[LS7B] week 8- carbon cycle

how is co2 added to atmosphere?

(1) geologic inputs, mainly from volcanoes and mid-ocean ridges; (2) biological inputs, especially *respiration*; (3) human activities, including deforestation and the burning of fossil fuels.

how is co2 removed from atmosphere?

(1) geologic removal, especially by chemical weathering in which CO2 in rainwater reacts with exposed rocks, and (2) biological removal, mainly through *photosynthesis* (needs sunlight)

how can we explain the temperature and CO2 increases coincide with the last great retreat of continental ice sheets?

*forests expand as glaciers shrink, so changes in forests cannot account for a pattern of increasing atmospheric CO2 with the retreat of glaciers. *Volcanism and weathering also fail to account for the observed pattern. There is no evidence that volcanic activity has waxed and waned in a pattern that could explain observed CO2 variations. And rates of weathering, which remove CO2 from the atmosphere, should increase as temperature rises, but CO2 levels have actually increased. => *climate can and does change without any input from humans*

Reservoirs and fluxes are key in long-term carbon cycling.

*reservoirs* of carbon include (terrestrial+marine) organisms, the atmosphere, soil, the oceans, and sedimentary rocks

Physical processes add and remove CO2 from the atmosphere.

*volcanism* -> add chemical reactions between air and exposed rocks, a process called *chemical weathering* -> remove

If plate tectonic processes form a chain of high mountains, would you expect atmospheric CO2 to increase or decrease?

All else being equal, increasing the elevation of mountains should increase rates of chemical weathering and erosion. As chemical weathering of continental rocks consumes CO2, atmospheric CO2 levels should decline.

terrestrial carbon cycle

C enters the cycle through photosynthesis and is transferred through food webs by consumers Decomposers complete the return of c to the atmosphere by co2

why do the two curves correlate closely?

CO2 is known to be an effective greenhouse gas, meaning that it allows incoming solar radiation to reach Earth's surface but traps heat that is re-emitted from land and sea. -> higher concentrations of CO2 result in warmer temperatures

How much carbon is stored in each of Earth's major reservoirs?

If we add up all the carbon contained in the total mass of organisms living on land, that amount of carbon is just a bit smaller than the amount of carbon stored as CO2 in the atmosphere. Soil stores as much carbon as do land organisms and the atmosphere combined, as slowly *decaying organic compounds*. In the oceans, the amount of carbon contained in living organisms is actually very small. Much more resides as *inorganic carbon* dissolved in the water—most of it in the deep oceans as CO2 and bicarbonate and carbonate ions The biggest carbon reservoir of all, however, lies beneath our feet, within sediments and sedimentary rocks. Calcium carbonate minerals (CaCO3), which form limestone, and organic matter preserved in sedimentary rocks dwarf all other carbon reservoirs combined by three orders of magnitude. *Fluxes* are the rates at which carbon flows from one reservoir to another.

How does this result influence the hypotheses we developed to account for increasing atmospheric CO2 levels?

In photosynthesis, CO2 containing the lighter isotope 12C is incorporated into biomolecules preferentially over CO2 containing 13C, and for this reason organic matter generated by photosynthesis (and the organic matter of organisms that eat photosynthetic organisms) differs in its proportions of 13C and 12C from CO2 from volcanic gases and inorganic carbon dissolved in the oceans.

How does the graph in Fig. 25.3 suggest that human activities have influenced CO2 levels in the atmosphere? What might be a plausible alternative hypothesis?

The coincidence in timing between the Industrial Revolution and the observed increase in CO2 levels—following a millennium of little change—suggests that human activities played a role in recent changes in atmospheric composition. Alternative hypotheses would focus on other processes that add CO2 to the atmosphere. Perhaps, for example, increased volcanic activity added more CO2 to the atmosphere, or perhaps warming induced by changes in solar radiation caused thawing of permafrost, facilitating increased respiration of soil organic matter at high latitudes. These hypotheses can be tested against the historical record of volcanic activity and measurements of solar radiation over the past century.

plate tetonic

The crust that descends into subduction zones carries with it sediments, including carbonate minerals and organic matter. Subduction removes carbon from Earth's surface, but this carbon will be recycled to the surface as CO2 emitted from volcanoes and mid-ocean ridges.

What is the major source of the CO2 that has accumulated in Earth's atmosphere over the past two centuries?

The ratio of 13C to12C in CO2 emitted by volcanoes is too high to account for the data, as is the ratio in CO2 released from the oceans. In contrast, organic matter formed by photosynthesis has just the right ratio of 13C to 12C to account for Suess's measurements. By itself, the changing abundance of 13C in the air could reflect the conversion of plant carbon to CO2 by burning, or it could reflect the burning of fossil fuels formed by the burial of plant and algal materials in the geologic past. follow up work: While the burning of vegetation to clear land for agriculture results in CO2 with the right ratio of 13C to 12C, modern vegetation has far too much 14C to account for observed changes in the 14C of air. Only fossil fuels have the right ratios of all three carbon isotopes—12C, 13C, and 14C—to account for the pattern of isotopic change in atmospheric CO2 measured by Suess and others.

how does the periodic growth and decay of continental ice sheets relevant?

The repeated climatic shifts recorded in ice cores reflect periodic variations in the amount and distribution of solar radiation on Earth's surface, which are caused by oscillating changes in Earth's orbit around the sun.

Which of the following could explain why 18O:16O in glacial ice cores decreases as temperature decreases? Question 4 choices Choice A., Water that contains the heavy isotope of oxygen, H218O, requires slightly more heat input to evaporate from the ocean and is thus less common in snowflakes at lower temperatures. Choice B., Water that contains the heavy isotope of oxygen, H218O, does not readily form ice crystals (snowflakes) after evaporation, so it is less common in glaciers. Choice C., Water that contains the heavy isotope of oxygen, H218O, evaporates more easily from the ocean, therefore it is more common in snowflakes at lower temperatures.

a

On an annual basis, what is the net result of the contributions/removals of CO2 from the atmosphere due to global photosynthesis and respiration (both terrestrial and marine)? Question 1 choices Choice A., approximately 3 gT yr-1 decrease Choice B., approximately 9.5 gT yr-1 increase Choice C., approximately 209 gT yr-1 increase Choice D., approximately 212 gT yr-1 decrease

a => the small difference btwn the amount fixed by photosynthetic organism and the amount return by heterotroph to the envr as co2 can be accounted for by organic matter that gets buried in sediments after the organism died

fossil fuel

a natural fuel such as coal or gas, formed in the geological past from the remains of living organisms.

Measuring the isotopic composition of atmospheric CO2 shows that the burning of fossil fuels has led to _____ CO2 levels over the last 200 years. decreasing increasing

b

Recall that *photosynthetic rates remain relatively constant in regions near the equator*. Imagine that tropical environments persisted throughout Earth's northern and southern hemispheres (i.e., Earth's entire climate mirrored that near the equator). If Keeling had collected his atmospheric CO2 data on such an Earth, what would you expect the Keeling Curve to look like? a sinusoidal curve sloping upward (atmospheric CO2 levels would fluctuate seasonally, but would increase over time) a straight line sloping upward (atmospheric CO2 levels would not seasonally oscillate, but would have increased over time) a straight line sloping downward (atmospheric CO2 levels would not seasonally oscillate, but would have decreased over time) a sinusoidal curve sloping downward (atmospheric CO2 levels would fluctuate seasonally, but would have decreased over time) a straight line without a slope (atmospheric CO2 levels would have remained constant over time)

b

The CO2 level is _____ during winter in the northern hemisphere compared to levels in the summer. the same higher lower

b

What conclusion would you draw if the historical peaks in temperature did not coincide with the peaks in CO2 concentration? Question 5 choices Choice A., Glacial ice cores are not a useful indicator of temperature and CO2 concentration. Choice B., Atmospheric CO2 concentration is probably not the cause of global temperature variation. Choice C., There must be significant time lag between temperature rise and its impact on CO2 concentration.

b

annual oscillation of Kneeling curve?

biological process

Measurable increases in carbon dioxide in the atmosphere over the past 200 years have resulted primarily from human addition of carbon dioxide from: Question 2 choices Choice A., burning fossil fuels for heating, cooling and transportation. Choice B., clear cutting forests. Choice C., Both of these choices are correct.

c

Net human inputs to atmospheric carbon dioxide per year are estimated at approximately 8.8 gigatons. This is considerably less than the 209 gigatons of carbon dioxide added to the atmosphere by the respiration of all marine and terrestrial organisms. Given this, should we be concerned? Question 3 choices Choice A., We shouldn't be concerned; this addition is much too small to make a difference. Choice B., We shouldn't be concerned; variations in CO2 levels are naturally occurring events. Choice C., We should be concerned because this addition is not counterbalanced by photosynthesis or other removal processes and therefore can accumulate or build in the atmosphere over many years leading to climate change. , [ Choice D., We should be concerned because this addition is not counterbalanced by respiration or other removal processes and therefore can accumulate or build in the atmosphere over many years leading to climate change.

c

The _____ cycle deals with geological processes and carbon reservoirs, some of which have been built up (or take place) over a period of millennia. In contrast, the _____ cycle involves biological processes that are carried out on a daily and/or seasonal basis. long-term carbon; intermediate carbon short-term carbon; rapid carbon long-term carbon; short-term carbon short-term carbon; long-term carbon

c

Which of the following organisms would be classified as primary producers? A. the fish B. the copepod C. the phytoplankton D. humans, through the production of CO2

c

Why is the ratio of 18O:16O in glacial ice cores a good indicator of past temperature? Question 3 choices Choice A., It tracks CO2 concentration very closely. Choice B., 18O is isotopically unstable, even at cold temperatures. Choice C., The ratio decreases as temperature decreases. Choice D., This ratio causes a proportional change in CO2 concentration.

c

Variations in atmospheric CO2 over hundreds of millions of years reflect plate tectonics and evolution

climate, are determined in large part by geologic processes: changes in the rate of organic carbon burial in sediments, continental weathering of rocks uplifted into mountains, and volcanic gas release. All these processes reflect the action of Earth's great physical engine: plate tectonics.

Recall that during the Paleozoic Era, atmospheric CO2 levels decreased. Why? Volcanism decreased during this period. New, extensive mountain ranges formed and chemical weathering decreased. Earth's plate tectonics changed dramatically, and subduction increased. Photosynthetic, woody plants first appeared during this period.

d

Increased rate of photosynthesis is the most likely explanation as to why atmospheric CO2 levels appear lowest during glacial periods of Earth's history. True False

f

which processes play key roles in the carbon cycle on timescales of centuries and longer?

geologic

global co2 oscillation

global atmospheric CO2 declines through the northern summer, when the ratio of photosynthesis to respiration is highest, and then increases through fall and winter, when the ratio is reversed.

The regular oscillation of CO2 reflects the seasonality of photosynthesis in the Northern Hemisphere. why does northern hemisphere show a more obvious pattern than Southern?

land is distributed asymmetrically, with more land—and hence more plants—in the Northern Hemisphere than in the Southern Hemisphere.

Photosynthesis and respiration + human activites are key processes in short-term carbon cycling

photosynthesis -> *remove* aerobic respiration -> *add* +ATP human activities -> *add* => account for the increase in co2 over the year

*Except in a narrow band around the equator*, rate of photosynthesis and respiration

photosynthesis is *seasonal*, with higher rates in the summer and lower rates in the winter. Respiration *constant* through the year.

why does CO2 concentration in the air reached its annual high point in spring and then declined in fall? *seasonal oscillation*

summer remove winter increase

The long-term and short-term carbon cycles are connected by the fact that some of the carbon contained in C6H12O6 (or other organic molecules) in plants is incorporated into sedimentary rocks or oil; it is not immediately reintroduced back into the atmosphere. True False

t