Earth Systems and Climate

Prof. Gorring
GEOS 112 PHYSICAL GEOLOGY
Dec. 7-10, 1998 Earth Systems and Global Climate

1. Earth Systems
- Interior, Lithosphere, Atmosphere, Hydrosphere and Biosphere.
- Geochemical model includes all of these systems (reservoirs) and associated fluxes (in/out).

Plate tectonics controls flux between interior (mantle) and lithosphere.

Weathering, transport, and sedimentation link hydrosphere and lithosphere.

Evap., Precip, and biol. gas exhange link biosphere, hydrosphere, and atmosphere

2. Formation of Earth Systems: (Ga = giga-anum or one billion years)

Interior and Lithosphere (including crust):

Thermal cooling at ~4.4 Ga led to chemically differentiated planet. Plate tectonics takes over ~4.0 Ga when lithosphere cools sufficiently. Continental crustal evolution. igneous processes continually extract light materials (mostly at subduction zones) to gradually form continental crust.

Atmosphere and Hydrosphere:

About 85-90% of atmosphere produced early on (4.4 to 4 Ga), the rest gradually by volcanic degassing of the mantle (N₂ > CO₂> CO » CH₄, H₂O » SO₂ > O₂). Water condensed out to form hydrosphere.

Biosphere:

Probably originated ~4 Ga; although only have good fossil evidence for bacteria at 3.5 Ga. Assembly of large organic molecules (amino acids) from reaction of methane (CH₄), ammonia (NH₃), and strong UV radiation. Clay minerals and/or organic molecules from meteorites may have helped as templates to form intial "RNA-world", quickly evolved into DNA.

Photosynthesis: ( first organisms ~3.5 Ga) - Organisms with chlorophyll use sunlight to convert H₂O and CO₂ into organic matter ("CH₂O") and O₂. Respiration is the opposite reaction. Cycles CO₂ and O₂ through Earth systems. The burial (and removal) of organic matter increases atmospheric O₂. Significant increase O₂ at about 2.5 Ga, but didn't reach near PAL until the end of the Proterozoic (~1 to 0.5 Ga). Ozone (O₃) production led to filtering of some UV radiation, allows land life to develop.

3. Climate and Earth Systems

Earth systems are generally in steady state (equilibrium). Pertubations of any one system can cause climate change. Climate is controlled by the response of the hydrolgic and atmospheric systems to changes within or to the other systems.

Examples of Some Pertubations:

increased volcanism, meterorite impact
tectonics, mountain building
fossil fuel buring, deforestatio

Greenhouse Effect:

Incidient short-wave radiation (ie. visible, UV) heats Earth's surface. Long-wave radiation (IR) is emitted and absorbed by H₂O vapor, CO₂, CH₄ and other gases and is reflected back down. Increase any of these gases causes global T rise.

4. The Carbon Cycle
        Atmospheric CO₂ is linked to climate. Main evidence:
            (1) CO₂ is a greenhouse gas
            (2) ice core records show correlation w/other climate indicators (e.g. O isotopes, etc.).
            (3) CO₂ correlates well with paleoclimates inferred from geologic record

Long-Term Carbon Cycle

Largest reservoirs for C are mantle, carbonate seds, and sed. organic carbon.

Important long-term fluxes of CO₂ into the atmosphere:

- volcanism

- carbonate metamorphism CaCO₃ + SiO₂ >>>> CaSiO₃ + CO₂

- carbonate sedimentation 2HCO₃^- + Ca²⁺ >>>> CaCO₃ + CO₂ + H₂O

- weathering of organic-rich seds. ("respiration"; CH₂O + O₂ >>>> CO₂ + H₂O)

        Important long-term fluxes of CO2 out of the atmosphere:
                - carbonate rock weathering     CaCO₃ + CO₂ + H₂O >>>> 2HCO₃- + Ca²⁺
                - silicate rock weathering     CaSiO₃ + 2CO₂ + H₂O >>>> 2HCO₃^- + Ca²⁺ + SiO₂
                - silcate weathering + carb sedimentation     CaSiO₃ + CO₂ >>>> CaCO₃ + SiO₂
                - burial of organic matter         ("photosynthesis"; CO₂ + H₂O >>>> CH₂O + O₂)

*Fluxes are very small relative to reservoir masses; controls long-term changes in
atmospheric CO₂ (>1 Ma).

Short-Term Carbon Cycle

Important reservoirs are deep ocean, soil C, surface ocean, atmosphere, terrestrial biota in order of abundance. Deep ocean is much larger than others.

*Reservoirs are smaller than in long-term cycle, BUT fluxes are much larger in this system; controls short-term (<1 Ma) variations in atmospheric CO₂.

Anthropogenic flux of CO₂ (fossil fuel burning, deforestation) is currently ~7-8 Gt/yr; (Gt = gigatons 10¹⁵ grams). ~3 Gt/yr goes into the atmosphere to account for 0.7% annual rise in atmospheric CO₂. 100% shutoff would take ~200 yr. to reduce CO₂ to pre-Industial levels.

Major Question: Where is the "missing" CO₂ going?

"We think" (remains debatable in scientific community) that ~2 Gt/yr has been adsorbed by the deep ocean and the rest (~2-3 Gt/yr) has been taken up by the terrestrial biota. This appears to negate the 2 Gt/yr from tropical deforestation, primarily from reforestation of the northern hemisphere.