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(despite their scatter) a level distribution broadly lying
within the range of present-day carbonate sediments.
There were two intervals of Earth history, however,
when marine carbonate isotope compositions under-
went wild excursions - both positive and negative -
from this steady-state trend. Each episode is believed to
represent an explosion of photosynthesis, when a
blooming of biota across the globe drew down a signif-
icant proportion of the atmosphere's CO
2
and, upon
death, these organisms deposited their remains as
reduced
carbon in sediments accumulating on the ocean
floor. This 'organic carbon' locked up in sedimentary
rocks has the distinctive negative δ
13
C values that are
the fingerprint of Rubisco's role in photosynthesis and
its preference for
12
C. The removal of this negative δ
13
C
carbon left the complementary atmospheric CO
2
reser-
voir enriched in
13
C during these episodes, and by
exchange with the oceans this led to the deposition
of the high-δ
13
C carbonate sediments shown in
Figure 10.13.
The older of these two excursions - at the beginning
of the Proterozoic eon - marked the first sustained
global appearance of O
2
in the Earth's atmosphere,
which up to this point had consisted only of gases like
H
2
O, CO
2
, CO and H
2
(see Chapter 11). Oxygen's
appearance had profound consequences, and accord-
ingly this first period of δ
13
C excursions is known as
the
Great Oxidation Event
(Figure 10.13). Although the
atmospheric O
2
concentration generated at this time
was far lower than today's level (Figure 11.8), the
draw-down of CO
2
required to release this photosyn-
thetic oxygen reduced the natural atmospheric 'green-
house effect' sufficiently to bring about major
worldwide glaciations (a phenomenon known loosely
as
Snowball Earth
). The wild fluctuations in δ
13
C dur-
ing this episode (Figure 10.13) reflect alternation of
(i) periods in which photosynthesis and organic matter
deposition predominated (
raising
atmospheric δ
13
C),
with (ii) colder periods in which collapsing biological
productivity allowed oxidation of organic matter to
exceed the rate of carbon burial,
depressing
atmospheric
δ
13
C values (Kump
et al.
, 2011).
The second excursion (between 850 and 500 Ma ago)
coincided with the time when photosynthesizing life
first colonized the land surface towards the end of the
Neoproterozoic era, initiating a steeper growth in
atmospheric O
2
content towards the breathable range
we depend on today (Figure 11.8).
20
Modern
carbonate
sediments
10
0
Pre-industrial
Present-day
Atmospheric CO
2
-10
-20
Plants
Neoproterozoic
'
Great oxidation
'
4000
3000
2000
Age/Ma BP
1000
0
-35
Figure 10.13
The δ
13
C record in marine carbonates from the oldest-known rocks to the present day; open symbols signify
samples with less precise ages (uncertainty >50 Ma). The shaded bars on the left indicate the isotopic ranges of
present-day
carbon reservoirs; 'pre-industrial' shows the isotope composition of atmospheric CO
2
prior to the industrial revolution.
(Source: Adapted from Shields and Veizer, 2002. Reproduced with permission of the American Geophysical Union.)
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