The Biogeochemical Carbon Cycle: CO 2,the greenhouse effect, & climate feedbacks. Assigned Reading: Kump et al. (1999) The Earth System, Chap. 7.

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1 The Biogeochemical Carbon Cycle: CO 2,the greenhouse effect, & climate feedbacks Assigned Reading: Kump et al. (1999) The Earth System, Chap. 7.

2 Overhead Transparencies

3 Faint Faint Young Sun Paradox Young Sun Paradox 4 1 H--> 4 He Incr. density= incr. luminosity Liquid H 2 O existed >3.5 Ga (sed. rocks, life, zircon d 18 O)

4 Simple Planetary Energy Balance Simple Planetary Energy Balance Likely solution to FYSP requires understanding of Earth s energy balance (& C cycle)

5 Energy Absorbed

6 Neither Albedo or Geothermal Heat Flux Changes Can Keep the Earth from freezing w/ 30% lower S

7 Lower S compensated by larger greenhouse effect

8 The Electromagnetic Spectrum

9 Incoming UV, Outgoing IR Greenhouse Gases absorb IR radiation efficiently

10 Molecules acquire energy when they absorb photons

11 Carbon Cycle: Strong driver of climate on geologic timescales

12 geochemical The Biocarbon Cycle

13 Chemical Weathering = chemical attack of rocks by dilute acid CO 2 + H 2 O <---> H 2 CO Carbonate Weathering: 2+ - CaCO 3 + H 2 CO 3 --> Ca + 2 HCO 3 Geochemical Carbon Cycle #2 2. Silicate Weathering: CaSiO H 2 CO 3 --> Ca HCO SiO 2 + H 2 O x CO 2 consumption for silicates Carbonates weather faster than silicates

14 Carbonate rocks weather faster than silicate rocks!

15 Net Reaction of Rock Weathering + Carbonate and Silica Precipitation in Ocean CaSiO 3 + CO 2 --> CaCO 3 + SiO 2 CO 2 consumed (~ 0.03 Gt C/yr) Would deplete atmospheric CO2 in 20 kyr Plate tectonics returns CO2 via Volcanism and Metamorphism Carbonate Metamorphism Net reaction of geochemical carbon cycle (Urey Reaction) CaCO 3 + SiO 2 --> CaSiO 3 + CO 2 CO 2 produced from subducted marine sediments

16 Geologic record indicates climate has rarely reached or maintained extreme Greenhouse or Icehouse conditions... Negative feedbacks between climate and Geochemical Carbon Cycle must exist Thus far, only identified for Carbonate- Silicate Geochemical Cycle: Temp., rainfall enhance weathering rates (Walker et al, 1981) (I.e., no obvious climate dependence of tectonics or organic carbon geochemical cycle.) How are CO 2 levels kept in balance? Feedbacks Adapted from Kump et al. (1999)

17 A Closer Look at the Biogeochemical Carbon & the Organic Carbon Sub-Cycle

18 BIOGEOCHEMICAL CARBON CYCLE fixation dissolution sink OCEANS ATMOSPHERE CO 2 rock weathering sink CONTINENTAL EROSION exhalation CO2 DIC = HCO CO 3 2- Uplift IGNEOUS ROCKS BIOSPHERE SEDIMENTS lithification CaCO3 METAMORPHIC ROCKS organic matter SEDIMENTARY ROCKS Corg TECTONICS

19 Earth's Carbon Budget Biosphere, Oceans and Atmosphere 3.7x10 18 moles Crust Corg 1100 x mole Carbonate 5200 x mole Mantle 100,000 x 1018 mole

20 Steady State & Residence Time Steady State: Inflows = Outflows Any imbalance in I or O leads to changes in reservoir size Inflow: Outflow: 60 Gton C/yr Atmospheric CO 2 60 Gton C/yr Respiration 760 Gton C 1 Gton = 10 9 * 1000 kg = g Photosynthesis The Residence time of a molecule is the average amount of time it is expected to remain in a given reservoir. Example: t R of atmospheric CO 2 = 760/60 = 13 yr

21 Carbon Reservoirs, Fluxes and Residence Times Species Amount (in units of gc) Residence Time (yr)* d 13 C %o PDB** Sedimentary carbonate-c ~ 0 Sedimentary organic-c ~ -24 Oceanic inorganic-c ~ Necrotic-C ~ -27 Atmospheric-CO ~ -7.5 Living terrestrial biomass ~ -27 Living marine biomass ~ -22

22 Carbonate-Silicate Geochemical Cycle CO 2 released from volcanism dissolves in H 2 O, forming carbonic acid H 2 CO 3 CA dissolves rocks Weathering products transported to ocean by rivers CaCO 3 precipitation in shallow & deep water Cycle closed when CaCO 3 metamorphosed in subduction zone or during orogeny.

23 Simple Carbon Cycle Modeling Total C entering atm. & oceans = Total C buried in sediments closed system 13C into atm. & oceans = 13 C being buried in sediments conservation of isotopes d in =f org * d org + f f carb + f org = 1 e TOC = d carb d org carb * d carb ~ 5 the avg. value for crustal C isotopic difference between inorganic and organic carbon (Dc) f org = (d carb + 5) / e TOC Hayes et al., Chem Geol. 161, 37, 1999; Des Marais et al., Nature 359, 605, 1992

24 Carbon Isotopic Excursions Ma More complete sediment record + Improved chronology = More detailed picture showing abrupt and extreme C-isotopic shifts A global composite of d 13 C data shows 4 excursions Plus one at the pc-c boundary

25 Carbon Isotopic Excursions Ma More complete sediment record + Improved chronology = More detailed picture showing abrupt and extreme C-isotopic shifts

26 Modeling the Proterozoic Carbon Cycle ß d carb and d org through time Ma in 100Ma increments ß note the constancy of d carb while d org decreased. Why? biochemistry or pco 2 ß forg increased through this time, was episodic and was linked to periods of rifting and orogeny ß also associated with extreme glaciations ß Increase in the crustal inventory of C requires increases in the inventories of crustal Fe 3+, crustal and marine sulfate, nitrate and atmospheric oxygen. ß SO - 4, NO 3 and O 2 increases changed patterns of respiration ß NO - 3 would have forced productivity changes

/ Past and Present Climate

/ Past and Present Climate MIT OpenCourseWare http://ocw.mit.edu 12.842 / 12.301 Past and Present Climate Fall 2008 For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms. The Faint Young