Lecture 29 – The Late Paleozoic, Ice Age

Q: Why do Cretaceous climate models fail to match geological evidence, even with 4× present CO₂?

Models still produce polar regions that are too cold and tropics that are slightly too warm. Increasing CO₂ alone does not resolve the mismatch — the real problem is insufficient modeled heat transport to the poles.

Q: What are potential problems with the geological data used to reconstruct Cretaceous climate?

Cretaceous organisms may have responded differently to temperature than modern ones; most fossils come from moderate coastal regions (biasing toward warmer readings); and post-depositional alteration of geochemical/isotopic signals may produce erroneous reconstructions.

Q: What are potential problems with the climate models used for the Cretaceous?

Cloud effects are difficult to model; ocean circulation is not well simulated; and ocean circulation may have been fundamentally different in the Cretaceous.

Q: How is heat transported to polar regions today?

The atmosphere transports approximately two-thirds and the oceans transport approximately one-third of the heat reaching polar regions.

Q: What is the ocean heat transport hypothesis for the Cretaceous?

In the Cretaceous, strong evaporation in the subtropics (~30°N) produced very saline surface water dense enough to sink at low latitudes, creating a “warm deep-water conveyor” that delivered significantly more heat to polar regions than today’s thermohaline circulation.

Q: Besides the warm deep-water conveyor, what other factors helped warm Cretaceous polar regions?

Widespread shallow inland seas (from high sea level) allowed warm equatorial water to travel poleward, and reduced albedo due to the absence of ice further warmed the poles.

Q: What is the present-day Mediterranean an analog for regarding Cretaceous oceans?

It illustrates how strong subtropical evaporation can produce high-salinity surface water that sinks — a process that may have driven a fundamentally different style of ocean circulation in the Cretaceous.

Q: When did the Paleozoic Era span?

From 541 million years ago to 252 million years ago.

Q: When did the Late Paleozoic Ice Age (LPIA) occur, and how long did it last?

From approximately 325 to 265 Ma — over 50 million years, making it the longest icehouse period in the Phanerozoic.

Q: Where was the LPIA primarily centered?

Around the south pole region, comparable in size to Pleistocene glaciations, and composed of numerous independent ice sheets that advanced and retreated.

Q: What was Gondwana’s role in the LPIA?

Gondwana (mainly South America, southern Africa, Antarctica, and Australia) moved into a south polar position; the first ice sheets appeared around 320 Ma when Gondwana was at the pole.

Q: What supercontinent had formed by ~280 Ma, and how?

Pangea, formed by combining Gondwana, Euramerica, and Siberia; it was almost fully assembled by that time.

Q: What did continent-continent collisions between Gondwana and Euramerica produce?

Mountain chains at least the size of the Himalayas in equatorial positions. Today’s remnants include the Appalachians, Atlas Mountains, Bohemian Massif, and Massif Central (France).

Q: How did equatorial mountain chains contribute to LPIA cooling?

They enhanced silicate weathering, which removes CO₂ from the atmosphere, driving cooling.

Q: What indicators suggest low sea floor spreading rates during the LPIA?

Aragonitic oceans during the Carboniferous–Permian and MgSO₄ evaporite deposits in the Carboniferous and Permian — both indicate low mid-ocean ridge activity and low sea floor spreading.

Q: What was the possible trigger for the start of the LPIA?

Low sea floor spreading rates, which were slowing down at the beginning of the LPIA and remained low beyond its end.

Q: What were the four main drivers of long-term climate change during the LPIA?

Continents in polar position (yes); sea floor spreading rate (low); continent-continent collisions (yes); atmospheric CO₂ content (likely low). All pointed toward icehouse conditions.

Q: What proxy data are used to estimate atmospheric CO₂ during the late Paleozoic?

Paleosols, phytoplankton carbon isotopes, fossil leaf stomata density, and planktonic foraminifera boron isotopes. These broadly agree with geochemical models like GEOCARB.

Q: What were estimated CO₂ levels between 320 and 260 Ma?

Likely as low as today’s levels.

Q: What do geochemical models like GEOCARB combine?

The carbon cycle, the oxygen cycle, and the sulfur cycle.

Q: How did Carboniferous swampy forests develop, and what did they produce?

Monsoon rain in front of equatorial mountain ranges facilitated dense swampy forests, resulting in widespread coal deposits as organics were buried in the sedimentary record.

Q: How did the expanding Carboniferous biosphere affect atmospheric composition?

CO₂ consumption by photosynthesis and burial of carbon in vast coal deposits removed CO₂ from the atmosphere, while high rates of photosynthesis and reduced O₂ consumption raised atmospheric O₂ to ~30% (compared to today’s 21%) — the highest in the Phanerozoic.

Q: What were the active drivers of cooling during the LPIA?

Enhanced photosynthesis on land (removing CO₂ and burying carbon as coal) and silicate weathering from equatorial mountain chains — both actively removed atmospheric CO₂.

Q: When did Gondwanan ice sheets begin to disappear?

Around 260 Ma.

Q: When did the End Permian mass extinction occur?

Approximately 250 Ma, after a very long icehouse period — it is one of the worst mass extinctions in the geologic record.

Q: What environmental change accompanied the End Permian mass extinction?

A massive (~10×) increase in atmospheric CO₂, occurring after a prolonged period of low CO₂ during the LPIA.