PNNL

Abstract

The calcite shells, or tests, of foraminifera provide a window into Earth history because they are well preserved in marine sediments and contain useful geochemical proxies for paleoceanography. Previous observations of diurnal heterogeneity in proxies like Mg/Ca demonstrate a complex relationship between environmental conditions and test composition. The definitive causes for this diurnal banding and the potential impact for proxy interpretation in systems other than Mg/Ca have yet to be determined. Here we use time-of-flight secondary ion mass spectrometry (ToF-SIMS), a chemical and isotope mapping technique with a spatial resolution of 300 nm, to show that Na banding is a consistent feature in the tests of cultured Orbulina universa, a species of planktic foraminifer. This banding occurs in two distinct forms: (1) sharp Na bands associated with embedded organic sheets and (2) regular, thicker, but lower-amplitude Na bands that are found throughout the test. In Orbulina bilobata, a morphotype of O. universa that develops a second partial spherical chamber attached to the primary sphere, we use the first type of banding to show that a second organic sheet is used as a template to form this second chamber. Thus, sodium banding links test geochemistry with foraminiferal growth models, allowing us to explain the pattern and timing of chamber formation as well as a specific type of geochemical banding in foraminifera. Furthermore, the magnitude, timing, and coherency between Na and Mg bands can help constrain mechanistic explanations for geochemical banding in foraminifera in general.