Developments in Drilling and Thinking in the Late 1970s

Although DVDP had concluded its work, the NZ Antarctic Programme was persuaded that a further effort to drill in western McMurdo Sound was justified. This was named the McMurdo Sound Sediment and Tectonic Studies Project (MSSTS), and led by New Zealand but with US and Japanese scientific participation. The United States provided the ''Longyear 44'' rig from DVDP and significant logistic support. A camp for the MSSTS-1 hole was set up on the sea ice soon after the late August winter flight to the ice, allowing the drilling to begin in October 1979. Despite the cold and difficult conditions, core was recovered to a depth of 227 m below the sea floor, when operational problems terminated drilling. After passing through a few tens of metres of Plio-Pleistocene strata, and an interval of virtually no recovery, the strata below 110 m bsf cored well. They provided a record of striking facies changes between diamictites, sand and mud, along with a pollen record and a beech leaf indicating a vegetated coastline and a cold temperate continental margin in those times. Both lithofacies and biofacies were seen as reflecting cyclic changes in extent of the continental ice-sheet margin advancing and retreating across the drill site, with associated changes in sea level by tens of metres through glacioeustacy (Barrett and McKelvey, 1986; Barrett et al.. 1987). Initially, the strata were thought to extend back to Paleocene times (Webb, 1983) but further study of the sparse faunal and floral assemblages and recognition of reworked older microfossils led to a late Oligocene age assignment (Webb et al., 1986).

While the NZ programme had focussed on drilling in McMurdo Sound, the US programme developed a project to core through the Ross Ice Shelf 420 km from the ocean (Clough and Hansen, 1979). The Ross Ice Shelf Project (RISP) in two successive seasons drilled through 430 m of ice to measure and sample the properties of the 230 m water column, and take cores and photographs of the sea floor beneath (Webb, 1978, 1979). The sea floor cores revealed a few tens of centimetres of Late Quaternary mud overlying a metre of mid-Miocene glaciomarine mudstone with diatomite clasts several millimetres across indicating an interglacial period of ice- and sediment-free biogenic sedimentation at 82°S (Webb et al., 1979; Scherer et al., 1988). Terrestrial palynomorphs from the clasts indicate coastal beech forests at this time also.

The MSSTS-1 results also provided a glimpse of Antarctic glacial history from the Ross Sea margin, and although there were some issues to be resolved with the drilling technology, scientific advances in the wider world were supporting the case for a better physical record of past Antarctic climatic events. These included extensive records of seismic stratigraphy from continental margins, along with a new type of analysis that recognized sequences developed as a consequence of coastal advance and retreat (Vail et al., 1977). These coastal movements were taken to imply variations in ice volume causing sea level changes of hundreds of metres, and mainly from the mid-Oligocene on. At the same time, the newly developed deep-sea isotope record provided a different basis for inferring ice-sheets on Antarctica, and these indicated an earliest Oligocene initiation (Kennett, 1977). Kennett (1982) incorporated these new concepts and insights from sequence and isotope stratigraphy into a comprehensive synthesis of history and knowledge of the Earth as a system, albeit with a focus on the marine realm, an approach that is now widely accepted and practised. Neither isotopes nor sequence stratigraphy could provide direct evidence of the behaviour and history of the Antarctic Ice Sheet itself, but the first coastal Oligocene cores, from MSSTS-1 in 1979, confirmed the cyclic behaviour of ice sheet and sea level (Barrett et al., 1987). Core chronology was not yet adequate to constrain their frequency, but the role of orbital forcing in driving high-frequency Quaternary glacial cycles (Hays et al., 1976) was plainly relevant to early Antarctic Ice Sheets.

While significant progress was being made in documenting past ice-sheet behaviour, a key event of the 1970s was the publication of John Mercer's hypothesis for the likely future behaviour of the Antarctic Ice Sheet as a consequence of rising CO2 emissions (Mercer, 1978). He observed that a large area of West Antarctica lay below sea level, the ice sheet being thus inherently unstable, and that much of the ice-sheet margin was buttressed by ice shelves. Observing that present-day ice shelves form only where the January summer isotherm is below 0°C, he concluded that a rise of 5-10°C, projected as likely in the following 50 years or so, would be sufficient to cause the disintegration of the major ice shelves buttressing the West Antarctic Ice Sheet leading to disintegration of the ice sheet itself. Although there was significant disagreement among glaciologists on both the buttressing hypothesis and the immediacy of the threat, the hypothesis provided new impetus for seeking a geological record of the history of both East and West Antarctic Ice Sheets.

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