Uptake of ammonium and nitrate by calcium carbonate polymorphs

Gabitov, Rinat I. and Wynn, Peter M. and Wearing, Catherine and Carlson, Logan and Ariunbold, Gombojav and Jimenez-Arroyo, Angel L. and Bell, Elizabeth and Halcovitch, Nathan and Thirumalai, Rooban V.K.G. (2026) Uptake of ammonium and nitrate by calcium carbonate polymorphs. Geochimica et Cosmochimica Acta. ISSN 0016-7037 (In Press)

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Abstract

The abundance of nitrogen ionic species in natural waters is linked to drivers of environmental change at the global and local scales throughout the anthropogenic era. The potential for calcium carbonate minerals as a resistant host material to record the reactive nitrogen composition of natural waters is driving a requirement to understand incorporation mechanisms and to develop absolute analytical techniques that operate at the µm scale. To assess the capability of calcium carbonate for the uptake of dissolved nitrogen compounds, we conducted an experimental study on the incorporation of ammonium and nitrate into inorganic carbonate crystals. Calcium carbonate polymorphs were synthesized in nitrogen-containing growth media and were used to quantify nitrate and ammonium uptake by calcite, aragonite, and vaterite phases. Automated colourimetry analyses yielded partition coefficients between mineral and fluid ranging from (D(NO3)) of 2.12·10-2 to 6.51·10-1 regardless of CaCO3 polymorph (i.e. calcite and aragonite). Secondary ionisation mass spectrometry (SIMS) demonstrated ammonium is more readily incorporated into polymorphs of vaterite and aragonite (D(NH4) = 8.77·10-3 and D(NH4) = 1.63·10-2, respectively), as opposed to calcite (D(NH4) = 3.09·10-3). SIMS was also used to assess the degree of nitrogen heterogeneity within crystal phases. Calcite rhomboids were found to have high internal heterogeneity, with an almost two orders of magnitude shift in N concentration due to inferred sectoral zoning. The polycrystalline nature of aragonite spherulites mitigated intra-spherulite heterogeneity, resulting in analytical repeatability of <10% standard deviation (1 s.d.) when N(CaCO3) = 164–169 ppm. Inter-spherulite variability for aragonite polymorphs was calculated to average 7.6% (1 s.d.), thus demonstrating a potential for development of reference materials for high precision analytical techniques. Despite the heterogeneity observed across the various synthesized CaCO3 polymorphs, the technique appears to have potential for use as a SIMS calibration dataset and for estimating absolute nitrogen concentrations in the carbonate palaeorecord at the µm scale.