A chemical analysis of sediment cores from the North Pacific Ocean show a consistent pairing of volcanic ash and hypoxia, a low ocean oxygen interval spanning thousands of years, during times of rapid climate warming at the end of the last ice age, new research shows.A chemical analysis of sediment cores from the North Pacific Ocean show a consistent pairing of volcanic ash and hypoxia, a low ocean oxygen interval spanning thousands of years, during times of rapid climate warming at the end of the last ice age, new research shows.