Geophysical data acquisition - OGS

Orbitally-Controlled rhythmic sedimentation
in the Wild Drift, Antarctica (ODP Leg 188, Site 1165)
M. REBESCO
This research, conducted in collaboration with J. Gruetzner (Bremen University,
Germany) is an outcome of the participation of an OGS researcher (Michele
Rebesco) to the ODP Leg 188 in Prydz Bay – Cooperation Sea (Antarctica). Leg 188
cruise began in Fremantle (Australia) on 10 January 2000 and ended in Hobart
(Australia) on 11 March 2000. Three sites were drilled on continental shelf, slope
and rise to document onset and fluctuations of East-Antarctic glaciations.
Site 1165 is situated in a water depth of 3357m on the continental rise in front of
the outlet of the Lambert glacier-Amery Ice Shelf system that today drains 22% of
East Antarctica. The site cored a 999-m lower Miocene-Holocene section into an
elongate sediment body (Wild Drift) formed by the interaction of westward-flowing
currents with the sediment supplied from the shelf. Alternations with wavelengths
ranging from cm to m size between a greenish grey diatom bearing clay facies and
dark grey clay facies with silt laminations are apparent throughout the hole back
to early Miocene time. Furthermore the greenish intervals are characterised by
lower density, susceptibility and iron content. The dark grey intervals are
interpreted as contouritic facies deposited during maximum ice advances whereas
the greenish sediments indicate hemipelagic sedimentation under warmer climate
conditions.
Analysis of high resolution colour photo-spectrometer data reveals that the colour
cycles are best described by the ratio of the reflectivity in the green colour band
and the average reflectivity (grey).
Spectral analyses on depth and time series of the investigated parameters over
selected intervals demonstrate that variance is dominated by orbital frequencies as
predicted by the Milankovitch theory. The detected obliquity and precession cycles
allow a refined evaluation of sedimentation rates.
Subsidence at the Cape Roberts drill sites
(Ross Sea, Antarctica) from backstripping techniques
L. DE SANTIS
G. BRANCOLINI
The tectonic subsidence of the western margin of the Victoria Land basin has been
estimated from the physical properties and ages of the sediments in the Cape
Roberts Project drill cores 2/2A and 3, using backstripping techniques, assuming a
local isostatic compensation. The sediment load effects is removed from the total
subsidence and the tectonic contribution through time at each location is
calculated.
The analysis indicates a total tectonic subsidence of about 660 m at this location
between 34 Ma and the present time. Two main trends are defined, i) about 230
m/m.y. from 34 Ma to 32.5 Ma, and ii) about 23 m/m.y. from 32.5 Ma to 21 Ma.
Since 21 Ma, the subsidence is not well constrained. Extrapolation indicates a very
low subsidence rate, but uplift within this period may have greatly affected the
estimate.
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