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| Titel |
Improving the accuracy and precision of TIMS U-series ages of modern corals from the Great Barrier Reef, Australia |
| VerfasserIn |
Tara Clark, Jian-xin Zhao, Yuexing Feng, Terry Done, Stacy Jupiter, Janice Lough, Eric Matson, John Pandolfi, George Roff |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250044478
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| Zusammenfassung |
| The main limiting factor in obtaining precise and accurate Uranium-series ages of modern
corals (e.g. since European settlement of northern Australia around 1850 AD), is the ability to
constrain and correct for initial or non-radiogenic 230Th. This is becoming particularly
important in paleoecological research where accurate chronologies are required to pinpoint
changes in community structure and the timing of mortality events in order to identify
possible drivers. In this study, thermal ionisation mass spectrometry (TIMS) Uranium-series
dating of 61 samples collected from living and non-living Porites spp. from the near shore
region of the GBR was performed to spatially constrain initial 230Th/232Th (230Th/232Th0)
variability.
In the living Porites corals, the majority of 230Th/232Th0 values were higher than the
bulk-Earth value (~4.4Ã10-6) generally assumed for non-radiogenic 230Th corrections
where the primary source of initial thorium is terrestrially derived. Despite samples being
taken from regions adjacent to contrasting levels of land modification, no apparent difference
was found in 230Th/232Th0 between regions exposed to varying levels of sedimentation
during runoff events. However, 230Th/232Th0 variability is evident between reefs within each
region. Overall, most samples across the entire region give 230Th/232Th0 values in the range
of 6±1Ã10-6.
An examination of 232Th/238U versus 230Th/238U from living and non-living corals
revealed mainly two components contributing to initial 230Th in the non-living coral samples.
High 232Th concentrations found in the majority of non-living coral samples suggest that a
significant amount of Th may have been incorporated into the coral skeleton through
post-mortem non-carbonate sediment infiltration. The results of this study demonstrate that
accurate U-series ages cannot be achieved where single non-radiogenic thorium correction
values are used interchangeably for samples taken from different hydrological settings and
for both living and dead Porites. For non-living corals, we propose the use of a specific
correction value calculated based on 232Th levels for individual samples to account for
both terrestrially-derived detrital and hydrogenous Th. Furthermore, we have also
developed a vigorous cleaning procedure to reduce the level of infiltrated sediment
adhering to the non-living coral skeleton. Using both methods we are able to achieve
precise and accurate U-series ages (up to ±1 year) for corals less than 200 years old. |
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