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SCAR Report 16

Appendix 7

DEEP STRATIGRAPHIC DRILLING IN THE ANTARCTIC OUTSIDE ODP
P. J. Barrett, School of Earth Sciences, Victoria University of Wellington,
P O Box 600, Wellington

Deep stratigraphic drilling (>25 m) is an essential means of sampling strata if we are to build up a historical record of climatic and tectonic events over the last 100 million years from the Antarctic region. It is expensive in time, money and logistic effort but provides data and as a consequence insights that can be obtained in no other way. The value of continuously coring stratigraphic sequences has been widely appreciated through knowledge gained from the deep ocean basins and continental margins since the inception of the Deep Sea Drilling Project in 1968. The Antarctic margin has benefitted from two DSDP/ODP campaigns (1972-73, and 1987-88), but the small number of holes (8 in total) and the poor core recovery from strata drilled on the continental shelf persuaded Antarctic scientists to look for other ways of sampling the Antarctic stratigraphic record, which it was acknowledged contained the most direct record of the term history of the ice sheet. As a consequence Antarctic scientists and operators have developed experience in cold climate stratigraphic drilling both onshore and offshore to depths of several hundred metres.

The first deep drilling project on the Antarctic continent was the Dry Valley Drilling Project, a joint project by Japan, NZ and USA, between 1970-75. The project used a land-based Longyear 44 wireline diamond drilling system to core continuously to depths of over 300 m in valley fill (e.g. Taylor Valley), in volcanic flows and hyaloclastites (e.g. at McMurdo Station) and in basement granite (e.g. Lake Vida, Victoria Dry Valley). The project showed how it was possible to core through permafrost with almost 100% recovery (but not how to continue beneath the permafrost), and also tested the feasibility of using the same rig to drill offshore from a sea ice platform for drilling strata to date the beginning of Antarctic glaciation in the Ross Sea region.

Several further attempts were made by the NZ Antarctic Programme to drill deep enough to recover pre-glacial strata, the most successful being the CIROS-1 drill hole, which reached a depth of 702 m in 1986 with 98% core recovery. The history from the core is now known to span the period from around 20 to 35 Ma ago with glacier ice calving at sea level in the oldest cores, but grounded ice probably no older than 30 Ma. In the next two field seasons a similar drilling system will be used for the Cape Roberts Project to sample the same sequence 70 km north and continue into another 1000 m into older strata, seeking the extend back the climatic and tectonic history of the region to the Cretaceous.

The CIROS-1 drill hole showed that deep core could be recovered not only from offshore but also onshore sites under the climatic constraints of Antarctic conditions. The Cape Roberts Project will take deep drilling a stage further, by attempting to drill in water over twice as deep (400+ m), and by developing a portable rig, camp and laboratories. The main components are built in and around standard shipping (ISO) containers, and can be shipped, off-loaded and sledged (but not flown) into place. The cost of the planned 3 holes yielding 1500 m of core in two seasons is $US 4.3 million for all aspects of the logistics, the drilling system and core recovery. Scientific work on the core is expected to cost another $2 million.

Once the Cape Roberts Project is completed, hopefully in 1999, then the camp and drilling system will be available for other tasks. Past experience indicates roughly the following times for planning such projects. (lead time is 4-7 years from conception to drilling):

1-2 years: Site surveys. Although some preliminary seismic, gravity and magnetic surveys may have been carried out to define drilling targets with reasonable confidence, site specific surveys will be needed.

2-3 years: Organisational development. Getting core scientific and logistic team to agree on both science goals and operational approach, to secure funding required, and organise comprehensive environmental evaluation. This could run in parallel with the site surveys.

2 years: Assembly of drilling system and shipping to a location close to drill site area

1-2 years: Drilling activity and scientific study. If on sea-ice then the risk of delay due to ice conditions

1 year: Site clean up

Sequences worth considering for coring with a drilling system of this type include the following, but there are certain to be many others: