Geoscience Reference
In-Depth Information
key role in defining the nitrogen isotope compositions of surface material. Fixation con-
verts atmospheric nitrogen into ammonium needed for amino acids with variable depletions
in
15
N, i.e.
15
N
. Nitrification is an exothermic reaction that turns organic
ammonium from mostly reduced amino (NH
2
) groups into nitrite then nitrate. The yield
of nitrification being close to 100 percent, the end products are variably depleted in
15
N.
Reduction of marine nitrate into N
2
, called denitrification, is a very active process taking
place in interstitial waters below the water-sediment interface: isotopically light N
2
is lost
to the atmosphere, while the heavier residual nitrates are flushed back into the ocean. This
explains the
δ
=
0to
−
27
15
Nof
of deep-sea nitrates.
Nitrogen is surprisingly abundant in diamonds in which its
δ
≈+
5
15
Nis
δ
≈−
7
. There is a
clear difference in nitrogen isotope compositions between mid-ocean ridge (
−
4) and ocean
island basalts (
3), yet with significant overlap. The interpretation of these variations is
still unclear but they seem to be related to how much recycled lithosphere is present in the
source of each type of basalt.
+
3.8 Other elements
Recent progress in analytical techniques has opened up new grounds for stable isotope
geochemistry. Besides boron, which we will see later can be used as a pH-meter of ancient
oceans, and silicon, which is currently revived as a paleoceanographic tracer, the use of the
isotope compositions of some metals has recently attracted considerable attention. Lithium
has an interesting cycle giving information on fluid-rock interactions and subduction-
zone processes. Magnesium and calcium are relevant to weathering and erosion cycles,
molybdenum to redox conditions in the ocean and in diagenesis, Fe, Cu, Zn to biological
processes and ore-body genesis, and selenium to environmental studies in changing redox
conditions.
Exercises
1. Consider oxygen gas and the reaction
16
O
2
+
18
O
2
⇔
2
16
O
18
O. What is the number
of distinguishable configurations of each of the molecules involved in the reaction? If
oxygen isotopes were distributing themselves at random in proportion to their abun-
dances in the system and without any other effects, calculate the
α
value of this
reaction.
2. The
18
O value of modern seawater is 0
δ
while the average value of the polar ice cap
18
O value of an
is
−
45
. The ice cap holds 2 wt% of the oceanic water. Calculate the
δ
ice-free ocean. Other water reservoirs can be neglected.
3. The
18
O values of benthic carbonates (PDB scale) decreased from
δ
−
1.2
to
−
2.5
between the Early Eocene and the Oligocene. Assume an ice-free ocean (no salinity
variation) and determine the cooling of the deep ocean over the same period using
