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Abstract

The conditions of mass extinction at the boundary of the Ordovician and the Silurian are different from those of the other four great mass extinctions of the Phanerozoic. Although all of the major factors leading to other cases of mass extinctions also took place: the sea level fluctuations and climate variations, volcanism and impact events that caused emission of harmful gases, ashes, aerosols, and hence, the greenhouse effect, darkening of the atmosphere, reduction of photosynthesis and bioproductivity, disruption of food chains, and anoxia. The prerequisite stress conditions were the movement of the lithospheric plates and the rapid development of continental glaciation of Gondwana caused by its position in the south polar region. Cooling, hydrodynamics change throughout the entire thickness of the ocean waters, and the corresponding decrease in sea levels reduced the shelf–space which was the basic ecological niche of the Ordovician marine biota. A very important factor in the Phanerozoic was strong and long volcanic eruptions in the subduction zones and island arcs around the Iapetus Ocean during the collision of Laurentia and Baltica. A unique role was played by the emergence and development of terrestrial plants and the development of microphytoplankton that in the process of photosynthesis fixed atmospheric CO2, which contributed to the disappearance of the greenhouse effect and the global climate system transition from the greenhouse to the glacial type. In this case, the biota had a powerful influence on all the outer realms of the Earth.
ISSN 1028334X, Doklady Earth Sciences, 2014, Vol. 456, Part 2, pp. 667–669. © Pleiades Publishing, Ltd., 2014.
Original Russian Text © M.S. Barash, 2014, published in Doklady Akademii Nauk, 2014, Vol. 456, No. 6, pp. 680–683.
667
During the Phanerozoic (since 542 Ma) overall
progressive development of biota occurred, which was
disturbed by more or less significant episodes of biodi
versity decrease. Especially catastrophic extinctions
on the Earth when more than 75% of the species dis
appeared within geologically short intervals are
referred to the great mass extinctions. During the
Phanerozoic five great extinctions are recognized:
—the Ordovician, at the boundary of the Ordovi
cian and Silurian;
—the Devonian at the boundary of the Devonian
and Carboniferous;
—the Permian, at the Permian–Triassic boundary;
—the Triassic, at the late Triassic–early Jurassic
boundary;
—the Cretaceous at the boundary of the Creta
ceous and the Tertiary.
These episodes are of greatest interest, since their
detailed study provides an opportunity to identify the
abiotic factors that affect or could affect the evolution
of the biota.
The first of the five “great mass extinctions” of the
Phanerozoic occurred at the end of the Ordovician, at
the Hirnantian Stage. It began at the boundary of the
Hirnantian and Katian stages of the Ordovician
(445.6 Ma) and ended at the boundary of the Ordovi
cian and Silurian about 443 million years ago. More
than 25% of families, 49–57% of genera, and 86% of
species became extinct [1]. Extinction affected all
pelagic forms (chitinozoa, acritarchs) and many taxa
in the deepwater shelf. Out of the total of about
100 species of the Ordovician conodonts, only 20 spe
cies are found in the Silurian. In the Gondwana high
latitude sections, they disappeared completely. The
trilobite genera decreased from 113 in the Late Ordov
ician to 45 in the Early Silurian. It is believed that all
the trilobites that had larvae or mature forms of pelagic
the mode of living became extinct [2].
In the brachiopod communities, 150 of 180 genera
became extinct. The crisis also affected bryozoans, but
to a lesser extent than trilobites and brachiopods. On
the Laurentian and Baltic paleocontinents, about 44%
of genera went extinct. The bivalves, actively evolved
in the Ordovician, particularly in coastal communi
ties, in the crisis lost 24 of 40 genera [3]. Among the
Nautiloidea 80% of genera became extinct. Corals lost
60–70% of the genera. As a consequence of the
extinction of the majority of reefbuilding organisms,
the development of reef structures almost completely
stopped for several million years.
What were the factors in the Ordovician mass
extinction? The general paleogeographic conditions
of extinction were formed as a result of the movement
of Gondwana toward the South Pole, causing global
cooling and continental glaciation. Spreading of the
continental plates in the Ordovician formed extensive
areas of young crust, and the sea reached the highest
level during the Phanerozoic, perhaps more than 200 m
above the present level [4]. After an interval of green
house climate, typical for this period, cooling in the
late Ordovician began, which is believed by many
researchers, along with the decrease in the sea level, to
have led to the mass extinction. The first pulse of
extinction occurred at the boundary of the Katian and
Hirnantian stages and was very close to the interval of
rapid growth of the ice sheet on Gondwana. The
eustatic lowering of the sea level and cooling in the
tropical region played an important role. At this time,
the sea level dropped, according to various estimates,
by 40–100 m. The isotope paleothermometry of the
late Ordovician tropical ocean sediments gave temper
ature estimates at 32–37°C, with the exception of
brief cooling by 5°C. At the same time, the volume of
continental glaciation reached or exceeded the Late
Pleistocene glacial maximum, resulting in a major
perturbation of the carbon cycle and the mass extinc
tion [5]. These data were obtained through analysis of
samples of Rugosa and Tabulata corals, brachiopods,
and trilobites. All samples were collected from depos
its that accumulated on shelves and epicontinental
Environmental Conditions of the Mass Extinction of Marine Biota
at the End of the Ordovician
M. S. Barash
Presented by Academician Yu.M. Pushcharovskii March 18, 2013
Received March 25, 2013
DOI:
10.1134/S1028334X14060312
Shirshov Oceanology Institute RAS, Moscow, Russia
email: barashms@yandex.ru; barash@fromru.com
GEOLOGY
668
DOKLADY EARTH SCIENCES Vol. 456 Part 2 2014
BARASH
basins in the tropical latitudes, at which in the Late
Ordovician–Early Silurian Laurentia was located.
Variations in the stable isotope oxygen and carbon
ratios at this time suggest abrupt environmental changes
[6], the development of anoxic conditions and with
drawal of the light oxygen isotope from the waters
through fixing it in the Gondwana ice sheet. The sea
regression related to the glaciation would have led to a
sharp reduction in the shallow coastal ecological niches.
The biphasal nature of the Late Ordovician extinction,
as proposed [6], is associated with a sudden glaciation
of Gondwana, and then its rapid disappearance.
With the onset of glaciation offshore of subpolar
Gondwana, cold, oxygenrich water began to form,
sink, and spread over the bottom of the ocean, like the
modern Antarctic and Arctic nearbottom water
masses. Oxygenation of bottom waters was stressful for
organisms adapted to lowoxygen, but nutrientrich
conditions (graptolites, trilobites, brachiopods, etc.).
The second phase of the Late Ordovician extinction
was greater and more strongly affected biota in the
middle and outer shelves [3].
In southern China, the whole interval over the
Ordovician and Silurian boundary, except for the Hir
nantian Stage, contains abundant organic matter [7].
Fluctuations of
13
С
org
and
δ
34
S
sulf
showed major cli
matic changes, sea level fluctuations, and repeated
occurrence of anoxic conditions in the water column,
which could have caused the biotic crisis. Possibly
toxic water rose to the surface, causing mass extinction
of graptolites, which occurred simultaneously with the
mass deaths of brachiopods and trilobites. Thus, cli
matic fluctuations, along with multiple shifts of the sea
level and the emergence of anoxia, caused stepwise
extinction during the Late Ordovician crisis.
Before the Hirnantian glaciation, the Katian global
warming occurred, as evidenced by the displacement
of the lowlatitude benthic fauna of trilobites and bra
chiopods in the high latitudes and the rise of ende
mism at low latitudes [8]. Warming episodes (sea level
rise) and cooling (glaciation growth and sea level low
ering) alternated several times, consistently reducing
biodiversity.
The simulation results considering continental ice
and general water circulation show that the expansion
and reduction of ice sheets in the Late Ordovician
were heavily dependent on fluctuations in atmospheric
p
CO
2
and insolation parameter changes with the fre
quency of 30000–40000 years [9]. Analysis of African
glacial–marine sediments suggests that the full develop
ment of the two phases of the ice sheet on the continental
shelf represents a full cycle of glaciation, the beginning of
which is associated with the onset of the mass extinction.
During the retreat of the ice cover from the shelf, the sec
ond phase of the Late Ordovician mass extinction started.
The minimum duration of the first extinction phase was
about 0.3 Ma [10].
There is no doubt that an important reason for the
advent of cooling was the rapid development of the
first land plants, which in the process of photosynthe
sis absorb CO
2
from the atmosphere, as well as large
amounts of calcium, magnesium, phosphorus, and
iron from the soil. Removal of Ca
2+
and Mg
2+
lead to
the formation of new minerals, particularly carbonates
that fixed atmospheric carbon dioxide. In turn, the
input of P and Fe into the sea triggered rapid develop
ment of living organisms, in particular microphy
toplankton–Acritarchs, which also began to absorb
CO
2
out of the atmosphere. As a result, significant
changes occurred in the carbon cycle of the planet that
lead eventually to reduction of the CO
2
content in the
atmosphere. The atmospheric concentration of CO
2
in the Ordovician is 15–20 times higher than the mod
ern level. At the end of the period, it began to plummet
and later during the Phanerozoic never rose to the
Early Paleozoic values. As a result, the typical Ordov
ician greenhouse climate changed to glacial with some
consequent glacial periods.
The mass extinction at the end of the Ordovician
could have been affected by volcanic activity. During
this period, the southern continents were assembled in
the Gondwana supercontinent. In the Early Ordovi
cian, the continents of Laurentia, Siberia, and Baltica
were independent, but Baltica began to move towards
Laurentia through the Iapetus ocean, which was com
pletely closed by subduction, forming terranes [11].
The major orogenic process was the Taconiian orog
eny, which began in the Cambrian. At the beginning of
the Late Ordovician 460–450 Ma, volcanoes along the
subduction zone at the Iapetus Ocean margin and
island arcs threw into the atmosphere huge amounts of
volcanic ash and large amounts of CO
2
, causing the
greenhouse effect. This volcanic island arc joined with
the Proto–North America (Laurentia), forming the
Appalachian Mountains.
Common in southern China and North and South
America, as well as in Europe, bentonite clays contain
minerals that are typical of felsic volcanic ash and are
a product of their diagenesis in marine environments.
These ash layers are usually less than 20 cm thick, but
in some cases are 1–2 m or more, and can be traced
over areas of millions of km
2
. Their sources were vol
canic arcs and subduction zones along the eastern
edge of the Laurentian Shield. These eruptions were
probably the largest in the Phanerozoic [12] and had
to have had a strong influence on the atmosphere and
the biosphere. The oxygen isotope ratio in conodont
apatite showed an increase
δ
18
O
at 1.5 directly
above the thick ash layer that reflects the sudden but
short glaciation before the main Hirnantian one, sug
gesting that a major volcanic event affected the Late
Ordovician climate [13] .
In the Late Ordovician, several impact events hap
pened close to or coinciding with the mass extinction
interval. In Estonia, the meteorite crater Kyardla is
7 km in diameter and has an age of about 455 Ma. The
DOKLADY EARTH SCIENCES Vol. 456 Part 2 2014
ENVIRONMENTAL CONDITIONS OF THE MASS EXTINCTION 669
Lockne crater located in Sweden has a diameter of
7.5–13.5 km and a similar age of 455 Ma. The crater
Rock Elm Disturbance located in Wisconsin (United
States) is 6 km in diameter with the meteorite size esti
mated at 170 m. Its age has been determined in the
range 455–430 Ma, i.e., from the Middle Ordovician
to the Early Silurian, and overlaps with the time of
mass extinction. In northern Canada there is the
meteorite impact crater Pilot of 6 km diameter and age
of 445 ± 2 Ma [14]. The Slate Islands in the province
of Ontario in Canada are the central uplift of an
impact crater with a diameter of 32 km and an age of
about 450 Ma. However, according to other determi
nations, this impact event occurred in the Proterozoic
or Early Paleozoic. These relatively small impact
events could not have had an effect on deterioration of
the global environmental and the significant influence
of asteroid strikes onto the great Ordovician extinction
does not yet have firm evidence.
A review of the available information shows that
the conditions of mass extinction at the boundary of
the Ordovician and the Silurian are different from
those of the other four great mass extinctions of the
Phanerozoic. Although all of the major factors leading
to other cases of mass extinctions also took place: the
sea level fluctuations and climate variations, volcan
ism and impact events that caused emission of harmful
gases, ashes, aerosols, and hence, the greenhouse
effect, darkening of the atmosphere, reduction of pho
tosynthesis and bioproductivity, disruption of food
chains, and anoxia [15].
The prerequisite stress conditions were the move
ment of the lithospheric plates and the rapid develop
ment of continental glaciation of Gondwana caused
by its position in the south polar region (see figure).
Cooling, hydrodynamics change throughout the
entire thickness of the ocean waters, and the correspond
ing decrease in sea levels reduced the shelf–space which
was the basic ecological niche of the Ordovician marine
biota. A very important factor in the Phanerozoic was
strong and long volcanic eruptions in the subduction
zones and island arcs around the Iapetus Ocean during
the collision of Laurentia and Baltica.
A unique role was played by the emergence and
development of terrestrial plants and the development
of microphytoplankton that in the process of photo
synthesis fixed atmospheric CO
2
, which contributed
to the disappearance of the greenhouse effect and the
global climate system transition from the greenhouse
to the glacial type. In this case, the biota had a power
ful influence on all the outer realms of the Earth.
ACKNOWLEDGMENTS
The author is grateful to Academician A.P. Lisitsyn
for useful advice.
This work was performed under program number 28
of the Presidium of the Russian Academy of Sciences
“The origin of life and the formation of the biosphere.”
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Translated by A. Larionov
Plate motion. The South
Pole location
of Gondwana
Flora development.
Abrupt СО
2
reduction
The Gondwana
continental
glaciation.
Temperature fall Ocean
hydrodynamics change.
Ventilation
of deep waters
Sea regression
(70
100 m).
Reduction
of shelf area
Volcanism
The First
Extinction Stage
Ocean
hydrodynamics change.
Development of deep
water hypoxia
Warming.
Deglaciation.
Transgression,
Shelf Flooding
The Second
Extinction Stage
Sequence of causes of the marine biota extinction in the
Late Ordovician.
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