Dias nummer 1

1
Sulfur isotopes in chalk pyrite Indications of
early diagenetic processes Malene Dolberg Hansen
and Bjørn Buchardt Geological Institute,
University of Copenhagen, dolberg_at_stud.ku.dk
The purpose of this study was to explain the
process of sulfur isotope fractionation by
sulfate reducing bacteria in the marine
environment and the diagenetic implications in
concretionary pyrite formation. This was
exemplified by a study on concretionary pyrite
from Stevns Klint, Denmark. The sulfur isotope
distribution in the concretions was measured for
two purposes 1. The sulfur isotope variation
across the Cretaceous/ Tertiary boundary and 2.
The sulfur isotope variation in selected
concretions.
Schematic figure showing the pyrite forming
processes the numbers are the amount that
fractionates through bacterial sulfate reduction.
The figure to the left shows the measurements
from the concretion centre, these reflect the
primary pyrite the d34S-values are set in
stratigraphical order. There is a remarkable
shift against more positive values on the
K/T-boundary from about 30 in the Chalk to 19
in the Fish Clay.
The figure to the right shows the d34S-variation
through a concentric and zoned concretion. The
concretion is from the Chalk, a Maastrichtian
deposit. There is a clear difference from the
edge to the centre. The values in the centre are
more negative than at the edge. This trend shows
a shift from open to closed conditions with
further burial. Under closed conditions is d34S
distributed by the Rayleigh distillation.
Conclusions The pyrite forming processes are
exemplified by the d34S measurements from Stevns
Klint, Denmark, where open followed by closed
conditions is shown.
The measurements were made on dried, frozen and
cut concretions. This was made on an Isoprime,
Dual Inlet, Gas Mass Spectrometer, as work
standard was Ag2S-IAEA-std3 used.