Abstract Particulate organic carbon transported to the deep ocean forms a critical component of the Earth’s carbon cycle. The concentration of these particles is generally parameterized in models to decrease continuously with depth, but isolated layers of small particles (≤ ${le} $20 μm ${upmu }mathrm{m}$) observed within the mesopelagic challenge this assumption. Since their global distribution and contribution to sub‐surface carbon stocks is unknown, we have developed a new algorithm for detecting these layers in backscattering data collected by BGC‐Argo floats. Our results show that these layers are present in 9% of 70,668 profiles and are distributed widely across the global ocean. They are most abundant in oxygen minimum zones (OMZs), where they contribute ∼ $mathit{sim }$16% toward mesopelagic particulate carbon stocks and often coincide with nitrate deficits, suggesting a role of denitrifying bacteria. Outside OMZs, subduction only explains 12.5% of layers, implying the main cause remains unidentified.