Bridging the Faraoni and Selli oceanic anoxic events: late Hauterivian to early Aptian dysaerobic to anaerobic phases in the Tethys

V. Matera,D. Fleitmann,S. Bodin,J. E. Spangenberg,N. Jammet,A. Godet,P. Froidevaux,K. B. Föllmi,T. Adatte,M. Bôle

doi:10.5194/cp-8-171-2012

Abstract

Abstract. A detailed geochemical analysis was performed on the upper part of the Maiolica Formation in the Breggia (southern Switzerland) and Capriolo sections (northern Italy). The analysed sediments consist of well-bedded, partly siliceous, pelagic carbonate, which lodges numerous thin, dark and organic-rich layers. Stable-isotope, phosphorus, organic-carbon and a suite of redox-sensitive trace-element contents (RSTE: Mo, U, Co, V and As) were measured. The RSTE pattern and Corg:Ptot ratios indicate that most organic-rich layers were deposited under dysaerobic rather than anaerobic conditions and that latter conditions were likely restricted to short intervals in the latest Hauterivian, the early Barremian and the pre-Selli early Aptian. Correlations are both possible with organic-rich intervals in central Italy (the Gorgo a Cerbara section) and the Boreal Lower Saxony Basin, as well as with the facies and drowning pattern in the Helvetic segment of the northern Tethyan carbonate platform. Our data and correlations suggest that the latest Hauterivian witnessed the progressive installation of dysaerobic conditions in the Tethys, which went along with the onset in sediment condensation, phosphogenesis and platform drowning on the northern Tethyan margin, and which culminated in the Faraoni anoxic episode. This episode is followed by further episodes of dysaerobic conditions in the Tethys and the Lower Saxony Basin, which became more frequent and progressively stronger in the late early Barremian. Platform drowning persisted and did not halt before the latest early Barremian. The late Barremian witnessed diminishing frequencies and intensities in dysaerobic conditions, which went along with the progressive installation of the Urgonian carbonate platform. Near the Barremian-Aptian boundary, the increasing density in dysaerobic episodes in the Tethyan and Lower Saxony Basins is paralleled by a change towards heterozoan carbonate production on the northern Tethyan shelf. The following return to more oxygenated conditions is correlated with the second phase of Urgonian platform growth and the period immediately preceding and corresponding to the Selli anoxic episode is characterised by renewed platform drowning and the change to heterozoan carbonate production. Changes towards more humid climate conditions were the likely cause for the repetitive installation of dys- to anaerobic conditions in the Tethyan and Boreal basins and the accompanying changes in the evolution of the carbonate platform towards heterozoan carbonate-producing ecosystems and platform drowning.

Highlights

The Early and early Late Cretaceous represents a time interval of considerable paleoenvironmental change, which found its expression in the repeated installation of widespread dysto anaerobic conditions in outer-shelf and basinal settings (Schlanger and Jenkyns, 1976; Jenkyns, 1980; Weissert and Erba, 2004)
We present new insights on the time interval spanning the late Hauterivian and earliest Aptian based on data from the Breggia and Capriolo sections in southern Switzerland and northern Italy, respectively (Fig. 1)
The highest total organic carbon (TOC) values in the Breggia section are registered in two layers below the Hauterivian-Barremian boundary

Summary

Introduction

The Early and early Late Cretaceous represents a time interval of considerable paleoenvironmental change, which found its expression in the repeated installation of widespread dysto anaerobic conditions in outer-shelf and basinal settings (Schlanger and Jenkyns, 1976; Jenkyns, 1980; Weissert and Erba, 2004). One of the oldest “oceanic anoxic episodes” (OAE) of the Cretaceous dates from the latest Hauterivian and is known as the “Faraoni event” (Cecca et al, 1994). This episode was originally identified in the central Italian Apennines, where it is preserved in the form of a welldistinguishable interval of thin and closely spaced organicrich mudstone layers in pelagic carbonate (Cecca et al, 1994; Coccioni et al, 1998, 2006; Baudin et al, 2002; Baudin, 2005). Evidence for the presence of a Faraoni equivalent was not excluded for the Argentinean Neuquen Basin (Tyson et al, 2005)

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