CIROS1 Dynamic Oligocene Ice Sheets

Offshore, the most successful drilling for the elusive early Cenozoic pre-glacial record took place in late 1986. The CIROS-1 drill hole cored to 702m below the sea floor with 98% recovery. The core reached back to the late Eocene and spanned the period to the early Miocene, based on magnetostratigraphy underpinned by biostratigraphic datums largely from diatoms, but including foraminifera, palynomorphs and nannofossils (Harwood et al., 1989; Hannah et al., 1997; Wilson et al., 1998; Roberts et al., 2003). However, the record did not quite reach the warm middle Eocene, despite coring the early Oligocene-late Eocene transition in the lower half of the core. The strata were a deep-water turbidite facies with lonestones, indicating some glacial influence. In contrast, the late Oligocene-early Miocene upper half comprised sequences of diamictite, sand and mud (Hambrey et al., 1989), seen in a much smaller way in MSSTS-1, and typical of near-shore glaciomarine sedimentation with cyclic ice margin advance and retreat and corresponding changes in sea level. The depositional patterns clearly showed Antarctic Ice Sheets to be dynamic and reaching the Ross Sea coast from at least late Oligocene times onwards.

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