Old radiocarbon ages in the southwest Pacific Ocean during the last glacial period and deglaciation

Marine radiocarbon (C-14) dates are widely used for dating oceanic events a nd as tracers of ocean circulation, essential components for understanding ocean-climate interactions. Past ocean ventilation rates have been determin ed by the difference between radiocarbon ages of deep-water and surface-wat er reservoirs, but the apparent age of surface waters (currently similar to 400 years in the tropics and similar to 1,200 years in Antarctic waters(1) ) might not be constant through time(2), as has been assumed in radiocarbon chronologies(3,4) and palaeoclimate studies(5). Here we present independen t estimates of surface-water and deep-water reservoir ages in the New Zeala nd region since the last glacial period, using volcanic ejecta (tephras) de posited in both marine and terrestrial sediments as stratigraphic markers. Compared to present-day values, surface-reservoir ages from 11,900 C-14 yea rs ago were twice as large (800 years) and during glacial times were five t imes as large (2,000 years), contradicting the assumption of constant surfa ce age. Furthermore, the ages of glacial deepwater reservoirs were much old er (3,000-5,000 years). The increase in surface-to-deep water age differenc es in the glacial Southern Ocean suggests that there was decreased ocean ve ntilation during this period.