The impact and implications of aragonite-to-calcite transformation on speleothem trace element composition

Abstract

Trace elements in cave carbonates (both calcite and aragonite) have been considered major proxies in paleo-hydroclimatic research. However, their imprints in authigenic minerals might be altered by neomorphic processes (e.g., aragonite-to-calcite transformation). Therefore, it is crucial to understand the impacts of neomorphism on the trace element compositions of secondary calcite and understand the mechanisms of trace elements redistribution during neomorphic processes. In this study, two aragonite stalagmites (SN3 and SN15) from Shennong Cave, part of which were transformed into calcite, were further inspected and analyzed in terms of petrological observations, trace element geochemistry, and Mg compositional mapping. The results show that (1) the aragonite-to-calcite transformation in SN3 and SN15 occurs by a dissolution-precipitation mechanism and is influenced by several factors, including internal fluid compositions, microstructure (such as porosity and permeability) of the parent material, and crystal defects. (2) The primary aragonite is enriched in Sr but depleted in Mg compared with secondary calcite because of the crystallographic disparity. The non-equilibrium absorption of Mg and slow diffusive element transport may contribute to the extremely high Mg/Ca ratios in secondary calcite near the mineral phase boundary. Incorporation competition among trace elements also plays an important role in affecting the distribution of Sr and Mg in neomorphic calcite. Considering the previous study on the alteration of isotopes (δ18O, δ13C) associated with the aragonite-to-calcite transformation, we propose that intervening fluids play an important role in altering the secondary calcite oxygen isotopic composition and Mg/Ca ratio, while the composition of primary aragonite dictates the secondary calcite carbon isotopic composition and Sr/Ca ratio, provided that the transformation of aragonite to calcite caused by a dissolution–reprecipitation process occurring in trapped pore water. These results highlight the indispensability of petrographic analysis of speleothem specimens before using trace elements and isotopes for paleoclimate research.