Note: No formal accuracy tests were conducted and these data are disseminated to allow discussion related to methods. Sample Analyses: Samples were processed at both the USGS in Menlo Park, CA, and at UC Berkeley following established methodology for separating organic material from sinter (Howald et al., 2014; Lowenstern et al., 2016; Slagter et al., 2019). First, the exterior surface of each sample was removed using a rock saw, and then any further material was removed if there was any visible algal material in the interior of the sample. Second, samples underwent a series of chemical baths. Samples were crushed and soaked in 30% hydrogen peroxide for 48 hours to remove any remaining modern algae. Once cleaned, samples were decanted and rinsed three times with milliQ water, and then bathed in 1M hydrochloric acid (HCl) for 72 hours to dissolve any inorganic carbonates. Samples were once more decanted and rinsed three times in milliQ water before being soaked in concentrated (48%) hydrofluoric acid (HF) until all silicates were dissolved. The remaining organic material was placed in centrifuge tubes with milliQ water and sent to the Keck-Carbon Cycle Accelerator Mass Spectrometry Laboratory (Keck-CCAMS) at the University of California, Irvine. There each sample was centrifuged, the milliQ water decanted, and then placed in a vacuum oven so that the remaining material would fully dry. Then, 5-10 mg of the sample was placed into a labeled quartz tube, along with a stick of silver to capture free radicals, and ~60 mg of CuO. The quartz tubes were evacuated and sealed before being placed into a furnace at 900 °F overnight to combust their contents, producing CO2 gas. The amount of carbon dioxide produced by each sample was quantified and if concentrations were above 0.19 mg C then a small aliquot was separated for δ13C measurements. The remaining CO2 was reduced to elemental graphite for 14C measurement. The Keck-CCAMS runs a 500 kV compact AMS unit from the National Electrostatics Corporation (NEC 0.5MV 1.5SDH-2). Radiocarbon dates were calibrated using the UCIAMS atmospheric IntCal13 dataset from Reimer et al. (2013). Database Contents: The data file (carbon_isotope_data_Supplementary.csv) contains the ẟ13C, fraction modern, D14C, 14C age, and calibrated 14C age (calibrated with Reimer et al., 2013) for all reported samples. Radiocarbon concentrations are given as fractions of the Modern standard, D14C, and conventional radiocarbon age, following the conventions of Stuiver and Polach (Radiocarbon, v. 19, p.355, 1977). All results have been corrected for isotopic fractionation according to the conventions of Stuiver and Polach (1977), with ẟ13C values measured on prepared graphite using the AMS spectrometer. These can differ from ẟ13C of the original material, and are not shown. The entries in the data file appear in the following columns: A. Sample ID B. Location C. Testing Date D. Material Dated E. ẟ13C (‰) F. ẟ13C (‰) ± G. Fraction Modern H. Fraction Modern ± I. D14C (‰) J. D14C (‰) ± K. 14C age (yr. BP) L. Calibrated Age (cal yr. BP) M. Calibrated Age (cal yr. BP) ± N. Combusted wt (mg) O. % Carbon References Howald, T., Person, M., Campbell, A., Lueth, V., Hofstra, A., Sweetkind, D., Gable, C.W., Banerjee, A., Luijendijk, E., Crossey, L. and Karlstrom, K., 2014. Evidence for long timescale (> 103 years) changes in hydrothermal activity induced by seismic events. Geofluids. 15(1-2), 252-268. Reimer PJ, Bard E, Bayliss A, Beck JW, Blackwell PG, Bronk Ramsey C, Buck CE, Cheng H, Edwards RL, Friedrich M, Grootes PM, Guilderson TP, Haflidason H, Hajdas I, Hatté C, Heaton TJ, Hogg AG, Hughen KA, Kaiser KF, Kromer B, Manning SW, Niu M, Reimer RW, Richards DA, Scott EM, Southon JR, Turney CSM, van der Plicht J. IntCal13 and MARINE13 radiocarbon age calibration curves 0-50000 years calBP. Radiocarbon 55(4). DOI: 10.2458/azu_js_rc.55.16947 Lowenstern, J.B., Hurwitz, S., and McGeehin, J.P., 2016. Radiocarbon dating of silica sinter deposits in shallow drill cores from the Upper Geyser Basin, Yellowstone National Park. Journal of Volcanology and Geothermal Research. 310, 132-136. Slagter, S., Reich, M., Munoz-Saez, C., Southon, J., Morata, D., Barra, F., Gong, J., Skok, J.R., 2019. Environmental controls on silica sinter formation revealed by radiocarbon dating. Geology. 47 (4), 330–334. doi: https://doi.org/10.1130/G45859.1