New evidence for NE Atlantic pre-PETM volcanism (original) (raw)

Rendiconti online della Società Geologica Italiana, 2014

Abstract

ABSTRACT A recently exposed section on the Fur island in Denmark spans the base of the Paleocene-Eocene Thermal Maximum (PETM). The section represents an offshore facies of the North Sea Basin. The base of the CIE of the PETM is established in the section by carbon isotope analysis of TOC and of a specific biomarker for marine Thaumarchaeota (Schoon et al., 2013). Several lithologic features in this section indicate major environmental change coinciding with the beginning of the PETM. The most important of these features are two quadruple volcanic ash layers occurring immediately below a thin density flow deposit (see below). The lowest sample showing the δ 13 C excursion is taken just above this density flow deposit. The total thickness of these pre-PETM ash layers is ca.10 cm and the ash layers must therefore represent an important episode of NE Atlantic Paleogene volcanism. They are followed by two thin ash layers occurring a few centimeters above the base of the CIE. The pre-PETM ash layers are older than the previously known, and well studied, ash layers of the NE Atlantic volcanism, the so-called positive and negative numbered ash series of Bøggild (1918). The main part of the Bøggild ash series occurs in post-PETM sediments (Pedersen et al., 2011). No other ash layers are known from the underlying Upper Paleocene sediments (Østerrende Clay and Holmehus Formation) in this or other previously studied Danish sections. The occurrence of ash layers prior to the onset of the CIE lends support to previous suggestions that NE Atlantic volcanism was a trigger of the global warming of the PETM (e.g. Svensen et al., 2004). In the Fur section, sea-floor conditions changed from oxic (indicated by bioturbation and absence of lamination) to anoxic (indicated by a fine lamination) immediately above the upper quadruple ash layer, i.e., within sample resolution simultaneous with the start of the CIE. Previous observations suggest a similar shift to anoxic bottom waters in the offshore settings of the entire North Sea Basin at the beginning of the PETM (Refe). Organic geochemistry of the Fur section indicates that the euxinia at this site reached the photic zone during the entire PETM (Schoon et al., in review). The lowermost 5 cm of the PETM section is a small density flow deposit witnessing transient sea-floor instability right at the onset of the PETM. The density flow may have been triggered by an increased sediment load, as there is evidence for an increased sedimentation rate during the PETM in the Danish Subbasin (CH-C unpublished data). This is the only observation of a density flow in Danish onshore sections of the entire Upper Paleocene and Lower Eocene. It is noteworthy that sea-floor instability has previously been shown to coincide with the beginning of the PETM in the western North Atlantic (Katz et al., 1999). The local nature of the density flow in the Fur section may be due to a local sloping of the seafloor caused by movement in one of the nearby salt structures. The dinoflagellate genus Apectodinium was probably favoured by warmth and high nutrient levels, and showed a global abundance peak in neritic environments during the PETM (Crouch et al., 2001). Apectodinium increases in the Fur section from less than 2% below, to ca. 50% within the density flow and in the overlying anoxic clay of the CIE (Stolleklint Clay of the Ølst Formation). In summary, the new Fur section shows ash layers of a significant volcanic episode immediately below sediments from the PETM, supporting NE Atlantic volcanism as a trigger of the PETM. The base of the PETM is a local density flow deposit, which was probably caused by an increased sedimentation rate during the PETM. The bottom waters became anoxic and in the surface waters a bloom of Apectodinium began at the start of the PETM.

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