Mesoscale eddies influence carbon export through production, subduction, mixing, and particle redistribution. In this study, a BGC-Argo float observed a deep particle layer during the rapid decay of a cyclonic eddy in the Northwest Pacific. The layer occurred below the subsurface chlorophyll maximum and the main pycnocline, remained confined to a narrow density range near 26 kg m^-3, and often exhibited a particulate backscattering signal comparable to or stronger than that associated with the overlying subsurface biomass maximum. It lacked a spiciness anomaly and showed optical signatures of small, pigment-depleted particles. An idealized model suggests that the slow sinking of particles into the deeper eddy periphery likely results from outward radial ageostrophic transport along sloping isopycnals during eddy relaxation. Together, the observations and model identify an efficient isopycnal pathway for exporting small particles into the twilight zone.