Author: Nicholas Benetti Sullivan

Publisher:

Published: 2022

Total Pages: 0

ISBN-13:

The study of deep time is crucial for contextualizing present and future environmental change. This is particularly true for the high southern latitudes, a linchpin of the global climate system. However, assembling and compositing a complete, high-resolution sedimentary archive in these regions is often difficult, owing to a range of logistical, environmental, and stratigraphic challenges. Two quantitative methodologies are brought to bear on these problems. The first is stratigraphic constrained optimization (CONOP), a tool for compiling individual, imperfect, local stratigraphic sections into a parsimonious composite succession. The second is astrochronology, a branch of cyclostratigraphy focused on the recognition of astronomically influenced climate rhythms (Milankovitch Cycles) in the stratigraphic record. The findings presented here, using these approaches, provide insight into past environmental and oceanographic change occurring at timescales ranging from millennia to millions of years. Chapter 1 of this dissertation presents strategies for calibrating timescales generated by CONOP with the geochronological information encoded in astrochronology. A synthesis of new and published cyclostratigraphic records in the Southern Ocean are reviewed. These are then used to help refine and calibrate a CONOP output, and four strategies for integrating these interpretations are discussed. Various case studies are used to build a refined composite history for the Southern Ocean. Chapter 2 presents a new Early Miocene cyclostratigraphic analysis of Miocene strata at two ice-proximal sites, IODP U1521 and AND-2A, in the Ross Sea in the Southern Ocean. Astrochronological analysis was conducted on clast abundance data that reflect the proportion of ice-rafted debris. The variable strength of observed obliquity and eccentricity forcing in data across two distinct time intervals (17.4-17.7 Ma and 18.0-18.5 Ma) informs our uncertainty of ice sheet dynamics during the warm climate of the Miocene and helps characterize the different responses of marine influenced versus terrestrial-based ice sheets. These interpretations are then used to inform and develop a new hypothesis relating the evolution of the Antarctic cryosphere to multi-million year "grand cycles" in eccentricity and obliquity. Finally, chapter 3 presents the discovery of astronomically-influenced millennial (1-15 kyr) cycles in early Miocene strata in the core AND-2A from McMurdo Sound. This discovery shows the ubiquity of millennial-scale cyclicity in the southern cryosphere, even during the comparatively warmer Miocene climates that predate a large Northern Hemisphere ice sheet. These findings help us understand the intrinsic behavior of the Antarctic ice sheet and its response to external forcing at a range of temporal scales. The chapters of this dissertation contribute to a better understanding of the temporal dynamics of the Southern Ocean and the Antarctic cryosphere, a complex and dynamic system that may respond to modern climate change in unexpected ways. This has been achieved with the development of new ways of using astrochronology and CONOP together, a methodological advancement that shows promise for application at all levels of the geologic timescale.