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Late pleistocene palaeoceanographic and geochemical evolution of the South Tasman Rise

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Moy, Andrew David (2005) Late pleistocene palaeoceanographic and geochemical evolution of the South Tasman Rise. PhD thesis, University of Tasmania.

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Abstract

Global ocean circulation affects climate through its influence on transport and the
carbon cycle. Past changes in global ocean circulation may have played an important
role in past glacial-interglacial cycles. This study examines changes in deep-water
circulation and carbonate chemistry in the Southwest Pacific sector of the Southern
Ocean over the past 160 kyrs using benthic foraminiferal stable isotopes and
carbonate dissolution proxy records. Two sediment cores (located at 3310 m and
4002 m water depths) are the focus of this research and are positioned south of the
Subtropical Front south of Australia in modern Circumpolar Deep Water. The
chronostratigraphy of the cores is established by accelerator mass spectrometry
(AMS) 14C dates, benthic and planktonic foraminiferal δ180 and lithostratigraphy.

Results of this study provide new, deep-water Southern Ocean δ13C values from sites
unlikely to be impacted by productivity effects and establishes that glacial (Last
Glacial Maximum (LGM); 18-24 kyrs and Stage 6.2; 134-137 kyrs) benthic
foraminifera (Cibicidoides spp.) δ13C values are depleted compared to interglacial
(Holocene; 0-10 kyrs and Stage 5.5; 118-124 kyrs) values. Previously, depleted
Southern Ocean glacial δ13C values have implied a reduced contribution of northern
source deep-water input into the Southern Ocean [Oppo and Fairbanks, 1987; Oppo
et al., 1990; Charles and Fairbanks, 1992; Ninnemann and Charles, 2002].
However, the global δ13C reconstruction in this study, suggests Southern Ocean δ13C
values were lower than northern source deep-water values and greater than or equal
to Pacific δ13C values, implying there was no 'shut down' of deep-water circulation
patterns during the glacial intervals over the past 160 kyrs.

Comparisons between LGM δ13C records from the South Tasman Rise (STR) and
published records from Atlantic, Pacific, Indian and Southern Oceans suggests: ( 1)
horizontal δ13C gradients were maintained in the LGM between the Atlantic, Pacific
and Southern Oceans above ,..,3500 m, (2) LGM Southern Ocean δ13C values
resemble LGM Pacific b13C values at water depths below ,..,3500 m, and (3) LGM
bottom waters in the Atlantic, Pacific and Southern Ocean converge to similar δ13C
values below ,..,4000 m. A criterion to exclude benthic δ13C records possibly affected
by productivity overprints (Δδ13CcIb-DIc> -0.2 %0) enabled this interpretation.

Planktonic foraminiferal shell weights for Globorotalia injlata (355-425 μm and
300-355 μm) and Globerigina bulloides (355-425 μm) are calibrated to depthnormalised
carbonate ion concentrations. LGM shell weights estimate the deep-water
carbonate ion concentration ([C03=]) at the STR was ,...,10-13 μmol/kg greater than
during the Holocene. Together with a revised shell weight calibration in the
equatorial Atlantic and Pacific, the LGM deep-water [C03 =] reconstruction suggests:
(1) Southern Ocean [C03=] were intermediate to Atlantic and Pacific [C03=] above
,...,4000 m and, (2) LGM bottom waters in the Atlantic, Pacific and Southern Ocean
converge to similar [C03=] below ,...,4000 m, suggesting a similar deep-water source.

Carbonate preservation indices (%CaC03 and %whole planktonic foraminifera) at
the STR indicate three dissolution maxima at ,_,159 kyrs, ,....,66 kyrs (Stage 4), and ,...,27
kyrs. The dissolution records display similar timing to those recorded in the deep
equatorial Pacific [Le and Shackleton, 1992] and deep South Atlantic [Hodell et al.
2001]. The synchroneity between the carbonate dissolution events at the STR, the
equatorial Pacific and the deep South Atlantic 'lndo-Pacific' carbonate stratigraphy
[Hodell et al. 2001], along with a similar deep-water [C03=] for these basins below
,_,4000 m, suggest a similar deep-water source at these locations during glacial
intervals.

The oxygen and carbon isotopic records were obtained from Globigerina bulloides
(300-355 μm) at MD972106 and GC34. The two cores are located close to the
modem calcite saturation horizon in modern Circumpolar Deep Water (CPDW),
providing a test of the effects of carbonate dissolution on G. bulloides stable isotopic
composition. There are differences between the stable isotopic compositions (δ180
and δ13C) between the two cores, particularly during interglacial intervals. G.
bulloides δ180 and shell weights are tightly correlated to each other and to
atmospheric pC02 , and are most likely due to the combined effects of the selective
dissolution of foraminiferal shell calcite and calcification rates of G. bulloides.
Isotopic and shell weight gradients between the two cores show large changes at
glacial-interglacial transitions. The complex relationship between post-depositional
dissolution and calcification rates of G. bulloides suggest both influence its stable
isotopic composition. It is difficult to isolate the effects of each process on G.
bulloides δ180 and δ13C records at both sites.

Item Type: Thesis (PhD)
Keywords: Ocean circulation, Carbonate rocks, Geochemistry
Copyright Holders: The Author
Copyright Information:

Copyright 2005 the Author

Additional Information:

Thesis (Ph.D.)--University of Tasmania, 2005. Includes bibliographical references

Date Deposited: 03 Feb 2015 03:05
Last Modified: 11 Mar 2016 05:54
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