How to digest a carbonate

Photo showing a Permian-Triassic sample section in the U.A.E. These are relatively pure carbonates that hold important information on the driver of the greatest mass extinction in history. Photo credit R. Wood

Carbonates as an archive

We use carbonate rocks, either from outcrops or core samples, as an archive for ancient ocean chemistry. The carbonate precipitates from seawater and can incorporate metals directly into the crystal lattice. If we can isolate the carbonate, we can reconstruct trends in seawater isotopic compositions.

Unfortunately carbonate rocks also contain some silicate minerals and Mn-oxide coatings, which also contain metals. This makes it very difficult to isolate the pure carbonate phase and can bias or overwhelm the carbonate signatures.

Leaching experiments

Researchers typically used dilute acids to try and isolate the pure carbonate component from a bulk sample. Whilst these often largely avoid breaking down detrital components themselves, trace metals can still be released because they are more mobile.

In this work we undertook leaching experiments with a range of acid dilutions and acid types, with and without pre-cleaning treatments. The cleaning was designed to remove Mn-oxides from the samples. This helps us to chemically characterise the different components of a sample, and identify the best way to treat the sample.

In the end the carbonate fraction is only a very small phase for metal isotopes, and care must be taken to avoid any contributions from other phases. This is shown in the right hand figure where the blue fractions represent carbonate. Depending on the isotopic composition of each component, and the relative concentrations, the assumed carbonate isotopic signature can be easily contaminated.

Figure showing the relative amounts of U and Mo in different phases of a sample. We aim to isolate the carbonate bound metals (blue), and avoid all the rest. But this is very difficult and can lead to a bias or distortion of the true carbonate concentrations and isotopic signatures. Figure taken from Clarkson et al., (2020)

Recommendations

We now use a dilute, pH buffered, weak acid (1M Ammonium Acetate) which does the best job at avoiding detrital phases such as silicates and clays. In addition, for metals like Mo, we need to use a reductive cleaning pre-treatment in order to remove Mn-oxides. This step is not always needed because some metals (e.g. U) are not concentrated in the Mn-oxide phases.

MOC acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 795722

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