THE EXPERIMENTAL INVESTIGATION OF ACETIC ACID EXTRACTION OF NON-CARBONATE Mg IN SEDIMENTS: AN EXAMPLE FROM THE MID-MIOCENE TO PLIOCENE SEDIMENTS OF XINING BASIN
One important precondition for the carbonate composition-based quantitative analyses for paleo-reconstructions is to choose adequate selective leaching reagents for carbonates extractions. The widely adopted diluted acetic acid leaching method is being challenged by contributions from non-carbonate fractions based on multiple clues including strontium isotopes. Such investigations for non-carbonate magnesium in acetic acid leachate, however, are quite limited,hindering the future application of bulk sample carbonate Mg isotope into the studies of weathering and climate change. Herein, we use a series of experimental analyses to investigate the sources of non-carbonate Mg in diluted acetic acid leachate. The materials are from MJZ section(36°41'07〃N,102°04'15"E) in the northeast part of Xining Basin,NW China. This section with 336 m in thickness includes mid-Miocene to Pliocene fluvial sediments, spanning from 12.8 Ma to 4.8 Ma according to detailed paleomagnetism analyses. First,we choose five representative mudstone/siltstone samples,and analyze their inorganic carbon content by carbon analyzer (SHIMADZU TOC-L). The 5 samples are named as ES-1, ES-2, ES-3, ES-4 and ES-5 in sequence (from 116.4 m, 158.8 m, 186.4 m, 89.7 m and 216.7 m of the MJZ section, respectively) according to their carbon content from high to low. And then the carbon contents are compared with the CO_2 amount calculated by the Ca and Mg concentrations (ICP-OES, Leeman Labs Prodigy-H) in acetic acid leachate. The results show good correlation among samples with high carbonate contents but poor among those with low carbonate contents, suggesting that there are significant amounts of non-carbonate Mg and Ca leached by acetic acid in the low carbonate samples. Therefore, we design a 4-step sequential extraction to check the sources of these non-carbonate Mg. We first separate each of the 5 samples into 3 parallel samples, one is left as blank and other two are treated with KCl and NH4Ac solutions, respectively, in order to extract the exchangeable fractions and examine the difference between the two exchangeable agents. Next, we use 1 mol/L acetic acid to extract the carbonate fractions. After that, continuous 1 mol/L acetic acid leaching for 2 times are applied in order to check the capability of acetic acid to dissolving non-carbonate fractions when carbonates are exhausted. The final step is 2 mol/L HCl leaching under 80 ℃. According to the result of exchangeable fraction leaching,the difference in agents leads to little difference in the composition of leachates except that some carbonate may dissolve in NH4 Ac. By comparing the first acetic acid leachates with and without exchangeable fraction extractions, the contributions of exchangeable fractions to the Mg in acetic acid leachate are estimated to be more than 30% for all the samples and more than 50% for low carbonate content samples. Another source of non-carbonate Mg might be from silicate, since other fractions such as those bounded to organic matters or Fe-Mn oxides are not likely to provide Mg cations to acetic acid leachate. The silicate fractions provide as an endmember high in Mg/Ca ratios, supported by the generally inverse correlation between Mg/Ca ratios and carbonate content in the first acetic acid leaching step, and the increasing Mg/Ca ratios during continuous acetic acid leaching.