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You are in:  Home » Publications » Reports » Report 16 » Appendix 7

SCAR Report No 16,

Appendix 7

WEDDELL SEA, LAZAREV SEA, AND RIISER-LARSEN SEA
Wilfried Jokat
Alfred Wegener Institute, Bremerhaven, Germany

From the present knowledge of geodynamic movements, the Weddell Sea faced the earliest rifting events, which ended in the break-up of the Gondwana super continent. After South America and Africa had separated from Antarctica, the rift process continued into the recent Lazarev and Riiser Larsen seas to split off India. The break-up of these continental masses resulted into the creation of new restricted basins. At approximately 130 Myr a major reorganisation of the sea floor spreading occurred. Altimeter data indicate an almost continuos herring bone pattern of gravity anomalies, which are interpreted to result from a drastic decrease of spreading velocities. Around the same time the formation of a large volcanic feature, the Maud Rise, was in progress. From ODP holes 693 and 692 it is known that a large hiatus started at 110-120 Myr and ended at approximately 40 Myr. During the same time span oceanic crust formed along the South Atlantic/Indian ocean sector of East Antarctica. Madagascar and India split off latest 110 Ma ago. The rifting continued with the separation of Australia and New Zealand. Till 30 Myr Antarctica still was connected with the South American continent preventing the establishment of a Circum-Antarctic current system as it is present today. The separation of both continents afterwards led to an oceanographic isolation of the Antarctic continent, which might have accelerated and/or initiated the large scale glaciation of the whole continent. Till today several glacial/interglacial periods with different strengths occurred. No age control exists to describe this glacial events in greater detail, e.g. when did the Filchner-Ronne Ice Shelf advance to the shelf break.

This brief description of the geodynamic history of the South Atlantic/Indian ocean sector is of course not complete but summarises also the main scientific objectives of geophysical programmes in this area. In general it is the understanding of the tectonic and glacial history of this region. In the following some specific areas will be described in greater detail, some to a lesser degree as only few geophysical data are available.

North-Western Weddell Sea (64°S 60°W/70°S 50°W)

This area is extremely poor investigated as here the most difficult ice conditions are found. The Larsen Shelf between 64°S and 66°S has been surveyed during two American expeditions in 1991 and 1993. Based on these results the shelf can be divided into three major units:

In the west chaotic reflection pattern are interpreted to represent Jurassic basalts.

East to this unit faulted and slightly eastward tilted (3-5°) sediments are found. In a zone which is up to 60 km wide the Cretaceous to Oligocene (?) sediments are outcropping at the sea floor and/or are only covered by a thin veneer of sediments.

The easternmost unit is interpreted to represent Miocene to Pleistocene(?) mainly glacial deposits.

Filchner-Ronne Shelf

Like the previous region this area is poorly and unsystematically sampled due to the varying ice conditions. Only three MCS lines crossing the whole Ronne Shelf in N-S direction exist. The seismic data show mostly flat-lying sediments across the whole 500 km wide Ronne Shelf at least down to 5 km depth. At the margins towards the Antarctic Peninsula in the west and East Antarctica old sediments of most likely Jurassic/Cretaceous age are outcropping and/or are covered by a thin veneer of sediments.
The glacial deposits across the Ronne Shelf along 35¡W show no large variation in structure for more than 400 km. Even MCS lines perpendicular (E-W direction) to them show no structural changes. This might be interpreted as evidence for a grounded but static ice shield or for the presence of a non-grounded ice shelf on most of the Ronne Shelf. There might be areas where a complete glacial sedimentary record can be recovered by shallow drilling.

Clear evidence for grounded ice is found in the Ronne and Filchner Troughs bordering the huge shelf area. The troughs are strongly eroded by glaciers.

South-Western Weddell Sea Off Filchner Trough Mouth Fan

Seismic surveys of several institutions have mapped at least four large scale channel/leve complexes having their origin along the Crary Trough mouth fan. The full extent of these structures is now known due to the new Weddell Sea bathymetric map (in press). However, the age, development and dynamic relations of the channel/levee complexes are unknown at all. It is evident that most of the glacial material of this part of the Weddell Sea have been drained through the Crary Trough mouth fan.

Continental Margins between 20°W and 30°E

From bathymetric mapping it is known that the non-ice covered shelf is quite narrow. At some locations the present ice shelf edge is almost above the bathymetric shelf break. Between 19¡W and 0¡ a prominent escarpment, the Explora Escarpment, is present at water depths of less than 2000 m. Here, two ODP holes were drilled on the shoulders of the Wegener Canyon. The oldest drilled rocks were of Cretaceous age. A 70 Myr hiatus between 110 Myr and 40 Myr prevented the recovery of a more complete geological record. However, these holes delivered the only age information for the Weddell Sea, which have been used for seismic stratigraphy interpretations. Borehole information had to be extrapolated over more than 1000 km and consequently the interpretation is quite vague.

Further to the east two large submarine ridges, Astrid (14¡E) and Gunnerus (28¡E) ridges have been mapped with seismics, gravity and magnetics. They are regarded as remnants of early break-up processes during the separation of Antarctica from Africa and India. The sedimentary column is in parts quite thin but most likely continuos since its formation.

Central Weddell Sea

Altimeter data show a herring bone gravity anomaly pattern at 68¡S running in E-W direction across the whole Weddell Sea basin. These anomalies have been interpreted to result from a drastic drop in spreading velocity at approximately 130 Myr. They were considered to be caused by rough oceanic basement, which is typically for slow mid-ocean spreading ridges. This structural interpretation has been confirmed by a recent geophysical survey (seismic, gravity). Approximately 100 to 200 km to the north the oceanic basement ridges are outcropping and can be sampled directly.

Summary

The whole problem in understanding any geodynamic and glacial processes in the areas described is the missing age and structural control on prominent seismic units. The existing ODP holes on Maud Rise and on the Explora Escarpment are either too shallow to detect basement for dating the break-up and/or are difficult to correlate with the deep sea deposits. An ODP hole in the center of the Weddell Sea is of lesser value for any seismic interpretation as dating is quite problematic. Even dating by magnetic spreading anomalies is not possible as only few data exist. Most of the geodynamic history is extrapolated from the conjugate margins off South Africa and the onshore volcanic record. In brief, in large areas no direct age control exists. Therefore, any published ages concerning the geodynamic and glacial history of this area are not validated by samples and are tentatively. This is in strong contrast to the huge amount of geophysical data which were collected in the last decades.

Current Mapping Projects

• Within a British-Russian project all aeromagnetic data of both countries have been gathered, mapped and are published. Areas covered: Antarctic Peninsula, Filchner Ronne Shelf, Coats Land, Dronning Maud Land.
• Within a Russian-German project all potential field and radar data (gravity, magnetic, ice thickness) from earlier expeditions have been gathered and maps will be produced. There is only some overlap with the British-Russian maps for the aeromagnetic.
• Gravity data from all marine expeditions (Germany, Norway) have been gathered, processed, adjusted.
• Most of the multichannel seismic data in the Weddell Sea, Lazarew and Riiser Larsen seas have been gathered at AWI and have been interpreted in terms of tectonic and glacial history.
• Recently altimeter data from ERS1 and ERS2 satellites are available, showing the gravity field over all areas under discussion.
• The marine magnetic data in the Weddell Sea have been gathered at BAS to understand the origin of the herring bone gravity anomalies.
• American-Argentine-Chile aeromagnetic data are partly published now.
• A new bathymetric chart of the southern Weddell Sea is in press
• Sub ice topography map of the Filchner-Ronne Shelf is published. Data from Russian, British and German expeditions were used.

Scientific Problems/Questions Which Might Be Answered In The Next Decade

l. Glacial History
Time span: 0-60 Myr
Eastern Weddell Sea, Lazarev Sea and Riiser-Larsen Sea

What are the differences between the sediment input during glacial/interglacial periods comparing margins covered by large ice shelves (Filchner, Ronne, Larsen shelves) with margins covered by minor ice shelves (East Antarctic coast between 20¡W and 30¡E)? This would allow to judge in which periods the shelves were covered by floated, grounded ice or uncovered by ice shelves.

• Action: Systematic bathymetric, Side scan sonar and seismic mapping of the margins, Shallow coring,
• Data: Not available

South Western Weddell Sea

• When did the glaciation of East Antarctica start?
• How sensitive did the East Antarctic Ice shield behaved in the past due to climate changes? How stable and/or unstable were the East and West Antarctic ice shields in the past?
• Are there any time delays in the reaction of the different areas in Antarctica in terms of ice shield growth or retreat due to climate changes?
• How did the opening of the Drake Passage influence the glacial record of the Weddell Sea? Did it initiate the glaciation of the continent or did it only accelerate it?
• Action: ODP drilling into the channel/levee complexes and on Polarstern seamounts
  Presite Data: All necessary data are available
• Was the Ronne Ice shelf grounded in the past (at least during the last glacial)? Verifying of numerical modelling.
• Further understanding of links between shelf and deep sea glacial deposition.

  Action: Shallow drilling on the Ronne Shelf; ODP will due to ice coverage not be able to drill in this area.
  Presite Data: Most of the data are available; some more seismic data are needed to give better constraints on the drill location.

• How does the continental climate record from ice cores correlate with the marine data from gravity or piston cores? Which kind of information can be derived concerning growth and disintegration of the East Antarctic ice shield during the last 300ky .
• Action: Drilling on the East Antarctic ice shield. Gravity cores from the Weddell Sea.
• Data: European Drilling programme EPICA is intended to start in 1999. Gravity cores are already available.

11. Tectonic/Paleoceanographic History
Time span: 0-180 Myr
Eastern Weddell Sea,Lazarev Sea, and Riiser-Larsen Sea

• When did the break-up of Gondwana start in the Weddell Sea and how did it continue eastward?
• How did the restricted anoxic Mesozoic basins evolve with time? How was the productivity and paleobiogeographic development of Mesozoic Antarctic faunas and floras?
• How was the development of high latitude and global Mesozoic climates?
• When did gateways open after break-up of Africa and/or India to connect the Weddell, Lazarew and Riiser Larsen seas to the world oceans? Which role did the Maud Rise, the Astrid and Gunnerus ridges play? For this a detailed knowledge of the magnetic spreading anomalies in this area is essential.

  Action: ODP drilling along the margins and the adjacent deep seas to date volcanic wedges and the adjacent deep sea magnetic anomalies.
  Data: MCS seismic is available to choose drilling locations along the margins. The magnetic anomaly pattern is extremely poorly charted. EMAGE project intends to collect aerogravity and aeromagnetic data within the next 6 years along the margin to map the potential field anomalies across the adjacent deep seas (20°W to 30°E).

Western and South Western Weddell Sea

Shallow drilling into the outcropping old sequences along the Larsen Shelf, Ronne and Filchner troughs would provide first information on the Mesozoic environment of the Weddell Sea. It would also put strong constraints on any geodynamic models for this area. Furthermore, it would provide first information on the presence of paleo-gateways in the Filchner-Ronne Shelf area.
Is the southern Weddell Sea (Filchner Ronne shelves) an extensional basin or is it a part of a highly mobile terrane (Ellsworth Withmore mountains) having moved into its position in Jurassic/Cretaceous times?

Action: Shallow drilling into outcropping sequences
Presite Data: All necessary data are available; enough alternate sites can be specified

Central Weddell Sea

• Determination of the age and composition of the outcropping basement ridges in the northern deep Weddell Sea. Dating of the youngest magnetic anomalies present in the Weddell Sea.
• What is the age of the E-W gravity anomaly at 68¡S in the Weddell Sea? If the age of this event is known it might play an important role for geodynamic models.Action: ODP drilling; dredging of outcropping basement ridges in the deep sea part of the Weddell Sea Data: Some presite data are available; no dredging of the ridges have been performed so far.

The short summary clearly demonstrates that the most valuable information in understanding the glacial and geodynamic history of the areas described can be expected from deep and shallow drilling. Without that information most of the above mentioned interpretations will continue to be pure speculation.
Not mentioned at all are activities over ice performing e.g. seismic experiments on the Filchner-Ronne Ice shelf and on the East Antarctic craton. Various experiments (seismics, hot water drill etc.) can be carried out here to increase our understanding for the geodynamic history and recent glacial processes (e.g. dynamics of the grounding line, sediment thickness etc.).