Sequence Stratigraphy, Chronostratigraphy and Zircon Geochronology of the CIROS-1 Drill Core, Ross Sea, Antarctica: Implications for Cenozoic Glacial and Tectonic Evolution
Authors:   , Van de Ven Evelien 
Published: November 2021

<p>Antarctica plays a central role in the global climate system. Understanding the continent's past climate interactions is key to predicting its future response to, and influence on, global climate change. In recent decades, sediment cores drilled on the Antarctic continental margin have provided direct evidence of past climatic and tectonic events. Drilled in 1986 from sea ice in western McMurdo Sound, the pioneering 702 m-long CIROS-1 core extended back to the Late Eocene and provided some of the first evidence of the antiquity and history of the Antarctic ice sheets. The CIROS-1 drill core recovered a depositional history of the western margin of the Victoria Land Basin adjacent to the Trans-Antarctic Mountains. It was located directly offshore from where the Ferrar Glacier, which drains the East Antarctic Ice Sheet, discharges into the Ross Sea. Consequently CIROS-1 contains a record of both the glacial and tectonic Cenozoic evolution of the Antarctic margin. This thesis provides a timely re-evaluation of the CIROS-1 core with new analysis techniques that enable further insights into the glacial and tectonic history of the western Ross Sea region, and includes three key objectives:  (1) Re-examine CIROS-1 sedimentology and stratigraphy and provide a new facies and sequence stratigraphic analysis using modern methods developed from recent drilling projects (e.g. CRP, ANDRILL).  (2) Develop a new integrated chronostratigraphic model through an assessment and compilation of previous studies, which provides a context for the interpretation of detrital zircon data, climate and tectonic history. (3) Undertake a detailed examination of the provenance of CIROS-1 sediments using cutting edge in situ analysis techniques of detrital zircons (U-Pb and trace element analysis using LA-ICP-MS).  Glaciomarine sequence stratigraphic analysis identifies 14 unconformity-bound sequences occurring in two distinctive stratigraphic motifs. The four sequences located beneath the 342 mbsf unconformity contain relatively complete vertical facies succession. They were deposited in shallow marine, fluvio-deltaic conditions with distal glaciers terminating on land, and possibly calving into the ocean in adjacent valleys as evidenced by occasional ice-rafted debris. The ten sequences located above ~342 mbsf have a fundamentally different architecture. They are incomplete (top-truncated), contain subglacial and ice proximal facies grading upsequence into distal glaciomarine and shelf conditions. Top truncation of these sequences represents overriding of the CIROS-1 site by the paleo-Ferrar Glacier during glacial phases.  A revised age model for CIROS-1 is presented that utilises new calibrations for Antarctic diatom zones and compiles three previously published age models for different sections of the core (Roberts et al., 2003; Wilson et al., 1998; Hannah et al., 1997). The new age model allows correlation of Late Oligocene cycles with coeval cycles in CRP-2/2A, 80 km to the north. A fundamental orbital control on the dynamics of these East Antarctic Ice Sheet outlet glaciers is evident from this comparison. Both glacier systems respond in-phase to longer-period orbital components (e.g. eccentricity 100 kyr and 400 kyr), but differ in their sensitivity to precession (20 kyr). It appears that during the Late Oligocene the Ferrar catchment responded to 20 kyr precession cycles, whilst the larger MacKay Glacier, which is more directly connected to the East Antarctic Ice Sheet, responded to longer duration 125 kyr (eccentricity) forcing.  CIROS-1 zircons group into four distinct geochemical suites. Zircons formed in felsic igneous environments dominate the CIROS-1 population, with 89 % of zircons analysed showing geochemical characteristics inherent to granitic/rhyolitic zircons. Approximately 7 % of CIROS-1 zircons have a highly trace element enriched igneous provenance and were most probably sourced from enriched enclaves in granitic/rhyolitic units or from pegmatites. Approximately 3 % of CIROS-1 zircons show a metamorphic geochemical signature, and ~1 % formed in trace element depleted igneous environments. The zircons were sourced from the local basement (Koettlitz, Granite Harbour Groups), the Beacon Supergroup, and potentially, lithologies of the East Antarctic Craton located under the ice, or components of the Trans-Antarctic Mountains located under the current baseline of geologic exposure.  Large-scale, systematic temporal trends in zircon characteristics have been divided into three distinct climatic periods: Zone 1 (702-366 mbsf, Late Eocene), Zone 2 (366-250 mbsf, Late Oligocene) and Zone 3 (< 250 mbsf, Late Oligocene and Early Miocene). Zircons deposited during these periods show unique properties. During Zone 1, Antarctica experienced a relatively warm temperate climate and alpine style glaciers flowed eastwards through the Trans-Antarctic Mountains. Zircons in this zone contain a subtle record of unroofing of geochemically zoned Granite Harbour and Koettlitz units located in the Ferrar Valley. During Zone 2 deposition, glaciers flowed though the Trans-Antarctic Mountains draining a large and ephemeral EAIS, which oscillated on orbital time scales. Zircons in this interval show variable properties, high numbers and were most probably deposited as the paleo-Ferrar Glacier deeply incised the Ferrar Fiord. In contrast, Zone 3 is characterised by a flux of McMurdo Volcanic Complex derived sediments, together with systematic changes in zircon characteristics. These patterns indicate a Late Oligocene shift in ice flow to the site (above ~250 mbsf). Due to a cooling that culminated in the Mi-1 glaciation, ice flow to the site changed from an eastward to a northward flow, in response to an increased ice volume in the Ross embayment.</p>