Ice cores gathered across the Arctic Ocean reveal microplastics at concentrations two to three times higher than previously recorded.
As sea ice melts with climate change, the plastic will be released back into the water, with unknown effects on wildlife, say German scientists.
Traces of 17 different types of plastic were found in frozen seawater.
Microplastics are tiny plastic pieces under five millimetres long. They can be eaten by filter-feeding animals and passed up the food chain.
A considerable amount of microplastic is released directly into the ocean by the gradual breakdown of larger pieces of plastic. But microplastics can also enter the sea from health and beauty products, washing synthetic textiles or abrasion of car tyres.