Future Ship Based Drilling

Core recovery from ship-based drilling on the Antarctic continental shelf is typically poor (a few to 30%) in unconsolidated diamicton, but can be high (>95%) in calm seas and either fine-grained mud lacking lonestones or lithified sediment (including diamictite). Sites in deeper water (>400m) are easier for operational reasons, but depth penetration has rarely been much more than 400 m in practice, on account of drill bit failure. However, hole re-entry and drill bit replacement is possible. Ship-based drilling is normally subject to more ice and weather constraints than floating sea-ice or ice-shelf

12,000m

10,000m

8000m

6000m

12,000m

m

m

drilled

rec'd

"Slimhole"

7438

6963

94%

DSDP/ODP

4144

1397

34%

SHALDRIL

176

113

64%

11758

8473

ANDRILL-2

ANDRILH

| 5 year hiatus

4000m i

ODP Leg 178

CIROS-2

MSSTS-1

Jt I

10 year hiatus

2000

1970

DSDP Leg 28

1980

5 year hiatus

1990

2000

2100

Figure 3.9: Cumulative metres of sedimentary strata drilled on the Antarctic continental shelf since drilling began in 1973, based on data from Table 3.2. Note that only the ODP holes drilled on the shelf are shown here - significant holes were drilled on the slope and rise also. The graph shows the episodic nature of progress in recovering core, reflecting the time and effort required from a relatively small community in both developing proposals and projects and then seeing them through to publication. The better recovery from ''Slimhole'' drilling is a consequence of the ability to recirculate drilling fluid, a smaller hole diameter and a stable drilling platform.

drilling. The upcoming Integrated Ocean Drilling Program (IODP) Expedition 323, which plans to occupy both a Neogene and a Holocene site on the continental shelf off Wilkes Land in early 2009, will be a useful test of technology and experience gained from previous legs (IODP, 2007). It will also provide the first long time scale geological history for this sector of the Antarctic margin, of interest because it will provide a test for the histories developed separately from drilling in the McMurdo region of the Ross Sea and in Prydz Bay.

Despite the quality and length of core it yields, the floating ice technology can be used only in coastal or inland locations, leaving open huge areas of the shelf that are now extensively covered with seismic surveys. Considerable effort in the past three decades has gone into ship-based coring programmes to penetrate the layer of compact diamictite deposited when the continental shelf was largely covered with grounded ice during the Last Glacial Maximum, but with little success. SHALDRIL, a US Antarctic initiative (http://www.arf.fsu. edu/projectsShaldril.cfm), has been a recent development to address this need and help correlate the extensive seismic stratigraphy between the various sectors of the Antarctic continental shelf with other regions and other continental margins. The record thus far (Table 3.2) indicates that further technological development is needed for successful sampling.

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