Complementary roles of orbital insolation and North Atlantic Deep Water during Late Pleistocene interglacials

Abstract

A long‐standing dilemma of the Milankovitch hypothesis involves the observation that the Holocene deglaciation was comparable in extent to previous late Pleistocene interglacials even though insolation forcing was less. In this paper we propose an explanation that is based on an observed complementary relationship between eccentricity and North Atlantic Deep Water (NADW) production rates. A δ13C record from the South Atlantic indicates that NADW production rates are high when eccentricity is low (stages 1, 11), and vice versa. For the Holocene, the inferred stronger thermohaline circulation injects approximately the same heat into the high latitude North Atlantic (∼9 W/m²) as the insolation differences between the early Holocene and last interglacial peak. We utilize an energy balance model to calculate that the excess heat from the Holocene thermohaline increase can explain at least 50–65% and 100% of the energy budget “shortfall” from Milankovitch forcing over the Laurentide and Fennoscandian ice sheets, respectively. We suggest that the observed complementary relationship between Milankovitch and NADW reflects a coupling due to a “reverse monsoonal” effect over the North Atlantic. During times of high eccentricity and strong land‐sea contrast, enhanced heating over land results in increased subsidence over the ocean, which causes circulation and precipitation changes. These processes could lower ocean salinities and decrease thermohaline circulation. Thus deglaciation can progress through either mechanism, with the total heat balance for each interglacial being approximately the same. However, the combined effect of increased insolation and reduced thermohaline circulation for the last interglacial yields slightly warmer (∼1.0°C) temperatures in mid‐ and high‐latitude northern hemisphere land areas than in the Holocene. This result is approximately in line with geologic data from these regions.