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Fabrics, trace element partitioning and stable isotope ratio fractionation into speleothem calcite depend on crystallization processes. We documented that in warm and wet tropical climate settings, high supersaturation and pH may shift from layer growth to nanoparticle/nanocrystal attachment. This suggests that both classical and non-classical growth mechanisms may be operating. We have observed that both non-classical amorphous calcium carbonate (ACC) nanoparticle or even nanocrystal attachment and spiral (classical) growth occur in calcite farmed in the caves of Atiu (Cook Island Archipelago). Depending on localized shifts in SIcc and pH, “impurities” may be preferentially incorporated as non-monomer species (non-classical) or monomer (classical) species. This gives rise to “random” lateral distribution of some trace elements that would otherwise be expected to follow the classical “sector zoning” pattern. The “random” distribution is typical of porous columnar fabric, whilst the sector zoning distribution in Sr characterizes compact columnar calcite. In the porous columnar calcite fabric, the occurrence of non-classical ACC particle attachment also influences the stable oxygen isotope composition of its fluid inclusions, which is more negative than expected from dripwater (Global Meteoric Water Line) values. In the compact columnar fabric, there is not enough fluid inclusion water for measurements.When applied to Sr incorporation, our findings suggest that its uptake into speleothem calcite is a function of SIcc and pH (which influence non-classical pathways) rather than growth rate, as already hypothesized by Wasylenki et al. (2005). However, by having in mind only a classical crystallization mechanism, SIcc becomes a measure of growth rate. Consequently, the argument of Wasylenki et al. (2005) would not explain why Wassenburg et al. (2021) did not find a relation between Sr uptake and stalagmite growth rate. If SIcc is taken as a measure of the transition from monomer-by-monomer to ACC nanoparticle attachment, then Sr uptake becomes dependent on processes that govern the transformation from ACC to calcite at the site of attachment. The presence of growth inhibitors (inorganic and organic) that may be incorporated as non-monomer species and observed in Atiu’s speleothems, may explain why Sr uptake may either depend on growth rate or not, as well as its lateral heterogeneous variability. Ultimately, in porous columnar fabric, Sr uptake is dictated by both non-classical pathways and the presence of growth inhibitors incorporated as non-monomers.References:Wassenburg, J.A., Scholz, D., Jochum, K.P., Cheng, H., Oster, J., Immenhauser, A., Richter, D.K., Haeger, T., Jamieson, R.A., Baldini, J.U.L. and Hoffmann, D., 2016. Determination of aragonite trace element distribution coefficients from speleothem calcite–aragonite transitions. Geochimica et Cosmochimica Acta, 190, pp.347-367.Wasylenki, L.E., Dove, P.M. and De Yoreo, J.J., 2005. Effects of temperature and transport conditions on calcite growth in the presence of Mg2+: Implications for paleothermometry. Geochimica et Cosmochimica Acta, 69(17), pp.4227-4236.