Explaining Trace Metal and Metal Isotope Variability in Coral

Explaining Trace Metal and Metal Isotope Variability in Coral

Models of coral biomineralization usually invoke a privileged space closed to the external environment, the calcifying fluid. Despite decades of microscopy and other approaches, the basic structure of the calcifying environment is still largely unknown. The presence or absence of a closed calcifying space can be tested using multiple compositional tracers. Indeed, we find that Metal/Calcium (Me/Ca) tracers follow closed-system behavior during skeletal growth, as measured in deep-sea coral.  This closed-system model of coral biomineralization can be used to explain non-environemntal tracer variability in coral, can be used to improve paleoproxies, and can quantify how coral control calcifying fluid pH in response to ocean acidification.  Ongoing work measuring calcium isotopes in deep-sea coral indicates that the closed-system model can also explain the behavior of a wide range of non-traditional stable isotope tracers in coral.

Deep-sea coral are an ideal system to study the complex process of biomineralization. In many locations the deep-sea acts as a “culture medium” of virtually constant composition over the ~100 year lifetime of a coral. In these locations there is essentially no environmental variability.  Thus, compositional variability in these coral can be attributed entirely to the process of biomineralization. This is the signal we quantify and characterize as part of a geochmical approach to biomineralization.

Figure 1. Morphology and sampling of the solitary deep-sea coral Desmophyllum dianthus.  (a&b) To clearly separate different skeletal regions the coral is cut in a pie shape wedge and a thin section is made containing several septa.  (c) Transmitted light photomicrograph of a portion of an exert septa shows the opaque central band filled with disorganized crystals corresponding to the centers of calcification (COCs). The rest of the septa is composed of organized bouquets of c-axis aligned crystals (Gagnon et al., EPSL 2007).

Our results show that tracer behavior strongly follows skeletal architecture (Figures 1&2.). Centers of calcification (COCs) are small regions of disorganized crystals thought to be the initial stage of skeletal extension. Within the COCs, unlike the rest of the skeleton, Mg/Ca ratios vary more than two fold while Sr/Ca is near constant.

Figure 2. Mg/Ca doubles in the ~100 µm wide central band, composed of COCs, compared to the surrounding septal material. Background image is a negative transmitted light photomicrograph of the sampled septal thin section with vertical dashed lines designating separate micromilled regions subsequently analyzed by isotope-dilution ICP-MS. Analytical error bars are smaller than line thickness (Gagnon et al., EPSL 2007).

Consistent with closed-system behavior during precipitation, Mg/Ca increases with decreasing Sr/Ca in the skeleton outside of the COCs (Figure 3). A process other than closed-system behavior dominates Mg/Ca behavior in the COCs and our data provide new constraints on a number of possible mechanisms for precipitation of this aragonite.

Figure 3. Inversely correlated Sr/Ca and Mg/Ca are consistent with closed system (Rayleigh) behavior during precipitation of outer septa material (black squares), but not precipitation of the central band (open squares) (Gagnon et al., EPSL 2007).  The pattern of increasing Mg/Ca with decreasing Sr/Ca has been confirmed in several other modern individuals of Desmophyllum dianthus since publication of the original paper.

Read more: Gagnon, Adkins, Fernandez, and Robinson. (2007).  Sr/Ca and Mg/Ca vital effects correlated with skeletal architecture in a scleractinian deep-sea coral and the role of Rayleigh fractionation.  Earth and Planetary Science Letters.