ESH Ch. 10: Geochemical Cycles

Negative feedback loop (Climate)

Chemical weathering accelerates as temperatures rise, extracting greenhouse gases and cooling the planet.

Geochemical cycle levels

These cycles process matter as elements, compounds, and partition isotopes at all levels.

Methanogenesis

The creation of methane via anoxic subsurfaces, wetlands, or animals like cows.

Fast carbon cycle

Movement through biological processes like photosynthesis, respiration, and methanogenesis.

Slow carbon cycle

Carbon movement primarily through geological processes.

Decomposition

Bacteria and fungi break down matter via respiration, returning CO₂ to the environment.

Burial (Carbon)

Organic matter trapped in low-oxygen environments, locking carbon into sedimentary rock or coal.

Effect of excess burial

Removes CO₂ from the photosynthesis-respiration cycle and increases O₂ levels.

Biological pump

Movement of C between atmosphere and deep ocean via 'fecal rain' and then lithification.

Anoxia

Low-oxygen to entirely oxygen-depleted conditions in water. Indicated by dead phytoplankton because no photosynthesis, forming carbon-rich black ocean shales.

Positive Oxygen Isotope Excursion

18O > 16 O. Indicates rapid cooling and glacial expansions. 16O evaporates, trapped in expanding glaciers. 18O rich ocean water.

The δ18O (Stable oxygen isotope ratio) records:

Ancient water temperatures and the volume of global glaciers.

a

δ13C (Stable carbon isotope ratio)

Records changes in global organic carbon burial rates relative to weathering rates.

a

Acid rain formation

Atmospheric CO₂ dissolves into water vapor (H₂O) to create weak carbonic acid (H₂CO₃).

Chemical weathering (Silicates)

Acid rain dissolves rocks, releasing calcium and bicarbonate ions into the ocean.

Carbonate rock formation

Marine organisms use calcium and carbonate ions to build shells; these sink and compress into limestone.

Factors increasing weathering

Higher temperatures, increased rainfall, and mountain building/glaciation (increased surface area).

Positive feedback loop (Climate)

As global glaciation increases, albedo increases, cooling down the temperature.

Negative carbon isotope excursion

Signifies a rapid injection of light carbon (12C), often from sudden warming, volcanism, or methane melting.

Negative oxygen isotope excursion

Indicates rapid global warming or sudden deglaciation dumping light water into seas.

Positive carbon isotope excursion

Primary producers prefer 12C > 13C. Carbon burial increases, creating limestone with high 13C.