Spatial variability of initial 230Th/ 232Th in modern Porites from the inshore region of the Great Barrier Reef

• Published in: Geochimica et cosmochimica acta Vol. 78; pp. 99 - 118
• Main Authors: Clark, Tara R., Zhao, Jian-xin, Feng, Yue-xing, Done, Terry J., Jupiter, Stacy, Lough, Janice, Pandolfi, John M.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 01-02-2012
• Subjects: Age; Bands; community structure; Corals; Dating; Dating techniques; Earth; Error correction; Freshwater; ionization; mass spectrometry; mortality; paleoecology; Porites; Reefs; rivers; runoff; sediments; Australia; Great Barrier Reef; ISEW, Australia, Queensland, Great Barrier Reef; IW, Pacific
• ISSN: 0016-7037; 1872-9533
• DOI: 10.1016/j.gca.2011.11.032

The main limiting factor in obtaining precise and accurate uranium-series (U-series) ages of corals that lived during the last few hundred years is the ability to constrain and correct for initial thorium-230 (
230Th
0), which is proportionally much higher in younger samples. This is becoming particularly important in palaeoecological research where accurate chronologies, based on the
230Th chronometer, are required to pinpoint changes in coral community structure and the timing of mortality events in recent time (e.g. since European settlement of northern Australia in the 1850s). In this study, thermal ionisation mass spectrometry (TIMS) U-series dating of 43 samples of known ages collected from living
Porites spp. from the far northern, central and southern inshore regions of the Great Barrier Reef (GBR) was performed to spatially constrain initial
230Th/
232Th (
230Th/
232Th
0) variability. In these living
Porites corals, the majority of
230Th/
232Th
0 values fell within error of the conservative bulk Earth
230Th/
232Th atomic value of 4.3
±
4.3
×
10
−6 (2
σ) generally assumed for
230Th
0 corrections where the primary source is terrestrially derived. However, the results of this study demonstrate that the accuracy of
230Th ages can be further improved by using locally determined
230Th/
232Th
0 values for correction, supporting the conclusion made by
Shen et al. (2008) for the Western Pacific. Despite samples being taken from regions adjacent to contrasting levels of land modification, no significant differences were found in
230Th/
232Th
0 between regions exposed to varying levels of sediment during river runoff events. Overall, 39 of the total 43
230Th/
232Th
0 atomic values measured in samples from inshore reefs across the entire region show a normal distribution ranging from 3.5
±
1.1 to 8.1
±
1.1
×
10
−6, with a weighted mean of 5.76
±
0.34
×
10
−6 (2
σ, MSWD
=
8.1). Considering the scatter of the data, the weighted mean value with a more conservative assigned error of 25% (i.e. 5.8
±
1.4
×
10
−6) that encompasses the full variation of the 39
230Th/
232Th
0 measurements is recommended as a more appropriate value for initial
230Th corrections for U-series dating of most
Porites samples from inshore regions of the GBR. This will result in significant improvement in both the precision and accuracy of the corrected
230Th ages related to those based on the assumed bulk Earth
230Th/
232Th
0 value of 4.3
±
4.3
×
10
−6. However, several anomalously high
230Th/
232Th
0 values reaching up to 28.0
±
1.6
×
10
−6 occasionally found in some coral annual bands coinciding with El Niño years imply high
230Th/
232Th
0 sources and highlight the complexities of understanding
230Th/
232Th
0 variability. For U-series dating of young coral samples from such sites where anomalous
230Th/
232Th
0 values occur, we suggest replicate dating of multiple growth bands with known age difference to verify age accuracy.