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

• Glacial deep ventilation ages in the Southwest Pacific were as much as ∼3500 yr.
• Oldest glacial 14C ages were at mid water (∼2500–3500 m) not abyssal depths.
• Benthic-minus-planktonic differences ages mask old ventilations ages.
• Deep ventilation decreases in progressively deeper water across the deglaciation.
• A modern profile of ventilation ages is established by ∼15 ka.

Marine radiocarbon (14C) is widely used to trace ocean circulation and the 14C 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 Δ14C, 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 Δ14C minimum centered on ∼2500 m. Glacial radiocarbon ages of intermediate depth waters (600–1200 m) were ∼800 to 1600 14C years, about twice modern and persisted through the early deglaciation. Notably, in the glaciation shallower depths were significantly more enriched in 14C than waters between 1600 and 3800 m, which were ∼4000 to 6200 14C years, or about 3–5 times older than modern. Abyssal waters deeper than 4000 m were also more 14C rich than the overlying deep water. With radiocarbon ages of 1800–2300 14C years, this was similar to modern values. In the early deglaciation, Δ14C 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 ∼14 ka a modern-type Δ14C profile for deep ocean water masses was in place. Our results 1) confirm a deep 14C 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 Δ14C 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.