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

E Geerken,G J Reichart,A Roepert,L Polerecky,L J De Nooijer,H E King

doi:10.1038/s41598-019-40298-y

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.

Highlights

Element incorporation into foraminiferal calcite provides a valuable tool for reconstructing seawater variables
Images of elements associated with organics (N and P) show thin bands at the same positions as the brighter bands in the scanning electron microscopy (SEM) images (Fig. 1)
Peak locations overlap for all elements measured here and coincide with the location of the organic linings separating the calcite lamellae of the shell walls

Summary

Introduction

Element incorporation into foraminiferal calcite provides a valuable tool for reconstructing seawater variables. A key feature of foraminiferal geochemistry is systematic intra-shell chemical heterogeneity, which has the potential to inform us about the underlying mechanisms of shell formation This variability is most characteristically manifested in so-called banding, i.e., alternation of high and low concentrations of a given minor or trace element in the direction parallel to the surface of the shell wall[33,34,35,49,50]. Mg banding has been hypothesized to result from (i) diurnal oscillations in pH in the foraminiferal micro-environment as a function of symbiont activity, which is assumed to have a kinetic effect on Mg incorporation[2,32], (ii) day-night patterns due to mitochondrial sequestration of Mg35, or from (iii) alternating calcification pathways during chamber formation[26,37] These factors may well be interacting and their relative contribution may vary between species, possibly explaining inter-species differences in Mg banding and average Mg/Ca values. Comparing element banding between different species and specimens could provide unique insights into the biological controls on element partitioning

Methods

Results

Discussion

Conclusion