Chapter 3 - What Rocks Say Flashcards
Why did Kelvin assume the earth could be no more than 20 million years old?
He surmised through his research, based on the heat of the earth, that the rate the earth cooled meant it could not be more than 20 million years old. However, he assumed the earth was a rigid sphere, not a circle with dynamic insides.
If Kelvin was right, what kind of isotopes would be found in rocks?
Many short-halflived, undecayed isotopes. However, we rarely find these - we typically just find their daughters, alluding to a very old, ancient earth.
How can geologists estimate a rocks age?
They measure concentrations of stable and radioactive isotopes in a sample. Because decay rate is steady, scientists can use this to determine how long decay has been occurring in the rock and thus the rocks age.
What is radiometric dating?
A technique that allows geologists to estimate the precise time when one geologic formation began and another ended.
Why will the fossil record never be complete?
Most organisms don’t fossilize. Lots of dead bodies are scavenged or decayed, which makes fossilization a rare event.
Whats the significance of Anchioris?
Scientists were able to study it and determine exactly what colors its feathers were due to the presence of melanosomes in it’s fossil. It had striking black, white and grey plumage.
What are Langerstatten?
Exceptionally well preserved fossils at a specific site and same period of time, usually including soft tissue. The Burgess Shale is a Langerstatten.
How did the Burgess Shale form?
505 million years ago, a rich community of marine animals were occasionally buried by mudslides from the steep cliff they lived under. Anoxic conditions and sediment covering them preserved these creatures. This happened many times, making a very diverse snapshot of marine life.
How does coal form?
Millions of years ago, plant matter fell into swamps where it was buried, and then over time, with heat and pressure, turns into coal.
What are biomarkers?
Molecular evidence of life in the fossil record, like dna fragments, molecules like lipids, or even specific isotope ratios.
What is okenane and it’s significance?
It is a molecule and biomarker produced only by purple sulfur bacteria, which is very rare today. Looking at the presence of okenane in rocks tells us that 1.94 billion years ago, purple sulfur bacteria dominated and the oceans were likely toxic.
Why is the ratio of carbon-12 to carbon-13 lower in plants than in the rest of the atmosphere?
Because carbon-13 is heavier than carbon-12. This is why scientists can exam the ratio in rocks and determine if it was from a biological source.
What is the carbon-12/carbon-13 ratio like in animals like cows and horses vs giraffes or elephants?
Cows and horses graze mostly on c4 grasses, which means their ratio is higher than that of their c3 leaf eating cousins. C4 plants have higher amounts of carbon-13 than C3.
What did Cerling discover in the tooth enamel of an east african hominin?
He discovered that it contained a low ratio of carbon-13 to carbon-12, implying that he had a diet rich in c3 plants, similar to chimpanzees.
Other fossils that showed c4 plants growing around the same time told Cerling that his hominin was actively selecting for c3 plants.
What did Cerling discover when he studied more ancient hominin species?
That their diet shifted more from c3 to c4 plants. This is supported by the fact that some hominins had large jaws and teeth for eating tough grasses, but also implies they could have got it from the meat of c4 grazing animals.
Why can’t scientists ever hope to find conventional fossils from early earth?
The crust began to break up, with rocks eventually sinking into the earth and melting away. Scientists have shifted their attention to isotopes in ancient rocks.
