affect the processes with a d
13 C signature furthest away from
the oceanic value, in other words, the oxidation of the
organic sedimentary carbon exposed on land or the burial of
organic carbon in sediment. However, it is impossible to
discriminate between them using the d
13 C signal alone.
However, geochemical interpretation is possible for some
major excursions. In particular, the positive and very large
Carboniferous excursion (a positive excursion of 4‰
throughout the Carboniferous (Veizer et al. 1999) is interpreted as the recording of the burial of a very large amount
of organic carbon (this was the time large coal beds were
being deposited in Europe, Russia and North America),
which extracted preferentially the
12 C from the
ocean-atmosphere system. This resulted in a higher consumption of atmospheric CO 2 and a cooling of the climate
which coincides reasonably well with the PermoCarboniferous ice age. This observation encouraged the
association of any positive excursion of d
13 C with a cooling
of the climate, especially since many positive excursions
coincide with episodes of extensive burial of organic matter
(episodes of global anoxia and black shale formation), such
as the end of the Devonian (Frasnian-Famennian episode,
circa 375 Ma) or the global anoxic events of the Cretaceous
(Aptian circa 120 Ma and Turonian circa 90 Ma). This
interpretation, however, is not conclusive. To illustrate the
difficulty of interpreting the d
13 C signal in geochemical and
climate terms, it should be noted that certain anoxic episodes, during the formation of gray or black shales, are
accompanied by negative excursions of the oceanic d
13 C,
such as the Toarcian anoxic episode. These events probably
indicate favorable conditions (anoxic environment) for the
preservation of organic matter, but the total amounts of
organic matter ultimately buried are low. The Toarcian
negative excursion can be interpreted as the result of a large
CO 2 outgassing from the mantle when the Karoo-Ferrar
traps were established in South Africa and could therefore be
linked to a warming of the climate by the greenhouse effect.
It should also be pointed out that, at shorter intervals during
glacial-interglacial Quaternary oscillations, the d
13 C of the
ocean decreased during glacial periods due to a reduction in
the biosphere, a consequence of aridity on the continents
(Part III, Chaps. 1 and 2).
Finally, some very rapid and pronounced negative
excursions could be attributed to the destabilization of
methane hydrates contained in marine sediments. The
methane released by the sediments is characterized by a d
13 C
of −60‰ and these negative values allow very pronounced
excursions of d
13 C. In general, it is assumed that the CH 4
contribution to the exosphere is short (10
4 to 10
5 years) and
intense, resulting in rapid negative excursions (e.g. the
thermal maximum of the Palaeocene-Eocene transition
(McInerney and Wing 2011).
It is even possible to imagine a combination of effects: the
Karoo traps were established in the coal-rich sediments of
Gondwana 183 million years ago. As a result, a massive
degassing of reduced carbon, low in
13 C, towards the
atmosphere, causing the negative excursion in d
13 C
observed during the Toarcian (McElwain et al. 2005).
Finally, a database of all the d
13 C values measured on
benthic foraminifera from 40 ODP and DSDP drillings
provides high-resolution coverage of the entire Cenozoic
(Zachos et al. 2008). The dominant signal from this curve is
the decrease in d
13 C of about 2% from mid-Miocene
(15 Ma) onwards. The reasons for this reduction remain
obscure. It could be a sign of a global decrease in the burial
of organic carbon over the last 15 million years. However,
over the same period, isotopic fractionation e MO decreased
by approximately 8‰, whereas it remained relatively
Fig. 27.5 d
13 C of calcite
sediment from the Phanerozoic.
% of data points
364
Y. Goddéris et al.
13 C signature furthest away from
the oceanic value, in other words, the oxidation of the
organic sedimentary carbon exposed on land or the burial of
organic carbon in sediment. However, it is impossible to
discriminate between them using the d
13 C signal alone.
However, geochemical interpretation is possible for some
major excursions. In particular, the positive and very large
Carboniferous excursion (a positive excursion of 4‰
throughout the Carboniferous (Veizer et al. 1999) is interpreted as the recording of the burial of a very large amount
of organic carbon (this was the time large coal beds were
being deposited in Europe, Russia and North America),
which extracted preferentially the
12 C from the
ocean-atmosphere system. This resulted in a higher consumption of atmospheric CO 2 and a cooling of the climate
which coincides reasonably well with the PermoCarboniferous ice age. This observation encouraged the
association of any positive excursion of d
13 C with a cooling
of the climate, especially since many positive excursions
coincide with episodes of extensive burial of organic matter
(episodes of global anoxia and black shale formation), such
as the end of the Devonian (Frasnian-Famennian episode,
circa 375 Ma) or the global anoxic events of the Cretaceous
(Aptian circa 120 Ma and Turonian circa 90 Ma). This
interpretation, however, is not conclusive. To illustrate the
difficulty of interpreting the d
13 C signal in geochemical and
climate terms, it should be noted that certain anoxic episodes, during the formation of gray or black shales, are
accompanied by negative excursions of the oceanic d
13 C,
such as the Toarcian anoxic episode. These events probably
indicate favorable conditions (anoxic environment) for the
preservation of organic matter, but the total amounts of
organic matter ultimately buried are low. The Toarcian
negative excursion can be interpreted as the result of a large
CO 2 outgassing from the mantle when the Karoo-Ferrar
traps were established in South Africa and could therefore be
linked to a warming of the climate by the greenhouse effect.
It should also be pointed out that, at shorter intervals during
glacial-interglacial Quaternary oscillations, the d
13 C of the
ocean decreased during glacial periods due to a reduction in
the biosphere, a consequence of aridity on the continents
(Part III, Chaps. 1 and 2).
Finally, some very rapid and pronounced negative
excursions could be attributed to the destabilization of
methane hydrates contained in marine sediments. The
methane released by the sediments is characterized by a d
13 C
of −60‰ and these negative values allow very pronounced
excursions of d
13 C. In general, it is assumed that the CH 4
contribution to the exosphere is short (10
4 to 10
5 years) and
intense, resulting in rapid negative excursions (e.g. the
thermal maximum of the Palaeocene-Eocene transition
(McInerney and Wing 2011).
It is even possible to imagine a combination of effects: the
Karoo traps were established in the coal-rich sediments of
Gondwana 183 million years ago. As a result, a massive
degassing of reduced carbon, low in
13 C, towards the
atmosphere, causing the negative excursion in d
13 C
observed during the Toarcian (McElwain et al. 2005).
Finally, a database of all the d
13 C values measured on
benthic foraminifera from 40 ODP and DSDP drillings
provides high-resolution coverage of the entire Cenozoic
(Zachos et al. 2008). The dominant signal from this curve is
the decrease in d
13 C of about 2% from mid-Miocene
(15 Ma) onwards. The reasons for this reduction remain
obscure. It could be a sign of a global decrease in the burial
of organic carbon over the last 15 million years. However,
over the same period, isotopic fractionation e MO decreased
by approximately 8‰, whereas it remained relatively
Fig. 27.5 d
13 C of calcite
sediment from the Phanerozoic.
% of data points
364
Y. Goddéris et al.