Click for next page ( 24


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 23
2 Understanding Deep Submergence Science HIGHLIGHTS This chapter Documents the diverse nature of deep submergence science (to establish the need for a mix of expertise and approaches) Documents the significance of this research for efforts to ad- dress some of the most compelling questions in science, not just ocean science (to demonstrate the soundness of supporting this ef- fort despite the cost of working at depth) Describes the geographic extent over which this research must be done (to demonstrate the need to expand the ability to provide a suite of platforms over a wide expanse of the ocean) Describes the variable water depths at which this science must be done (to demonstrate the need for a suite of platforms capable of support work at a range of depths not all on the seafloor) Presents a coherent and logical definition of deep submer- gence science (to support calls for consistent, equitable, and trans- parent mechanisms to provide access to scientific assets) Research carried out at depth in the ocean over the last 40 years has provided dramatic and unique insights into some of the most compelling scientific questions ever posed. Understanding the nature of planetary 23

OCR for page 23
24 FUTURE NEEDS IN DEEP SUBMERGENCE SCIENCE processes and the fundamental constraints on the nature and existence of life has driven scientific inquiry to remote areas of Earth and the solar system. Yet when compared to the vast distances involved in space, the deep ocean lies essentially at "our back door." The remoteness and isola- tion of deep ocean environments makes this region of inner space a par- ticularly fertile field for scientific inquiry. Fundamental contributions to the understanding of processes responsible for plate tectonics and ocean chemistry and physics, as well as the origins of life and mechanisms for speciation, have been made by scientists working at depth in the ocean. There is reason to believe that further discoveries can be expected if ad- equate access to these regions can be provided. In the mid-1970s, during an investigation of geothermal plumes of water along a mid-ocean ridge spreading center of a tectonic plate, a com- pletely unique community of life forms was discovered (Corliss et al., 1979~. Hydrothermal vents, with their unique chemosynthetic communi- ties and with their chemistry profoundly impacting ocean chemistry, are among the most important discoveries of the twentieth century. They have formed the basis for major research programs on the evolution of seawa- ter and on the chemical, biological, evolutionary, and ecological relation- ships of vent organisms; they have revolutionized our understanding of processes controlling seawater chemistry and the origins and evolution of life on Earth; and they have stimulated new hypotheses regarding the possibility of life on other planets (Rothschild and Mancinelli, 2001~. The numerous scientific planning activities carried out by the Ocean Science Division of the National Science Foundation (NSF/OCE) and other organizations over the last decade have identified a number of rea- sons for investigating the depths of the ocean (I. Yoder, National Science Foundation, Arlington, Va., written communication, 2003~. Public com- ments received during the course of this study from human occupied ve- hicle (HOV) and remotely occupied vehicle (ROY) users further identify areas of inquiry with significant scientific potential. (Examples of some of the comments received that pertain to deep submergence science are found in Box 2-1.) Deep submergence science is a diverse field of study involving bio- logical, chemical, geological, and physical oceanography, as well as ma- rine archaeology. Habitats range from the vast midwater environments (Figure 2-1) to the ocean floor; from continents to the depths of the ocean basins; from plate boundaries at spreading ridges and ocean trenches to the remains of ancient and recent civilizations. The diverse nature of deep submergence science necessitates the use of a mix of expertise, ap- proaches, platforms, and tools. There is an acute need for observations, sampling, and conducting of interactive and manipulative experiments in geographically diverse areas, from passive and convergent margins, to

OCR for page 23
UNDERSTANDING DEEP SUBMERGENCE SCIENCE 25

OCR for page 23
26 FUTURE NEEDS IN DEEP SUBMERGENCE SCIENCE ridge crests, ocean basins and the water column above, including as yet poorly studied ocean regions such as the Southern and Arctic Oceans. The ocean margins and ridges are ideal global laboratories to study dynamic interactions among physical, chemical, and biological processes. Submer- gence science provides a powerful way for conducting important research in the geosciences and biological sciences. The subsequent sections dis- cuss only a few examples of the most compelling scientific challenges that call for access to the ocean depths. NOTABLE AREAS FOR POTENTIAL CONTRIBUTION WITHIN THE GEOSCIENCES Enhanced observation, sampling, and interactive experimental capa- bilities will lead to new findings and will thus open up new research per-

OCR for page 23
27 .. . _~ . :_: OF HI rat , i' ~1 ... . :) ~ ~ :~ - . ~ ~ ~ . ~ ) ~ ~ ~ ~ !' , . V) .~ o bC o o _I _I au V) au 5- au 5- V) au o O O ~ 5-, au J ._, ~ . . ~ O

OCR for page 23
28 FUTURE NEEDS IN DEEP SUBMERGENCE SCIENCE spectives and visions. For example, these findings will lead to a better understanding of interactions between the hydrological regimes in ocean margins and the consequences for earthquakes, slope stability, arc volca- nism, ocean chemical balances, the global carbon cycle, clathrate hydrate formation and dissociation, ocean resources (e.g., hydrocarbon reservoirs associated with organic matter maturation in margins), and benthic chemosynthetic ecosystems sustained primarily by natural expulsion of nutrient-bearing fluids from overpressured sediments and rocks. Cold seeps, mud, and serpentine dippiest and the associated chemosynthetic ecosystems were discovered with submersibles, and the great majority of what is known about this new frontier of science was obtained from the use of deep submergence vehicles. In the late 1960s, scientific understanding of how Earth works was completely revolutionized by the newly accepted concept of plate tecton- ics and seafloor spreading. The surface of Earth consists of vast, rigid, lithospheric plates that are in relative motion. The driver of plate move- ment resides at depth, in the mantle. Large convection cells result in the slow motion of the mantle in the solid state. The mantle rises beneath mid-ocean ridges, melts, and erupts to form new ocean crust. The 60,000- km-long mid-ocean ridge system is therefore the location at which plates are generated. Newly created seafloor spreads at rates on the order of 1 to 20 cm per year. The seafloor generated at mid-ocean ridges covers two- thirds of the planet. Old seafloor is consumed at subduction zones, where two adjacent plates moving in opposite directions meet. The denser plate bends and plunges into the mantle, while the overriding plate is com- plexly deformed. At these convergent margins, the subduction of a plate causes differential partitioning of strain and generates earthquakes and volcanism. It is accompanied by dehydration reactions that release fluids, generating partial melting and thus, abundant magmatic activity. Eventu- ally, the subducted plate is recycled into the mantle. Due to its constant renewing, the age of the oldest intact oceanic crust, located in the western Pacific, does not exceed 180 million years, and is therefore very young compared to the age of Earth. This new concept was built on the visionary hypothesis of continental drift proposed by Alfred L. Wegener (1880-1930) at the beginning of the twentieth century to explain the match in shape and geological structure of continents across an ocean such as the Atlantic. In the plate tectonic scenario, continents do not drift but are anchored in the plate and en- trained with it. However, because they are less dense than the ocean crust, i"Diapir" refers to a general class of geologic structure that occurs when a lower-density layer of material is overlain by a higher-density layer of material. Salt domes are a classic example, as are shale diapirs and mud volcanoes.

OCR for page 23
UNDERSTANDING DEEP SUBMERGENCE SCIENCE 29 they are not entrained in subduction. Consequently, the distribution of continents has changed during geological times. The initiation of plate rifting results in continental breakup: passive margins represent the tran- sition between the continent and the newly formed ocean basin. The open- ing of a new basin eventually results in the closure of an older one and the collision of two continental masses. This process has occurred many times in the past and resulted in the building of successive mountain belts that have been subsequently eroded. At present, the closure of the Tethys ocean basin is responsible for the building of the Alps and the Himalayas. A basic principle of plate tectonics is that plates are essentially rigid and do not deform. Therefore, most of the active areas are located at plate boundaries, where the plates are either constructed (mid-ocean ridges) or consumed (trenches associated with subduction). Another essential im- plication is that because these areas are located underwater, the under- standing of processes governing plate tectonic requires exploration, mea- surement, and sampling of the seafloor. Among the various ways of investigating the seafloor, direct approach has proved to be critical. Mid- ocean ridges, passive and active plate margins, and hot spots are the key areas. Their investigation has benefited by, and still requires the use of, submersibles. All of the work accomplished at sea during the last 30 years has contributed to verifying the concept of plate tectonics. There are still a number of processes, however, that are not fully understood. Mid-ocean Ridge Processes With the widespread acceptance of plate tectonic theory, the value of direct observation of the seafloor became obvious to the whole scientific community. The next major step was the discovery of active hydrother- mal vents at the end of the 1970s. "Black smokers" (Plate 4a) venting flu- ids at temperatures in the range of 350C build up hydrothermal chim- neys and sustain completely original chemosynthetic biological colonies that feed on chemical compounds dissolved in hydrothermal waters (Plate 4b). These vents result from seawater convection cells that are activated by magmatic heat, either from magma chambers located beneath the ridge or from cooling magmatic rocks. Mid-ocean ridges then became a major field of interest not only for earth scientists but also for fluid chemists and biologists (Plate 5~. This new scientific excitement led to the creation of the Ridge InterDisciplinary Global Experiments (RIDGE) program by NSF/OCE to study the relationship between geological processes and the biology at mid-ocean ridge systems. In the last 10 years, the focus of ridge studies has evolved from explo- ration and sampling toward in situ experimentation and long-term moni-

OCR for page 23
30 FUTURE NEEDS IN DEEP SUBMERGENCE SCIENCE taring. This has been made possible through the development of instru- ments associated with submersibles new types of samplers and sensors. It is clear that mid-ocean ridges function as systems; the aim now is to understand the linkages between what is happening in the mantle, the nature and distribution of RIDGE biospheres, and ultimately the compo- sition of seawater. "From mantle to microbes" is the leading idea that drives the new RIDGE 2000 program. To answer this fundamental ques- tion, multidisciplinary experiments must be conducted at specific sub- merged locations. This has led the program to select a limited number of sites that represent the entire spectrum of ridge types: fast-; intermediate-; and slow-spreading ridges; hot-spot-influenced ridges; and backarc ba- sins. In these sites, coordinated, multidisciplinary experiments by geolo- gists, geophysicists, chemists, biologists, and oceanographers will require long-term monitoring to determine their temporal and spatial evolution. Some of these "integrated study sites" have already been selected by the community while others are still under discussion, but it is obvious that all will be at depths shallower than 4,500m. Because of improved research techniques these studies will require repeated cruises and more submers- ible time to conduct experiments and to deploy and recover instruments on the seafloor. Logistically, some of the selected sites Juan de Fuca Ridge, East Pacific Rise) are located within the areas where Alvin normally works. The Lau basin, however, in the western Pacific, is very far away. The RIDGE 2000 Science Plan (Ridge 2000, 2003) seeks to address the following questions: How is melt transport organized within the crust and mantle? How does hydrothermal circulation affect characteristics of the melt zone, the crustal structure, and ridge morphology? How does biological activity affect vent chemistry and hydrother- mal circulation? What are the forces and linkages that determine the struc- ture and extent of the hydrothermal biosphere? What is the nature and space-time extent of the biosphere from deep in the sub-seafloor to the overlying ocean? How and to what extent does the hydrothermal flux influence the physical, chemical, and biological characteristics of the overlying ocean? These questions require submersibles to conduct detailed surveys and sampling programs, as well as long-term monitoring. Convergent Margin Dynamics Fluids contained in the subducting plate and overlying sediment and rocks are expelled at various levels under the influence of increasing pres-

OCR for page 23
UNDERSTANDING DEEP SUBMERGENCE SCIENCE 31 sure and temperature, often manifest at the seafloor as fluid seeps, and mud or serpentine volcanoes, associated with prolific chemosynthetic eco- systems (Kulm et al., 1986~. Fluid is carried into subduction zones, both trapped in the pores of sediment and rocks and bound in hydrous miner- als. The fate of the fluid varies from location to location (e.g. Oliver, 1986; Langseth and Moore, 1990; Peacock, 1990; Kastner et al., 1991; Moore and Vrolijk, 1992~. At about half of all convergent margins, most of the sedi- ment carried into the subduction zone is detached and added to the over- riding plate to form an accretionary prism. Rates of fluid flow through these prisms are generally too low to significantly alter the thermal struc- tures of subduction zones. It has become increasingly clear in many other ways, however, that the consequences of the presence and flow of fluids is profound. Where flow is sufficiently focused and fluxes of carbon and sulfur species are high, benthic biological communities are supported on energy derived chemosynthetically. In nonaccretionary subducting mar- gins (e.g., the Mariana-Izu Bonin and Costa Rica subduction systems), the significant pore fluid volume in the underthrust sediments that is carried to a great depth (~15 km) potentially provides a large-volume source of fluid that is released primarily by compaction. Water bound in hydrous minerals in the sediment and altered oceanic basement is carried to even greater depths and is driven off by increasing temperatures and pressures. Decarbonation also occurs at greater depths. The relative importance of these various sources of fluids is beginning to be understood, but the quantities involved and the impact on ocean and mantle chemistries and microbiology are poorly constrained. Early in the evolutionary stages of coarse-grained accretionary prisms, diffuse flow may be an important means of fluid expulsion (Kastner et al., 1993~. In fine-grained, highly consolidated, or well-cemented sediments having low permeabilities, disequilibrium between production and drain- age hence overpressure or super-hydrostatic fluid pressures evolve, causing fracture and fault zones to carry most of the flow (Carson et al., 1990~. Under certain conditioning, hydrofracturing may result (Knipe and McCaig, 1994~. Several chemical and isotopic tracers for tracking fluid flow, such as methane, chlorinity, and helium isotopes, are established. Fluids originating from compaction of sediments transported into the sub- duction system and from dehydration processes of the sediments and the subducting slab are expelled by tectonic consolidation and burial. Extreme pressures can create zero effective pressure (grain to grain contact) condi- tions and effectively fluidize sediments and rock, probably manifest in the formation of mud volcanoes. High fluid pressure may also reduce the effective strength of faults and earthquakes that cause fracturing, facilitat- ing focused fluid flow. Submarine cold seepages, mud, and volcanoes (Fryer, 1992) are thus widespread in margin slopes and trenches, provid-

OCR for page 23
32 FUTURE NEEDS IN DEEP SUBMERGENCE SCIENCE ing life-sustaining nutrients for large microbial communities within a broad range of temperatures, pressures, and diverse chemical environ- ments, from methane-rich to sulfide-rich, and from seawater salinity to brines, most previously unknown. Some of the bacteria may become im- portant for biomaterials. Exploration is an extremely important component in both the RIDGE 2000 and the MARGIN programs. Only a small fraction of plate bound- aries have been explored, and exciting new findings resulted each time a new segment was investigated. Although many similarities exist in the Atlantic and Pacific Ocean hydrothermal vent systems and associated benthic communities, they are rather distinct in some aspects. The same is true for margins. For example, the existence of serpentine and mud volca- noes and cold seeps was a surprising discovery. Their spatial distribution and geochemical significance are as yet unknown. The following are but a few of the very intriguing and high-priority scientific questions dealing with convergent margins that can be ad- dressed through deep submergence science (MARGINS, 1998, 2000a,b): What is the role of fluids in seismicity in how earthquakes influ- ence fluid flow? What is the mass exchange of fluid between the ocean and mantle, and what are the consequences of this flow for the balance of ocean chem- istry, benthic biology, and mantle heterogeneity? How does the partitioning of fluid fluxes between the shallow outer forearc regions and depth determine where fluids contribute to melt pro- duction, arc volcanism, and mantle metamorphism? What is the influence of chemical fluxes on the mechanical proper- ties and deformation of the overriding plate? What is the relative importance of transient versus steady-state hydrological processes? What is the role of organic carbon in the oceanic carbon cycle (margins account for more than 80 percent of the organic carbon in the ocean)? Passive Margin Processes Several important processes can be addressed using submersibles on passive margins. These include determining the mass flux of meteoric water versus seawater recycling in the ocean; the consequences of various flow regimes for ocean chemistry, benthic biology, and slope stability; the sedimentological and tectonic controls on hydrology; the influence of hy- drology on accumulation and migration of hydrocarbons and gas hy- drates; and the magnitude and spatial distribution of the driving forces on fluid flow. The hydrological regimes of passive rifted margins are

OCR for page 23
UNDERSTANDING DEEP SUBMERGENCE SCIENCE 33 largely unknown. Although fluxes, including those from meteoric sys- tems on land, are predicted to be large as revealed by 226Ra enrichment (Moore and Vrolijk, 1992), understanding of fluid flow regimes remains highly incomplete. Fluid flow in passive margins influences the migra- tion and accumulation of hydrocarbons, and, thus, gas hydrate forma- tion and accumulation, slope stability, and the morphology of the margins. Driving forces are derived from thermal pressure and salinity- density contrasts, as well as from topographic heads, but little is known about the magnitudes of the forces or the differences from location to location or with time. Rates of flow are only beginning to be quantified, in a few instances through the use of tracers from meteoric water (e.g., Cable et al., 1996~. The lateral and depth extent of passive margin fluid flow systems is not well understood. Major advances in understanding these processes, and the environmental consequences will depend on accessibility to existing and new platforms for deep submergence sci- ence. Direct measurements of flow rates through sediments, at focused and nonfocused flow sites (using flowmeters deployed by submersibles, for example), can provide essential information on several of the pro- cesses mentioned above. Sampling bottom waters in areas of compli- cated bathymetry represents a technical challenge for conventional wire- line water sampling equipment. ROV-mounted isotopic systems would be ideal for overcoming this challenge. The potential for a large-scale failure of the shelf upper slope, off Vir- ginia and North Carolina, where landslide scars exist, is an example of the acute need for access to innovative submergence science in an environ- ment and geographic region previously neglected. Enigmatic asymmetric cross-cutting "crack"-like features up to 50m deep, arranged in an ech- elon fashion along a 40-km section of the shelf edge have been docu- mented with high-resolution side-scan seafloor imaging chirp subbottom profiling. The cracks appear to result from massive expulsion of gas through the seafloor, creating permeable pathways for updip-upslope gas migration. Gas accumulations in marine sediments have profound global environmental, geotechnical, hazard, and resource significance. Using submersibles to study these features along the shelf edge of the mid-At- lantic margin would provide data on new, previously unknown, methane venting processes and sites in the marine environment. Ocean Geochemistry The mass flux of fluid into the ocean from cold seeps and ridge flanks, and from elevated temperature seeps and hot vents at ocean ridges and hot spots, profoundly influences seawater chemistry. A volume equiva- lent to the entire ocean cycles through the ridge crest and off-axis venting

OCR for page 23
34 FUTURE NEEDS IN DEEP SUBMERGENCE SCIENCE systems in 1 million to 3 million years and through convergent margins in a few hundred million years. Fluid expulsion through the ocean sediments and basement is driven by temperature, pressure, and density gradients. These fluids support rich benthic biological communities on energy derived chemosynthetically, mostly from sulfur and carbon compounds. Life on Earth may have origi- nated at such seeps and/or vents. Global fluid fluxes and their impact on ocean geochemical budgets are unknown. Instrumenting and sampling focused fluid flow sites for short- and long-term monitoring of chemistry and fluxes, and observing the effects of flux fluctuations on the benthic and benthopelagic communities, require submergence facilities (i.e., ve- hicles, special sensors, specialized instruments). The seeps and hydrother- mal vents span a wide depth range, from a few hundred meters at margin slopes, to ~2,500m at ridges and >6,000m at trenches. The fluids emanating from such focused sites into the ocean propa- gate laterally and vertically. Observing their interactions with seawater, in particular the rates and processes leading to mineralization and scav- enging of trace elements (e.g., phosphorus by iron oxyhydroxides), and their effect on the near-bottom and midwater biological communities re- quire submersibles with special capabilities. Gas Hydrate Occurrence and Formation Clathrate hydrates are crystalline compounds in which an expanded ice-like lattice forms cages that contain gas molecules. The hydrates are stable only when gas molecules occupy the cages at moderate to high pressures and low temperatures. Their stability also depends on the com- position of the gas molecule and the pore fluid chemistry (Handa, 1990; Sloan, 1990; Dickens and Quinby-Hunt, 1997~. Methane hydrate is the most common natural gas hydrate on Earth (Sloan, 1990; Kvenvolden, 1995~. Pressure and temperature constraints and the availability of meth- ane restrict methane hydrates to two main environments on Earth: (1) in the modern ocean temperature regime in water depths exceeding 500 m (shallower beneath colder Arctic seas), and (2) on land beneath high-lati- tude permafrost. Even conservative estimates, within an order of magni- tude, converge on the value of ~10,000 gigatons (21 x 10~5 m3) of methane (Kvenvolden, 1988), approximately 3,000 times the amount of methane in the atmosphere. The mass of carbon (C) in methane hydrate is about twice the amount of all fossil fuels on Earth, and thus comprises a major pool linked to the oceanic carbon budget. Methane hydrate is considered important for a variety of reasons. First, its dissociation may be caused by natural tectonic, oceanic, and/or climate processes in the sediments that can lead to large-scale instability

OCR for page 23
UNDERSTANDING DEEP SUBMERGENCE SCIENCE 35 by creating overpressures that can trigger submarine landslides, thus con- trolling natural seafloor mass movements and generating tsunamis (Kayen and Lee, 1991; Booth et al., 1994~. Understanding the dynamics of gas hydrates is important for ocean chemistry and biology. Catastrophic landslides caused by bottom-water temperature and/or pressure fluctua- tions could abruptly release much methane from dissociating hydrates to the ocean, impacting the dissolved oxygen content in the water column. Second, when released to the atmosphere, methane is a powerful greenhouse gas that may have had impact on past climates. Along with CO2 and water vapor, methane is one of the most effective greenhouse gases. Because of the potential for hydrate decomposition as a conse- quence of a greenhouse gas-induced global warming, a small change in stability regime, such as bottom water temperature variations, could re- sult in large and potentially rapid climatic feedback effects (e.g., Raynaud et al., 1993; Behl and Kennett, 1996~. If only a small fraction of the methane hydrate in the world occurs in extractable concentrations, it might be a significant energy resource for hydrocarbon fuel. Although it will con- tinue to increase the global greenhouse effect in the atmosphere, as a fuel it would be significantly less polluting than other fossil fuels. Because methane hydrate is unstable at typical surface temperature and pressure and is highly susceptible to dissociation when perturbed, studies of the dynamics of its nucleation, formation, and dissociation in response to environmental perturbations are very challenging. They re- quire careful new observations, sampling, and recovery at in situ condi- tions. Novel seafloor and water column measurements and experiments, such as in situ interactive and perturbation experiments, as well as devel- opment and testing of new measurement tools, are essential and require the use of submersibles. Studies of gas hydrates in the natural environment involve the skills of a diverse community of scientists and the use of a wide range of exist- ing and newly developing techniques. Some examples of these techniques are high-resolution geophysical seafloor observations and mapping sys- tems; uncontaminated chemical and biological sampling under natural conditions and in situ biogeochemical analyses; and development and testing of new in situ logging for physical and chemical parameters, such as nuclear magnetic resonance (NMR) measurements. Careful integration of well-designed, long-term field studies with an array of submersible vehicles, particularly ROVs closely linked with autonomous underwater vehicles (AUVs), is needed in order to refine quantitative estimates of gas and hydrates in the continental margins, to test various hypotheses for the formation of hydrates, and to understand the circumstances and envi- ronmental consequences of their dissociation.

OCR for page 23
36 FUTURE NEEDS IN DEEP SUBMERGENCE SCIENCE NOTABLE AREAS FOR POTENTIAL CONTRIBUTION WITHIN THE BIOLOGICAL SCIENCES Discoveries made using submersible vehicles have radically altered our understanding of life on Earth, from its origins and present diversity to its evolutionary processes and its future. Underlying most of the bio- logical research in the deep sea are questions of ecological structure and dynamics, whether the fauna occurs on the abyssal plain, in the oceanic water column, at a cold seep, or at a hydrothermal vent. The following important questions remain to be studied and answered: How are these communities structured and how are they organized into functional ecological groups? How have the constituent species adapted to conditions in these habitats that appear to us to be so extreme? benefit? How can we use what we learn about such adaptations for human Evolution and Ecology of the Deep-Sea Benthos Much of the current research is focused on understanding the evolu- tionary and ecological processes that determine the spatial and temporal structure of deep-sea communities. For benthic communities, this includes studies of larval dispersal, patterns of diversity, colonization, and specia- tion (Plate 6a,b). The intellectual drivers for studies of evolution at hydro- thermal vents and cold seeps and, by extension, other deep-sea habi- tats can be summarized by four questions: 1. What are the forces that direct the evolution of chemosynthetic communities and of the deep-sea fauna in general? 2. How do chemosynthetic fauna disperse between ephemeral sea- floor localities that are isolated along or between ridge axes? 3. Do topographic features such as transform faults disrupt gene flow along a ridge system? 4. Do cold seeps, wood, carrion, and whale falls provide stepping stones between isolated vent habitats? Are they responsible for the ob- served levels of diversity? These questions and attendant ecological studies of the factors that structure the diversity of deep-sea communities require in situ observa- tions and experimental manipulations. Capabilities necessary to support this work include the following:

OCR for page 23
UNDERSTANDING DEEP SUBMERGENCE SCIENCE 37 High-resolution quantitative imaging and mosaic mapping Manipulative functions for placement of experimental packages and arrays Discrete, high precision measurements of fluid chemistry at a range of temperatures In situ, real-time characterization of habitats in terms of tempera- ture, chemistry, geology, and flow Sampling tools to provide faunal collections for genetics and for productivity estimates. Increased exploration reveals that biodiversity in the deep sea is far greater than could have been imagined even a decade ago. Understand- ing the broad-scale patterns and processes of biodiversity, on the seafloor and in the water column, is one of the most challenging areas of modern deep-sea biology. Carbon Dynamics Linking Midwater and Benthic Communities A fundamental puzzle of deep-sea biology concerns the transfer of en- ergy from the primary producers at the top of the water column to the animals that inhabit the deep seafloor below, over an average distance of 4,000m. Measurements of this transfer are traditionally based on sediment traps, which collect the slowly sinking, small particles of detritus that are believed to be the principal source of food for the deep. Recently, measure- ments made by deep submergence vehicles of the metabolic energy being utilized by deep benthic communities revealed a significant discrepancy. Substantially more energy is being utilized than can be accounted for by the traditional means of measuring its arrival (Smith and Kaufmann, 1999~. The challenge of this situation is to accurately trace the routes and rates of energy transfer through the water column and its complex, resi- dent midwater communities and accurately measure the input to the benthic community. Preliminary evidence, also acquired by deep submer- gence vehicles, suggests that the spectrum of detritus that reaches the benthos includes large, rapidly sinking particles that are not collected by sediment traps. The in situ perspective provided by undersea vehicles is helping to resolve this global-scale issue, just as these vehicles initially revealed the problem (Druffel and Robison, 1999~. At the interface between the base of the oceanic water column and the deep-sea floor is a region of heightened biomass and resuspended par- ticulate matter known as the benthic boundary layer. Within this "benthopelagic" transition zone live a variety of both active swimmers and passive drifters. In addition, there is a potentially enormous biomass from microbial populations of the underlying abyssal and hadal sediment

OCR for page 23
38 FUTURE NEEDS IN DEEP SUBMERGENCE SCIENCE column and associated basement rock. Because this community is vertically narrow, it is difficult to tow nets accurately within its boundaries. Soft mesh nets suitable for capturing delicate pelagic animals seldom survive contact with the bottom, and trawls rugged enough to survive contact typically de- stroy the soft-bodied fauna they catch. What is required to accurately study this region are vehicles that can measure and control their height above the seafloor, while observing, recording, and sampling the resident fauna. Ecological Structure of Midwater Communities Between the sunlit upper layers of the open ocean and the dark floor of the deep sea is the largest living space on Earth. Within this immense midwater habitat are the planet's largest animal communities largest in terms of distribution area, numbers of individuals, and biomass. It is the least explored of all the Earth's major habitats. Midwater animals are adapted to a fluid, three-dimensional world without solid boundaries (Plate 7a,b). Conducting research in this context presents unique chal- lenges to scientists and to the tools they use. The ecological structure of midwater communities is rooted in the phytoplankton that occupy just the upper 2 to 5 percent of the water col- umn. The sunlight that powers photosynthesis near the surface also serves to illuminate the midwater habitat, at least during daylight hours. The eyes of visually cued predators are surprisingly effective even at dimly lit depths. As a result, a large number of midwater animals follow changes in the light field to stay at light levels optimal to their predator avoidance and prey capture strategies. Although these daily vertical migrations cover at most several hundred meters per individual, cumulatively they are the largest mass migrations on Earth and are a profoundly important dynamic aspect of oceanic ecology. Yet while we have been aware of these migrations since the advent of SONAR (sound navigation and ranging), we know surprisingly little about them. What is needed is the ability to track and observe the migrators throughout their daily cycles, to monitor their activity, and to examine aggregations and species interactions. The means to accomplish these goals are undersea vehicles that can travel through the water column as freely as the animals themselves, with a de- gree of stealth that will not disrupt the natural patterns. Another common adaptation to the reduced midwater light regime is transparency, a highly effective predator avoidance strategy. As much as a third of all macroscopic animals in the upper 1,000 meters are essen- tially invisible. Imaging systems with side-lighting, back-lighting, or po- larization capabilities, in combination with human eyes and high-resolu- tion cameras, are required for observations and enumeration.

OCR for page 23
UNDERSTANDING DEEP SUBMERGENCE SCIENCE Communication in the Deep Sea 39 Biologically produced light, or bioluminescence, appears to be one of the most common form of communication in the oceanic water column and, by extension, on Earth (Widder, 1997~. Midwater animals use biolumines- cence in a variety of ways. While the diversity of luminous organisms has been revealed by conventional sampling methods, the dynamics of biolu- minescence are still unknown. The most promising method for understand- ing the full scope of bioluminescent communication in the deep-sea will be through in situ work with undersea vehicles through direct observation of the behavior of individual organisms and the interactions of multiple or- ganisms that can be compared to bioluminescent patterns. Communication via pheromones and other chemical cues is also thought to be widespread. Such excreted chemicals travel rapidly along fine-scale horizontal structures in the water column. Mapping of such vertically stratified fine-scale structures along a horizontal plane can be done only with nontethered platforms incorporating a real-time feed- back mechanism. Autonomous vehicles and HOVs are the ideal plat- forms for such studies. Gelatinous Animals and the Pelagic Food Web One of the most important discoveries by midwater researchers in re- cent years has been that gelatinous animals form a dominant ecological component of midwater communities worldwide (Plate 8a,b,c,d,e,f). The segments of the pelagic food web occupied by jellyfish (cnidarians, cteno- phores, etc.) includes at least three trophic levels, from primary consumers to top-level carnivores. Because they are perfectly adapted to their fluid habitat, these animals are soft bodied and frequently very fragile. As a re- sult, they were seriously underestimated by conventional sampling meth- ods such as trawl nets. It was not until midwater-capable undersea vehicles became available that the extent and importance of this "jelly Web" was realized. This part of the food web is a partially closed system because so many gelatinous animals feed on other jellies, thus sequestering a substan- tial fraction of pelagic biomass in their bodies. Even less well known is how the organic material consumed by this portion of the overall web is cycled back into the rest of the deep-sea community. The copious amounts of mu- cus produced by some gelatinous animals and their ability to capture small particulates hint at a major role in marine snow production; cnidarians are also known to take up dissolved organic matter. The significance of this aspect of midwater ecology requires further exploration and discovery. Just as undersea vehicles were critical to opening these lines of research, they will be vital to the measurement and process studies to follow.

OCR for page 23
40 FUTURE NEEDS IN DEEP SUBMERGENCE SCIENCE Speciation and Biodiversity The evolutionary, geological, and historical factors driving speciation in the pelagic environment are radically different from those operating in terrestrial and seafloor communities due to the fluid nature of ocean cur- rents and consequent mixing effects. On a geological time scale the isolation of communities from each other can be ephemeral, and understanding how speciation occurs in the open ocean, particularly in the more stable deep sea midwater environment, will undoubtedly force us to rethink paradigms based on terrestrial systems. Factors contributing to the extremely high di- versity seen in pelagic communities, concerning the number of species co- existing at any given survey point, can be elucidated only when the entire community is sampled. For the gelatinous component of this community, this is possible only using undersea vehicles. The sampling of gelatinous organisms using submersible-mounted equipment allows fragile animals to be collected in pristine condition, which in turn allows accurate taxo- nomic data to be collected for these animals. It is only with accurate taxo- nomic data that cryptic species, subspecies, and life stages of some organ- isms can be identified. Without these data we cannot have an accurate description of the biodiversity in the midwater environment. Just as Alvin revolutionized deep-sea benthic biology and geology by providing an in situ perspective, submersibles better suited to operate in the midwater are enabling scientists to radically change the way we see and understand that vast ecosystem. These HOVs and ROVs provide ac- cess to the midwater habitat to enable direct observation and intervention for research. This new viewpoint lets us see midwater animals in the con- text of their environment, instead of being hauled to the surface in the bottom of a net. It allows studies of the dynamic aspects of biology, in- cluding behavior, in situ physiological measurements, and the interac- tions between species none of which were possible before. OCEAN EXPLORATION An important element in the use of deep submergence vehicles is ocean exploration. A recently released National Research Council report, recognized the necessity of an ocean exploration program to identify and describe the ocean's resources. Such a program would "provide opportunities for in- vestigating new regions and that draws on research from a variety of dis- ciplines, would speed discovery and application of new information," in- cluding climate change predictions, beneficial new products, informed policy choices, and enhanced ocean stewardship (NRC, 2003b, p. 3~. The report pointed out that in the United States ocean scientists "rely

OCR for page 23
UNDERSTANDING DEEP SUBMERGENCE SCIENCE 41 on relatively few, large, carefully managed assets ships, submersibles, and laboratory facilities." Because the assets available to ocean science are already stretched thin, any new ocean exploration program that would enhance current efforts "will require substantial assets. New . . . assets would increase the effectiveness of the program, while minimizing inter- ference with the current research endeavors" (NRC, 2003b, p. 14~. As required by its statement of task, the committee on Exploration of the Seas developed budget scenarios for supporting an exploration pro- gram. Three levels of support were described, and the highest level of support recommended including "a ship, three HOVs, five ROVs, and ten AUVs " (NRC, 2003b, p. 140~. In addition, a separate flagship capable of supporting simultaneous ROV and HOV activities was recommended to "maximize program capabilities [and] ensure access and scheduling flex- ibility" (NRC, 2003b, p. 143~. It is important to note that any assets required by an enhanced ocean exploration program are above and beyond those discussed and recom- mended in this report. FUTURE NEEDS Advances in all of the areas discussed above will require highly fo- cused, high-resolution studies that integrate results from various disci- plines. The selection of representative sites must be based on scientific objectives and not dictated by the need to do all research in restricted geographic regions. Great effort must be made to find examples of pro- cesses that are simple as well as representative so that results are as un- equivocal as possible and have utility in a global context. To bear fruit, the majority of the research described above requires only greater access to deep submergence platforms. In some instances, especially midwater work or work at greater depth, research will require more advanced platforms with greater capabilities. Development of an adequate pool of assets necessary for continued deep submergence re- search will require long-term commitments. A well-equipped oceano- graphic fleet, including HOVs, ROVs, and AUVs, is well beyond the de- velopment phase, and its worth to a broad base of marine scientists has been well demonstrated. In many cases, the development and testing of highly specialized and novel instrumentation will be required. In others, experiments may require several years to complete. Clearly some means of achieving an appropriate balance must be found to provide continuity for longer-term projects, experiments, and responsiveness to promote in- novation and access to support monitoring efforts.

OCR for page 23
The scale of inveshnent required for the building or purchasing of an AUV is much smaller than for ROVs or HOVs and the operational cost is also much lower. Therefore, AUVs are often owned and operated at the institution level rather than at the national level. They are very powerful tools, complementary to HOVs and ROVs, ant! are often used in association with other assets. Because of their limiter! payload and power supply, they are generally assigned for specific tasks. The continuing technological development, however, makes them more and more efficient. Therefore, they undoubtedly should be a component of available assets for creep submergence science. FIXED OCEAN OBSERVATORIES Ocean Observatories Initiative In the last decade significant elements of the oceanographic research community have embraced a strategy for sustained time-series investigations, primarily to unclerstanct temporal variability and causality in Earth and ocean processes. In deep submergence science this trenc! is represented! by the recent evolution from exploratory deep submergence expeditions toward regular repeat visits at selected sites. In the United States, the overall trench probably will culminate in the very near fixture in a major investment in long-term seafloor observatory infrastructure by the NSF Ocean Observatories Initiative (OOI). The OOI is the outgrowth of national and international community planning efforts, and various elements of seafloor/ocean observatory science have been addressed in two recent NRC reports. The first of these reports, entitled Illuminating the Hidden Planet: The Future of Seafloor Observatory Science (NRC, 2000), clocumented the need! for long-term fixer} observatory sites in the oceans for conducting basic research to address a broad range in fundamental scientific issues in both ocean ant} Earth science, and concluded that establishing such observatories is feasible in concept. The second, entitled Enabling Ocean Research in the 21st Century. Implementation of a Network of Ocean Observatories (NRC, 2003a) adciressecT in more detail the implementation of a seafloor observatory network for multidisciplinary ocean research in the context of the pencting NSF OOI, and specifically examines! the impact on both the UNOLS fleet and the pool of deep submergence assets in the research community. These observatories, if constructed, will have a significant impact on deep ocean research, especially for time-series studies. DSVs needed to support OOT are separate from, and, would be in addition too, those discussed in this report. THE NATIONAL DEEP SUBMERGENCE FACILITY As discusser! in Chapter I, the NDSF was created in 1974 by the National Oceanic ant! Atmospheric Administration (NOAA), the Office of Naval Research (ONR), and the National Science Foundation (NSF) to provide the nation with a core operational creep submergence team. The first 21 years of NDSF operations included only HOV work with Alvin. During this period various towed geophysical packages and ROVs were developed at a Hunter of U.S. oceanographic institutions, inclucling WHOI. In 1995, the NDSF collection of submergence assets was expanded to include some of the tethered vehicles that had been developed at WHOI, including the ROV system Jason II/Medea and the Argo towed can1era system. The assets currently provided by the NDSF include the HOV AiViM, the ROV system Jason II/Medea, the Argo-II towed camera system (Photo 3-4), and the DSL-120A side-scan sonar system. Use statistics for NDSF vehicles are summarized in Figure 3-l . 42