Kiel 4 carbonate device

For oxygen isotope analysis a second section was taken from each tooth. Surface dirt and any dentine was removed using a dental burr. The sample was ground to a fine powder in an agate mortar under acetone. The powder was leached for 10 minutes in 10% acetic acid to remove diagenetic carbonate then centrifuged and washed in 18MΩ H₂O three times and dried. Two 5 μg aliquots were weighed for analysis.
Oxygen isotope analyses were performed on a Thermo Scientific Kiel IV Carbonate device coupled with a MAT253 isotope ratio mass spectrometer on c. 0.6mg samples. Samples were calibrated and drift corrected by bracketing with NBS18 and 19. Results are quoted as δ values relative to PDB. Quality control standards give reproducibilities at 1σ of ±0.01 for δ¹³C and ±0.05 for δ¹⁸O. For comparison with published measurements on enamel phosphate oxygen isotopes and rainwater δ18O, structural carbonate values were converted to phosphate values and then to drinking water equivalents [51 (link),52 ].

Isotopes of oxygen and carbon in carbonate phases were measured at the UC Santa Cruz Stable Isotopes Laboratory by acid digestion using an individual vial acid drop Thermo Fisher Scientific Kiel IV carbonate device interfaced to a Thermo Fisher Scientific MAT 253 dual-inlet isotope ratio mass spectrometer (iRMS). We loaded ∼100-μg fractions into individual vials that were dried overnight in a 70°C vacuum oven. Samples reacted at 75°C with H₃PO₄ (1.92 g/cm³ specific gravity) to generate CO₂ and H₂O. The latter is cryogenically separated, and noncondensible gases are pumped away before introduction of the CO₂ analyte into the iRMS. Samples are measured concurrently with replicates of NBS-18 limestone standard reference material and a Carrara Marble in-house standard (CM12, calibrated to NBS-18 and NBS-19). Carbonate δ¹⁸O and δ¹³C values are calculated relative to Vienna PeeDee Belemnite (VPDB) with a two-point calibration between CM12 and NBS-18 to correct for offset and linearity. Reproducibility and independent quality control are monitored with measurements of “Atlantis II” powdered coral. We convert δ¹⁸O_(VPDB) to δ¹⁸O_(VSMOW) with δ¹⁸O_(VSMOW) = 1.03091 × δ¹⁸O_(VPDB) + 30.91, after (71 (link)).

Stable isotope analysis (δ18O and δ13C) was conducted by acid digestion using an individual vial acid drop ThemoScientific Kiel IV carbonate device interfaced to ThermoScientific MAT-253 dual-inlet isotope ratio mass spectrometer at the University of California, Santa Cruz, USA and Nanjing University, Nanjing, China. Ratios listed in Table S1 for the 11 samples used for developing the geochronology of JC001 refer to activity ratios normalized to the corresponding ratios measured for the secular-equilibrium HU-1 standard. ²³⁰Th ages are calculated using Isoplot/Ex 3.75^{53 (link)}. Non-radiogenic ²³⁰Th correction was applied assuming non-radiogenic ²³⁰Th/²³²Th atomic ratio = 4.4 ± 2.2 × 10⁻⁶ (bulk-earth value), and ²³⁸U, ²³⁴U, ²³²Th and ²³⁰Th are in secular equilibrium. Non-radiogenic ²³⁰Th correction results in large age error magnification for samples with low ²³⁰Th/²³²Th ratios.
Using the 11 dates presented in Table S1, the age model for JC001 was developed using the StalAge algorithm^{66 ,67 (link)}. An age of 2012 (−62 BP) was used for the tip of the stalagmite as this was the date it was removed. The age and 95%-confidence limits were calculated in StalAge with Monte-Carlo simulations. Here the final age model was determined through an ensemble of 30 iterations of the algorithm to ensure a representative age model using ensemble mean values (Supplementary material Table S2).