THE ELEMENTS IN THE UNIVERSE - Chemical Fundamentals of Geology and Environmental Geoscience - page 224

reservoirs with distinct geochemical signatures
(Figure 10.8).
• The Earth is the only planet to reside in the Solar
System's 'habitable zone' where liquid water exists
sustainably at the surface, a key factor in the devel-
opment of life. Subduction of oceanic crust modified
by chemical reaction with seawater probably lies
behind another unique feature of the Earth: the
chemically evolved continents.
• The Earth's early atmosphere consisted of combin-
ations of H 2 O, CO 2 , CO, N 2 , H 2 and CH 4 . Oxygen, the
product of photosynthesis by living things,
first appeared in the atmosphere at the end of
the Archaean eon, remained at a low concentration
through most of the Proterozoic eon, and then built
up progressively to the present level through the
Phanerozoic eon (Figure 11.8a), alongside the evol-
ution of plants.
• The cumulative influence of photosynthesis in
drawing down atmospheric levels of CO 2 and other
greenhouse gases has been the key to the Earth's
present benign climate, and has led to the depos -
ition of a large crustal reservoir of reduced hydro-
carbons that we exploit as 'fossil fuels'. Our current
rate of releasing fossil carbon back into the atmos-
phere poses huge risks to the future of mankind.

Taylor, S.R. and McLennan, S.M. (1985) The Continental Crust:
Its Composition and Evolution . Oxford: Blackwell.
Weinberg, S. (1993) The First Three Minutes , 2nd edition. New
York: Basic Books.
White, W.M. (2013) Geochemistry . Chichester: Wiley-
Blackwell.

Exercises

11.1 Examine the values of Z and N up to the element
lead in Figure 10.1.1 (nuclide chart, Box 10.1) for
which there are no naturally occurring nuclides.
What do they have in common? Why?
11.2 The decay of the short-lived isotope 26 Al (half-life
0.7 million years) is thought to have provided an
important source of heating during the early his-
tory of the Solar System. Calculate (a) the decay
constant of 26 Al, and (b) the time required for the
rate of 26 Al heat production to fall to a hundredth
of its initial value.
11.3 Identify the elements represented by the six
unlabelled data points in Figure 11.2 (enlarged
inset). To which subgroups in the Periodic Table
do they belong?
11.4 Why are volatile elements more depleted in the
Moon than the Earth (Figure 11.5)? Why does the
Moon have a small core compared with other
planetary bodies?
11.5 Highlight the mistakes in the following
statements:
(a) Nuclides sharing the same number of neu-
trons are called isotopes.
(b) Chemical elements are the products of fusion
reactions in stars.
(c) Dark matter consists of chemical elements
that just don't emit light.
(d) The s-process occurs only in supernovae.
(e) Siderophile elements in the Earth occur only
in the core.
(f) The beginning of photosynthesis led immedi-
ately to atmospheric oxygen.

Further reading

Albarède, F. (2009) Geochemistry. An Introduction , 2nd edition.
Cambridge: Cambridge University Press.
Bethe, H. and Brown, G. (1985) How a supernova explodes.
Scientific American 252 (5), 40-48.
Ferreira, P.G. (2006) The State of the Universe - A Primer in
Modern Cosmology . London: Weidenfeld and Nicolson.
Lenton, T. and Watson, A. (2011) Revolutions that Made the
Earth . Oxford: Oxford University Press.
Lin, D.N.C. (2008) The genesis of planets. Scientific American
298 (5), 50-59.
Riordan, M. and Schramm, D.M. (1993) The Shadows of
Creation - Dark Matter and the Structure of the Universe , 2nd
edition. Oxford: Oxford Paperbacks.