1. School of Environmental and Marine Sciences and Department of Geology, University of Auckland Private Bag 92019 Auckland, New Zealand
2. Antarctic Cooperative Research Centre, GPO Box 252-80, Hobart, Tasmania, Australia
3. Institute for Antarctic and Southern Ocean Studies, University of Tasmania, GPO Box 252-77, Hobart, Tasmania, Australia
4. Centre for Accelerator Mass Spectrometry, Lawrence Livermore National Laboratory L-397, 7000 East Avenue, Livermore, California 94551, USA
5. Department of Earth & Planetary Sciences, 20 Oxford Street, Harvard University, Cambridge, Massachusetts 02138, USA

Correspondence to: Elisabeth L. Sikes^1,2 Correspondence and requests for materials should be addressed to E. L. S. (e-mail: Email: e.sikes@auckland.ac.nz).

Marine radiocarbon (¹⁴C) dates are widely used for dating oceanic events and as tracers of ocean circulation, essential components for understanding ocean–climate interactions. Past ocean ventilation rates have been determined by the difference between radiocarbon ages of deep-water and surface-water reservoirs, but the apparent age of surface waters (currently 400 years in the tropics and 1,200 years in Antarctic waters¹) might not be constant through time², as has been assumed in radiocarbon chronologies^{3, 4} and palaeoclimate studies⁵. Here we present independent estimates of surface-water and deep-water reservoir ages in the New Zealand region since the last glacial period, using volcanic ejecta (tephras) deposited in both marine and terrestrial sediments as stratigraphic markers. Compared to present-day values, surface-reservoir ages from 11,900 ¹⁴C years ago were twice as large (800 years) and during glacial times were five times as large (2,000 years), contradicting the assumption of constant surface age. Furthermore, the ages of glacial deep-water reservoirs were much older (3,000–5,000 years). The increase in surface-to-deep water age differences in the glacial Southern Ocean suggests that there was decreased ocean ventilation during this period.