Chapter 11
Orbital-Scale Changes in
Carbon Dioxide and Methane
Reporter : 陳少華
Date : April 22, 2004 (Thursday)
Introduction
• Earth provides us an
important archive of
climate change : ice core.
• Two important
greenhouse
gases :Methane (CH4)
and carbon dioxide (CO2)
have be found in the ice
core.
Introduction
• Methane (CH4) and carbon dioxide (CO2) have varied
over orbital time scales.
• Methane levels have fluctuated mainly at the 23,000-
year 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
repeatedly dropped by 30 % .
Ice Cores
Drilling and Dating Ice Cores
Ice coring location
Trapping Gases in the Ice
Ice Cores
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.
Deposition (0.5-1m/year) →a few
hundred years.
Deposition (0.05-0.1m/year) →10002000 years.
sintering
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.
315ppm
1958
Figure 11-4. Ice core and instrumental CO2 and CH4 .
Orbital-Scale
Change in
Methane
minima
maxima
Figure 11-5. Methane and
the monsoon
How would changes in the strength of
low-latitude monsoons produce
changes in atmospheric methane
concentrations?
• One possible link is the impact monsoon fluctuations have on the
amount of precipitation that falls in Southeast Asia.
• 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
Figure 11-6. Long-term CO2 changes
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 1.2o/oo, atmospheric CO2 levels
increase 11 ppm .
Physical Oceanographic Explanations of
CO2 Changes
Only a small part of
the 90-ppm change
actually observed.
We are still left with 90%(79ppm) of the CO2
decrease to account for by other mechanism.
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
180gigatons
530gigatons
300gigatons
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.
Orbital-Scale Carbon Reservoirs
gigatons
Vegetation and soil
-530
Atmosphere
-180
Ocean mixed layer
-300
Deep ocean
1010
Earlier we found that temperature and salinity changes of surface waters can
explains only a little over 10% of the total glacial reduction in atmospheric CO2
levels ,leaving almost 90% yet to explained. And now we know that the atmospheric
carbon was stored in the deep sea.
How did the carbon get into the deep
ocean?
Why was more carbon stored in the
deep ocean during glacial times
than today?
Tracking Carbon through the Climate
System
BOX 11-1. Carbon Isotope Ratios
We need a quantitative way to track its movement ,two carbon
isotopes exits in nature.
( C / C ) sample −( C / C ) s tan dard
13
δ C=
13
(0
00
)
12
13
13
12
12
( C / C ) s tan dard
× 1000
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
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.
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%)
Inorganic C Mean
in ocean
δ13C
+ (530)
(-25%) =
C added
Mean
from land δ13C
(38,530)
(x%)
Glacial ocean Mean
carbon total
δ13C
x=-0.34
• We can analysis δ13C value in the shells.These bottom-dwelling
organisms should record regional deep-ocean δ13C values.
-0.35~-0.4
Can δ13C
Evidence
Detect
Glacial
Changes in
Carbon
Reservoirs?
Figure 11-12
Carbon transfer
during glaciations
Can δ13C Evidence Detect Glacial Changes
in Carbon Reservoirs?
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
How could such a transfer of carbon
from surface to deep water occurs?
• Ocean carbon pump hypothesis (Wally Broecker):
• Carbon was exported from surface waters to the deep
ocean by higher rates of photosynthesis and biologic
productivity.
• CO2+H2O
CH2O+O2
Organic tissue
What provides the source of added
nutrient to stimulate greater
photosynthesis?
Pumping of Carbon into the Deep
Ocean during Glaciations
Figure 11-14. Annual carbon production in the modern surface ocean
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 deep ocean δ13C patterns
Changes in the
Circulation of
Deep Water
during
Glaciations
Present-Day
Controls on
Regional
δ13C Values
Figure 11-19.
Regional δ13C
difference
<0.5 o/oo
Core of the flow
Today
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
1.5m
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.
0.9
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