Data for U-Th dated coral from reef matrix cores and death assemblage from Mazie Bay, North Keppel Island, Southern Great Barrier Reef (NERP TE 1.3, UQ)

This dataset provides the results of U-Th dating of coral samples obtained from reef matrix percussion cores and death assemblages from Mazie Bay, North Keppel Island, Southern Great Barrier Reef. Data is presented for 117 coral samples ranging in age from 6900 years before present (yr.BP) to modern. 

A U-Th dating approach to understanding past coral reef dynamics and geomorphological constraints on future reef growth potential; Mazie Bay, Southern Great Barrier Reef - Reconstructing coral reef histories at multiple temporal scales provides a window of understanding into their response to changing environments. Using high precision Uranium-Thorium dating of corals from various reef zones we have reconstructed a complete growth history of Mazie Bay reef (North Keppel Island) in the Southern Great Barrier Reef. Our results show that Mazie Bay reef has been dominated by fast-growing branching Acropora spp. corals for the past 7000 years, and that during the mid-Holocene coral growth rapidly filled available accommodation space. The modern veneer of living coral is subject to periods of disturbance and recovery driven by various climatic influences including cyclones, floods and bleaching. Although loss of coral at Mazie Bay in the past was followed by relatively rapid recovery (~15 years), continued or chronic decimation of adult Acropora spp. coral populations could be catastrophic for this region due to the lack of connectivity to reefs outside of the Keppel Islands region.

Methods:
This tables contain data for U-Th dating of coral samples collected in February 2012 and June 2013 from a fringing reef in Mazie Bay, North Keppel Island, Southern Great Barrier Reef, including reef flat and slope matrix percussion cores and death assemblage (coral rubble at the sea/sediment interface next to the reef slope). For reef matrix percussion cores, ten-centimetre diameter aluminium cores were manually percussed into the reef matrix in November 2012 and November 2013. Four cores (two proximal to the beach and two distal) were taken from the emergent reef flat. Nine reef slope matrix cores from three discrete location across ~400 m of the modern reef slope were manually percussed on SCUBA at a depth of ~3 metres below lowest astronomical tide (mLAT) following methods described by Roff et al. (2015). Coral fragments were selected for U-Th dating from the base of each core where coral material was present, then up core where suitable skeletal material was available (i.e. from corals with enough unaltered aragonite for U-Th dating purposes). As compaction of the reef matrix occurs during percussion coring internal and external measurements of the cores were taken in the field prior to extracting the cores. Compaction of the cores was calculated as; core length = (Total Length of the core (initial) - External) and percentage compaction of the reef material inside the cores [(Internal-External)/((Total Length (initial) -External) – (Internal-External))*100]. Coral depths are based on linear uncompacted core length and reported relative to depth metres lowest astronomical tide (mLAT) based on 2012/2013 tide data from Maritime Safety Queensland for Rosslyn Bay (Station-024011A). Death assemblages were collected at three sites, adjacent to the reef slope cores, along four consecutive 20 m transects running parallel to the reef front at depths of ~- 3 to - 6 mLAT. Coral rubble was excavated from the benthos within 5 metre intervals of each transect and placed into calico bags (40 cm x 20 cm). Samples of death assemblages for U-Th dating were selected randomly from the calico bags, with sub-samples for U-Th dating being taken as close to the top (most recent) growth section of the corals so as to represent the timing of mortality.

U-Th dating
All coral samples were prepared and U-Th dated on a Nu Plasma Multi-Collector Inductively Coupled Plasma Mass Spectrometer (MC ICP-MS) at the Radiogenic Isotope Facility, The University of Queensland following methods described in detail in Clark et al.  (2014) and Leonard et al. (2016). U-Th data was calculated using Isoplot 3.75 (Ludwig, 2012). Activity ratios were calculated from atomic ratios using decay constants; lambda 238 = 1.55125 × 10-10 yr-1 (Jaffey et al., 1971), lambda 234 = (2.8262 ± 0.0057) × 10-6 yr-1,  lambda 230 = (9.158 ± 0.028) × 10-6 yr-1 (Cheng et al., 2000) and corrected using the two component mixing equation of Clark et al. (2014) with a detrital 230Th/232Th value of 0.62 based on Keppel Islands specific isochron data from Leonard et al. (2016). For ease of comparison to previously reported data from reef matrix cores we report all dates prior to 1950 as yr.BP (years before present; where present is 1950), but consider all ages &gt;1950 from slope core tops and death assemblage as “modern” and report as AD. 

Format:
The dataset consists of three excel spreadsheets; one for samples U-Th dated to older than 1950 AD (calculated as years before present where present is 1950), and the second is calculated as years AD (post 1950 samples). The third spreadsheet provides lat/long of sample sites.

Data Dictionary:
•   Labcode – sample specific labcode in Radiogenic Isotope Facility, The University of Queensland
•   Sample Name – Sample field name – NK = North Keppel; S = site number; AB/CD = core section followed by uncompacted core depth where samples was taken. 
•   Genus – Genus of coral
•   Date of Chemistry – Date column chemistry was completed
•   U (ppm) – Uranium concentration in parts per million
•   232Th (ppb) – Thorium 232 concentration in parts per billion 
•   (230Th/232Th); (230Th/238U); (234U/ 238U) - activity ratios calculated from atomic ratios using decay constants; lambda 238 = 1.55125 × 10-10 yr-1 (Jaffey et al., 1971), lambda 234 = (2.8262 ± 0.0057) × 10-6 yr-1, lambda 230 = (9.158 ± 0.028) × 10-6 yr-1 (Cheng et al., 2000).
•   uncorr. 230Th Age (ka) - Uncorrected 230Th age was calculated using Isoplot/EX 3.75 program (Ludwig, 2012). All values have been corrected for laboratory procedural blanks and all errors are 2 sigma.
•   corr. 230Th Age (ka) - 230Th ages were corrected using the two-component correction method of Clark et al. (2014) using 230Th/232Thhyd and 230Th/232Thdet activity ratios of 1.08 ± 0.23 and 0.62 ± 0.14, respectively.
•   delta 234U - = [(234U/238U) - 1] × 1000
•   Depth (mLAT) – Uncompacted core sample depth relative to metres lowest astronomical tide 

References:
Cheng, H., Edwards, R.L., Hoff, J., Gallup, C.D., Richards, D.A. and Asmerom, Y., 2000. The half-lives of uranium-234 and thorium-230. Chemical Geology, 169(1): 17-33.

Clark, T.R., Roff, G., Zhao, J.-x., Feng, Y.-x., Done, T.J. and Pandolfi, J.M., 2014. Testing the precision and accuracy of the U–Th chronometer for dating coral mortality events in the last 100 years. Quaternary Geochronology, 23(0): 35-45.

Jaffey, A., Flynn, K., Glendenin, L., Bentley, W.t. and Essling, A., 1971. Precision measurement of half-lives and specific activities of U 235 and U 238. Physical review C, 4(5): 1889.

Leonard, N.D., Zhao, J.-x., Welsh, K.J., Feng, Y.-x., Smithers, S.G., Pandolfi, J.M. and Clark, T.R., 2016. Holocene sea level instability in the southern Great Barrier Reef, Australia: high-precision U–Th dating of fossil microatolls. Coral Reefs, 35(2): 625-639.

Ludwig, K., 2012. Isoplot/Ex Version 3.75, a Geochronological Toolkit for Microsoft Excel. Berkeley Geochronology Center. Special Publications. Berkeley Geochronology Center, Berkeley, CA.

Roff, G., Zhao, J.-x. and Pandolfi, J.M., 2015. Rapid accretion of inshore reef slopes from the central Great Barrier Reef during the late Holocene. Geology, 43(4): 343.

Data Location:

This dataset is filed in the eAtlas enduring data repository at: data\NERP-TE\1.3_Coral_Cores

Organization: Australian Ocean Data Network