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

Overhead Transparencies

Faint Young Sun Paradox 4 1 H→ 4 He Incr.density = Incr.luninosity Liquid H 2 O existed >3.5 Ga (sed. Rocks, life, zircon  18 O)

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

Energy Absorbed

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

Lower S compensated by larger greenhouse effect

The Electromagnetic Spectrum

Incoming UV, Outgoing IR “Greenhouse Gases” absorb IR radiation efficiently

Molecules Acquire Energy when they Absorb Photons

Carbon Cycle: Strong driver of climate on Geologic timescales 1.CO 2 Feedbacks: Geochemical Carbon cycle ‧ Transfer of C between rocks and ocean/atmosphere (> yr) can perturb CO 2 greenhouse effect ‧ Ocean/atmosphere C reservoir small w.r.t. rock reservoir and the transfer rates between them 2.Evidence for Long-Term CO 2 -Climate Link 3.Case Studies: Permo-Carboniferous Glaciations Warm Mesozoic Period Late Cenozoic Cooling

The Bio- Geochemical carbon Cycle The Geochemical Carbon Cycle 1. Organic Carbon Burial and Weathering 2. Tectonics: Seafloof spreading Rate Mantle CO 2 from Mid-Ocean Ridges Chemical Weathering Consumes CO 2 Carbonate Metamorphism Produces CO 2 3. Carbonate-Silicate Geochemical Cycle

Geochemical Carbon Cycle #2 Chemical Weathering = chemical attack of rocks by dilute acid 1.Carbonate Weathering: 2. Silicate Weathering: consumption for silicates Carbonates weather faster than silicates

Carbonate Rocks Weather faster than Silicate rocks!

Net Reaction of Rock Weathering Carbonate and Silica Precipitation in Ocean consumed Would deplete atmospheric Plate tectonics returns and Metamorphism via Volcanism Carbonate Metamorphism produced from subducted marine sediments Net reaction of geochemical carbon cycle (Urey Reaction)

Geologic record indicates climate has rarely reached or maintained extreme Greenhouse or Icehouse conditions.... Negative feedbacks between climate andGeochemical 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)

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

BIOGEOCHEMICAL CARBON CYCLE ATMOSPHERE CO 2 dissolution sink OCEANS rock weathering sink Exhalation CO2 fixation SEDIMENTSBIOSPHERE lithification Organic matter SEDIMENTARY ROCKS Uplift Corg CONTINENTAL EROSION IGNEOUS ROCKS METAMORPHIC ROCKS

Earth's Carbon Budget Biosphere, Oceans and Atmosphere Crust Mantle

Steady State & Residence Time Steady State: Inflows = Outflows Any imbalance in I or O leads to changes in reservoir size Atmospheric Outflow: Inflow: RespirationPhotosynthesis The Residence time of a molecule is the average amount of time it is expected to remain in a given reservoir. Example of atmospheric

Carbon Reservoirs, Fluxes and Residence Times Sedimentary carbonate-C Sedimentary organic-C Oceanic inorganic-C Necrotic-C Atmospheric-CO 2 Living terrestrial biomass Living marine biomass SpeciesAmount Residence Time

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

Simple Carbon Cycle Modeling Total C entering atm. & oceans = Total C buried in sediments closed system conservation of isotopes 13 C into atm. & oceans = 13 C being buried in sediments ~5 the avg. value for crustal C isotopic fifference between inorganic and organic carbon Hayes et al., Chem Geol ; Des Mariais et al., Nature

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

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

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