Does an asymmetric thermohaline-ice-sheet oscillator drive 100 000-yr glacial cycles?

Abstract

A hypothesis is presented that late Quaternary 100 000-yr glacial cycles are driven by an asymmetric thermohaline–ice-sheet oscillator that emerged in the global climate system 650 000–950 000 yr ago, perhaps when the main source of Northern Hemisphere deep-water production shifted south from the Arctic into the Nordic seas. It is hypothesised that the asymmetry is due to the increasing difficulty after 950 000 years ago of resetting an interglacial mode of the critical Nordic limb of the salinity conveyor once it switches off and an ensuing iceberg flux enters the areas of downwelling. A possible reason for both a southward shift and the resulting asymmetry is uplift of the Greenland–Scotland submarine ridge from activity of the Iceland mantle plume. In this hypothesis an individual 100 000-yr glacial cycle begins when the northernmost limb of the salinity conveyor in the Nordic seas is curtailed, or even switched off, perhaps due to the growing strength of competing Antarctic Bottom Water (AABW) generated by interglacial recession of the West Antarctic Ice Sheet (WAIS) from the West Antarctic Rift System. Such recession produces southern marginal seas where dense shelf water can collect and overflow into the abyss. When northern ice sheets, nucleated by this circulation switch, develop marine components that calve icebergs into the Nordic seas, the salinity conveyor can no longer revert to an interglacial mode from orbital forcing, as it did prior to 950 000 yr ago. In order to reset an interglacial circulation mode of the conveyor, ice sheets must continue to grow for 100 000 years until they capture enough excess volume to produce a gravitational collapse of marine-based components, so massive that all grounded ice is flushed from North Atlantic continental shelves. The outburst of icebergs produced by this collapse cripples the glacial mode of overturning in the northern North Atlantic. Once this collapse ends, however, the Nordic seas become nearly free of icebergs for the first time in 100 000 years because of the depletion of adjacent marine-based components. As a consequence, North Atlantic salinity increases rapidly, switching the conveyor into a vigorous interglacial mode of operation and hence terminating the glacial cycle. By lowering sea-level, the prolonged growth of Northern Hemisphere ice sheets during each 100 000-yr cycle drives Antarctic grounding lines seaward across continental shelves, squeezing off the source of densified shelf waters that feed AABW. Sea-level rise and increased basal melting, however, caused by the subsequent collapse of northern ice sheets and the reintroduction of North Atlantic Deep Water into the Southern Ocean, reverses the process, forcing retreat of Antarctic grounding lines from their advanced last-glacial maximum positions. This retreat opens marginal seas for renewed formation of dense shelf water. By expanding marginal seas and hence the source of dense shelf water, ongoing recession of the WAIS strengthens AABW during the course of an interglaciation, eventually forcing a thermohaline circulation switch in the Nordic seas and initiating yet another 100 000-yr glacial cycle. The 100 000-yr duration of each cycle is set by two factors. Inertia is built into the system by the long time required for ice sheets to grow to the excess volume necessary for a marine collapse that resets the salinity conveyor into an interglacial mode. Eccentricity-driven changes in the amplitude of the precession or tropical half-precession signal give rise to warming events that trigger such a collapse of excess ice about each 100 000 yr.