How soon will landfilled plastics integrate into the geological carbon cycle?

Approximately half of plastic waste ends up in landfills, where fragmentation leads to the leakage of microplastics, nanoplastics, and petrogenic carbon back into ecosystems. However, the timeframe for plastic re-entry into the geological carbon cycle remains unknown. Using landfill-derived field data, we developed a model predicting fragmentation of various polymers into macroplastics, microplastics, fine microplastics, and nanoplastics. We find total waste plastic concentrations range from 85 to 414 mg g^-1, with microplastic, fine microplastic, and nanoplastic generation rates of 2-69, 0.5-36.8, and 0.04-1.9 mg per g of plastic, respectively. Plastic distribution depends more on landfill depth than disposal age. Polyethylene terephthalate fragments faster than polypropylene or polyethylene. Our model predicts peak microplastic and fine microplastic fractions within 157-382 and 412-2118 years, respectively, with approximately half of the plastic-derived carbon available for geological cycling in 80-208 years. This research helps clarify the environmental fate of pervasive plastic pollution.