Changes in Environmental Conditions as the Cause of the Marine Biota Great Mass Extinction at the Triassic–Jurassic Boundary

Abstract

In the interval of the Triassic–Jurassic boundary, 80% of the marine species became extinct. Four main hypotheses about the causes of this mass extinction are considered: volcanism, climatic oscillations, sea level variations accompanied by anoxia, and asteroid impact events. The extinction was triggered by an extensive flooding of basalts in the Central Atlantic Magmatic Province. Furthermore, a number of meteoritic craters have been found. Under the effect of cosmic causes, two main sequences of events developed on the Earth: terrestrial ones, leading to intensive volcanism, and cosmic ones (asteroid impacts). Their aftermaths, however, were similar in terms of the chemical compounds and aerosols released. As a consequence, the greenhouse effect, dimming of the atmosphere (impeding photosynthesis), ocean stagnation, and anoxia emerged. Then, biological productivity decreased and food chains were destroyed. Thus, the entire ecosystem was disturbed and a considerable part of the biota became extinct.

Full text

ISSN 1028334X, Doklady Earth Sciences, 2016, Vol. 466, Part 2, pp. 119–122. © Pleiades Publishing, Ltd., 2016.
Original Russian Text © M.S. Barash, 2016, published in Doklady Akademii Nauk, 2016, Vol. 466, No. 6, pp. 688–691.
119
The Triassic period (251–199.6 Ma) was a time
when marine biota recovered and developed after the
Great Mass Extinction at the Paleozoic–Mesozoic
boundary. However, one of the five great Phanerozoic
mass extinctions occurred at the end of the Triassic
and involved both marine and terrestrial assemblages.
It is supposed that 47% of genera and 80% of species
became extinct within the interval of 600 ka to 8.3 Ma
[1, 2]. The major tectonic background events for these
processes were the breakup and rifting of Pangaea dur
ing opening of the preAtlantic Ocean.
At the end of the Rhaetian, at the Triassic–Jurassic
boundary (TJB), 48% of invertebrate species (mainly
cephalopods, bivalves, gastropods, brachiopods)
became extinct [3]. Within the limits of presentday
Europe, the territory of which is the most well studied,
92% of species and 42% of genera of marine organisms
went extinct in the Late Triassic. In some local terres
trial ecosystems, about 50% of tetrapods and up to
95% of plant species died off (see overview in [4]).
At the end of the Triassic, global extinction of many
reefforming organisms (first of all, scleractinial corals
and sponges) occurred: 50 genera became extinct,
while 11 survived. Coral reefs and almost all (16 of
18 families) calcareous sponges disappeared. Thus,
reef ecosystems were completely destroyed. Mass
extinction also involved ostracods, foraminifera, coc
colithofora, radiolaria, and dinoflagellates.
The Late Triassic mass extinction was not a single
event. Detailed stratigraphic data have shown that the
extinction of ammonites, brachiopods, and conodonts
was extended in time. Particular events are distin
guished, with higher extinction rates with approaching
the end of the Triassic: in the Middle and Late Norian
and at the end of the Rhaetian.
There are four main hypotheses about the causes of
the Late Triassic mass extinction: intensive volcanism
with the accompanying events, climatic variations, sea
level oscillations with the accompanying anoxia, and
impact events (asteroid impacts). It is supposed that
the extinctions of both marine and terrestrial biota
were triggered by some global event. The extensive
flood basalts within the Central Atlantic Magmatic
Province (CAMP), with the maximum estimated to be
at 199.0 ± 2.4 Ma fits in time with the TJB (~200 Ma)
mass extinction. This flood basalt effusion was the
greatest, at least in area: it covers more than 7000 000
km
2
, spanning from France to Southern Brazil, with a
volume of more than 2.5
×
10
6
km
3
[5].
The evidence for intensive tectonic events accom
panied by the volcanic activity is found in the eastern
margin of North America, in South America, in
northwestern Africa, and in southeastern Europe. This
is attributed to the main phase of rifting, which pre
ceded the opening of the preAtlantic Ocean. Erup
tions were characterized by the release of ashes and
Changes in Environmental Conditions
as the Cause of the Marine Biota Great Mass Extinction
at the Triassic–Jurassic Boundary
M. S. Barash
Presented by Academician A.P. Lisitsyn February 28, 2014
Received March 11, 2014
Abstract
—In the interval of the Triassic–Jurassic boundary, 80% of the marine species became extinct. Four
main hypotheses about the causes of this mass extinction are considered: volcanism, climatic oscillations, sea
level variations accompanied by anoxia, and asteroid impact events. The extinction was triggered by an exten
sive flooding of basalts in the Central Atlantic Magmatic Province. Furthermore, a number of meteoritic cra
ters have been found. Under the effect of cosmic causes, two main sequences of events developed on the
Earth: terrestrial ones, leading to intensive volcanism, and cosmic ones (asteroid impacts). Their aftermaths,
however, were similar in terms of the chemical compounds and aerosols released. As a consequence, the
greenhouse effect, dimming of the atmosphere (impeding photosynthesis), ocean stagnation, and anoxia
emerged. Then, biological productivity decreased and food chains were destroyed. Thus, the entire ecosystem
was disturbed and a considerable part of the biota became extinct.
DOI:
10.1134/S1028334X16020173
Shirshov Institute of Oceanology, Russian Academy
of Sciences, Nakhimovskii pr. 36, Moscow, 117213 Russia
email: barashms@yandex.ru
GEOLOGY

120
DOKLADY EARTH SCIENCES Vol. 466 Part 2 2016
BARASH
considerable amounts of CO
2
and SO
2
during degas
sing of basalts, so this should have affected the compo
sitions of both the atmosphere and the ocean.
The new highprecision U/Pb geochronological
technique on volcanic ashes from independent marine
stratigraphic crosssections has shown that the TJB
biological crisis correlates with the beginning of effu
sive volcanic activity on land within the CAMP in a
period of less than 150 ka [6]. It was shown that the
accompanying sea level drop and the negative
δ
13
С
peak at the very end of the Triassic occurred over less
than 290 ka. These rapid sea level fluctuations on a
global scale suggest that the global cooling was closely
related to the TJB extinction and potentially caused by
volcanism in the CAMP.
Mineralogical study of the uppermost Rhaetian
layer in the northern Austrian Alps [7] revealed that
ash layer traces that had been interpreted as direct evi
dence for volcanism in the CAMP coincided with or
immediately preceded the mass extinction and the ini
tial negative carbon isotope anomaly. Above the ash
boundary, the clay mineral composition indicates the
intensive erosion under the hot and humid greenhouse
climate conditions. This supports the idea that volca
nism in the CAMP caused the climatic and other envi
ronmental changes, which, in turn, is reflected in the
carbon isotope anomalies and led to the mass extinc
tion.
The evidence for the significant influence of volca
nic activity in the CAMP on the change in environ
mental conditions at the TJB in the marine environ
ment is the shift in the ratio between osmium isotopes
and the increase in the rhenium and osmium concen
trations [8].
It is believed that both Late Triassic and Early
Jurassic climates were warm. In the Late Triassic,
greenhouse conditions expanded over the entire Earth
so that humid forests reached polar zones. This was the
only period during the Phanerozoic for which no signs
of glacier activity have been revealed. Some data, how
ever, indicate temperature variations at the boundary
of epochs.
The significant variations in the ratio between car
bon isotopes in carbonates and organic matter verify
the considerable changes in the global carbon cycle at
the TJB [9]. This ratio in marine limestones of North
ern Italy showed a negative peak coinciding with the
disappearance of benthic fauna and the subsequent
positive shift started above the TJB.This ratio in
marine limestones of Northern Italy showed a negative
peak coinciding with the disappearance of benthic
fauna and the subsequent positive shift started above
the TJB. The negative peak could have been caused by
the release of gas hydrates.
The great negative isotope carbon shift defined on
marine carbonates, organic matter, and terrestrial
wood is explained by the fact that degassing of volcan
ogenic CO
2
caused the release of a significant amount
of CH
4
from gas hydrates in marine deposits into the
atmosphere. Oxidation of CH
4
must have additionally
increased
p
CO
2
of the atmosphere up to 2500 ppmv by
the beginning of the Jurassic [10].
Based on the paleobotanic data, the TJB was char
acterized by a fourfold increase in the atmospheric
CO
2
concentration and 3–4°C increase in tempera
ture and by settling of greenhouse conditions. Tempera
tures exceeded the living ranges for many plants, leading
to the extinction of 95% of megaflora species [11].
In the ocean, wind circulation and meridional
water exchange must have decreased (the latter, four
fold), whereas stratification must have increased, on
the contrary. With respect to an increase in tempera
ture, oxygen solubility (and hence its concentration in
water) decreased, causing anoxia that led to extinction
of marine fauna.
The sea level oscillations at the end of the Triassic
are well supported by the geological data on Europe
and North America. The large regression was rapidly
replaced by the transgression in the Early Jurassic. For
example, deposits of the western Tethys Ocean in the
Iberian Craton demonstrate a regression–transgres
sion cycle of about 12 Ma long: it started in the Late
Triassic and finished in the Early Jurassic. The sea level
drop was accompanied by reduction of the shelf seas.
Transgressions of lowoxygen or deoxygenated waters
into shelf zones and subsequent regressions destroyed
the environments of bivalves and caused their mass
extinction. Relatively small sea level drops at the TJB
were defined by tectonic movements, because no
traces of continental glaciations were found. However,
sea level oscillations could have intensified the deteri
orating effect of the main causes.
The Jurassic basal layers in Europe and the Ameri
cas are rich in organic matter and contain no benthic
organisms, or contain impoverished assemblages indi
cating the stagnation conditions. The well proved oce
anic anoxia can explain the mass extinction of marine
biota; however, the decline of the terrestrial fauna and
flora suggests that more universal causes took place.
The relationship between the Late Triassic mass
extinction and the impact events has certain evidence.
The impact time for the Manicouagan crater, Quebec,
is defined at 214
±
1 Ma (an alternative estimate is
210
±
4 Ma; hereinafter, the ages of impact structures
are after [12]).
Impact craters of the same age have also been found
in France (Rochechouart) and England (near Bristol).
In the latter case, chromium isotopes have indicated the
cosmic origin of the impactites formed after the event.
According to the Armethod, the age of these impac
tites is 214
±
8 Ma. This time precedes the period of
stepwise extinction in the Middle Norian.
It was reported in [13] that there is a stratum of Late
Triassic deformed deposits (seismites) of 2–4 m thick,
occupying several thousand square kilometers, in
England. These deposits were produced by a seismic

DOKLADY EARTH SCIENCES Vol. 466 Part 2 2016
CHANGES IN ENVIRONMENTAL CONDITIONS 121
event of extraordinary magnitude. M.J. Simms thinks
that they indicate an impact of a fireball of several kilo
meters in crosswidth: it produced a crater of several
tens of kilometers in diameter and triggered intensive
seismic waves. The epicentral position is supposed to
be more than 500 km westnorthwest of central Brit
ain, on the shelf west of Ireland, where the possible
crater is buried beneath young deposits of 2–3 km
thick.
There are a number of craters on the Earth’s sur
face with an age of about 214 Ma: Rochechouart
(France), Manicouagan and Saint Martin (eastern
and western Canada, respectively), Obolon (Ukraine),
and Red Wing (western United States). These five
impact structures were probably formed during a mul
tiple impact event due to the Earth’s collision with a
fragmented comet or asteroid [14].
The Rochechouart crater in France is hosted in the
Hercynian rocks of the Central Massif and not
reflected in the topography (its diameter is about
25 km). The Manicouagan crater is about 100 km in
diameter. The Saint Martin crater is about 40 km in
diameter and was revealed from gravimetric and mag
netic data, with subsequent verification by drilling.
The Obolon and Red Wing craters are about 15 and
9 km in diameter, respectively, with the latter having
an age of 200
±
25 Ma (based on the stratigraphic
data). In the United States (Tennessee), the Wells
Creek crater has also been found: its diameter is
13.7 km and the approximate age is 200 Ma. The mass
extinction at the TJB in Italy is associated with the
shale horizons containing the impact quartz; in the
Newark Basin, eastern United States, it is linked with
the iridium anomaly.
Some researchers emphasize the greater or lesser
gaps between volcanism, impact events, and mass
extinctions. In the author’s opinion [15], these gaps
are insignificant because events of cosmic origin trig
ger the occurrence of unfavorable conditions lasting
millions of years. Mass extinctions occur within these
intervals, when some combination of hostile environ
mental conditions reaches the level that makes exist
ence of certain organisms or their assemblages impos
sible.
Under the effect of causes that are likely induced by
orbital movement of the Solar System around the cen
ter of the Milky Way, two sequences of events develop
on the Earth: the terrestrial ones that lead to intensive
volcanism and the cosmic ones that are related to
impacts of large asteroids and comets (Fig. 1). How
Orbital movement of the Sun around the galaxy's center
Intersection of the galaxy's branches Oscillations relative to the plane
Tectonics Mantle
plumes
Volcanism
Asteroids Comets
Impact events
Ash, aerosolsСО
2
, SO
2
, Cl, F, CH
4
emissions
Dimming of the atmosphere,
reduction of the UV radiation
Greenhouse effect
Ocean stagnation Reduction of photosynthesis
and bioproductivity
Anoxia Destruction of the food chains
Mass extinction
Climatic
fluctuations
Sea level
Processes that led to the mass extinction at the TJB.

122
DOKLADY EARTH SCIENCES Vol. 466 Part 2 2016
BARASH
ever, mass extinctions may occur in the presence of
effects from only one sequence of these two, either
volcanismrelated or that related to large impact
events. In this case, extinctions are probably less
extensive.
As is shown from the scheme in Fig. 1, volcanism
and impact events produce similar effects. In both
cases, poisonous chemical compounds and aerosols
are released into the atmosphere. The greenhouse
effect settles followed by global warming and dimming
of the atmosphere; in turn, dimming impedes penetra
tion of UVradiation and thus reduces photosynthesis,
ocean stagnation, and anoxia. Bioproductivity
reduced and food chains were destroyed. In the end,
all important lifesupporting processes stop for most
of the fauna and mass extinction occurs.
ACKNOWLEDGMENTS
This work was supported by the Presidium of the
Russian Academy of Sciences (Program no. 28 enti
tled “The origin of life and the formation of the bio
sphere”).
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Translated by N. Astafiev