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

a r t i c l e i n f o a b s t r a c t Keywords: radiocarbon reservoir ages Southern Ocean last glaciation CO 2 climate change Marine radiocarbon (14 C) is widely used to trace ocean circulation and the 14 C levels of interior ocean water masses can provide insight into atmosphere–ocean exchange of CO 2 the since the last glaciation. Using tephras as stratigraphic tie points with which to estimate past atmospheric 14 C, 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 14 C minimum centered on ∼2500 m. Glacial radiocarbon ages of intermediate depth waters (600–1200 m) were ∼800 to 1600 14 C years, about twice modern and persisted through the early deglaciation. Notably, in the glaciation shallower depths were significantly more enriched in 14 C than waters between 1600 and 3800 m, which were ∼4000 to 6200 14 C years, or about 3–5 times older than modern. Abyssal waters deeper than 4000 m were also more 14 C rich than the overlying deep water. With radiocarbon ages of 1800–2300 14 C years, this was similar to modern values. In the early deglaciation, 14 C 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 14 C profile for deep ocean water masses was in place. Our results 1) confirm a deep 14 C 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 14 C 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.