Abstract
AbstractThe stratigraphic variability and geochemistry of Llandovery/Wenlock (L/W) Series boundary sediments in Poland reveals that hemipelagic sedimentation under an anoxic/euxinic water column was interrupted by low-density bottom currents or detached diluted turbid layers that resulted in intermittent seafloor oxygenation. Total organic carbon values and inorganic proxies throughout the Wilków 1 borehole section suggest variable redox conditions. U/Mo ratios > 1 throughout much of the Aeronian and Telychian stages, together with an absence of pyrite framboids, suggest oxygenated conditions prevailed. However, elevated total organic carbon near the Aeronian/Telychian boundary, together with increased U/Th and V/(V + Ni) ratios and populations of small pyrite framboids are consistent with the development of dysoxic/anoxic conditions at that time. U/Th, V/Cr and V/(V + Ni) ratios, as well as Uauthigand Mo concentrations, suggest that during the Ireviken black shale deposition, bottom-water conditions deteriorated from oxic during Telychian time to mostly suboxic/anoxic immediately prior to the L/W boundary, before a brief reoxygenation at the end of the Ireviken black shale sedimentation in the Sheinwoodian Stage. Rapid fluctuations in U/Mo during the Ireviken Event are characteristic of fluctuating redox conditions that culminated in an anoxic/euxinic seafloor in Sheinwoodian time. Following Ireviken black shale deposition, conditions once again became oxygen deficient with the development of a euxinic zone in the water column. The Aeronian to Sheinwoodian deep-water redox history was unstable, and rapid fluctuations of the chemocline across the L/W Series boundary probably contributed to the Ireviken Event extinctions, which affected mainly pelagic and hemipelagic fauna.
Highlights
The Ireviken Event (IE) records a little-studied, but important Silurian extinction at the Llandovery/Wenlock (L/W) series boundary (Telychian/Sheinwoodian Stage boundary) (e.g.Calner, 2008)
Due to the relatively high thermal maturation of the sediments, our research has focused on redox proxies that are not susceptible to alteration by heat, such as pyrite framboid petrography and trace metal concentrations
The older part of the succession consists largely of grey to dark grey and subordinate black shales that belong to two informal lithostratigraphic units (Fig. 2)
Summary
The Ireviken Event (IE) records a little-studied, but important Silurian extinction at the Llandovery/Wenlock (L/W) series boundary (Telychian/Sheinwoodian Stage boundary) (e.g.Calner, 2008). Munnecke et al, (2003) suggested that the anoxia responsible for the extinction losses originated in the deep oceans, before invading the shallower shelf seas. The reconstruction of palaeoenvironments during the IE has been based on stable carbon and oxygen isotope studies (Munnecke et al, 2003; Cramer & Saltzman, 2005, 2007; Noble et al, 2005; Loydell & Fryda, 2007; Lehnert et al, 2010; Vandenbroucke et al., 2013), but interpretations have not always been consistent with other sedimentological and geochemical observations Major discrepancies exist in proposed sea-level histories (see compilations in Munnecke et al, 2010; Melchin et al, 2012) and in the origins of redox changes (Jeppsson, 1990; Bickert et al, 1997; Page et al, 2007)
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