Supplemental Information s14
Supplemental Information
Palaeoecological evidence of a historical collapse of corals on the inshore Great Barrier Reef following European settlement
George Roff*, Tara R Clark, Claire Reymond, Jian-xin Zhao, Yuexing Feng, Laurence J McCook, Terence J Done, John M Pandolfi
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Contents:
Figure S1
Supporting Methods
Tables S1, S2
1. Supporting Figures
Figure S1. Representative photographs of life and death assemblages among sites at Pelorus Reef. Site A and C show death assemblages dominated by thick arborescent Acropora framework, while Site B shows high cover of foliaceous Pavona cactus and the transition of Pavona to Acropora assemblages through asexual fragmentation of Pavona.
2. Supporting Methods
Sample selection and U-series dating
Approximately 1g of carefully cleaned material from each sub-sample was used for U-series dating using a VG Sector-54 WARP-filtered high-abundance-sensitivity thermal ionisation mass spectrometer (TIMS) at the Radiogenic Isotope Facility (RIF), the University of Queensland, following the analytical protocols described in detail in Zhao et al. [1] and Yu et al. [2]. Each sub-sample was spiked with a 229Th-233U-236U mixed tracer and dissolved in double-distilled nitric acid. After complete digestion, a few drops of H2O2 were added, and the sample beakers tightly closed and placed on a hotplate at 90°C overnight to decompose any trace amounts of organic matter and to ensure complete tracer-sample mixing. After standard Fe hydroxide co-precipitation to pre-concentrate U and Th, the precipitates were redissolved in 0.2 ml 7 M double-distilled HNO3 and purified using standard ion-exchange methods [3]. U and Th fractions were loaded separately onto carefully examined ultra-pure zone-refined rhenium single filaments, sandwiched into two graphite layers (see Zhao et al. [4] for filament purity and detailed procedures for dating very young carbonates). Th isotopic ratios 232Th/229Th and 229Th/230Th were measured manually, whereas 233U/235U, 234U/235U and 233U/236U, automatically on the Daly ion counter of the TIMS in peak-jumping mode. The U isotopic ratios were corrected for mass fractionation by normalising to 233U/236U = 0.96341 in the spike [2]. These ratios, together with the sample and mixed tracer weights, were used to calculate 230Th/238U and 234U/238U activity ratios as well as U and Th concentrations. U-series ages were then calculated using Isoplot/Ex 3.0 Program [5]. Decay constants used are those reported by Cheng et al. [6]. The operating abundance sensitivity of the TIMS measured at one mass distance in the mass range of U and Th is less than 70 ppb, suggesting the tailing effect on 230Th from 232Th is negligible. During routine U-series isotope measurements on the TIMS in our laboratory, an international uraninite standard HU-1 was frequently measured to monitor the instrument conditions and analytical reproducibility. Long-term repeated measurements of 234U/238U atomic ratios in the HU-1 standard yielded a mean value of 54.888 ± 0.050 (2σ, N = 26), which is identical to the analytical uncertainty obtained by Cheng et al. [6].
For TIMS measurements of samples with very young ages, one of the main contributors to age error is the procedural blank from sample preparation and the colloidal graphite used to load samples onto filaments. The procedural 230Th blank which is mainly derived from the chemistry, was determined to be 1.52 ± 0.31 ×10-9 nmol (n = 12); contributing 0.35 to 0.96 yr to the 230Th ages of the samples in this study, varying according to the weights and U levels of individual samples. Procedural blanks for 238U were 0.5-2.0 ×10-5 nmol, which is negligible for coral samples containing ~3 ppm U. Procedural blanks for 232Th were higher and more variable, with a mean of 6.25 ± 3.04 ×10-9 nmol (N=12). We attribute this relatively high 232Th blank to the new colloidal graphite used in the lab as recent multi-collector ICP-MS analysis of similar procedural blanks show 232Th to be 5-10 times lower. 230Th blank in the graphite is negligible as monitoring of the blank graphite filaments shows that the ion counts on mass 230 are consistently low; indistinguishable form the dark noise of the TIMS, which is ~0.2 cps [4]. 230Th, 232Th and 238U blank contributions have been extracted when calculating 230Th/232Th and 230Th/238U ratios and corresponding 230Th ages of the samples.
In addition to TIMS measurements, a large number of samples were also dated more recently by our newly installed Nu Plasma multi-collector inductively coupled plasma mass spectrometer (MC-ICP-MS). Compared with TIMS, the chemical separation procedure for MC-ICP-MS is simplified, and only 250-500 mg material and a 229Th-233U mixed tracer were used. After chemical separation, the Th fraction was mixed with a small percentage of U (sufficient to achieve 3-4 volts of 238U signal) and the mixture was diluted to 3-ml in 2% HNO3. The U-Th mixed solution was then injected into the MC-ICP-MS through a DSN-100 desolvation system with an uptake rate of around 0.12 ml per minute. U-Th isotopic ratio measurement was performed on the Nu Plasma MC-ICP-MS following the analytical protocol described by Hellstrom [7], with minor modifications to the detector configuration (e.g. the 235U on IC0 sequence of Hellstrom [7] was not used in our protocol, instead we used 233U for IC0/Faraday gain calibration). The natural 238U/235U value of 137.88 was used for mass fractionation correction for both U and Th isotopic ratio measurements. The second electron multiplier IC2 was filtered with a fitted retardation lens to increase the abundance sensitivity by 10 folds to minimize large tailing of the 232Th signal into the 230Th peak, which is potentially very serious for impure carbonates with high 232Th/230Th. The working abundance sensitivity on IC2 is <500 ppb at one mass distance from the large peak in the U-Th mass range, thus tailing from 232Th to mass 230 is typically <5% of 230Th peak even for the dirtiest samples with 232Th/230Th >100,000. For samples with 232Th/230Th <10,000, this tailing is typically <1%. The tailing contribution was extracted from 230Th signal by simultaneous measurements of baselines at masses 230.5 and 229.5. The geometric mean of the two half-mass-unit baseline measurements was used in the online acquisition program. The working sensitivity of the instrument is about 0.5-0.6 volt per ppb for U and 0.4-0.5 volt per ppb of Th. Each sample took about 20 minutes to measure. In between samples, a vigorous 15-minute cleaning procedure was undertaken and all isotopes were monitored and raw counts measured on their respective detectors to ensure no carry-over memories from previous samples. Measurements of samples, standards, and carryover memories were performed fully automatically using a CETAC ASX-110 autosampler. A long-term monitoring of the carryover memories over 15 months shows that 230Th memory is consistently less than 0.1 counts per second, which is negligible for most samples. The memories for all other isotopes are also negligible. To ensure the reproducibility of the data, we spiked our CRM-112A (or called STM-960) U metal standard solution with the same mixed tracer and carried out column chemistry together with the samples. Repeated measurements of the U metal standard gave an age of 74.8±0.1 years (~1936 AD), within error of the result reported by McCulloch and Mortimer [8]. Similarly we spiked a 10-ppb CRM-112A solution and measured it directly without column chemistry (with 230Th signal only a few counts per second). The results gave dates identical within error of the above age. The procedural blanks on our MC-ICP-MS are considerably smaller, with mean measured 238U, 232Th and 230Th blanks of 1.6x10-5, 1.7x10-5 and 2.5x10-10 nmol (or 3.7 pg, 3.8 pg, 0.06 fg), respectively. 230Th blank extractions for our samples of ~0.5 g in size correspond to only about 0.2 year.
Initial 230Th correction
As the samples in this study are extremely young, with very low radiogenic 230Th, the proportion of the initial 230Th component is much higher, making a significant contribution to the measured 230Th/238U ratios, which must be corrected for in order to work out a reliable 230Th age of the sample. Three different 230Th/232Th0 ratios were applied for the initial 230Th correction, including a mean value of 0.65±25% calculated from ICP-MS Th/U measurements of ~40 local sediments from the Burdekin River catchment area, which is confirmed by direct measurements of uncleaned sediment-rich coral samples of known ages (Clark et al., in prep), a mean value of 1.0±25% calculated from 14 direct measurements of 230Th/232Th0 in live corals of known ages in the Palm Islands region, and a model value calculated for each sample using a two-component mixing equation (see Supplementary Table 4 footnote) that accounts for the proportion of a terrestrially derived 230Th component (represented by the sediments from the Burdekin River catchment area with 230Th/232Th0 = 0.65±25%) incorporated into the skeleton post-mortem, and a hydrogenous 230Th component incorporated into the skeleton during growth (represented by measured 230Th0 in live corals in the region with 230Th/232Th0 = 1.0±25%). The corrected 230Th ages based on the above two-component mixing model are listed in Table S2. Despite vigorous cleaning applied to all the dated samples, a large number of samples still contain high 232Th indicative of high-level contamination from terrestrial sediments, resulting in low measured 230Th/232Th activity ratios in these samples. As a result, initial 230Th corrections for these high-Th samples have a significant impact on the corrected 230Th ages and their age uncertainties, with error magnifications up to >10 times if compared with the uncorrected 230Th age uncertainties (typically ~1% or less), which are only related to analytical errors.
3. Supporting Tables
Table S1. Benthic cover across sites at Pelorus Island, central Great Barrier Reef (percent cover ± SE)
Benthic category / Site A / Site B / Site CLiving coral / 3.5 ± 0.6 / 36.3 ± 7.7 / 4.4 ± 0.8
Dead coral / 77.2 ± 3.7 / 31.3 ± 5.4 / 59.4 ± 4.4
Soft coral / 10.6 ± 2 / 12.5 ± 1.2 / 26.8 ± 7.4
Algae / 2.0 ± 1.8 / 5.4 ± 4.4 / 0.2 ± 0.1
Substrate / 6.5 ± 3.9 / 14.4 ± 7.9 / 9.2 ± 3.1
Table S2. U-series ages of corals obtained from death assemblages and sediment cores. Coral samples obtained from historical assemblages and cores from sites at Pelorus Reef, central Great Barrier Reef.
1
Sample Name / Site / Genera / Assemblage / Method / U (ppm) / 232Th (ppb) / (230Th/ 232Th) / (230Th/238U) / (234U/ 238U) / Uncorr, 230Th age (a) / Corr. 230Th age (a) / Date of chemistry / corr. 230Th Age (AD) /PN1AE / A / Acropora / Core / TIMS / 2.8159 ± 0.0034 / 8.484 ± 0.036 / 4.67 ± 0.10 / 0.004637 ± 0.000086 / 1.1461 ± 0.0013 / 441.3 ± 8.2 / 377 ± 15 / 2009.9 / 1633 ± 15 /
PN3AA / A / Acropora / Core / TIMS / 3.6752 ± 0.0048 / 1.318 ± 0.003 / 44.66 ± 0.46 / 0.005278 ± 0.000050 / 1.1465 ± 0.0017 / 502.2 ± 4.8 / 492.8 ± 5.2 / 2009.9 / 1517.1 ± 5.2 /
PN3iAB / A / Acropora / Core / TIMS / 3.3090 ± 0.0040 / 5.273 ± 0.030 / 9.56 ± 0.14 / 0.005020 ± 0.000052 / 1.1460 ± 0.0015 / 477.8 ± 4.9 / 442.9 ± 8.7 / 2009.9 / 1567.1 ± 8.7 /
PN4AA / A / Acropora / Core / TIMS / 3.7792 ± 0.0047 / 2.378 ± 0.010 / 25.97 ± 0.29 / 0.005387 ± 0.000045 / 1.1478 ± 0.0011 / 512.1 ± 4.3 / 497.2 ± 5.3 / 2009.9 / 1512.7 ± 5.3 /
Pel.A.AC / B / Acropora / Core / TIMS / 3.2717 ± 0.0026 / 3.120 ± 0.021 / 1.27 ± 0.028 / 0.000398 ± 0.000008 / 1.1491 ± 0.0014 / 37.7 ± 0.7 / 15.3 ± 4.5 / 2008.2 / 1992.9 ± 4.5 /
Pel.A.AP / B / Acropora / Core / TIMS / 3.2409 ± 0.0037 / 0.617 ± 0.002 / 89.99 ± 0.83 / 0.005650 ± 0.000040 / 1.1460 ± 0.0015 / 537.9 ± 3.8 / 532.0 ± 4.1 / 2008.2 / 1476.2 ± 4.1 /
Pel.B.AC / B / Acropora / Core / TIMS / 3.3313 ± 0.0027 / 4.123 ± 0.035 / 2.58 ± 0.08 / 0.001053 ± 0.000024 / 1.1462 ± 0.0013 / 100.1 ± 2.3 / 72.0 ± 6.11 / 2008.2 / 1936.2 ± 6.1 /
Pel.B.AR / B / Acropora / Core / TIMS / 3.2218 ± 0.0032 / 3.361 ± 0.022 / 26.56 ± 0.27 / 0.009131 ± 0.000042 / 1.1461 ± 0.0012 / 870.7 ± 4.1 / 847.3 ± 6.4 / 2008.2 / 1160.9 ± 6.4 /
PB1AP / B / Acropora / Core / TIMS / 3.6231 ± 0.0051 / 3.865 ± 0.022 / 12.24 ± 0.20 / 0.004304 ± 0.000055 / 1.1437 ± 0.0013 / 410.4 ± 5.3 / 386.3 ± 7.2 / 2009.9 / 1623.6 ± 7.2 /
PB2ANP / B / Acropora / Core / TIMS / 3.5122 ± 0.0050 / 3.553 ± 0.011 / 3.10 ± 0.16 / 0.001033 ± 0.000054 / 1.1457 ± 0.0013 / 98.2 ± 5.1 / 74.8 ± 7.0 / 2009.9 / 1935.1 ± 7.0 /
PB3ATQ / B / Acropora / Core / TIMS / 3.6610 ± 0.0046 / 1.578 ± 0.003 / 5.96 ± 0.09 / 0.000846 ± 0.000014 / 1.1473 ± 0.0013 / 80.3 ± 1.3 / 68.9 ± 2.6 / 2009.9 / 1941.0 ± 2.6 /
PelPB1 / B / Acropora / Core / MC / 2.9626 ± 0.0029 / 6.994 ± 0.007 / 24.01 ± 0.06 / 0.001853 ± 0.000020 / 1.1447 ± 0.0008 / 1796.3 ± 5.0 / 1744.4 ±11.3 / 2010.5 / 266.0 ± 11.5 /
PelPB2 / B / Acropora / Core / MC / 3.0659 ± 0.0037 / 7.204 ± 0.007 / 23.67 ± 0.07 / 0.001193 ± 0.000024 / 1.1445 ± 0.0008 / 1764.6 ± 6.0 / 1713.1 ±11.8 / 2010.5 / 297.4 ± 11.9 /
PS1AC / C / Acropora / Core / TIMS / 3.9037 ± 0.0057 / 5.005 ± 0.023 / 29.53 ± 0.24 / 0.012479 ± 0.000063 / 1.1461 ± 0.0014 / 1191.5 ± 6.2 / 1164.2 ± 8.4 / 2009.9 / 845.8 ± 8.4 /
PS3AA / C / Acropora / Core / TIMS / 3.8586 ± 0.0051 / 3.628 ± 0.016 / 12.87 ± 0.28 / 0.003987 ± 0.000075 / 1.1460 ± 0.0016 / 379.4 ± 7.2 / 358.1 ± 8.4 / 2009.9 / 1651.8 ± 8.4 /
Sample Name / Site / Genera / Assemblage / Method / U (ppm) / 232Th (ppb) / (230Th/ 232Th) / (230Th/238U) / (234U/ 238U) / Uncorr, 230Th age (a) / Corr. 230Th age (a) / Date of chemistry / corr. 230Th Age (AD)