3.5 Biofacies
41
(1997) made a first attempt to determine the age of sediment layers from the D/Lratio of asparagin acid in the shells of bivalves (Cerastoderma edule, Mya arenaria). The shells of these two species were analysed from two I m long sediment
cores sampled in the Mytilus bed. The maximum age was determined for a 2000
year old C. edule, the youngest specimen was estimated to be a living M. arena ria
belonging to the O-group. From the relationship between age of the shell and
sediment depth the authors estimated the mean sedimentation rate to be 34 cm per
century. According to this, the sediment depth of 35 cm analysed by Hertweck
would represent a century, while he himself estimated the age of successive MytiIus facies to be a maximum of 50 years. This points to the need for further development of methods, such as the amino acid method, for dating sediment layers in
highly dynamic environments.
3.5.4
Effects of Ice Winters
Beside the mapping of sediment composItion and biofacies in the Spiekeroog
backbarrier tidal flats (Hertweck 1995), a more detailed study was carried out at
the Swinnplate in 1988 (Kurmis 1995). The results are in good agreement with
those from Hertweck (1995). After the ice winter of 1995/96, the biofacies distribution was modified. In the western part of the Swinnplate no L. conchilega were
found and A. marina became the dominating species (Haberer 1997). At two stations in the southern part, the polychaete H. filiformis, who prefers muddy habitats,
was observed together with a local increase of sediments <63 /lm up to 5-10 % of
the total sediment (surrounding stations 0-5 %). Mytilus beds, dominating the
centre of the Swinnplate were destroyed by ice-scour. The mud deposited within
the mussel beds was removed. At the eastern part of the Swinnplate a sparse to
dense colonization of L. conchilega was described. It could not be distinguished if
these represented either empty tubes of animals living there in preceding years, or
juveniles (date of sampling: Marchi April 1996; Haberer 1997).
As opposed to the Swinnp1ate, L. canchilega was only observed at three sampling points at the Groninger Plate (sampling dates April-June 1996; Engelbrecht
1997). On the large-scale, the area was dominated by A. marina with some patches
of Pygospio elegans. In the area of the tidal channel protruding from the southwest
into the Groninger Plate, H. jiliformis was found after the ice winter. This differs
from observations made in 1992 when the polychaete did not occur at this area
(Flemming & Bartholoma, unpublished; Mi.iller & Widdel, unpublished). As on the
Swinnplate, this finding co-occurred with an increase of sediments <63 /lm at these
stations which could be due to ice-scour.
These changes in the spatial distribution of biofacies and sediments due to a
natural disturbance event help with the interpretation of facies sequences in depth
profiles.
41
(1997) made a first attempt to determine the age of sediment layers from the D/Lratio of asparagin acid in the shells of bivalves (Cerastoderma edule, Mya arenaria). The shells of these two species were analysed from two I m long sediment
cores sampled in the Mytilus bed. The maximum age was determined for a 2000
year old C. edule, the youngest specimen was estimated to be a living M. arena ria
belonging to the O-group. From the relationship between age of the shell and
sediment depth the authors estimated the mean sedimentation rate to be 34 cm per
century. According to this, the sediment depth of 35 cm analysed by Hertweck
would represent a century, while he himself estimated the age of successive MytiIus facies to be a maximum of 50 years. This points to the need for further development of methods, such as the amino acid method, for dating sediment layers in
highly dynamic environments.
3.5.4
Effects of Ice Winters
Beside the mapping of sediment composItion and biofacies in the Spiekeroog
backbarrier tidal flats (Hertweck 1995), a more detailed study was carried out at
the Swinnplate in 1988 (Kurmis 1995). The results are in good agreement with
those from Hertweck (1995). After the ice winter of 1995/96, the biofacies distribution was modified. In the western part of the Swinnplate no L. conchilega were
found and A. marina became the dominating species (Haberer 1997). At two stations in the southern part, the polychaete H. filiformis, who prefers muddy habitats,
was observed together with a local increase of sediments <63 /lm up to 5-10 % of
the total sediment (surrounding stations 0-5 %). Mytilus beds, dominating the
centre of the Swinnplate were destroyed by ice-scour. The mud deposited within
the mussel beds was removed. At the eastern part of the Swinnplate a sparse to
dense colonization of L. conchilega was described. It could not be distinguished if
these represented either empty tubes of animals living there in preceding years, or
juveniles (date of sampling: Marchi April 1996; Haberer 1997).
As opposed to the Swinnp1ate, L. canchilega was only observed at three sampling points at the Groninger Plate (sampling dates April-June 1996; Engelbrecht
1997). On the large-scale, the area was dominated by A. marina with some patches
of Pygospio elegans. In the area of the tidal channel protruding from the southwest
into the Groninger Plate, H. jiliformis was found after the ice winter. This differs
from observations made in 1992 when the polychaete did not occur at this area
(Flemming & Bartholoma, unpublished; Mi.iller & Widdel, unpublished). As on the
Swinnplate, this finding co-occurred with an increase of sediments <63 /lm at these
stations which could be due to ice-scour.
These changes in the spatial distribution of biofacies and sediments due to a
natural disturbance event help with the interpretation of facies sequences in depth
profiles.