UTRECHT MICROPALEONTOLOGICAL BUllETINS

stabilizing bottom conditions. The maximum diversity in samples 1393 and
1395 is probably caused by the addition of allochthonous faunal elements
to the indigenous fauna. The relatively greater, though variable proportion of
badly preserved specimens throughout this lower part of the section indicates
that the source of supply of reworked material had not yet been·
drowned. The source area probably receded rapidly, to vanish completely
after a relatively short duration of Trubi sedimentation.
Benthonic life evidently suffered during the periods of laminated deposition
and silica-enrichment, not only in number of species and in diversity
but also in size of the individuals. Within the upper parts of the second,
third and fifth laminated intervals the species diversity seems to increase
again, somehow anticipating the improved environments, while sedimentation
of laminated sediments still continued. These changes must pertain to
bottom-bound processes which are thought to have been influenced by
primary changes in the overlying water column. As soon as the supply of
nutrients in the surface waters falls off a gradual decrease of the organic
sedimentation rate will take place, resulting from the decreasing supply
of planktonic elements to the bottom. The lower rate of deposition will in
turn result in consumption of less oxygen at the bottom owing to the smaller
amount of decomposing organic material. Benthonic life will become
re-established and diversity will tend to increase before silica-enrichment
comes to a complete stop.
An opposite effect may explain the drops in diversity below the laminated
intervals 1 and 3. As soon as nutrients increase in the surface waters
the phytoplankton production will tend to increase, but it may take some
time before it leads to (near-)oxygen depletion at the bottom.
Silica enrichments shown by high quantities of diatoms and radiolarians
are not completely correlated with poor bottom conditions resulting in
laminated sediments. Some of the laminated intervals contain but few
siliceous organisms, whereas for instance in the homogenized limestone
intercalation between intervals 3 and 4 radiolarians and diatoms abound.
Apparently burrowing organisms were able to homogenize this thin interval,
indicating that bottom conditions were not too poor. If increased planktonic
life caused low oxygen contents of the bottom water, we may conclude that
blooms did not always effect non-calcareous, calcareous and siliceous planktonic
groups simultaneously in the same relative proportions. Inversely,
increase of planktonic life did not always lead to seriously deteriorating
bottom conditions.
The highly diverse communities that seem to have lived during deposition
of the upper part of the section at the level of samples 107 A-G appar-