Element banding and organic linings within chamber walls of two benthic foraminifera

Abstract

Trace and minor elements incorporated in foraminiferal shells are among the most used proxies for reconstructing past environmental conditions. A prominent issue concerning these proxies is that the inter-specimen variability in element composition is often considerably larger than the variability associated with the environmental conditions for which the proxy is used. Within a shell of an individual specimen the trace and minor elements are distributed in the form of bands of higher and lower concentrations. It has been hypothesized that differences in specimen-specific element banding patterns cause the inter-specimen and inter-species variability observed in average element composition, thereby reducing the reliability of proxies. To test this hypothesis, we compared spatial distributions of Mg, Na, Sr, K, S, P and N within chamber walls of two benthic foraminiferal species ( Amphistegina lessonii and Ammonia tepida ) with largely different average Mg content. For both species the selected specimens were grown at different temperatures and salinities to additionally assess how these parameters influence the element concentrations within the shell wall. Our results show that Mg, Na, Sr and K are co-located within shells, and occur in bands that coincide with organic linings but extend further into the calcite lamella. Changes in temperature or salinity modulate the element-banding pattern as a whole, with peak and trough heights co-varying rather than independently affected by these two environmental parameters. This means that independent changes in peak or trough height do not explain differences in average El/Ca between specimens. These results are used to evaluate and synthesize models of underlying mechanisms responsible for trace and minor element partitioning during calcification in foraminifera.

Organic Heterogeneities in Foraminiferal Calcite Traced Through the Distribution of N, S, and I Measured With NanoSIMS: A New Challenge for Element-Ratio-Based Paleoproxies?

“…while class 3 represents the highest 32 S − / 40 Ca 16 O − ratios outside these bandings that is supposed to represent organicrich heterogeneities of the test. According to Geerken et al (2019), the edges of the test pores and the edges of the test walls show the strongest topography differences on foraminiferal cross-sections after measurements with NanoSIMS. Since these topography differences likely impact the ion yield during the NanoSIMS measurements, test pores and wall edges were avoided for the definition of ROIs.…”

Coupled calcium and inorganic carbon uptake suggested by magnesium and sulfur incorporation in foraminiferal calcite

“…This might, in part, control the ultimate E/Ca if these species are used to precipitate the foraminiferal shell. During chamber addition, pH changes externally to the foraminifera (pH < 8; Glas et al, 2012;Toyofuku et al, 2017) and internally at the site of calcification or in seawater vacuoles (de Nooijer et al, 2009). Availability of SO 2− 4 remains similar when pH changes, while presence of MgCO 3 increases with pH, especially when pH > 8 ( Fig.…”

“…Two exercises using PHREEQC showing activity of SO 2− 4 , Ca 2+ and MgCO 3 at (a) different temperatures (temperature range of controlled culture experiment indicated in grey) and (b) different pH. Chosen pH range reflects external and internal pH shift during chamber formation (Glas et al, 2012;de Nooijer et al, 2009;Toyofuku et al, 2017). crease in [CO 2− 3 ] will lead to lower SO 2− 4 /CO 2− 3 values in the site of calcification.…”

“…With a seawater concentration of ∼ 30 mM [SO 2− 4 ], it is very likely that the SO 2− 4 : CO 2− 3 at the site of calcification is > 1, but it is dependent on the carbonate chemistry at the SOC. Laboratory experiments have revealed that the internal pH of a foraminifera is elevated, with (species-specific) values ranging from ∼ 8.75 (Bentov et al, 2009) to ≥ 9 (de Nooijer et al, 2009) at the start of shell formation due to proton pumping (Toyofuku et al, 2017), which lowers the pH in the microenvironment surrounding the foraminifer (Glas et al, 2012). When assuming the SO 2− 4 and inorganic carbonate concentration at SOC is equal to natural seawater at 400 ppm CO 2 (∼ 2650 mg L −2 [SO 2− 4 ], ∼ 2100 µmol L −1 DIC), the elevation of internal pH to 9 creates a SO 2− 4 : CO 2− 3 of ∼ 25, leading to the stabilization of vaterite and a band enriched in SO 2− 4 close to the primary organic sheet, which we observe in the chamber wall distribution of all three of our species (Fig.…”

“…In perforate hyaline species, studies have shown that the site of calcification is separated from the surrounding seawater by a protective envelope. Although observations for Ammonia suggest this envelope might not be closed at the start of calcification (Nagai et al, 2018), the carbonate chemistry at the site of calcification is proposed to be controlled by the foraminifer and, for example, characterized by a high internal and low external pH (de Nooijer et al, 2009;Glas et al, 2012;Toyofuku et al, 2017), as well as the chemistry of the calcification fluid itself (Erez, 2003;Bentov and Erez, 2006;Evans et al, 2018). Control-ling the physicochemical conditions at the site of calcification is necessary to overcome inhibition of calcite nucleation and growth by sulfate and magnesium ions (e.g., Zeebe and Sanyal, 2002;Fernández-Díaz et al, 2010;Reddy and Nancollas, 1976).…”