Response of Early Eocene deep-water benthic foraminifera to volcanic ash falls in the Polish Outer Carpathians: Palaeocological implications

Lower Eocene sediments of the Subsilesian Unit of the Polish Carpathians contain bentonite layers that were deposited in a deep-water basinal environment. Foraminiferal assemblages occurring within the bentonites and in the surrounding shales show important differences in taxonomical composition, the proportions of ecological groups, and test composition. Foraminiferal assemblages collected from bentonites are impoverished, display smaller test size, are dominated by a single opportunistic taxon, and have lower numbers of suspension feeders in comparison with assemblages surrounding sediments. In addition, bentonite assemblages show a test composition comprised of white fine-grained quartz, and include a greater number of juvenile forms with test abnormalities. The extent of such discrepancies is variable and depends on the thickness of the bentonite layers. The deposition of volcanic ash layers (bentonites) represented catastrophic events for benthic foraminifera. After each ash fall event a kill layer was formed, and settled by an opportunistic foraminiferal group dominated by specimens of the epifaunal Glomospira charoides, leading to the formation of nearly monospecific foraminiferal assemblages. In the case of thin ash layers some infaunal species are found. These organisms probably survived the deposition of volcanic ash and associated unfavourable conditions beneath the surficial sediment layer. The studied sections of the Subsilesian Unit include intervals containing several bentonite layers, indicating that the deposition of volcanic ash was a recurrent phenomenon. Following the deposition of volcanic ash, a relatively quick recovery of the foraminiferal community structure took place, and changes in the foraminiferal community proceeded each time according to the same sequence.