History of mortality in Great Barrier Reef coral communities since European settlement

Clark, Tara (2011). History of mortality in Great Barrier Reef coral communities since European settlement PhD Thesis, School of Earth Sciences, The University of Queensland.

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Author Clark, Tara
Thesis Title History of mortality in Great Barrier Reef coral communities since European settlement
School, Centre or Institute School of Earth Sciences
Institution The University of Queensland
Publication date 2011-12
Thesis type PhD Thesis
Supervisor Jian-xin Zhao
John Pandolfi
Terry Done
Total pages 201
Total colour pages 16
Total black and white pages 185
Language eng
Subjects 040303 Geochronology
040203 Isotope Geochemistry
060205 Marine and Estuarine Ecology (incl. Marine Ichthyology)
Formatted abstract
Despite reported declines in coral cover and shifts in community structure in inshore reef communities on the Great Barrier Reef (GBR), Australia, there remains considerable debate as to whether these changes reflect a degraded ecosystem as a result of anthropogenic disturbances or the dynamic nature of inshore reefs. This is primarily due to a lack of historical information prior to the onset of long-term monitoring in the late 20th century. In order to make any valid interpretations, a baseline understanding of coral community structure and disturbance history is required. Here the uranium series (U-series) dating method has been further developed in order to place these changes in an accurate chronological timeframe.

However, dating young corals (<200 yrs old) from the inshore region of the GBR is not without its limitations. Such samples contain proportionally higher initial 230-thorium (230Th0) compared to older samples and those from open ocean settings, and can result in meaningless 230Th ages if left uncorrected. U-series dating of 43 samples of known ages collected from living Porites sp. from the far northern, central and southern inshore regions of the GBR, revealed 230Th/232Th0 atomic values to mostly range from 3.5 ± 1.1 to 8.1 ± 1.1 ×10-6, with a weighted mean of 5.76 ×10-6 (± 25%). This value is much more constrained than the bulk Earth value routinely used in the past to correct for initial 230Th/232Th in speleothem and old coral samples. 230Th/232Th0 values varied from one site to another and several anomalously high initial 230Th/232Th values coinciding with historic El Niño events were also found; highlighting the importance of understanding 230Th/232Th0 variability in order to obtain a suitable value for 230Th0 correction.

For dead corals from inshore reef settings, both hydrogenous and detrital 230Th incorporated during growth as well as post-mortem, respectively, needs to be accounted for when correcting 230Th ages. Examination of 230Th/232Th in dead Porites revealed samples to lie on a mixing line between a detrital value of 3.3 ± 0.1 ×10-6 (2) and a hydrogenous value of 5.76 ×10-6 reported in live Porites corals. Through a combination of locally determined initial 230Th/232Th values and vigorous cleaning procedures, it is possible to reduce the age error associated with initial 230Th correction to less than ± 1 yr in dead coral samples from turbid inshore environments receiving multiple sources of 230Th0 using a two-component correction scheme. As the analytical uncertainty is about 1%, collectively, an age precision of ± 1-3 yrs is achievable for corals less than 200 years old, enabling the detection of historical mortality events at an unprecedented resolution.

The congruence between the 230Th age data collected from 41 individual Acropora colonies corrected for initial 230Th using a sample specific two-component equation and the documented catastrophic loss of Acropora at Pandora Reef as a result of the 1998 bleaching event, is evidence to 9 support the applicability of U-series in accurately dating dead coral skeletons from the death assemblage.

Ecological surveys performed across five reefs in the Palm Islands region revealed that large expanses of dead Acropora rubble were not limited to Pandora Reef. U-series dating of these adjacent sites revealed the timing of mortality to be spatially heterogeneous among reefs, spanning from ~1930 to 1998 AD. At these locations, coral communities that were once characterised by the formerly abundant branching Acropora have now been replaced by soft corals, macroalgae and other coral genera typical of inshore reef environments. The loss of large arborescent colonies prior to 2000 AD and the low abundance of living colonies imply a regionally depressed state of Acropora communities in accordance with recent accounts.

To determine differences in the timing and cause of mortality among different coral species, samples of massive Porites colonies were also collected from the Palm Islands region. U-series dating revealed the timing of mortality to have occurred over two periods from 1994.6 ± 2.3 to 1999.4 ± 3.4, and more recently from 2006.4 ± 1.8 to 2008.3 ± 2.0 A.D. High resolution measurements of Sr/Ca, Mg/Ca and Ba/Ca obtained from well preserved specimens enabled the timing of mortality to be further constrained to a seasonal resolution, and suggest that thermal bleaching in 1998 and increased sedimentation from flood plume waters in 2008 and 2009 may have been the primary causes for mortality.

Overall, the results of this thesis demonstrate the high fidelity of U-series dating in both precisely and accurately determining the timing of mortality in coral death assemblages. Moreover, a combination of U-series dating and elemental geochemistry represents a potentially important method for discerning the likely cause of mortality not only in Porites colonies, but also in adjacent coral communities. This may prove to be a powerful tool to reef managers in evaluating the timing of mortality events as well as recovery performance in a quantified historical context at reefs where long-term ecological monitoring data are absent.
Keyword Coral reef
Great Barrier Reef
Additional Notes Colour pages: 64, 67, 93, 95-96, 113, 125, 150-152, 155-156, 166 , 192-194. Landscape pages: 55-59, 87-89, 97-99, 119-120, 123, 162-165, 189-190.

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Created: Fri, 29 Jun 2012, 10:46:32 EST by Tara Clark on behalf of Scholarly Communication and Digitisation Service