Biology Reference
In-Depth Information
to better calculate temperature values and provide an improving curve for
the past 100 million years.
Temperatures were warm in the Middle Cretaceous and were beginning
to cool in the Late Cretaceous when the asteroid hit, causing a brief re-
bound, then they continued to decline. Temperatures suddenly rebounded
dramatically in the early Tertiary, in part, because of an explosive emis-
sion of methane (a greenhouse gas) from the Norwegian Sea. Another fac-
tor was the continuing albeit more gradual release of CO
2
and additional
methane from volcanism in East Greenland (Storey et al. 2007) associated
with activity along the Mid-Atlantic Ridge. In total, some 1500 gigatons
of CO
2
were emitted, temperatures rose by 5°C-6°C, and ocean acidity
increased, resulting in the extinction of 30-50 percent of benthic (deep-
water) foraminifera species. The warming lasted for about 220,000 years,
with the most rapid increase in the fi rst 20,000 years, and temperatures
ultimately reached maximum values at about 55 Ma (the EECL/LPTM).
Tropical biotas attained their most extensive geographic distribution dur-
ing this interval, while temperate organisms, such as deciduous trees
and shrubs, were mostly restricted to high latitudes and high altitudes.
If ocean waters in tropical regions also experienced substantial warming,
reaching 35°C-40°C for sea surface temperatures (see review by Huber
2008), this would sustain strong ocean circulation via thermal gradients.
It would also cause some marine organisms of surface and mid-depth
habitats to shift their ranges northward and southward to escape physi-
ologically incompatible equatorial temperatures. To the extent that low-
to midelevation terrestrial temperatures in tropical regions were higher
than the approximate present temperature of 28°C, this would be a fac-
tor in forcing the extinction of some tropical species, in addition to a shift
in range of others to the north and south (migration) because of unsuit-
ably warm climates in the lower latitudes. These species would be in addi-
tion to those expanding their range in response to warming temperatures
in the higher latitudes. There is now some evidence that temperatures in
the tropics may have been substantially warmer in the Paleocene than at
present. As noted, the discovery of a spectacular giant boid snake thirteen
meters long and weighing over a ton from the Cerrejón Formation of Co-
lombia at 60 Ma (Head et al. 2009), prior to the even warmer tempera-
tures at 55 Ma, is interpreted to indicate MATs of 30°C-34°C, or 6°C-
8°C warmer than at present and higher than previous estimates based on
paleobotanical data.
As the input of CO
2
and the effects of the methane excursion waned
(Pearson et al. 2009), temperatures dropped until about 35 Ma, then
