New Zealand Journal of Geology and Geophysics abstracts
Textural variations in Neogene pelagic carbonate ooze at DSDP Site
593, southern Tasman Sea, and their paleoceanographic implications
Penelope
J. Cooke1
Campbell
S. Nelson1
Martin
P. Crundwell1,2
Brad
D. Field2
E.
Shirley Elkington1
Harold
H. Stone1
1Department of Earth
Sciences
University of Waikato
Private Bag 3105
Hamilton, New Zealand
email:
p.cooke@waikato.ac.nz
2Institute of Geological
& Nuclear Sciences
P.O. Box 30 368
Lower Hutt, New Zealand
Abstract Changes in Neogene sediment texture in
pelagic carbonate-rich oozes on the Challenger Plateau, southern Tasman
Sea, are used to infer changes in depositional paleocurrent velocities.
The most obvious record of textural change is in the mud:sand ratio.
Increases in the sand content are inferred to indicate a general
up-core trend towards increasing winnowing of sediments resulting from
increasing flow velocity of Southern Component Intermediate Water
(SCIW), the forerunner of Antarctic Intermediate Water. In particular,
the intervals c. 19–14.5 Ma, c. 9.5–8 Ma, and after 5 Ma are
suggested to be times of increased SCIW velocity and strong sediment
winnowing. Within the mud fraction, the fine silt to coarse clay sizes
from 15.6 to 2 µm make the greatest contribution to the
sediments and are composed of nannofossil plates. During extreme
winnowing events it is the fine silt to very coarse clay material
(13–3 µm) within this range that is preferentially removed,
suggesting the 10 µm cohesive silt boundary reported for
siliciclastic sediments does not apply to calcitic skeletal grains. The
winnowed sediment comprises coccolithophore placoliths and spheres,
represented by a mode at 4–7 µm.
Further support for seafloor winnowing is gained from the presence
in Hole 593 of a condensed sedimentary section from c. 18 to 14 Ma
where the sand content increases to c. 20% of the bulk sample.
Associated with the condensed section is a 6 m thick orange unit
representing sediments subjected to particularly oxygen-rich, late
early to early middle Miocene SCIW. Together these are inferred to
indicate increased SCIW velocity resulting in winnowed sediment
associated with faster arrival of oxygen-rich surface water subducted
to form SCIW. Glacial development of Antarctica has been recorded from
many deep-sea sites, with extreme glacials providing the mechanism to
increase watermass flow. Miocene glacial zones Mi1b–Mi6 are identified
in an associated oxygen isotope record from Hole 593, and correspond
with times of particularly invigorated paleocirculation, bottom
winnowing, and sediment textural changes.
Keywords Tasman Sea; carbonate; texture; Neogene;
DSDP Site 593; winnowing
G03041; Received 16 April 2003; accepted 24 February 2004; Online
publication date 1 December 2004
New Zealand Journal of Geology & Geophysics, 2004, Vol. 47:
787-807
0028-8306/04/4704-0787© The Royal Society of New Zealand 2004
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