5:15 PM - 7:15 PM
[MIS14-P30] Examination on cleaning methods of foraminifera tests for paleoenvironmental reconstruction using carbonate associated sulfate
Keywords:sulfur, foraminifera, cleaning
Accurate measurement of sulfur isotopic composition of dissolved sulfate in the ocean is crucial for understanding the global sulfur cycle. Sulfur isotope ratios of carbonate-associated sulfate (CAS) in biogenic carbonates can provide a long-term continuous records of marine sulfate sulfur isotopic records, because carbonate-shelled microfossils can occur continuously in marine sediments throughout geologic time. Foraminifera, including planktonic species such as surface-dweller and thermocline-dweller, and benthic species have constrained their habitat depth (Paris et al., 2014). Therefore, it is possible to determine the depth distribution of sulfate sulfur isotope ratios in the water column (e.g., Paris et al., 2014).
Because Fe-Mn-oxides and pyrite adhered on the surface of carbonate minerals via diagenetic processes in marine sediments can be a source of contamination, accurate determination of CAS sulfur isotopes ratios requires cleaning of foraminifera tests to remove those contaminants. Pyrite is a major host of diagenetic sulfur, and even small amounts can affect the sulfur isotopic ratio of CAS. Fe-Mn-oxides have also been reported to contain sulfur in amounts that can alter sulfur isotopes ratio of CAS (Rennie et al. 2018). Therefore, it is important to evaluate cleaning methods for determining sulfur isotope ratio of CAS in biogenic carbonates. However, few studies have examined cleaning treatments of foraminifera tests for CAS sulfur isotope analysis.
Here, we evaluated the cleaning methods for foraminifera sulfur isotope analysis. Samples were Pleistocene sediment cores collected on the Ontong Java Plateau. From this core, subsamples of the five stratigraphic level were selected. It were separated on a 150-µm-diameter sieve and the fraction larger than 150 µm-diameter was used. The samples were divided using a splitter, and then mineral grains and benthic foraminifera tests were removed by handpicking under a microscope. Mixed species of planktonic foraminifera were collected with this procedure (hereafter this fraction is called “Raw” fraction). Foraminifera tests in the samples were then gently crushed and ultrasonicated in ultrapure water (Milli-Q) and methanol, and then clay minerals were removed under a microscope (MM fraction). Subsequently, oxidative cleaning (Ox Rennie fraction) and sequential reductive-oxidative cleaning (Red Rennie fraction) were performed using the method of Rennie et al. (2018). To compare this method with that of Cheng et al. (2000), the oxidative cleaning process of Rennie et al. (2018) was replaced with that of Cheng et al. (2000) and oxidative (Ox Cheng) and sequential reductive-oxidative cleaning (Red Cheng) were performed. In total, six fractions were analyzed. The element ratios of Na/Ca, Mg/Ca, Sr/Ca, Fe/Ca, Mn/Ca, and Ba/Ca in each fraction were measured by ICP-MS, and the sulfate ion concentrations were determined by ion chromatography, applying a CAS extraction method with cation exchange reigns (Kochi et al., 2023).
The results of element analyses showed that Fe/Ca and Mn/Ca decreased significantly from the Raw fractions to the MM fractions, indicating that the physical treatments such as ultrasonication removed Fe and Mn effectively. It was confirmed that Fe/Ca and Mn/Ca were progressively reduced from the MM fractions to the oxidative cleaning treatments (Ox Rennie and Ox Cheng fractions) and from the MM fractions to the reductive-oxidative cleaning treatments (Red Rennie and Red Cheng fractions). It suggests that Fe-Mn- oxides were removed progressively. A similar trend was observed for Ba/Ca, suggesting that Ba is largely derived from Fe-Mn-oxides. On the other hand, the sulfate ion concentration remained nearly constant at approximately 1000 ppm across the five fractions, excluding the Raw fraction. This result suggests that the contribution of sulfate ion derived from Fe-Mn-oxides and pyrite is small and is unlikely to significantly affect CAS concentration measurements. However, to account for the potential influence of pyrite and organic sulfur, which may not have been completely removed during the cleaning process, on sulfur isotope ratios, a detailed evaluation through isotope ratio measurements is necessary. In this presentation, we provide a comprehensive discussion based on these measurement results.
Cheng, H., et al. (2000) Geochim. Cosmochim. Acta, 64(14) 2401-2416. Rennie, V.C.F., et al. (2018) Nat. Geosci., 11, 761–765. Paris, G., et al. (2014) Geochem., Geophys., Geosys., 15(4) 1452-1461. Kochi, T., et al. (2023) Geostandard and Geoanalytical Research., 48(1) 77-89.
Because Fe-Mn-oxides and pyrite adhered on the surface of carbonate minerals via diagenetic processes in marine sediments can be a source of contamination, accurate determination of CAS sulfur isotopes ratios requires cleaning of foraminifera tests to remove those contaminants. Pyrite is a major host of diagenetic sulfur, and even small amounts can affect the sulfur isotopic ratio of CAS. Fe-Mn-oxides have also been reported to contain sulfur in amounts that can alter sulfur isotopes ratio of CAS (Rennie et al. 2018). Therefore, it is important to evaluate cleaning methods for determining sulfur isotope ratio of CAS in biogenic carbonates. However, few studies have examined cleaning treatments of foraminifera tests for CAS sulfur isotope analysis.
Here, we evaluated the cleaning methods for foraminifera sulfur isotope analysis. Samples were Pleistocene sediment cores collected on the Ontong Java Plateau. From this core, subsamples of the five stratigraphic level were selected. It were separated on a 150-µm-diameter sieve and the fraction larger than 150 µm-diameter was used. The samples were divided using a splitter, and then mineral grains and benthic foraminifera tests were removed by handpicking under a microscope. Mixed species of planktonic foraminifera were collected with this procedure (hereafter this fraction is called “Raw” fraction). Foraminifera tests in the samples were then gently crushed and ultrasonicated in ultrapure water (Milli-Q) and methanol, and then clay minerals were removed under a microscope (MM fraction). Subsequently, oxidative cleaning (Ox Rennie fraction) and sequential reductive-oxidative cleaning (Red Rennie fraction) were performed using the method of Rennie et al. (2018). To compare this method with that of Cheng et al. (2000), the oxidative cleaning process of Rennie et al. (2018) was replaced with that of Cheng et al. (2000) and oxidative (Ox Cheng) and sequential reductive-oxidative cleaning (Red Cheng) were performed. In total, six fractions were analyzed. The element ratios of Na/Ca, Mg/Ca, Sr/Ca, Fe/Ca, Mn/Ca, and Ba/Ca in each fraction were measured by ICP-MS, and the sulfate ion concentrations were determined by ion chromatography, applying a CAS extraction method with cation exchange reigns (Kochi et al., 2023).
The results of element analyses showed that Fe/Ca and Mn/Ca decreased significantly from the Raw fractions to the MM fractions, indicating that the physical treatments such as ultrasonication removed Fe and Mn effectively. It was confirmed that Fe/Ca and Mn/Ca were progressively reduced from the MM fractions to the oxidative cleaning treatments (Ox Rennie and Ox Cheng fractions) and from the MM fractions to the reductive-oxidative cleaning treatments (Red Rennie and Red Cheng fractions). It suggests that Fe-Mn- oxides were removed progressively. A similar trend was observed for Ba/Ca, suggesting that Ba is largely derived from Fe-Mn-oxides. On the other hand, the sulfate ion concentration remained nearly constant at approximately 1000 ppm across the five fractions, excluding the Raw fraction. This result suggests that the contribution of sulfate ion derived from Fe-Mn-oxides and pyrite is small and is unlikely to significantly affect CAS concentration measurements. However, to account for the potential influence of pyrite and organic sulfur, which may not have been completely removed during the cleaning process, on sulfur isotope ratios, a detailed evaluation through isotope ratio measurements is necessary. In this presentation, we provide a comprehensive discussion based on these measurement results.
Cheng, H., et al. (2000) Geochim. Cosmochim. Acta, 64(14) 2401-2416. Rennie, V.C.F., et al. (2018) Nat. Geosci., 11, 761–765. Paris, G., et al. (2014) Geochem., Geophys., Geosys., 15(4) 1452-1461. Kochi, T., et al. (2023) Geostandard and Geoanalytical Research., 48(1) 77-89.