HR: 17:30h
AN: B32D-07    [PDF]
TI: Geochemical Records of Bleaching Events and the Associated Stressors From the Great Barrier Reef
AU: * Roark, E B
EM: ebroark@socrates.berkeley.edu
AF: Department of Geography, University of California,, Berkeley, CA 94720-4740 United States
AU: McCulloch, M
EM: Malcolm.McCulloch@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AU: Ingram, B L
EM: ingram@socrates.berkeley.edu
AF: Department of Earth and Planetary Science, University of California,, Berkeley, CA 94720-4767 United States
AU: Marshall, J F
EM: John.Marshall@anu.edu.au
AF: Research School of Earth Sciences, Australian National University, Canberra, ACT 0200 Australia
AB: The health of coral reefs world-wide is increasingly threatened by a wide array of stressors. On the Great Barrier Reef (GBR) these stressors include increased sediment flux associated with land use changes, increased sea surface temperatures (SST) and salinity changes due to large floods, the latter two of which are factors in an increased number of bleaching events. The ability to document long-term change in these stressors along with changes in the number of bleaching events would help discern what are natural and anthropogenic changes in this ecosystem. Here we present results of an initial calibration effort aimed at identifying bleaching events and the associated stressors using stable isotopic and trace element analysis in coral cores. Three $\sim$15-year time series of geochemical measurements ($\delta$$^{13}$C, $\delta$$^{18}$O, and Sr/Ca) on {\it Porites} coral cores obtained from Pandora Reef and the Keppel Islands on the GBR have been developed at near weekly resolution. Since the $\delta$$^{13}$C of the coral skeletal carbonate is known to be affected by both environmental factors (e.g. insolation and temperature) and physiological factors (e.g. photosynthesis, calcification, and the statues of the symbiotic relationship between corals and zooxanthellae) it is the most promising proxy for reconstructing past bleaching events. The first record (PAN-98) comes from a coral head that had undergone bleaching and died shortly after the large-scale bleaching events on Pandora Reef in 1998. A second core (PAN-02) was collected from a living coral within 10m of PAN-98 in 2002. Sr/Ca ratios in both cores tracked even the smallest details of an {\it in situ} SST record. The increase in SST that occurred three to four weeks prior to bleaching was faithfully recorded by a similar decrease in the Sr/Ca ratio in PAN-98, indicating that calcification continued despite the high SST of 30-31$\deg$C. The $\delta$$^{13}$C values decreased by about 5$\permil$, one week after the SST increase, and remained at this value for about 4 weeks until the coral died. In 1994 and 1995, there are decreases in the $\delta$$^{13}$C values of 3$\permil$. In 1994, a flood plume from the Burdekin River reached Pandora Reef and bleaching was reported. In 1995 we note a 4-5 week period of elevated SST based on the Sr/Ca results, which may have been sufficient to cause stress or bleaching of the coral. No clear decreases in $\delta$$^{13}$C values associated with any bleaching event was evident in the PAN-02 record, however there is a clear growth hiatus that lasted several months during the 1998 bleaching event. $\delta$$^{18}$O results in both records show many of the same details as the Sr/Ca and SST record, suggesting temperature changes as the dominant control. However, during flooding events (1996, 1997, and 1998), the $\delta$$^{18}$O values were decreased by increased freshwater input to the reef. The associated salinity changes were determined by subtracting the temperature component from the $\delta$$^{18}$O signal using Sr/Ca ratios and compared with the weekly average flow records from the Burdekin River and a Ba/Ca record (McCulloch et al. 2003) of sediment flux to the reef. Similar results were obtained in a third record from the Kepple Islands which included one of the largest floods of the century and a bleaching event in 1991.
DE: 1635 Oceans (4203)
DE: 4215 Climate and interannual variability (3309)
DE: 4825 Geochemistry
DE: 4870 Stable isotopes
SC: Biogeosciences [B]
MN: 2003 Fall Meeting