Transcript Ch 5
Greenhouse Earth: 100 Ma GEOL 1130 Paleoclimate Research • Two components – Observations • i.e. fossils, sediments, chemical proxies – Modeling • using observations to test climatic conditions, • see if observations are consistent with each other, • predict other climatic conditions that might be recorded but not yet found. • Observation – modeling comparisons extremely important to moving research forward 100 Ma observations 1. Broadleaf (breadfruit-type tree) fossils at polar locations a. Today found in tropical evergreen locations 2. Cold blooded animals to poles 3. Corals found between 40 N and S a. Today found between 30 N and S 4. Sea level higher by at least 200 m 1 & 2 suggest that the atmosphere and ocean were much warmer 3 suggests that the oceans were much warmer Activity: Was heat distribution on Earth more or less efficient in the Cretaceous than it is today? Modeling Cretaceous Climate • Activity: What are boundary conditions in this model that would be different from today? – Continental position – Sea level – Atmospheric CO2 concentration Modeling Results • Ran model with changes in geography only and with changes in both geography and CO2 – Which one does a better job? • Does either one get the right amount of latitudinal heat transport? • What is a potential solution? Cretaceous ocean circulation? • Today’s deep oceans filled with cold (~1-4 C), somewhat saline water • Cretaceous deep ocean filled with warm (>10-15 C?) water? • Warm water transported from the tropics to the poles would have dramatically increased the latitudinal heat transport Sea Level Change • Two types of sea level change in geology – Relative – local tectonic or isostatic effects on coast lines – Eustatic – global sea level changes • In terms of climate change, eustatic sea level change is what we are interested in • Terminology: – Transgression – rise in sea level – Regression – fall in sea level Sea Level Change • Activity: If a continental shelf has a slope of 1:1000, how much far inland will the ocean travel if sea level rises by 1 meter? – 1000 m or 1 km! • Is sea level today low or high? That is, is sea level at the continental break in slope or up on the shelf? • How much would sea level rise if all the ice caps were to melt? • How much land would we lose if sea level rose 70 m? Note today’s low sea level!! Has sea level remained constant over time? Past Sea Level Five potential causes for sea level change 1. Tectonic – mid ocean ridges – Activity: How does spreading rate affect the volume of the ocean basins? – Thin vs fat mid ocean ridges: • • • • Ocean floor depth (m) = original depth + 350*age1/2 Today’s ridge depth = 2500 m Activity: What is the square root of 5, 10, 20, 40, 60, 80, 100? How much change in depth occurs over each period? Crust that is more than 60 Ma old has reached a relatively stable depth of 5500 m Plate spreading driver of sea level • Faster spreading makes ocean basins more shallow, causing high sea level (ocean pushed up on to continents) • Slower spreading…(visa versa) Drivers of sea level change 2. Tectonics – collisions – When continents collide, some of the continental land mass is thickened (mountains and plateaus), shrinking continents and by default expanding oceans • Modern example? – Tibetan Plateau started 55 Ma 3. Volcanic Plateaus – Take up room on ocean floor, displacing water Drivers of sea level change 4. Climate: – Ice on land takes water out of the ocean – What is total range between greenhouse world 100 Ma and 20,000 years ago? • ~300 m 5. Thermal expansion/contraction of sea water – 1 part in 7000 for each degree C