DOI: 10.5285/72ee3175-0a64-43b0-9308-b488979a59e9
title: Drake Passage deep-sea coral U-Th and 14C data for Southern Ocean corals

description: Changes of circulation pattern in the Southern Ocean have been invoked to explain a significant portion of the increase in the atmospheric carbon dioxide during the last deglaciation. However, the accurate timing and thus underlying mechanisms of these changes are still controversial, requiring knowledge of different water masses movements with absolute age constraints. Aragonitic scleractinian deep-sea corals, recovered from a broad range of depths in the Drake Passage, provide a unique opportunity to investigate Southern Ocean ventilation with precise U-Th age control. A rapid age-screening technique achieved by coupling a laser system to Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS) enables us to get an approximate age distribution of the coral samples in order to select appropriate specimens for more accurate isotope-dilution age and radiocarbon age determination. Thus far more than 1800 deep-sea corals from the Drake Passage have been dated using this and other techniques, and about 400 samples have been dated precisely using isotope-dilution method. The age results show that deep-sea corals can be found across nearly the whole of the last deglaciation across a wide range of depths and locations. With known radiocarbon contents and U-Th ages of the deep-sea corals, the ventilation state of different water masses in the past can be assessed based on their decay-corrected 14C activities. This data submission includes all U-Th and 14C data available for the Drake Passage corals. Funding was provided by the NERC standard grant NE/N003861/1.
description: 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 normalized to a delta13C value of -19per mille (Olsson, 1970)). The blank correction was done by subtracting the Fm of a 14C-dead deep-sea coral (~145 ka, Fm = 0.0023+/-0.0011 (n=23, 2 S.D.)) from the measured samples. REFERENCES Adkins, J.F., Griffin, S., Kashgarian, M., Cheng, H., Druffel, E., Boyle, E., Edwards, R.L., Shen, C.-C., 2002. Radiocarbon dating of deep-sea corals. Radiocarbon 44, 567-580. Bradtmiller, L.I., Robinson, L.F., McManus, J.F., Auro, M.E., Bostock, H.C., 2009. The distribution of 231Pa and 230Th in paired water column and surface sediment samples. Geochimica et Cosmochimica Acta Supplement 73, A154. Burke, A., Robinson, L.F., 2012. The Southern Ocean's Role in Carbon Exchange During the Last Deglaciation. Science 335, 557-561. Burke, A., Robinson, L.F., McNichol, A.P., Jenkins, W.J., Scanlon, K.M., Gerlach, D.S., 2010. Reconnaissance dating: A new radiocarbon method applied to assessing the temporal distribution of Southern Ocean deep-sea corals. Deep Sea Research Part I: Oceanographic Research Papers 57, 1510-1520. Chen, T., Robinson, L.F., Beasley, M.P., Claxton, L.M., Andersen, M.B., Gregoire, L.J., Wadham, J., Fornari, D.J., Harpp, K.S., 2016. Ocean mixing and ice-sheet control of seawater 234U/238U during the last deglaciation. Science, aag1015. Chen, T., Robinson, L.F., Burke, A., Southon, J., Spooner, P., Morris, P.J., Ng, H.C., 2015. Synchronous centennial abrupt events in the ocean and atmosphere during the last deglaciation. Science 349, 1537-1541. Cheng, H., Adkins, J., Edwards, R.L., Boyle, E.A., 2000. U-Th dating of deep-sea corals. Geochimica et Cosmochimica Acta 64, 2401-2416. Cook, M.S., Keigwin, L.D., 2015. Radiocarbon profiles of the NW Pacific from the LGM and deglaciation: Evaluating ventilation metrics and the effect of uncertain surface reservoir ages. Paleoceanography 30, 174-195. Edwards, R., Gallup, C., Cheng, H., 2003. Uranium-series dating of marine and lacustrine carbonates. Reviews in Mineralogy and Geochemistry 52, 363-405. Olsson, I.U., 1970. Radiocarbon variations and absolute chronology. Robinson, L.F., Adkins, J.F., Keigwin, L.D., Southon, J., Fernandez, D.P., Wang, S.-L., Scheirer, D.S., 2005. Radiocarbon Variability in the Western North Atlantic During the Last Deglaciation. Science 310, 1469-1473. Spooner, P.T., Chen, T., Robinson, L.F., Coath, C.D., 2015. Rapid Uranium-Series Age Screening of Carbonates by Laser Ablation Mass Spectrometry. Quaternary Geochronology. Vogel, J.S., Southon, J.R., Nelson, D., Brown, T.A., 1984. Performance of catalytically condensed carbon for use in accelerator mass spectrometry. Nuclear Instruments and Methods in Physics Research Section B: Beam Interactions with Materials and Atoms 5, 289-293.
description: Multi-Collector Inductively Coupled Plasma Mass Spectrometry (MC-ICPMS, Neptune) Accelerator Mass Spectrometry
description: 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.