GEOCHEMICAL CYCLES

 
Photosynthesis / Respiration Cycle (Plants)

Photosynthesis CO₂ + H₂O => CH₂O + O₂

No change

Respiration CH₂O + O₂ => CO₂ + H₂O

Tissue = stores CO₂ and water as sugars

Animal Respiration

Inhale O₂ to oxidize plant sugars. Exhale CO₂.

No change

Oxygen consumed = oxygen released during sugar production

Decomposers

Decompose organic debris by respiration

Burial of plant debris

Coal swamps =>sedimentation (burial) => C stored (removed from the system) =>

Decrease of atmospheric CO₂

Anoxia - acidic waters =>  C debris  =>   preserved

                =>    removed from decomposers

Aquatic Ecosystems

Algae (producers) buried in deltas or decomposed

Warmer climates prevent polar sinking  = deep sea anoxia

Burial in epicontinental areas = > oil

Carbon Isotopes: ¹² C and ¹³ C

Photosynthesis uses ¹² C ( ¹² C is high in plants)

• C isotope balance through     =>Photosynthesis - respiration

                                            => burial - weathering

Burial - weathering

• If burial > weathering, the atmosphere and oceans are enriched in ¹³ C
• High ¹³ C in limestones and organic sediments indicate abundant burial

 

Atmospheric O₂

Removal of organic matter by burial (less to decompose) increases atmospheric O₂.

High ¹³ C in Carboniferous limestones indicate that O₂ was twice as abundant as today

 

Atmospheric CO2

Affected by weathering, metamorphism and precipitation of CaCO₃

Ca CO₃in platforms (a stable environment) = stored longer

Weathering removes atmospheric CO₂

Accelerated by high temperature, precipitation and vegetation

Negative feedback produced by temperature and precipitation: What is negative feedback?
 

CO₂ builds up for some reason) => greenhouse increase in T=> more weathering =>CO₂ is removed

Precipitation & High T:
T increase =>more moisture =>more rain =>more weathering = CO₂ is removed

                    forests expand => more weathering = CO₂ is removed

Increased weathering with warmer and/or wetter climates prevent a runaway greenhouse effect

 

Variation of the Oxygen Isotope Ratio of Skeletons with Temperature

Temperature: Skeleton ratio different from the ratio in the environment.

At low temperature, skeletons incorporate more of the heavy isotope (Oxygen 18)

Salinity: Lighter isotope (Oxygen 16) evaporates more readily and leaves hypersaline, isotopically heavy seawater.
 

These two opposite behaviors obscure interpretations of past climate temperatures based on oxygen isotopes ratios.

 

 
 
 

Effect of Glacial ice: Moisture is isotopically light => ocean is enriched in Oxygen 18

Combination of a temperature drop and glacier expansion result in isotopically heavy shells.

 

Ocean Chemistry and Skeletal Mineralogy

Based on Mg/Ca ratio
• High Mg/Ca ratio precipitation of aragonite and Mg rich calcite
• Low Mg/Ca ratio precipitation of normal calcite
Ca is transferred from the ocean rocks to seawater; Mg from the water to the ocean rocks

If MOR activity expands, sea level rises, Mg/Ca ratio decreases precipitation of normal calcite

If MOR activity reduced, sea level drops, Mg/Ca ratio increases precipitation of aragonie and Mg rich calcite