Depositional patterns at Drift 7, Antarctic Peninsula: Along-slope versus down-slope sediment transport as indicators for oceanic currents and climatic conditions
Introduction
Thermohaline-driven bottom currents (contour currents) and their related deposits, generally termed contourites, gained an increasing amount of attention during the last three decades. This is due to the fact that drift deposits contain a record of palaeoenvironmental information about ice sheet development and oceanography, and this archive is hence used to gain knowledge on the palaeoclimatic development of a certain region. “Drift” is a more general term compared to “contourite” and refers to larger sediment deposits with an often complex internal architecture, which are generated by persistent currents of thermohaline origin (Pickering et al., 1989). Publications of the last years concentrated on the recognition and classification of bottom-current-controlled deep-sea deposits in sediments and seismic data (Viana et al., 1998) and their interaction with turbidites (Faugères and Stow, 1993, Stow et al., 1998, Faugères et al., 1999, Stow et al., 2002). A recent publication by Shanmugam (2000) critically reviewed the facies models proposed for deep-water processes during the last 50 years. This discussion shows that the interplay between contour and turbidity currents is not yet fully understood (Faugères and Stow, 1993, Faugères et al., 1999). Nevertheless, it has become increasingly clear that both down-slope and along-slope processes play crucial and interactive roles in the construction and shaping of continental margins.
Transport, erosion and deposition of sedimentary particles are fundamental processes in the benthic boundary layer because they represent the link between oceanographic processes in the water column and the documentation of these processes in the sedimentary record. Harris et al. (2001) proposed that the sea ice regime and production of bottom water are closely related, and thus, the amount of deposited material in a sediment drift is connected to glacial–interglacial cycles. Sedimentary structures and textures hence constitute archives of the depositional and re-depositional environment and processes. By an inversion of those features into the generating process, the analysis of sedimentary structures can lead to a deciphering of the acting oceanographic conditions and, thus, to a better understanding of the development of both oceanographic currents and the climate in a particular area.
Variations in the volume and extent of Antarctica's ice sheets and sea ice cover in the Southern Ocean have a strong effect on Earth's climate via the planetary albedo, eustatic sea level, atmospheric and oceanic circulation. The puzzle of Antarctica's Neogene glacial history is still unsolved (e.g., Stroeven et al., 1998); many pieces, such as the question of major deglaciation during the warm Pliocene period, remain to be unravelled. In order to learn more about the Neogene history of bottom currents and the extension of Antarctica's ice sheet marine geophysical data sets of a sedimentary drift in the Pacific Sector of the continental margin of Antarctica were analysed.
A system of 12 sediment drifts west of the Antarctic Peninsula have been extensively surveyed (Rebesco et al., 1996, Rebesco et al., 1997, Camerlenghi et al., 1997, Lucchi et al., 2002, Giorgetti et al., 2003). About 4000 km of multichannel seismic reflection data (Rebesco et al., 1996) and three ODP Leg 178 site (Barker et al., 1999, Barker et al., 2002) allow a detailed investigation of the different sedimentary units. I will discuss the distribution of the seismic units of Drift 7, which was sampled at ODP Leg 178 Sites 1095 and 1096 and present a model for the interplay of down-slope and along-slope sediment transport. This in turn is interpreted with respect to the controlling oceanographic and climatic conditions.
Section snippets
Geological and oceanographic background
On the Pacific margin of the Antarctic Peninsula, a thick sequence of late Cenozoic to Holocene sediments records the history of bottom-water flow and West Antarctic glaciation (Tucholke et al., 1976). Of specific interest is a series of large mounds aligned along the continental rise between 63° and 69°S (McGinnis and Hayes, 1995, Rebesco et al., 1998). Twelve sedimentary mounds have been identified, separated by large channels (Fig. 1). Their shape generally is asymmetric with steeper,
Seismic stratigraphy
In general, I followed and applied the seismostratigraphic model as defined by Rebesco et al. (1997). Using the information supplied by ODP Leg 178 Sites 1095 and 1096 for Drift 7 (Shipboard Scientific Party, 1999a, Shipboard Scientific Party, 1999b) and Site 1101 for Drift 4 (Shipboard Scientific Party, 1999c) the age model for the seismic units was refined. P-wave velocities supplied by Volpi et al. (2002) were used to convert the geological information from depth into two-way traveltime.
Results
To better define and describe the sedimentary environments, the seabed reflection and interfaces M1/M2, M2/M3, M3/M4, M4/M5, M5/M6, and M6/basement were tracked, thicknesses of the different units computed, and maps of reflector depth and unit thickness compiled. Since reliable velocities were only at hand at the locations of ODP Leg 178 Sites 1095 and 1096 the maps of reflector depth are still in milliseconds two-way traveltime (TWT) and strictly do not represent depth. The term is just used
Discussion
Rebesco et al. (1997) interpreted unit M6 as a mainly turbiditic sequence near the margin. They report that the unit thins away from the slope. My observations, a depocentre close and parallel to the slope, also point towards material derived from the continental shelf, which was moved down the slope. There, the sediment was deposited primarily close to the slope and SW of the crest of interface M6/basement. This indicates a process which involved less energy than a big mass movement event,
Depositional model
My interpretation of sedimentary structures and maps of reflector depth and unit thickness led to the construction of a self-consistent model for the deposition at Drift 7 since the Oligocene. Fig. 6 shows the different stages of the scenario from unit M6 (36–25 Ma, Fig. 6a) to unit M1 (3 Ma–present, Fig. 6f). The shaded areas show the main outline of the unit's base horizon (rms values, see Results) while the hatched areas show the depocentres (rms values, see Results) of the unit. The bold
Conclusions
Maps of reflector depth and unit thickness of Drift 7 were interpreted with respect to depositional patterns. Those depositional patterns were considered to give information on the sediment transport processes (along-slope versus down-slope) active at this part of the continental margin of the Antarctic Peninsula during the Neogene. I also hypothesise on oceanographic and climatic conditions.
I compiled a self-consistent depositional model, which showed that the along-slope transport was the
Acknowledgements
I thank C.-D. Hillenbrand for tracking the seismic horizons and collaborating on the age model, and M. Rebesco for permitting to use the Italian seismic data on Drift 7. Additionally, the discussion with both helped my work enormously, even though we could not derive a common hypothesis to write this publication together. Furthermore, I am grateful for the comments of David Piper, Peter Harris and an anonymous reviewer. This is Alfred-Wegener-Institut Contribution No. awi-n 16023. This research
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