Sheri N. White

Optical modem technology for seafloor observatories

Regional cabled observatories will bring broadband Internet to the seafloor around areas that include hydrothermal vent sites and other scientifically interesting features. The ideal platform for exploring these sites in response to episodic events is a remotely-piloted, autonomous underwater vehicle (AUV) that is capable of sending back high-quality video or other high-rate sensor data. The combined requirement of remote command/control and high data rates argues for a bi-directional optical communications link capable of streaming data at 1-10 Mbit per second rates. In this paper, we present a preliminary design for an optical modem system based on an omnidirectional source and receiver. The functional requirements and system constraints driven by use case scenarios are first reviewed. This is followed by a discussion of the optical transmission properties of seawater and the resulting impact on detection in high-rate communications, including coding considerations. A link budget and the data rate versus range relationship are developed. Validation results in a test tank and in the ocean will then be reviewed

Raman Spectroscopy in the Deep Ocean: Successes and Challenges

The deep ocean is a demanding environment to the analyst, one that contains fine particles of degraded organic matter and is characterized by high pressures (about 360 atmospheres at 3.6 km depth), low temperatures (down to ;2 8C), and a high concentration (3.5 wt %) of corrosive salts. Such conditions make it difficult to analyze in situ interesting and important geologic materials and dynamic pro-

In situ Raman analyses of deep-sea hydrothermal and cold seep systems (Gorda Ridge and Hydrate Ridge)

The Deep Ocean Raman In Situ Spectrometer (DORISS) instrument was deployed at the Sea Cliff Hydrothermal Field and Hydrate Ridge in July 2004. The first in situ Raman spectra of hydrothermal minerals, fluids, and bacterial mats were obtained. These spectra were analyzed and compared to laboratory Raman measurements of standards and samples collected from the site. Spectra of vent fluid (∼294°C at the orifice) at ∼2700 m depth were collected with noncontact and immersion sampling optics. Compared to spectra of ambient (∼2°C) seawater, the vent fluid spectra show changes in the intensity and positions of the water O-H stretch bands due to the elevated temperature. The sulfate band observed in seawater spectra is reduced in vent fluid spectra as sulfate is removed from vent fluid in the subseafloor. Additional components of hydrothermal fluid are present in concentrations too low to be detected with the current Raman system. A precision underwater positioner (PUP) was used to focus the laser spot on opaque samples such as minerals and bacterial mats. Spectra were obtained of anhydrite from actively venting chimneys, and of barite deposits in hydrothermal crusts. Laboratory analysis of rock samples collected in the vent field also detected the presence of gypsum. Spectra of bacterial mats revealed the presence of elemental sulfur (S8) and the carotenoid beta-carotene. Challenges encountered include strong fluorescence from minerals and organics and insufficient sensitivity of the instrument. The next generation DORISS instrument addresses some of these challenges and holds great potential for use in deep-sea vent environments.

Qualitative and quantitative analysis of CO2 and CH4 dissolved in water and seawater using laser Raman spectroscopy.

Laboratory experiments have been performed using laser Raman spectroscopy to analyze carbon dioxide (CO2) and methane (CH4) dissolved in water and seawater. Dissolved CO2 is characterized by bands at ∼1275 and 1382 Δcm−1. Dissolved CH4 is characterized by a dominant band at ∼2911 Δcm−1. The laboratory instrumentation used for this work is equivalent to the sea-going Raman instrument, DORISS (Deep Ocean Raman In Situ Spectrometer). Limits of quantification and calibration curves were determined for each species. The limits of quantification are ∼10 mM for CO2 and ∼4 mM for CH4. A ratio technique is used to obtain quantitative information from Raman spectra: the gas bands are referenced to the O–H stretching band of water. The calibration curves relating band height ratios to gas concentration are linear and valid for a range of temperatures, pressures, and salinities. Current instrumentation is capable of measuring the highest dissolved gas concentration observed in end-member hydrothermal fluids. Further development work is needed to improve sensitivity and optimize operational configurations.

New Observations on the Distribution of Past and Present Hydrothermal Activity in the TAG Area of the Mid-Atlantic Ridge (26°08′ N)

Seafloor acoustic and photographic imagery combined with high- resolution bathymetry are used to investigate the geologic and tectonic relations between active and relict zones of hydrothermal venting in the TAG (Trans-Atlantic Geotraverse) hydrothermal field at 26°08′N on the Mid-Atlantic Ridge (MAR). The TAG field consists of a large, currently active, high-temperature mound, two relict zones (the Alvin and Mir zones), and an active low-temperature zone. The active mound and the Alvin relict zone lie along a series of closely-spaced, axis-parallel (NNE-trending) faults in an area of active extension east of the neovolcanic zone. The Alvin zone extends for 2.5 km along these faults from the valley floor onto the eastern wall, and consists of at least five mounds identified using DSL-120 sidescan sonar and bathymetric data. The existence of sulfide structures on most of these mounds is verified with near-bottom electronic still camera (ESC) images from the Argo-II deep-towed vehicle, and is confirmed in at least one case with collected samples. Two of these mounds were previously unidentified. The existence of these mounds extends the length of the Alvin zone by ~0.5 km to the south. Much of the Alvin relict zone appears to be buried by debris from a large mass wasting event on the eastern wall of the median valley. The Mir zone, located on normal fault blocks of the eastern valley wall, cannot be clearly identified in the sidescan data and no structural connections from it to the active mound or Alvin zone can be discerned. The active mound is located at the intersection of an older oblique fault set with the younger axis- parallel faults which extend into the Alvin relict zone, and no fresh volcanics are observed in the vicinity of the mound. The fact that both the active mound and the Alvin relict zone lie along the same set of active, axis-parallel faults suggests that the faults may be a major control on the location of hydrothermal activity by providing pathways for fluid flow from a heat source at the ridge axis.

Mineral-microbe interactions in deep-sea hydrothermal systems: a challenge for Raman spectroscopy

In deep-sea hydrothermal environments, steep chemical and thermal gradients, rapid and turbulent mixing and biologic processes produce a multitude of diverse mineral phases and foster the growth of a variety of chemosynthetic micro-organisms. Many of these microbial species are associated with specific mineral phases, and the interaction of mineral and microbial processes are of only recently recognized importance in several areas of hydrothermal research. Many submarine hydrothermal mineral phases form during kinetically limited reactions and are either metastable or are only thermodynamically stable under in situ conditions. Laser Raman spectroscopy is well suited to mineral speciation measurements in the deep sea in many ways, and sea-going Raman systems have been built and used to make a variety of in situ measurements. However, the full potential of this technique for hydrothermal science has yet to be realized. In this focused review, we summarize both the need for in situ mineral speciation measurements in hydrothermal research and the development of sea-going Raman systems to date; we describe the rationale for further development of a small, low-cost sea-going Raman system optimized for mineral identification that incorporates a fluorescence-minimizing design; and we present three experimental applications that such a tool would enable.

Variations in ambient light emission from black smokers and flange pools on the Juan De Fuca Ridge

Ambient light emitted by high-temperature black smokers and flange pools on the Juan de Fuca Ridge was imaged using a new spectral imaging camera. Most of the light is emitted at long wavelengths (700–1000 nm) and corresponds well to thermal radiation from a body at the same temperature as the vents/flanges. However, black smokers also emit time-varying radiation in the visible region (400–650 nm) which cannot be explained by a thermal source. This visible radiation is 1–2 orders of magnitude greater than would be expected for purely thermal radiation; it exhibits variation with time, despite relatively constant vent temperatures; and it is not associated with the hottest part of the plume (i.e. the orifice). Flange pools do not exhibit excess visible light over that for a thermal source, suggesting that the light at smokers is caused by mechanisms related to turbulence, mixing, or precipitation.

Laser Raman spectroscopy used to study the ocean at 3600‐m depth

Making geochemical measurements in the deep ocean is fundamentally difficult. For this reason, century-old technologies using water bottles and cores for sample recovery still provide the basic tools. With the development of research submersibles and remotely operated vehicles (ROVs), however, new opportunities for sophisticated sampling and analysis have arisen.

Precision underwater positioning for in situ laser Raman spectrographic applications

The Monterey Bay Aquarium Research Institute (MBARI) has developed and deployed a laser Raman spectrometer system (DORISS-Deep Ocean Raman In-Situ Spectrometer) for oceanic geo-chemical measurements of sea water specimens. Quality spectra have been obtained on standards carried to the seafloor. The next stage of this development involves the ability to obtain spectra from natural targets of interest in the deep ocean and to maximize signal return from the sample. To accomplish maximum signal return the DORISS probe head must be properly positioned and focused. In addition the probe head must be held steady for several minutes while spectra are being acquired. This is in contrast to laboratory work in which the sample is precisely positioned with respect to the probe head. This requirement has been the driver for a new device, the Precision Underwater Positioning system (PUP). The positioning system has strict requirements for motion about the target. The DORISS requires very exact and repeatable motions with positional accuracies in the <1 mm range over a large workspace. The device must also let the user see where they are focusing, be able to move without disturbing the base position, and to stay stable over a variety of terrains. In addition, the system must be adaptable to other vehicles. Another requirement is to return relative position from a known home position once the PUP is set in place. Moreover, the device has several operational constraints that impacted the design and operation of the system. This paper outlines the science drivers, the operational considerations, and the engineering trades that were made to build the first two stages of the PUP. The test results of this system are also included demonstrating the devices actual performance against the specifications. In conclusion the paper outlines the next tasks and the direction the program is taking to fulfill the complete precision underwater positioning system requirements.

Laser Raman Spectroscopy as a Tool for In Situ Mineralogical Analyses on the Seafloor

In situ sensors capable of real-time measurements and analyses in the deep ocean are necessary to fulfil the potential created by the development of deep-sea platforms such as AUVs and cabled observatories. Laser Raman spectroscopy is an optical technique which is capable of in situ molecular identification of solids, liquids and gases, and is well suited to extreme environments. At present, it is not possible to identify the chemical composition of minerals in situ . Raman spectroscopy has been used successfully for mineral identification in the laboratory. The development of a sea-going Raman system capable of deployment on an ocean observatory or AUV will allow in situ analyses of minerals that are deposited and precipitated at hydrothermal vent and other seafloor sites. This will provide insights into processes occurring in the subsurface which affect ocean chemistry. This paper presents both sea-going and laboratory Raman data of a variety of minerals found in the deep ocean, and instrument parameter issues for a sea-going system optimized for in situ mineralogical analyses on the seafloor