Effect of organic matrices on the determination of the trace element chemistry (Mg, Sr, Mg/Ca, Sr/Ca) of aragonitic bivalve shells (<i>Arctica islandica</i>)&mdash;Comparison of ICP-OES and LA-ICP-MS data

Abstract

The element chemistry of biogenic carbonates can provide important data on past environments. However, the Sr/Ca and Mg/Ca ratios as well as the Mg and Sr concentrations of biological carbonates, especially aragonitic bivalves often depart from apparent thermodynamic equilibrium. When measured in situ by means of LA-ICP-MS, the Mg concentration is often substantially enriched (two-to threefold) near the organic-rich, annual growth lines. To test the hypothesis that some organic components exert a major influence on the skeletal metal content, the element chemistry of different shell components (insoluble organic matrix, IOM; dissolved CaCO 3 and soluble organics, SOM) of Arctica islandica was measured by means of ICP-OES and LA-ICP-MS. The ICP-OES data indicate that the IOM is strongly enriched in Mg (130 ppm) and depleted in Sr and Ca (10 ppm and 0.22 wt%, respectively) when compared to the whole biomineral (Mg: 68 to 99 ppm, Sr: 860 to 1,060 ppm, Ca: ~35.72 wt%). Although the average relative abundance of the IOM barely exceeds 0.46 wt%, its chemical composition in combination with its heterogeneous distribution across the shell can significantly increase estimates of the Mg concentration if measured in situ by LA-ICP-MS. Depending on the distribution of the IOM, the Ca concentration may be significantly lower locally than the average Ca concentration of the whole shell (35.72 wt%). If this remains undetected, the Mg concentration of shell portions with higher than average IOM content is overestimated by LA-ICP-MS and, conversely, the Mg concentration is underestimated in shell portions with lower than average IOM content. Removal of the IOM prior to the chemical analysis by LA-ICP-MS or mathematical correction for the IOMderived magnesium concentrations is therefore strongly advised. The different chemistry of the IOM may also exert a major control on the trace element to calcium ratios. Shell portions enriched in IOM will show up to 200 times higher Mg/ Ca and up to two times higher Sr/Ca ratios than the average shell of A. islandica. Without removal of the IOM prior to the analysis, Mg/Ca and Sr/Ca ratios of shell portions with higher IOM content cannot be used as paleothermometers. Because it is currently impossible to remove the IOM prior to chemical analyses by LA-ICP-MS, we recommend the use of wet chemical techniques (= possibility to separate and measure individual shell components) such as ICP-OES at the expense of lower sampling resolution. The results of this study will significantly improve our understanding of shellbased climate and environmental proxies.

The Application of Long-Lived Bivalve Sclerochronology in Environmental Baseline Monitoring

“…Heterogeneities in the crystal fabric coincide with heterogeneous distribution of Sr/Ca, Mg/Ca, and Ba/Ca (Schöne et al, 2010Shirai et al, 2014). During the formation of the annual growth lines, extremely slow growth occurs and Sr/Ca and Mg/Ca are seemingly incorporated in equilibrium with the ambient seawater.…”

Environmental and biological factors influencing trace elemental and microstructural properties of Arctica islandica shells

“…Levels of strontium (Sr) and magnesium (Mg) in bivalve shells have been proposed as proxies for water temperature (Klein et al, 1996;Schöne et al, 2013). However, Sr/Ca and Mg/Ca ratios are still difficult to interpret because the incorporation of trace and minor impurities in the shell carbonate is partly physiologically controlled (e.g., Urey et al, 1951;Purton et al, 1999;Lorrain et al, 2005;Freitas et al, 2005Freitas et al, , 2006Schöne et al, 2010Schöne et al, , 2013Marali et al, 2017a;Geeza et al, 2018). Published results on the relationship between the Sr/Ca and Mg/Ca ratios of bivalve shells and temperature are highly ambiguous; previous studies have reported a positive relationship (Stecher et al, 1996;Hart and Blusztajn, 1998;Toland et al, 2000), negative relationship (Dodd, 1965;Stecher et al, 1996;Surge and Walker, 2006;Schöne et al, 2011), and no relationship (Gillikin et al, 2005;Strasser et al, 2008;Izumida et al, 2011;Wanamaker and Gillikin, 2018).…”

“…Previous studies of mollusks show that environmental parameters can also influence the microstructure of the shell (Lutz, 1984;Tan Tiu and Prezant, 1987;Tan Tiu, 1988;Nishida et al, 2012) and therefore, can serve as potential proxy for environmental conditions (Tan Tiu, 1988;Tan Tiu and Prezant, 1989;Schöne et al, 2010;Milano et al, 2017b). For example, the size and elongation of individual biominerals in Cerastoderma edule (Milano et al, 2017b) and the cyclical changes in thickness of the outer layer of Scapharca broughtonii (Nishida et al, 2012) shells are related to temperature changes.…”

Geochemical and microstructural characterisation of two species of cool-water bivalves (&lt;i&gt;Fulvia tenuicostata&lt;/i&gt; and &lt;i&gt;Soletellina biradiata&lt;/i&gt;) from Western Australia

“…Freitas et al, 2006;Gillikin et al, 2005b;Lowenstam and Weiner, 1989) and environmental conditions at the time of deposition (e.g. Ferguson et al, 2011;Gazeau et al, 2010;Hahn et al, 2014;Heinemann, 2011;Henkes et al, 2013;Lowenstam and Weiner, 1989;Schöne et al, 2011). Molluscs are globally distributed and demonstrate sequential growth, thus providing high-resolution seasonal and sub-seasonal records of environmental conditions (e.g.…”

“…Purton et al, 1999); organic matrix (e.g. Schone et al, 2010;Takesue et al, 2008) and microstructure (overall fabrics) (e.g. Schöne et 30 al., 2013;Shirai et al, 2014).…”