Off-campus UMass Amherst users: To download campus access dissertations, please use the following link to log into our proxy server with your UMass Amherst user name and password.
Non-UMass Amherst users: Please talk to your librarian about requesting this dissertation through interlibrary loan.
Dissertations that have an embargo placed on them will not be available to anyone until the embargo expires.
Open Access Dissertation
Doctor of Philosophy (PhD)
Year Degree Awarded
Month Degree Awarded
Robert M. DeConto
Climate | Geology
The 41,000-year variability of Earth’s glacial cycles during the late Pliocene-early Pleistocene is usually attributed to variations in Earth’s obliquity (axial tilt). However, a satisfactory explanation for the lack of precessional variation in marine d18O records, a proxy for ocean temperature and ice-volume, remains contested. Here, a physically based climate model is used to show that the climatic effect of precession is muted in global isotope records due to two different mechanisms, with each dominating as a function of eccentricity. At low eccentricities (e0.019), the time-integrated summer insolation and number of positive degree-days impacting ice sheets varies at precessional periods, but the variation is out-of-phase between the Northern and Southern Hemispheres. Each mechanism dominates at different times, leading to a net attenuation of precessional variability in globally integrated proxy records of ice volume.
Recently, several interglacials (MIS 9, 11, 31, 49, 55, 77, 87 and 91) have been identified as warmer than others and have been termed “Super-interglacials”. It has been shown that the warmest of these interglacials follow exceptionally low eccentricity periods, with a lag of ~50kyr. The explanation proposed for this low eccentricity preconditioning of the super interglacials is directly linked to the fact that the polar ice sheets respond differently to precessional changes at different eccentricities, as described above. Using a series of GCM and ice-sheet model simulations covering MIS 11 and 31, it is shown that Southern Hemisphere ice-sheets respond to Northern Hemisphere insolation at lower eccentricities, switching to local Southern Hemisphere insolation at higher eccentricities. This switch from northern forcing to southern insolation forcing leads to Antarctica missing a beat in its glacial-interglacial cycles, as northern and southern insolation intensities vary out-of-phase at 23 ka precessional periods. Thus, depending on the orbital conditions, Antarctica either has an unusually long glacial or interglacial period following a low eccentricity orbit. In the latter case, the prolonged warm conditions in the Southern Hemisphere preconditions the Polar Regions to produce a large response during the unusually warm interglacials like MIS 11 or 31.
Roychowdhury, Rajarshi, "Eccentricity Modulation of Precessional Variation in the Earth’s Climate Response to Astronomical Forcing: A Solution to the 41-kyr Mystery" (2018). Doctoral Dissertations. 1191.