JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. B1, 10.1029/2000JB000015, 2002
[2] Light has long been known to exist in the deep ocean in the form of bioluminescence (light emission from organisms) [e.g., Bradner et al., 1987] and Cerenkov radiation (light emission from radioactive decay particles traveling faster than the speed of light in the medium) [Belcher, 1953]. While bioluminescence is visible to the unaided eye, Cerenkov radiation produces subscotopic background radiation in the ocean (~150 photons cm-2 s-1 emitted into a half-space in the 300–630 nm range) [Roberts, 1979]. Recently, it was discovered that high-temperature hydrothermal vents are also a source of light in the deep ocean. This brings up interesting questions regarding the sources of this light and the possible effect on the surrounding biological communities: What is the sources (or sources) of vent light and what does it tell us about physical and chemical processes occurring there? Can organisms living in the vent environment detect this light? If so, how is this information used?
[3] The idea that hydrothermal vents emit visible light was first suggested by Van Dover et al. [1989] following the discovery of a novel photoreceptor on the dorsal side of the Mid-Atlantic Ridge vent shrimp Rimicaris exoculata. This large organ, which contains the photopigment rhodopsin, does not possess imaging capabilities, but it is uniquely designed to detect very low levels of light [O'Neill et al., 1995]. Pelli and Chamberlain [1989] calculated that the thermal (blackbody) radiation due to the high vent temperatures (~350°C) was significant enough to be detected by R. exoculata but not by humans. The existence of vent light was confirmed in 1988 when ambient light from a high-temperature black smoker on the Juan de Fuca Ridge was imaged with a charge-coupled device (CCD) camera [Smith and Delaney, 1989; Van Dover et al., 1994]. Subsequent investigations of vent light (at Snake Pit on the Mid-Atlantic Ridge and Hole-to-Hell on the East Pacific Rise) using a simple nonimaging photometer called Optical Properties Underwater Sensor (OPUS) focused on the red to near-infrared region of the spectrum (700–1000 nm) where thermal radiation was suspected to be strongest [Van Dover et al., 1996]. These studies found temporal variability and excess flux over that expected for a blackbody, both of which suggest sources other than thermal radiation. Later investigations with OPUS (in the 9°N area of the East Pacific Rise) revealed that visible light (in the 400–750 nm range) was also present that could not be explained by thermal radiation alone [White et al., 1996].
[4] Upon exiting a hydrothermal vent orifice, the high-temperature fluid immediately mixes with cold (~2°C) ambient seawater, creating a rising plume of cooling hydrothermal fluid in which metallic minerals precipitate. Thermal radiation is expected to be greatest at the orifice where the temperature is highest. However, other mechanisms of light emission may occur in other parts of the plume away from the orifice and are expected to have unique spectral characteristics. OPUS (or any other nonimaging spectrometer) lacks the ability to image vent light, thus limiting its ability to adequately characterize vent light. A new instrument, Ambient Light Imaging and Spectral System (ALISS), was developed to overcome this deficiency. ALISS is a low-light CCD camera with custom-designed optics which allow it to image a scene simultaneously in nine wavelength bands. By using long exposure times and image processing techniques, even very low levels of light (on the order of 103 photons cm-2 s-1 per unit solid angle at a distance of 50 cm in water) can be detected and analyzed, and the emitting region can be localized.
[5] White et al. [2000] presented preliminary ALISS data from the Main Endeavour Field of the Juan de Fuca Ridge. This paper discusses a broader set of data collected from two deployments of the ALISS camera system in the Pacific and goes a step further in processing the data to translate the count rates detected at the ALISS camera to photon fluxes (in photons cm-2 s-1 sr-1) at the vent orifice. Rough spectra obtained from flange pools, black smoker chimneys, and a beehive vent suggest that whereas thermal radiation is ubiquitous at high-temperature vents, temporally varying, visible radiation (400–700 nm) is also present and cannot be explained by thermal radiation alone. While a mechanism for this visible light has yet to be determined unequivocally, one or more of a number of sources (e.g., chemiluminescence, vapor bubble luminescence, triboluminescence, and crystalloluminescence) appear to be candidates.
Citation: Investigations of ambient light emission at deep-sea hydrothermal vents, J. Geophys. Res., 107(B1), 10.1029/2000JB000015, 2002.