JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B1, 10.1029/2000JB000015, 2002
6.1. Light Emission and Possible Sources
[35] ALISS data show that light is ubiquitous at all types of high-temperature hydrothermal vents (i.e., flange pools, black smokers, and beehives) in the East Pacific and that thermal radiation is the dominant source at wavelengths above 650 nm. This radiation is dependent entirely on the temperature of the fluid and its emissivity. Since flange pools and beehive structures are backed by hot, opaque rock, they are expected to have a higher emissivity than the semitransparent fluid of a black smoker. Emissivity was estimated from ALISS data and recorded vent temperatures by assuming that long-wavelength light emission was purely thermal radiation. Lobo Flange and Dudley Flange both emit light consistent with thermal radiation from a 332°C body of emissivity of 0.9. The temperature of black smoker fluid drops by 20–30°C on exiting the orifice, and the emissivity appears to be ~0.3. Clear hydrothermal fluid (such as that emanating from a beehive) may be as low as 0.1.
[36] Mechanisms other than thermal radiation are also responsible for light emission at hydrothermal vents. A number of these mechanisms are discussed in greater detail by Reynolds [1995] and White [2000]. Tapley et al. [1999] documented chemiluminescence, the direct conversion of chemical energy to light, during sulfide oxidation in seawater. Hydrothermal vent fluids contain significant concentrations of sulfide and undergo rapid mixing with oxygenated seawater entrained in the vent plume, making chemiluminescence a likely source of vent light. Light emission can be associated with both crystallization and fracturing of minerals, crystalloluminescence (XTL) and triboluminescence (TL), respectively. A number of minerals prevalent at hydrothermal vents, within the chimney structure and as precipitates in the plume, such as sphalertie (ZnS) and chalcopyrite (CuFeS2) [Haymon, 1983; Tivey and McDuff, 1990] are known to be TL and XTL active [Nelson, 1926; Reynolds, 1995, and references therein; Walton, 1977]. Chakravarty and Walton [2000] and Reynolds [2000] describe a newly identified type of luminescence (vapor bubble luminescence) associated with the condensation of macroscopic vapor bubbles in water, produced by injecting steam into water. Emission is observed both in freshwater and seawater and does not appear to be wavelength dependent in the 380–600 nm region. Because of the complexity of hydrothermal systems and the lack of detailed information on possible mechanisms, we cannot confirm quantitatively the extent to which any nonthermal source contributes to vent light.
[37] Although flanges were expected to emit purely thermal radiation, nonthermal emission was also observed. Some flanges have solid material protruding through the hot flange pool into cold ambient seawater. This is seen as dark features in the ambient light images (e.g., Figure 9). At Dudley Flange, excess light emission in the 500–550 nm band is observed in the vicinity of this material. It is possible that precipitation is actively occurring on these features or that they induce local mixing. Analyses of the protruding material may help to determine whether processes such as crystalloluminescence or chemiluminescence are occurring due to those mechanisms.
[38] The ALISS data show three cases of purely thermal radiation at black smokers: Peanut Vent and Puffer Vent (dive 3237) on the Juan de Fuca Ridge and Q Vent at 9°N on the East Pacific Rise. A number of black smoker vents also emit excess light over that predicted for thermal radiation in the visible region of the spectrum (<700 nm). Repeated measurements at Puffer Vent and Sully Vent suggest that the intensity of this light can vary over time [White et al., 2000]. The spatial and spectral data obtained with ALISS can be used to infer which sources are responsible for the nonthermal light. Most nonthermal light is associated with the cooler regions of the plume. This can be seen at Puffer Vent (dive 3234) and P Vent where significant light is observed away from the orifice (0–25, 25–50, and 50–75 percentile regions) at ~500–600 nm. This suggests mechanisms related to mixing (chemiluminescence) or precipitation (crystalloluminescence or triboluminescence) which do not occur directly at the orifice. As hydrothermal fluid exits a vent, it entrains and mixes with oxygenated, ambient (~2°C) seawater. This mixing and associated cooling leads to sulfide oxidation and the precipitation of minerals such as sphalerite in the vent plume [e.g., Baron, 1998].
[39] In some cases, excess light over thermal radiation is observed at the orifice (75–100 percentile region). At Sully Vent and L Vent, significant light is observed from 450 to 600 nm for the entire source area. These vents do not appear to be different from the other vents in any way (i.e., in chemistry, temperature, size), so their wideband emission is perplexing. The only proposed mechanism known to emit wideband radiation is vapor bubble luminescence. However, this mechanism is most likely to occur away from the orifice where quenching of the exiting fluid occurs. It is possible that the wideband light observed at these vents is due to a combination of sources. Experiments on chimney pieces revealed strong TL/XTL emissions from the inside of chimneys (predominantly chalcopyrite) at 450 nm (C. Z. Radziminski et al., unpublished report, 1997). Thus it is possible that at some vents we are observing the signal from mineral precipitation within the chimney conduit scattered outward. The remaining emission from the 500–600 nm region could be due to chemiluminescence or precipitation of other minerals.
[40] Only one beehive structure was imaged with ALISS. An attempt was made to analyze light emission from both the structure itself and the fluid emanating from it. A flux of ~104 photons cm-2 s-1 sr-1 was observed from both the sulfide structure and the fluid at 600 nm. This emission may be generated in the fluid layer (and thus superimposed on the structure emission). Because the fluid appears to be clear (i.e., no significant precipitation is observed), chemiluminesence (possibly due to sulfide oxidation) is the most likely source mechanism. ALISS data from the sulfide structure show significant light emission in the 500–600 nm band above that predicted for thermal radiation. This emission could be due to crystalloluminescence from the precipitation of minerals on the beehive structure.
[41] A summary of the ambient light data obtained with ALISS is given in Table 1. The photon fluxes determined from the ALISS measurements are less than those recorded by the OPUS instrument [Van Dover et al., 1996]. OPUS was only capable of measuring count rates for each filter. However, as discussed above, the interference filters used by both ALISS and OPUS allowed light leakage outside of the passband. Thus, without inverting the data (as was done with the ALISS measurements), accurate photon fluxes at the vent could not be obtained with OPUS.
[42] Hydrothermal vents are home to unique biological communities whose primary producers are chemosynthetic bacteria that extract energy from vent chemicals rather than sunlight [Jannasch, 1995]. These bacteria live symbiotically with more complex organisms such as tube worms or are free living and grazed upon by heterotrophic organisms [Hessler and Kaharl, 1995; Van Dover, 2000]. Because of the depths at which these communities live (thousands of meters below the sea surface) their environments were originally thought to be devoid of significant light. Hence it was not surprising to scientists to find that some shrimp (e.g., Rimicaris exoculata) lacked conventional eyes and eyestalks [Williams and Rona, 1986]. We now know that significant nonsolar light does exist at hydrothermal vents in the deep ocean and that the supposedly blind shrimp have unique nonimaging photoreceptors capable of detecting low levels of light [O'Neill et al., 1995; Van Dover et al., 1989]. The question must now be asked: How does vent light affect surrounding biological communities?
[43] Are vent animals capable of seeing vent light, and if so, how do they use this information? As discussed in section 1, the vent shrimp Rimicaris exoculata has developed a dorsal photoreceptor uniquely designed to detect low light levels [O'Neill et al., 1995]. Two other Mid-Atlantic vent shrimp (Chorocaris chacei and Mirocaris fortunata) were found to have similar photoreceptive organs [Van Dover, 2000, and references therein]. No comparable photosensitive macroorganisms have been found in the East Pacific where the present data were obtained. Calculations by Pelli and Chamberlain [1989] suggest that the photoreceptors of R. exoculata are capable of detecting a 350°C vent; however, they note that only behavioral evidence can prove that the shrimp actually see vent light. If R. exoculata does see vent light, it may have either positive or negative phototactic responses: being able to see vents may lead the shrimp to food (i.e., the chemosynthetic bacteria prevalent at vents), or away from vent fluid hot enough to cook them [Van Dover, 2000].
[44] A more challenging (and controversial) question is: Does photosynthesis occur at deep-sea hydrothermal vents? A number of phototrophic organisms are adapted to live in low-light conditions. These include organisms living at the bottom of the pelagic photic zone, in shallow benthic environments, and beneath polar ice. Bacteria appear to adapt well to low-light conditions. Five strains of a photosynthetic, brown, sulfur bacterium (Chlorobium phaeobacteriodes) have been isolated from a depth of 80 m in the Black Sea [Overmann et al., 1992]. All strains were found to be extremely low-light adapted, thriving at light intensities <6 × 1013 photons cm-2 s-1. In fact, the amount of light reaching 80 m depth was calculated to be on the order of 1011 photons cm-2 s-1. Species of green bacteria (e.g., genera Pelodictyon and Ancalochloris) and purple bacteria are also able to survive at low light levels (~6 × 1013 photons cm-2 s-1) and low sulfide concentrations [Pfennig, 1978]. However, even these extremely low-light levels are well above the levels observed at deep-sea vents.
[45] For an organism to be able to photosynthesize using vent light it must contain pigments that absorb at long wavelengths (where light emission is strongest), and it would have to live close to the vent fluid where attenuation effects are minimal. However, attenuation of light in seawater is strong at long wavelengths (~0.2 cm-1 at 950 nm). At a distance of 3.5 cm from the flange the intensity drops by half. At 11.5 cm away the intensity is reduced to one tenth of that at the flange pool. Thus the possibility of hydrothermally driven photosynthesis is marginal, and, at present, no known phototrophs have been identified at deep-sea vents [Kolber et al., 2000; Yurkov and Beatty, 1998]. While obligate photosynthesis is unlikely at deep-sea vents, facultative photosynthesis and photoheterotrophy cannot yet be ruled out.
Citation: Investigations of ambient light emission at deep-sea hydrothermal vents, J. Geophys. Res., 107(B1), 10.1029/2000JB000015, 2002.