GEOLOGICA CARPATHICA
, OCTOBER 2016, 67, 5, 451 – 462
doi: 10.1515/geoca-2016-0028
www.geologicacarpathica.com
Planktonic foraminiferal turnover across
the Cenomanian –Turonian boundary (OAE2)
in the northeast of the Tethys realm, Kopet-Dagh Basin
BEHNAZ KALANAT
1
, MOHAMMAD VAHIDINIA
1
, HOSSEIN VAZIRI-MOGHADDAM
2
and MOHAMAD HOSSEIN MAHMUDY-GHARAIE
1
1
Department of Geology, Faculty of Science, Ferdowsi University of Mashhad, Mashhad, Iran;
Be.kalanat@stu.um.ac.ir,
vahidinia@ferdowsi.um.ac.ir, mahmudygharaie@gmail.com
2
Department of Geology, Faculty of Science, University of Isfahan, Isfahan, Iran; avaziri7304@gmail.com
(Manuscript received December 5, 2015; accepted in revised form September 22, 2016)
Abstract: Two Late Cenomanian – Early Turonian (C–T) intervals of the eastern part of the Kopet-Dagh basin, NE Iran
have been investigated to evaluate the response of planktonic foraminifera to the geological event OAE2. The Gharesu
and Taherabad sections with the thicknesses of 30 m and 22.5 m are composed of shale and marl interbedded with
glauconitic sandstone. Three biozones Rotalipora cushmani, Whiteinella archaeocretacea and Helvetoglobotruncana
helvetica were recognized based on study of planktonic foraminifera, in these sections. We observed the patterns of
planktonic foraminiferal assemblage changes around the C–T boundary and divided this succession into several
successive intervals. This study confirms that OAE2 was a long term event. A gradual perturbation in the study
successions starts in the interval 1 with low abundance and diversity of planktonic foraminifera. An enhanced oxygen
minimum zone (OMZ) occurs in the interval 3 which coincides with a temporary absence of planktonic foraminifera
and sedimentation of framboidal pyrite. High diversity of planktonic foraminifera and appearance of new genera in the
interval 5 indicate return of normal conditions to the basin. A significant short-term sea surface temperature cooling is
also indicated by planktonic foraminiferal turnover and carbonate contents in the interval 2 which is comparable with
other parts of the Tethys Ocean, Boreal sea and Atlantic region.
Keywords: biostratigraphy, palaeoecology, planktonic foraminifera, Cenomanian – Turonian boundary, OAE2,
Tethyan realm , Kopet-Dagh basin.
Introduction
Oceanic anoxic events (OAEs) were episodes of widespread
marine anoxia. There were arguably between two and seven
OAEs during the Mid-Cretaceous (Leckie et al. 2002). The
most prominent and widespread of these OAEs spans the
Cenomanian – Turonian boundary interval and is called
Cenomanian–Turonian Boundary Event (CTBE) or OAE2
(Schlanger & Jenkyns 1976; Leckie et al. 2002).
The Cenomanian – Turonian boundary (~ 93.5 Ma) was
a time of transition in the nature of the ocean-climate system.
This interval is known as a typical greenhouse period caused
largely by increased CO
2
from elevated global igneous activity
(Jones & Jenkyns 2001; Poulsen et al. 2001). It is marked
by a major warming peak and globally averaged surface
temperatures more than 14 °C higher than those of today
(Tarduno et al. 1998), ~ 100 – 200 m higher sea level than that
of today (Haq et al. 1987; Miller et al. 2005), a lack of perma-
nent ice sheets and oceanic circulations (Frakes et al. 1992).
These conditions could have increased the rate of chemical
weathering, flux of nutrient to the ocean and biological pro-
ductivity (Pedersen & Calvert 1990). Production of a lot of
organic matter during the time of decreasing oceanic circula-
tions leads to an anoxic condition and superimposition of
three crises during the event: wide scale occurrence of black
shales (Arthur & Premoli Silva 1982; Luciani & Cobianchi
1999; Coccioni & Luciani 2005), faunal turnovers (Leckie et
al. 1998; Keller et al. 2001, 2008; Keller & Pardo 2004; Caron
et al. 2006; Soua et al. 2009, 2011) and a positive δ
13
C excur-
sion (Paul et al. 1999; Keller et al. 2004).
The C–T boundary has been studied worldwide (Luderer
& Kuhnt 1997; Leckie et al. 1998; Luciani & Cobianchi
1999; Keller et al. 2001, 2004, 2008; Coccioni & Luciani
2005; Mort et al. 2008 among others) but this boundary was
little known in Iran. Only a few studies have been conducted
in the Kopet-Dagh basin. Previous reports from the Shurab
and Hmam-Ghale sections in the east of the Kopet-Dagh
basin have suggested the presence of oxygen-restricted depo-
sition in the region, based on the presence of dark shale and
marl sediments and foraminiferal turnovers (Abdoshahi et al.
2010; Ghoorchaei et al. 2011). Other studies described this
boundary as a disconformity (Afshar-Harb 1994; Sadeghi &
Forughi 2004) or paraconformity (Vahidinia et al. 1999) in
the Kopet-Dagh basin.
The investigated sections in the Gharesu and Taherabad
localities are the only well described Cenomanian–Turonian
boundary intervals in the northeastern part of Tethys. It is
therefore a unique opportunity to study the changes in fossil
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content across the Cenomanian – Turonian boundary and the
palaeoceanographic development in this part of the Mid-
Cretaceous world.
The turnover of the planktonic foraminifera, is a particular
focus of this study because their perturbation has been a sen-
sitive monitor of events during the Cenomanian – Turonian
boundary in the previous studies.
Geographical and geological setting
of the studied areas
The Kopet-Dagh basin, which stretches over nearly 700
kilometres from the east of the Caspian Sea to NE Iran, Turk-
menistan and north Afghanistan (Fig. 1C) is composed of
a 5000 –7000 m thick sequence of gently folded rocks of
Middle Jurassic to Eocene age (Afshar-Harb 1994). The Cre-
taceous sediments were deposited on the northern shelf of
a deeper marine basin separating the Iran Plate from Eurasia
(Turan) Plate in the northeast of the Tethys realm (Fig. 1A, B).
This sequence reaches more than 3000 m in thickness and
seems to represent all stages of the Cretaceous (Stocklin
1968). The Aitamir and Abderaz formations are two major
lithostratigraphic units in the Kopet-Dagh basin with Lower
to Upper Cretaceous age (Afshar-Harb 1994).
The Late Cenomanian to Early Turonian interval in the
Gharesu and Taherabad sections spans the upper part of the
Aitamir Formation and lower part of the Abderaz Formation.
The Gharesu section is located a few kilometres west of the
city of Kalat (Fig. 1D). This section is composed of 30 m of
shale and marl interbedded with glauconitic sandstone. The
Taherabad section is 22.5 m thick and is located along the
Mashhad-Kalat main road (Fig. 1D). The lithology of this sec-
tion is similar to the Gharasu section except that the Gharesu
section has more sandstone beds than the Taherabad section.
Material and methods
After preliminary sampling to locate the C–T interval,
22 samples spanning 30 m of the Gharesu section and
49 samples spanning 22.5 m of the Taherabad section were
finally collected. The samples were taken at 20–50 cm
intervals close to the C–T boundary and at 1–2 m intervals
farther away from the boundary.
In the laboratory, samples were crushed and put into a mix-
ture of water and a small amount of H
2
O
2
, then washed
through a 53 µm sieve. For planktonic foraminiferal studies,
about 250–300 specimens were counted and identified except
in the samples around the C–T boundary where foraminifera
are rare or absent. Species identification follows that of
Caron (1985) and Premoli-Silva & Verga (2004) (Figs. 2, 3).
The carbonate contents were measured using the calcimeter
method in the laboratory.
Fig. 1. A — Palaeogeographic map of the Late Cretaceous (Late Cretaceous; © Blakey R.;
http://jan.ucc.nau.edu/~rcb7/RCB.html
) showing
the location of investigated sections in the Kopet-Dagh basin (1, 2) and important sections referred in the text (3 — Pont d’Issole,
4 — Eastbourn, 5 — Tarfaya). The area of map B is outlined; B — Cenomanian palaeogeographic map of the Middle East (modified after
Philip & Floquet 2000); C — structural units of Iran and location of the Kopet-Dagh basin in northeast Iran, southwestern Turkmenistan
and north Afghanistan (after Berberian & King 1981); D — the Gharesu and Taherabad sections along the Mashhad-Kalat main road.
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Fig. 2. SEM illustrations of planktonic foraminifera from the Taherabad section (‘a’ for spiral side, ‘b’ for lateral side, ‘c’ for umbilical
side). 1 a, b, c
— Rotalipora cushmani (Morrow), sample T4. 2 a, b, c — Rotalipora appenninica (Renz) sample T14. 3 a, b, c — Prae
globotruncana gibba Klaus, sample T48. 4 a, b, c — Helvetoglobotruncana helvetica (Bolli), sample T46. 5 a, b, c — Whiteinella prae
helvetica (Trujillo), sample T46. 6 a, b, c — Dicarinella hagni (Scheibnerova), sample T46. 7 a, b, c — Whiteinella archaeocretacea Pessagno,
sample T47.
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Fig. 3. SEM illustrations of planktonic foraminifera from the Taherabad and Gharesu sections (‘a’ for spiral side, ‘b’ for lateral side, ‘c’ for
umbilical side). 1 a, b, c — Marginotruncana pseudolinneiana Pessagno, sample gh21. 2 a, b, c — Rotalipora appenninica (Renz), sample
gh5. 3 a, b, c — Helvetoglobotruncana helvetica (Bolli), sample gh21. 4 a, b, c — Helvetoglobotruncana helvetica (Bolli), sample gh22.
5 a, b, c — Praeglobotruncana stephani (Gandolfi), sample gh6. 6 — Heterohelix globulosa (Ehrenberg), sample T47. 7 — Heterohelix
moremani (Cushman), sample T48. 8 — Guembelitria cenomana Keller, sample T46. 9 — Guembelitria cenomana Keller, sample gh21.
10 — framboidal pyrite, sample T2.
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Biostratigraphy
Despite ammonites, changes in the planktonic foramini-
feral assemblages are less indicative and seem to occur over
a broad interval of time coeval with the contemporary oceanic
environmental perturbation across the Cenomanian –Turonian
boundary.
The foraminiferal biostratigraphy of the Cretaceous has
been studied in great detail by many authors. Wonders (1980)
proposed that C–T boundary corresponds to the base of
the Whiteinella archeocretacea zone. But subsequent studies
(Caron 1985; Robaszynski et al. 1993; Robaszynski &
Caron 1995; Hardenbol et al. 1998; Premoli-Silva & Verga
2004; Zaghbib-Turki & Soua 2013) indicated that this boun-
dary is placed in the middle of the W. archeocrheetacea zone
(Fig. 4).
28 species belonging to 11 genera and 22 species belon-
ging to 10 genera of planktonic foraminifera were recognized
respectively in the Taherabad and Gharesu sections. The
stratigraphic distributions of these planktonic foraminifera
are plotted in Figs. 5 and 6.
Based on the planktonic foraminiferal zonal scheme of
Premoli-Silva & Verga (2004), the following planktonic
fora miniferal biozones were identified in the sections:
Rotalipora cushmani Total Range Zone: This zone is
defined by the first and last occurrences of Rotalipora
cushmani and corresponds to the Middle–Late Cenomanian.
Premoli-Silva & Verga (2004) divided this zone into two
subzones (Rotalipora greenhornensis and Dicarinella
algeriana) which are differentiated by the first appearance of
D. algeriana.
Rotaliporids are not present from the base of this zone in
our sections, but this group appears from a height of 2.5 m in
the Taherabad section and 10.5 m in the Gharesu section.
Rotaliporids (R. cushmani, R. appenninica) are scarce in this
biozone and are associated with whiteinellids, heterohelicids,
muricohedbergellids and dicarinellids.
In the Gharesu section the first appearance of rotaliporids
coincides with the first appearance of dicarinellids, but in the
Taherabad section D. algeriana appears before rotaliporids.
The absence of rotaliporids and dicarinellids in the base of
sections resulted from eutrophic condition and/or low oxygen
content or by shallower marine environmental conditions.
So we suggest the interval from the base of sections to the
last occurrence of
R. cushmani belongs to the D. algeriana
subzone.
Whiteinella archaeocretacea Partial Range Zone: This
zone spans the interval from the last occurrence of R. cush
mani to the first occurrence of H. helvetica and extends from
the latest Cenomanian to earliest Turonian.
This biozone appears from a height of 6.5 m in the
Taherabad section and 14 m in the Gharesu section. Assem-
blages in this interval, if present, consist of whiteinellids,
muricohedbergellids, heterohelicids and guembelitriids.
Helvetoglobotruncana helvetica Total Range Zone:
The base and top of the zone coincide respectively with the
first and last occurrences of the index-species H. helvetica.
This zone ranges from Early to Middle Turonian.
We recognized the species about 19.5 m and 29 m above
the base of the Taherabad and Gharesu sections, respectively.
The most important and common foraminifera in this zone
are: dicarinellids, whiteinellids, muricohedbergelids, hetero-
helicids and praeglobotruncanids. Also Marginotrucana
species are present in the Gharesu section.
Palaeo-ecology and planktonic
foraminiferal turnover
In the past 20 years, the palaeo-ecology of foraminifera
became a dominant field of micropalaeontology. Significant
changes in foraminiferal assemblages can be interpreted as
reflecting an ecological response to palaeo-oceanographic
variations. Among other foraminifera the ecology of plank-
tonic foraminifera plays an important role in the study of
oceanic systems (Nebrigic 2006).
Recent planktonic foraminifera reach their highest diver-
sity within a stratified water column with normal salinity,
Fig. 4. Comparison of standard zonations of the Tethyan Realm with study sections. Time scales of Gradstein et al. (1995) and
Hardenbol et al. (1998) are adopted.
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nutrient and oxygen content (Hart 1980 a, b; Leckie et al.
1998; Hallock et al. 1991; Keller et al. 2001; Keller & Pardo
2004). In these conditions large, keeled, deep-water dwelling
forms (K strategists, such as: Rotalipora and Helvetoglobo
truncana) increase. But when environmental conditions are
more extreme, diversity is low and opportunistic taxa
(r strate gists, such as: Heterohelix and Muricohedbergella)
are common (Keller et al. 2001; Coccioni and Luciani 2005).
These simpler morphotypes with little surface ornamentation
and thin test walls generally occupy the surface mixed layers
(above thermocline), or shallow waters with generally
un stable and/or eutrophic conditions (Hart 1980 a, b; Caron &
Homewood 1983; Leckie et al. 1998; Keller 1988; Li &
Keller 1998; Keller et al. 2001).
In order to interpret foraminiferal distribution across the
C–T transition, Coccioni and Luciani (2005) summarized
published data on stable isotopic analysis together with depth
ranking based on environmental inferences (morpho logy and
palaeobiogeographic distribution) in their article (Fig. 7).
Based on changes in
planktonic foraminiferal
distribution and diversity,
we divided the Gharesu
and Taherabad sec tions
into 5 discrete intervals:
Interval 1: This inter-
val (samples gh1–gh4
in Gharesu
section and
T0–T3 in the Taherabad
section, the lower part
of R. cushmani zone) is
characterized by low spe-
cies diversity and abun-
dance and low carbonate
content (Fig. 8). In the
Gharesu section plank-
tonic foraminifera are not
present or too scarce to be
counted. In the Taherabad
section simple morpho-
type of planktonic fora-
minifera with globular
chambers and trocho -
spiral, biserial and tri-
serial test (Whiteinella,
Murico hedbergella, Hete
rohelix and Guembelitria)
are present. Keeled forms
(Dicarinella algeriana)
are present in very low
abundance in sample T1
(Fig. 9).
We propose this interval
was deposited in a marine
environment with high
terri genous influx and
eutrophic condition. Framboidal pyrite (Fig. 3-10) is also
present in all samples of this interval. These framboids are
50 –150 µm in diameter and are composed of tiny crystallites
of 2–5 µm which are spheroidal, cubic, octahedra or irregular
in shape. The formation of framboidal pyrite in sediments or
during sedimentation requires an anaerobic environment
(Schallreuter 1982). Under low oxygen content conditions,
sulphate-reducing bacteria produce hydrogen sulphide which
reacts with available iron to form microconcretions of pyrite,
possibly by the mechanisms suggested by Berner (1969).
Interval 2: At the onset of this interval (samples gh5–gh7
in the Gharesu section and T4–T14 in the Taherabad section,
the upper part of the R. cushmani zone), rotaliporids appear
in both sections. A marked increase in diversity and abun-
dance of planktonic foraminifera occurs in this interval, as
the number of species in the Gharesu section increases from
1 to 10 and in the Taherabad section from 6 to 15 (Fig. 9), this
is accompanied by a relative increase of CaCO
3
content
(Figs. 8, 9) which suggests, terrigenous input decreases in
Fig. 5. Distribution and species richness of planktonic foraminifera in the Gharesu section.
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FORAMINIFERAL TURNOVER ACROSS OAE2, KOPET-DAGH, IRAN
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this interval. However, the
condition in this interval is
more stable, a relatively high
percentage of Whiteinella,
Murico hedbergella and Guem
belitria and low abundance of
keeled forms (2–5 % in sam-
ples which are pre sent) indi-
cate a general meso trophic
regime in interval 2.
Interval 3: This interval
(samples gh8–gh16 in the
Gharesu section and T15–T39
in the Taherabad section, the
lower part of the W. archaeo
cretacea zone) starts with the
disappearance of rotaliporids
and ends with a thick glauco-
nitic sandstone. At the base
of this phase, a dramatic
Fig. 6. Distribution and species richness of planktonic foraminifera in the Taherabad section.
Fig. 7. Inferred life strategy of upper Cenomanian and lower Turonian planktonic foraminifera derived
mainly from latitudinal distribution, abundance and depth ranking based on environmental inferences
(morphology and biogeographic distribution), and stable isotopic data, plotted against the oceanic
surface trophic resources continuum (modified after Coccioni & Luciani 2005).
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de crease in diversity and abun-
dance of planktonic foramini-
fera occurs. Planktonic fora-
minifera are absent except for
sample gh11 in the Gharesu
section and samples 17–19,
22–26 in the Taherabad sec-
tion. The assemblage contains
dwarfed and scattered
Guem
belitria and Whiteinella in
the Gharesu section and Guem
belitria, Heterohelix, Whitei
nella and Muricohedbergella
in the Taherabad section.
Keeled morphotypes are pre-
sent in very low abundance
(almost 2 %) in samples 23, 24,
26 in the Taherabad section
(Fig. 9).
All the above, low percen-
tage of carbonate, presence of
sandstone beds and framboidal
pyrite in the sediments, indi-
cate sedimentation occurs in
a marine environment with
high productivity and low oxy-
gen content.
The presence of benthic fora-
minifera in samples where
planktonic foraminifera dis-
appear temporarily indicates
that the water column and bot-
tom water is not extremely
depleted in oxygen content and
absence of planktonic forami-
nifera is a result of a high detri-
tal input and productivity
beside a developed oxygen
minimum zone (OMZ).
Interval 4: At the onset
of this interval (samples
gh17–gh20 in the Gharesu sec-
tion and samples T40–T45 in
the Tahera bad section, the
upper part of the W. archaeo
cretacea zone) planktonic fora-
minifera reappear with higher
abundance and diversity.
Heterohelicids dominate the
assemblage. Whiteinella and
Muricohedbergella are present
with high abundance. Guembe
litria are very rare in this
interval. Keeled forms are
present in all samples from the
Taherabad section and in
Fig.
8.
Species
richness
and
carbonate
content
profiles
for
Tethys,
Boreal
and
Atlantic
sites.
Tie
lines
are
based
on
biostratigraphic
correlation.
Horizontal
white
band
indicates
the
extent
of
the
sea
surface
temperature
cooling.
Horizontal
grey
band
shows
expansion
of
OAE2
in
the
sections.
Pont
d’Issole
data
are
from
Grosheny
et
al.
(2006)
and
Jarvis
et
al.
(201
1),
Eastbourne
data are from Keller et al. (2001) and
Tarfaya data are from Keller et al. (2008).
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sample 18 from the Gharesu section (although with low
abundance ~ 4–5 %) (Fig. 9).
This interval coincident with maximum Heterohelix abun-
dance. This event (the so called Heter
ohelix shift) was also
observed at the same stratigraphic interval in the world
(Leckie et al. 1998; Keller et al. 2001, 2008; Keller & Pardo
2004; Zaghbib-Turki & Soua 2013) and is an excellent
biomarker for global correlations. Heterohelix shift appears
to be associated with a global expansion of the oxygen
minimum zone (Keller & Pardo 2004, Zaghbib-Turki &
Soua 2013).
Increasing of planktonic foraminiferal abundance and
carbonate content, show that terrigenous input is low in
this interval.
Interval 5: This interval (samples gh21–gh22 in the
Gharesu section, samples T46–T49 in the Taherabad section,
H. helvetica zone) starts with the appearance of Helveto glo
botruncana helvetica. The diversity and abundance of plank-
tonic foraminifera are higher than in other intervals (Fig. 9).
The appearance of new genera (Helvetoglobotruncana and
Marginotruncana in the Gharesu section and Helvetoglo
botruncana in the Taherabad section) and abundance of
keeled forms indicate this interval is deposited under a relatively
normal salinity, oxygen content and oligotrophic condition.
Guembelitria disappear in this interval because this genus
thrived in eutrophic surface waters of shallow marginal
marine environments with variable salinities at times of
severe ecological stress (Keller 2002; Keller et al. 2002).
Correlation and discussion
Lower abundance and number of species of planktonic
fora minifera, lower keeled forms and more sandstone inter-
calation in the Gharesu section suggest that this section was
deposited in a shallower water environment than the Tahera-
bad section but all other changes in the sea level and appea-
rance and disappearance of species are comparable in these
sections (Fig. 9).
In the Gharesu and Taherabad sections, decrease of oxygen
content starts in interval 1 and continues in intervals 2, 3 and
4, as is revealed by the presence of framboidal pyrite in the
deposits and planktonic foraminiferal content. High numbers
of the opportunists and Heterohelix shift suggest that the
environmental perturbation related to the CTBE did not end
during interval 3 but continued into interval 4.
It seems that, recovery phase occurs in the Helvetoglo
botruncana helvetica zone (interval 5) and is marked by an
Fig. 9. Correlation of the Gharesu and Taherabad section based
on biostratigraphy, distribution pattern of planktonic foraminifera
and lithology.
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increase of both abundance and diversity of planktonic
foraminifera.
Good correlation between diversity of planktonic forami-
nifera and CaCO
3
content suggests that detrital input and pro-
ductivity beside oxygen content are effective factors in pat-
terns of change in planktonic foraminiferal assemblages in
the Gharesu and Taherabad sections, as in the intervals 1 and 3,
high detrital input leads to a eutrophic condition and decreases
the diversity of planktonic foraminifera (Fig. 9).
The Late Cenomanian sea surface
temperature cooling
On a global scale, the C–T boundary is characterized by its
warm climate interval accompanied by a major sea level
transgression (Hancock & Kauffman 1979; Haq et al. 1987;
Uličný et al. 1993; Hardenbol et al. 1998). However, many
authors have described an ephemeral but significant decrease
in sea surface temperatures at the onset of OAE2 in some
CTB sections (Lamolda et al. 1994; Forster et al. 2007; Jarvis
et al. 2011; Kaiho et al. 2014 among others). This cooling has
been attributed to a drop in atmospheric pCO
2
(Symbol for
the negative decadic logarithm of the CO
2
concentration)
levels which in turn was caused by enhanced carbon seques-
tration by burial of organic matter (Hasegawa et al. 2003;
Forster et al. 2007). This event was first recognized as an
incursion of boreal fauna in the shelf seas of NW Europe
(e.g., Jefferies 1962; Gale & Christensen 1996; Voigt et al.
2004) and called the Plenus Cold Event. In the Western
Interior Seaway of North America and in the equatorial
proto- Atlantic, this interval was characterized by repopu-
lation of the seafloor by benthic foraminifera (the Benthic
Oxic Event), indicating regional reoxygenation of bottom
waters (Lekie et al. 1998; Keller et al. 2001, 2008; Keller &
Pardo 2004).
This short-lived cooling event in the Gahresu and
Taherabad sections is characterized by a decreased detrital
input and increased carbonate production and diversity of
planktonic foraminifera in interval 2, at the end of the
R. cushmani zone (Fig. 9).
This event is compared with those at Pont d’Issole (France,
Tethys Ocean), Eastbourne (England, Boreal sea) and Tazra
(Morocco, Atlantic region) in Figure 8.
The record of Pont d’Issole in France (Tethys Ocean)
(Grosheny et al. 2006; Jarvis et al. 2011) displays two promi-
nent black shale intervals within an interbedded lime-
stone-grey marl succession. This succession is similar to
Gharesu and Taherabad in many ways (increase of carbonate
and planktonic foraminiferal diversity in cooling interval) but
differs from Gharesu and Taherabad by its further depth and
sedimentation of black shale before and after the cold event.
The CTB interval at Eastbourne section in England (Boreal
sea) consists of chalk and rhythmically bedded marls (Plenus
Marls Formation) (Gale et al. 1993). In the Plenus marls, the
relative abundance increase of keeled taxa and increase of
planktonic foraminiferal diversity, coincident with decreased
Heterohelix abundances, suggest a well-stratified water mass
and reduced oxygen minimum zone (Keller et al. 2001).
In contrast to Gharesu and Taherabad (Tethys realm), marls
and marly chalk deposits at Eastbourne (Boreal Sea), with
decreased calcite, increased phyllo-silicates, quartz and feld-
spar were deposited in the Plenus Cold Event period (Keller
et al. 2001). This difference in carbonate content of the
Plenus Cold Event interval may be explained by the different
palaeo-latitude of sections. The Gharesu and Taherabad
sections with their lower palaeo-latitude are less effected by
sea-level lowstand periods, increased erosion and accelerated
detrital input.
As before mentioned, synchronous with the Plenus Cold
Event, a period of extensive bottom water reoxidation
occurred throughout the Atlantic region. In this interval at the
Tazra section (Morocco, Atlantic Ocean), the low oxygen
tole rant Heterohelix populations dramatically decreased in
response to more oxygenated waters throughout the water
column, Guembelitira thrived and the surviving rotaliporids
species rapidly disappeared (Keller et al. 2008). Low Calcite/
Detritus ratios in the cooling event indicate increased detrital
input, linked to intensified upwelling in the western African
margin (Keller et al. 2008).
In fact, this cooling event is an unusual interlude in the long-
term trend of oceanic anoxia during the Cenomanian–
Turonian interval. At the start of OAE2, higher sea surface
temperature together with enhanced runoff and nutrient input
resulted in burial of organic matter. Burial of organic matter
(OM) lowered pCO
2
, cooling the greenhouse climate. At the
end of the OAE2, when carbon burial rates were reduced,
pCO
2
increased again (van Bentum et al. 2012). In all
described sections, faunal turnover suggests the expansion of
the oxygen minimum zone before and after the cold event
(Fig. 8)
Conclusion
The two Cenomanian –Turonian sections at Gharesu and
Taherabad in the Kopet-Dagh basin coincide with the upper
part of the Aitamir Formation and lower part of the Abderaz
Formation. The C–T boundary in these two sections is con-
formable based on presence of the three following zones:
Rotalipora cushmani Total Range zone, Whiteinella archaeo
cretacea Partial Range Zone and Helvetoglobotruncana
helvetica Total Range Zone. Planktonic foraminiferal assem-
blage changes show five intervals with different degrees of
environmental perturbation associated with the OAE2. In the
two sections, the most dramatic changes took place during
interval 3. Temporary disappearance of all planktonic
foraminifera in some samples, low carbonate content and
presence of framboidal pyrite indicate an increased detrital
input, surface productivity and an enhanced OMZ in
interval 3. High abundance of simple planktonic morpho-
types especially heterohelicids shows that low oxygen
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GEOLOGICA CARPATHICA
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2016, 67, 5, 451 – 462
content continues into interval 4. Appearance of the new
genera Helvetoglobotruncana and Marginotruncana in the
upper part of sections indicates that this part of the sediments
was deposited under more normal and stable condition.
A decreased run off also coincides with increased carbonate
contents and diversity of planktonic foraminifera in interval 2
(at the end of the R. cushmani zone) can be interpreted as
evidence of a short-term cooling in the studied succession,
which is comparable to the Plenus Cold Event in the Tethys
realm and benthic oxic zone in the Atlantic Ocean.
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