L. 7: Abrupt Climate Shifts & Fall of Civilizations, C. 6-Buckley

Question 1

Isotopes as Proxy for Climate

Answer 1

O18 is heavier than regular O16, has 2 more protons. O18 is less likely to be picked up by the sun in evaporation, and plants do not transpire O18 through stomata because more work. As water rises toward mountains, more O 18 falls out. Also, O18 means cooler temperature because less evaporated. That is, O16 travels farther inland and farther poleward than O18 as water leaves equator and travels toward poles.

Question 2

Fractionation

Answer 2

Breaking down water into different ratios, O18 and O16, can see where water came from.

Question 3

Sediment cores

Answer 3

Can tell 5 million years of temperature. Can use chemical ratios to understand past climates.

Question 4

Pollen (proxy)

Answer 4

When see more oak pollen than maple, must have been drier. Charcoal is a macrofossil like a shell.

Question 5

Testate amoebae (proxy)

Answer 5

Use to understand lake levels because there is a relationship between water table depth and concentration of amoebae in sediment using high resolution carbon dating. On land can go back 5-10K years, in oceans millions of years. But can be off 50-300 years as dating error. Doesn’t happen with tree rings.

Question 6

Leaf waxes, lipids, fat (proxy)

Answer 6

Ratio of leaf wax isotopes, heavy hydrogen, represent dry and wet conditions back through time. They don’t decay in sediment.

Question 7

Ice cores (proxy)

Answer 7

Greenland and Iceland have layers. Annual layers in some icebergs. Can go back in time and reconstruct past climate using ratio of trapped gas from when layer was formed.

Question 8

speleothems (proxy)

Answer 8

Limestone in caves collected in stalactites and stalagmites, measure O18, can go back 100K years. But can have multiple points of fractionation, and water resources from different regions can mix. Layers do not correlate with tree ring records, timing different.

Question 9

varves (proxy)

Answer 9

Lake sediments, layers form seasonally, first sand and sediment (inorganic) eroded from land in spring make light layer, then organic material like leaves that fall and decay form darker layer. Sometimes, though, layers smash and lose time resolution (also ice cores).

Question 10

sclerochronology (proxy)

Answer 10

Biological geo-records, like tree rings, mussel increments, fish earbones (otolith, 50-75 years). Each layer correlated to environmental condition. Mussels have more fractionation when really dry. Gray snapper otolith ring strongly correlated with SST (grow more when warmer).

Question 11

corals (paleoproxies)

Answer 11

Have stress bands, annual layers, often related to salinity, going back 4-500 years. Fast growth, no stress bands. Can cross date with isotopes.

Question 12

carbonate sediment (proxy)

Answer 12

Pacific northwest – dark layers formed during dry periods.

Question 13

Mongolian tree ring records

Answer 13

Show that Genghis Khan (1211-1225) did not venture out because of drought, but tree rings show an unusual wet period (pluvial), good for diet of meat and cheese which require a lot of water. Resources matter; cranium development linked to resource availability (shift wet to arid required adaptation to survive).

Question 14

Angkor, capital of Khmer Empire in Cambodia

Answer 14

9th-14th centuries. Two severe droughts (weaker summer monsoon rainfall from El Nino warm phase), followed by extreme wet flooding (La Nina cool phases).