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.