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The oxygen isotopic fractionation between water and calcite is the cornerstone for estimating the formation temperature of calcite [1, 2, 3]. The oxygen isotope thermometer is one of the most valuable tools in geothermometry, palaeoclimatology and palaeoceanography. Despite 60 years of laboratory experiments and search for carbonate material precipitating at (near-)equilibrium [4, 5], establishing the temperature-dependent calcite-water fractionation factor has proven challenging. Here we address this problem with coupled traditional and clumped isotope measurements from an unconventional carbonate archive -- subaqueously-precipitated carbonates (SPC) which form in cave pools and drip sites, as well as natural hot springs. SPC include cave pearls, cave pool rim carbonates, hot spring pisoids, and carbonate geodes. Their common feature is subaqueous formation from supersaturated waters with an unlimited pool of calcium bicarbonate and very slow CO$_2$-degassing, which promote (near-)equilibrium conditions. Here, we expand on earlier work that used SPC as calibration material for clumped isotope thermometry [6] by measuring \delta$^{18}$O of 20 (sub-)modern SPC samples from caves and hot springs with known temperatures (0.8$^{\circ}$C and 76$^{\circ}$C) with calcite or aragonite mineralogies, together with their host water. Our results enable determination of the fractionation factor between carbonate and water for a wide temperature range. Additionally, clumped isotope measurements confirmed isotopic equilibrium for both, \delta$^{18}$O and \Delta$_{47}$ of the analysed SPCs, supporting the equilibrium equations of Coplen [4] and Watkins et al. [7]. SPC grown at thermodynamic equilibrium provide potential standard materials for temperature calibration of clumped isotopes.\textbf{References}\\1 McCrea J. M. (1950) On the isotopic chemistry of carbonates and a paleotemperatures scale. \textit{J. Chem. Phys.} 18, 163--171.\\2 Epstein S. et al. (1953) Revised carbonate-water isotopic temperature scale. \textit{Bull. Geol. Soc. Am.} 64, 1315--1326.\\3 Kim S.-T. and O'Neil J. R. (1997) Equilibrium and nonequilibrium oxygen isotope effects in synthetic carbonates. \textit{Geochim. Cosmochim. Acta} 61, 3461--3475.\\4 Coplen T.B. (2007) Calibration of the calcite--water oxygen-isotope geothermometer at Devils Hole, Nevada, a natural laboratory. \textit{Geochim. Cosmochim. Acta} 71, 3948--3957.\\5 Demeny A. et al. (2010) Empirical equations for the temperature dependence of calcite--water oxygen isotope fractionation from 10 to 70$^{\circ}$C. \textit{Rapid Commun. Mass Spectrom.} 24, 3521--3526.\\6 Breitenbach S.F.M. et al. (2018) Combining Mg/Ca and clumped isotopes in foraminifera for redundant estimation of temperature. \textit{Geochim. Cosmochim. Acta} 236, 283-296.\\7 Watkins et al. (2014) The influence of temperature, pH, and growth rate on the \delta$^{18}$O composition of inorganically precipitated calcite. \textit{Earth Planet. Sci. Lett.} 404, 332--343. [ABSTRACT FROM AUTHOR]

As one of the most prominent seasonally recurring atmospheric circulation patterns, the Asian summer monsoon (ASM) plays a vital role for the life and livelihood of about one-third of the global population. Changes in the strength and seasonality of the ASM significantly affect the ASM region, yet the drivers of change and the varied regional responses of the ASM are not well understood. In the last two decades, there were a number of studies reconstructing the ASM using stalagmite-based proxies such as oxygen isotopes (\delta$^{18}$O). Such reconstructions allow examination of ASM drivers and responses, increasing monsoon predictability.In a review study [1], we focus on stalagmite \delta$^{18}$O records from India at the proximal end of the larger ASM region. Stalagmite-based time series are available from a number of caves in northern, northeastern and south Indian caves, as well as from the Andaman Islands. Despite widespread occurrence of carbonate rocks, data is lacking for Central and western India (the Vindhayans and Thar Desert), and Pakistan. We examine Indian stalagmite records collated in the Speleothem Isotope Synthesis and AnaLysis version 1 database [1] and support the database with a summary of record quality and regional climatic interpretations of the \delta$^{18}$O record during different climate states. Indian stalagmite \delta$^{18}$O records show well-dated, high-amplitude changes in response to the dominant drivers of the ASM on orbital to multi-centennial timescales, and indicate the magnitude of monsoon variability in response to these drivers.We suggest the most useful time periods (climatic events) and locations for further work using tools such as data-model comparisons, spectral analysis methods, multi-proxy investigations, and monitoring.References[1] Kaushal, N., Breitenbach, S.F.M., Lechleitner, F.A., Sinha, A., Tewari, V.C., Ahmad, S.M., Berkelhammer, M., Band, S., Yadava, M., Ramesh, R. and Henderson, G.M.: The Indian Summer Monsoon from a Speleothem $\delta^{18}$O Perspective -- A Review. Quaternary, 1, 29, https://doi.org/10.3390/quat1030029, 2018.\\[2] Atsawawaranunt, K., Comas-Bru, L., Amirnezhad Mozhdehi, S., Deininger, M., Harrison, S. P., Baker, A., Boyd, M., Kaushal, N., Ahmad, S. M., Ait Brahim, Y., Arienzo, M., Bajo, P., Braun, K., Burstyn, Y., Chawchai, S., Duan, W., Hatvani, I. G., Hu, J., Kern, Z., Labuhn, I., Lachniet, M., Lechleitner, F. A., Lorrey, A., P\'{e}rez-Mej\'{\i}as, C., Pickering, R., Scroxton, N., and SISAL Working Group Members: The SISAL database: a global resource to document oxygen and carbon isotope records from speleothems, Earth Syst. Sci. Data, 10, 1687-1713, https://doi.org/10.5194/essd-10-1687-2018, 2018. [ABSTRACT FROM AUTHOR]