Deciphering the upper Famennian Hangenberg Black Shale depositional environments based on multi-proxy record

Abstract

Presented for the first time in this paper are the results of a detailed multi-proxy investigation conducted on the Hangenberg Black Shale (HBS) from the Polish part of the Laurussian Shelf, which provide details about the environmental conditions in existence during deposition of the shale and the role played by anoxic conditions in the mass extinction events that occurred at the end of the Devonian times. Inorganic and organic redox indicators indicate that bottom water redox conditions changed periodically from being mainly anoxic/euxinic to oxic or being partially depleted in oxygen. U/Th values above 1.25, Ni/Co values above 7 and V/(V+Ni) values above 0.8 recorded from the lower part of the HBS all point to anoxic/euxinic conditions being present, as do high total organic carbon contents (TOC) above 10% and degree of pyritisation (DOP) values around 0.75%. However, the presence of benthic fauna over the lower part of the HBS attests to opportunistic colonisation of the seafloor during oxic episodes. Evidence for similar episodes has also been recognised over the middle part of the HBS, e.g., U/Th values below 1.25, Ni/Co values below 4, V/(V+Ni) values below 0.8, TOC values of 3% to 5.5% and DOP values of 0.4% to 0.75%, plus the common occurrence of benthic fauna. Anoxic conditions returned during the deposition of the upper part of the HSB, though they were not as well developed as when the lower part of the shale was laid down.Of interest is the presence of small pyrite framboids and isorenieratene biomarkers in all the analysed samples, which suggest that euxinic conditions persisted in the photic zone of the water column. A twenty centimetre thick layer of volcanogenic deposits (tuffites) is present in the middle part of the HBS, the geochemical characteristics of which are typical of ocean floor basalts. The occurrence of volcanogenic material below and within the HBS, together with an absence of calcium carbonate over its upper part, the presence of abundant tetrads just above the black shale and a drastic decrease in faunal frequency in the upper part of the HBS imply that volcanism may have caused oceanic acidification (or hypercapnia) that in turn potentially may have influenced the Hangenberg mass extinction event.