New Zealand Journal of Geology and Geophysics abstracts

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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. Crundwell^1,2
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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