Schol EVIDENCE Flashcards
What are stable isotopes?
Do not radioactively decay, hence standard naturally occurring ratio
How is relative abundance of isotopes found?
By specialised mass spectrometry
Describe O-16 and O-18 ratios
Focused on the ratio in water molecules.
Variations occur due to evaporation and condensation - H2O-18 is heavier. More difficult to evaporate and condenses first.
“Enriched” in H2O-18 or 16
What do oxygen isotope ratios tell us?
From ice cores: higher 18:16 warmer, lower 18:16 cooler.
Cooler climate = greater amount of condensation as moist air moves toward poles, more H2O-18 is lost. Ice formed at poles depleted in H2O-18.
Oxygen isotopes in trees and organisms
Dating tree rings used to find frequency of storms.
Storms produce ppt DEPLETED in H2O-18.
trees make glucose, will have cellulose depleted in O-18 in tree rings.
Organisms take H2O and incorporate into shells (eg. CaCO3 shells of plankton, deposit as limestone layers. Date layers for ocean temp)
Describe K-Ar dating
Decay of K-40 to stable Ar-40
Used to date volcanic strata (material contains K-40)
Ar present in rock before it solidified removed by intense heat, so any Ar present in solidified stratum must be from decay.
Half life over half a billion years.
Good for dating samples over 100,000 years.
Radiocarbon dating
C-14 to N-14
Short half life (5730 years), can measure up to 50,000 years.
Small amount of atm C-14. Taken up by plants - when alive, organism ratio C-14 to stable C is the SAME as atm.
Dies, organism does not take new C. Over time, C-14 decays, ratio changes.
Layer surrounding the fossil can be indirectly dated
What do dark and light bands of ice cores show
Dark = compacted winter snow
Light = summer snow (typically coarser grained, less tightly packed, hence more transmissive of light).
Can be used to compare AMOUNTS of snow over years.
When summer snow melts, refreezes without bubbles. Indicates warmer climate.
How can ice cores be used to find temperatures?
The hole left behind - insulated.
Use ice probe to compare temp.
(No ice before ice age - bottom layer heated by earth.)
Trapped materials in ice cores
Colder climate = less vegetation = stronger winds = increased dust in cores
Sea salt in ice - indicates wind strength and direction as it is blown from the sea
Gas bubbles or methane - produced by wetland methanogens and certain marine organisms —> terrestrial and oceanic ecosystems
Sediment cores layers
Alternating dark / light = deposition in winter and summer, depending on which area has more rainfall
Wider layers = increased deposition, maybe due to increased ppt.
Species trapped in sediment
Plankton species: Foraminifera (calcium carbonate) and diatoms (silicate)
Certain species live only in certain conditions - distribution of species —> conditions
Warmer oceans = lower diatom diversity
Greater Mg content in Foram shells
Pollen - distribution of plant species in an area —> climate
Eg mangrove = saltwater species, in sediment from freshwater source indicates higher sea levels
Evidence that earth’s climate has changed
Ice cores: BUBBLES show ratios of O18 to O16, and % of CO2 and CH4
Fossil Pollen: different species present
(Phyto)plankton: shows physical and chemical conditions of water
tree ring width
