Oceanic Anoxic Event 1d (late Albian) in deep-water sediments of the Outer Carpathians, Poland; Carbon isotope and agglutinated foraminiferal records

The Albian–Cenomanian transition is stratigraphically still poorly constrained in deep-water environments below the CCD. For this reason, the recognition of the OAE1d in such sedimentary records is extremely rare. Our high-resolution carbon-isotope (δ¹³C_org) stratigraphy of the Upper Albian and Lower Cenomanian turbidite/hemipelagic succession, accumulated in the marginal Silesian Basin of the Western Tethys, made it possible to identify the interval corresponding to the OAE1d. It has been recognized within two lithostratigraphic units of the Silesian Nappe of the Outer Carpathians (the Lower and Middle Lgota Beds), which are composed mostly of turbidite sediments containing a large amount of bioclastic material occurring in the silty and sandy fraction (locally over 70%). Bioclasts were redeposited from marginal shelf of the European Platform. The hemipelagic non-calcareous claystones which separate the turbidite sequences contain deep-water agglutinated foraminiferal (DWAF) assemblages, and are devoid of calcareous benthic foraminifers.

Using the analysis of the DWAF morphogroups, as well as changes in the benthos abundance and its taxonomic composition in relation to the characteristics (colour and TOC content) of hemipelagic sediments, we indicated changes in the environmental conditions that took place during the OAE1d at the bottom of the Silesian Basin. The most abundant horizons of organic-rich shales are characteristic of the lower part of the OAE1d succession corresponding to the Pialli Level from the Umbria-Marche Basin, although thin intercalations of black shales are also present along the upper part of this succession, where the hemipelagic sediments are dominated by green-coloured shales. The variability of organic matter in the studied sediments only slightly correlates with the abundance of the DWAFs and with their taxonomic composition. The more visible features in the latest Albian agglutinated benthos concern relative proportions of foraminiferal morphogroups which correspond to life-style and feeding strategies, and in this way reflect changes in selected environmental parameters. It seems that fluctuations in the morphogroup distribution along the OAE1d succession reflects the influence of two groups of factors: (i) oxygen concentration in bottom waters (low in the older part of the OAE1d, with fluctuations in the younger part of this isotope event), and (ii) the organic carbon flux that was linked to the onset of a massive redeposition of biogenic material from the European shelf. The last factor is related to the sea level fall during the 3-rd order regressive cycle.