New Zealand Journal of Geology and Geophysics abstracts
d13Corg chemostratigraphy of the Permian-Triassic boundary in the
Maitai Group, New Zealand: evidence for high-latitudinal methane
release
E. S. KRULL
Department of Geological Sciences
Indiana University
Bloomington, IN 47405
USA
G. J. RETALLACK
Department of Geological Sciences
1272 University of Oregon
Eugene, OR 97403-1272
USA
H. J. CAMPBELL
G. L. LYON
Institute of Geological & Nuclear Sciences
P.O. Box 30 368
Lower Hutt, New Zealand
Abstract Carbon isotopic studies from marine organic matter
of the Permian-Triassic Maitai Group, New Zealand, reveal a significant
d13Corg shift toward more negative values within the Little Ben
Sandstone Formation. These isotopic data chemostratigraphically define the
previously debated position of the Permian-Triassic boundary in the Maitai
Group. The Permian-Triassic record of the Maitai Group is also important
because of its high paleolatitudinal setting and the deposition at intermediate
depths in the ocean (c. 400 m) within a volcanic arc-related basin.
Marine Permian-Triassic strata deposited at water depths deeper than shelf
areas are rare. High latitude Permian-Triassic boundary sections document a
significantly larger isotopic offset across the boundary compared with lower
latitude settings.
Carbon isotopic values decrease rapidly by an average of 7[[perthousand]]
from homogeneous values (x -25‰) in the Tramway and lower Little Ben Sandstone
Formation to highly fluctuating and very depleted values (x -32‰) within
the Little Ben Sandstone Formation. The lowermost Big Ben and Tramway Formations
are considered to be Permian in age, based on their homogeneous and comparably
heavier carbon isotopic values and supported by fossil atomodesmatinid bivalves.
Based on the distinct d13Corg excursion toward negative values and
the concurrent onset of strong isotopic fluctuations, the Permian-Triassic boundary
is placed in the lower half of the Little Ben Sandstone Formation.
Very depleted d13C values in the Little Ben Sandstone Formation
of -38‰ indicate a contribution from isotopically light methane. A possible
methane source is clathrates, released by large submarine slides or warming-induced
melting of permafrost. The Little Ben Sandstone Formation has been interpreted
as a massive event deposit from a submarine slide (Landis 1980). This hypothesised
methane release could have been in part responsible for the larger Permian-Triassic
isotopic shift in high latitudes compared with low latitudes because large volumes
of clathrates are trapped in continental shelves and high-latitude permafrost.
Keywords Permian-Triassic boundary; Maitai Group; stable
carbon isotopes; methane
New Zealand Journal of Geology & Geophysics, 2000, Vol. 43: 21-32
0028-8306/00/4301-0021 $7.00/0 (c) The Royal Society of New Zealand
2000
PDF file of entire paper: medium quality (1247K); (scanned from paper original: notes about this process)
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