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

Abstract

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 chronologies3,4 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.

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Figure 1: Locations of sediment cores and modern samples used in this study.
Figure 2: Surface-reservoir ages for subtropical and subpolar waters.
Figure 3: Surface and deep reservoir ages in the southwest Pacific for three time slices.

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Acknowledgements

We thank E. Bard and J. Southon for discussions and comments. L. Carter kindly provided material from the core collection and modern shells were provided by W. Blom. L. Robertson helped produce the figures and P. Shane helped with tephra identifications. Radiocarbon analyses at CAMS were performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory.

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Correspondence to Elisabeth L. Sikes.

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Sikes, E., Samson, C., Guilderson, T. et al. Old radiocarbon ages in the southwest Pacific Ocean during the last glacial period and deglaciation. Nature 405, 555–559 (2000). https://doi.org/10.1038/35014581

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