Complex sentence: Dioxins have been repeatedly detected in clays from Europe and USA since 1996, with contamination of clay assumed to be the result of ancient forest fires or similar natural events with concentration of the PCDD/F during clay sedimentation.
Split into simpler sentences: Dioxins have been repeatedly detected in clays from Europe and USA since 1996. Contamination of clay is assumed to be the result of ancient forest fires or similar natural reasons, and enrichment during clay sedimentation.