Chemical Heterogeneity of Mg, Mn, Na, S, and Sr in Benthic Foraminiferal Calcite

Abstract

The chemical composition of fossil foraminiferal shells (tests) is widely used as tracers for past ocean chemistry. It is, therefore, important to understand how different (trace) elements are transported and incorporated into these tests from adjacent seawater. The elemental distribution within the walls of foraminiferal tests might be used to differentiate between proposed transport mechanisms. Here, the microdistribution of Mg, Mn, Na, S, and Sr in tests of three species of foraminifera, known to have contrasting test chemistry, is investigated by a combination of electron probe microanalysis (EPMA) and nanoscale secondary ion mass spectrometry (nanoSIMS), micro-X-ray fluorescence (µXRF), and micro-X-ray absorption near-edge structure (µXANES) analyses. The three investigated species are the symbiont-barren Ammonia sp. T6 and Bulimina marginata, which precipitate a low-Mg calcite test, and the symbiont-bearing species Amphistegina lessonii, which produces a test with intermediate Mg content. Because all analyzed tests were formed under controlled and identical laboratory conditions, the observed distributions of elements are not due to environmental variability but are a direct consequence of the processes involved in calcification or, in the case of A. lessonii, possibly symbiont activity. Despite some variability in elemental microdistribution between specimens from a given species, our combined dataset shows species-specific distributions of the elements (e.g., peak heights and/or band-widths) and also a systematic colocation of Mg, Na, S, and Sr for all three species, suggesting a coupled or simultaneous uptake, transport, and incorporation of these elements during chamber addition. The observed trace element patterns generally reflect a laminar calcification model, suggesting that heterogeneity of these elements is intrinsically linked to chamber addition. Although the incorporation of redox-sensitive Mn depends on the Mn concentration of the culture medium, the Mn distribution observed in Ammonia sp. suggests that Mn transport is similarly linked to laminar calcification dynamics. However, for B. marginata, Mn banding was sometimes anticorrelated with Mg banding, suggesting that (bio)availability, uptake, and transport of Mn differ from those for van Dijk et al. Chemical Heterogeneities in Foraminiferal Shells Ammonia sp. Our results from symbiont-bearing A. lessonii suggest that the activity of symbionts (i.e., photosynthesis/respiration) may influence the incorporation of Mn owing to alternation of the chemistry in the microenvironment of the foraminifera, an important consideration in the development of this potential proxy for past oxygenation of the oceans.

Temperature Impact on Magnesium Isotope Fractionation in Cultured Foraminifera

“…The overall trend is about three times more sensitive as observed for porcelaneous Amphisorus kudakajimensis (slope −0.01‰/°C; Maeda et al, 2019). Such a difference most likely reflects a difference in the biomineralization between these species, which is also evident in the contrasting Mg/Ca of these species: A. kudakajimensis typically has Mg/Ca ratios of ∼150 mmol/mol, whereas A. lessonii has an order of magnitude lower Mg/Ca ratio, typically of 20-30 mmol/mol (e.g., van Dijk et al, 2019a;Geerken et al, 2019). The sensitivity of the isotopic composition of incorporated Mg to temperature also varies between these species: A. lessonii being less sensitive (slopes of the calibration regression being 1.69 mmol/mol/°C in A. lessonii (van Dijk et al, 2019b) compared to 2.74 mmol/mol/°C in A. kudakajimensis (Maeda et al, 2017).…”

Nanoscale trace metal imprinting of biocalcification of planktic foraminifers by Toba’s super-eruption

“…2B and S6). In the case of inter-CL Mg banding, the high and thin Mg bands are observed within or close to the "outer organic linings", intercalated with the outer calcitic layers (OCLs) 6,12,13,37 . Previously, the "intra-CL" Mg banding was considered as an exception for the spherically-shaped planktic foraminifer O. universa, whose last chamber was known to continue thickening after its formation 5,7 .…”

“…To unravel the Mn and Zn trace patterns, a nanometric spatial resolution and high elemental sensitivity by nano-imaging is required, and is achieved here by synchrotron X-ray Fluorescence analyses (XRF) 29 . In previous works, foraminifer intra-test elemental distribution mapping reached spatial resolutions of 85-500 nm for elemental concentrations at the 100 ppm level (nanoscale Secondary Ion Mass Spectrometry-NanoSIMS), or of 1 µm with minor element sensitivities (Synchrotron micro-XRF and Electron Probe Micro Analysis-EPMA) for concentrations relative to the average Ca signal 12,13 . We have focused our nano-XRF measurements on two sampling choices of the last chamber walls: full wall bulk (10 to 11 µm thick) and patterns in transverse sections across the wall (~ 1 µm thick).…”

Mn Incorporation in Large Benthic Foraminifera: Differences Between Species and the Impact of pCO2

“…and Bulimina marginata, thin bands with high Mn/Ca are observed at the start of calcite lamellae (Van Dijk et al, 2019b), which could be due to absorption layers that are exposed to the surrounding seawater between chamber formation events, as hypothesized in Barras et al (2018). These high Mn/Ca layers are co-located with high Mg/Ca banding and are associated with a carbonateassociated species of Mn (Van Dijk et al, 2019b). Other processes related to seawater carbonate chemistry could also impact element incorporation (Ries et al, 2009).…”

“…Barras et al (2018) found that the two benthic species Ammonia tepida and Bulimina marginata, cultured under the same conditions, have a Mn partitioning of 0.08 and 0.6, respectively, an offset of one order of magnitude. Incorporation of Mn appears to be coupled to that of Mg (Van Dijk et al, 2019b), similar to what is observed for other elements (Van Dijk et al, 2017b), possibly due to crystal lattice strain (Mucci and Morse, 1983;Evans et al, 2015) or simultaneous uptake (Van Dijk et al, 2019a). Even if the relative sensitivity of Mn/Ca to oxygenation is the same for different species, as suggested for certain proxies (e.g., S/Ca; Van Dijk et al, 2017a), the absolute Mn/Ca value is still needed for reconstruction of oxygenation.…”