A high-resolution carbon and oxygen isotope analysis of Late
Oxfordian-Early Kimmeridgian deep-shelf sediments of southern Germany is
combined with investigation of nannofossil assemblage composition and
sedimentological interpretations in order to evaluate the impact of
regional palaeoenvironmental conditions on isotopic composition of
carbonates. This study suggests that carbonate mud was essentially
derived from the Jura shallow platform environments and also that the
isotopic signature of carbonates deposited in the Swabian Alb deep shelf
indirectly expresses the palaeoenvironmental evolution of the platform.
Short-term fluctuations in delta(13) C and delta(18)O are probably
controlled by changes in salinity (fresh-water input versus evaporation)
in platform environments. Long-term fluctuations in carbon and oxygen
isotope record throughout the Late Oxfordian-Early Kimmeridgian result
from the interplay of increasing temperature and decreasing humidity,
which both control the trophic level. Changes from mesotrophic to
oligotrophic conditions in platform environments and in the deep-shelf
surface waters are inferred.
During the Late Oxfordian (Bimammatum Subzone to Planula Zone), the
delta(13)C curve displays a positive shift of about 1 parts per
thousand, which is comparable in intensity to global perturbations of
the carbon cycle. This evident isotopic shift has not been documented
yet in other basinal settings. It can be reasonably explained by local
palaeoenvironmental changes on the Jura platform (salinity, temperature,
and nutrient availability) that controlled platform carbonate
production, and the geochemistry of overlying waters. However,
increasing carbonate production on the Jura platform and related
positive delta(13)C shifts recorded in the Swabian Alb deep shelf are
the regional signatures of climatic changes affecting other
palaeogeographical domains of Europe in which the carbonate production
increased throughout the Late Oxfordian. (C) 2002 Elsevier Science B.V.
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