Monitoring carbon and climate change in the deep sea

Despite its distance from the sunlit shallows, the deep seafloor is connected to the waters above. Bits of organic matter—including dead plants and animals, mucus, and excreted waste—slowly sink through the water column to the seafloor. The community of animals and microbes on and in the mud digests some of this carbon while the rest might get locked in deep-sea sediments for up to thousands of years.

The deep sea plays an important role in Earth’s carbon cycle and climate, yet we still know little about processes happening thousands of meters below the surface. MBARI is leveraging advancements in robotic technologies to address this. After 25 years of hard work and engineering innovation—navigating both failures and successes along the way—we have developed a long-term solution for monitoring the abyssal seafloor.

The sheer expanse of the deep sea and the technological challenges of working in an extreme environment make these depths difficult to access and study. Engineering obstacles like extreme pressure and the corrosive nature of seawater make it difficult to send equipment to the abyssal seafloor to monitor the animals and ecosystems deep underwater. Scientists actually know more about the surface of the moon than the deep seafloor.

An autonomous robotic rover, Benthic Rover II, has provided new insight into life on the abyssal seafloor, 4,000 meters (13,100 feet) beneath the surface of the ocean. This innovative mobile laboratory has further revealed the role of the deep sea in cycling carbon. The data collected by this rover are fundamental to understanding the impacts of climate change on the ocean.

This revolutionary rover is the result of the unique collaboration between MBARI engineers and scientists, led by Senior Scientist Ken Smith and Electrical Engineering Group Lead Alana Sherman.

Researchers deploy Benthic Rover II from MBARI’s vessel, the R/V Western Flyer. The ship’s crew gingerly lowers the rover into the water and releases it to free-fall to the ocean floor. It takes the rover about two hours to reach the bottom. Once it lands on the seafloor, the rover can begin its mission.

First, sensors check the currents flowing along the seafloor. When they detect favorable currents, the rover moves up or across the current to reach an undisturbed site to begin collecting data.

Cameras on the front of the rover photograph the seafloor and measure fluorescence. This distinctive glow of chlorophyll under blue light reveals how much “fresh” phytoplankton and other plant debris has landed on the seafloor. Sensors log the temperature and oxygen concentration of the waters just above the bottom.

Next, the rover lowers a pair of transparent chambers that measure the oxygen consumption of the community of life in the mud for 48 hours. As animals and microbes digest organic matter, they use oxygen and release carbon dioxide in a specific ratio. Knowing how much oxygen those animals and microbes use is crucial for understanding carbon remineralization—the breakdown of organic matter into simpler components, including carbon dioxide.

After 48 hours, the rover raises the respirometer chambers and moves 10 meters (32 feet) forward, careful not to cross its previous path, and selects another site to sample. It repeats this sampling pattern over and over for the duration of deployment, typically a full year.

At the end of each deployment, the R/V Western Flyer returns to recover the rover, download its data, swap out its battery, and return it to the deep seafloor for another year. Within each year-long deployment, the MBARI team launches another autonomous robot—the Wave Glider—from shore to return quarterly to check on Benthic Rover II’s progress. An acoustic transmitter on the Wave Glider pings the rover on the seafloor below. The rover sends status updates and sample data to the glider overhead. The glider then transmits that information to researchers on shore via satellite.

Data from Benthic Rover II have helped the MBARI team quantify when, how much, and what sources of carbon might be sequestered, or stored, in the abyssal seafloor.

For the past seven years, Benthic Rover II has been continuously operational at Station M, an MBARI research site located 225 kilometers (140 miles) off the coast of central California. Station M lies 4,000 meters (13,100 feet) below the sea surface—as deep as the ocean’s average depth—making it a good model system for studying abyssal ecosystems.

Over the past 32 years, Smith and his team have constructed a unique underwater observatory at Station M. Benthic Rover II and a suite of other instruments operate there 24 hours a day, seven days a week, for a full year without servicing.

Data collected at Station M show that the deep sea is far from static. Physical, chemical, and biological conditions can change dramatically over timescales ranging from hours to decades.

The surface waters of the California Current over Station M teem with phytoplankton in the spring and summer. These seasonal pulses in productivity cascade from the water column to the seafloor. Much of this sinking organic matter—known as “marine snow”—originated as carbon dioxide in the atmosphere.

Over the past decade, MBARI researchers have observed a dramatic increase in large pulses of marine snow falling to the seafloor at Station M. These episodic events account for an increasing fraction of the yearly food supply at this site. In seven years of operation at Station M, Benthic Rover II recorded significant weekly, seasonal, annual, and episodic events—all providing data that help MBARI researchers understand the deep-sea carbon cycle.

Between November 2015 and November 2020, Benthic Rover II recorded a substantial increase in the rain of dead phytoplankton and other plant-rich debris, or phytodetritus, landing on the abyssal seafloor from the waters overhead. A decrease in the concentration of dissolved oxygen in the waters just above the deep seafloor accompanied this windfall of organic matter.

Traditional short-term monitoring tools would not have detected the fluctuations that drive long-term changes and trends. Benthic Rover II has revealed a more complete picture of how carbon moves from the surface to the seafloor. The success of Benthic Rover II and MBARI’s ongoing work at Station M highlight how persistent platforms and long-term observations can further our understanding of the largest living space on Earth.