Iron speciation and annual records in black coral as new proxy for mining and environmental impacts

Highlights: • Iron can be efficiently accumulated in black coral in marine environments. • The major tris DOPA-Fe structure in black coral results in its strong stability. • Fe profile in black coral recorded the history of anthropogenic activities. • Profile of Fe in black coral may serve as a potential Fe proxy for marine system. Iron (Fe) is a micronutrient and plays an important role in regulating ocean primary production and consequently changing oceanic CO₂ uptake. However, approaches for high-resolution of Fe records in marine environment has been a great challenge. In this study, we report for the first time an annual Fe record on black coral organic skeleton from the northern South China Sea (SCS) as an archive to study the environmental change during the past century. In situ micro-Raman, synchrotron micro X-ray absorption near edge spectroscopy (μ-XANES) and synchrotron micro X-ray fluorescence (μ-XRF) were applied to investigate the Fe speciation and the radial Fe profile in black coral. The preliminary results from micro-Raman and synchrotron micro XANES analysis demonstrated that Fe in black coral was mainly combined with 3,4-dihydroxyphenylalanine (dopa) as tris-DOPA-Fe complex. Such spatial coordination structure of complexation makes Fe have high affinity with dopa in black coral. Furthermore, elevated Fe concentration in Fe profile recorded on synchrotron μ-XRF spectra with 2.5 μm resolution corresponded well to the exploitation history of the adjacent onshore Tiandu Iron Mine (Sanya, China) from 1939 to 1960. Other distinct Fe peak coincides with the war activities in 1970s. The findings presented in this work indicate that the high-resolution iron record with low annual growth rate (~17.8 μm year⁻¹) of black coral may serve as a proxy of marine environmental record.