CHAPTER 6
Greenhouse Era 100 Myr Ago
- See Fig. 6-1 shows the world 100 Myr ago during the Cretaceous period.
- Sea-levels were about 200 meters higher which caused some inland flooding. This flooding effectively reduced the amount of surface land.
- There is NO evidence that ANY permanent ice existed on Earth at this time, even at the poles.
- There is fossil evidence of tropical plants and animals living north of the Arctic Circle and south of the Antarctic Circle!
- Fig. 6-3 shows the temperature vs. latitude plot for both Cretaceous and present day time periods. This plot shows that the polar regions were 20oC to 40oC warmer than today.
- Antarctica today is very cold because the ice sheet rises up about 3-4 km in elevation. Furthermore, the ice has a high albedo which reflects incoming solar radiation.
Causes of Sea Level Change
- Over tectonic times, sea level has risen and fallen by several hundreds of meters. These rises and falls are called marine transgressions and regressions, respectively.
- These changes are small in comparison to the depth of the ocean (average depth is greater than 4000 m) but they are important in determining factors of climate change.
- Because continental margins (edges) are relatively flat, a rise in sea level of 1 m can cause sea water to move as much as 1000 m inland!
- Fig. 6-7 shows how sea level changes influence sediment deposition. When sea levels are high, sediment falls onto the continental shelf where it can be more easily studied.
- See Fig. 6-8. Sea level is relatively low present day.
- From 80 - 100 My ago, sea level was about 200 m higher. Fig. 6-10 shows sediment deposition. Much of North America and Europe were flooded. See Fig. 6-1 again.
- Since that time, sea level has dropped due to 1) plate tectonics and 2) changes in the volume of water due to climate changes.
- Fig. 6-11 shows how sea floor spreading rates influence the volume of sea water and therefore the sea level height. Faster spread rates cause higher sea levels.
- See Fig 6-13. Continental collisions influence sea levels. When continents collide, they cause a wider ocean basin which will cause a drop in sea levels. For example, the last major collision (still underway) is India colliding with southern Asia which began 55 My ago. This collision has increased the area of the ocean by 2,000,000 km2 which had dropped sea levels about 40 m!
- Fig. 6-14 shows how volcanic plateaus can increase sea levels. Present day there are much fewer plateaus than 80-100 My ago. Plateaus back then are thought to have caused a 10 m - 40 m sea level rise above today's level.
- Fig. 6-15 shows how water stored in ice sheets influences sea level. No ice sheets existed 80 - 100 Mya so sea levels had to be higher. Today, the Antarctic ice sheet holds enough water to cause a 66 m drop in sea level and the Greenland ice sheet hold about 6 m of sea level water.
- Thermal expansion/contraction of water also influences sea level. Warmer water expands so oceans 80 to 100 Mya would have "taken up more space" than the cooler water of today. It is estimated that cooler water today has caused a 7 m drop in sea level.
- Table 6-1 shows all of the sea level change factors. When all factors are added up, it would appear that sea levels today are 300 to 440 m lower than 80 - 100 Mya. However, sediment evidence shows sea level today should only be 100 - 300 m lower. Uncertainties in past spreading rates, volcanic plateaus, and sediment rates, along with possible other factors might account for this mismatch.
Sea Level Change and Climate Changes
- Higher sea levels flood continents. Water heats up and cools down more slowly than land, so a flooded continent would cause milder seasons (not as hot in summer nor as cold in winter.)
- As sea levels decreased, seasons would cause more extreme climate variations.
- Chapter 2 showed that summer melting was the major control on the amount of glaciation present on the Earth. Cooler summers yielded less melting which caused greater glaciation and climate cooling (due to higher albedos.) Therefore, when sea levels were higher and caused cooler summers, there should have been more glaciation. Today, when sea levels are lower, there should be less glaciation. Observation shows that both cases are completely opposite! Therefore, one can assume that sea level change does NOT control long-term climate change.
Asteroid Impacts and Climate Change
- Large impacts occur only every 50 to 100 My. Fig. 6-16 (iridium levels) and 6-17 (shock quartz) show evidence for a major impact in the Yucatan Peninsula about 65 My ago.
- This impact is thought to have caused 70% of animal species and 40% or plant species to become extinct.
- Fig. 6-18 shows how this impact could have caused climate change on the order of centuries and not millennia. Initially, there would be great shock waves causing water and rock to be vaporized and injected into the atmosphere and outer space. Over the next few years, this would have led to global cooling due to decreased solar radiation caused by the soot and dust. Over longer periods, the carbon injected into the atmosphere from large-scale fires would have led to increased levels of CO2 which would lead to global warming. Therefore, it is believed that large impacts are NOT long-term climate changers.
Helpful Links:
The Mid-Cretaceous Period
Sea Level and Climate
The Causes of Sea Level Change
