Heat Storage in the Deep Ocean as a Capacitor to Explain Deglaciations.
Abstract:Since the classic work of Hays, Imbrie, and Shackleton in 1976, we have known that glacial cycles are paced by the Milankovitch frequencies. However, it has also been long recognized that deglaciations, especially in the ‘100k-world’, are too abrupt to be a linear response of the climate system to this orbital forcing. To explain this ‘sawtooth’ behavior, rising pCO2 in the atmosphere has been proposed to be an amplifier of deglacial climate change. Yet this CO2 must come from somewhere and it does not seem to be an early responder in the deglacial sequence of events. Most ideas focus on the deep ocean as the only reservoir large enough to store the CO2 on G-I timescales, including the capacity to release it quickly.
Here we propose a new ‘capacitor’ for the climate system, deep ocean heat storage, that could provide the key physical mechanism to explain the important features of deglacial climate. There is a growing body of evidence, from carbonate stable isotopes and pore water salinity estimates, that the Last Glacial Maximum deep ocean was more stratified than today. Through thermobaricity in seawater’s equation of state (the pressure dependence of the thermal expansion coefficient), salt stratification can store heat in a water column that is locally statically stable. However, analogous to CAPE in the atmosphere, this heat energy is convectively available and can lead to large, abrupt deep-ocean mixing. Using clumped isotopes in deep-sea corals from Heinrich Event 1, we have found warmer water underneath colder water, about 800 years before the Bolling-Alerod warming recorded in Greenland ice cores. We propose that the abrupt nature of the Bolling is due to the discharge of this deep ocean thermal capacitor which then changes the deep circulation from a glacial to a modern pattern.