Response of oceanic organisms to abiotic events in the Paleogene

Abstract

Climate fluctuations with the optimum in the Early Eocene and subsequent cooling were the main abiotic factor that controlled
the development of the oceanic biota in the Paleogene. The Paleogene represented the transitional stage from the greenhouse
climate of the Mesozoic to the partly glacial Neogene and was characterized by changes in the distribution of the temperatures
in the ocean with the replacement of the dominant latitudinal thermal circulation by the largely meridional thermohaline one.
The climate changes were also determined by other factors: the opening and closure of seaways between basins, the position
of major currents, volcanic activity, the sea-level fluctuations, the composition of the hydro- and atmosphere, and others.
These changes were, in turn, determined by factors of higher order, primarily, by tectonic movements: vertical and horizontal
(motions of lithospheric plates). The contribution of impact events to this process is also highly probable. All these factors
influenced, via the hydrological and hydrochemical parameters of the water column, the evolution of the oceanic biota: their
distribution areas, the sizes of the organisms, the diversity of the communities, the bioproductivity, and the mass extinction
(for example, the extinction of 30–50% of the benthic foraminifers at the Paleocene-Eocene transition in response to the abrupt
temperature increase). The Eocene-Oligocene transition (38 Ma ago) was marked by a global biotic crisis, the most significant
one in the Cenozoic, when the abyssal part of the ocean was filled with cold water to form the psychrosphere. At least five
major impact events, which preceded the Oligocene mass extinction of the biota, occurred in the terminal Eocene (36–35 Ma
ago).

Full text

385
ISSN 0001-4370, Oceanology, 2009, Vol. 49, No. 3, pp. 385–395. © Pleiades Publishing, Inc., 2009.
Original Russian Text © M.S. Barash, 2009, published in Okeanologiya, 2009, Vol. 49, No. 3, pp. 418–429.
INTRODUCTION
The Paleogene represented the transitional stage
from the Mesozoic to Neogene and was characterized
by changes in the distribution of the temperatures in the
ocean with the replacement of the dominant latitudinal
thermal circulation by the largely meridional thermoha-
line one. It was intermediate between the ice-free to
glacial climatic systems. The climatic changes were
determined by other factors as well, had different con-
sequences, and were associated with other abiotic fac-
tors that controlled the development of organisms: sea-
level fluctuations, the opening and closure of seaways,
volcanic activity, the composition of the hydro- and
atmosphere, and others. These changes were, in turn,
governed by factors of higher order, primarily, by tec-
tonic movements: vertical (orogenic, growth of the
mid-oceanic ridges) and horizontal (motions of lithos-
pheric plates). The contribution of impact events (colli-
sion of the Earth with asteroids and comets) to this pro-
cess is also highly probable. All these factors influ-
enced, via the hydrological structure and the
hydrochemical hydrochemical properties of the water
column, the evolution of the oceanic biota: their distri-
bution areas, the sizes of the organisms, the diversity of
the communities, the bioproductivity, and the mass
extinction of organisms.
This study is dedicated the Paleogene period (65.5
to 23.03 Ma ago), which includes the Paleocene (65.5
to 53.8 Ma ago), Eocene (53.8 to 33.9 Ma ago), and
Oligocene (33.9 to 23.03 Ma ago) epochs. Hereinafter,
the ranges of the stratigraphic units and their ages are
given in accordance with the stratigraphic scale
adopted by the International Geological Congress [17].
The development of organisms in the epicontinental
and intracontinental seas, which are characterized by
their own complex geological history in the context of
the global changes, is omitted from this consideration.
The Cenozoic as a whole was characterized by a
cooling trend. It comprises two climatic stages: the
Paleogene and Neogene. The Paleogene represented
the transitional stage between the Mesozoic to Neogene
and was characterized by changes in the distribution of
temperatures in the ocean with the replacement of the
dominant latitudinal thermal circulation by the largely
meridional thermohaline one. It was intermediate
between the ice-free to glacial climatic systems. The
drastic changes in the development of the climatic and
paleoceanological settings at the Paleogene–Neogene
and Neogene–Quaternary transitions are explained by
the particular tectonic events.
In the Late Eocene (approximately 40 Ma ago), the
separation of Australia and Antarctica resulted in the
appearance of shallow water exchange between the
MARINE GEOLOGY
Response of Oceanic Organisms to Abiotic Events
in the Paleogene
M. S. Barash
Shirshov Institute of Oceanolgy, Russian Academy of Sciences, pr. Nakhimovskii 36, Moscow, 117997 Russia
E-mail: barashms@yandex.ru
Received April 14, 2008; in final form, May 25, 2008
Abstract
—Climate fluctuations with the optimum in the Early Eocene and subsequent cooling were the main
abiotic factor that controlled the development of the oceanic biota in the Paleogene. The Paleogene represented
the transitional stage from the greenhouse climate of the Mesozoic to the partly glacial Neogene and was charac-
terized by changes in the distribution of the temperatures in the ocean with the replacement of the dominant lati-
tudinal thermal circulation by the largely meridional thermohaline one. The climate changes were also determined
by other factors: the opening and closure of seaways between basins, the position of major currents, volcanic activ-
ity, the sea-level fluctuations, the composition of the hydro- and atmosphere, and others. These changes were, in
turn, determined by factors of higher order, primarily, by tectonic movements: vertical and horizontal (motions of
lithospheric plates). The contribution of impact events to this process is also highly probable. All these factors
influenced, via the hydrological and hydrochemical parameters of the water column, the evolution of the oceanic
biota: their distribution areas, the sizes of the organisms, the diversity of the communities, the bioproductivity, and
the mass extinction (for example, the extinction of 30–50% of the benthic foraminifers at the Paleocene–Eocene
transition in response to the abrupt temperature increase). The Eocene–Oligocene transition (38 Ma ago) was
marked by a global biotic crisis, the most significant one in the Cenozoic, when the abyssal part of the ocean was
filled with cold water to form the psychrosphere. At least five major impact events, which preceded the Oligocene
mass extinction of the biota, occurred in the terminal Eocene (36–35 Ma ago).
DOI:
10.1134/S0001437009030114

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southern areas of the Pacific and Indian oceans, which
provided the first immediate biogeographic contact
between the shallow-water and planktonic organisms of
these basins. The subsequent separation of the southern
continents (Antarctica, Australia, and South America)
was accompanied by the opening of the Tasman Sea,
the formation of the Drake Passage, and the develop-
ment of the circum-Antarctic circulation system. The
appearance of the Circum-Antarctic Current deter-
mined the thermal isolation of Antarctica and separat-
ing from it the warm waters of subtropical gyres. The
formation of the South Ocean stimulated the growth of
the Antarctic continental glaciation, which influenced
the climate evolution and resulted in cooling in many
world regions, including the northern polar latitudes.
The new climatic regime affected the environments and
biogeographic evolution of the biota in the World
Ocean. The cooling in both near-polar regions was
responsible for intensified thermal convection and
related process: the movement of the bottom waters
toward the low latitudes, the significant cooling of the
bottom water mass, and the corresponding changes in
the abyssal fauna.
The equatorial and low latitudes were, in contrast,
marked by the breaking of the formerly continuous cir-
cum-global equatorial current due to the continental
drift or formation of land massifs: the Tethys became
closed, Australia advanced northward, and the seaway
between North and South America was also closed.
These processes changed the width and intensity of the
equatorial current with the associated upwelling and
affected the balance of nutrients, the biological produc-
tivity, and the development of the oceanic flora and
fauna.
Judging from the oxygen isotope composition in
tests of planktonic and benthic foraminifers, the cool-
ing events were rapid. They occurred in the Early–Mid-
dle Eocene, at the Eocene–Oligocene transition, in the
Middle Miocene, and the Late Pliocene. The cooling
episodes alternated with warming periods, which were
particularly significant in the Eocene (Fig. 1). These
climatic fluctuations resulted in biogeographic and
other changes in the biotic communities of the ocean.
Figure 2 illustrates the relations between the global
tectonic and volcanic events, on the one hand, and the
climatic changes, on the other hand.
The analysis of the size variability in planktonic for-
aminifer tests during the last 70 My in response to envi-
ronmental changes [35] revealed that they decreased in
size at the Cretaceous–Paleogene boundary and
remained small in high-latitude assemblages. Accord-
ing to the latest research, the evolution of the test sizes
in low latitudes comprises three phases: the first phase
(65 to 42 Ma ago) is characterized by dwarf forms; the
second phase (42 to 14 Ma ago) is marked by small
average sizes with some fluctuations; and the third
phase (the last 14 My) is remarkable for the unprece-
dented growth of planktonic foraminiferal tests, which
remain large in recent communities. The analysis of the
correlation between the test sizes and the different
external factors reveals that the periods of the size
Fig. 1.
Climatic changes during the last 65 My (after [32] with modifications).
(Pal) Paleocene; (Eo) Eocene; (Ol) Oligocene; (Mio) Miocene; (Pli) Pliocene; (Plt) Pleistocene; (PETM) Paleocene–Eocene Ther-
mal Maximum; (Vostok) based on data derived from the ice core of the
Vostok
station.
0
1020304060 50 0
2
0
4
6
8
10
12
1
2
3
4
–2
–4
–6
–8 5
Pal Eo Ol Mio Pli
Plt
Polar ocean
∆
T
(
°
C)
êÖíå
Eocene
optimum
Ma
Pal Eo Ol Mio Pli
Plt
Antarctica
glaciation
Warming
in the Antarctic
Antarctica
glaciation
Glacial
cycles
Vostok
∆
T (
°
C)
Benthos
δ
18
O
‰

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RESPONSE OF OCEANIC ORGANISMS TO ABIOTIC EVENTS 387
Fig. 2.
Main Cenozoic abiotic events (after [43] with modifications).
growth coincide with the phases of global cooling
(Eocene and Neogene). These periods were character-
ized by enhanced latitudinal and vertical thermal gradi-
ents and high biodiversity (polytaxy). The most signif-
icant changes in the test sizes are observable in the low
latitudes, while the maximal temperature fluctuations
occurred in the polar latitudes. It seems that the test size
in the foraminiferal assemblages is determined most
likely by the latitudinal and vertical gradients rather
than by the water temperature per se.
Paleocene.
The events at the Cretaceous–Paleogene
transition that coincided with the collision of the Earth
with a large asteroid or comet [4] were responsible for
the global collapse of ecosystems and the mass extinc-
tion of organisms. Two groups of organisms that sur-
vived these events (dinoflagellates and benthic fora-
minifers in the stratotype section of the Cretaceous–
Paleogene boundary interval in El Kef [16]) provide
evidence for drastic changes in the oceanic circulations.
The oxygen isotope changes derived from tests of
planktonic and benthic foraminifers demonstrate that
both the surface and bottom water temperatures
decreased during the Paleogene (Fig. 3).
By the Middle Paleocene, the planktonic microor-
ganisms recovered after the crisis to become most
abundant and diverse in the Middle Eocene, when the
surface waters of the ocean were warm at all the lati-
tudes. For example, the diversity of planktonic foramin-
ifers at the beginning and end of the Paleocene was six
and 64 species, respectively [5]. During the Paleocene
climatic maximums approximately 62 and 60 Ma ago,
which are characterized by low meridional thermal gra-
dients, thermophilic taxa of planktonic foraminifers
migrated with warm boundary currents southward up to
the Falkland Plateau and northward up to western
Greenland and the North Sea.
There are numerous indications of significant
changes in the oceanic biota under the influence of abi-
otic factors (primarily, the rise in temperature) available
for the terminal Paleocene and Paleocene–Eocene tran-
sitional period. The Paleocene–Eocene thermal maximum
was characterized by rapid global warming by
6–8°
C and
a negative shift in the carbon isotope ratio by
3
‰
, as is
evident from both oceanic and terrestrial sections
[11, 19]. The authors assume rapid release of large
3
10
20
30
40
50
60
70
5
021
0
–1 0 1 2
4
0 4 8 12
°
C
3
Closure
of the Panama
seaway
Volcanism
of the Columbia River area
Accelerated
Tibet uplifting
Red Sea rifting
Plate reorganization
and Andes rise
Opening
of the DrakePassage
Opening
of the Tasman
Passage
Plate reorganization
and reduced
spreading
velocity
Rifting
and volcanism
in the North Atlantic
Collision
of India and Asia
Impact
West Antarctica
Intensification
of the Asian
monsoon
East Antarctica
Middle Miocene
climatic optimum
Mi-l
glaciation
Late Oligocene
warming
Oi-l
glaciation
Appearance
of ephemeral
ice shields
Early Eocene
climatic optimum
Late Paleocene
thermal maximum
Ice shields of Antarctica;
Ice shields of the Northern Hemisphere
δ
13
C(
‰
)
Tectonic
events
Climatic
events
δ
18
O (
‰
)
Ma
MioceneOligoceneEocenePaleocene
Glaciation:
partial or temporal
complete and permanent
Temperature
Plio
.
Plst
shield
shield

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methane volumes from gas hydrates into the atmo-
sphere, which resulted in the greenhouse effect.
Benthic foraminifers, which survived the mass
extinction in the terminal Cretaceous without signifi-
cant changes, suffered similar mass extinction at the
Paleocene–Eocene transition, when approximately
50% of their species disappeared (ODP Hole 690) [39].
It is conceivable that the changes in the bottom environ-
ments were much more abrupt and stronger at that time
than in the terminal Cretaceous. The temperature of the
deep waters increased by
6°ë
and was close to that of
the surface waters, which reduced the vertical gradient.
The benthic foraminifer and ostracode assemblages
were dominated by small forms with fragile tests. The
extinction among planktonic taxa and small marine
invertebrates was insignificant. This deepwater biota
extinction event is considered to be the most significant
one in the last 90 My. It was brief (<3 Ky) [21] and
caused by the abrupt warming of the deep waters and
the oxygen depletion in them.
The extinction event was followed by a period with
dominant development of small thin-walled forms
among the benthic foraminifers. The surviving taxa
such as
Nuttallides truempyi, Bulimina semicostata,
Pullenia bulloides
, and
Globocassidulina subglobosa
were dominant elements of their assemblages until the
Middle Eocene. Despite their radiation with a peak in
the initial Middle Eocene, benthic foraminifers have
never reached the diversity that was characteristic of
their Cretaceous and Early Paleocene assemblages.
In the Antarctic region, planktonic microfossils
demonstrate distinct changes: the diversity of plank-
tonic foraminifers, dinoflagellates, and calcareous
algae (“calcareous nannoplankton”) increased in the
middle and high latitudes. This is usually explained by
the intrusion of warm waters from low latitudes into the
South Ocean due to the surface water temperature ris-
ing. These areas are characterized by the appearance of
morozovellids and the bloom of discoasters. The abrupt
environmental changes are reflected in the brief excur-
sions of the stable oxygen and carbon isotopes in the
carbonate tests of foraminifers.
The abrupt warming of the deep waters was likely
determined by the rapid distribution of the warm saline
deep waters (WSDW) that were produced in the Tethys
middle latitudes against the background of the reduced
generation of deep waters in the high latitudes (the hal-
othermal circulation in contrary with the recent thero-
mohaline one) [21, 27, 39] (Fig. 4).
Kennett and Stott [21] define a combination of sev-
eral factors responsible for such events: the uniformly
warm climate, including the Antarctic region; the insig-
nificant meridional thermal gradient; the ice-free conti-
nents; and the distinct greenhouse conditions. The wide
Fig. 3.
The stable oxygen isotope ratio in marine microfossils from the Paleocene to Early Oligocene. (A) bottom waters; (B) surface
waters (after [26], simplified).
–1.03.0 2.5 2.0 1.5 1.0 00.5 –0.5
Epoch
ÄÇ
Early
Oligocene
Late
Eocene
Middle
Eocene
Early
Eocene
Late
Paleocene
Early
Paleocene
δ
18
O

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RESPONSE OF OCEANIC ORGANISMS TO ABIOTIC EVENTS 389
Tehtyan seaway north of Africa could have provided
large volumes of warm and saline deep waters. The
triggering mechanism of the ocean warming and biotic
reorganization in the terminal Paleocene remains, how-
ever, unclear so far. As follows from the model pre-
sented by the authors, they consider the oxygen defi-
ciency to be the immediate cause of the deep fauna
death.
Thomas [39] assumes that the mass release of meth-
ane from gas hydrates into the atmosphere was respon-
sible for the warming. She believes that gradual warm-
ing due to prolonged release of
ëé
2
into the atmo-
sphere during eruptions in the North Atlantic volcanic
province probably served as a triggering mechanism for
the gas hydrate dissociation. The temperature rise
resulted in enhanced dissociation of gas hydrates,
which increased, in turn, the temperature to produce
hyperthermal conditions, which explains the oxygen
isotope shift. The rapid global extinction of 30–50% of
the benthic foraminiferal taxa could have been caused
by the oxygen-deficient conditions due to the high tem-
perature or methane oxidation. The CaCO
3
content in
the sediments decreased almost to zero through the
entire World Ocean, which was noted in all the deep-
sea drilling holes from the abyssal zone to the outer
shelf. The main components of the assemblages imme-
diately after the extinction event were stagnation-resis-
tant taxa.
The Paleogene sediments of the Shatsky Rise reflect
the cyclicity of the changes. They demonstrate hyperther-
mal episodes in the initial Late Paleocene (58.4 Ma ago),
during the Paleocene–Eocene thermal maximum
(55.8 Ma ago), and in the Early Eocene (52.7 Ma ago).
During the thermal maximum, the temperature of the
surface waters increased by
5°
C, which was accompa-
nied by reorganization of the benthic and planktonic
communities and a notable brief lysocline rise [8].
There are geochemical data indicating enhanced pri-
mary production at that time sufficient for the reduction
of the greenhouse gases in the atmosphere and termina-
tion of the global warming [30], although some authors
assume depletion of the near-surface waters in biogenic
elements during the Paleocene–Eocene thermal maxi-
mum. The appearance of some planktonic foraminifer
species such as
Parasubbotina paleocenica, Acarinina
africana
, and
A. sibiyanesis
during the thermal maxi-
mum implies their wide expansion under the high pro-
ductivity of the surface waters in tropical and subtropi-
cal upwellings of the Atlantic and Pacific oceans. These
data are in accordance with the high productivity in
many benthic communities and dinoflagellate assem-
blages, as well as with the materials on the intensified
weathering; the influx of biogenic elements into the
ocean; and the enhanced productivity, which is evident
from the Sr/Ca value and the increased Ba accumula-
tion rates.
In the opinion of some researchers [10, 22], the
widely accepted concept of the Paleocene–Eocene ther-
mal maximum as resulting from the gradual warming
due to terrestrial factors is inconsistent with the sudden
onset of the climatic, geochemical, paleogeographic,
and biotic changes. These authors believe that the latter
were triggered by some catastrophic event such as the
collision of the earth with an asteroid or comet (impact
event).
This impact event could have caused oxidation of
terrestrial organic matter that accumulated in the termi-
nal Paleocene. This could have immediately affected
the carbon isotope ratio in the atmosphere and ocean.
The influx of significant quantities of carbon enriched
in the
12
ë
isotope could have resulted in the greenhouse
effect culminating in the thermal maximum and, prob-
ably, in dissociation of gas hydrates in bottom sedi-
ments. Indirect evidence in favor of an impact event is
the unusually high content of magnetic nannoparticles
in kaolinite-rich shelf sediments that accumulated at
the very beginning of the carbon isotope excursion.
Also noteworthy is the insignificant (although remark-
able) increase in the iridium concentration in the sedi-
ments. The authors consider the abrupt increase in the
kaolinite accumulation rates as resulting from the rapid
weathering of the dust cover that accumulated on the
continents during the impact event and the washing out
of its products into the ocean.
Eocene.
The initial Eocene approximately 2 My
after the Paleocene–Eocene thermal maximum was
marked by another hyperthermal event registered in
five deep-sea drilling holes (ODP Leg 208) on the
Walvis Ridge in the southeastern Atlantic [24]. In the
section, this event is reflected in a bed of red clay with
a sharply reduced
ë‡ëé
3
content, which indicates the
CCD rising to approximately 2 km. Tests of the plank-
tonic surface foraminiferal species
Acaranina
solda-
doensis
demonstrate the negative shift of
δ
l
3
C and
δ
18
O
by 1.5 and
2.5
‰, respectively. These data indicate an
abrupt temperature rise. The event was on a global
scale. The authors arrived at the conclusion that, similar
to the Paleocene–Eocene maximum, the latter was astro-
nomically determined and reflects 400- and 100-Ky
cycles of eccentricity variations. The Eocene warming
episode that culminated approximately 50 Ma ago was
likely the most significant one for the entire Cenozoic
and coincided with the extensive transgression, which
widened the habitat niches of marine organisms. The
species diversity of microorganisms was particularly
high, reflecting the peak of evolutionary radiation. The
assemblage of calcareous nannofossils in the South
Ocean was as diverse as 20–30 species [3]. Tropical
foraminifer taxa with small tests populated the oceanic
realm from the Antarctica to
55°
N [6].
Due to the particular position of the tectonic plates,
the Arctic basin was almost completely isolated from
the World Ocean in the Paleocene and Eocene. Accord-
ing to the isotope analysis of apatite from fish teeth
found in the section drilled on the Lomonosov Ridge
(ODP 302), the salinity of the seawater was very low in

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the Paleocene and increased up
21–25
‰ in the Early–
initial Middle Eocene to reach the present-day values
(
35
‰
) in the Miocene [41]. The end of the Eocene
thermal maximum during the Azolla Event approxi-
mately 48.7 Ma ago was marked by a positive
δ
13
ë
excursion, which reflects the extremely high productiv-
ity of the surface waters (
Azolla
is a plant that can dou-
ble its biomass under favorable conditions (long day-
light in polar areas) within two–three days and likely
lived in deltas of rivers flowing into the Arctic basin).
On the Lomonosov Ridge, the 100-m-thick
sequence initial Middle Eocene in age is represented by
thin-bedded sediments enriched in organic matter and
containing abundant marine and freshwater siliceous
microfossils. The largely endemic assemblages of
marine diatoms are buried together with freshwater
algae. Under the prevalent brackish-water settings, the
variations in the proportions of the different ecological
microfossil groups reflect the changes in the salinity,
stratification, and trophic relations [38].
The lithological and paleontological studies of Mid-
dle Eocene sediments (46 Ma ago) in the Arctic Ocean
(ODP 302) revealed a strong cyclic signal with periods
of 50 and 100 cm, which are correlated with Milanko-
vitch orbital cycles [33]. The organic components are
represented by angiosperm pollen and spores, cysts of
dinoflagellates and chrysophytes, and diatoms. The
Middle Eocene (approximately 44.4 Ma) and Lower
Miocene (approximately 18.2 Ma) sediments are sepa-
rated by a hiatus 26 My long [33]. Based on dinocysts,
pollen and spores, siliceous microfossils, and geochem-
ical data, it is established that, prior to the hiatus, the
temperature of the surface waters was relatively high
(approximately 8
°
C) and they were fresh or brackish.
The sediments underlying the hiatus are characterized
by a cooling trend with an increasing influence of fresh-
water and shoaling of the ridge up to its rising above the
sea level. Immediately above the hiatus, the sediments
contain reworked dinocysts from the Cretaceous to Oli-
gocene. The pollen indicates a relatively cold climate.
The temperature of the surface waters is, however, esti-
mated to be as high as
15–19°
C, which is explained, in
the opinion of the authors, by the intrusion of North
Atlantic waters after the Fram Strait opened in the Early
Miocene. These data imply the simultaneous influence
of fresh and saline waters and alternating well-aerated
and anoxic conditions. During the period correspond-
ing to the hiatus, the Lomonosov Ridge was above the
sea level owing to the tectonic movements and its fluc-
tuations.
The divergence in the test sizes of the planktonic fora-
minifers from the high and low latitudes of the World
Ocean began in the Middle Miocene (42 Ma ago). It is
considered as resulting from the growth of the thermal
gradient in response to the initiated cooling in Antarc-
tica [35, 43]. Since that time, the temperature of the sur-
face Antarctic waters dropped by approximately
12°ë
,
while, in the equatorial latitudes, it remained practi-
cally unchanged [36]. This stimulated the development
of the polar front and biogeographic provincialism,
Fig. 4.
The general circulation model of deep and intermediate waters for the time of the terminal Paleocene isotope excursion and
mass extinction compared with the general present-day oceanic circulation. (WSDW) warm saline deep water; (AAIW) Antarctic
Intermediate Water; (AABW) Antarctic Bottom Water; (NADW) North Atlantic Deep Water; (MED) Mediterranean water (after
[21] with modifications).
Ant. Atlantic Arctickm
0
1
2
3
4
5
6
0
1
2
3
4
5
6
S
80*70 60 50 40 30 20 10 0 10 20 30 40 50 60 70
°
N
Latitude
Ocean: present-day with psychrosphere
(thermohaline circulation)
Proteus: Eocene (halothermal circulation)
AABW
AAIW MED
Surface waters
AAIW
Tethys
Surface waters
NADW
WSDW ?

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RESPONSE OF OCEANIC ORGANISMS TO ABIOTIC EVENTS 391
which is confirmed by the distribution of the diatom
assemblages [9]. The foraminiferal assemblage of
morozovellids is gradually replaced by that consisting
of morphologically simpler globigerinids and globo-
quadrinids [7].
The Eocene global biotic crisis.
The Eocene–Oli-
gocene transition (38 Ma ago) was marked by the glo-
bal biotic crisis, the largest one in the Cenozoic, which
is termed the terminal Eocene event. At that time, the
abyssal part of the ocean was filled with cold water to
form the psychrosphere. According to the oxygen iso-
tope ratio in tests of benthic foraminifers from the Sub-
arctic, tropical Pacific, and deep North Atlantic regions,
the temperature of the bottom waters dropped rapidly
down to
4–5°ë
, which was probably related to the for-
mation of spacious sea ice areas in the Antarctic region.
This was accompanied by various hydrological pro-
cesses: the formation of the Antarctic bottom waters
and the thermohaline circulation similar to the present-
day one and the substantial CCD subsidence in all the
oceans. The cooling coincided with the regression. The
falling of the mean annual temperatures in the high lat-
itudes increased the thermal gradients between the
equatorial and polar areas. The Circum-Antarctic Cur-
rent that was responsible for the isolation of Antarctica
and its glaciation appeared owing to plate tectonic
movements that provided wide and deep seaways
between Antarctica and the nearest continents and
microcontients. The opening of a deep (at least 2000 m)
passage between the South Tasman Rise and Antarctica
is dated approximately back to the Eocene–Oligocene tran-
sition and that of the Drake Passage, to ~
31
±
2
Ma, i.e., the
beginning of the Oligocene [23]. The development of
the Circum-Antarctic Current stimulated the formation
of cold eastern boundary currents in all the oceanic
basins of the Southern Hemisphere [20]. The cooling of
the Antarctica continent resulted in intensified circula-
tion and upwellings.
In contrast to the terminal Cretaceous event, the ter-
minal Eocene one caused less significant changes in the
biota. Nevertheless, it affected the deepwater benthic
communities to a greater extent as compared with the
Cretaceous event. Significant changes in the foramin-
iferal communities occurred in all the oceans. With the
onset of the Antarctic glaciation at the Eocene–Oli-
gocene transition, the distribution areas of tropical
organisms shifted toward the equator. The quantitative
analysis of the Late Eocene–Early Oligocene radiolar-
ian assemblages revealed that their basic changes took
place in the Late Eocene (35.5–35.7 Ma ago), at the
Eocene–Oligocene boundary (33.7–33.8 Ma ago), and
in the early Oligocene (30.9–31.3 Ma ago) [15]. The
stratigraphic level of this reorganization follows the
Late Eocene impact event, although it was caused, in
the opinion of the authors, by the global cooling rather
than by the asteroid falling. The fauna of the Late
Eocene planktonic foraminifers with high diversity and
complex forms such as
Globigerapsis
and
Hantkenina
gave way to the low-diversity assemblages with domi-
nant primitive
Globigerina
and
Globorotalia
[18]. The
dominant role of simple forms makes the Oligocene
planktonic foraminifers similar to their Early Paleo-
gene assemblages that existed after the late Cretaceous
biotic crisis. The diatom assemblages of the Antarctic
region experienced changes at the Eocene–Oligocene
transition as well.
Significant environmental changes near the Eocene–
Oligocene boundary are also recorded in Hole 1090
drilled on the Agulhas Ridge in the Atlantic segment of
the South Atlantic during ODP Leg 177 [12]. Its section
hosts the interval (37.5–33.5 Ma) highly enriched in
biogenic opal indicating the development of upwelling,
an influx of biogenic elements into the surface layer of
the ocean, and high sedimentation rates of microfossils
with opal skeletons. Significant changes in the assem-
blages of calcareous dinoflagellates are also recorded
near the Eocene–Oligocene boundary in DSDP Hole
357 drilled in the Rio Grande Rise area (South Atlan-
tic). These changes are correlative with the abrupt tem-
perature drop registered in this section [7].
The terminal Eocene was marked by at least five
impact events, which formed following craters: the
Popigai in Siberia (100 km across), the Chesapeake in
the United States (85 km), the Mistastin in Canada
(100 km), the Logoisk in Byelorussia (17 km), and the
Vanalitei in Canada (7.5 km). Late Eocene tektites
(small glassy particles of rocks melted and thrown out
into the atmosphere) are found in sedimentary sections
of the Indian, Pacific, and Atlantic oceans; the Carib-
bean Sea; the Gulf of Mexico; and in the Weddell Sea
in the Antarctic region. The sources (Chesapeake Bay
and Siberia) and ages (35.5 and 35.7 Ma) have been
determined for tektites from deep-sea drilling holes in
the Atlantic, Indian, and Pacific oceans. With account
for the probable errors in the age determinations, the
asteroids collided with the Earth at practically the same
time [42] (Fig. 5).
These events preceded the Oligocene biotic extinc-
tions (Fig. 6).
It is conceivable that these impact events stimulated
the temperature rise and interrupted the Cenozoic cool-
ing trend from the Late Paleogene greenhouse to the
Oligocene glacial conditions. The termination of the
global warming related to the impact events was fol-
lowed by rapid (in geological terms) restoration of the
cooling trend, which resulted in the strong temperature
fall. The elevated
3
He content in the Upper Eocene sed-
iments is thought to be related to the influx of cosmic
dust into the Earth’s atmosphere [14]. According to the
last authors, this process started 1 My before the aster-
oid fall and terminated 0.5 My later. Thus, similar to the
Cretaceous–Paleogene transition 65 Ma ago, the cli-
matic and paleoceanologic changes in the terminal
Eocene caused by tectonic processes were synchronous
with extraterrestrial events: the Earth’s collision with
several asteroids or comets and the influx of dispersed
cosmic matter into its atmosphere.

392
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No. 3
2009
BARASH
Oligocene. The oxygen isotope composition in tests
of planktonic foraminifers from sections recovered by
deep-sea drilling in the southwestern Pacific [28]
reflects the development and strengthening of the lati-
tudinal zonality and the thermal gradients between the
Subantarctic and middle latitudes during the Oli-
gocene. The lithological materials of ODP Leg 189 [31]
demonstrate that the continuous current in the South
Ocean did not exist until the mid-Oligocene and its
rapid formation preceded the Early Miocene glaciation
in Antarctica. It is assumed that, in the terminal Oli-
gocene, the Faeroe–Iceland Ridge partly subsided
below the sea level with simultaneous formation of Ice-
land after changes in the spreading axis. These pro-
cesses resulted in the appearance of the water exchange
between the Arctic basin and the Greenland Sea
(approximately 25 Ma ago). Muller [29] cites various
micropaleontological data indicating a decrease in the
surface temperatures at the Oligocene–Miocene transi-
tion, which is substantiated by the oxygen isotope stud-
ies. The surface water temperature fall in the Antarctic
region affected the high-latitude fauna of planktonic
foraminifers, which acquired (in the Oligocene) fea-
tures typical of their present-day assemblages: low
diversity and simple morphology (materials of leg 28 of
D/V Glomar Challenger). This initiated the develop-
ment of biogeographic provincialism typical of the
present-day Antarctic biota. Simultaneously, planktonic
microorganisms suffered intense extinction and corre-
sponding diversity reduction in the low and middle lati-
tudes. By the mid-Oligocene, the belts populated with
subtropical and tropical communities of calcareous nan-
noplankton retreated northward to give way to temperate
and subpolar assemblages near Antarctica [3].
During the Oligocene warming episode 26.5 to
24.5 Ma ago, the thermal gradients decreased again, the
biogeographic provinces widened, and subtropical spe-
cies appeared again in the Antarctic region [7, 43],
which is reflected in the decrease of the test sizes up to
the Paleogene level [35].
The Paleogene–Neogene transition was marked by
climatic changes in all the oceans. The foraminiferal
assemblages reflect the gradual warming from the Late
Oligocene to the Early Miocene. This warming was
more notable in the high latitudes than in the zones of
equatorial upwellings. The terminal Oligocene (the
upper part of zone P22) was marked by a slight relative
cooling, and, until the Miocene (zone N4), the climate
was extremely unstable with globally intensified
upwellings [37].
CONCLUSIONS
The climatic changes with the optimum in the early
Eocene and subsequent gradual or stepwise cooling
served as the main abiotic factor that regulated the
biodiversity in the Cenozoic. These changes had differ-
ent consequences for the oceanic biota and acted
together with other abiotic factors: the sea-level
changes, the opening and closure of the seaways
between basins, the volcanic activity, and others. These
changes were, in turn, determined by factors of higher
order, primarily, by tectonic movements: vertical (oro-
genic with growth of the mid-oceanic ridges) and hori-
zontal (motions of the lithospheric plates). The contri-
bution of extraterrestrial factors such as impact events
(the collision of the Earth with large asteroids and com-
ets), the crossing of specific areas in cosmic space by
Fig. 5. Five known Late Eocene impact structures and ODP/DSDP holes with indication of impact events. The circle size is propor-
tional to the diameter of the impact structure. Shown on the left are the age ranges of the impact events (after [42] simplified).
60°
120°150°180°210°240°270°300°330°0°30°
40°
20°
0°
20°
40°
60°
90°
216
292 462
315A
94 612
RC9–58
Popigai
Chesapeake
Mistastin Logoisk
Vanapitei
N
Eocene Oligocene
30
35
40
45
Ma
Popigai
Chesapeake
Vanapitei
Mistastin
Logoisk
S

OCEANOLOGY Vol. 49 No. 3 2009
RESPONSE OF OCEANIC ORGANISMS TO ABIOTIC EVENTS 393
our solar system, and others to this process is also
highly probable. It should also be noted that the factors
responsible for some paleoclimatic and paleoceanolog-
ical changes, as well changes in the oceanic biota, have
no unambiguous explanation.
The finding of dinoflagellate and benthic foramin-
iferal assemblages in the western Tethys provides evi-
dence for drastic changes in the global circulation in the
initial Paleogene: a significant temperature fall in the
surface and deep waters of the ocean. The mass extinc-
tion of benthic foraminifers (50% of the species) at the
Paleocene–Eocene boundary was determined by warm-
ing of the surface and deep Antarctic waters. In con-
trast, the diversity of planktonic organisms such as for-
aminifers, dinoflagellates, and calcareous algae (nan-
noplankton) increased. The oxygen deficiency in the
deep waters was related to the principal reorganization
of the oceanic circulation: warm high-salinity deep
waters were forming in the middle latitudes (halother-
mal circulation, which is contrary to the present-day
thermohaline one).
The uniformly warm climate; insignificant meridi-
onal thermal gradient; ice-free continents; well-devel-
oped “greenhouse conditions;” and the existence of the
wide Tethys seaway north of Africa, which could sup-
ply large volumes of warm saline deep waters, are
thought to be responsible for the unique situation
55 Ma ago. The rapid release of ëé2 into the atmo-
sphere owing to volcanic and/or hydrothermal activity
and the mass influx of methane into the atmosphere
from gas hydrates dissociated owing to the gradual
warming caused by the above-mentioned unique fac-
tors could have served as the triggering mechanism.
The influence of impact events cannot be ruled out as
well. The rapid global extinction of 30–50% of the
benthic foraminiferal taxa could have resulted from
oxygen-deficient conditions due to high temperatures
or methane oxidation and variations in the primary pro-
duction.
In the opinion of some authors, the concept accord-
ing to which the Paleocene–Eocene thermal maximum
that culminated the gradual warming was determined
by terrestrial factors is inconsistent with the sudden
onset of climatic and other changes. They should have
been triggered by some catastrophic event such as a
collision with an asteroid or comet (impact event). The
terminal Eocene was marked by at least five impact
events that left impact craters. The asteroid falls
occurred practically at the same time 36–35 Ma ago.
These events preceded the Oligocene biotic extinctions.
The Eocene–Oligocene transition 38 Ma ago coin-
cides with the global biotic crisis, the most significant
one in the Cenozoic. This is the so-called thermal
Eocene event, when the abyssal zone of the ocean was
filled with cold water to form the psychrosphere. This
stimulated the formation of Antarctic bottom water and
the development of the thermohaline circulation similar
to the present-day one. The warm water deep circula-
tion was eventually replaced by the cold-water one at
the Eocene–Oligocene transition period, when the sub-
sidence of the Faeroe–Iceland threshold subsided to
permit water exchange between the north Atlantic and
Arctic basin. During the Oligocene warming episode
26.5–24.5 Ma ago, the thermal gradients decreased
again, the biogeographic provinces extended, and sub-
tropical species appeared again in the Antarctic region.
Similar to as in the Paleozoic and Mesozoic, some
Paleogene climatic and paleoceanologic changes
caused by tectonic factors occurred almost synchro-
nously with extraterrestrial events such as the collision
of the Earth with several asteroids or comets, the ejec-
tion of dispersed cosmic matter into the atmosphere,
and others, which were probably explained by common
causes of higher order, including factors originating
beyond the Solar system [1, 2].
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