Trace metal fluxes to ferromanganese nodules from the western Baltic Sea as a record for long-term environmental changes

Trace element profiles in ferromanganese nodules from the western Baltic Sea were analyzed with laser ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS) and synchrotron-based micro-X-ray radiation techniques (fluorescence: μSXRF and diffraction: μXRD) at high spatial resolution in growth direction. Zn showed the only significant enrichment of the trace elements studied (Zn, Cu, Cd, Ni, Co, Mo and Ba), with values in the outermost surface layers of up to sixfold higher than those found in older core parts. The high-resolution Zn profiles provide the necessary temporal resolution for a dating method analogous to dendrochronology. Profiles in various samples collected for two decades were matched, and the overlapping sections were used for an estimation of the accretion rates. Assuming a continuous accretion of these relatively fast-growing nodules (on the average of 20 μm year−1 over the last century), the Zn enrichment was assessed to have commenced around 1860/1870 in nodules from the Kiel Bight (BG) and in 1880/1890 in those from Mecklenburg Bight (MB). Apart from the obvious success with Zn, other anthropogenic trace metals like Cu and Cd are not enriched at all, which, together with the distinct early diagenetic Fe/Mn banding, weaken the potential of the nodules for retrospective monitoring.