Reduced ventilation and enhanced magnitude of the deep Pacific carbon pool during the last glacial period

Abstract

It has been proposed that the ventilation of the deep Pacific carbon pool was not significantly reduced during the last glacial period, posing a problem for canonical theories of glacial-interglacial CO 2 change. However, using radiocarbon dates of marine tephra deposited off New Zealand, we show that deep-(>2000 m) and shallow sub-surface ocean-atmosphere 14 C age offsets (i.e. 'reservoir-' or 'ventilation' ages) in the southwest Pacific increased by ∼1089 and 337 yrs respectively, reaching ∼2689 and ∼1037 yrs during the late glacial. A comparison with other radiocarbon data from the southern highlatitudes suggests that broadly similar changes were experienced right across the Southern Ocean. If, like today, the Southern Ocean was the main source of water to the glacial ocean interior, these observations would imply a significant change in the global radiocarbon inventory during the last glacial period, possibly equivalent to an increase in the average radiocarbon age >2 km of ∼700 yrs. Simple mass balance arguments and numerical model sensitivity tests suggest that such a change in the ocean's mean radiocarbon age would have had a major impact on the marine carbon inventory and atmospheric CO 2 , possibly accounting for nearly half of the glacial-interglacial CO 2 change. If confirmed, these findings would underline the special role of high latitude shallow sub-surface mixing and air-sea gas exchange in regulating atmospheric CO 2 during the late Pleistocene.

Response of a comprehensive climate model to a broad range of external forcings: relevance for deep ocean ventilation and the development of late Cenozoic ice ages

“…When this is combined with the pCO 2 effect on air-sea equilibration of radiocarbon, expected to be approximately 250 years for the LGM , it implies an apparent aging of the glacial ocean by 690 years relative to the pre-industrial. This is very similar to the whole-ocean average radiocarbon age increase of 600 years reported by Sarnthein et al (2013) and 700 years reported by Skinner et al (2015).…”

The influence of the ocean circulation state on ocean carbon storage and CO<sub>2</sub> drawdown potential in an Earth system model

“…Broecker et al, 1990;Sikes et al, 2000;Keigwin and Schlegel, 2002;Skinner et al, 2010Skinner et al, , 2015, it seems possible that the effect of increased mixing was indeed offset by some other process. One such factor could be that the global ocean was saltier and more stratified (Ballarotta et al, 2014).…”

Reconstructing deglacial ocean ventilation using radiocarbon : data and inverse modeling

“…In palaeoceanography, quantitative information about the exchange between surface and deep waters is often deduced from the "age" of deep waters relative to that of the surface waters or the atmosphere (for recent studies see, e.g., Burke and Robinson, 2012;Chen et al, 2015;Keigwin and Lehman, 2015;Rae et al, 2014;Skinner et al, 2015). The age of deep ocean waters over the past 40 kyrs or so is typically estimated from measurements of the radiocarbon (half-life of 5,700±30 yr; Audi et al, 2003) activity of fossil samples of benthic foraminifera and deepdwelling corals.…”