Hidden “tails” slow marine snow, impacting deep sea carbon transfer and storage

Newly discovered microscopic mucus tails – trailing from particles of marine snow particles – slow these particles’ descent into the deep ocean, research finds. This doubles the particles’ residence time in the ocean's upper layers and significantly alters estimates of how much carbon is sequestered in the deep sea. The oceans serve as a vast reservoir and critical sink for atmospheric carbon dioxide. A key process driving carbon sequestration in the ocean is the biological pump, where photosynthetic activity at the ocean’s surface leads to the formation of “marine snow” – a shower of organic material falling from upper waters to the deep ocean. The steady accumulation of these particles on the ocean floor has the potential to store carbon in the deep sea for millennia. The formation of marine snow involves dynamic biogeochemical processes, and the particles’ properties, including size, shape, and sinking speed, are central to their role in carbon cycling. However, due to the biological and physical complexity of marine snow particles and a lack of direct observations, a mechanistic understanding of marine snow sedimentation has remained elusive, introducing significant uncertainties in carbon flux estimates in some climate models.

Using ship-based sediment trap sampling, scale-free vertical tracking microscopy, and a model of particle-flow interactions, Rahul Chajwa and colleagues observed the hydrodynamics of marine snow in the Gulf of Maine. Chajwa et al. discovered that marine snow particles are coated with an invisible layer of mucus, forming comet-like tails as they sink. The drag imposed by these hidden mucus tails greatly slows their sinking speed, nearly doubling the residence time of these particles in the upper ocean and reducing the amount of carbon they transport to the deep ocean for sequestration. By integrating these newly discovered dynamics into a theoretical sedimentation model, Chajwa et al. provide a more accurate understanding of marine snow sedimentation. “Chajwa et al.’s model suggests that the difference between the atmospheric carbon dioxide calculated with and without comet tails is on the order of 40 parts per million. Equivalently, the sinking particles with mucus tails sequester a couple hundred gigatonnes less carbon from the atmosphere,” write B.B. Cael and Lionel Guidi in a related Perspective. “Although such estimates are very approximate at this stage, the large magnitude change in carbon sequestration highlights the need to study this phenomenon further.”