Formation of calcite in the presence of dissolved organic matter: Partitioning, fabrics and fluorescence

Abstract

Dissolved organic matter (DOM) is omnipresent in natural waters and is commonly incorporated into carbonates. Records of DOM from speleothems (secondary carbonates found in caves) have often been interpreted to reflect groundwater DOM concentrations.However, the fidelity of these records is largely untested. An understanding of the relationship between dripwater and speleothem DOM is thus required to allow speleothems to be reliably used as archives of DOM concentration.We precipitated calcite (CaCO3) crystals from weak solutions of (NH4)2CO3, CaCl2 and NH4Cl. These solutions also contained peat DOM (from 0 to 15 mgC/Lppm). Fluorescence 3D excitation-emission matrix (3D EEM) analysis showed a strong, positive correlation between [DOM] in the parent-solutionaq, and [DOM]calcite in the calcite. Calcite precipitation was reduced at high DOM concentrations, potentially indicating inhibition of crystallisation. Partition coefficient values showed that DOMaq was subtly preferentially incorporated into calcite. Scanning electron microscope images indicated that the crystal structures were heavily influenced by DOM adsorption with finer, smooth-faced, rhombohedral crystals forming in growth solutions with low aqueous [DOM]aq (0-5 mgC/Lppm), and prismatic, 'impure' crystals produced at high aqueous [DOM]aq (10 and 15 mgC/Lppm). Overall, our results indicate that authigenic carbonates are likely to faithfully record variations in aqueous [DOM]aq within the natural range of DOM concentrations in representative freshwater systems (caves, soil water), and that crystal habits are altered by aqueous [DOM]aq within their growth solutions.We also applied our findings to three flowstones collected from three New Zealand caves which vary in climatic, vegetation and hydrological regimes. We conclude that differences in initial aqueous [DOM]aq do indeed control incorporation of DOM into calcite, and thus 3D EEM fluorescence can be used to reconstruct original aqueous [DOM]aq from authigenic carbonates.

Sub-annual fluorescence measurements of coral skeleton: relationship between skeletal luminescence and terrestrial humic-like substances

“…This is in good agreement with both our canonical understanding of the spectral characteristics of terrigenous HULIS in aquatic environments (Coble 2007), and also with the previous coral fluorescence study by (Matthews et al 1996). The linear concentration dependence of the HULIS fluorescence intensity and the lack of pH sensitivity between pH 2 and 7 are both consistent with prior studies of terrestrial HULIS fluorescence in water samples, in abiogenic calcite (Pearson et al 2020) and in coral skeleton (Matthews et al 1996). Unlike Matthews et al (1996), the samples in this study were dissolved and measured directly without first extracting the organic matter.…”

“…Moreover, extractions also subject the organic matter to more extreme changes in solution pH. Pearson et al (2020) examined the incorporation of dissolved organic matter (DOM) from groundwater into stalagmites in caves, using a similar technique to ours: they precipitated abiogenic CaCO 3 from water samples that were amended with peatland DOM, and measured fluorescence EEM spectra after dissolving the CaCO 3 samples. Their findings with calcite precipitation are similar to our results with aragonite-the fluorescence signal in the precipitated CaCO 3 was dominated by the humic-like Peak C (ex/em 330-350/420-480 nm (Coble 1996).…”

Late Quaternary Climate Variability and Change from Aotearoa New Zealand Speleothems: Progress in Age Modelling, Oxygen Isotope Master Record Construction and Proxy-Model Comparisons

“…Nettlebed cave speleothem trace element chemistry, ultraviolet luminescence intensity (UVL), and Uranium isotopic data support the interpretation of δ 13 C as a palaeohydrology proxy. Millennial-scale UVL variations closely mimic coeval δ 13 C patterns and have been hypothetically linked to seepage water organic acid content flux that impacts speleothem fluid inclusion density, an assertion now supported by experimental evidence [149]. In this situation, increased (or decreased) speleothem growth rates that contribute to calcite formation should align to increases (or decreases) in percolation water flow rates, elevated (or reduced) biological activity in and above the epikarst, and warmer (or cooler) temperatures.…”

Drying increases organic colloidal mobilization in the karst vadose zone: Evidence from a 15‐year cave‐monitoring study

“…The measurement parameters for 3D‐EEM were set to a 1.0 s integration time, a measurement range of 240–600 nm excitation wavelength with a step‐size of 3 nm, and a 246–824 nm emission wavelength with a defaulted step‐size. To eliminate instrument‐specific biases, each matrix was corrected for inner‐filter effects, and scatter lines were Rayleigh masked using Aqualog's in‐situ data processing software and protocols (Pearson et al, 2020). Because of the difference in the spectrometers between the two‐stage experiments, the spectra were normalized to the corresponding mean Raman unit (RU) of distilled deionized water with the integration intensity of the Raman peak range of 371–428 nm emission at 350 nm excitation to ensure unit uniformity (Lawaetz & Stedmon, 2009).…”