230Th age results for 104 individual dead coral samples obtained from Keswick, Round Top, Scawfell and Derwent Island in 2006 (MTSRF 1.1.4, NERP TE 1.3, UQ)

This dataset consists of the raw data used to calculate 230Th ages for 104 individual dead coral samples obtained from Keswick, Round Top, Scawfell and Derwent Island (Mackay region, Great Barrier Reef) in 2006. Samples were measured using both thermal ionisation mass spectrometry (TIMS) and multi collector inductively coupled plasma mass spectrometry (MC ICPMS) at the Radiogenic Isotope Facility, The University of Queensland.

In the Mackay region of the south-central GBR, surveys of coral community composition over the past 19 years have revealed a loss in Acropora cover at several inshore reefs. The aim of this study was to reconstruct historical coral community dynamics at Round Top, Keswick, Scawfell and Derwent Island (20°34'S, 149°08'E) by dating dead corals collected from the death assemblage using high-precision U-Th methods developed specifically for high-detrital inshore environments. This data will help to understand when and why a decline in ecologically important Acropora corals occurred.



This dataset is currently under embargo.


Methods:

In this study, dead coral assemblages or ‘death assemblage’ consisting of loose dead coral rubble only were collected in March 2006 at two sites from Round Top, Keswick, Scawfell and Derwent Island (Fig. 1; Table 1) at a depth of 1-5 m on SCUBA. At each site, three grab samples were collected along each of two 25 m transects (total of six) where coral rubble was present. Approximately one litre of coral rubble was collected in 5 mm mesh bags, excavated from the same point to a depth of 10 cm and within a 2 m radius. Coral rubble was then soaked overnight in 10% bleach, rinsed and then sun dried.

A total of 104 individual dead coral rubble colonies were randomly selected from the mesh bags for U-Th dating (see raw data table). Where a colony was of a branching growth form, material for dating was selected from the very centre of the colony and within 6 cm from the branch tip to represent the area of most recent growth. Replicate samples (between two to four replicates) were also taken from 15 colonies for quality control and for establishing 230Th/232Th versus 238U/232Th isochrons for detrital correction; this resulted in a total of 130 samples for 230Th age dating.

Twenty-five dead coral rubble samples were prepared for U-Th dating prior to 2009 and were cut into small fragments using a diamond trim saw and stainless steel bone cutters, ultrasonically cleaned in Milli-Q water and then dried. These samples were dated using a VG-Sector-54 WARP-filtered high-abundance-sensitivity thermal ionization mass spectrometer (TIMS) at the Radiogenic Isotope Facility, The University of Queensland.

A further 79 dead coral rubble samples were prepared from 2009-onwards and were cut into small fragments using a diamond trim saw, dried, and then crushed in to ~1 mm grain size using an agate mortar and pestle. Crushed material was then soaked overnight in pre-cleaned glass beakers containing 10% AR grade H2O2. Samples were then repeatedly sonicated in Milli-Q water until the water remained clear, the water decanted and the sample dried on a hot plate overnight at 40˚C. Prior to U-Th chemistry procedures, samples were carefully vetted using a binocular microscope and any contaminants (shell, coralline algae etc) or altered material was removed using fine forceps. Samples were dated using a Nu Plasma I Multi Collector Inductively Coupled Plasma Mass Spectrometer (MC-ICP-MS) at the Radiogenic Isotope Facility, The University of Queensland. Following blank, instrumental drift and mass bias fractionation correction, all 230Th age data were calculated in Excel with the Isoplot Ex 3.75 add-in. All data were corrected for locally derived extraneous sources of detrital and hydrogenous 230Th using a two-component mixing model. 

Further information can be found in this publication:
Clark TR, Leonard ND, Jupiter SD, Roff G, Henley B, Zhao J, Feng Y, Quaintance JK McCulloch M, Pandolfi JM (accepted) Past Acropora mortality on the Great Barrier Reef linked to climate variability and anthropogenic impacts. Nature Communications Earth and Environment. 

Limitations of the data:

This U-Th data table lists all samples run on the TIMS and MCICPMS at The University of Queensland, including repeated samples as well as those deemed unreliable to be used in interpretation (i.e. initial 234U/238U was outside the range of modern seawater values, uranium concentrations were outside the range of modern corals). 
Refer to Clark TR, Leonard ND, Jupiter SD, Roff G, Henley B, Zhao J, Feng Y, Quaintance JK McCulloch M, Pandolfi JM (accepted) Past Acropora mortality on the Great Barrier Reef linked to climate variability and anthropogenic impacts. Nature Communications Earth and Environment for a list of the samples that were considered reliable. 

The GPS reading failed for two Derwent Island transects (DWAT2 and DWBT1), so no coordinates were recorded for them in Table 1. In the derived spatial dataset (GeoPackage/CSV), samples on these two transects were assigned the coordinates of the neighbouring transect at the same site (DWAT2 from DWAT1, DWBT1 from DWBT2) so that they could be displayed on the map. These approximate positions are flagged in the coord_source field as “approx_sibling”; all other samples carry their own transect coordinate (“transect”).

Format of the data:

The sampling locations and U-Th age data are presented as a table in the following .xls files: ‘Table 1. Sampling Locations.xls’ and ‘Table 2. Raw UTh data.xls’.

Data dictionary:

Table 1. Sampling locations.xls:

Reef – Samples were collected from either Keswick, Round Top, Scawfell or Derwent Island. Refer to map of study location in the Mackay (Cumberland Islands) region, south-central GBR.

Site – Two sites were sampled at each reef – Site A and Site B.

Transect – Samples were collected along two 25 m transects [Transect 1 (T1) and Transect 2 (T2)] laid parallel to the reef flat at each site. Transect codes combine the site code with the transect number, e.g. RTAT1 = Round Top Site A, Transect 1. Site codes: RTA – Round Top Site A; RTB – Round Top Site B; KIA – Keswick Island Site A; KIB – Keswick Island Site B; SCA – Scawfell Island Site A; SCB – Scawfell Island Site B; DWA – Derwent Island Site A; DWB – Derwent Island Site B. Note that the GPS reading failed for two Derwent Island transects (DWAT2 and DWBT1), so Table 1 records no coordinates for them; in the derived spatial dataset these samples were mapped using the coordinates of the neighbouring transect at the same site (see eAtlas Processing).

Date – Date of sampling of death assemblage on SCUBA.

Depth_m – Depth samples were collected on SCUBA metres (m)

Lat – Latitude (decimal degrees)

Long – Longitude (decimal degrees)

Position – Location of where GPS co-ordinates (Lat/Long) were taken at either the start and/or end of each transect. For in situ colonies sampled from St Bees Island, GPS co-ordinates were taken at the location of the colonies.

Description – Further information on sampling location. 

Table 2. Raw UTh data.xls:

*All age data was calculated using the Isoplot/Ex 3.00 Program (Ludwig 2012)
Sample number – sample number out of 104 individual coral samples. Samples were occasionally repeated to ensure reproducibility and to construct 230Th/238U isochrons. For example, three subsamples were taken from sample one.

Reef – Samples were collected from either Keswick, Round Top, Scawfell or Derwent Island. Refer to map of study location in the Mackay (Cumberland Islands) region, south-central GBR.

Site – Two sites were sampled at each reef – Site A and Site B.

Transect – Samples were collected along two 25 m transects [Transect 1 (T1) and Transect 2 (T2)] laid parallel to the reef flat at each site.

Laboratory code – Name given to each individual sample analysed in the Radiogenic Isotope Laboratory, UQ. Note that samples were analysed over a period of 20 years by various geochemists, hence the variation in the naming system.

Original date &amp; time – Date &amp; time each sample was analysed as recorded by the MC ICP MS. Only the date was recorded for samples measured by the TIMS.

Date &amp; time – Standardised date &amp; time of measurement of each sample measured by both instruments. The time of measurement for samples measured on the TIMS is reported as 00:00.

Instrument - MC ICP MS – Multi Collector Inductively Coupled Plasma Mass Spectrometer, TIMS – Thermal Ionisation Mass Spectrometer

Sample weight (g) – Weight of coral material sampled from an individual colony used in U-Th chemistry procedures.

Spike weight (g) – Weight of 229Th–233U–236U mixed tracer added to sample prior to U-Th chemistry procedures.

Measured_238d233, Measured_238d234, Measured_232d229, Measured 229d230 – measured ratio of 238U/233U, 238U/234U, 232Th/229Th, 229Th/230Th isotopes on the instrument after mean laboratory blank extraction and correction for mass fractionation.

SEpercent_238d233, SEpercent_238d234, SEpercent_232d229, SEpercent_229d230 – Percentage standard error of 238U/233U, 238U/234U, 232Th/229Th, 229Th/230Th isotopes measured on the instrument detectors while the sample is being atomised and ionised.

Th230_nm_g, Th232_nm_g – concentration of 230Th and 232Th isotopes in nanomoles per gram (nm g-1) of sample.
U_ppm – uranium (U) concentration in parts per million (ppm)

232Th_ppb – 232Th concentration in parts per billion (ppb)

230Thd232Th_activity, 232Thd238U_activity, 230Thd238U_activity, 234Ud238U_activity – Measured isotopic ratios, sample and mixed tracer weights as well as U and Th concentrations were used to calculate the 230Th/232Th, 232Th/238U, 230Th/238U and 234U/238U activity ratios. Activity ratios were calculated from atomic ratios using the decay constants of Jaffey et al. 1971 and Cheng et al. 2000.

Uncorrected 230Th age ka - Uncorrected 230Th age thousands of years (ka)

Uncorrected_initial_234Ud238U_activity – Uncorrected 234U/238U activity ratio representing the isotopic state of the sample at the moment of formation.

Detrital_232Thd238U_activity – The detrital fraction which represents background terrigenous material calculated using the elemental abundances of U (ppm) and Th (ppb).

Corrected_230Thd238U_activity – 230Th/238U ratio corrected for a detrital component, using 232Th as an index isotope.  

Corrected_234Ud238U_activity – 234U/238U ratio corrected for a detrital component, using 232Th as an index isotope.  

Corrected_230Th_Age_ka - 230Th age thousands of years (ka) corrected for hydrogenous and terrestrially derived 230Th0 using a sample specific non-radiogenic (230Th/232Th) value using the following equation:
(230Th/232Th)_mix =
((232Th_live/232Th_dead) * (230Th/232Th)_live) +
(((232Th_dead - 232Th_live)/232Th_dead) * (230Th/232Th)_sed)
where 232Thdead is the measured 232Th value (ppb) in the individual dead coral sample. 232Thlive is the mean 232Th value (ppb) measured in live Porites coral samples collected from the Palm Islands region (no live corals were collected from the Mackay region) determined to be 0.95 ppb (N=12). 230Th/232Thlive represents or approximates the isotopic composition of the hydrogenous component in the live coral skeleton during growth with an activity value of 1.08 ± 20%. 230Th/232Thsed represents the detrital component incorporated into the coral skeleton post-mortem with a mean activity value of 0.69 ± 20% based on y-intercept values of 230Th/232Th versus 238U/232Th isochron.
Corrected_initial_234Ud238U_activity – Determined using the measured activity ratios 230Th/238U and 234U/238U through the U-series age equation. Values deviating away from an average initial activity ratio of ~1.145 may suggest open system behaviour.

Date_of_chemistry – Year/month as a decimal year when samples were spiked with a 229Th–233U–236U mixed tracer in preparation for U-Th chemistry procedures.

Corrected_Age_CE – Age of the sample in years Common Era (CE) calculated as follows: Date of chemistry – [Corrected 230Th age (ka) × 1000]

Refer to the following publications for further information:
Clark TR, Zhao J, Feng Y, Done TJ, Jupiter S, Lough J, Pandolfi JM (2012) Spatial variability of initial 230Th/232Th in modern Porites from the inshore region of the Great Barrier Reef. Geochimica et Cosmochimica Acta, 78, 99-118.
Clark TR, Leonard ND, Jupiter SD, Roff G, Henley B, Zhao J, Feng Y, Quaintance JK McCulloch M, Pandolfi JM (accepted) Past Acropora mortality on the Great Barrier Reef linked to climate variability and anthropogenic impacts. Nature Communications Earth and Environment.
Ludwig KR (2012) User's Manual for Isoplot 3.75: A Geochronological Toolkit for Microsoft Excel. Berkeley Geochonological Centre, Berkeley, United States of America.

eAtlas Processing:

Excel was used to load the XLS files and saved as CSV files. These were processed using a Python script to join the sampling locations, Table 1, with the coral dating data, Table 2, to produce a GeoPackage suitable for visualisation in GeoServer. The samples on the two Derwent Island transects whose GPS reading failed (DWAT2 and DWBT1) were assigned the coordinates of the neighbouring transect at the same site so they could be mapped; the underlying data values were not altered, and these approximate positions are flagged in the coord_source field. No further modifications to the data were made, just format conversion.

References:

Clark TR, Zhao J, Feng Y, Done TJ, Jupiter S, Lough J, Pandolfi JM (2012) Spatial variability of initial 230Th/232Th in modern Porites from the inshore region of the Great Barrier Reef. Geochimica et Cosmochimica Acta, 78, 99-118

Clark TR, Leonard ND, Jupiter SD, Roff G, Henley B, Zhao J, Feng Y, Quaintance JK McCulloch M, Pandolfi JM (accepted) Past Acropora mortality on the Great Barrier Reef linked to climate variability and anthropogenic impacts. Nature Communications Earth and Environment.

A. H. Jaffey, K. F. Flynn, L. E. Glendenin, W. C. Bentley, A. M. Essling, Precision measurement of half-lives and specific activities of U-235 and U-238. Physical Review C 4, 1889-+ (1971).

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

Location of the data:
This dataset is filed in the eAtlas enduring data repository at: data\custodian\2011-2014-NERP-TE\1.3_Coral-cores\in-2026-06_out-2026-06_230Th

Tags: Marine
Organization: Australian Ocean Data Network