Most marine plastic pollution originates on land. However, once plastic
is at sea, it is difficult to determine its origin. Here we present a
Bayesian inference framework to compute the probability that a piece of
plastic found at sea came from a particular source. This framework
combines information about plastic emitted by rivers with a Lagrangian
simulation, and yields maps indicating the probability that a particle
sampled somewhere in the ocean originates from a particular source. We
applied the framework to the South Atlantic Ocean, focusing on floating
river-sourced plastic. We computed the probability as a function of the
particle age, at three locations, showing how probabilities vary
according to the location and age. We computed the source probability of
beached particles, showing that plastic found at a given latitude is
most likely to come from the closest source. This framework lays the
basis for source attribution of marine plastic.