Porifera-microbialites of the Lower Liassic (Northern Calcareous ...

Geochemical Analyses 87
8. Geochemical Analyses
The early Jurassic sponge communities on the top of sunken late Triassic reefs represent the first
recovery of a complex metazoan benthic community. Therefore it is important to reconstruct the
paleoenvironmental conditions. Since sequences at or close to the T-J boundary are scarce in the
world, the here presented data provide additional information close to the critical interval and its
aftermath. Like at Adnet locality and Steinplatte buildup slope, most spiculites are characterized
by low-diverse faunas and reduced sedimentary input consolidated by microbialites that formed
during early diagenetic taphonomic processes. Thus, their composition displays ancient
structures already known from ancestral communities, especially from Paleozoic mud mounds
some of which also show the onset of benthic associations following mass extinction events like
it is the case at Devonian mud mounds of the Australian Canning Basin (Playford 1980;
Stephens and Sumner 2003). Since the causes for the T-J event are not firmed and recovery
strategies are badly understood so far, geochemical proxies are one important possibility to
evaluate probable influences on sponge settlement e.g. by cold seeps or by hydrothermal fluids.
The origin of some Liassic alpine ferromanganese crusts was already discussed as being either
hydrologically or hydrothermally controlled (Böhm et al. 1999), so that the finding of Fe/Mn
precipitates also at Steinplatte locality led to further studies of the crust’s composition. Hereby
measurements of stable isotopes and element analyses were additionally correlated with those of
corresponding sediments, because all crusts are formed by alternating sedimentary and
ferromanganese laminae that both are diagenetically overprinted (Germann 1971; Drittenbass
1979; Fels 1995). Liassic crusts were sampled at 4 localities in the Osterhorn block (part of the
Tyrolic Nappe, SE of Salzburg). The Upper Hettangian/Lower Sinemurian marmorea crust from
Adnet reef slope was sampled at the Lienbacher Quarry (locality S3-a) and the Rot-Grau-Schnöll
Quarry (locality S3-b) both situated NE of Adnet near Hallein. Another crust that formed in
Lower Sinemurian time was also sampled at Adnet / Lienbacher Quarry (locality S3-a). Further
samples encompasse a Hettangian crust from Luegwinkel (locality S3-c) situated SE of Golling,
about 150 m NNE of the road to Paß Lueg, and a Pliensbachian/Toarcian crust from Tannhauser
Berg (locality S3-d, SSW of Golling, at the northern slope of the Hagengebirge).
8.1. Stable Isotopes
The record of stable isotopes (C, O) is one of the most important proxy to understand the former
e.g. CO2 and CH4 conditions. The T-J boundary is one of the most prominent extinction event
within the Phanerozoic and this critical interval is geochemically characterized by a slight
negative excursion of the delta 13 C both in carbonates and in organic matter (Morante and