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
C-org:P-tot 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.