Rano Raraku geochemical data

Published: 16-11-2020| Version 1 | DOI: 10.17632/k93rp3p4pd.1
Contributors:
Elena Argiriadis,
Mara Bortolini,
Natalie Kehrwald,
Marco Roman,
Clara Turetta,
Shapara Hanif,
Evans Erhenhi,
Jose Miguel Ramirez-Aliaga ,
Dave McWethy,
Amy Myrbo,
Anibal Pauchard,
Carlo Barbante,
Dario Battistel

Description

A geochemical survey of the water, suspended particulate matter, sediments and soil of the Rano Raraku crater lake basin in Rapa Nui (Easter Island) was performed in order to assess if the lake water could have been a viable fresh water source for early inhabitants, as well as better understanding past interactions between humans and environmental conditions as archived in lake sediments. At the time of sampling in September 2017, lake maximum depth was 1.5 m. The lake level has substantially declined in the subsequent years, with the lake drying almost completely in January 2018. The lake is currently characterized by highly anoxic conditions, with a predominance of ammonium ions on nitrates, a high concentration of organic carbon in the water-sediment interface and reducing conditions of the lake, as evidenced by Mn/Fe and Cr/V ratios. At the time of sampling, water pH was 7.8.

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Major ions (Cl-, SO42-, NO3-, NH4+, Na+, K+, Ca2+ and Mg2+) in water samples were analyzed using an ion chromatography system (883 Basic IC plus, Metrohm) equipped with a Metrosep Cation 1-2 (particle size 7 µm; eluent: HNO3 3 mM) and a Metrosep Anion supp/4 (particle size 5 µm; eluent: HCO3-/CO32-buffer 1.7/1.8 mM) column for cation and anion analysis, respectively. Liquid samples were directly injected into the chromatographic system after filtration through 0.45 µm syringe filters. Quantification of trace elements (TEs) and REEs in water samples were performed using an inductively coupled plasma - sector field mass spectrometer (ICP-SFMS model Element-XR, Thermo Scientific, Bremen, Germany). Water samples were acidified with ultrapure grade HNO3 and analyzed 24 hours later. TEs and REEs were quantified using external calibration curves. Method accuracy was assessed through the analysis of a certified reference material (TMRAIN95). The isotopic composition of water samples was determined with a DeltaV-Advantage mass spectrometer (Thermo Scientific) equipped with a gas-bench. The samples were first flushed with a gas mixture of 2 % H2 in He and analyzed to determine δ2H. The same samples were then flushed with a gas mixture of 0.4 % CO2 in He and analyzed to determine δ18O after 20 hours of equilibration. All samples were measured at least in triplicate. The SMOW2 and SLAP2 isotopic standards were used as references together with a laboratory standard that was analysed every 9 samples to evaluate the stability of measurements. The isotopic composition is expressed asδR= [(R_s-R_r)/R_r ┤]×1000 where δR is δ2H or δ18O, R is 2H/H or 18O/16O in the sample (s) and in the reference (r) respectively. Sediment, soil and SPM samples were digested with a mixture of HNO3, HCl and HF (6:2:1 mL for 0.2 g of sample, suprapure grade acids, Romil), using an Ethos 1 microwave oven (Milestone) and a temperature-controlled program up to 200 °C in pressurized Teflon vessels. The digests were diluted in 50 mL of ultra-pure water, randomized and analyzed within 24 hours for determination of major, trace and rare earth elements. The analysis was carried out by ICP-MS (iCAP RQ, Thermo Scientific) equipped with an ASX-560 autosampler (Teledyne Cetac Technologies), PFE cyclonic spray chamber thermostatted at 2.7 °C, sapphire injector, quartz torch, Ni cones and 1550 W of plasma RF power. Triplicate acquisitions were performed in kinetic energy discrimination (KED) – high matrix mode (collision gas He). Quantification was obtained by external calibration with standard prepared by mixing the multi-elemental solutions IMS-101, IMS-102 and IMS-104 (UltraScientific), and using on-line spiked Rh as the internal standard. Accuracy was assessed by contextual mineralization and analysis of the certified reference materials NIMT/UOE/FM/001 (peat) and BCR-667 (estuarine sediment).