JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B1, 10.1029/2000JB000015, 2002
[23] ALISS imaged two flange pools (Lobo Flange and Dudley Flange) during the 1998 Alvin cruise to the Endeavour Segment of the Juan de Fuca Ridge. The relative stability of flange pools provides an excellent environment in which to image pure hydrothermal fluids that are not subject to turbulence, mixing with ambient seawater, and rapid precipitation of minerals. Many of the possible sources of vent light other than thermal radiation such as chemiluminescence, vapor bubble luminescence, crystalloluminescence, and triboluminescence (described in section 6) are dependent on these processes. Hence it can be postulated that light emission from flange pools is solely due to thermal radiation.
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[24] Processed images of Lobo Flange taken with both filter arrays are shown in Figure 8. In each tile the upper portion of the image is brighter than the lower portion. This is due to the oblique orientation of the camera during imaging (see Figure 3). Most of the ambient light is emitted between 700 and 900 nm (at wavelengths longer than 900 nm the attenuation in seawater becomes dominant). Ambient light images from both Lobo and Dudley (Figure 9) show dark features that appear to be sulfide material protruding through the flange pools into cold ambient seawater (as noted by observers on the dive); thus the material does not emit significant thermal light.
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[25] As predicted, light from Lobo Flange appears to be purely thermal radiation. The inversion spectrum from Lobo Flange is shown in Figure 10. Only the brightest/closest portion of the image, ~20 cm2, was used in the spectral calculation. The temperature of the flange pool was measured to be a steady 332°C with the Alvin temperature probe. Because the flange pool is not an ideal blackbody, it has an emissivity of <1; emissivity is the ratio of the thermal radiation of a body to that of an ideal blackbody at the same temperature. The photon flux from Lobo Flange corresponds well to the predicted flux from a 332°C blackbody with an emissivity of 0.9. The high emissivity suggests that the dominant light emission is from the hot rock backing the flange pool rather than the fluid itself; a dark opaque object has a higher emissivity than a semitransparent one. It must be noted that emissivity most likely varies with wavelength, but our analysis is unable to resolve wavelength dependence.
[26] For Dudley Flange, two portions (~11 cm2) of the image were analyzed, differentiated by the presence or absence of dark features in the ambient images (Figure 9). Inversion spectra from these areas are compared to the spectrum of Lobo Flange in Figure 10. The area without dark features corresponds to the emission from Lobo (i.e., purely thermal radiation). However, the area with dark features shows excess emission in the 500–550 nm region. This excess flux is above the detection level of the camera and cannot be explained by thermal radiation. The mechanism by which solid material protruding through a flange pool leads to visible light emission is not obvious. However, the existence of such material may induce local vertical mixing or precipitation on the substrate, which could be associated with nonthermal emission mechanisms.
[27] Black smoker vents were imaged at both 9°N on the East Pacific Rise and the Endeavour Segment of the Juan de Fuca Ridge. Vents in both areas (e.g., Peanut Vent in the Main Endeavour Field and Q Vent in the Venture Hydrothermal Field) emitted light that appeared to be purely thermal radiation. However, other vents showed significant emission at some wavelengths above what is expected for thermal radiation alone. Puffer Vent in the Main Endeavour Field was imaged on two dives 3 days apart. On Alvin dive 3234, thermal radiation and excess visible light was observed; however, on dive 3237, only thermal radiation was detected. We will show data from four black smoker vents, which emit radiation in the visible region greater than that predicted for thermal radiation alone. These vents fall into two categories: (1) excess narrow-band emission in the 500–600 nm region and (2) excess wideband emission.
5.2.1. Narrow-band emission: Puffer Vent and P Vent
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[28] Puffer Vent (dive 3234) in the Main Endeavour Field and P Vent in the Venture Hydrothermal Field have narrow-band emission in the 500–600 nm range. Puffer Vent appeared to be boiling, and a maximum temperature of 372°C was measured inside the orifice. Unlike most black smokers, where the fluid exits from the top of a chimney-like structure, fluid at Puffer Vent is directed downward from a large sulfide structure and then rises buoyantly (Figure 11a). It is difficult to see the orifice under illuminated conditions due to the smoky fluid; however, ambient light images show clearly where the highest temperature (and hence the orifice) is located (Figure 11b). The ambient light image at Puffer Vent was divided into four regions based on the intensity quartiles (as described in section 4.1) (Figure 11c, inset). Pixels in the highest intensity quartile (75–100 percentile) are clustered around the orifice, while pixels in the lowest intensity quartile (0–25 percentile) are found along the fringes of the source mask (~5 cm from the orifice). The inversion spectra from these regions are plotted in Figure 11c. The highest-intensity quartile has an emission spectrum that resembles a blackbody. The temperature just outside of the orifice (the highest temperature that ALISS can “see”) was measured to be ~330°C with the Alvin temperature probe. An emissivity of 0.3 is required to match the thermal radiation from a 330°C source to the flux observed at the orifice of Puffer Vent. Indeed, the semitransparent nature of black smoker fluid would suggest a relatively low emissivity as compared to a flange pool backed by hot opaque rock.
[29] Emission spectra from the regions of Puffer Vent away from the orifice correspond to thermal radiation at long wavelengths (>650 nm). The intensity of the thermal emission decreases with increasing distance from the orifice and hence decreasing temperature. At shorter wavelengths, excess light emission is observed in the 500–550 nm region (Figure 11c). This excess emission is >104 photons cm-2 s-1 sr-1 in the 50–75 percentile region and decreases to near the detection limit in the 0–25 percentile region. Puffer was imaged 3 days later and showed no excess light emission at that time. Thus nonthermal emission in the visible region can vary with time.
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[30] P Vent (Figure 12a) had a maximum orifice temperature of ~377°C. Its inversion spectra (Figure 12b) are very similar to those of Puffer Vent. The flux at the orifice (75–100 percentile region) corresponds to a 340°C blackbody with an emissivity of 0.3 with no excess emission in the visible region. In the regions farther away from the orifice, thermal radiation observed at long wavelengths decreases. In the 50–75 and 25–50 percentile regions an excess flux >104 photons cm-2 s-1 sr-1 is observed in the 500–550 nm band. The spectrum from the 0–25 percentile region shows a flux of ~104 photons cm-2 s-1 sr-1 at 550 nm. These photon fluxes in the visible region of the spectrum must be caused by a source mechanism other than thermal radiation.
5.2.2. Wideband emission: Sully Vent and L Vent
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[31] Sully Vent in the Main Endeavour Field and L Vent in the Venture Hydrothermal Field both emit excess wideband light in the visible region. Sully Vent (Figure 13a) had a maximum orifice temperature of ~373°C and appeared to be boiling. The temperature just outside of the orifice was ~340°C as recorded by the Alvin temperature probe. Inversion spectra are shown in Figure 13b. As with previous black smoker vents, emission at long wavelengths is consistent with thermal radiation. Given a maximum temperature of 340°C, an emissivity of 0.3 is required to fit the observed data in the highest quantile region. At shorter wavelengths (<600 nm), light from the entire source area is on the order of 104 photons cm-2 s-1 sr-1. Light at the orifice (75–100 percentile region) is slightly higher than the lower percentile regions. In addition to the 5-min exposure images, sixty 30-s exposures were also obtained at Sully Vent. The time series obtained from those exposures shows rapid (<30 s) shifts in photon flux in the visible region of the spectrum [White et al., 2000].
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[32] L Vent (marker AdV 4–9) in the Venture Hydrothermal Field (Figure 14a) had internal orifice temperature of ~315°C. The inversion spectrum from the orifice (75–100 percentile region) corresponds to a 295°C blackbody with an emissivity of 0.3. Measurements made at L Vent during an April 1996 cruise confirm a temperature of ~295°C at the orifice corresponding to an internal orifice temperature of ~311°C. Like Sully Vent, inversion spectra from L Vent (Figure 14b) correspond to thermal radiation at the long wavelengths (>600 nm) and are on the order of 104 photons cm-2 s-1 sr-1 at the short wavelengths (<600 nm). However, unlike Sully Vent, emission at L Vent in the 0–25 percentile region is only above the detection limit in the 500–550 nm band.
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[33] One beehive structure was imaged with ALISS at Q Vent in the Venture Hydrothermal Field. The beehive is visible on the left third of the ALISS image (Figure 15a). Sectioning the source area by intensity does not isolate significant portions of the beehive (and the spectra from those regions do not vary significantly). Thus the entire structure was analyzed as one unit (“beehive structure”; Figure 15b). In an effort to analyze pure fluid without rock behind it the area outside of the source region but within 1 or 2 cm of the beehive was isolated (“fluid layer”; Figure 15b).
[34] Inversion spectra from the beehive and the fluid layer are shown in Figure 15b. Photon flux from the beehive corresponds to a 265°C blackbody of emissivity of 0.9 at wavelengths >650 nm. No temperature measurements were made at the beehive. An emissivity of 0.9 was used in the blackbody calculations because the fluid imaged is backed by hot opaque rock, similar to a flange pool. Assuming the light from the fluid layer is dominated by thermal emission from fluid in contact with the beehive structure (i.e., ~265°C fluid), emissivity of the fluid may be as low as 0.1. This low value of emissivity is not surprising given the low absorptivity of clear water. The spectra from the beehive and the fluid layer both show a photon flux of ~104 photon cm-2 s-1 sr-1 in the 600–650 nm region. In the 500–600 nm region the beehive has a flux of 104 photons cm-2 s-1 sr-1, while that of the fluid layer is below the detection level. Comparing light emission from the beehive structure to that from the fluid itself suggests that excess light at 600–650 nm is due to a fluid-related mechanism, such as mixing or turbulence, while the 500–550 nm excess emission observed only at the beehive may be related to mineral-related mechanisms, such as precipitation. The spectrum from the fluid layer shows a dip in emission at 800 nm. While it is not easily seen in the inversion spectrum, ALISS data from the beehive structure indicate a similar dip in the 800-nm channel. This type of decrease in long-wavelength light was not observed at other vents and cannot be easily explained.
Citation: Investigations of ambient light emission at deep-sea hydrothermal vents, J. Geophys. Res., 107(B1), 10.1029/2000JB000015, 2002.
