The Great Detective Science Flashcards
Why is reconstructing the Earth’s History called ‘the Great Detective Science’
Little of the evidnece is straightforward to interpret, because it is old, squashed, altered, and in code
What two methods are coming together to help piece together the Earth’s history
The field has mature (geology, fossils) and immature (DNA, biomarkers) methods, increasingly forming a powerful combination
Where is Sandstone predominatly found
Desert, Coastal Dunes, Growing delta, River channels
Describe Sandstone
Grains mostly quartz, various cements
Metamorphoses to quartzite if pure
Usually few fossils (moderate to high energy)
Few chemical forensic clues
Where is Limestone predominatly found
Nearshore, lagoon, reef
Deep sea
Describe Limestone
Grains and cement mostly CaCO₃ (Mg⁺)
Metamorphoses to marble if pure
Few to many to all fossils
Many chemical forensic clues
Where are Siltstone, mudsone and shale predominatly found
Quiet, gentle environments
Lakes, Lagoons
Far continetal shelf, slope, deep margin
Deep sea
Describe Siltstone, mudstone and shale
Grains mostly quartz, various fines and cements
Metamorphoses to slate then schist then gneiss
Few to many fossils
Many chemical forensic clues
Where is Ooze (and Red Clay) predominatly found
Deep sea
Where fossils are rare, mostly windblown dust; red clay
Describe Ooze
Brains mostly microfossils: CaCO₃ or silica
Rarely found metamorphosed (subducted)
Mostly microfossils
Many chemcial foresic clues
Why is fossil evidence not found on high ground
Because it will experience high erosion and weathering
Why is fossil evidence found in low ground and marine (low basins, contiental margins etc)
Good chances of deposition and long-term survival with physical and chemical integrity
Why is fossil evidence found on oceanic crust
It has a good chance of deposition
However there is issues with its long term survival
Where hence are the best places to find fossils
Deep sea
Continental margins
Subsiding basin
Delta (moderate to good)
Lake, Swap and channel (moderate to good)
Palaeosol (=fossil soil) (moderate)
Desert (moderate)
Tradiditonal environmental reconstruction is from
Grain characterisitcs, rock structures and fossils
How can the K-Ar ratio be used to give the age of volcanic rock
When a volcanic rock solidifies, it contains K with no appreciable Ar
K decay to Ar at a rate known to physicits (~1.2 By)
This is unaffected by heat or pressure
Ar can escape and contamination can be a good problem, so care and good sample selection needed
What are characterisitc of good ‘marker’ fossil
- Abundant
- Good preservation potential) organisms with hard parts
- Widespread (global)
- Lives/winds up depsotied in different environments - planktonic graptolites idea
- Appears and become abundant and widespread quickly
- Extinct everywhere at once quickly, preferably not at a mass extinction boundary
- Short duration
Didymograptus leintwardinensis once existed in the sea, floating passively near the surface
Very abundant and widespread, so found in many types of marine rock nearly everywhere
Died out, cause unknown, everywhere at the same time
How can this be used in relative dating
Rocks containing D.leintwardinensis everywhere are all the same age; lower rocks are older and higher rocks are yonger
Rubisco can be used as a biomarker
What is it main biological use
All photosynthesisers use rubisco to make carbon compounds
It is the most abundant enzyme on earth, which in involved in the first major step of carbon fixation, through catalysing a reaction between carbon dioxide and RuBP
If something is an inorganic carbon compound
What it it’s basic formula going to be
CH₂O
What is an inorganic carbon compound
Something that can be precipitated by some living things in the form of, for example, shells
e.g. CaCO₃
If a record said “Δ ¹³C of -20%”
What does this mean
Deficient in ¹³C to the tune of 20%
Relative to typical carbonate carbon ratio
If the carbon compounds in a rock have a Δ ¹³C of -20%, what has caused this?
The most likely explanation is that the compound was made using rubisco, which means photosynthesis and life
Why do we predominatly look at isotopes in rocks when analysing early life
Because early life was soft bodied and hence would not fossilise well
The Carbon + Rubisco signature is one example of fractionation:
What is this?
Some process that distorts the ratios of elements or chemicals away from their otherwise expected values
Many elements have isotopes that are fractionated away from their expected values by one process or set of conditions or another
How can iron be used as a chemical indicator or the environment and early life
Much Iron is in rocks and its oxidation state gives us clues about the oxidation state if the environment of deposition (if oxygen was around or not)
Why are biomarkers a useful way to learn about early life
Biomarkers have great potential to tell us things we want to know. Different groups of lifeforms produce different organic compounds as end or intermediate products
We can now measure organic compounds present in rocks in minuscule amounts - using gas chromatography mass spec
What is the problem with biomarkers however
Other scientists have pointed out that compounds could have migrated into the focal rock from younger ones
How can comparision of genes be used to look at evolutionary relationships
The more similar the genetic code between current living organisms and ones previously lived
Differences between codes can be converted to estimates of times of divergence: so-called ‘molecular clock’ estimates
Molecular clock analyses must be ‘tied down’ by fossil evdience to be worth much
What is the Signor-Lipps effect
since the fossil record of organisms is never complete, neither the first nor the last organisms in a given taxon will be recorded as a fossil
The Signor-Lipps effect is often applied specifically to cases of the youngest-known fossils of a taxon failing to represent the last appearance of an organsism
But overall its hard to know the full life-time of a species because of this
