Link between light-triggered Mg-banding and chamber formation in the planktic foraminifera Neogloboquadrina dutertrei

The relationship between seawater temperature and the average Mg/Ca ratios in planktic foraminifera is well established, providing an essential tool for reconstructing past ocean temperatures. However, many species display alternating high and low Mg-bands within their shell walls that cannot be explained by temperature alone. Recent experiments demonstrate that intrashell Mg variability in Orbulina universa, which forms a spherical terminal shell, is paced by the diurnal light/dark cycle. Whether Mg-heterogeneity is also diurnally paced in species with more complex shell morphologies is unknown. Here we show that high Mg/Ca-calcite forms at night in cultured specimens of the multi-chambered species Neogloboquadrina dutertrei. Our results demonstrate that N. dutertrei adds a significant amount of calcite, and nearly all Mg-bands, after the final chamber forms. These results have implications for interpreting patterns of calcification in N. dutertrei and suggest that diurnal Mg-banding is an intrinsic component of biomineralization in planktic foraminifera.


Supplementary Discussion:
LA-ICP-MS results of specimens exposed nightly to 87 Sr label 87 Sr/ 88 Sr ratios (blue lines, Supplementary Fig. 1) were used to distinguish between calcite that precipitated in the ocean and calcite formed under culture conditions during day and night periods.
Calcite that grew in the ocean and prior to the first night transfer into 87 Sr-labeled seawater (grey boxes, Supplementary Fig. 1) had ambient 87 Sr/ 88 Sr ratios (~0.084). At collection, these specimens were thinly calcified (chamber walls were <5 μm thick) and had a newly formed final chamber (Supplementary Table   1, Supplementary Figs 1a-f). Laser spot analyses revealed variable Mg/Ca ratios and evidence of Mgbanding in the cultured calcite. Several high Mg-bands that formed in culture were enriched in 87 Sr indicating that they formed at night. The number of high Mg-bands is generally consistent with the number of day/night cycles the specimens experienced in the laboratory (Supplementary Table 1).
NanoSIMS imaging (following section) reveal that thin Mg-bands that are closely spaced or 'wavy' in nature are not always resolvable using LA-ICP-MS, which is expected given the nature of how the two types of profiles are generated. For NanoSIMS, an approximate 1.5m region of a 2D NanoSIMS image is integrated (data is integrated perpendicular to the line profile) to generate the line profiles (e.g. white boxes in Figure 2). In contrast, the laser ablation profiles are generated using a 30-50 m round or square laser spot (~700 to 2000 micron area) that pulses through the shell starting from the flat inner shell surface to the outer shell surface, thus, the laser depth profiles integrate a larger amount of data per pulse compared to the nanoSIMS line profiles. Owing to the nature of the Mg-bands in N. dutertrei, the integrated data using the laser may contain a mixture of high and low Mg/Ca calcite, which yields a mixed signal. This is due, in part, to the nature of how the Mg/Ca bands form in N. dutertrei. Initially, Mg-bands in N. dutertrei form parallel to the inner shell wall, but they become 'wavier' towards the outer portion of the shell wall. The 'waviness' of the outer Mg-bands is correlated with the surface topography of the shell.
As the laser ablates through the shell, it first encounters trace element bands that are perpendicular to the laser beam (and thus Mg-bands are easily resolvable). As ablation progresses the beam encounters wavy Mg-bands and, therefore, a mixture of low and high Mg/Ca calcite. Because both high and low Mg/Ca calcite is ablated at the same time, the amplitude of the Mg-banding is reduced in the resulting depth profile. Additionally, as the laser penetrates through the shell wall, the beam can interact with the sidewall of the spot being ablated, which can further obscure, or mix, the trace element signal. Laser ablation profiles are, therefore, initially higher-resolution at the beginning of the depth profile with abrupt transitions between trace element banding. As ablation progresses, the combination of the wavier trace element bands and any sidewall interactions yield a more mixed signal and reduced amplitude banding with smoothed transitions between trace element variations. We refer to this as an 'analytically mixed' signal.
We simulate the effects of a laser-based 'mixed signal' by generating two line profiles (integrated perpendicular to the line profile) through nanoSIMS image from Specimen 299. The first profile is generated using a 1.5-m portion of the intensity image, which yields high amplitude Mg-bands in the line profile (blue lines, Supplementary Figs 1e,f). In contrast, a 24-m thick line profile yields line profiles that have diminished trace element banding because the integrated regions contain a mixture of both high and low Mg/Ca calcite (red lines, Supplementary Figures 1e,f). Laser profiles integrate an even larger area and therefore may yield even lower amplitude, or obscured, banding.

Supplementary Figure 4. SEM of a polished cross-section of specimen 152B
24 Mg/ 40 Ca NanoSIMS images are superimposed to identify the locations of the enlarged NanoSIMS images detailed in Supplementary Fig. 5. See Supplementary Fig. 5 for enlarged images. Note: warm colors = higher ratios. The 'X' in the image below marks the location of a NanoSIMS image that was not completed. These images represent the summation of a stack of 20-40 NanoSIMS frames that were shift-corrected; the hue scale bar represents the ratio of the summed 24 Mg, 87 Sr, and/or 88 Sr counts divided by 40 Ca counts × 10000.