Reduced deep ocean ventilation in the Southern Pacific Ocean during the last glaciation persisted into the deglaciation

Marine radiocarbon (C-14) is widely used to trace ocean circulation and the C-14 levels of interior ocean water masses can provide insight into atmosphere-ocean exchange of CO2 the since the last glaciation. Using tephras as stratigraphic tie points with which to estimate past atmospheric Delta C-14, we reconstructed a series of deep radiocarbon ages for several time slices from the last glaciation through the deglaciation and Holocene in the Southwestern Pacific. Glacial ventilation ages were much greater in magnitude than modern and had a strong mid-depth Delta C-14 minimum centered on similar to 2500 m. Glacial radiocarbon ages of intermediate depth waters (600-1200 m) were similar to 800 to 1600 C-14 years, about twice modern and persisted through the early deglaciation. Notably, in the glaciation shallower depths were significantly more enriched in C-14 than waters between 1600 and 3800 m, which were similar to 4000 to 6200 C-14 years, or about 3-5 times older than modern. Abyssal waters deeper than 4000 m were also more C-14 rich than the overlying deep water. With radiocarbon ages of 1800-2300 C-14 years, this was similar to modern values. In the early deglaciation, Delta C-14 depleted waters were flushed from shallower depths first and replaced with progressively younger waters such that by 18 ka, the deep to intermediate age difference was reduced by half, and by similar to 14 ka a modern-type Delta C-14 profile for deep ocean water masses was in place. Our results 1) confirm a deep C-14 depleted water mass during the LGM and early deglaciation, and 2) constrain the extent of this old water in the Southern Pacific as between 1600 m and 3800 m. The availability of atmospheric ages from tephras reveals that the presence of older surface reservoir ages in the glaciation caused the estimation of ventilation ages from simple benthic-planktonic offsets to significantly underestimate the depletion of Delta C-14 in deep waters. This may have had a role in masking the large change in reservoir ages since the glaciation when using benthic-planktonic reservoir age estimates. (C) 2016 Elsevier B.V. All rights reserved.