Page 118 - Earth's Climate Past and Future

94 PART II • Tectonic-Scale Climate Change

were the long-range effects on Earth’s climate? Surpris- Deep-sea temperature (˚C)
ingly, little long-term climatic effect is evident. Earth’s 10 12 14 16
climate was in a warm greenhouse state at the time of
the impact, and there it remained afterward.
Several problems make it difficult to determine the 54.8
climatic effects of such a brief impact event. One is that
rapid changes in sedimentary records are blurred and
smeared by burrowing animals and by bottom currents
(Chapter 2). Distinctive impact-related features such as 54.9
the iridium layer can still be detected despite this blurring
(see Figure 5-13A) because they contrast clearly with the
material in which they are deposited. But for most kinds Myr ago
of sedimentary archives, the signal from a single year (or
even a decade or century) of climate that is warmer or 55.0
cooler than normal will be mixed into and combined with
the signals left by “normal” years and blurred beyond
recognition.
A second problem with detecting the climatic effect 55.1
of the impacts of even huge asteroids is that climate
change was already occurring for other reasons before FIGURE 5-16 Unusual warmth 55 Myr ago A pulse of
the impact. Records from marine sediments show a large unusual warmth that developed near 55 Myr ago and persisted
(3º–4ºC) warming and then cooling during the 500,000 for tens of thousands of years warmed the deep ocean by
years before the impact, but little or no additional several degrees Celsius.
change after it. Some land vegetation records suggest a
long-term warming for hundreds of thousands of years
after the impact, but it has not been demonstrated that changes associated with this relatively abrupt warming
this warming was related to the impact event. The included a major acidification of ocean waters that
increase in CO levels caused by the immediate effects of caused widespread dissolution of CaCO sediment on
2 3
the event seems unlikely to have persisted for that long. the seafloor and the extinction of nearly half of the
species of benthic foraminifera living on the seafloor.
IN SUMMARY, impact events such as the one at the Also observed at this time is a large shift toward more
Cretaceous-Tertiary boundary have clearly had negative carbon isotopic values in marine plankton. Such
apocalypse-like effects on the environment, including a change requires a major release of carbon enriched in
13
mass extinctions of organisms that transformed life the C isotope (see Appendix 2). Early attention focused
on Earth. Despite this environmental apocalypse, the on methane, which exists in the atmosphere as a familiar
background state of the climate system 65 Myr ago greenhouse gas (Chapter 2) but is also present in solid
seems to have been changed little or not at all. form frozen (methane clathrates) into a slushy mixture
not far below the ocean floor. If the subsurface ocean
13
were to warm significantly, this C-rich slush could be
Large and Abrupt Greenhouse Episode converted to a gas and released to the ocean and then to
near 50 Myr Ago the atmosphere.
13
The large shift in δ C values showed, however, that
The warm greenhouse world was still in existence when the available methane sources 55 Myr ago were inade-
a relatively brief episode of even warmer climate began quate to account for the amount of carbon in the atmos-
near 55 Myr ago. Within about 10,000 years, ocean and phere and that immense additional releases of CO were
2
terrestrial climate warmed by 5ºC at low latitudes and also required. The source of this huge amount of extra
9ºC at high latitudes. The excess warmth persisted at or carbon is not entirely clear, but one obvious candidate is
near full strength for about 70,000 years and then the deep ocean, which is currently a very large carbon
slowly faded away over the next 100,000 years. Com- reservoir. Also unclear at this point is the initial trigger
pared to the slow scale of tectonic changes, this thermal for the carbon and methane releases. In any case, the
maximum was a brief event. addition of large amounts of CO and methane consid-
2
Evidence for the warming comes from the spread of erably warmed climate for several tens of thousands of
plants and mammals into high latitudes and also from years. The recovery from this thermal perturbation took
the decreasing values of the oxygen isotope “paleother- about 100,000 years. The most plausible mechanism for
mometers” explained in Appendix 1 (Figure 5-16). The removing the extra carbon from the atmosphere is