Pleistocene Ice Sheet Variability on Orbital to Millennial Scales from Northwest Greenland Stratigraphy

Abstract

Past variability of the Greenland Ice Sheet (GrIS) and the role it plays in global ice dynamics is poorly constrained due to a paucity of proximal climate records. With sea level rise patterns controlled by the source and relative timing of ice sheet melt, understanding the response of individual ice sheets to climate forcings is critical for forecasting future sea level changes. This thesis utilises the newly cored Site U1604 from International Ocean Discovery Program (IODP) Expedition 400, on Greenland’s northwestern continental margin to resolve GrIS variability through the Pleistocene (c. 1 Ma to present). Following a multi-proxy approach well established in Antarctica, visual observations, sedimentological variables (grain size distribution and ice-rafted debris abundance), physical property measurements (magnetic susceptibility and natural gamma radiation), and geochemical compositions (X-ray fluorescence) were used to create an integrated lithofacies model. Ice sheet margin proximity exerts a strong control on nearby sedimentary processes and thereby sedimentary deposits. Consequently, the alternating packages of bioturbated mud, wavy and rhythmically laminated mud, interstratified mud and sand, calcareous muddy diamict, and interstratified diamict and mud reflect alternating predominant sedimentary processes related to the advance and retreat of the GrIS through the mid-late Pleistocene. While stratigraphy alone does not show simple, regular, and repeating lithofacies successions at Site U1604, periods of ice margin stability and instability are defined based on the rapidity of lithofacies changes, the presence of calcareous muddy diamict sourced from North America, and millennial variability of ice rafted debris and magnetic susceptibility records. The timing and frequency of these periods of stability and instability are variable during the last million years. From c. 600 ka to present, periods of instability occur during interglacials and are preceded by calcareous muddy diamict deposits indicative of North Baffin Bay Ice Shelf collapse and subsequent surging of ice streams sourced from the Laurentide Ice Sheet. Throughout this interval, the amplitude of millennial variability is modulated by ~100 kyr cycles that are in-phase with eccentricity and global ice volume. Prior to this (c. 975 – 600 ka), periods of instability are not coherent with eccentricity and global ice volume, indicating the GrIS acted independently of proximal Northern Hemisphere Ice Sheets. Importantly, the first indication of a North Baffin Bay Ice Shelf collapse occurs at c. 830 ka, 190 kyr earlier than analogous Heinrich events recorded in the Labrador Sea that are sourced from the more southern Hudson Strait. This timing supports mid-Pleistocene Transition hypotheses that invoke an increasing southern extent of Northern Hemisphere ice sheets to explain the switch to 100 kyr glacial cycles. Ultimately, this research emphasises the complex interplay between the Greenland and Laurentide Ice Sheets throughout the last c. 1 million years and highlights the role of the North Baffin Bay Ice Shelf in glacial dynamics of the 100-kyr world.