David Hutchinson: High CO2 in the Miocene drives oxygen loss in the abyssal ocean and oxygen gain in intermediate waters
David K. Hutchinson [1], Benjamin W. Anthonisz [1], Katrin J. Meissner [1], Samar P. Khatiwala [2], Benoit Pasquier [1], Pearse Buchanan [3]
[1] Climate Change Research Centre, University of New South Wales, Sydney, Australia
[2] School of International Liberal Studies, Waseda University, Tokyo, Japan
[3] CSIRO Environment, Hobart, Australia
We have developed new simulations of the Miocene Climate Optimum (approx. 15 million years ago), using ACCESS-ESM1.5. We focus on the impact of greenhouse warming, using 1x, 2x and 3x pre-industrial CO2 forcing, on deep ocean circulation and ocean oxygenation. First, we find that abyssal overturning formed in the Southern Ocean is suppressed at higher CO2. Deep ocean circulation below ~2000 m largely collapses, leaving the abyssal ocean largely unventilated. Despite this, Atlantic deep water continues to form, albeit at a weaker rate and a reduced depth of overturning. In contrast, intermediate water formation is significantly enhanced in the North Pacific, especially at 3x pre-industrial CO2. In addition, Tethys (i.e. “paleo-Mediterranean”) Intermediate Water is also enhanced at higher CO2. These overturning changes lead to an overall decrease in ocean oxygen, with the abyssal ocean becoming de-oxygenated, but not completely hypoxic, due to a concurrent reduction in remineralisation. The North Pacific, and parts of the equatorial subsurface Pacific become better ventilated at higher CO2, contrary to the abyssal and global mean oxygen trends. Some aspects of these results could help to explain Miocene Pacific proxy records of oxygen, while some disagreement remains in other regions.