Sediment cores were collected from the East Antarctic margin, aboard the Australian Marine National Facility R/V Investigator, during the IN2017_V01 voyage from January 14th to March 5th 2017 (Armand et al., 2018). This marine geoscience expedition, named the “Sabrina Sea Floor Survey”, focused notably on studying the interactions of the Totten Glacier with the Southern Ocean through multiple glacial cycles. The cores were collected using a multi-corer (MC) and a Kasten corer (KC). The MC were sliced every centimetre, wrapped up in plastic bags, and stored in the fridge. The KC was sub-sampled using a u-channel; and sliced every centimetre once back the home laboratory (IMAS, UTAS, Hobart, Australia).
About 200 mg of dried and ground sediment were weighed into a clean Teflon vial and oxidized with a mixture of concentrated HNO3 and 30% H2O2 (1:1). The resulting solutions were gravimetrically spiked with ~ 24 pg of 229Th (NIST 4328C, National Institute of Standards and Technology, USA) and ~ 2 ng of 236U (IRMM-3660a, Institute for Reference Materials and Measurements, European Union) and left to equilibrate overnight. Samples were then digested in open vials using an acid mixture comprising 10 mL HNO3, 4 mL HCl, and 2 mL HF, at 180°C until close to dryness. Digested residues were converted to nitric form before being oxidised with a mixture of 1 mL HNO3 and 1 mL HClO4 at 220°C until fully desiccated. Samples were finally re-dissolved in 4 mL 7.5 M HNO3.
Thorium and uranium were isolated from the sediment digest using AG1-X8 anion exchange resin (Bio-Rad, USA), following the procedure described in Negre et al., (2009). Prior to analysis, purified samples were filtered using Pall® Acrodisc® ion chromatography syringes and 0.45 μm filters (Sigma-Alderich®, USA). 229Th, 230Th, 234U and 235U were analysed by Sector Field Inductively Coupled Mass Spectrometry (SF-ICP-MS, Thermo Fisher Scientific, Bremen, Germany) at the Central Science Laboratory (UTAS, Hobart, Australia). Samples were introduced in the ICP using an Aridius® II desolvating nebulizer (DSN, CETAC Technologies, USA) and with the capacitive guard electrode turned on to limit the oxide formation and to enhance sensitivity. Samples were analysed in batches of three and bracketed by a natural uranium standard (Certified Reference Material CRM 145, New Brunswick Laboratory, USA) and two acid blanks (2% HNO3, 0.1% HF). The sample introduction system was rinsed for 5 minutes between each sample with a matching 2% HNO3 and 0.1% HF solution.
The raw intensities of 230Th and 234U were corrected for procedural blank, tailing and mass bias (Anderson et al., 2012; Shen et al., 2002). The intensity of 230Th was corrected from the tailing of 232Th using the log mean intensities of the half masses 229.5 and 230.5. The mass bias was determined by the measurements of the 235U/234U ratio of the CRM-145. Concentrations were calculated using isotope dilution equations (Sargent et al., 2002).
References
Anderson, R. F., Fleisher, M. Q., Robinson, L. F., Edwards, R. L., Hoff, J. A., Moran, S. B., … Francois, R. (2012). GEOTRACES intercalibration of 230Th, 232Th, 231Pa, and prospects for 10Be. Limnology and Oceanography: Methods, 10(4), 179–213. https://doi.org/10.4319/lom.2012.10.179
Armand, L. K., O’Brien, P. E., Armbrecht, L., Baker, H., Caburlotto, A., Connell, T., … Young, A. (2018). Interactions of the Totten Glacier with the Southern Ocean through multiple glacial cycles (IN2017-V01): Post-survey report. ANU Research Publications, (March). https://doi.org/http://dx.doi.org/10.4225/13/5acea64c48693
Negre, C., Thomas, A. L., Mas, J. L., Garcia-orellana, J., Henderson, G. M., Masque, P., and Zahn, R. (2009). Separation and Measurement of Pa , Th , and U Isotopes in Marine Sediments by Microwave-Assisted Digestion and Multiple Collector Inductively Coupled Plasma Mass. Analytical Chemistry, 81(5), 1914–1919. https://doi.org/10.1126/science.276.5313.782.(3)
Sargent, M., Harrington, C., and Harte, R. (2002). Guidelines for Achieving High Accuracy in Isotope Dilution Mass Spectrometry (IDMS). Guidelines for Achieving High Accuracy in Isotope Dilution Mass Spectrometry (IDMS). Royal Society of Chemistry. https://doi.org/10.1039/9781847559302-00001
Shen, C.-C., Lawrence Edwards, R., Cheng, H., Dorale, J. A., Thomas, R. B., Bradley Moran, S., … Edmonds, H. N. (2002). Uranium and thorium isotopic and concentration measurements by magnetic sector inductively coupled plasma mass spectrometry. Chemical Geology, 185(3–4), 165–178. https://doi.org/10.1016/S0009-2541(01)00404-1
