Raexcess/Ba growth rates and U-Th-Ra-Ba systematic of Baltic Mn/Fe crusts

We analyzed ²³⁸U, ²³⁴U, ²³²Th, ²³⁰Th, and ²²⁶Ra by thermal ionization mass spectrometry (TIMS) and Ba by inductively coupled plasma optical emission spectrometry (ICP-OES) on eight Mn/Fe crusts from the Mecklenburg Bay (SW Baltic) and on one from the Bothnian Bay (N Baltic) to test the ²²⁶Ra_ex/Ba ratio as potential geochronometer. ²²⁶Ra_ex/Ba ratios decrease as a function of depth within the concretions in all analyzed profiles. Calculated diffusion coefficients are relatively low (∼9 · 10⁻⁷ cm²/yr for Ra and 5 · 10⁻⁷ cm²/yr for Ba) and suggest that diffusion is negligible for the Ra and Ba record. In addition, ²²⁶Ra_ex/Ba ages are consistent and independent from the growth rate and growth direction within a crust. Thus, the decline in ²²⁶Ra_ex/Ba ratio is most likely due to radioactive decay of ²²⁶Ra_ex, although the influence of varying oxic conditions has still to be evaluated. ²²⁶Ra_ex/Ba growth rates range from 0.021 to 0.0017 mm/yr and tend to be lower than those calculated and based on stratigraphic methods (1 to 0.013 mm/yr). ²²⁶Ra_ex/Ba ages of concretions from shallow water environment (20 m depth, Mecklenburg Bay/SW Baltic) cover a time interval from 990 ± 140 yr to 4310 ± 310 yr BP corresponding to the stabilization of the sea level close to the present position about 5500 to 4500 yr ago. One sample from greater depth (70 m, Bothnian Bay-/N Baltic) showed a higher ²²⁶Ra_ex/Ba age of 6460 ± 520 yr BP.