A deglacial ventilation history of Northeast Pacific intermediate waters

The transfer of carbon from the deep ocean to the atmosphere may have driven the 80-90 ppm rise in atmospheric CO₂ recorded in Antarctic ice cores during the last glacial-interglacial transition (25-10 ka). The prevailing hypothesis is that this oceanic carbon reservoir, thought to be located in the abyssal Southern and Pacific Oceans, ventilated through the Southern Ocean, and released its CO₂ to shallower depths, and eventually to the atmosphere. While proxy data supports a deglacial increase in Southern Ocean ventilation, evidence for a sufficiently large glacial abyssal carbon reservoir or an intermediate-depth circulation pathway for newly ventilated Southern Ocean waters is sparse and/or complicated. There is also evidence for increased subarctic North Pacific ventilation during every deglaciation since the onset of Northern Hemisphere glaciation at 2.7 Ma. Thus, a bipolar increase in deglacial ocean ventilation could potentially account for the decline in atmospheric ¹⁴C activity during the Mystery Interval (17.5-14.5 ka) and resolve the source of poorly ventilated intermediate waters on the NE Pacific margin. Here we evaluate the possibility that ventilation of abyssal waters in the subarctic North Pacific also contributed to atmospheric CO₂ rise during the last deglaciation (18-11 ka). We reconstruct the deglacial ventilation history for the intermediate NE Pacific using paired ¹⁴C measurements (n=14) of planktonic and benthic foraminifera from a high-resolution sediment core MD02-2496 (48°58N, 127°02W; 1243 m water depth), collected offshore of Vancouver Island, Canada. The site lies near the base of the present day oxygen minimum zone in the transitional zone between the North Pacific subtropical and subpolar gyres. We find a mean deglacial benthic-planktonic (B-P) age of 1,198±122 years and a mean ¹⁴C ventilation age of 2152±572 years (n=14) This is not significantly different to that found at sites on Gorda Ridge (2710m depth) and off Baja California (705m depth) or to modern values. Although all NE Pacific sites exhibit a general decline in radiocarbon activity from 18 to 11 ka, the magnitude of the decline at 1243 m (Vancouver margin; 276‰) is less than at 705 m (Baja; 343‰) and more than at 2710 m (Gorda; 200‰). During the Mystery Interval, Δ¹⁴C at 1243 m decreased by ~130‰, but values are more similar to Gorda Ridge than to Baja. However, our record reveals an increase in Δ¹⁴C at the onset of the Bølling-Allerød (~14.7 ka), which is similar to that off Baja but not Gorda Ridge. Our results suggest that a relatively young water mass influenced intermediate depths of the NE Pacific at this time. This was replaced by an older (lower Δ¹⁴C) watermass during the Younger-Dryas. Our results provide further support for increased ventilation of the subarctic North Pacific at the onset of the Bølling-Allerød.