Greenland’s seaweed is locking carbon away deep in the ocean

An international, interdisciplinary study has – for the first time – confirmed the oceanographic pathways that transport floating macro-algae from the coastal waters of Southwest Greenland and into deep-sea carbon reservoirs, revealing a potential mechanism in long-term global carbon storage.

Macro-algae – a group that includes kelp and other seaweeds – form some of the ocean’s most productive coastal ecosystems, absorbing large quantities of atmospheric carbon dioxide. Previous research suggests that between four and 44 teragrams of carbon – carried by macro-algae – may be reaching depths beyond 200 metres each year, where it can remain sequestered for at least a century.

Quantifying this contribution, however, has remained challenging, not least due to the diversity of macro-algal species, complex physical transport processes, and limited evidence tracking detached seaweed once it leaves rocky shorelines.

To address these uncertainties, researchers combined satellite observations, ocean drifter data, numerical modelling and advanced turbulence analysis. Together, these approaches demonstrate that large mats of floating macro-algae can be transported hundreds of kilometres offshore before eventually sinking to depth, where their organic carbon may be stored long-term.

Analysis of data from 305 surface oceanographic drifters showed that buoyant macro-algae can be carried from coastal zones into deeper waters over ecological timescales of approximately 12 to 64 days, before the seaweed degrades.

Satellite observations provided further confirmation. More than 1,300 high-resolution Sentinel-2 images, processed via the EU’s Copernicus programme, revealed nearly 8,000 distinct floating macro-algae patches across the Greenland shelf sea and the adjacent Labrador Sea, highlighting the widespread offshore presence of detached seaweed.

Cooling surface waters drive intense ocean mixing, submerging floating macro-algae to depth. Under high pressure, buoyancy structures within the seaweed collapse, causing the material to sink and transport carbon into the deep ocean.