Permian-Triassic boundary beds in Antarctica are unusual
and intriguing, and will be a primary focus of this research.
Boundary beds have already been collected from the Permian-Triassic
contact in South Africa (Lootsberg Pass, Carlton Heights and
Bethulie sections of Smith & Ward 2001), Australia (Coalcliff
and Wybung Head near Sydney; Retallack 1999), New Zealand
(Wairoa Gorge and Productus Creek: Krull et al. 2000) and
Antarctica (Mt Crean in southern Victoria Land and Graphite
Peak in the central Transantarctic Mountains; Retallack et
al. 1998a; Retallack & Krull, 1999). In all of these localities,
the boundary is represented by a ‘claystone breccia’.
Its clayey soil clods and weathered volcanic rocks average
4 mm in size. The breccias looks like a product of massive
deforestation and soil erosion, and are quite unlike beds
above and below this stratigraphic level. In South Africa,
Australia and Antarctica the claystone breccias are thin (6-15
cm) and form the base of a paleosol, but in marine sections
of New Zealand they are thick (50-60 m). Some of these beds
have low amounts of alkali and alkaline earths as if leached
by acid, with the exceptionally high value of 30 for the ratio
of alumina/bases in the Sydney Basin examples. They also contain
fossil plant debris and coal clasts in Antarctica and Australia,
and shell and bryozoan fragments in New Zealand.
Our science objective is to collect samples for laboratory
analysis that will reveal events that caused extinction of
most life forms on Earth at the Permian-Triassic boundary
250 million years ago. Suggested causes include meteorite
impact, massive volcanic activity, continental shelf collapse
and a massive dissociation of methane hydrates. We intend
to discriminate between these causes through a variety of
field and laboratory approachs. Fossil soils and sediments
can reveal ecosystem changes. Carbon isotopic studies can
reveal methane inputs and plant productivity changes. Iridium,
helium, fullerenes and shocked quartz can reveal the role
of impact. Our party includes a variety of experts in fossil
soils (Retallack and students), carbon isotopic analysis (Jahren
and Hagopian) and fullerenes, helium and iridium (Becker and
Poreda).
Recognizing the Permian-Triassic boundary in Antarctica
A fundamental problem for this project is being able to recognize
the Antarctic Permian-Triassic boundary, which has long been
regarded as diachonous and disconformable. During Late Permian
and Early Triassic time the current region of the Transantarctic
Mountains was a non-marine backarc basin inboard of an Andean-style
volcanic arc separating it from the Eopacific Ocean. Palaeocurrents
in these alluvial sediments indicate an arc-parallel drainage
basin extending from the central Transantarctic Mountains
through southern Victoria Land into northern Victoria Land,
Antarctica. This extensive river system breached the Ross
High that separated central Transantarctic and Victoria Land
depocenters. Local palaeocurrents directed onto the craton
and away from the volcanic arc represent local alluvial fans
from the flanking volcanic-metamorphic highlands (Isbell et
al. 1990, Collinson et al. 1994). Estimates of the extent
and duration of disconformities in these alluvial deposits
now come from biostratigraphy, paleosols, carbon isotopic
studies, iridium, fullerene and helium isotopic analyses.
|
