Chapter 11
Orbital-Scale Changes in
Carbon Dioxide and
Methane
Reporter : Yu-Ching Chen
Date : May 22, 2003 (Thursday)
Outline



Introduction
Ice Cores

Drilling and Dating Ice Cores
Trapping Gases in the Ice

Orbital-Scale Change in Methane

Methane and the monsoon


Orbital-Scale Change in CO2

Physical Oceanographic Explanations of CO2 Changes
Orbital-Scale Carbon Reservoirs
Tracking Carbon through the Climate System

Can δ13C Evidence Detect Glacial Changes in Carbon Reservoirs?


Pumping of Carbon into the Deep Ocean during Glaciations
Changes in the Circulation of Deep Water during Glaciations

Conclusion

Introduction

Methane (CH4) and carbon dioxide (CO2) have varied
over orbital time scales.

Methane levels have fluctuated mainly at the 23,000year orbital rhythm of precession, and we will evaluate
the hypothesis that these changes are linked to
fluctuations in the strength of monsoons in Southeast
Asia.

During glaciations, atmosphere CO2 value have
0 00
repeatedly dropped by 30 0 00.
Ice Cores
Drilling and Dating Ice Cores
Ice Cores
Trapping Gases in the Ice
Air moves freely through snow and ice in
the upper 15m of an ice sheet, but flow is
increasingly restricted below this level.
Bubbles of old air are eventually sealed off
completely in ice 50 to 100m below the
surface.
Figure 11-3. Sintering: Sealing air bubbles in ice
Ice Cores
Measurements of CO2 (top) and
methane (bottom) taken on bubbles in
ice cores merge perfectly with
measurements of the atmosphere in
recent decades.
Figure 11-4. Ice core and instrumental CO2 and CH4 .
Orbital-Scale Change in Methane
550~770 maxima
350~450 minima
12500-10000/5=23000 years/cycle
Methane record from Vostok ice
in Antarctica shows regular
cycles at Intervals near 23,000
years (left).
This signal closely resembles the monsoonresponse signal driven by low-latitude
insolation (right).
Figure 11-5. Methane and the monsoon
How would changes in the strength of
low-latitude monsoons produce
changes in atmospheric methane
concentrations?

Heavy rainfall in such regions saturates the ground,
reduces its ability to absorb water, and thereby increases
the amount of standing water in bogs.

Decaying vegetation uses up any oxygen in the water
and creates the oxygen-free conditions needed to
generate methane.

The extent of these boggy areas must have expanded
during wet monsoon maxima and shrunk during dry
monsoon minima.
Orbital-Scale Change in CO2
A 400,000-year record of CO2 from
Vostok ice in Antarctica shows four
large-scale cycles at a period of
100,000 years similar to those in the
marine δ18O record.
280-300ppm maxima 180-190 minima
Abrupt increases in CO2 occur during
time of rapid ice melting.
Figure 11-6. Long-term CO2 changes
Orbital-Scale Change in CO2
A record of the last 160,000
years of CO2 variations from
Vostok ice in Antarctica
(left)resembles the marine δ18O
record (right).
CO2 concentrations in the
atmosphere changed by 30 0 00
just a few thousand years.
Figure 11-7. The most recent CO2 cycle
What factors could explain the
observed 90-ppm drop in CO2 levels
during glacial Intervals from the
levels observed Interglacial intervals?
Physical Oceanographic Explanations
of CO2 Changes




One possibility is that changes in the physical
oceanographic characteristics of the surface ocean-its
temperature and salinity.
CO2 dissolves more readily in colder seawater,
atmospheric CO2 levels will drop by 9 ppm for each 1℃
of ocean cooling.
CO2 dissolves more easily in seawater with a lower
salinity.
During glaciations, the average salinity of entire ocean
increased by about 1.2o0/00oo, atmospheric CO2 levels
increase 11 ppm .
Physical Oceanographic Explanations
of CO2 Changes
Orbital-Scale Carbon Reservoirs
Figure 11-8. Exchange of carbon The large changes in atmospheric CO2 in ice
cores over intervals of a few thousand years must involve rapid exchanges of
carbon among the near-surface reservoirs.
Orbital-Scale Carbon Reservoirs
Figure 11-9. Interglacial-glacial changes in carbon reservoirs During the glacial maximum 20,000
years ago, large reductions of carbon occurred in the atmosphere, in vegetation and soils on
land, and in the surface ocean. The total amount of carbon removed from these reservoirs
(> 1000 gigatons) was added to much larger reservoir in the deep ocean.
Tracking Carbon through the Climate System
Figure.11-11 Photosynyhesis and carbon isotope factionation Photosyntheis on land and in the
surface ocean converts inorganic carbon to organic form and causes large negative shifts in
δ13C values of the organic carbon produced.
Tracking Carbon through the Climate System
Figure 11-10. Carbon reservoir δ13C values The major reservoirs of carbon on Earth have
varying amounts of organic and inorganic carbon, and each type of carbon has
characteristic carbon isotope values.
Tracking Carbon through the Climate
System
BOX 11-1. Carbon Isotope Ratios
( C / C ) sample ( C / C ) s tan dard
13
 C
13
 C
13
(13C / 12C ) sample (13C / 12C ) s tan dard
13
12
( C / C ) s tan dard
(0
00
)
12
 1000
13
13
12
12
( C / C ) s tan dard
1000
Can δ13C Evidence Detect Glacial Changes
in Carbon Reservoirs?

We can use a mass balance calculation to estimate the
effect of adding very negative carbon to the inorganic
carbon already present in the deep sea:
(38,000)
(0%)
+ (530)
(-25%) =
(38,530)
(x%)
Inorganic C Mean C added Mean
Glacial ocean Mean
in ocean
δ13C from land δ13C
carbon total δ13C
x=-0.34
Can δ13C Evidence Detect Glacial Changes
in Carbon Reservoirs?

Fig. 11-12
Pumping of Carbon into the Deep
Ocean during Glaciations
During glaciations(A), 12C-enriched
from the land to the ocean at the
same time that 16O-enriched water
vapor is extracted from the ocean
and stored in ice sheets.
During interglaciations (B), 12C-rich
carbon returns to the land as 16Orich water flows back into the ocean.
Figure 11-13. Glacial transfer of 12C and 16O
Pumping of Carbon into the Deep
Ocean during Glaciations

Ocean carbon pump hypothesis

Carbon was exported from surface waters
to the deep ocean by higher rates of
photosynthesis and biologic productivity.

CO2+H2O
CH2O+O2
Pumping of Carbon into the Deep
Ocean during Glaciations
Figure 11-14. Annual carbon production in the modern surface ocean
DO wind Fertilize the Glacial Ocean?
BOX 11-2. Iron fertilization of ocean surface waters
Pumping of Carbon into the Deep
Ocean during Glaciations
Photosynthesis in ocean surface waters
sends 12C rich organic matter to the deep
sea, leaving surface waters enriched in
13C (left).
At the same time, photosynthesis
extracts nutrients like phosphate (PO4--2)
from surface waters and sends them to
deep sea. As a result, seawater δ13C
values and phosphate concentrations are
closely correlated (right).
Figure. 11-17. Link between nutrients and δ13C values
Pumping of Carbon into the Deep Ocean during
Glaciations
Figure 11-16. Measuring changes in the ocean carbon pump
Pumping of Carbon into the Deep Ocean during
Glaciations
If the ocean carbon pump
affects atmospheric CO2 levels,
the net difference between
surface and deep-water δ13C
values should increase when
CO2 levels are low.
Measured δ13C differences
show some correlation with
past changes in atmospheric
CO2
Figure 11-17. Past changes in the carbon pump
Changes in the Circulation of Deep
Water during Glaciations
Figure 11-18 Modern deepocean δ13C patterns
Changes in the Circulation of Deep Water during
Glaciations
Present-Day Controls on Regional δ13C Values
Figure 11-19. Regional δ13C difference
Changes in the Circulation of Deep Water during
Glaciations
Past Changes in Regional δ13C Values
Figure 11-20. Change in deep Atlantic circulation during glaciation
Changes in the Circulation of Deep Water during
Glaciations
The percentage of deep water
Originating in the North
Atlantic and flowing to the
equator during the last1.25
Myr has been consistently
lower during glaciations than
during interglaciations.
Figure 11-21 Changing sources of Atlantic deep water.
Changes in the Circulation of Deep Water during
Glaciations
Changes in Ocean Chemistry
Figure 11-22. Carbon system controls on CO2 in the glacial atmosphere
Conclusion
Thanks For Your
Attention