Past climate change - Climate Change: Biological and Human Aspects - page 85

western Canada, a crater 100 km wide in Manicouagan in eastern Canada and a
25 km crater in Rochechouart, France. The arguments against this are that the craters
have different magnetic fields. (As a crater cools so its magnetic field aligns with
the Earth's, but the Earth's field takes a thousand years or more to re-set, which
suggests that the craters were formed at least that far apart and not simultaneously.)
The evidence against this argument is that the largest crater had such a volume of
molten rock that it would have taken more than a thousand years to cool to the Curie
point (580 ◦ C for pure magnetite), when the Earth's magnetic field would have become
embedded in the rock (Vent and Spray, 1998).
An alternative explanation for the end-Triassic extinction is volcanic action due to
the break up of the old continent of Pangea. This would have injected carbon dioxide
(warming) and/or sulphates (cooling) into the atmosphere, causing a climate blip that
resulted in the extinction. Another explanation is that both these scenarios happened
simultaneously. What is known is that the geological strata of the volcanic Chon Aike
province of South America and Antarctica were created by a super volcano 170 mya,
as was the Karoo-Ferrar province 183 mya. These eruptions lasted millions of years
and would have had an impact on the global climate. So, from the end of the Triassic
through to the first half of the Jurassic there were a number of global climate-related
extinction events of varying degrees.
3.3.7 Toarcianextinction(183mya)
As noted above, the first half of the Triassic saw a number of events but there has
been particular recent interest in the Toarcian (early [late lower] Jurassic) extinction
due to both its magnitude and growing evidence for its similarity to that of the
Eocene thermal maximum, or Palaeocene-Eocene Thermal Maximum (see section
3.3.9). For a brief time (120 000 years), in the geological sense, during the Toarcian
(183 mya), many of the Earth's ecosystems experienced severe disruption. Carbon
laid down in Toarcian strata reveals a pronounced decline of approximately 5-7‰ in
the proportion of 13 C. This suggests that a pulse of 12 C diluted the carbon pool. The
source of this 12 C includes carbon from marine carbonates, marine organic matter
and terrestrial plant material (Cohen et al., 2007).
This decline in 13 C is referred to as a carbon isotope excursion (CIE). As discussed
in Chapters 1 and 2 the majority (98.9%) of stable carbon exists as the 12 C isotope but
some exists as other isotopes and radioactive (hence unstable) 14 C,whichisusedin
carbon dating. Around 1.1% of stable carbon exists as 13 C, but the Rubisco enzyme
in photosynthesis has a higher affinity for 12 C. This means that organic matter (photo-
synthetic organisms and the animals that live off of them) has proportionally far more
12 C than 13 C compared to non-organic geological strata. This relates to the isotopic
balance in photosynthetically fixed carbon (even if the carbon is from animal remains
further up the food chain or on a higher trophic level). However, there is also carbon in
carbonate ions that is not photosynthetically fixed 12 C and here the isotopic balance is
subject to the environmental sources (of which previously photosynthesised carbon
is still one) and sinks of the various carbon isotopes. This carbonate is present in
creatures such as the Foraminifera which have shells containing carbonate. Analysis
of the carbon isotopes in such species provides an indication of the isotopic car-
bon balance in their environment. The implication for the Toarcian CIE (and for the