ANSWERS TO EXERCISES - Chemical Fundamentals of Geology and Environmental Geoscience - page 232

cannot be formed by fusion (Figure 11.3.1) and
are instead products of progressive neutron
capture in stars and supernovae.
(c) Perhaps 5-10% of dark matter consists of dark
baryonic matter ('ordinary stuff that doesn't
shine', Ferreira, 2006); the remaining 90-95%
is believed to consist of exotic subnuclear
particles yet to be identified .
(d) T he s(=slow)-process neutron capture reactions
can occur in stellar interiors . The heaviest
nuclides are however formed by more rapid
(' r-process ') neutron capture that can only
operate in the very high neutron fluxes that
occur in supernovae .
(e) Siderophile elements are indeed concentrated
in the Earth's metallic core, but many (notably

Fe and Ni) also have lithophile and/or chalcophile
tendencies (as shown by the overlapping fields
in Figure 11.4 and multicoloured cells in
Plate 7) and therefore can be found in silicate
and sulfide parts of the Earth too.
(f) Oxygenic photosynthesis is believed to have
begun in the oceans around 3.5 Ga ago, but
the oxygen initially formed there is believed
to have been immediately mopped up in
oxidizing the accumulated reduced species
in seawater like Fe 2+ (as implied by BIF
sediments). Only when this 'oxygen sink'
had been completely oxidized could excess
oxygen escape, around the end of the
Archaean, to form an oxygenated atmosphere
(Figure 11.8).