This Week: Biogeochemical Cycles

This Week: Biogeochemical Cycles

Hydrologic CycleCarbon Cycle

Announcements

• Reading: Chapters 4 (p. 74 – 81) and 8

• Another Problem Set (Due next Tuesday)

• Exam 2: Friday Feb 29

• My office hours today and next week rescheduled to Thurs 3 - 4 pm

Water Cycle “Box Model”Burden: amount of material in reservoir

Reservoir: region where material stored; each box

Source/Sink: flow rate into/out of reservoir

There’s ~ 1.3x1018 m3 of H2O in the oceans. About 3.6x1015 m3 of H2O per year evaporate

from the oceans. How long does the average H2O molecule reside in the ocean?

36

year

s

360

yea

rs

360

0 ye

ars

5%

35%

61%1. 36 years2. 360 years3. 3600 years

Precipitable Water Distribution

Precipitable water greatest over oceans, in tropics.

Spatially Resolved Precipitation Rate

Subsiding branches of Hadley Cells

Precipitation Rate Minus Evaporation Rate

green (positive): net water gainyellow/brown (negative): net water loss

Water Cycle and Terrestrial Biosphere

Tundra

Forest

Grassland

Temperature0oC

Pre

cip

itati

on

Desert

Precipitation Changes with Global Warming

Figure SPM.7

Climate Model Predictions

Announcements

• Problem Set 4 Due Tuesday Feb 26

• Group Choices by Friday

• Seminar on Thursday 12:30 310c ATG

Carbon Cycle

• Short-term biosphere-driven cycle– Terrestrial biosphere– Marine biosphere

• Long-term inorganic cycle– Weathering-volcanism thermostat

Global Biogeochemical Cycles

EARTHSURFACE

Emission

Reduced gasOxidized gas/particle

Oxidation

Uptake

Reduction

Deposition

Atmospheric CO2 Seasonal Cycle

1980 1981 1982 1983 1984 1985330

332

334

336

338

340

342

344

346

348

350

CO

2 M

ixin

g R

atio

(p

pm)

at M

au

na

Lo

a

Time

Monthly Mean CO2 Mixing Ratio

Atmosphere responds to biosphere on short timescales

Mass Units of Geologic Scale

• 1 Gigaton = 1x109 tons = 1x1012 kg = 1 x 1015 grams = 1 Petagram

• 1 Gigaton = 1 Petagram

Terrestrial Biosphere C Cycle

Atmosphere780 Gtons

Living Biomass750 Gtons

Litter110 Gtons

Soil 2000 Gtons

52

38

NPP: 60 Decay/resp: 49

Decay/resp: 11

Steady State?Longest lifetime?Shortest lifetime?

Flows in units ofGtons C per year

The effect of deforestation on atmospheric CO2 is

to in

crea

se it

(sou

rce)

to d

ecre

ase

it (s

ink)

neg

ligib

le

94%

2%5%

1. to increase it (source)

2. to decrease it (sink)3. negligible

Under conditions of higher CO2 (and sufficient nutrient supply), plants

have been shown to increase photosynthesis rates.

Posi

tive

feed

back

Neg

ativ

e fe

edbac

k

Not a

feed

back

27%

15%

58%

1. Positive feedback2. Negative feedback3. Not a feedback

Marine Organic C Cycle

death/fecal matterOrgC sinks

OrgC andnutrients

Surface Ocean

Deeper Ocean

CO2(aq) + nutrients

Photosynthesis

CO2(aq)

O2

a tiny bit to sediments

CO2

decay

Biological OrgC Pump: Key Points

CO2 and nutrientspumped down by biota

Thermo-Haline Circulation

1. Surface depleted (relatively) in C and nutrients

2. Deep ocean enriched in C and nutrients

3. Atmospheric CO2 responds to net pumping

Surface Ocean

Deeper Ocean

~1000 yrs

OrgC + O2 CO2(aq)

Marine ProductivityGlobal map of ocean color from SEAWIFS satellite

chlorophyll phytoplankton (where the nutrients are)

remember upwelling and convection?

Reservoirs of (Inorganic) Carbon

Atmosphere790 Gtons

Ocean3.7x104 Gtons

Lithosphere4x107 Gtons

Mainly as CO2

Mainly as HCO3-

Mainly as CaCO3

Carbon in the oceans is mostly in an inorganic form

Shell Formation

Surface Ocean

Deep Ocean

CO32- + Ca2+

Shell formation(CaCO3(s))

death/shells sink

CaCO3(s)DIC

slow

SlowTHC

Small amnt to sediments

Carbonate shell formation ___________ the ocean’s capacity to take up

atmospheric CO2

incr

ease

s

dec

reas

es

18%

82%1. increases2. decreases

The “Ultimate” Inorg C Cycle

CaSiO3 + CO2 CaCO3 + SiO2

Silicate Weathering

CaSiO3 + CO2 CaCO3 + SiO2

CaSiO3 Rock

CO2 (atm)

CaSiO3 RockCaCO3 + SiO2

Oceans

Ca2+ CO3

2- SiO2

Rain/runoff

Chemical Weathering RateFaster with higher CO2, higher T, higher rainfall

Volcanic Degassing

Volcanism causes reverse of weathering

CaSiO3 + CO2 CaCO3 + SiO2

Tectonic activity converts CaCO3 rocks back to silicate rocks in the mantle (magma).

CO2 released finds way to atmosphere via vents

HEAT

Silicate Weathering “Thermostat”CO2

CaSiO3 CaCO3 + SiO2

Precip/runoff

Chemical weathering

CaCO3 + SiO2

burial

CaCO3 + SiO2

CaCO3 + SiO2 CaSiO3 + CO2

This cycle operates on 0.5 – 1 million year timescale.

Is the silicate weathering – volcanism InorgC cycle a positive or negative

feedback?

Posi

tive

Neg

ativ

e

42%

58%1. Positive2. Negative

Silicate Weathering Feedback

Initial ForcingCO2

+ Silicate Weathering Rate

+

-

Temperature/ Precipitation

+

Negative Feedback—Stabilizing Climate

Does the silicate weathering feedback loop explain the glacial-interglacial

cycling of atmospheric CO2?

Yes N

o

45%

55%

1. Yes2. No