Long-term reworking of volcanic ash deposited in the abyssal ocean based on uranium and thorium isotope measurements

A red-clay core from north of New Zealand contains a 21 cm-thick layer of the 340 thousand-year old Rangitawa ash. Uranium and thorium isotope measurements on this core show that ash continuously contributes to sediments overlying the ash layer, ranging from ~ 58% immediately above the ash to ~ 8% at the core top. Rather than from an upward bioturbation or subsequent volcanic eruptions, the ash in sediments is most likely sourced from suspended Rangitawa ash in the nepheloid layer that has steadily re-deposited at the core site ever since the Whakamaru super-eruption. We suggest that this lateral transport bears important influence on interpreting marine sediment proxy records.