ISOTOPIC RECORD OF ANCIENT AND PRE-HISTORIC LEAD POLLUTION IN LAGUNA ROYA LAKE SEDIMENTS, NORTHWESTERN SPAIN

This study presents a 4,000 year lake sediment record of atmospheric lead deposition from Laguna Roya (42° 8'8.42"N, 6°47'38.95”E), a small glacial lake located in the Sierra Segundera of Northwestern Spain. An increase in lead concentrations, and a decrease in ²⁰⁶Pb/²⁰⁷Pb ratios, after 900BC in the sediment record. This is coincident with archeological evidence that suggest the onset of the Iron Age in the region at this time. An abrupt acceleration of atmospheric lead deposition, accompanied by a dip in ²⁰⁶Pb/²⁰⁷Pb ratios occurs shortly after the Roman conquest of the region in 29BC. These shifts reach a peak at ~130AD. After the fall of the Roman Empire, the anthropogenic lead deposition input decreases but remains relatively constant, with depleted ²⁰⁶Pb/²⁰⁷Pb ratios compared to the background signature. Another lead deposition peak of interest occurs in the late middle ages ~1000AD. The timing of this increased lead input is inconsistent with existing local archeological and historical mining records but coincides with the findings of similar multiproxy studies in the region. Source appointment of atmospheric pollution will be estimated using isotopic fingerprinting techniques using ratios of lead’s four stable isotopes ²⁰⁴Pb, ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸ Pb. Ratios of these four lead isotopic compositions measured in the Laguna Roya sediment samples, will be compared with known lead isotope values from ore bodies of ancient mining sites throughout Spain and Europe. It is often difficult to determine accurate timing, or intensity of mining and metallurgical activities due to lack of archeological evidence. Lake sediments, and isotopic fingerprinting, in such cases, can serve as valuable archives through which past regional environmental conditions can be elucidated.