148
Zonneve1d et al.
Keupp (1981, 1982, 1991) and Keupp and Mutterlose (1984) imply for Lower Cretaceous assemblages of the Boreal Realm that morphological
structures ofthe calcareous cyst wall and cyst form
might vary with changing temperatures and/or
salinities. They suggest that in relation to reduced
temperatures the wall thickness and amount of wall
layers reduce, crystal size increases, paratabulation
might become more pronounced and cyst shape
become less elongated. Studies investigating the
extent of which these trends in other time intervals
are currently in progress (Willems and Janofske
pers. comm.). Besides a relation to temperature and
salinity, inshore - offshore trends might also be reflected by the calcareous cyst association and
morphology of cysts. For Lower and Upper Cretaceous material of the Boreal Realm, Keupp (1992,
1993, 1995a, 1995b) and Zligel (1994) observe alow
diversity cyst association during peak transgressive
phases related to offshore environmental conditions,
dominated by thin-walled species with a presently
extinct "Pithonelloid" crystal orientation (Keupp
and Versteegh 1989; Kohring 1993). According to
these authors, the outer shelf is characterized by a
somewhat higher diversity and by species with a
radial crystal orientation. Inshore environments are
characterized by the highest diversity and thick!
double-walled specimens with an oblique crystal orientation. A detailed study of Middle and Upper
Eocene material by Kohring (1993) generally supports these findings. Kohring (1993) observes, however, that species with radial crystal orientation geographically replace the species with a "Pithonelloid"
crystal orientation, whereas the Eocene inshore
associations are characterized by a low diversity.
Transport
To date, little information is available on (primary
or secondary) lateral transport of calcareous cysts
by ocean currents. Sinking experiments have shown
that sinking rates of individual cysts ofthe species
Scrippsiella trochoidea (Stein) Loeblich III are
about 0.013 cmls allowing individual cysts to reach
an ocean bottom of3 000 m depth in about 58 days
(Anderson etal. 1985a). This suggests that they are
vulnerable to lateral transport by ocean currents.
However, it is unlikely that calcareous dinoflagellates
sink down as individual specimens. A recent study
on sediment trap material from the Bedford Basin
(Nova Scotia) has shown that flocculated organic
matter and faecal pellets in the traps contain many
organic-walled dinoflagellate cysts (Mudie 1996).
Flocculated material and faecal pellets aggregate
dinoflagellate cysts, increasing their sinking rates
and decreasing the change of lateral transport by
ocean currents. Assuming similar transport mechanisms for organic- and calcareous-walled
dinoflagellate cysts, this suggests that calcareous
cysts may also sink to the ocean floor incorporated
in faecal pellets of flocculated material.
Secondary transport by bottom currents has
been suggested to be a major factor influencing the
organic-walled cyst associations (Dale and Dale
1992; Versteegh 1995; Dale 1996). However, the
distribution of organic-walled cysts in surface sediment samples seems largely environmentally determined (e.g. Wall and Dale 1973; Wall et al. 1977;
Turon 1984; Matthiessen 1994; Zonneveld 1996,
1997; Marret and De Vernal 1997). The influence
of secondary transport on the calcareous cyst
assemblages in bottom sediments depends on several factors such as, for instance, sedimentation
rates and bottom current velocity. The effect of secondary transport on the final cyst recovery is therefore probably region-dependent and cannot therefore be generalized. Until now information has been
insufficient to determine the rate at which these
processes influence calcareous cyst distribution.
Future studies on recent distribution patterns in
surface sediments and sediment traps will provide
greater clarity concerning such potential transport
mechanisms.
Preservation
The wall of some calcareous resting cysts is formed
by an organic inner layer and a calcitic outer layer.
Consequently, the preservation of these cysts in
sediments may be sensitive to several processes.
The calcite crystals can dissolve as a result ofthe
undersaturation of bottom waters with respect to
CO;2. Sediments below the lysocline are therefore
often depleted in calcareous cysts. Recently,
Zonneveld et al. (1997) have shown that organicwalled dinoflagellate cysts can severely degrade in
Zonneve1d et al.
Keupp (1981, 1982, 1991) and Keupp and Mutterlose (1984) imply for Lower Cretaceous assemblages of the Boreal Realm that morphological
structures ofthe calcareous cyst wall and cyst form
might vary with changing temperatures and/or
salinities. They suggest that in relation to reduced
temperatures the wall thickness and amount of wall
layers reduce, crystal size increases, paratabulation
might become more pronounced and cyst shape
become less elongated. Studies investigating the
extent of which these trends in other time intervals
are currently in progress (Willems and Janofske
pers. comm.). Besides a relation to temperature and
salinity, inshore - offshore trends might also be reflected by the calcareous cyst association and
morphology of cysts. For Lower and Upper Cretaceous material of the Boreal Realm, Keupp (1992,
1993, 1995a, 1995b) and Zligel (1994) observe alow
diversity cyst association during peak transgressive
phases related to offshore environmental conditions,
dominated by thin-walled species with a presently
extinct "Pithonelloid" crystal orientation (Keupp
and Versteegh 1989; Kohring 1993). According to
these authors, the outer shelf is characterized by a
somewhat higher diversity and by species with a
radial crystal orientation. Inshore environments are
characterized by the highest diversity and thick!
double-walled specimens with an oblique crystal orientation. A detailed study of Middle and Upper
Eocene material by Kohring (1993) generally supports these findings. Kohring (1993) observes, however, that species with radial crystal orientation geographically replace the species with a "Pithonelloid"
crystal orientation, whereas the Eocene inshore
associations are characterized by a low diversity.
Transport
To date, little information is available on (primary
or secondary) lateral transport of calcareous cysts
by ocean currents. Sinking experiments have shown
that sinking rates of individual cysts ofthe species
Scrippsiella trochoidea (Stein) Loeblich III are
about 0.013 cmls allowing individual cysts to reach
an ocean bottom of3 000 m depth in about 58 days
(Anderson etal. 1985a). This suggests that they are
vulnerable to lateral transport by ocean currents.
However, it is unlikely that calcareous dinoflagellates
sink down as individual specimens. A recent study
on sediment trap material from the Bedford Basin
(Nova Scotia) has shown that flocculated organic
matter and faecal pellets in the traps contain many
organic-walled dinoflagellate cysts (Mudie 1996).
Flocculated material and faecal pellets aggregate
dinoflagellate cysts, increasing their sinking rates
and decreasing the change of lateral transport by
ocean currents. Assuming similar transport mechanisms for organic- and calcareous-walled
dinoflagellate cysts, this suggests that calcareous
cysts may also sink to the ocean floor incorporated
in faecal pellets of flocculated material.
Secondary transport by bottom currents has
been suggested to be a major factor influencing the
organic-walled cyst associations (Dale and Dale
1992; Versteegh 1995; Dale 1996). However, the
distribution of organic-walled cysts in surface sediment samples seems largely environmentally determined (e.g. Wall and Dale 1973; Wall et al. 1977;
Turon 1984; Matthiessen 1994; Zonneveld 1996,
1997; Marret and De Vernal 1997). The influence
of secondary transport on the calcareous cyst
assemblages in bottom sediments depends on several factors such as, for instance, sedimentation
rates and bottom current velocity. The effect of secondary transport on the final cyst recovery is therefore probably region-dependent and cannot therefore be generalized. Until now information has been
insufficient to determine the rate at which these
processes influence calcareous cyst distribution.
Future studies on recent distribution patterns in
surface sediments and sediment traps will provide
greater clarity concerning such potential transport
mechanisms.
Preservation
The wall of some calcareous resting cysts is formed
by an organic inner layer and a calcitic outer layer.
Consequently, the preservation of these cysts in
sediments may be sensitive to several processes.
The calcite crystals can dissolve as a result ofthe
undersaturation of bottom waters with respect to
CO;2. Sediments below the lysocline are therefore
often depleted in calcareous cysts. Recently,
Zonneveld et al. (1997) have shown that organicwalled dinoflagellate cysts can severely degrade in