Centimetre-scale variability of redox-sensitive elements in Tremadocian black shales from the eastern Baltic Palaeobasin.

The high-resolution study of vertical geochemical variability of shallow-water Tremadocian black shales of the Türisalu Formation targeted two drill core sections from Suur-Pakri Island, NW Estonia. Altogether 374 samples from 4.6 m thick shale were analysed by XRF. The metalliferous and organic-rich black shales revealed significant centimetre-scale variation in the concentration of redox-sensitive trace metals--U, Mo and V. The V profiles show cyclic variations in half a metre- to metre-scale and the strongest correlation with loss on ignition (LOI) 500 °C matter abundance). The abundance of Mo presents high values near the lower and upper contacts of black shale and otherwise moderate covariance with LOI. The distribution of U is not coupled with LOI, being characterized by irregular local enrichment anomalies in the profiles of both sections. This suggests that sequestration of U may have been time-dependent and possibly favoured by dissimilatory U-reduction at the sediment-water interface under iron-reducing conditions. Significant depositional variability of the studied organic-rich muds apparently supported dynamic physicochemical and biological microenvironments at the sediment-water interface and thus temporally and spatially diversified the paths and efficiency of synsedimentary redox-sensitive trace element enrichment. [ABSTRACT FROM AUTHOR]