 3x109 m3 the “standing crop ” of Mn in the halo is 3,400 kg. To replenish the Mn in the halo would thus take 11 days, based on the crater export flux of 300 kg/day. Given the evidence cited earlier that the halo can be disrupted on very short time-scales, but that it also appears very persistent over the 4-8 day time scales (in two different years that we have observed it), an 11 day replenishment time seems ineffectively long; it is likely that disruption by mean flows is rare. We suggest that in fact the halo may be the result of internal waves “trapped” to the summit of the volcano, as has been observed at other seamounts (Codiga and Eriksen, 1997; Eriksen, 1991).
I. Hydrothermal Heat Flux
All of the outside water below 765 m is heavier than the crater bottom water, thus any inflow below 765 m will flow directly to the bottom of the crater; inflow above this depth will reach neutral buoyancy levels above the floor of the crater, and spread laterally. Given our simple uniform-velocity flux model, it is not possible to specify the fluxes as a function of depth in the breaches, though it is clear that virtually none of the flow through the 765 m deep SE breach will reach the crater bottom. Consistent with our flow model, we will use the total integrated temperature excess over the depth interval between 730 m (bifurcation depth) and 795 m (deepest breach depth); this average temperature difference (inside-outside) is 0.153 °C. The geothermal/hydrothermal heating represented by this temperature excess is:
Power = Import flux (1157 m3/sec) x ΔT (0.153 °C) x heat capacity (4.3x106 j/m3/°C)
Power = 760 megawatts
This is a large power output when compared to active hydrothermal fields on spreading ridges, where individual hot smokers typically produce 6 MW (Bemis et al., 1993) and hydrothermal “fields” (diffuse plus discrete flows) typically produce 75 MW per km ridge length (Baker, 1996; Baker et al., 1996). The crater output is comparable to that recently measured with ABE at the Main Endeavor vent field (600 MW; Yoerger et al., 2001; Yoerger et al., 2001). It is also comparable to the lower estimates of overall output of the TAG field (both diffuse and discrete flows: 780-2,513 MW; Schultz and Elderfield, 1997), but smaller than the Rainbow field (estimated at 2,300 MW; German et al., 1999a; 1999b; Thurnherr, 2001).
Conclusions
In this paper, we were able to show that Vailulu’u seamount supports an active hydrothermal venting system with both significant thermal and chemical mass fluxes. In particular, we l summarize:
This study reveals Vailulu’u seamount as a source of significant hydrothermal activity and it shows that off-axis volcanoes have great potential for enhancing our understanding the relationships between magmatism and hydrothermalism. It also demonstrates that a complex phenomenon, like a submarine hydrothermal system, is best studied in an integrated scientific approach. In our case, we used monitoring data with in situ oceanographic studies and geochemistry, in particular, four one-year temperature records on the crater rim and inside the crater, a very large number of CTDNF hydrocasts and the analysis of waters for Mn and 3He. Above all, we have shown that this can be done with an icebreaker in tropical waters, not a perfect experiment, but still pretty successful.
Acknowledgements
We are very indebted to the National Science Foundation for funding the seismic monitoring effort and the US Coast Guard for providing ship time opportunity on the ice breakers Polar Sea and Polar Star. We thank the crews of Polar Sea and Polar Star, and their science crews Bill Woityra, Scott Chen, Jim Tallman and Troy Friehammer and April A. Isley, Sean McPhilamy, Chris James, Drew Egressey, David Otani, Ryan Moraros and Rachael Smith, respectively. Captain J. Jackson is complimented for his innovative use of renewable energy for ship propulsion. During the 2001 cruise, we benefited from Gary Klinkhammer and his ZAPS team (Joe Bussell, Karhryn Brooksforce, Charlie Friedericks and Fred Martwick). We thank Paul Cassens and Taulealo Vaofusi for their on-board help, and Tau for the name of Vailulu’u. We thank Peter Craig, Tisa and Candyman for help with local logistics and Phil McGillivary (for Coast Guard logistics and his help with his instructions on offerings to Tangaloa and Mafui’e in keeping the volcano from erupting). We thank Dan Fornari for advice on VEMCO temperature loggers, and the loan of his wax-corer, Josh Curtice and Dempsey Lott for help with the helium measurements, and Jurek Blusztajn and Lary Ball for help with the manganese analyses. We are especially grateful to Jack Whitehead and Karl Helfrich for many tutorials on physical oceanography, and especially their patience while we utilized their derivation of the sill-flow equation.
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