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Sediment Cores and the Rock Record
The geologic record of Antarctic Ice Sheet evolution is obscured by the
mist of time and by the ice sheet itself, which covers over 95 percent of
the continent. There is a virtual absence of rocks that record Antarctica's
climatic evolution during the time interval from the Cretaceous through
the Eocene (approximately 130 to 45 million years ago). Rocks from continents
that were attached to, or adjacent to, Antarctica during this time interval
contain little evidence for glaciation.
Because the rock record on Antarctica is so sparse, geologists have focused
their attention on the offshore sedimentary record. These studies have led
to several important observations.
Ice Rafted Debris
Ice rafted debris is the sediment that is carried in glacial ice and dropped
to the seafloor. Ice rafting can occur near the grounding line of an ice
sheet, or several hundred kilometers away in association with a far-traveling
iceberg. Ice rafted debris commonly shows up in the deep sea sedimentary
records as a slight increase in sand-size material that is of a unique mineralogic
composition.
The expansion of the zone of ice rafting around Antarctica is believed by
some glacial geologists to coincide with the expansion of glaciers. A larger
ice sheet and colder temperature results in farther-drifting icebergs, and
thus a wider-range distribution of ice-rafted debris. The initial appearance
of ice-rafted debris in the geologic record of the Southern Ocean Waters
is believed to mark the initiation of the Antarctic Ice Sheet. Subsequent
increases and decreases in the quantity of ice rafted debris are interpreted
to indicate expansions and contractions of the Antarctic Ice Sheet. It
should be noted, however, that there is considerable debate over the interpretation
of ice rafted material, its initial appearance in sedimentary cores, and
its significance.
There is an overall increase in the amount of sand and gravel in deep sea
sediment cores in sediments younger than about 34 million years. This sand
and gravel is assumed to have been dropped by icebergs (ice-rafted debris)
and therefore indicates the existence of glaciers on Antarctica.
Sediment Cores
Many glacial environments produce sediment. Because the physical and chemical
setting varies in each of the different glacial environments, a different
sediment is produced. For example, under the ice sheet abrasion and plucking
produce till. Till has very specific characteristics. Till is a poorly
sorted, well mixed sediment with angular clasts. It does not contain any
modern microfossils. In contrast, ice shelf sediment, the sediment that
originates under the ice shelf, is very fine grained. It may contain microfossils
from some organisms that can make a living under the ice shelf, but it will
contain no diatoms because the shelf restricts light. The open ocean sediment
also is fine grained, but it does contain diatom microfossils. It also
may have occasional coarser sediment that was carried by icebergs.
These sediment packages from the different glacial settings are stacked
on top of each other, indicating changing glacial environments through time.
A retreating glacier will leave subglacial till behind. This will be capped
by an ice shelf sediment layer. Open-marine sediment will form the top
layer of the sequence. The geologist can sample the stacked sediment sequence
of the sea floor remotely, by taking a sediment core. The information the
geologist acquires from the core will tell him about the environmental conditions,
and changes in the environmental conditions, during the time period of deposition
of the sediment. Critical dating of the sediment in the core, and depositional
events represented by the sediment, possibly may be acquired.
Piston Cores. Sediment piston corers vary in size and engineering,
but most consist of a hollow pipe (up to 10 meters long) and a weight that
is lowered over the side of a ship to the sea floor. The weight pushes
the pipe into the sediment layers. When the pipe is extracted and raised
to the ship, it brings the sediment sample with it. Scientists then open
the pipe to reveal the sediment. Coring preserves the layering of the deposits.
The layering provides geologists with clues to the glacial environment.
If till is overlain directly by open-ocean sediment, then it is possible
that an ice shelf did not exist at the location of the sediment core.
Drilling. Drilling is another way to collect sediment cores. Drilling
implies a rotating coring system with a drill bit that actually cuts through
consolidated or lithified sediment. Drilling allows the collection of very
long cores, and thus the sedimentary layers that are sampled can be very
old.
The Ocean Drilling Program (previously the Deep Sea Drilling Program) is
an international effort that focuses on drilling the global ocean basins
to answer questions about basin structure and development. Four Ocean Drilling
Program or Deep Sea Drilling Program studies have concentrated on drilling
the Antarctic margin. Ross Sea, Weddell Sea, Prydz Bay (Amery Ice Shelf)
and Kerguelen Plateau have hosted drill ships. The objectives have focused
on understanding basin development, reconstructing past climates and oceanography,
and on assessing the initiation of glaciation. Cores acquired by the Ocean
Drilling Program are more expensive and require more time and effort to
collect compared to piston cores. However, the information obtained in
the extended geologic record is priceless.
Department of Earth Science, Rice University, Houston, TX
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