Correlative geochemical imaging of Desmophyllum dianthus reveals biomineralisation strategy as a key coral vital effect

Abstract

The chemical and isotopic composition of stony coral skeletons form an important archive of past climate. However, these reconstructions are largely based on empirical relationships often complicated by "vital effects" arising from uncertain physiological processes of the coral holobiont. The skeletons of deep-sea corals, such as Desmophyllum dianthus, are characterised by micron-scale or larger geochemical heterogeneity associated with: (1) centres of calcification (COCs) where nucleation of new skeleton begins, and (2) fibres that thicken the skeleton. These features are difficult to sample cleanly using traditional techniques, resulting in uncertainty surrounding both the causes of geochemical differences and their influence on environmental signals. Here we combine optical, and in-situ chemical and isotopic, imaging tools across a range of spatial resolutions (~ 100 nm to 10 s of μm) in a correlative multimodal imaging (CMI) approach to isolate the microstructural geochemistry of each component. This reveals COCs are characterised by higher organic content, Mg, Li and Sr and lower U, B and δ¹¹B compared to fibres, reflecting the contrasting biomineralisation mechanisms employed to construct each feature. CMI is rarely applied in Environmental/Earth Sciences, but here we illustrate the power of this approach to unpick the "vital effects" in D. dianthus, and by extension, other scleractinian corals.

Figure 1

D. dianthus DY081-914DD: (a) removal of septa from specimen for both bulk solution geochemistry (Stewart et al.; Kershaw et al.; this study) and CMI; (b) optical image of the surface area of DY081-914DD where CMI was performed (approximated by dashed box). The primary COC is visible running vertically down the centre of the optical image as a white band, surrounded by dark fibrous aragonite on each side. Secondary COCs consist of a mixture of COC-like white and fibre-like dark material.

Figure 2

Optical and geochemical imaging of DY081-914DD collected on a range of modalities, along with an aligned, composite image where a greyscale version of the organic content image is overlain by lines identifying locations of low pH (red), high Mg/Ca (blue), and high DIC (green) (where low and high constitutes the lower and upper 50% of values respectively). Structural component boundaries identified using the optical image are shown by bold white lines.

Figure 3

Boxplots showing the geochemical composition of the primary COC and fibrous aragonite of DY081-914DD. Results of size effect analysis (Cohen’s d) are shown in the top left corner of each panel: four stars = large d-value, three stars = medium d-value, two stars = small d-value, and one star = negligible d-value.

Figure 4

Cross plots of selected geochemical variables for DY081-914DD. All pixel data are plotted as semi-transparent black data points to give a sense of data density. Data for the COC and fibrous aragonite are contoured at confidence levels of 95%, 68%, 16%, and 2.5% (solid turquoise and dashed purple lines), respectively.

Figure 5

Rayleigh fractionation models (red dashed line) for Li, B, Mg and Sr incorporation into the aragonitic skeleton of DY081-914DD. All pixel data are plotted as semi-transparent black data points. Data for the COC and fibrous aragonite are contoured at confidence levels of 95%, 68%, 16%, and 2.5% (solid turquoise and dashed purple lines), respectively.

Figure 6

[CO₃^2–]_cf and [DIC]_cf versus pH of DY081-914DD. The trend lines show models for the evolution of the carbonate fluid as: (i) HCO₃^– is added, simulated by increasing [DIC]_cf and ALK in a 1:1 ratio, (ii) metabolic CO₂ diffuses in, increasing [DIC]_cf with alkalinity remaining constant, (iii) a reduction in the removal of protons by the CaATPase enzymatic pump, decreasing alkalinity with [DIC]_cf remaining constant, (iv) admixture of seawater into the calcifying space, and (v) precipitation of CaCO₃ that decreases alkalinity and [DIC]_cf in a 2:1 ratio. Regarding COC and fibrous aragonite, filled symbols denote mean compositions (± 1 SD) whilst open symbols are that of individual pixels.