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

Geochemical Analyses 97
>28 to >56 which points to either low or absence of contamination by terrigenous matter,
whereas the Y/Ho ratio in own samples show values of 0.87-1.67.
Disregarding possible shifts by the terrigenous input, analyses of REE were first of all carried
out on crusts to evaluate their kind of formation in comparison to the “normal” sediments below
and above. Except for one, all crust samples show distinct positive Ce-anomalies and minor
positiv Gd-anomalies. In the assumption of their hydrogenic origin the pattern indicates normal
marine-oxic conditions (sensu Neuweiler and Bernoulli 2005). Because of the high amount of
Fe- and Mn-oxihydroxides (mean values of Fe range about 30% in the crusts, in contrast to
0.5-1.5% in adjacent sediments), that usually absorb Ce in its oxic state (Ce 4+ ) the crusts should
have served as a sink for cerium. Only the REE patterns at Plattenkogel hill exhibit strikingly a
different pattern by showing a significant negative Ce-anomaly. Apparently the signal is not
caused by contamination with intercalated sediment, because the negative anomaly was also
captured by the associated spiculite matrix as well as by the “Frutexites” structures protruding
from the crusts surface (laser ICP-MS, supplement to Fig. 46). At first glance, the occurrence of
the anomaly in different horizons suggests a postsedimentary influence, whereas its Y/Ho-ratio
(~1.67) and Zr/Hf-ratio (~40) does not really correspond to values of hydrothermal origin (Bau
1996). Otherwise hydrothermal systems could produce positive Eu-anomalies at low
Y enrichments (Bau and Dulski 1999, Hongo and Nozaki 2001) as indicated in the pattern of the
Plattenkogel hill crust. Furthermore the “Frutexites” structures in the lumachelle layer as well as
manganese impregnations in the clastic sequence imply special redox conditions, possibly to be
explained by fluids leaking through cracks or fissures of the subjacent reef body in early Liassic
time. High barium values (presumably fixed by biofilms), relatively low Ni+Cu+Co values, and
the low Fe/Mn-ratio support this assumption (Bonatti et al. 1972). At Steinplatte locality the
crusts only occur locally and do not extend over long distances, thus scavenging over long
periods can be excluded. If oxidized Ce, as assumed by Moffett (1990), is obviously microbially
mediated, then negative values could also be the result of elevated sedimentation rate or
accelerated precipitation rate due to syngenetic hydrothermal fluids or bacterial mats (Bau et al.
1996).
8.3. Biomarkers
Autochthonous micrites (microbialites) are assumed to be strongly involved in the calcification
of detected collapsed sponge remains, respectively in the formation and lithification of the
Lower Liassic Schnöll limestone. Biomarker analyses were carried out to evaluate and prove the