Science Questions

The OOI is based upon a community vision resulting from two decades of workshops, meetings, and reports. Science questions helped inform the location of OOI’s arrays, the type of instruments deployed, and the data collected. The OOI was designed to maximize scientific discovery, with the flexibility to help answer the following questions:

What is the ocean’s role in the global carbon cycle?

What are the dominant physical, chemical, and biological processes that control the exchange of carbon and other dissolved and particulate material (e.g., gases, nutrients, organic matter) across the air-sea interface, through the water column, and to the seafloor? What is the spatial (coastal versus open ocean) and temporal variability of the ocean as a source or sink for atmospheric CO2? What is the seasonal to interannual variability in particulate flux? What is the impact of decreasing pH to ocean chemistry and biology?

One of the most striking geochemical patterns observed in the twentieth century is the rising concentration of atmospheric CO2. This discovery, made possible through sustained decadal observations, can be compared to changes in the ocean in only a few places because of the lack 9 of comparable observational capabilities, despite the fact that the ocean plays a dominant role in the global carbon cycle and represents the largest reservoir of carbon on Earth. Observations suggest complex processes that make the ocean a carbon sink are being modified as a result of increasing atmospheric CO2 loading and climate change (20, 21). The exchange of CO2 between the atmosphere and ocean is mediated by air-sea mixing and ocean ventilation, carbonate equilibrium (the solubility pump), and the conversion of dissolved CO2 into particulate and dissolved organic carbon by marine phytoplankton and respiratory pathways (the biological pump). The fraction of the biologically fixed carbon that becomes sequestered in marine sediments is mediated by the structure of pelagic ecosystems. These ecosystem processes are predicted to change as increasing ocean CO2 concentrations decrease ocean pH. Changes to high-latitude food webs, especially in the North Pacific and Southern Ocean, are disproportionately important regions of marine CO2 biogeochemistry and appear to be particularly sensitive.

The air-sea flux of CO2 and biological carbon fixation and sequestration rates are highly variable, and understanding interactions between biology and geochemistry in the ocean is a major challenge for the research community. Addressing this question requires data collected at high sampling rates (hours to days) to quantify the importance of episodic events such as storms to air-sea fluxes and carbon-fixation rates. Adaptive sampling capabilities are needed to adjust data-collection strategies during and after an “event.” Long-term, decadal time-series data are needed to quantify subtle changes in the location and timing of biologically controlled carbon sinks and air-sea sources.

The Northeast Pacific Ocean and Southern Ocean are subpolar/polar regions considered highly vulnerable to the effects of reduced pH and are regions that are sinks for atmospheric CO2. Thus, they are prime locations to site sensors that can obtain long time series. At these sites, surface moorings will support core meteorological sensors and in-water sensors with the capability to accommodate additional sensors. Subsurface moorings will be equipped with profilers that can provide high-vertical-resolution data (e.g., physical, chemical, biological core sensors) in the upper ocean to sea surface, and extended vertical profiles to the seafloor. Data collected by gliders will fill in the gaps between the moorings and will provide a mesoscale context required for data interpretation. To complement these data, and to provide a system-wide regional perspective spanning the coastal ocean to the deep sea, time series of the spatial and temporal variability and trends in CO2 flux, primary productivity and particulate flux, and changes in ocean systems will also be collected over a range of shelf systems.

How important are extremes of surface forcing in the exchange of momentum, heat, water, and gases between the ocean and atmosphere?

What is the effect of extreme (high wind and waves) surface forcing on air-sea fluxes of mass and energy? What is the effect of extreme wind on the structure of the upper mixed-layer? What are the air-sea fluxes of aerosols and particulates?

Improving the knowledge of the mechanisms underlying air-sea exchange is crucial to the interpretation of larger-scale physical and biogeochemical processes. The lack of observations at the air-sea boundary during high wind and sea states is a serious impediment to our understanding of air-sea exchange during extreme atmospheric forcing. Ships are not generally effective sampling platforms in severe weather conditions. Thus, measurements of the exchange of mass (including gases, aerosols, sea spray, and water vapor), momentum, and energy (including heat) across the air-sea interface during high wind conditions (> 20 ms -1) are rare. The availability of these data has been identified as critical to improving the predictive capabilities of storm forecasting and climate-change models, and for estimating energy and material (e.g., carbon, nitrogen) exchange between the upper and deep ocean. Severe storms and other extreme events can greatly affect coastal populations, and so are of particular interest to federal operational partners such as the National Oceanic and Atmospheric Administration (NOAA) and the Department of Homeland Security (DHS).

Continuous measurements above and below the air-sea boundary for periods of years to decades will provide the data needed to understand extreme surface forcing of episodic, seasonal, annual, and decadal processes. Measurements will be taken just above and below the sea surface that in the past have been difficult to obtain with standard moored sensors, especially in regions of high wind. The OOI platforms will have sufficient stability and power to support a suite of rugged meteorological and in-water sensors to enable studies of the dynamics of marine storms, upper ocean circulation, primary productivity, ocean carbon fluxes, and climate. Real-time communications will enable adaptive sampling of subsurface measurements to assess the efficacy of the gas exchange during the storm events, and real-time data will be used to derive parameterizations for coupled air-sea models.

In what ways do severe storms and other episodic mixing processes affect the physical, chemical, and biological water-column processes?

What are the effects of variable strength storms on surface boundary layer structure and nutrient injection into the photic zone? How do storm-induced nutrient injections influence primary productivity, and vertical distribution and size structure of particulate material?

Water column mixing is central to driving ecosystem productivity by replenishing nutrients to the euphotic zone; however, if mixing is too vigorous, overall productivity is suppressed by light limitation. The nonlinear interaction between mixing and light availability, and the corresponding ecosystem response, remains a central question to biological and chemical oceanography. These nonlinear processes impact overall phytoplankton community composition, which in turn affects entire food webs. In the past it has been difficult to measure the impact of mixing on ecosystem dynamics. Traditional approaches have not allowed scientists to maintain a persistent presence in the ocean to quantify the role of high- and low-frequency mixing events. The relative role of episodic and seasonal mixing events on the overall productivity of marine ecosystems remains an open question; their importance relative to large 11 cyclical phenomena (El Niño Southern Oscillation, Pacific Decadal Oscillation, North Atlantic Oscillation) remains difficult to evaluate.

The OOI will provide the infrastructure to persistently observe mixing processes in the ocean and assess the corresponding impact on the marine ecosystems. The distributed OOI assets will measure parameters necessary for studying air-sea exchange processes, mixed-layer depth dynamics, material exchange across the base of the mixed layer, internal wave dynamics, the evolution of benthic boundary layers, and changes in the composition and size distribution of the phytoplankton. The measurements will be made on horizontal scales of meters to kilometers and vertical scales of millimeters to meters. Water column data will be collected at high frequency by profiling moorings, including the critical upper 200 m. Data collected by the profiling moorings will be spatially extended by running coordinated transects of AUVs and gliders. Observations of resuspension and benthic boundary layer dynamics will be enabled with sensors mounted at several depths above the seafloor.

The broadly distributed OOI sensor network will measure numerous parameters, from the deep sea to the near-shore coastal ocean, which will enable comparisons of a range of ecosystems. The high-latitude subpolar sites are representative of regions with severe weather, high CO2 flux, and oligotrophoic and High Nutrient Low Chlorophyll (HNLC) areas. Data from the Pacific Northwest will provide information on a prototypical wind-driven upwelling/downwelling system characterized by a narrow continental shelf, highly variable seafloor topography, buoyant river plumes, and immediate connectivity with the open ocean. These data will be compared to those collected from the contrasting regimes such as the Middle Atlantic Bight, which has a broad continental shelf where mixing is impacted by warm core rings, detached bottom boundary layers at the shelf-slope front, and the frequent passage of severe storms impacting one of the most strongly stratified ocean systems on Earth (often a temperature gradient of over 20 degrees within 2–3 meters in the summer).

How does plate-scale deformation mediate fluid flow, chemical and heat fluxes, and microbial productivity?

What are the temporal and spatial scales over which seismic activity impacts crustal hydrology? How do the temperature, chemistry, and velocity of hydrothermal flow change temporally and spatially in subsurface, black smoker, diffuse, cold seep, and plume environments? How are these systems impacted by tectonic and magmatic events?

The oceanic crust is the largest fractured aquifer on the planet. Thermally driven fluid circulation through the oceanic crust profoundly influences the physical, chemical, and biological evolution of the crust and ocean. Fluid circulation within this aquifer provides heat and nutrients that sustain a vast biosphere at and below the seafloor. Despite some progress in sampling these environments, many of the most important fundamental questions remain, such as what is the depth and extent to which microbial life occurs within the subseafloor, and what is the 12 relationship between submarine plate-tectonic and sedimentary processes and seafloor and subseafloor ecosystems? Transient events such as magmatic eruptions at mid-ocean ridges increase nutrient (e.g., carbon dioxide) output and water venting volume by as much as a factor of one hundred, resulting in extensive microbial blooms and the growth of chemosynthetic communities at the seafloor. Organisms sampled from high-temperature ecosystems at deep-sea hydrothermal vents have challenged our understanding of the physical and biochemical conditions under which life not only exists, but thrives. Biotechnical research of this vast biosphere is leading to the development of pharmaceuticals important in fighting disease and infections, and biocatalysts (enzymes) that are more efficient, thermally stable, and cost-effective than synthetic catalysts important in material processing for industries.

An OOI sensor network on the Juan de Fuca Plate will examine the connection among seafloor spreading, volcanic activity, hydrothermal flow, and the subseafloor biosphere. Because volcanic and tectonic activities are episodic, instruments will be maintained for long time periods so that episodic events can be captured. Some sensors will be located where several-year time series studies have documented temporal changes in microbial communities following an underwater eruption. Instruments will also supply near-real-time data that will document the connection between crustal strain and subseafloor fluid flow. The Juan de Fuca Plate hosts the highest density of instrumented Ocean Drilling Program (ODP) and Integrated Ocean Drilling Program (IODP) sites of any place within the ocean crust. Similar to how NEPTUNE (NorthEast Pacific Time-Series Undersea Networked Experiments) Canada will connect to instruments in the Site 1027 borehole, the OOI will bring near-real time access to data from other ODP/IODP borehole experiments within reach, particularly when instruments provided by individual investigators are considered.

It is becoming increasingly apparent that the effects of magmatic and tectonic events are not limited to the near field. Stress changes induced by fault motions and the passage of seismic waves from distant earthquakes may trigger earthquakes and perhaps even volcanic eruptions. These events have been shown to perturb hydrothermal systems and methane seep environments. To more accurately correlate individual vent or seep activity to plate-scale events, the OOI infrastructure on the Juan de Fuca Plate will acquire data from seismometers, flow gauges, and chemical and biological sensors in arrays. Seafloor observatories are equally important for understanding the progressive changes in venting systems and the biological communities that occur between major events. These observatories can also be used to examine shorter-term perturbations in flow such as those that arise from tides.

What are the forces acting on plates and plate boundaries that give rise to local and regional deformation and what is the relation between the localization of deformation and the physical structure of the coupled asthenosphere-lithosphere system?

What is the style of deformation along plate boundaries? What are the boundary forces on the Juan de Fuca Plate and how do the plate boundaries interact? What are the causes and styles of intraplate deformation? What is the return flow from the ridge to the trench? How much oceanic mantle moves with and is coupled to the surface plate? How and why do stresses vary with time across a plate system?

Tectonic plates are the fundamental building blocks of the lithosphere, and their active boundaries display abundant earthquake and volcanic activity. The Juan de Fuca Plate in the Northeast Pacific Ocean exhibits all major types of oceanic plate boundary—subduction zone, mid-ocean ridge, and transform fault—within a relatively small area. As a consequence, wholeplate seismological and geodynamic observations there enabled by a network of broadband seismometers with associated hydrophones will permit investigation of processes that control the formation, evolution, and destruction of the plate and of the interactions of that plate with the leading edge of a continental margin. Observations of seismicity—particularly, its temporal and spatial distribution, and variations in intensity—will also provide insight into the nature and causes of stress variations with time across the entire plate, the styles and causes of intra-plate earthquakes, the episodic nature of submarine volcanism, and the coupling of forces across plate boundaries. Additionally, such a network will provide a regional context for other, more local experiments with apertures on the order of kilometers.

A plate-scale seismic array will also facilitate studies of the structure and evolution of the lithosphere-asthenosphere system by enabling finer resolution of the seismic velocity structure in three dimensions. In combination with data from land-based studies, a plate-scale seismic array will allow imaging of the deep and shallow structure that accompanies plate formation, evolution, and subduction. Such work will contribute to our understanding of mantle melting, the mechanical coupling of the asthenospheric mantle to the lithosphere, the pattern of return flow from trench to ridge, the nature of mantle flow near contrasting plate boundaries, the rheology of the mantle, and the importance of three-dimensional plate-scale structure for localizing and influencing seismogenic deformation.

How do tectonic, oceanographic, and biological processes modulate the flux of carbon into and out of the submarine gas hydrate “capacitor,” and are there dynamic feedbacks between the gas hydrate reservoir and other benthic, oceanic, and atmospheric processes?

What is the role of tectonic, tidal, and other forces in driving the flux of carbon into and out of the gas hydrate stability zone? What is the significance of pressure change on hydrate stability and methane fluxes due to winter storms and pressure pulses, and bottom currents interacting with topography? What is the fate of hydrate/seep methane in the ocean and atmosphere?

A significant amount of the methane near Earth’s surface is locked into gas hydrates in shallow sediments on continental margins. These gas hydrates may act as a capacitor in the carbon cycle by slowly storing methane that can be suddenly released into the ocean and atmosphere. It is important to understand the degree to which gas hydrates permeate seafloor and subseafloor environments, and the role they play in modulating carbon flux among the solid Earth, hydrosphere, atmosphere, and biosphere. Long-term observations are required to constrain hypotheses about system evolution and response to transient internal and external forcing events.

Hydrate Ridge in the central Cascadia accretionary complex is one of the best-studied gas hydrate deposits. Seafloor venting and formation of gas-rich hydrate deposits near the seafloor have been documented there through ODP drilling and by a series of seafloor studies using submersibles and Remotely Operated Vehicles (ROVs). These studies provide a basis for understanding how gas hydrates are distributed in marine sediments and the processes that lead to heterogeneity in this distribution. In this area, the subseafloor has been imaged with three-dimensional seismic data, which define a focused plumbing system that provides clear targets for observatory instruments. Data from observatory sensors will help to define the temporal evolution of this gas hydrate system, determine material fluxes from the earth into the ocean, and understand biogeochemical coupling associated with gas hydrate formation and destruction.

The stratigraphically controlled plumbing system at Hydrate Ridge contrasts with the gas hydrate system explored on the northern Cascadia margin in scientific ocean drilling programs. Here, fluid flow appears to be controlled by structures that cut across stratigraphic horizons. The central and northern Cascadia margins also provide a strong contrast in lithology, with much greater abundance of coarse-grained sediments in the north, which affects the nature of gas hydrate deposition. Collecting long-time-series data at both Hydrate Ridge and central and northern Cascadia will lead to a comprehensive understanding of gas hydrate processes as a function of lithology, stratigraphy, and structure. The observatory will include fluid samplers, seafloor cameras, in situ chemical sensors (e.g., CH4, H2S), IODP borehole sensors with increased sampling rates, and repeat high-resolution seafloor bathymetric, imaging, and plume surveys by autonomous vehicles with docking capabilities. As many of the key processes associated with gas hydrate systems occur over short time scales (e.g., gas hydrate release due to small and large earthquakes), real-time data transmission and the capability for adaptive response and sampling adjustment are fundamental “transformational” requirements to enable advancements in this research.

How do cyclical climate signals at the El Niño Southern Oscillation, North Atlantic Oscillation, and Pacific Decadal Oscillation time scales structure the water column, and what are the corresponding impacts on ocean chemistry and biology?

What are the effects of climate signals on variability in water column structure, nutrient injection in the photic zone, primary productivity, and vertical distribution and size structure of particulate material? Are secular climate change trends detectable in the oceans?

Atmospheric forcing at seasonal to inter-decadal time scales strongly influences the structure of marine food webs. Understanding when and how marine ecosystems shift between equilibrium states is a widely debated and central issue for both the research and marine resource management communities because many ocean food webs appear to be undergoing major shifts. For example, limited time series in the subtropical North Pacific Ocean show substantial changes in the phytoplankton, zooplankton, and pelagic fish biomass during the mid-1970s and 1980s. Early evidence indicates another shift may have occurred in the late 1990s. The North Atlantic has also exhibited recent shifts in circulation, with corresponding declines in copepod and cod stocks.

Untangling and understanding the causes, processes, and consequences of the different ocean climate cycles is an important problem confronting oceanographers. These cycles span years to decades upon which episodic events (e.g., storms) are superimposed. Thus, scientists require high-frequency (minutes), sustained (decades) time-series data across a range of ecologically relevant spatial scales. High-frequency data are needed to resolve the physical structuring of marine food webs, which can be heavily influenced by short-lived episodic events. The OOI network of distributed assets will measure atmospheric and in situ physical, chemical, and biological properties. Vertical resolution of the system will range from less than one meter to tens of meters; horizontal resolution will range from meters to hundreds of kilometers. For many of the ecosystem questions, OOI data will resolve the chemistry and the particulate matter in the upper 200 m of the water column. Given that many of the signatures of these large-scale processes are resolved at local and regional scales, the network will include coastal and global sites. Network nodes in the North Pacific will obtain data on the El Niño Southern Oscillation and Pacific Decadal Oscillation cycles, while nodes in the subpolar and subtropical Atlantic will resolve the impact of the North Atlantic Oscillation. A node in the subpolar Southern Ocean will fill critical gaps in current data sets.

How does topography-driven mixing maintain the observed abyssal stratification?

What processes are responsible for enhanced near-boundary mixing? How is heat transported into the ocean interior? What is the role mean seasonal versus episodic processes? What is the importance of the abyssal stratification and how is it maintained?

To date, the available data implies that there is insufficient turbulence in the deep interior oceans, away from the boundaries, to produce enough vertical mixing to account for the vertical profile of temperature. In short, the deep ocean is warmer than it should be given observed levels of in situ turbulence that can mix surface heat downward. Surface heat in the tropics and subtropics has to be mixed downward to balance the upwelling of the cold bottom waters formed at high latitudes that disperse into all ocean basins. Without some mechanism of downward mixing of heat, in about 3000 years the entire ocean beneath the thermocline would fill up with the very cold water created at high latitudes. Formation of such a deep, thick isothermal layer would have serious physical, biological, and climatic implications, including the cessation of the very convection of cold water that created it, thus halting the pull of warmer subtropical water toward the poles, especially in the far North Atlantic. This Meridional Overturning Circulation (MOC), considered an important contributor to the relatively warm climate of Europe, would cease to exist. This, at least, is one highly regarded hypothesis of the importance of the abyssal stratification. Understanding how the abyssal stratification is maintained is a critical issue for understanding potential variations in the MOC and resultant climatic impacts.

Observations in the past 15 years have uncovered enhanced levels of turbulence near the water-earth boundary in regions of rough and acute topography. The proposition, now well established, is that the required vertical mixing to maintain the abyssal stratification occurs near regions of strong topography, with the mixed products being distributed horizontally by mesoscale eddies and “mean” currents. Many physical mechanisms have been identified as possible contributors to the topography-catalyzed mixing. Through rapid sampling of physical variables over long periods of time, scientists will be able to sort out the nature of these mechanisms, their relative importance, their temporal and spatial variability, their dependence on changes in environmental factors (e.g., mesoscale current strength, internal wave energy levels), and how they can be parameterized in models of the ocean’s general circulation and climate. The OOI Network will provide continuous and plentiful power to support numerous sensors, high-bandwidth data communications, and the needed observational infrastructure to tackle these important questions.

What are the dynamics of hypoxia on continental shelves?

What are the relative contributions of low-oxygen, nutrient-rich source water, phytoplankton production from local upwelling events and along-shore advection, and local respiration in driving shelf water hypoxia? What are the impacts of shelf hypoxic conditions on living marine resources? How are wind-driven upwelling, circulation, and biological responses in the coastal zone affected by the El Niño Southern Oscillation, water mass intrusions, and inter-decadal variability?

Unlike hypoxic events fueled by anthropogenic nutrients and limited circulation of semi-enclosed estuaries or embayments, hypoxia on the continental shelf is driven by atmospheric forcing, upwelling/downwelling, and variability in ocean circulation. Low dissolved oxygen concentrations have been documented in the coastal waters off Oregon during late spring to 17 summer of 2002 to 2007. Some events were accompanied by mass die-offs of commercially important shellfish and finfish. Large regional-scale oxygen depletions have also been documented on the Middle Atlantic Bight. Upwelling brings nutrient-rich, oxygen-poor deep waters onto the shelf, fueling phytoplankton blooms, which, in turn, reduce oxygen levels in the near-seafloor water column through decomposition. In contrast, the 2002 event was triggered by an invasion of low-oxygen, subarctic water from the Gulf of Alaska, depressing dissolved oxygen levels in offshore waters from Vancouver Island to southern Oregon. The formation and duration of hypoxic areas are subject to climate variability and variations in upwelling/downwelling and oceanic flow on seasonal, interannual, El Niño Southern Oscillation, and inter-decadal scales. Understanding hypoxic events and impacts to marine ecosystems requires the ability to observe physical, chemical, and biological conditions across the continental shelf to slope waters, for periods spanning years (seasonal to interannual change) to decades (El Niño Southern Oscillation and Pacific Decadal Oscillation shifts). This is an especially pressing problem as the impact of the low-oxygen water can trigger mass mortalities in high tropic levels.

OOI’s distributed network of fixed and mobile platforms will permit studies of the frequency, intensity, and mechanisms driving the invasion of low dissolved oxygen water on continental shelves. Large, three-dimensional data volumes collected by gliders will provide detailed information for making maps of the low dissolved oxygen waters; gliders will also adaptively map the spatial extent and morphology of the low dissolved oxygen intrusion. For studies of coastal ocean processes, from event-scale changes, to interannual variability, to interdecadal trends, data will be collected by permanent and movable instrumented arrays that have sufficient power and bandwidth to support multidisciplinary sensors. These nodes will also collect time series of the gradients in physical and biogeochemical properties across the continental shelf and slope. By combining these data with simultaneous observations of the meteorological forcing and oceanic flows measured at high vertical resolution, scientists will be able to study the corresponding chemical and biological response to the low dissolved oxygen water.

How do shelf/slope exchange processes structure the physics, chemistry, and biology of continental shelves?

What processes lead to heat, salt, nutrient, and carbon fluxes across shelf-break fronts? What is the relationship between the variability in shelf-break frontal jets and along-front structure in coastal organic matter? What aspects of interannual variability (in stratification, offshore circulation patterns, jet velocities, and wind forcing) are most important for modulating shelf/slope exchange of dissolved and particulate materials?

There are numerous shelf/slope exchange processes that transfer significant amounts of heat, salt, and organic matter between continental shelves and the deep sea. These mechanisms are highly variable in space and time; many operate over kilometer scales in the horizontal dimension, but may only span meters in the vertical. Exchange may last only a few days; extreme events such as 18 storms appear to play a large role in sustaining exchange and dissipating heat, salt, and organic matter. Traditional shipboard sampling cannot provide sufficient spatial and temporal resolution to constrain, much less quantify, these shelf/slope processes and as a result it is not possible to derive robust budgets for carbon, heat, salt, and other properties on continental shelves. These deficiencies contribute significantly to our inability to quantify the flux of carbon between continental shelves and the deep sea. These shelf/slope exchanges are also critical in structuring continental shelf food webs because megafauna are often known to congregate at locations of intense exchange.

High-frequency spatial and temporal data collected by the OOI Network on the U.S. east and west coasts will enable scientists to quantify these exchange mechanisms and identify their impact on shelf/slope biogeochemistry. Because exchange can vary in location along the shelf/slope due to the passage of offshore features (e.g., warm/cold core rings), collecting realtime, in situ data will permit adjustment of AUV volumetric sampling patterns. Profilers will collect high-frequency data to characterize the water column, from the seafloor to the sea surface, to capture the properties of water masses as they pass through the array. Meteorological measurements will be taken so that the impact of wind forcing on exchange can be studied. Given that the many of the shelf/slope waters are optically complex, the sampling strategy will include optical characterization of dissolved and particulate material to describe the distribution of constituents (e.g., sediment, phytoplankton, detritus). To enable future studies of other shelves and processes, the array itself will redeployable at the completion of the initial study.