Lead profiles in red coral skeletons as high resolution records of pollution in the Mediterranean Sea
Lead concentrations in long-lived Corallium species of known age, from the Mediterranean Sea, Atlantic and Pacific Oceans, were determined by laser ablation, inductively coupled plasma mass spectrometer (LA-ICPMS). Lead concentrations in a 2000-year-old sub-fossil Mediterranean C. rubrum are ca 0.09 ± 0.03 μg/g. For the period 1894–1955, lead concentrations in C. rubrum skeletons from the Mediterranean are stable within the range 0.2–0.4 μg/g; concentrations increase to about 1–1.2 μg/g during the period 1960–1978, then decrease progressively to stabilize and reach values in the range 0.2–0.4 μg/g in present-day corals. These variations can be related to the lead gasoline pollution event that (1) started in the early 1950s with the increase of the numbers of cars in the world, and (2) was mitigated by the implementation of new regulations starting in 1975, leading to a return to pre-1950 levels in 2000. In the Pacific, lead concentrations in C. japonicum and C. konojoi are lower than in the Mediterranean C. rubrum, with values close to 0.17 ± 0.03 μg/g. The lowest lead concentrations in present-day samples (0.11 μg/g) are found in C. johnsoni and C. niobe from the Azores islands in the Atlantic, and in a Mediterranean C. rubrum from Montecristo Island, one of the least accessible and most protected areas in the Mediterranean Sea. Using lead concentrations in C. rubrum and in the Mediterranean seawaters, a partition coefficient Kd = [Pb/Ca]calcite / [Pb/Ca]seawater of 13 ±3 is estimated; it allows calculating past and present lead contents in seawater in which corals grew. Application to Coralliumspecies indicates that values endangering human health or threatening the preservation of aquatic ecosystem on long terms were nearly reached or exceeded in Mediterranean seawaters at the maximum of the lead gasoline pollution event in the 1980s. Measurements in C. rubrum from different places in the Mediterranean indicate that present-day seawater concentrations vary between 40 and 200 pmol/kg. As expected, the lowest concentrations come from protected areas insulated from human activities, while the highest come from places close to lead mining or processing sites.