Geoscience Reference
In-Depth Information
another hallmark of our planet with respect to its neighbors, started carrying renewable
resources to the ocean for life to grow stronger and stronger. A water world may not have
bred life with the same efficiency as our dual world of oceans and continents which is so
unlike other planets. The early emergence of plate tectonics may have been instrumental
in defining the conditions for long-lasting biological activity.
Continents may have been paid back for the help they provided life. We have seen and
will discuss again in the next chapter that biological activity strongly interferes with diage-
nesis. It actually increases the efficiency of erosion, helps process more detrital sediments,
and in the long run increases the proportion of hydrous material, such as clay minerals,
which are much more prone to melting than their anhydrous equivalents. Biological activ-
ity may therefore have strengthened the production of orogenic magmas and eventually
the amount of continental crust produced. The feedback loop between plate tectonics and
the emergence of life is likely to have endowed our planet with the main features of its
biogeochemical dynamics.
Exercises
1. In the range 0-20
◦
C, the vapor pressure of water at saturation
P
sat
H
2
O
changes with
temperature
T
as ln
P
sat
5365.37
T
−
1
26.06, where pressure is in Pa and tem-
perature in K. Assume that atmospheric water vapor forms above the ocean at low
latitudes at 15
◦
C and calculate a relationship between the residual fraction
f
of water
vapor (
Table 3.2
). Assuming that 1000 ln
H
2
O
=−
+
18
O
1.0779 10
6
T
−
2
α
=
−
2.796, infer a rela-
18
O values of rain water and their precipitation temperature. Find
appropriate linear approximations to all these equations.
2. Use the Mg/Ca ratios from
Table 9.1
to calculate the sea-surface temperature of the
Eastern Tropical Pacific over the last glacial cycle. Use
T
(
◦
C)
tionship between the
δ
0.667
x
2
=−
+
7.76
x
+
8.73 with
x
=
Mg/Ca. Which cycles can you identify?
Table 9.1
Mg/Ca in
Globigerinoides ruber
from the Eastern Tropical Pacific for
t
(y)
Mg/Ca
t
(y)
Mg/Ca
t
(y)
Mg/Ca
t
(y)
Mg/Ca
1800
2.99
27800
2.32
73 500
2.61
116 400
2.86
4400
3.07
32200
2.59
78 600
2.79
121 500
3.20
6900
3.16
39800
2.56
83 600
2.78
125 400
3.53
9500
3.06
44100
2.55
88 700
2.55
131 000
3.19
12 000
2.84
50900
2.53
93 700
2.63
134 100
2.79
14 300
2.72
55000
2.63
98 800
2.87
138 000
2.54
16 500
2.52
58600
2.53
101 300
2.95
141 400
2.51
18 800
2.38
65700
2.40
106 300
2.93
144 900
2.52
23 300
2.44
69200
2.45
111 400
2.78
148 400
2.63
