The ocean is a complex system, with mechanisms that absorb, circulate, sequester and also release CO2, depending on the conditions. Understanding these interlinked processes can give researchers a better idea of what to expect as the climate changes
Among these scientists is UC Santa Barbara geochemist and geobiologist Morgan Raven, who, thanks to a Faculty Early CAREER award from the National Science Foundation, is set to explore a lesser-known mechanism of ocean carbon sequestration — one that might become more conspicuous as the oceans warm.
Carbon sequestration in modern, oxygen-rich oceans is a choreography of physical, biological and gravitational processes. Most of it occurs as a result of the ‘biological pump’ — tiny phytoplankton at the ocean’s surface take in carbon dioxide in the air and dissolved carbon in the water to photosynthesize and to make their shells. They then get eaten by zooplankton which, if they don’t in turn get consumed by larger animals first, conduct a massive migration to the deep ocean where they deposit organic carbon and respire carbon dioxide before returning to the ocean’s surface the next night to repeat the journey.
That process, however, doesn’t explain the abundance of carbon in the sediments at the bottom of ocean anoxic zones, regions of the ocean where there isn’t enough oxygen to sustain zooplankton or any other animal. In recent years Raven and colleagues have discovered that another mechanism was at play. Large particles — sticky blobs of organic matter such as dead phytoplankton, fecal matter and other small organisms — host microenvironments where interesting process can occur, including microbial sulfur cycling, a process that impacts the carbon cycle.
“I’m particularly interested in how sulfides can effectively pickle organic matter,” Raven said. “So if you get a little bit of sulfide in the middle of these particles, you can change the carbon in those particles to a form that’s more likely to survive on these geologic time scales, more likely to make it all the way to the mud in the first place.” In oxygen-rich zones, thanks to zooplankton and other animals, organic carbon often becomes CO2 gas again as a result of eating and respiring. But in these oxygen-free zones, pickled organic particles are far away from animals and protected from enzymes and other substances that may break them down as they drift ever so slowly to the floor.
The process is actually an ancient one — it was much more widespread 145.5 – 65.5 million years ago during the Cretaceous period, when oceans were warmer and therefore had less oxygen, especially at depth. Low-oxygen conditions led to the buildup of organic matter on the anoxic sea floor, deposits we now call black shale.
As the ocean warms, it will hold less oxygen, which has major implications for the biological pump. Anoxic ocean zones will continue to grow, which in turn could make this pickling process more prevalent. Raven and her team are interested in understanding this cryptic process in greater detail.
Through a set of ocean expeditions to the Eastern Tropical North Pacific (ETNP) oxygen deficient zone — a naturally occurring low-oxygen zone off the coast of Mexico in the Pacific Ocean — and some custom-built particle traps, Raven’s team plans to collect particles floating down to the seafloor. Just getting the particles is in itself a feat.
“We want to connect what’s happening in the water column to what actually gets in the mud,” said Raven, who plans to use part of her grant to start an undergraduate research and training class to help students gain the special skills needed to participate in oceanic research expeditions. Undergrads will also be a large part of the field work, she said. The first expeditions are expected to take place in 2023 or 2024.