Chapter 11 Orbital-Scale Changes in Carbon Dioxide and Methane Reporter : Yu-Ching Chen Date : May 22, 2003 (Thursday)

Outline Introduction Introduction Ice Cores Ice Cores   Drilling and Dating Ice Cores   Trapping Gases in the Ice Orbital-Scale Change in Methane Orbital-Scale Change in Methane   Methane and the monsoon Orbital-Scale Change in CO 2 Orbital-Scale Change in CO 2   Physical Oceanographic Explanations of CO 2 Changes   Orbital-Scale Carbon Reservoirs   Tracking Carbon through the Climate System   Can δ 13 C 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 Conclusion

Introduction Methane (CH 4 ) and carbon dioxide (CO 2 ) have varied over orbital time scales. Methane (CH 4 ) and carbon dioxide (CO 2 ) 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. 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 CO 2 value have During glaciations, atmosphere CO 2 value have repeatedly dropped by 30. repeatedly dropped by 30.

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 Sintering: Sealing air bubbles in ice

Ice Cores Measurements of CO 2 (top) and methane (bottom) taken on bubbles in ice cores merge perfectly with measurements of the atmosphere in recent decades. Figure Ice core and instrumental CO 2 and CH 4.

Orbital-Scale Change in Methane 550~770 maxima 350~450 minima /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 monsoon- response signal driven by low-latitude insolation (right). Figure 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 CO 2 A 400,000-year record of CO 2 from Vostok ice in Antarctica shows four large-scale cycles at a period of 100,000 years similar to those in the marine δ 18 O record ppm maxima minima Abrupt increases in CO 2 occur during time of rapid ice melting. Figure Long-term CO 2 changes

Orbital-Scale Change in CO 2 A record of the last 160,000 years of CO 2 variations from Vostok ice in Antarctica (left)resembles the marine δ 18 O record (right). CO 2 concentrations in the atmosphere changed by 30 just a few thousand years. Figure The most recent CO 2 cycle

What factors could explain the observed 90-ppm drop in CO 2 levels during glacial Intervals from the levels observed Interglacial intervals?

Physical Oceanographic Explanations of CO 2 Changes One possibility is that changes in the physical oceanographic characteristics of the surface ocean-its temperature and salinity. One possibility is that changes in the physical oceanographic characteristics of the surface ocean-its temperature and salinity. CO 2 dissolves more readily in colder seawater, atmospheric CO 2 levels will drop by 9 ppm for each 1 ℃ of ocean cooling. CO 2 dissolves more readily in colder seawater, atmospheric CO 2 levels will drop by 9 ppm for each 1 ℃ of ocean cooling. CO 2 dissolves more easily in seawater with a lower CO 2 dissolves more easily in seawater with a lower salinity. salinity. During glaciations, the average salinity of entire ocean During glaciations, the average salinity of entire ocean increased by about 1.2 o / oo, atmospheric CO 2 levels increase 11 ppm. increased by about 1.2 o / oo, atmospheric CO 2 levels increase 11 ppm.

Physical Oceanographic Explanations of CO 2 Changes

Orbital-Scale Carbon Reservoirs Orbital-Scale Carbon Reservoirs Figure Exchange of carbon The large changes in atmospheric CO 2 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 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 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 δ 13 C values of the organic carbon produced.

Tracking Carbon through the Climate System Figure 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 Carbon Isotope Ratios

Can δ 13 C 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: 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%) (38,000) (0%) + (530) (-25%) = (38,530) (x%) Inorganic C Mean C added Mean Glacial ocean Mean Inorganic C Mean C added Mean Glacial ocean Mean in ocean δ 13 C from land δ 13 C carbon total δ 13 C in ocean δ 13 C from land δ 13 C carbon total δ 13 C x=-0.34 x=-0.34

Can δ 13 C Evidence Detect Glacial Changes in Carbon Reservoirs? Fig Fig

Pumping of Carbon into the Deep Ocean during Glaciations During glaciations(A), 12 C-enriched from the land to the ocean at the same time that 16 O-enriched water vapor is extracted from the ocean and stored in ice sheets. During interglaciations (B), 12 C-rich carbon returns to the land as 16 O- rich water flows back into the ocean. Figure Glacial transfer of 12 C and 16 O

Pumping of Carbon into the Deep Ocean during Glaciations Ocean carbon pump hypothesis Ocean carbon pump hypothesis Carbon was exported from surface waters Carbon was exported from surface waters to the deep ocean by higher rates of to the deep ocean by higher rates of photosynthesis and biologic productivity. photosynthesis and biologic productivity. CO 2 +H 2 O CH 2 O+O 2 CO 2 +H 2 O CH 2 O+O 2

Pumping of Carbon into the Deep Ocean during Glaciations Figure Annual carbon production in the modern surface ocean

DO wind Fertilize the Glacial Ocean? BOX Iron fertilization of ocean surface waters

Pumping of Carbon into the Deep Ocean during Glaciations Photosynthesis in ocean surface waters sends 12 C rich organic matter to the deep sea, leaving surface waters enriched in 13 C (left). At the same time, photosynthesis extracts nutrients like phosphate (PO ) from surface waters and sends them to deep sea. As a result, seawater δ 13 C values and phosphate concentrations are closely correlated (right). Figure Link between nutrients and δ 13 C values

Pumping of Carbon into the Deep Ocean during Glaciations Figure Measuring changes in the ocean carbon pump

Pumping of Carbon into the Deep Ocean during Glaciations Figure Past changes in the carbon pump If the ocean carbon pump affects atmospheric CO 2 levels, the net difference between surface and deep-water δ 13 C values should increase when CO 2 levels are low. Measured δ 13 C differences show some correlation with past changes in atmospheric CO 2

Changes in the Circulation of Deep Water during Glaciations δ 13 C Figure Modern deepocean δ 13 C patterns

Changes in the Circulation of Deep Water during Glaciations Present-Day Controls on Regional δ 13 C Values Figure Regional δ 13 C difference

Changes in the Circulation of Deep Water during Glaciations Past Changes in Regional δ13C Values Figure Change in deep Atlantic circulation during glaciation

Changes in the Circulation of Deep Water during Glaciations Figure Changing sources of Atlantic deep water. 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.

Changes in the Circulation of Deep Water during Glaciations Changes in Ocean Chemistry Figure Carbon system controls on CO 2 in the glacial atmosphere

Conclusion

Thanks For Your Attention