Baltic Mn/Fe precipitates:a record of 4300 years of natural and anthropogenic Pb isotope variations

Ferromanganese precipitates are widespread phenomena of the Baltic Sea, predominantly caused by seasonal and decadal variations of the redox conditions within the water column and the sediments. ²²⁶Ra_excess/Ba-dated Mn/Fe-precipitates of the Mecklenburg Bay (20 m water depth) indicate that their growth started at about 4300 years BP. Confirmation of the ²²⁶Ra_excess/Ba-ages is provided by high resolution electron-microprobe (EMP) multi-element analyses. Annual growth cycles are deduced from the seasonal changes of the Mn/Fe ratios which allowed the calculation of growth rates (about 12 um/year) that are in agreement with the results from the ²²⁶Ra_excess/Ba chronometer. In an attempt to reconstruct the long-term heavy metal input into the Baltic Sea we measured Pb concentrations and Pb isotope ratios of a selected Mn/Fe-precipitate from the Mecklenburg Bay by TIMS and by Laser-MC-ICP-MS. During the time interval from 4300 to 2800 years BP the ²⁰⁷Pb/²⁰⁶Pb ratio remained almost constant at a value of 0.78 similar to the average ²⁰⁷Pb/²⁰⁶Pb ratio of the Archean Scandinavian rocks (0.74 to 0.77). Between 2800 and 1800 years BP a significant increase of the ²⁰⁷Pb/²⁰⁶Pb ratio to values close to 0.82 is observed. This time period corresponds to the beginning of human mining activities in Europe and the Middle East. After that period the ²⁰⁷Pb/²⁰⁶Pb ratio slightly decreases to values ranging from 0.81 to 0.82. Between 1000 and 800 years BP ²⁰⁷Pb/²⁰⁶Pb ratios sharply increase to a value around 0.835. This maximum is followed by decreasing ratios to a value of 0.81. About 200 years BP corresponding to the onset of the industrial revolution in Europe ²⁰⁷Pb/²⁰⁶Pb values increase to a maximum present day value of 0.84. Our ²⁰⁷Pb/²⁰⁶Pb record is in chronological accord with other ²⁰⁷Pb/²⁰⁶Pb records from independent archives like peat-bogs and ice cores. In addition, this study adds support to the reliability of the ²²⁶Ra_excess/Ba chronometer. Furthermore, this indicates that the Baltic Mn/Fe precipitates are excellent archives for post-glacial and anthropogenic changes in the Baltic Sea and the circum Baltic area.