Orbital Theory, Marine Isotope Stage 11, and the Holocene Problem

As pointed out by Hays, Imbrie and Shackleton (1976), orbitally controlled variations in seasonal insolation provide a likely pacing for the Pleistocene ice ages. The interglacial intervals that serve as respites from the recurring glaciations are also of particular interest, since we are living within the Holocene, the most recent of these interglacial episodes. A key question is the timing of the inevitable return to glacial conditions that may be required by the orbital pacemaker. Another open question is how the climatic progression will proceed, now that mild conditions have persisted for approximately one half of an orbital (precession) cycle. Some insights to these questions may be derived from similar past intervals. Among the Pleistocene interglaciations, marine isotope stage 11 (MIS 11) may constitute the best analogue for the development and eventual demise of the Holocene, due to its comparable orbital configuration of minimal eccentricity. Detailed evidence from ODP Sites 980 and 983 in the North Atlantic indicates a markedly similar regional and global climate regime during MIS 11 and the Holocene, suggesting a consistent, if anomalous, response in excess of the presumed high northern latitude summer insolation forcing. Sea-surface temperature proxies based on planktonic foraminifera assemblages and (δ 18O) indicate stable ocean warmth for an extended interval during MIS 11. A variety of age models yield a minimum duration of 30 kyr for these regional conditions, or one and a half orbital (precession) cycles. This length suggests the possibility that the climate system may skip a beat when precession cycles have diminished amplitude due to minimal eccentricity-modulation. Both planktonic and benthic (δ 18O) during MIS 11 are indistinguishable from Holocene values. The simplest interpretation of these results is that regional hydrography in the North Atlantic, and global ice volume, were very similar during the two interglaciations. Only the duration of MIS 11 differs significantly from the elapsed portion of the Holocene. If the duration of the Holocene and MIS 11 is as similar as the presumed forcing and the climatic response of the two intervals, then the expected natural return to glacial conditions may be in the distant future.