On the Efficiency and Durability of Purposefully Sinking Seaweed Biomass as a Marine Carbon Dioxide Removal Strategy - Preprint

[Geoengineering News]

https://essopenarchive.org/doi/full/10.22541/essoar.176306401.14802254/v1

Authors: Michaela Sten, Kana Yamamoto, Timothy DeVries, Sebastian Krause, David A Siegel

13 November 2025

Abstract

Large-scale farming and purposeful sinking of seaweed has been suggested as a marine Carbon Dioxide Removal (mCDR) strategy. Farmed seaweed uptakes dissolved inorganic carbon (DIC) from the mixed layer, resulting in a CO2 deficit that causes an influx of atmospheric CO2 into the surface ocean. The carbon-rich seaweed is then harvested and conveyed to depth where the sequestered biomass is either eventually remineralized back to DIC or incorporated into sediments on the seafloor. To explore the efficiency and durability of seaweed mCDR, we simulate the advection and mixing of a DIC deficit through the ocean with a steady state global ocean circulation model that includes an interacting atmosphere with realistic air-sea gas exchange. We find that for many locations in the global ocean seaweed sequestration is nearly as efficient as direct air capture (DAC) and can be durable (sequestered for >100 years) if the biomass is conveyed rapidly (≥ 1,000 m d-1) to the seafloor at deep water sites (depths ≥ 2,000 m). Durability timescales and efficiencies are significantly reduced for slower vertical conveyance speeds (< 100 m d-1) and at shallower sites (depths < 500 m). Seaweed CDR efficiency and durability metrics are determined by local rates of vertical mixing, global circulation pathways, and the time required for CO2 air-sea gas exchange. Understanding the factors that control seaweed mCDR efficiency and durability including location (deep waters), conveyance strategy (rapid sinking), and the fate of biomass on the seafloor (long remineralization timescales) is critical for effective seaweed mCDR.

Source: ESS Open Archive