The Challenge

Ocean nutrient fertilization offers potentially cost-effective biological mCDR, but it requires verification that carbon is exported to depth rather than recycled through surface food webs, and then released into the atmosphere.

Monitoring, reporting, and verification (MRV) is widely considered a main limitation to biological mCDR. Absent established MRV, understanding a perturbation relies on tracking the fertilized water mass. Dissolved tracers track water masses but not sinking carbon (e.g., clumps of plankton), and existing methods for measuring sinking particles cannot directly separate mCDR-enhanced sinking from natural sinking. No single tracer or observing platform captures the full range of spatial and temporal scales involved.

Constraining the volume of ocean impacted by nutrient fertilization, quantifying the fraction of carbon exported to depth, and minimizing secondary environmental impacts are all needed for enhanced fertilization to be considered a viable mCDR method.

Solution: Proposed Optical Tracer Framework

Inert sinking particle tracers could be deployed alongside ship-based mCDR. They would be designed to be detectable across three existing observing systems:

• PACE satellite: surface, daily
• BGC-Argo floats: depth profiles, every few days
• UVP5 imaging: in situ snapshots of large, fast-sinking particles

If observed tracer distributions differ from predictions, that constrains models of physical transport. If observed size-class ratios differ from those deployed, that reveals biological processes such as aggregation. Together, these help attribute enhanced export to a specific deployment, which matters for both scientific validation and carbon credit verification.

What we tested

Three candidate materials span the cost and environmental impact trade-off: recycled glass, quartz sand, and designed glass microspheres. Minimizing that impact matters, as a tracer’s own footprint offsets the carbon it verifies. Each was modeled for optical detectability (Mie scattering) and sinking rate at sizes of 15–105 µm.

Key Findings

• PACE: All three materials have spectral signatures distinct from natural particles. Recycled glass is the more practical choice.
• BGC-Argo: All three materials exceed natural deep-ocean backscatter variability. Microspheres perform best by roughly 3–4×.
• UVP5: The 105 µm imaging limit is the binding constraint. Above it, microspheres are brightest, and glass outperforms quartz.
• Sinking: Tracers sink at 9–15 m/day (15 µm) up to 420–640 m/day (105 µm). This spans the range of natural particles from small detritus to marine snow, with the largest size sinking faster than even marine snow..
• No single material is optimal across all three platforms.

Based on the poster presented at IEEE Oceanic Engineering Society Conference OCEANS 2026 Monterey CA:

This work was presented at a conference but has not been peer reviewed.