Long distance kelp-rafting of rocks around southern New Zealand

Abundant rafts of kelp Durvillaea antarctica float northeast past the southeast coast of the South Island of New Zealand, commonly with attached rocks. Most rafts pass offshore unseen, but strong easterly winds during a series of five cyclonic storm events in 2017–2018 forced rafts carrying exotic rocks onto Dunedin city beaches. The kelp-rafted rocks ranged from boulders weighing several kilograms to centimetre-scale veneers. Kelp genetics confirm a southern New Zealand source and colonising barnacles indicate 10–20 days’ travel time. Distinctive limestones, granite-derived calcareous sandstones and amphibolite facies gneisses were derived from southwestern Fiordland 300–400 km from Dunedin. Most of the exotic rocks were granitoids derived from southwestern Fiordland and/or Stewart and Snares islands. Mafic volcanogenic breccia and pyroxenite were derived from the coast of Foveaux Strait. Kelp rafting is not a major source of coastal sediment but may have been responsible for some exotic clasts in the late Cenozoic geological record in inland Otago.