Geochemical heterogeneity of Estonian graptolite argillite

This paper describes vertical fine-scale geochemical heterogeneity of Estonian graptolite argillite (GA). GA samples from Pakri and Saka outcrop sections were collected at 20 cm intervals for chemical analysis of major and trace elements, including rare earth elements. The study indicates GA enrichment in U, V, Mo and Pb with respect to the average black shales and thus confirms the formerly reported data on GA geochemistry in general. However, the content of enriched elements and other trace metals was recorded to vary greatly across the sequences suggesting that trace metal distribution in GA is notably more heterogeneous than previously assumed. The origin of the observed complex distribution of trace elements was likely controlled by the interplay of different primary metal supply-sequestration factors/processes, such as synsedimentary redox-driven sequestration of redox sensitive elements, the provenance of clastic input, the postsedimentary redistribution, etc.