Stanford study finds Arctic currents sink carbon carrying phytoplankton four times faster

Phytoplankton are tiny plantlike organisms that form the base of marine food webs topped by orcas, whales and sharks. In most of the ocean, these organisms are either eaten or sink slowly toward the seafloor, carrying carbon with them as they descend. That journey from the sunlit surface layers typically takes weeks. In the Arctic's Chukchi Sea, however, scientists have documented a much faster route to the bottom, and the implications reach from the sea ice to the sediment below.

A Stanford-led team found that a convergence of ocean currents called a front pushes phytoplankton from beneath the ice to the seafloor at about four times the usual speed. The results appear in two papers published on September 23, 2026, in the Journal of Geophysical Research: Oceans, a publication of the American Geophysical Union. Together the studies describe both the growth of an unusually dense under-ice bloom and the physical forces that drive its remains downward.

The research was carried out in the lab of Stanford biological oceanographer Kevin Arrigo, the Donald and Donald M. Steel Professor in the Stanford Doerr School of Sustainability. Measurements gathered in the Chukchi Sea during the summer of 2023 aboard the research vessel R/V Sikuliaq suggest that sunlight filtering through cracks in the ice was sufficient to power photosynthesis in the water below, an environment once considered too dark for such growth.

"We're observing phytoplankton in huge populations underneath substantial sea ice," said Earth system science Ph.D. student Claudette Proctor, lead author of one of the papers. "They are growing in an environment that we previously thought was inhospitable." The published author lists also include researchers such as Manu Prakash, Robert Pickart and James Lauer, reflecting a collaboration that combined biological sampling with physical oceanography.

Key Facts

Phys.org reported on September 23, 2026, that the two papers detail an unusual descent of carbon-carrying phytoplankton and reveal that blooms can be 10 times denser in the darker waters below thick sea ice than in open water. Beneath the ice, the researchers observed one of the densest phytoplankton blooms ever recorded, even in areas where the ice was up to 2 meters (6.6 feet) thick. That thickness matters, because it shows the bloom was not confined to thin, translucent ice.

The under-ice blooms were up to 10 times more concentrated than those sampled a month later in open water, after the ice had begun to recede because of seasonal melt. To reach those conclusions, the team took seawater samples to measure nutrients and two indicators of phytoplankton density, carbon and chlorophyll, and placed floating sediment traps in seawater, both in open water and in holes cut within the ice, to catch sinking phytoplankton as it fell.

Across much of the sea, phytoplankton sank after their blooms peaked at a rate of about half a meter a day. In other locations, swift waters pushed the plankton down about four times faster on average because of the confluence of currents known as a front. Brightsurf reported on September 23, 2026, that the data show blooms withered and began to sink as they used up most of the available nutrients, especially nitrate.

The Journal of Geophysical Research: Oceans reported on September 23, 2026, that the Proctor-led paper tracked an under-ice bloom across almost its entire growing season during the summer 2023 cruise of the R/V Sikuliaq. The companion paper, led by James Lauer, used floating sediment traps and current data to show that a frontal convergence subducted the sinking phytoplankton toward the seafloor roughly four times faster than gravitational sinking alone. The Proctor paper carries the DOI 10.1029/2025JC023528, while the Lauer paper carries the DOI 10.1029/2025JC023507.

The mechanism, as Lauer described it, involves cold, salty water just below the ice drifting south with the current and colliding with warmer, fresher opposing water in the open sea. "That cold, salty water mass takes a dive, and the warmer, fresher water mass rides up on top," he said, describing how the collision sends the plankton careening downward. The Stanford Doerr School of Sustainability reported on September 23, 2026, that the work was supported by the U.S. National Science Foundation.

Analysis

The bigger picture here is that the Arctic's carbon cycle may be moving carbon to the seafloor faster than standard models assume, at least in the Chukchi Sea. The papers do not claim that every bloom behaves this way, and the authors themselves stress that many unknowns remain. But the measured contrast is hard to ignore: a typical sinking rate of about half a meter per day versus roughly four times that speed where a front forms.

The density finding is equally striking. Blooms up to 10 times more concentrated under ice than in open water challenge the old assumption that thick sea ice blocks photosynthesis. Arrigo and collaborators first discovered phytoplankton blooming under Arctic ice during a 2011 mission, and the ice has thinned and cracked over the past few decades as air and sea surface temperatures have climbed, allowing more light through.

What this really means is that the timing and location of carbon export in the Arctic depend heavily on physical oceanography, not just on biology. A bloom that grows under ice can be extraordinarily dense, and when a front intercepts the sinking material, the carbon reaches the bottom far sooner than a slow gravitational descent would allow. The faster the carbon sinks, the less time there is for it to be consumed or recycled in the water column above.

That said, the study is a snapshot from one sea in one season, and the authors are careful not to overstate it. The two papers together indicate increased carbon export to Chukchi Sea sediments, but questions remain about how widespread frontal subduction is across the Arctic and how it might change as ice cover continues to decline. Even so, the agreement between two independent lines of evidence, biological sampling and sediment traps, strengthens the case that the mechanism is real rather than an artifact of one instrument.

Why It Matters

The findings help explain areas of the Arctic seafloor with unexpectedly high populations of clams, brittle stars, walruses and whales that feed on them. Lauer said the discovery "helps to explain where the food sources to support that benthic biomass might be coming from."

The results also suggest an overall increase in carbon absorbed by phytoplankton and eventually stowed away in sediments, according to Arrigo, the senior author of both papers. That matters for global climate accounting, because carbon that reaches the seafloor and remains there is effectively removed from the atmosphere for long periods.

For a region warming faster than most of the planet, the Chukchi Sea is becoming a natural laboratory for how light, nutrients and currents interact under thinning ice. The 2023 cruise captured a bloom from start to finish, an opportunity that is rare in polar science. "We got to see the bloom come up, peak and then start to come down," Arrigo said. "You almost never get a chance to do that."

Next Up

Many unknowns remain. The authors point to open questions about how often frontal subduction occurs, how much carbon ultimately reaches the sediments, and how the system will respond as sea ice continues to thin and recede. The two papers appeared together on September 23, 2026, under the auspices of the American Geophysical Union.

For now, the work establishes that the Chukchi Sea can export carbon to the seafloor far faster than the textbook picture of a slow, steady marine snowfall would suggest. Future cruises and modeling efforts will be needed to determine whether this accelerated pathway is a regional quirk or a broader feature of a changing Arctic. The answers could reshape estimates of how much carbon the Arctic Ocean buries each year.