Why we have better maps of Mars than of the seafloor—and what USGS is doing to change that
Compared to the terrestrial surfaces of our own planet, or even those that are millions of miles away in the solar system, we know so little about the topography of the seafloor that new discoveries of massive features like seamounts—such as this one off the coast of California—continue to be made today.
At the root of this is a problem of technology. The tools we use to image land above water rely on sending and recording wavelengths of light through methods such as satellite imaging or LiDAR. These can produce highly accurate and detailed pictures of the surface of the earth in a matter of days.
But light does not penetrate through seawater beyond the first few hundred feet. With the maximum depth of the ocean plunging to more than 36,000 feet, we need alternative methods for surveying the seafloor.
Mapping the seafloor with sound
While light is rapidly attenuated in seawater, sound can travel for thousands of miles. Seafloor mapping technology relies on sonar devices
mounted either to the hull of a ship or on remotely operated and autonomous underwater vehicles. These devices emit pulses of sound in a fan shape across the seafloor, measuring the time it takes for the sound to travel and reflect off the seafloor, then return to the ship. With as many as 60 pulses of sound per second, the swath of the sonar survey can collect millions of data points of water depth as the ship travels.
Computer software transforms these data into a visual image of the seafloor. In November 2023, scientists from the USGS sailed approximately 350 miles from Honolulu, Hawai’i, to explore a little-surveyed, crescent-shaped area of the seafloor south of the Hawaiian Islands. Working in partnership with BOEM and the NOAA Ocean Exploration Cooperative Institute, the USGS conducted bathymetric surveys and environmental DNA sampling aboard the ten-day expedition. This is the first time this partnership has investigated and characterized the seafloor geology and biology of the Hawaiian abyssal plain.
The deeper the seafloor, the more challenging it is to obtain a high-resolution map, because the sound must travel farther. Abyssal plains are often neglected in mapping efforts since they are deeper than the average depth of the seafloor and therefore challenging to map; they also generally have less charismatic fauna than shallower seamounts. Further, mapping efforts often focus on resolving features that can be seen through low-resolution satellite imagery, and at this resolution abyssal plains appear flat and featureless. However, abyssal plains are the largest ocean floor environment, and their sediments preserve the history of oceanic processes and support crucial biogeochemical processes, including carbon sequestration.
How bathymetry shapes seafloor processes
Bathymetric surveys allow us to understand not only the ocean depths and the shape of the seafloor (is it flat or steeply sloping?) but also tells us about its composition (is the surface texture hard or soft?) based on the relative intensity of the sound reflection. This information is crucially important because the shape and composition of the seafloor affects the physical and biological processes within the ocean. Bathymetric maps are the foundation for deep-sea exploratory missions aiming to understand regional biology, geology, and chemistry, and serve as important tools for defining marine protected areas.
Much in the way that a house with stairs as opposed to ramps to walk up and down—or soft, plush carpet versus hardwood floors—would affect your comfort and mobility in your home “ecosystem”, the dynamic topography and structure of the seafloor shapes the types of ecosystems found on the seafloor. The sharp vertical relief offered by submarine canyons and continental shelves is also what drives ocean currents that cycle vital nutrients and oxygen to sustain marine food webs.
As the nation’s lead scientific mapping agency, the USGS brings a wealth of expertise in the interpretation of bathymetric survey data to understand the shape and geologic structure of the seafloor. Accurate, modern, high-resolution bathymetric maps are essential for delineating and protecting sensitive marine habitats, guiding the wise use of marine resources, ensuring safe maritime navigation, and detecting seafloor geologic hazards that can threaten coastal populations and infrastructure.
A collaborative effort to map the U.S. seafloor
Because of the importance of this information and the existing data gaps, the U.S. is supporting global advancements in ocean mapping through a consortium of federal agencies forming the National Ocean Mapping, Exploration, and Characterization Council (NOMEC). As co-chair of the NOMEC council, the USGS helps lead the charge in coordinating and implementing collaborative mapping, exploring, and characterization of the nation’s ocean waters. With this Hawaiian expedition, the USGS contributes to mapping the gaps in U.S. waters and provides insight into this understudied ecosystem by characterizing its deep-sea biology and geology.
Strategic partnerships developed with other federal agencies such as BOEM and NOAA, as well as with private and academic collaborators such as the Scripps Institution of Oceanography, Schmidt Ocean Institute, and Ocean Exploration Trust, have greatly expanded mapping and exploration of hazards and deep-sea ecosystems off the west coast of the United States.
The USGS has a unique role and mission as the federal provider of research expertise on marine geology, geophysics, and the processes that form and alter seabed and sub-bottom environments. USGS capacities in marine geology, geologic and oceanographic processes, and marine biology, ecology and geochemistry are all essential to NOMEC goals to map and characterize marine hazard and resource potential, seabed ecosystems, and the consequences of human and natural change.
2023 E/V Nautilus Expedition
Working in partnership with BOEM and the NOAA Ocean Exploration Cooperative Institute, scientists from the USGS embarked on a 10-day voyage to the outer limits of the U.S. Pacific waters south of Hawai’i to conduct seafloor mapping and autonomous environmental DNA sampling in order to investigate and characterize the geology and biology of the Hawaiian abyssal plain.
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