Ocean currents accelerate carbon sinking in the Arctic’s Chukchi sea

A pair of studies led by biological oceanographers at the Stanford Doerr School of Sustainability reveals that ocean currents in the Arctic’s Chukchi Sea are accelerating the descent of carbon-carrying phytoplankton to the seafloor at four times normal speeds

Published in the Journal of Geophysical Research: Oceans, the findings show that massive phytoplankton blooms thrive under thick sea ice and, upon dying, are driven rapidly down to benthic ecosystems by colliding ocean currents—a mechanism with major implications for global ocean carbon modelling.

Massive under-ice blooms and photic adaptation

During a summer 2023 expedition aboard the research vessel R/V Sikuliaq, the research team documented the full life cycle of under-ice phytoplankton blooms. Contrary to long-held assumptions that thick ice starves marine plant life of light, the measurements demonstrated that sunlight filtering through sea ice cracks fuels extreme photosynthetic activity.

Extreme Bloom Density

Phytoplankton populations beneath sea ice up to 2 meters thick were up to 10 times denser than blooms sampled weeks later in open, ice-free waters.

Nutrient Depletion and Sinking

As the under-ice microalgae depleted available nitrates, the blooms withered and began their downward descent toward the ocean floor.

Frontal dynamics drive 4x acceleration in carbon export

While phytoplankton across most global ocean regions sink slowly at roughly half a meter per day, the Stanford team identified specific localised hydrodynamic zones, known as ocean fronts, where carbon export is dramatically amplified:

1. Hydrodynamic collision:

   • As cold, dense, nutrient-depleted water carrying under-ice plankton drifted south, it collided with warmer, fresher, less-dense open-water currents.

2. Subduction downwelling:

   • The collision forces the cold, salty water mass, along with its suspended organic carbon, to plunge rapidly downward toward the ocean floor, carrying phytoplankton down at four times normal speeds.

3. Benthic ecosystem support:

   • This rapid downwelling explains how rich seafloor communities of clams, brittle stars, walruses, and bowhead whales thrive in localized regions of the Arctic shelf despite harsh conditions above.

Implications for climate change and global carbon cycles

The Arctic is warming at four times the global average rate. As ice recedes, the timing and physical dynamics of under-ice blooms could alter the balance between pelagic (surface) and benthic (seafloor) food webs:

• Carbon sequestration:

  • The rapid sinking mechanism increases the volume of atmospheric carbon absorbed by surface algae and permanently sequestered in deep-sea sediments.

• Food web misalignment:

  • If warming causes under-ice blooms to peak earlier, surface zooplankton and migratory predators may arrive after the primary food supply has already plunged to the seafloor, favouring bottom-feeding organisms over surface-dwelling species.