Coupling fragmentation to a size-selective sedimentation model can quantify the long-term fate of buoyant plastics in the ocean.

Plastic pollution in the ocean is a global environmental issue, with buoyant debris accumulating at the surface and posing long-term ecological threats. Although sediments are the ultimate sink for plastics, a mismatch between observed surface concentrations and estimated inputs implies the understanding of vertical sedimentation mechanisms and rates are inaccurate. Here, we present a coupled fragmentation-sedimentation model that quantitatively predicts the vertical transport and long-term fate of buoyant plastic debris and microplastics (MPs, less than 5 mm). Using a representative 10 mm polyethylene (PE) particle, we show that fragmentation into small MPs is essential for their incorporation into marine snow aggregates (MSAs) and subsequent settling. Even after 100 yr, ca. 10% of the initial plastic mass still remains at the surface providing a continual source of small MPs to ocean surface waters. This study provides the first mechanistic framework linking large plastic degradation to size selective sedimentation, demonstrating that plastic pollution will persist at our ocean surfaces for over a century even if inputs cease. Our findings highlight the need for mitigation strategies beyond input reduction and ocean clean-up, addressing the long-term removal of existing ocean plastics.This article is part of the Theo Murphy meeting issue 'Sedimentology of plastics: state of the art and future directions'.