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Carbonate - Δ47

Overview

Carbonate clumped isotope analysis starts when carbonate samples (6-8 mg) are digested in a common bath of phosphoric acid (specific gravity 1.9-1.95) held at 90 °C for 10 minutes. The evolved CO2 is cryogenically separated from water on an automated nickel / stainless steel vacuum line using an ethanol-dry ice slush trap, isolated in a liquid N2 trap, and passed through a Porapak Q trap (50/80 mesh, 122 cm long, 6.35 mm OD) held at -20 °C. The CO2 is transferred through the Porapak Q trap using helium as the carrier gas with a flow rate of ~35 mL / min for a total transfer time of 20 minutes then isolated cryogenically and transferred into a Pyrex break seal. Every 5-8 carbonate sample unknowns, a solid carbonate standard (C64, C2, coral, or ETH 1-4) or CO2 reference frame gas is purified on the vacuum line and transferred into a Pyrex break seal. The reference frame gases were created by equilibrating CO2 that originated from corn fermentation or fossil fuel combustion in Pyrex break seals with South Pole ice core water, local tap water, or evaporatively-enriched water (such that the δ47 range is approximately 80 ‰) held at 4 °C and 60 °C, or by heating CO2 in quartz break seals in a box furnace at 1000 °C. Break seals containing CO2 purified on the vacuum line are loaded into an automated 10-port tube cracker inlet system on a Thermo MAT 253 configured to measure mass / charge (m/z) 44-49, inclusive. To start each sample analysis, sample bellows are fully expanded and evacuated. Sample gas is expanded into the sample bellows and pressure is measured. Following sample gas filling, evacuated reference bellows at 100 % expansion is filled to a pressure equal to that measured in the sample bellows with UW ‘fermented corn’ reference CO2 (δ13C VPDB = -10.2‰, δ18O VPDB = -6.0‰; values calibrated by NBS-19 international carbonate standard). Following bellow fill, the m/z - 45 signal is used for peak centering and bellows are compressed for pressure adjustment that produced a m/z 44 signal of 16 V (equivalent to ~ 2500 mV for m/z 47). Pressure baseline (PBL) is automatically measured similar to the method of He et al. (2012) with the measurement made 0.08 kV left of peak center. Sample CO2 m/z 44-49 is measured against reference CO2 for 6 acquisitions of 15 sample-reference comparison cycles with 26-second integration times. Masses 44-46 are measured with standard amplification (3x108, 3x1010, 1x1011, respectively); masses 47-49 are measured with 1x1012 Ω amplification. At the end of each 6-acquisition sample measurement, water backgrounds are measured by peak centering on the mass-45 faraday collector and measuring m/z 18 of both sample and reference. Δ47 values are calculated using established methods (Santrock et al., 1985; Eiler and Schauble, 2004; Huntington et al., 2009, Brand et al. 2010, Daëron et al. 2016, Schauer et al. 2016) and are corrected to the carbon dioxide equilibrium scale (CDES) of Dennis et al. (2011) using CO2 equilibrated with a suite of waters and at three temperatures (4 °C, 60 °C, 1000 °C). We also use a suite of internal carbonate standards as well as the four ETH carbonates to track precision and accuracy in Δ47, δ13C, and δ18O.

Standard Operating Procedures

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