Lecture 22 - Causes of Long-Term Climate Change (p.2)

Q: What is the only type of chemical weathering that removes CO₂ from the atmosphere?

Silicate weathering. Carbonate weathering does not result in net removal of CO₂.

Q: What are the five factors that control the rate of silicate weathering on continents?

Temperature, precipitation (runoff), vegetation cover, physical weathering (increased surface area), and type of silicate rock (mafic weathers faster than felsic).

Q: Why is there more silicate weathering near the equator?

Higher temperatures, greater precipitation, and more vegetation in tropical regions all increase silicate weathering rates.

Q: How does vegetation enhance silicate weathering?

Vegetation delivers organic carbon to soils, which produces CO₂ (via respiration) and carbonic acid, enhancing chemical weathering by a factor of 2 to 10.

Q: How does physical weathering enhance silicate weathering?

It breaks rocks into smaller pieces, increasing the surface area available for chemical reactions.

Q: Why do mafic silicate rocks weather faster than felsic silicate rocks?

Mafic silicates are more chemically reactive and break down more readily, so they remove CO₂ from the atmosphere faster than felsic silicates.

Q: What happens to silicate weathering when Earth is warmer (Scenario 1)?

Higher temperatures, wetter climate, and more organic productivity lead to more chemical weathering and more CO₂ removal from the atmosphere.

Q: What happens to silicate weathering when Earth is colder (Scenario 2)?

Lower temperatures, drier climate, and less organic productivity lead to less chemical weathering and less CO₂ removal from the atmosphere.

Q: What type of climate feedback does silicate weathering represent?

Negative feedback — it moderates climate by increasing CO₂ removal during warming and decreasing CO₂ removal during cooling, acting as a thermostat.

Q: How do silicate weathering and volcanism work together as a thermostat?

Silicate weathering removes CO₂ from the atmosphere while volcanism returns it. Together they regulate atmospheric CO₂ and moderate long-term climate.

Q: What may cause an imbalance in carbon fluxes between weathering and volcanism?

Changes in plate tectonic activity — more active mid-oceanic ridges or more subduction zones increase volcanism and CO₂ output.

Q: What is the Phanerozoic Eon?

The eon spanning from 541 million years ago to today, representing about 12% of the geologic record. It is subdivided into three eras: Paleozoic, Mesozoic, and Cenozoic.

Q: What three approaches are used to map ancient climates?

Paleoclimatology (mapping rock types like tillites, coals, and evaporites), paleobiogeography (distribution of plants and animals), and paleogeography (using linear magnetic anomalies and paleomagnetism).

Q: What do calcrete, laterite, bauxite, and mangroves indicate as paleoclimate proxies?

Calcrete indicates arid conditions (calcium carbonate layers), laterite indicates warm and wet conditions (residual clay minerals), bauxite indicates intense tropical weathering (aluminum/iron oxides), and mangroves indicate warm coastal environments.

Q: How is sea floor spreading monitored?

Through paleomagnetism (magnetic reversals recorded in ocean floor basalts) and radioactive dating of those basalts.

Q: How often does Earth's magnetic field reverse?

Approximately every 250,000 years.

Q: Why can sea floor magnetic anomalies only be used for the last ~200 million years?

Because the oldest ocean floor is about 200 million years old — anything older has been subducted beneath continental plates.

Q: What is magnetic inclination and how is it used for rocks older than 200 Ma?

Magnetic inclination is the angle (dip) that Earth's magnetic field makes with the horizontal surface — near 0° at the equator and close to 90° at the poles. When volcanic rocks cool, magnetic minerals lock in this inclination, recording the latitude of the continent at that time.

Q: How confident are reconstructions of continental positions through time?

Reasonable certainty for the last 300 million years, less certainty between 300–540 Ma, and large uncertainties in the Neoproterozoic.

Q: What were climate conditions during the Lower Cambrian?

Continents were all in the Southern Hemisphere. Evaporites indicate warm and very dry (arid) conditions, with some warm-wet indicators closer to the equator.

Q: What characterizes the Middle–Upper Ordovician climate?

Evidence for glaciation appears (tillites), with ice sheets on Antarctica and arid conditions at ~30°N and ~30°S.

Q: What were climate conditions during the Middle Devonian?

No evidence of cold or dry conditions. Abundant arid indicators (calcrete) with evidence for warm and wet conditions.

Q: What characterizes the Lower Carboniferous climate?

Coals are widespread, indicating warm temperate conditions in the tropics with abundant organic deposits. Continents were slowly forming Pangea.

Q: What were climate conditions during the Lower Permian?

The supercontinent Pangea existed. Major glaciations occurred in the Southern Hemisphere (icehouse period), reaching as far as 40°S. Arid conditions persisted in the tropics, and organic deposits later became coal.

Q: What characterizes the Middle Triassic climate?

A warm period — evidence of glacial deposits had disappeared. Arid belts persisted alongside tropical and temperate conditions.

Q: What were climate conditions during the Upper Cretaceous?

Continents were approaching modern positions. Temperate regions at 40–60°S were much warmer than today, with arid belts at ~40°N and ~40°S marked by evaporites and calcites.

Q: How much did temperatures vary during icehouse conditions in the Phanerozoic?

Approximately 10–12°C.