Mediterranean underwater volcano unearths Australian lead
August 2006, CIESM News
Submarine hydrothermal vents form an unique marine environment. Even on such remote structures one can find traces of man’s impact. An international team of geologists (Vesselin M. Dekov, George D. Kamenov, Carlo Savelli and Jens Stummeyer) examined deposits from a site of an underwater volcano located about 100 km off the south-west Italian coast. They discovered that lead, one of the key components, did not come from the underlying rock nor from anywhere close by, but originated from New South Wales, Australia. Their study is published in the June issue of Marine Geology * .
The volcanic Marsili seamount ** rises some 3000 m above the abyssal plain in the south-eastern Tyrrhenian Sea. On its highest peak (~500 m deep), an extensive area of hydrothermal deposition has been explored via a remote TV survey and submersible observations.
In hydrothermal deposits metals like iron, lead, zinc and copper as well as several trace elements were found. Seawater seeps through cracks into the deep ocean floor, reaches high-temperature areas or magma and gets heated. Usually, high pressure inhibits the water to evaporate, keeps it fluid and allows to ascent again while extracting and dissolving various components from the volcanic surrounding. Once the heated and loaded water reaches again the seafloor, dissolved metals and trace elements precipitate.
In this case, the results of the analyses yielded a surprise: the lead (Pb) isotopic compositions did neither correspond to the underlying volcanic rocks nor match any other natural sources in the region. The measurements conducted on the Marsili ochre samples indicate that the lead is predominantly of anthropogenic origin, excluding European ore deposits as the source. Furthermore, the mineralogical and geochemical composition of these samples suggest a rapid precipitation from hydrothermal solutions of only low temperatures.
So, where does the lead in the Marsili deposits come from? Their isotopic signature, a relatively low 206Pb/ 207Pb ratio, reflects an incorporation of Pb from Australian ores and is, as Dr Kamenov suggests, typical for Broken Hill (New South Wales). This leads to the conclusion that the deposits were formed only within the last ~150 years, i.e. since the onset of the industrial revolution around 1850 AD, when Australian lead started being imported to Europe.
Today, the “Broken Hill” no longer exists: the orebody, the largest silver-lead-zinc mineral deposit in the world, has been mined away. “Lead was once mined there, then shipped to Europe and added to gasoline, burned in cars and subsequently released in the atmosphere”, explains Dr Kamenov as the most likely scenario of how the Australian lead reached the top of the Marsili Seamount. He continues: “From the atmosphere it ended in the Mediterranean and became sort of "recycled" in the Marsili seamount”. "Recycled" because the lead was hydrothermally precipitated at Broken Hill deposit around 1.6 billion years ago, and now part of this Pb is again precipitated in a modern hydrothermal system.
* Vesselin M. Dekov, George D. Kamenov, Carlo Savelli and Jens Stummeyer (2006): Anthropogenic Pb component in hydrothermal ochres from Marsili Seamount (Tyrrhenian Sea). Marine Geology, 229, 199-208.
** The Marsili seamount was also mapped during the CIESM SUB1 cruise. On its south-western side, the geophysical group recorded high resolution bathymetric and seismostratigraphic data. Sediment samples were collected at the basis of the Marsili seamount for analyses of biodiversity.