Foraminifera Iodine to Calcium Ratios: Approach and Cleaning

Abstract

Oxygen concentrations [O 2 ] in the oceans have been decreasing and oxygen minimum zones (OMZ) expanding since 1960 (Schmidtko et al., 2017). Climate models also show this decrease and predict that this trend will not only continue but accelerate in the future (Oschlies et al., 2008(Oschlies et al., , 2018. To better understand the longer-term oxygen cycle in the past, on time scales typically exceeding centuries to millennia, we can use proxy reconstructions (Moffitt et al., 2015). Most proxy reconstructions assess in-situ bottom water [O 2 ] and rely on sedimentary samples (e.g.,

Planktic foraminifera iodine/calcium ratios from plankton tows

“…We note that none of the species analyzed *Was stabilized using tetramethylammonium hydroxide but cleaned using the same method. For a comparison of stabilization techniques please refer to Winkelbauer et al (2021).…”

“…We assume that the cleaning method resulting in the lowest planktic foraminifera I/Ca ratios, was the most adequate at removing organic material within and adhered to the foraminifera tests. Overall, the method of Winkelbauer et al (2021), using 1% (v/v) buffered H 2 O 2 acquires the lowest I/Ca result, whereas the method using bleach showed the highest value (Table 2). We therefore used the method of Winkelbauer et al (2021) to remove organic material, which involved six repetitions of soaking the crushed samples in buffered 1% (v/v) H 2 O 2 at boiling point for ten minutes.…”

“…Overall, the method of Winkelbauer et al (2021), using 1% (v/v) buffered H 2 O 2 acquires the lowest I/Ca result, whereas the method using bleach showed the highest value (Table 2). We therefore used the method of Winkelbauer et al (2021) to remove organic material, which involved six repetitions of soaking the crushed samples in buffered 1% (v/v) H 2 O 2 at boiling point for ten minutes. The same sample from ODP 1088, 0-1 cm was also used to measure planktic foraminifera with different preservation states, i.e., semi-transparent tests and fully opaque encrusted tests.…”

“…Glock et al (2016) showed that organic material in benthic foraminifera could be a significant source of iodine. We carried out a set of experiments on core-top samples (ODP Site 1088, 0-1 cm, 41.14°S, 13.56°E, 2082 m below sea level) to assess which method may remove organic material/iodine contamination most effectively, using A) 1% buffered H 2 O 2 x 6, following Winkelbauer et al (2021), B) 50% (v/ v) buffered H 2 O 2 for 30 minutes (Anand et al, 2003), C) soaking in bleach, and D) combustion at 450 °C. Before measuring I/Ca ratios in planktic foraminifera each sample was initially rinsed in ultra-pure de-ionized water.…”

Simultaneous Determination of I/Ca and Other Elemental Ratios in Foraminifera: Comparing Results From Acidic and Basic Solutions

“…This method was adopted for measuring I/Ca in foraminifera (e.g., Glock et al., 2014; Zhou et al., 2014). I/Ca is extracted from the calcitic foraminiferal tests by dissolution of the tests with dilute nitric acid, followed by the addition of an organic alkaline buffer, typically TMAH or TA, to the acidic solution to raise the sample solution's pH, thereby minimizing iodine volatility before and during the analysis by ICP‐MS (Glock et al., 2014; Lu et al., 2010; Takaku et al., 1995; Winkelbauer et al., 2021). This is different from the measurement of other element/Ca (El/Ca) ratios that are routinely used in paleoceanographic research such as Mg/Ca, B/Ca, and Cd/Ca, analyses that are usually performed in acidic solutions (pH < 2.5) because metals are more stable in acidic solutions and may precipitate in alkaline solutions where the CaCO 3 saturation is very high (Ω > 10).…”