The oceans are complex systems, with mechanisms that can absorb, circulate, sequester, and release carbon dioxide. Understanding how these tightly interlinked processes are functioning can give scientists a better idea of what to expect as the climate changes. Since the oceans are the Earth’s largest carbon sink – sequestering roughly 10 petagrams per year – clarifying the mechanisms of ocean carbon sequestration is an important step forward in tackling the pressing issue of global warming.
Recently, geochemist and geobiologist Morgan Raven, an assistant professor at the University of California, Santa Barbara has received a Faculty Early CAREER award from the National Science Foundation (NSF) in order to explore a cryptic, lesser-known mechanism of ocean carbon sequestration, which could become more conspicuous as the oceans warm: particle-hosted sulfurization and its impact on sedimentary carbon burial.
Most of carbon sequestration occurs as a result of the oceans’ “biological pumps.” Tiny phytoplankton at the surface of the ocean absorb carbon dioxide from the air and dissolved carbon from the water during the process of photosynthesis. Afterwards, they get eaten by zooplankton which conduct a massive migration to the oceans’ depths where they deposit organic carbon and respire carbon dioxide before returning to the surface the following night to repeat the process.
However, these mechanisms cannot explain the abundance of carbon in the sediments at the bottom of ocean anoxic zones, where there is not enough oxygen to support zooplankton or other animals. Recently, Professor Raven and her colleagues have discovered another mechanism playing a fundamental role in carbon sequestration. Large particles – sticky blobs of organic matter consisting of dead phytoplankton, fecal matter, and various small organisms – host microenvironments which are crucial for microbial sulfur cycling, a process that impacts deeply the carbon cycle.
“I’m particularly interested in how sulfides can effectively pickle organic matter,” explained Molly Crotteau, one of Professor Raven’s doctoral students, who will also be involved in the project. “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.”
According to the scientists, this mechanism is an ancient one, being much more widespread during the Cretaceous Period, when oceans were warmer and contained less oxygen (145.5 to 65.5 million year ago). Such low-oxygen conditions led to the buildup of organic matter on the anoxic sea floor, in deposits of black shale. Nowadays, as the oceans warm again, they will once more hold lesser amounts of oxygen, which could make particle-hosting sulfurization more prevalent. In this new project, Professor Raven and her team aim to investigate in greater detail this fascinating process.
“We want to know in finer detail how much carbon gets preserved on the seafloor, and how significant it is. And how other factors will come into play, such as ocean temperatures, circulation and storms,” concluded Professor Raven.