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.
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.
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.
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.