Geoscience Reference
In-Depth Information
Phytoplankton, which dominates primary productivity at the surface of the oceans, has a
13
C value of about
δ
. Where organic-matter productivity is intense, there is a corre-
sponding depletion of surface water in
12
C. The carbon of carbonates that form the tests of
calcareous algae (such as, in particular, coccoliths that form chalk) and of foraminifera that
graze on plankton (such as globigerina) reflects the intensity of this depletion. The
−
25
13
C
values of foraminifera in carbonate sediments serve as a measuring rod for the biological
productivity of ancient oceans where the sedimentation occurred.
The
δ
13
C values of most diamonds (known as peridotitic), carbonatites (magmas essen-
tially made of carbonates such as those produced by the Oldoinyo Lengai volcano,
Tanzania) and volcanic gases cluster around
δ
. This isotope composition is considered
to be representative of deep-seated carbon. It is coincidentally very similar to atmospheric
carbon. Undegassed mid-ocean ridge basalts also have similar values, but CO
2
outgassing
drives the
−
7
13
C of the residual carbon towards
δ
−
20
.
3.6 Sulfur
Sulfur has four isotopes of masses 32, 33, 34, and 36 with mean abundances of 95.0,
0.75, 4.21, and 0.02 percent, respectively. Because of their small abundances, isotope 33
is rarely measured while isotope 36 is left out, so that the bulk of the geochemical infor-
mation comes from the
34
S/
32
S ratio only. Most of the S element inventory is confined to
the mantle and the core, but sulfate is one of the major components of seawater while cal-
cium sulfate and iron sulfide make up an important component of sediments. For decades,
the accepted standard was the Canyon Diablo Troilite (CDT). Troilite (FeS) is a sulfide
found as large blebs in the Canyon Diablo iron meteorite, which excavated Meteor Crater
in Arizona. After evidence was found that CDT was not truly homogeneous, it has been
replaced by a synthetic silver sulfide (Ag
2
S) produced by the International Atomic Energy
Agency. Because the surface reservoirs are presumably small with respect to the deep ones,
the mean terrestrial
34
S is probably very close to zero.
As for carbon, the most interesting aspects of sulfur stable isotopes are related to compet-
ing dominant states of oxidation, H
2
S, SO
2
, and SO
3
in the gas phase, S
2
−
(sulfide), SO
2
3
(sulfite), and SO
2
4
(sulfate) in solutions and ionic compounds. As for carbon, isotopic
equilibrium is rarely attained and kinetic isotope effects are important whenever redox
and biological processes prevail. The trend is the same as with carbon: oxidized species
are heavy (high
δ
34
S values), whereas the reduced species are light. It is easier to discuss
fractionation in the gas phase; SO
2
is heavier than H
2
Sby38permilat0
◦
C, 15 per mil
at 200
◦
C, and 4 per mil at 700
◦
C. Under the same conditions, SO
3
, which hydrates into
sulfate, is heavier than H
2
S, which hydrates into different sulfide ions, by 80, 33, and 4
per mil, respectively. In general sulfide and sulfate in equilibrium at ambient temperature
precipitate from solutions with a difference of about
δ
+
20 in favor of the sulfate. Today's
34
S of about
marine sulfate has a
, and so do modern marine evaporites: the very
large isotopic difference with CDT demands the existence of a very large deep reservoir of
34-S depleted sulfur and therefore reflects the fact that the dominant mass of sulfur is in the
δ
+
20
