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SCAR Report 16

Appendix 7

ANTOSTRAT ANTARCTIC MARGIN ODP INITIATIVE
Peter Barker, British Antarctic Survey, Cambridge, UK

The current ANTOSTRAT involvement with ODP has developed slowly over the past 3-4 years, and seems likely to last for a further 3-4 years. In this paper I set it in context, outlining both the science and what might be called the politics of the enterprise.

The Science

We now understand the essentials of sediment transport and deposition under a fully-fledged glacial regime, as seen on Antarctica. The Antarctic continental shelf is over-deepened and generally inward-sloping because of the erosive action of a grounded ice sheet. The ice sheet “drains” largely through ice streams, whose rapid flow is enabled by the presence of a basal layer of deforming till. The ice sheet appears to have been grounded to the continental shelf edge during most glacial maxima, and extensive progradational wedges at the continental margins of Antarctica have largely been created from this deforming till. In the simplest model, till is deposited on the upper continental slope during maxima, and on the shelf whenever a grounding line retreats and/or the ice base lifts. Pelagic and hemipelagic interbeds are deposited on shelf and slope during interglacials, when the grounding line is inshore. Ice sheet re-advance can lead to erosion of shelf deposits.

To some extent, the essentially unsorted glacial sediments deposited on the upper continental slope directly ahead of the grounding line are unstable, giving rise to debris flows and turbidity currents that cross the lower slope and rise. These processes involve gravitational sorting and, with the ambient bottom current regime, can lead to the development of sediment drifts on the continental rise. Some of these incorporate turbidites transported directly from the upper slope, whereas in other cases direct turbidity current flow by-passes the drifts, which comprise only the fine-grained component, entrained in a nepheloid layer of the ambient bottom currents and deposited down-current.

It is now clear that a record of ice sheet history is preserved within all three depositional environments: shelf topsets and slope foresets of the prograding wedge, and sediment drifts of the continental rise. The foreset record is direct, and the accessibility of the continental shelf during the present interglacial has focussed the greatest amount of attention onto its upper surface, but over geological time it is discontinuous because of erosion. Slope foresets provide a more continuous record, but both topsets and upper slope foresets are largely unsorted, which can present problems of sediment recovery during sampling. The continental slope has received comparatively little attention. The record of glaciation contained in drifts on the continental rise is less direct, being dependent additionally on non-glacial processes (slope instability, bottom currents), but may be easier to extract because the sorted sediments are more easily sampled.

Much of the development of this understanding has taken place within the period of ANTOSTRAT activity, and has owed much to the research environment created, in which seismic data held by several groups could be freely exchanged and communally interpreted, and sediment sampling by piston coring has in some cases successfully built on the seismic interpretations. The rapid progress made in understanding the modern Antarctic offshore environment, compared with relatively slow progress in the Northern Hemisphere despite much longer and more intensive study, may be attributed to the relative simplicity and accessibility of the Antarctic environment: an approximate radial symmetry, a fully glacial regime without significant fluvio-glacial activity, the absence of political barriers to collaboration, the fortuitous coincidence of national bases (and thus ship tracks and seismic data) in sheltered indentations in the coastline with the major ice drainage channels and thence prograded wedges.

Drilling Proposals

It became clear that the ANTOSTRAT data set could support a co-ordinated examination of Antarctic glacial history, by means of ODP drilling. Glacial history is of global importance, because of ice sheet interaction with ocean and atmospheric circulation, and its effects on sea level and oxygen isotopic determination of palaeotemperatures. The margin sediment record is crucial because existing low-latitude proxy estimates of ice sheet history are, and will remain, ambiguous and conflicting.

When the plan was first discussed at the August 1994 ANTOSTRAT Symposium in Siena there was only one ODP proposal for margin drilling extant, for the Antarctic Peninsula margin. However, four additional Letters of Intent were submitted to ODP for the end-December 1994 ODP proposal deadline, and tied together by a Summary Document. By end-1995, after a second meeting in Siena in September 1995, we had 5 full-blown proposals and a revised Summary. These were:

The results of these proposals can be combined to produce a complete glacial history, by use of numerical models of ice sheet development. Drilling on 3 or 4 margins would provide the complete story.

The ODP is an independent organisation with its own review procedures and considerable pressures upon and within it for drilling all over the world, to examine a wide range of problems in geoscience. The ANTOSTRAT proposals have undergone several stages of thematic review, and at times it has not been easy to detect progress. However, in May 1996 ODP set up an Antarctic Detailed Planning Group (ADPG) to examine and rank a range of proposals related to Antarctic margins. It recommended an investigation of Antarctic glacial sediments by means of a co-ordinated and prioritised suite of drilling campaigns around the continental margin. It recognised that more than one leg would be required to achieve the objective, and it provided a leg priority listing within a multi-leg plan. Its conclusions were very close to those of ANTOSTRAT. Antarctic Peninsula drilling had the additional role of helping understand the relationships between depositional environments so that drilling strategy on subsequent legs could be refined.

This step was extremely important: the ADPG membership included members of ODP Panels, and its recommendations were accepted by the Planning Committee, the dominant ODP science committee at the time. The ADPG is disbanded, and ODP has since been re-organized, but it is committed to those recommendations in theory and (so far as we know) in practice. One of the central reasons for continued formal SCAR recognition of ANTOSTRAT beyond the demise of its parent Group of Specialists last year was to assist interaction with ODP to maintain this commitment.

All is not plain sailing. One leg has been scheduled so far: the Antarctic Peninsula margin will be drilled during Leg 178 in February to April 1998. An ice support ship will be chartered for this leg, and is an additional expense for ODP, which has said it cannot afford that expense every year. In fact, drilling on the Kerguelen Plateau has been scheduled for early 1999, in the Antarctic margin’s ice and weather window. We have received an undertaking that there will be no procedural barriers to consideration of EITHER a revised Prydz Bay proposal OR a revised Wilkes Land/Ross Sea proposal pair, for drilling in the ice and weather window in early 2000. However, it is up to us to ensure that the science quality of those proposals is sufficiently high to merit drilling. The next deadline for proposal submission is 1st September 1997, and at the end of this week (12 and 13 July) we shall be spending time specifically in helping improve the existing proposals.

Drilling in a particular region is dependent on the existence of other highly-ranked proposals in the vicinity: there is a limit to the extent to which a single highly-ranked proposal can direct drill-ship movement, but “proposal pressure” is a reality. It seems most likely that Prydz Bay and the Wilkes Land/Ross Sea pair will be within reach of the drill ship in early 2000. However, we cannot guess the future movements of the drill ship and would be advised to ensure that the Weddell Sea proposal (now combined with a proposal to drill Mesozoic black shale and basement targets) is optimal also.

The Future

I see the future as a continuation of present strategies for a limited time, but no less difficult than the recent past, and needing continued care and effort.

In science terms, the central goal of this initiative should remain the elucidation of Antarctic glacial history. If it succeeds this will be a tremendous achievement, the solution of a difficult problem of long standing and of global significance. I think the strategy is right, and we stand a good chance of success.

The main difficulty most probably lies in maintaining ODP interest and enthusiasm. In essence, 90-95% of the ODP community would rather the ship was elsewhere. Some of those interested in the problem doubt our ability to produce a solution. To a large extent, Antarctic Peninsula drilling is a test of viability, particularly in the matter of recovery in tills, and there are those who are unhappy at sanctioning further drilling until viability is demonstrated. However, time is short, because of the pressure for the drill ship to head for other, more distant parts of the world. Our immediate aim is to work towards scheduling an ANTOSTRAT leg or legs for drilling in early 2000, and ensuring that leg is adequately supported (for example, with site survey data matching ODP regulations). Beyond that we should consider how to bring things to a satisfactory conclusion. What can the Antarctic community do to help things along? How do we ensure that those members of the ANTOSTRAT community NOT from ODP subscriber countries remain involved in the enterprise? What will be the likely contributions to the elucidation of glacial history from operations OTHER THAN ODP (Cape Roberts drilling, shallow shipboard drilling) that will be active within the same period?

It will be useful, in the context of this Workshop, to consider the possibility of any additional, future use of ODP drilling capability, in pursuit of objectives other than Antarctic glacial history. My views on this are twofold:

In short, it is unrealistic to make plans that involve the use of ODP resources to tackle other problems, in the short and medium term. For the next 5 to 10 years, we must work with what we in the Antarctic community can create ourselves.

These comments are not intended to eliminate ALL possible changes to existing proposals. The community within global marine geoscience that is interested in the high-resolution record of Holocene climate change (and correlation with the continental record in ice cores) has very successfully adopted the strategy of piggy-back studies on planned ODP legs. Recent examples include sites in Saanich Inlet and the Santa Barbara and Cariaco Basins. In particular, a proposal (#502) headed by Eugene Domack to sample a 70m ultra-high-resolution Holocene record in the Palmer Deep on the inner shelf of the Antarctic Peninsula just south of Anvers Island, will occupy 1-2 days of Leg 178. It will provide an interesting complement to the principal objectives and depositional environments of the leg, and we may learn much from it that will inform our interpretation of older sediments. The overdeepened shelf elsewhere around Antarctica may preserve similar records, and their sampling could be a legitimate target of an ANTOSTRAT drilling leg. The relevant community is meeting in the ANTIME Workshop, nearby.