Geology Reference
In-Depth Information
fossil carbon which - transformed by diagenesis and
thermal maturation over many millions of years - has
provided the coal, petroleum and natural gas resources
that fuel industrial societies today. Equally important
from the 'greenhouse' point of view, however, are the
countless carbonate-secreting organisms that over the
eons of geological time have fixed atmospheric carbon
dioxide in the form of limestone (the product of acc-
umulation of calcareous biogenic debris). Together
these carbon reservoirs in the crust account for the
marked difference in atmospheric CO
2
content between
Earth and Venus (0.03% versus 96.5%) and for the
temperate climate that we enjoy on Earth.
Human society as we understand it today will surv-
ive only if sound climate science prevails
soon
over
business-as-usual politics.
Review
• The average chemical composition of baryonic
matter making up the Solar System can be deter-
mined (a) from spectral analysis of the light reach-
ing us from the Sun and other stars, and (b) from
laboratory analyses of primitive meteorite samples
(Box 11.1). On the scale of galaxies, however, such
'visible' matter is outweighed many times by 'dark
matter' of uncertain composition.
• The Solar System abundance curve (Figure 11.2)
shows that H and He are the most abundant elem-
ents in the cosmos. Except for Li, Be and B, heavier
elements become progressively less abundant with
increasing
Z,
but with peaks around
Z
= 26 and 52.
Even-
Z
nuclides are generally ten times more abun-
dant than neighbouring odd-
Z
nuclides.
• H, He and Li are predominantly products of the Big
Bang, but heavier elements have been formed pro-
gressively by fusion (
Z
< 26) and neutron capture
reactions (
Z
> 26) in multiple generations of stars
(Figure 11.3).
• Element behaviour in the Solar System may be
described in terms of four overlapping cosmochem-
ical categories (Figure 11.4): lithophile, siderophile,
chalcophile and atmophile. Elements are also sub-
divided according to whether they - or their oxides -
are refractory or volatile (Figure 11.5).
• The four 'terrestrial' planets closest to the Sun
have rocky, metal-rich, volatile-depleted compos-
itions (Figure 11.6), modified in the case of Mercury
and the Earth-Moon system by post-accretion giant
impacts. The larger outer planets have lower-density
atmophile-rich compositions closer to the Solar
System average.
• Planetesimal accretion, core-segregation and the
Moon-forming giant impact (~4522 Ma ago) brought
about extensive partial melting in the early Earth,
promoting efficient partition of siderophile elem-
ents into the core. Heat accumulated from these
early exothermic events still contributes to surface
heat-flow today. Magmatism through Earth history
and extraction of crust have generated mantle
Future prospects
Life on Earth has survived a range of climates over the
course of geological history, encompassing mean
global temperatures 6-8 °C higher than at present dur-
ing Cretaceous times to 10 °C cooler in the depths of
the recent ice age. Despite the evident resilience of life
in general to such changes, one must recognize that
human
civilization has developed almost entirely dur-
ing the relatively constant and benign inter-glacial
climate of the last 11,700 years (the Holocene epoch), to
which our agriculture, settlement patterns and econ-
omies are now finely tuned. Through our reliance on
fossil fuels, however, we have been returning reduced
carbon from the crustal reduced-carbon reservoir into
the atmosphere, partially undoing the cumulative
work of photosynthesis over the last billion years.
Today we are releasing greenhouse gases back into the
atmosphere at a faster rate than any in recent geolog-
ical history, and the mean atmospheric CO
2
content is
higher today than at any time for the last half-million
years (IPCC, 2013) and probably higher than the last
15 million years (see, for example, Tripati
et al
., 2009).
In the words of one climate expert:
We are
(re)
creating a prehistoric climate in which human societies
will face huge and potentially catastrophic risks. Only by urgently
reducing global emissions will we be able to avoid the full conse-
quences of turning back the climate clock by 3 million years.
17
Bob Ward, policy director at the Grantham Research Institute
on Climate Change at the London School of Economics.
http://
bit.ly/1eEJyN2
. See also
www.epa.gov/climatestudents/
17
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