A mobile lab methodically charts deep-sea carbon cycles
Like the robotic rovers Spirit and Opportunity, which wheeled tirelessly across the dusty surface of Mars, a new robot spent most of July traveling across the muddy ocean bottom, about 40 kilometers (25 miles) off the California coast. This robot, the Benthic Rover, has been providing scientists with an entirely new view of life on the deep seafloor. It will also give scientists a way to document the effects of climate change on the deep sea. The Rover is the result of four years of hard work by a team of engineers and scientists led by MBARI project engineer Alana Sherman and marine biologist Ken Smith.
About the size and weight of a small compact car, the Benthic Rover moves very slowly across the seafloor, taking photographs of the animals and sediment in its path. Every three to five meters (10 to 16 feet) the Rover stops and makes a series of measurements on the community of organisms living in the seafloor sediment. These measurements will help scientists understand one of the ongoing mysteries of the ocean—how animals on the deep seafloor find enough food to survive.
Most life in the deep sea feeds on particles of organic debris, known as marine snow, which drift slowly down from the sunlit surface layers of the ocean. But even after decades of research, marine biologists have not been able to figure out how the small amount of nutrition in marine snow can support the large numbers of organisms that live on and in seafloor sediment.
This autonomous system collects high-resolution color images that allow identification of animals to the species level and visual records of significant seafloor features. Using the successful, Dorado-class AUV as its starting point, the Benthic rover is comprised of a high-resolution still camera, two xenon strobe lights, an obstacle-avoidance sonar, an acoustic modem and a navigation sonar. The rover can conduct benthic animal time-series surveys in one third the amount of time it takes to run them with the ROV. The benthic rover will also be used for exploratory investigations in tandem with the mapping AUV.
The rover’s camera fires every 1.6 seconds and produces overlapping images four meters wide that can be merged into one continuous mosaic. The resolution of each image at that altitude is sharp enough to resolve even the tiniest of crab antennae. The light from the twin strobes is carefully controlled to evenly illuminate the seafloor beneath the AUV. The high resolution images and illumination greatly enhance the manual animal identification tasks as well as the accuracy of the computer-based Automated Visual Event Detection system.
At the Monterey Accelererated Research System (MARS) site 891 m (nearly 3,000 feet) below the surface, it’s pitch dark. With no light around, there are no plants making food. So where do all the animals – the brittle stars, clams, rat-tails, lingcods, and rockfish, to name a few – come from? And what are they eating?
The bottom of the food web here is the soft blizzard of marine snow – dead organic matter – drifting down from the productive waters above. Small sea-bottom (benthic) creatures eat the marine snow, fatten up, and become food for larger invertebrates and fish.
Tracking carbon supply and demand
This deep-sea dinner party is one of the last remaining mysteries of the carbon cycle. Scientists know how carbon travels around most of the world, including how living things build their bodies with it and how carbon returns to the earth, air, and ocean – in ways such as decomposition or the burning of fossil fuels.
But beyond about 500 m (1,600 feet) deep in the ocean, scientists have only a fuzzy understanding of supply and demand for carbon. At sea bottom, they’re not even sure if the supply (from sources such as marine snow) balances the demand (from all the benthic organisms). The benthic rover is designed to put some precise numbers on these processes.
The rover is a mobile physiology lab. In a series of three-day experiments, the rover measures how much oxygen seafloor animals are using. Precise motors lower two 30-cm-wide (12-inch) sample chambers into the sediment, where probes record oxygen levels. Two acoustic scanners use ultrasound (in 4-MHz pulses) to look 10 cm (4 inches) deep into the sediment for large animals like worms.
Measuring oxygen consumption helps scientists calculate the seafloor’s demand for carbon. To measure the supply, the rover uses fluorescent scanners. The scanners detect still-active chlorophyll in plant cells, a sign that the cells have only recently settled out from sunlit surface waters.
Perhaps it should be called the benthic creeper
For each experiment, the rover camps out at a single location for 3 days, then moves about 5 m (15 feet) to begin another sample. Before moving, the rover consults a current meter and waits until the current shifts so that its next predecided study location lies upstream. This patient approach allows the rover to keep from stirring up sediment into the area it is about to measure. By the end of 50 measurements, it will have stayed underwater for six to nine months and moved a total distance of about one and a half football fields.
The rover is about the size of a riding lawnmower. It drives across the soft seafloor on caterpillar treads, which distribute the rover’s weight, grip better, and make less impact on the bottom than wheels. Although the rover weighs more than a ton in air, buoyant foam panels reduce that to 45 kg (100 pounds) in water. For the rover to function for up to a year at its maximum depth of 6,000 m (3.7 miles), it is made entirely of titanium and plastic.
To understand carbon cycling in the deep ocean, scientists need rovers that work on their own in much deeper, more remote waters than at MARS. As MBARI engineers tackle these stiff design requirements, MARS lets them work on one problem at a time.
With a constant data link to shore, engineers can test the drive mechanisms before they have perfected the automatic steering programs. They can also test sensor accuracy before the instrument goes down on a year-long stay. If problems crop up, the R/V Rachel Carson and ROV Ventana can be on site to fix things in less than two hours.