Methodology:

U-Th dating and data processing

More than 1800 deep-sea corals (Desmophyllum, Caryophyllia, Flabellum, Balanophyllia and Gardineria) were been reconnaissance dated with two different methods: laser-ablation U-Th dating (Spooner et al., 2015) and 14C dating (Burke et al., 2010).

Isotope-dilution U-Th dating was carried out following the established protocols in the Bristol Isotope Group at the University of Bristol (Chen et al., 2015). Briefly ferromanganese and organic coatings or re-mineralized parts of the coral samples were carefully removed with a Dremel tool before ~0.2 gram samples were cut for chemical cleaning (Cheng et al., 2000). Cleaned samples were weighed and dissolved in Teflon beakers using 2ml 7M distilled HNO3. A 236U-229Th mixed spike (Burke and Robinson, 2012; Chen et al., 2016; Chen et al., 2015; Robinson et al., 2005) was added to each sample and dried down at 180°C on the hotplate. Iron co-precipitation and anion-exchange columns were used to purify and separate the U and Th fractions from the matrix. Typically, ten samples were processed together with one U standard (Harwell uraninite standard, HU1) and one blank (4ml 7M distilled HNO3) for every batch. U and Th isotope ratios were measured using the standard-sample-bracketing method using a Multi-Collector Inductively Coupled Plasma Mass Spectrometer (Neptune) connected with an Aridus de-solvation system (Cetac). The U112a standard was used to bracket samples during U isotopes measurements whereas an in-house standard (SGS) was used for Th isotope measurements. Quality control was conducted by measuring HU1 and Th standards (ThB) at the beginning of every batch and after every 4 samples during analysis. We obtained a long-term external precision of ~1per mille for 234U/238U ratios and 2per mille for 229Th/230Th ratios based on the replicate measurements of both standards. A single uranium spike (236U) was added to the Th fraction and measured on a Faraday cup to normalize the signals during peak jumping between 229Th and 230Th (Burke and Robinson, 2012). The long-term external reproducibility of [230Th/238U] (activity ratio), which was monitored by repeated measurements of HU1 standard processed through column chemistry, was better than 3per mille (n=36, 2 S.D.). For the coral samples, the initial 230Th was corrected based on the modern-day 230Th/232Th ratios of the seawater collected near the dredge sites (Bradtmiller et al., 2009) and the measured 232Th concentration. The ages were resolved using the U-series age equations (Edwards et al., 2003) and the errors, including those associated with mass bias corrections, procedural blanks and initial 230Th corrections, were propagated using a Monte Carlo method (Chen et al., 2015).

Radiocarbon analysis and data processing

The radiocarbon data in this study were measured at the National Ocean Sciences Accelerator Mass Spectrometry (NOSAMS) facility, the UC Irvine Keck-CCAMS facility or the Bristol Radiocarbon Accelerator Mass Spectrometry Facility (BRAMS). Sample preparation for radiocarbon analysis followed previously established protocols for the deep-sea corals (Adkins et al., 2002; Burke and Robinson, 2012; Burke et al., 2010; Chen et al., 2015; Robinson et al., 2005). Briefly, about 20 mg of each sample was first leached with 0.1N HCl to remove potentially adsorbed CO2 (Adkins et al., 2002). Residual samples (~12 mg) were then dissolved in concentrated phosphoric acid in a pre-vacuumed 5ml tube. The generated CO2 gas was graphitized following the hydrogen reduction method (Adkins et al., 2002; Vogel et al., 1984). The 12C, 13C and 14C isotopes were measured by accelerator mass spectrometry simultaneously and the 14C results were normalized to a delta13C value of -25per mille and were reported as Fraction modern (Fm) (where modern is defined as 95% of the 1950 AD 14C concentration of the NBS Oxalic Acid I norm...(35)

Data collection:

Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS, Neptune)

Accelerator Mass Spectrometry

Data quality:

The long-term external reproducibility of [230Th/238U] (activity ratio) was better than 3per mille (n=42, 2 S.D.). Due to the uncertainty in the modern-day 230Th/232Th atomic ratio (2±2 × 10-4, 2 S.D.) that we applied to correct the initial 230Th and do the error propagation, the final age uncertainties largely depend on the 232Th concentrations. To minimize the influence of initial 230Th contamination on the final age uncertainties, coral samples with high 232Th concentrations were duplicated to get the lowest possible 232Th concentration.