Facies (2005) 51: 584–591
DOI 10.1007/s10347-005-0050-0
ORIGINAL ARTICLE
Mikael Calner
Silurian carbonate platforms and extinction events—ecosystem
changes exemplified from Gotland, Sweden
Received: 12 January 2005 / Accepted: 2 March 2005 / Published online: 21 April 2005
C Springer-Verlag 2005
Abstract Recent and ancient carbonate platforms are major marine ecosystems, built by various carbonate-secreting
organisms with different sensitivity for environmental
change. For this reason, carbonate platforms are excellent
sensors for changes in contemporaneous marine environments. A variety of ecosystem changes in carbonate platforms have previously been recognised in the aftermath
of mass extinction events. This paper addresses how two
Silurian extinction events among graptolites, conodonts,
and pentamerid brachiopods can be related to changes in
the style of carbonate production and general evolution of
low latitude carbonate platforms in a similar way as previously reported from the major five mass extinctions of the
Phanerozoic. Strata formed on Gotland during the Mulde
and Lau events share remarkably many similarities but are
strikingly different in composition compared to other strata
on the island. The event-related strata is characterised by
the sudden appearance of widespread oolites, deviating reef
composition, flat-pebble conglomerates, abundant microand macro-oncoids, stromatolites, and other microbial facies suggesting decreased bioturbation levels in contemporaneous shelf seas. Importantly, these changes can be tied
to high-resolution biostratigraphic frameworks and global
stable isotope excursions. The anomalous intervals may
therefore be searched for elsewhere in order to test their
regional or global significance.
Keywords Carbonate platform . Extinction event .
Microbial resurgence . Oolite . Gotland . Silurian
Introduction
various groups of marine taxa. As presently known, these
events are of relatively minor magnitude although certain
groups were severely affected, notably the conodonts and
graptolites (e.g., Jeppsson 1990; Jaeger 1991; Koren’ 1991;
Jeppsson et al. 1995; Jeppsson 1997; Jeppsson and Aldridge
2000; Lenz and Kozłowska-Dawidziuk 2001; Jeppsson and
Calner 2003; Por˛ebska et al. 2004). The increased use of
carbon and oxygen stable isotopes in stratigraphic studies
has unequivocally demonstrated the global signature
related to each of the three major Silurian events—the
latest Llandovery Ireviken Event, the Late Wenlock
Mulde Event, and the Late Ludlow Lau Event (e.g., Kaljo
et al. 1997, 2003; Samtleben et al. 2000; Saltzman 2001;
Munnecke et al. 2003). Although in many aspects these
events are unexplored, and adequately demonstrated
mainly from stable isotopes and microfaunal assemblages,
they have certainly changed our view of the Silurian Earth
as more dynamic than believed previously (Boucot 1991).
New important insights in to these events come from the
study of the particular ecosystems that some of the affected
groups of taxa really inhabited-carbonate platforms. These
are marine ecosystems with a life cycle and they are likely
to respond to extinction events in the same way as any group
of taxa, with an initial stage of termination or stress, followed by a low diversity catastrophe stage and a recovery
stage. Studies of the strata of Gotland (Fig. 1) have recently
demonstrated that two of the most profound Silurian events
are associated with changing composition and structure of
carbonate platforms (Calner and Jeppsson 2003; Calner
et al. 2004a; Calner 2005). The present paper aims to address the potential importance of such ecosystem changes
and reviews especially the main changes associated with
the Mulde and Lau events.
Over the last 15 years it has become evident that the
Silurian Period yields recurrent and abrupt changes in
Geological setting
M. Calner ()
GeoBiosphere Science Centre, Lund University,
Sölvegatan 12, SE-223 62 Lund, Sweden
e-mail: [email protected]
Tel.: +46-46222-73-79
Fax: +46-46222-44-19
The exposed sedimentary rocks on Gotland reflect a series
of stacked carbonate platform generations that formed in
the intra- to pericratonic Baltic basin at low latitudes south
of the equator (see Calner et al. 2004b for a recent review
585
A
Hangvar Formation
B
Tofta Formation
Hogklint Formation
FENNOSKANDIAN SHIELD
Slite Group
Lower and Upper
Visby formations
YTTERHOLMEN
Halla Formation
HORSNE
Klinteberg Formation
60 N
Frojel Formation
KLINTEHAMN
Hemse Group
BALTIC BASIN
LAU OUTLIER
BURGEN OUTLIER
SARMATIAN
SHIELD
N
Eke Formation
GOTLAND
Active margins
Burgsvik Formation
Depressions
Hamra Formation
Highs
20 E
Sundre Formation
N
20 km
Fig. 1 Silurian palaeogeography and location of Gotland. A Paleogeography of Scandinavia and the East Baltic showing Gotland within
the square (modified from Baarli et al. 2003). B Stratigraphic units of Gotland
and references). The basin was situated on the southern
margin of the Baltic Shield and the East European platform
(Fig. 1a). After extension, followed by tectonic quiescence
in the earliest Palaeozoic, the south-western margin of
the Baltic Shield was active from the latest Ordovician
when the Avalonia Composite Terrane was amalgamated
to Baltica (Pharaoh 1999). A substantial change in basin
tectonics that may have affected the Gotland area is noted
in the Silurian, particularly during Ludlow time (Poprawa
et al. 1999). During the Silurian, the Baltic basin was
fringed by a carbonate platform system whereas shales
with graptolites formed in the central and deeper parts of
the basin. The lack of major tectonic structures and the
exceptionally good preservation of the rocks (Munnecke
et al. 1999) enable good control on temporal and spatial
facies change, i.e., on platform architecture. To the south,
the Rheic Ocean separated the Baltic Shield form the
main Gondwana continent. This palaeogeographic setting
makes the Baltic basin an excellent target for studies of the
eventstratigraphic aspects of carbonate platform evolution.
Silurian extinction events
The two particular extinction events discussed herein are
briefly outlined below.
The middle Silurian Mulde Event
The crisis among the graptolites in the Late Wenlock [the
‘Big Crises’ of Jaeger (1991); the ‘lundgreni Event’ of
Koren’ (1991); Mulde Event of Jeppsson et al. (1995)
terminated 12 out of 14 graptolite species, leaving only
two as originators for the subsequent Palaeozoic graptolite
fauna (Lenz and Kozłowska-Dawidziuk 2001; see also
Jeppsson and Calner 2003; Por˛ebska et al. 2004). Also
conodont faunas were affected by this event (Jeppsson
et al. 1995; Jeppsson and Calner 2003; Fig. 2). After the
first recognition in shale and phosphorite (Jaeger 1991),
environmental and faunal disturbances attributed to this
event have been recognized from several palaeocontinents.
Studies of Silurian stable isotopes have revealed a positive
carbon isotope excursion (CIE) across the event (e.g.,
Samtleben et al. 1996; Kaljo et al. 1997; Samtleben et al.
2000; Saltzman 2001; Fig. 3) and confirmed the global
importance of the Mulde Event. On Gotland, the start of
the event has been identified as the last appearance datum
(LAD) of the conodont Ozarkodina sagitta sagitta at the
base of the Fröjel Formation (Jeppsson and Calner 2003).
The Late Silurian Lau Event
The Late Ludlow Lau Event (Talent et al. 1993; Jeppsson and Aldridge 2000; Calner 2005) started late in
the Polygnathoides siluricus conodont Chron. The event
caused extinctions and changes in community structures
(Jeppsson and Aldridge 2000) and records a substantial reduction in the diversity of pentamerid brachiopods (the Pentamerid Event of Talent et al. 1993). Among conodonts, no
platform-equipped taxon survived, and disaster conodont
faunas dominated by a single taxon developed during the
most severe part of the event (Jeppsson and Aldridge 2000).
The Lau Event is associated with a major positive carbon
isotope excursion (Samtleben et al. 1996, 2000; Lehnert et
al. 2003; Kaljo et al. 2004). The CIE and the LAD of the
zone fossil P. siluricus (Fig. 2) have permitted identification
586
419 Ma
L
U
L
D
F
A
T
L
E
U
S
I
L
U
R
I
D
L
O
W
A
N
N
L
K
Monograptus
Sundre Fm
formosus
Monograptus
balticus
10 m
Hamra Fm
( Kockelella
crassa
Zone)
G
L
E
oolites-oncolites
3.4-32 m wrinkle structures
E
D
O
N
W
H
I.
C.? ludensis
Formation
Ozar kodina
sagitta sagitta
Zone
K. o. or tus Zone
Cyrtograptus
6
8
10
CIE-interval
K?
stromatolites/oncolites
LAD
Polygnathoides
siluricus
Thickness
of strata
not to
scale
STROMATOPOROIDS
STROMATOLITES
ONCOIDS
CORALS
OOIDS
UNCONFORMITY K = KARST
MARL AND
ARGILLACEOUS LIMESTONE
SKELETAL LIMESTONE
Klinteberg
Zone
4
100 m
C.? gerhardi /
Ctenognathodus
C.? deubeli
murchisoni
Zone
Kockelella
Colonograptus?
or tus absidata
praedeubeli
Zone
G. nassa
Ozar kodina
P. d. parvus
bohemica longa
δ13C (‰)
40 m
Burgsvik Fm
Eke Fm
Neodiversograptus
nilssoni
GENERAL
FACIES
STABLE ISOTOPES
carbonate platform
2
with major
reef complexes
Lau Event
O
C
Ozar kodina
cr ispa
Zone
GOTLAND
CARBONATE
STRATIGRAPHY PLATFORM STRESS
Neocucullo0.4-14 m flat-pebble cong.
graptus
flat-pebble cong.
Polygnathoides
kozlowskii
silur icus
u. Saetograptus
leintwarZone
m.
dinensis
Oulodus
Bohemograptus
G silur icus acme
bohemicus
carbonate platform
A. ploeckensis
tenuis
with major
O
Z.
reef complexes
l?
R Kockelella ?
Hemse Group
v. var iabilis Zone
S
Lobograptus
Post-O. ex . n.ssp.S
T
scanicus
Ozar kodina
I
Lobograptus
excavata
progenitor
n. ssp. S
A
N
E
GRAPTOLITE
ZONATION
Ozar kodina
snajdr i
O.
Zone
U. Icr iodontid Sz.
M. Icr iodontid Sz.
L. Icr iodontid Sz.
423 Ma
W
CONODONT
ZONES & FAUNAS
carbonate platform
with major
reef complexes
CALCAREOUS
MUD-, SILT-, SANDSTONE
1
2
3
4
70 m
Mulde Event
oncolites
Halla
deviating reef
composition
Formation
20 m
Frojel Fm 10 m
lundgreni
Slite Marl
C. pernei
CIE-interval
oolites-oncolites
siliciclastics
carbonate platform
with major
reef complexes
K
LAD
Ozarkodina
sagitta sagitta
Thickness
of strata
not to
scale
Fig. 2 Middle to Late Silurian stratigraphic framework of
Gotland. The stratigraphic range intervals of the Mulde and Lau
events are shown with light grey shading. The interval with notable effects on the composition and structure of contemporaneous platforms are indicated with a darker grade of shading (extended from Calner 2005). Note that the exact stratigraphic level
for the start of karst weathering during the Lau Event is uncer-
tain, and that its position on the top of the Eke Formation is inferred. CIE - Carbon Isotope Excursion; data from Samtleben et al.
(2000). Biostratigraphic data from various sources, e.g., Calner and
Jeppsson (2003), Jeppsson and Aldridge (2000), and L. Jeppsson
(pers. comm. 2005). The Fröjel Formation has been detailed by
Calner (1999, 2002) and Calner et al. (2004a)
of the event in widely spaced sections, e.g., in the Mušlovka
section of the peri-Gondwanan Prague basin (Lehnert et al.
2003), and in the Broken River area, Queensland (Talent
et al. 1993).
Bottjer 1992; Sheehan and Harris 2004), oncoids (Whalen
et al. 2002), and subtidal wrinkle structures (Hagadorn and
Bottjer 1997; Pruss et al. 2004; Calner 2005). Reduced
bioturbation levels may also facilitate the formation of flatpebble conglomerates (Sepkoski 1982). In the context of
carbonate platforms as ecosystems, these largely biologically controlled facies can be regarded as ‘stress indicators’ or ‘disaster forms’. Such sedimentary facies, formed
during time intervals of substantially reduced grazing and
infaunal activity, may resemble facies from earlier times in
Earth history, when bioturbation was less pronounced and
are then referred to as anachronistic facies (Sepkoski et al.
1991). Other important ‘stress indicators’ are controlled by
geobiochemical changes in the marine environment resulting in anomalous carbonate precipitation (e.g., Grotzinger
and Knoll 1995; Woods et al. 1999). More recently and
based on the early Triassic and Silurian, it has been suggested also that widespread oolites may be a response to the
combined effects of extinction events (Groves and Calner
2004).
Extinction events and carbonate facies
There is a clear relationship between extinction events and
carbonate-producing biota, and a number of anomalous
depositional facies in carbonate platforms have been discussed as related to the low diversity ecosystems that develop in the aftermath of extinction events. Such facies
has hitherto only been discussed from major crises such
as the end-Ordovician, end-Permian, and the FrasnianFamennian mass extinctions. The anomalous facies are
generally associated with microbial resurgence and interpreted as related to the decreased rates of grazing and infaunal activity by marine benthos immediately after an extinction, and include level-bottom stromatolites (Schubert and
587
Fig. 3 Photographs showing various microbial facies related to
Silurian extinction events in carbonate platform strata. A Detail
of micro-oncoidal grainstone in the lower Halla Formation at
Klintebys 1. B Oncoid packstone from Gothemshammar 2.
C Columnar stromatolites from the Eke Formation at Gumbalde 1
(from Calner 2005). D Oncoids from the uppermost Eke Formation
(Ronehamn-1 drillcore). E Flat-crested kinneyia ripples from the uppermost Burgsvik Sandstone at Valar 2. F Micro-oncoidal rudstone
from the basal Burgsvik Oolite at Valar 2. All scale bars = 1 cm.
Hand lens in E=21 mm×28 mm
Several of these ‘stress indicators’ and anachronistic facies appear suddenly and simultaneously with the Mulde
and Lau events (particularly during the Lau Event) and
the associated carbon isotope excursions. The stratigraphic
position of these facies, their brief (time) occurrence and
their rarity in enclosing strata implies for the first time,
that also ‘minor events’ of the Phanerozoic, at least locally are associated with substantial ecosystem changes
(Calner 2005). Future studies are needed to demonstrate
whether the Silurian extinction events are of greater magnitude than presently understood or whether the magnitude
of taxonomic extinction is not related to the magnitude of
ecological change.
for each event differ in character and in the relative timing
of first occurrence. However, the events are similar in that
they:
Deposition during the Mulde and Lau events
In a broad sense the carbonate platform development during the Mulde and Lau events share unexpectedly many
common features that otherwise are rare or absent on
Gotland (Munnecke et al. 2003; Table 1; Fig. 2). It should
here be noted that strata associated to the separate events
include abundant crinoidal debris, brachiopods, trilobites,
and corals, suggesting that normal-marine conditions prevailed during the events. The anomalous sedimentary facies
1. start below regionally important unconformities, locally
with karst development, that is, in the late stages of
carbonate platform development,
2. are associated with influx of coarser siliciclastic material
to the basin,
3. are associated with thin units of oolites and oolitic strata,
4. are associated with increased preservation (resurgence)
of microbial facies such as micro- and macro-oncoids,
stromatolites and biomats,
5. are at least locally associated with reefs of deviating
composition. The most important reef-builders in preand post-extinction strata, stromatoporoids, were rarer
during the events and occurred predominantly in small
forms during the extinction intervals,
6. correlate in time with low-diversity faunas of conodonts
and graptolites,
7. overlap in time with major but short-lived, positive carbon stable isotope excursions.
Together, points 1 and 2 confirm that the extinctions
are associated with relative changes in the sea level. The
carbonate and siliciclastic depositional systems respond
588
fundamentally different to the relative sea-level change.
Siliciclastic material is transported to basins during relative
lowstand when the continent-sea gradient increases, and
carbonate platforms produce most of their sediments
during relative highstand of sea level when the area of
production increases. This depositional bias of the two
systems is referred to as lowstand and highstand shedding,
respectively (Schlager 1991; Schlager et al. 1994). Coarser
siliciclastic material was transported to the Gotland part
of the basin as a result of depositional bias and occurs
only at two stratigraphic levels; during the Mulde Event
(Calner 1999) and immediately after the Lau Event (Long
1993; Calner 2005). The paucity of siliciclastics elsewhere
on Gotland make these two sea-level falls stand out and
it is reasonable that they were of greater magnitude than
other relative sea-level falls of Wenlock and Ludlow age.
It should be noted that detailed stratigraphic work across
the range of the Mulde Event indicates that the first extinctions pre-dates the sea-level fall (Jeppsson and Calner
2003).
Points 3, 4, 5, and 6 have previously been largely overlooked in the study of Silurian extinction events but they are
important since they imply that major ecological changes
took place in carbonate platforms (i.e., on shallow-water
tropical shelves) during the separate events (see Calner and
Jeppsson 2003; Munnecke et al. 2003; Calner 2005). Importantly, these factors also suggest that extinctions among,
e.g., graptolites and the very substantial changes in carbonate platform deposition occur simultaneously. For this
reason, a causal linkage is probable.
Point 7 above unequivocally shows that the noted changes
are associated with a global signal in the marine environment (stable isotope data from Samtleben et al. 2000).
Facies anomalies during the Mulde Event
Strata formed during the Mulde Event on Gotland belong
to the Fröjel and Halla formations (Jeppsson and Calner
2003; Fig. 2). In the northeast, in proximal platform environments, the event is characterised by a shift from preextinction skeletal carbonate production with abundant,
large stromatoporoid reefs in the uppermost Slite Group
(Manten 1971), to dominantly non-skeletal carbonate production immediately after the extinction interval in the Bara
Oolite Member of the Halla Formation (Calner and Säll
1999; Fig. 3a). To the southwest, in a more distal platform
environment, the siliciclastic-rich Fröjel Formation overlies
marls that are lateral equivalents to the reefs in the northeast (see profile in Fig. 2). Where bedding is pronounced,
the Fröjel Formation locally yields notable bedding plane
trace fossil assemblages (epichnial grooves and hypichnial
ridges) but generally limited infaunal (endichnial) bioturbation. A basin-regional unconformity at the top of the
Fröjel Formation indicates the level for the last phase of
the extinction (Datum 2 of Jeppsson and Calner 2003) and
the maximum lowstand of the mid-Homerian sea-level drop
(Calner 1999).
The transgressive Bara Oolite Member formed immediately after the extinction phase represents a ‘low-diversity
stage’ of the carbonate platform. It consists of microoncoidal grain-rudstone (Fig. 3a) and oolitic strata from the
Klintehamn area near the west coast of Gotland (Fig. 1b)
across the island to the Hörsne area on east-central Gotland,
where these deposits are ca 5 m thick (Calner and Säll 1999)
and further northeastwards to the area for the Ruhnu (500)
core off Estonia (Põldvere 2003). Hence, the change in
carbonate production was a basin-regional phenomenon.
Preliminary correlation indicates that these anomalous
strata correlate with conspicuously laminated mudstones
in the deeper parts of the basin from Gotland to Latvia and
Poland (Calner et al. 2004c).
Stromatoporoids, the common reef-builders in Silurian
strata, decrease in diversity from the upper Slite Group to
the Halla Formation (Mori 1968) and predominantly occur
with small forms in the latter unit (Manten 1971). The first
reefs that establish after the extinction interval occurs in
the middle part of the Halla Formation on central Gotland
and on the small islet Ytterholmen (Fig. 1b; Calner et al.
2004a). The metre-sized reefs at Ytterholmen have not
been investigated with regard to species content. However,
the reefs on central Gotland have a deviating species
composition compared to most other reefs of Gotland.
Common characters of these earliest post-extinction reefs
are ‘small size, peculiar species content, and the absence
of direct analogs of them in the Wenlock of the East Baltic’
(Klaamann and Einasto 1977). They ‘differ from the
others in large number of small dendroid and encrusting
tabulates and bryozoans’ (Klaamann and Einasto 1977).
On western Gotland the earliest post-extinction reef
is a low-diversity halysitid-heliolitid autobiostrome—
dominated by dendroid heliolitids—that differs from all
other Wenlock-Ludlow reefs on Gotland (Calner et al.
2000). During this interval, stromatolites are important in
the East Baltic part of the basin (Calner et al. 2004a).
As in the lowermost Halla Formation, microbial carbonates continue to form an important part of the strata in
the upper part of the formation on eastern Gotland were
oncoidal packstone is the dominating facies (Fig. 3b).
Renewed carbonate platform progradation and reef development occur in the succeeding Klinteberg Formation.
Facies anomalies during the Lau Event
The strata formed during and shortly after the Lau Event
on Gotland belong to the uppermost Hemse Group and
the Eke, Burgsvik and basal Hamra formations (Fig. 2).
These strata yield a wealth of rapidly changing facies,
spanning from marlstone deposited below storm wavebase to oolites of peritidal origin. Intriguing stratigraphic
discontinuities occur at several levels, locally associated
with karst (Cherns 1982). The most profound sedimentary change, however, is indicated by the 3.4–31.0 m thick
Burgsvik Sandstone that is wedged within the platform
carbonates.
589
The strata have in common that they include evidence
for reduced grazing rates and infaunal activity during the
event interval (Calner 2005). This is first noted by the occurrence of flat-pebble conglomerates in the upper Hemse
Group and in the lower Eke Formation. The proximal parts
of the Eke Formation, which outcrops in the Lau outlier (Fig. 1b), also yield domal and columnar stromatolite
colonies that reach a height of a few centimetres (Fig. 3c).
These grew as isolated or laterally interconnected colonies,
in places forming a bulbous carpet. The stromatolites cooccur with branch-formed heliolitid corals, brachiopods,
and abundant crinoidal debris in stratified sediments associated to argillaceous mounds, indicating normal-marine,
subtidal conditions (Calner 2005). In the Burgen outlier
to the south (Fig. 1b), the Eke Formation is only 0.40 m
thick and mainly composed of Rothpletzella and Wetheredella boundstone. Rothpletzella oncoids with thick to very
thick cortices are abundant, occasionally rock-forming, in
laterally equivalent subtidal strata deposited seaward of the
shallow platform area (Fig. 3d). The reefs of the Eke Formation are highly argillaceous and, as during the Mulde
Event, stromatoporoids are smaller and less abundant than
in the immediate pre- and post-extinction strata (Mori 1970;
Manten 1971). Of the 23 species of stromatoporoids in the
Hemse Group only five are recorded from the Eke Formation (Mori 1970; Table 1).
The overlying hummocky cross stratified sandstones of
the Burgsvik Formation include various forms of wrinkle structures, e.g., flat-crested kinneyia ripples (Calner
2005; Fig. 3e). Based on the studies of recent sedimentary environments, Hagadorn and Bottjer (1997, 1999)
have provided evidence for a microbial origin for VendianCambrian wrinkle structures preserved in siliciclastic
rocks. Such structures represent fossil biomats and are
common in strata prior to the Ordovician metazoan radiation and thereafter restricted to highly stressed intertidalsupratidal environments due to increased bioturbation levels (Hagadorn and Bottjer 1999). Thereafter they reappear
in subtidal siliciclastic environments in the aftermath of the
Lau Event (Calner 2005) and the end-Permian mass extinction (Pruss et al. 2004), indicating a prolonged ecosystem
collapse after each of these events. The Burgsvik Sandstone
is followed by conspicuous micro-oncolitic and oolitic
strata (Fig. 3f) across the entire outcrop belt on Gotland
in the Burgsvik Oolite and the basal Hamra Formation.
Similar strata, inferably of coeval age, occur also in other
parts of the basin (in the Łysogóry Region of the Holy
Cross Mountains; Kozłowski 2003).
Renewed carbonate platform progradation and reef
development occur in the succeeding middle Hamra to
Sundre formations.
Discussion and conclusion
The growing body of faunal and stable isotopic evidence
for the global character of Silurian extinction events is
presently one of the foremost and most interesting developments of Silurian stratigraphy. However, the extinctions
have hitherto only been adequately demonstrated from mi-
crofossil assemblages (e.g., conodonts and graptolites) and
little is known from other groups and about contemporaneous environmental changes in the marine realm. As discussed herein, carbonate and siliciclastic sediments formed
during and immediately after the Late Wenlock Mulde
Event and the Late Ludlow Lau Event include anachronistic and various microbial facies. Hence, sedimentary
evidences suggest a decline in grazing and infaunal activity, and thereby stress, in low-latitude marine ecosystems during the studied time periods (Fig. 2). At this point,
palaeontological evidence for reduced grazing and infaunal
activity is limited, and detailed taxonomic studies are necessary in order to identify to what extent vagrant grazing
benthos (e.g., gastropods) were affected during these two
Silurian events. The most profound sedimentary changes
are associated with the Lau Event (Calner 2005) and strata
formed during this event yield at least the gastropod genus
Bellerophon at a few localities (Hede 1925: 68). Thus,
Stel and de Coo (1977: Table 1) erroneously indicated that
the Eke Formation is devoid of molluscs. The latter authors also indicated that the Eke Formation was devoid of
trace fossils (Stel and de Coo 1977: Table 1). Later studies
(Samtleben et al. 2000) and personal observations have in
part changed this picture but a decline of both gastropods
and trace fossils is likely. If the microbial resurgence not
correlates with a notable extinction among grazing benthos, other factors limiting their occurrence must be considered. Two models have been put forward to explain the
observed faunal and lithological changes in the Silurian
(Jeppsson 1990; Bickert et al. 1997). The models predict
arid climate conditions in low latitudes during the Mulde
and Lau events. Only limited sedimentary circumstantial
evidences exist for such conditions, e.g., local dolomitization (Calner 1999) and inferred evaporite minerals (the
calcite laths of Calner 2002) in strata formed during the
Mulde Event. Substantially increased salinity is unlikely
with regard to the general shelly fauna of these intervals.
Also, the increased deposition of fine terrigenous clastics
related to each event indicates increased rates of chemical
weathering and thereby humid climatic conditions rather
than arid. It can be concluded that the role of climate
change for the observed sedimentary changes is poorly
understood.
The Mulde and Lau events are both related to substantial
sea-level drops. The siliciclastic influx to the basin associated with these base-level changes could theoretically
lead to nutrient excess (Hallock et al. 1988) and thereby
would favour microbial communities. However, the relative timing of microbial resurgence and siliciclastic influx
does not show any such relationship. Nor does this explain age-equivalent microbial communities elsewhere in
the basin, e.g., in southernmost Sweden (Wigforss-Lange
1999). The co-occurrence of the global stable isotope excursions, the microbial resurgence, and the extinctions among
conodonts, graptolites, and pentamerid brachiopods rather
suggest that Silurian events to some extent resulted in low
diversity ecosystems in the same principal way as in the
aftermath of the end-Ordovician and end-Permian mass
extinctions.
590
Acknowledgements I am grateful to reviewer Axel Munnecke,
who also provided the palaeogeographic map, and to editor André
Freiwald for helpful comments to the manuscript. This research
was funded by the Swedish National Research Council (VR) and
Crafoordska stiftelsen.
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