Biogeochemical Cycles: What controls CO and O levels in the atmosphere?

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22 Φεβ 2014 (πριν από 3 χρόνια και 5 μήνες)

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Biogeochemical Cycles: What controls CO
2

and O
2

levels in the atmosphere?

1.

What controls CO
2

in the atmosphere?

2.

What controls O
2

in the atmosphere?

These questions are linked (see Keeling plots)

Why is O
2

changing seasonally?

Why is it out of phase b
etween N and S hemispheres?

Why is it slowly decreasing?

We're seeing the seasonal shift between photosynthesis and aerobic respiration
.


To understand survival of either gas at Earth's surface, must examine the cycling of C
around the surface and between
the crust and surface. Must consider four concepts


1.

Reservoirs
: temporary repositories for material that flows through them.

At
steady
-
state for mass
, a reservoir is neither growing nor decreasing.

Either it has
no

inflows or outflows, or those flows ar
e
balanced
.

Reservoir size is indicated either by mass units or in moles.


2.

Residence Time
: the average length of time a substance spends in a given reservoir.

Residence Time = Reservoir Size at Steady State/Inflow or Outflow Rate

Residence time gives an

indication of how fast a reservoir will respond to perturbations in its
inflow or outflow.

The atmosphere contains 760 Gt C as CO
2
. Every year it exchanges ~60 Gt C with the
ocean, and 60 Gt with the land biosphere via photosynthesis
-
respiration. The resi
dence
time of C in the atmosphere is ~6.3 years. If the activity of the terrestrial biosphere
dropped, it would take ~6 years for atmospheric CO
2

to reach a new steady state.

Gt = Gigaton = 1 billion (10
9
) metric tons. A metric ton = 1000 kg, so a Gt = 10
15

gm


3.

Feedbacks


linkage of two or more system components to form a round
-
trip flow of
information. Some maintain balanced flow (i.e., steady state) (negative feedback). Others
lead to run away acceleration or deceleration of flow from system (positiv
e feedback).

Negative feedback example:
thermostats, photosynthesis
-
pCO
2

Positive feedback example:
CO
2

solubility
-
temperature


4.

Reduced vs. Oxidized Carbon

Oxidized substances are electron
-
poor; reduced substances are electron
-
rich.

Organic Carbon, whic
h is rich in C
-
C, C
-
H, and C
-
N bonds, is reduced.

Inorganic Carbon, which is rich in C
-
O bonds, is oxidized.

Knowing these concepts, we can understand what it takes to build O
2

in the atmosphere.

Once O
2

is created, its survival is controlled by reaction w
ith reduced organic C.

Organic C must build up in a reservoir with a long residence time if O
2

is to accumulate.


Large C reservoirs at the Earth's surface are:

1.

Dissolved marine CO
2

& HCO
3
-
: 37,740 GtC (oxidized)



600 GtC in the surface ocean, remainde
r in the deep ocean

2.

Marine carbonate sediments
: 2500 GtC (oxidized

3.

Organic C in marine and terrestrial sediments
: 1600 GtC (reduced)

4.

Atmospheric CO
2
: 760 GtC

5.

Primary producers

(marine and land plants): 600 GtC (reduced)

6.

Atmospheric methane
:
10 GtC (reduced)


Large C reservoirs in the crust are:

1.

Carbonate sedimentary rocks
: 40x10
6

GtC (oxidized)

2.

Organic C in sedimentary rocks
: 10x10
6

GtC (reduced)


Main processes affecting organic C reservoirs are:

Short
-
term processes:

i.e., those that

involve surface reservoirs w/short residence times.

1.

Photosynthesis
: CO
2

& H
2
O links oxidized atmosphere to primary producers

2.

Death of Primary Producers
: Deposition in shallow sediments or eaten by consumers.

a.

Consumption leads to respiration, whic
h converts reduced C back to CO
2
.

b.

Deposition of organic matter in sediments. Nearly all of this is immediately decomposed
by microbes and from organic C back to CO
2
.

Long
-
term processes:

i.e., those that involve large reservoirs w/long residence times.

3.

Burial of Organic C
: A small amount of organic C (0.05 GtC/yr) survives short
-
term cycling
to be buried in sedimentary rocks.

4.

Weathering of Sedimentary Rocks
: Long
-
term steady state is maintained by the oxidative
weathering of a small amount of sedim
entary organic C every year.


BOTTOM LINE FOR OXYGEN:
O
2

builds up in the atmosphere because of this leak of organic
C (and other reduced substances) from the rapidly cycling Earth surface reservoirs to the
slowly reacting crustal reservoir. If we lost thi
s flux, or if sedimentary rock organic C was
oxidized faster than organic C was buried, the Earth's atmosphere would quickly be stripped of
O
2
. Steady state is maintained by this balance.


What feedbacks link organic C burial and weathering to maintain bal
ance?

More efficient burial of organic matter at times of low O
2
?

More efficient weathering/combustion of organic matter at time of high O
2
?

Less efficient photosynthesis at times of high O
2
?


What might knock these feedbacks out of whack?

Initial increase

in O
2

required that these feedbacks were not complete


greater rate of
burial than weathering in the late Archean/early Proterozoic.

Dramatic increase in sedimentation rate, sequesters organic C, increases O
2
.

New compound classes that are hard to degrad
e, increases org C burial, increases O
2
.

Example: Coal forest of Carboniferous. Nothing could eat them. Giant insects.


Burial of organic carbon also removes carbon from the Earth's surface, which has some
effect on the amount of CO
2

in the atmosphere. A
tmospheric CO
2

are also influenced by
the inorganic cycling of carbon.


Short
-
term processes:

1.

CO
2

exchange between surface ocean and atmosphere

2.

Dissolution of CO
2

in the ocean and carbonate equilibria


CO
2

+ H
2
O

2
CO
3


H
2
CO
3



H

+

+ HCO
3
-



2H
+

+

CO
3
2
-

3.

Biogenic precipitation of calcium carbonate




Ca
2+

+ 2HCO
3
-



CaCO
3

+ CO
2

+ H
2
O

4.

Deposition of carbonate in marine sediments

5.

Dissolution of carbonate from marine sediments


Bottom Line: Little inorganic C survives trip to ocean floor and co
rrosive bottom waters.
Only 0.2 GtC/yr is converted into CO
3

sedimentary rock.


Long
-
term processes:

Carbonate
-
Silicate Geochemical Cycle

1.

Carbonate weathering




CaCO
3

+ CO
2

+ H
2
O


Ca
2+

+ 2HCO
3
-

2.

Silicate weathering




CaSiO
3

+ H
2
CO
3


+ CO
2


Ca
2+

+ 2HCO
3
-

+ SiO
2

3.

Metamorphism & Volcanism




CaCO
3

+
SiO
2



CaSiO
3

+ CO
2


Bottom line: Silicate weathering, followed by marine carbonate precipitation, consumes
CO
2

at Earth's surface. Carbonate weathering, followed by marine carbonate
precipitation,
has no net effect on CO
2

levels.


On the long
-
term, weathering of silicates balances volcanic input of CO
2
. Steady state is
maintained by this balance.


What negative feedbacks link the rate of silicate weathering with the amount of
volcanism, via the CO
2

concentration of the atmosphere?


Why is silicate weathering sensitive to the amount of atmospheric CO
2
?


Increased CO
2
, stronger greenhouse, increased T, faster reactions.


Increased CO
2
, stronger greenhouse, increased rainfall, faster reactions.


Increas
ed CO
2
, more acidic rainwater, faster reactions.