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mantle. We can also expect that diagenetic reduction of marine sulfate, whether inorganic
or biologically mediated, results in sulfides (often present in organic-matter-bearing sedi-
ments such as pyrite FeS
2
) with
34
S values a few tens of per mil below the marine value
largely due to kinetic isotope effects. The
δ
34
S value of marine sulfate has fluctuated by
several per mil over the Phanerozoic (see
Chapter 9
), which reflects changes in the burial
of sedimentary pyrite and atmospheric oxygen pressure.
Sulfur isotopes have developed into a tracer of sulfide ore genesis thanks to the complex
speciation of this element in solution. First, the diprotic acid H
2
S dissociates into HS
−
and
S
2
−
as a function of pH and the dissociation constants vary with, among other parameters,
temperature. Second, fractionation of sulfur isotopes between sulfate and sulfide species,
either at equilibrium or kinetically controlled, is assumed. Depending on the pH and redox
potential of the solution, different proportions of these species coexist and each of them
has a different
δ
34
S value. Sulfides, such as pyrite, and sulfate, such as barite, can precip-
itate with a range of
δ
34
S that reflects these conditions. Using these principles, it can be
demonstrated that the sulfur from black smokers is largely - although non exclusively -
contributed by reduced basaltic sulfide and not by marine sulfate.
True igneous sulfides are rare because they are quickly oxidized by hydrothermal flu-
ids. Nickel sulfide (pentlandite) is an all-too-rare truly magmatic sulfide in peridotites and
basalts and its
δ
34
S is often close to zero.
Recently, the minor isotopes 33 and 36 of sulfur have found novel applications.
First, photochemical reactions between solar ultraviolet radiations and SO
2
in the upper
atmosphere create strong deviations from the mass-dependent fractionation relationships
δ
δ
34
S. We will return to this point upon discussion of the
rise of atmospheric oxygen. Second, because isotope fractionation between sulfides and
sulfates is often large, the linear approximation of the Rayleigh law, which we met for O
and H isotopes in meteoric waters, breaks down at the level of precision obtained by mod-
ern mass spectrometers. Small non-mass-dependent effects can thus be detected that seem
to be powerful tracers of microbiological activity.
33
S
34
S and
36
S
=
(1
/
2)
δ
δ
=
2
δ
3.7 Nitrogen
Nitrogen has two isotopes 14 and 15, with mean abundances of 99.63 and 0.37 percent,
respectively. The standard is, not surprisingly, atmospheric nitrogen (AIR). Although N
2
is
the main component of the atmosphere, it has been argued that a substantial proportion is
in the mantle. In addition,
15
N/
14
N in extra-terrestrial material varies a lot as a result of loss
from planetary atmospheres. We will therefore abstain from proposing a mean terrestrial
value of
15
N.
As with carbon and sulfur, fractionation is dominated by nitrogen species of different
oxidation states, N
2
,NH
3
, NO, NO
2
in the gaseous state and, for the last three, their dis-
solved equivalents, ammonium NH
4
, nitrite NO
2
, and nitrate NO
3
. At equilibrium under
surface temperature conditions, nitrites and nitrates are isotopically heavier, and NH
3
is
lighter than N
2
. Again, kinetic isotope effects are important. Biological processes play a
δ
