How we Reconstruct
the Past
the Past
What are the 3 major types of proxies (signatures) in rocks that we use to reconstruct Earth's past conditions?
Physical, chemical, & biological
Although we think of plants and animals when we think about 'life' today, what was the majority of life like during the Precambrian (which makes up ~90% of Earth's history!) ?
Microbial
When did the GOE occur?
~2.4 billion years ago
When did the NOE occur?
~750-650 million years ago
Why is studying Precambrian global climate change important to us?
It helps us better understand the factors that cause global climate change and predict trends in climate on Earth today & in the future.
Give one example of each major 'type' of proxy (signature in rock record).
Physical: texture, structure, etc.
Chemical: isotopes, trace metal enrichments, etc.
Biological: fossils, biomarkers, etc.
When did the first life on Earth appear?
How did the GOE affect global climate & why?
How: caused cooling
Why: C sequestration
How did the NOE affect global climate & why?
How: Snowball Earth (global ice age)
Why: C sequestration
What is the major similarity between these Precambrian cooling periods & climate change that is occurring today?
Both were/are caused by perturbations in the carbon cycle.
Stable isotopes are useful tools for studying past climate change events because they move through chemical reservoirs at different rates depending on their ______. This 'fractionation' of isotopes across chemical reservoirs allows us to use their ratios to gain insight into past periods of climate/ environmental change.
mass
When did the first Eukaryotes appear?
When did the first multicellular animals appear?
Eurkaryotes appeared around ~2.1-2.7 billion years ago.
Multicellular animals appeared around ~630 million years ago.
Mass extinction of anaerobic microbes (organisms that cannot tolerate oxygen)
About how long did the Snowball Earth (global ice age associated with the NOE) last?
~70 million years total!
The Snowball Earth event during the NOE emphasizes the importance (and potential consequences of) __________ __________ mechanisms when they get out of control.
Positive feedback mechanisms
(These can cause 'tipping points' where climate starts to 'run away')
What is the primary process responsible for stable carbon isotope fractionation on Earth? And why does that make these isotopes useful for us?
Photosynthesis (and other carbon metabolisms)
Organisms preferentially take up 'light' carbon, leaving the more 'heavy' carbon for carbonate producation.
So, during periods of cooling due to rapid carbon uptake (photosynthetic activity), the carbonate rock record preserves heavier C isotope values (more positive).
And during periods of warming due to carbon emissions, the carbonate rock record preserves lighter C isotope values (more negative).
(a) What is the difference between eurkaryotes & prokaryotes?
(b) What is the difference between eurkaryotes & multicellular animals?
(a) Prokaryotes are simple cells (bacteria, archaea, etc), whereas, Eukaryotes are more complex cells (with nuclei, mitochondria, etc.).
(b) While all multicellular animals are Eurkaryotes, not all Eurkaryotes are multicellular animals. Simpler, single-celled Eurkaryotes evolved much sooner than multicellular animals.
The GOE was caused by the evolution & global spread of photosynthesizing cyanobacteria (in a world that had not previously had photosynthesis & thus, O2 production).
However, we see the first robust evidence of cyanobacteria ~3.5 billion yrs ago. And the GOE occurred ~2.4 billion yrs ago. Why did it take so long for O2 to rise after the evolution of cyanobacteria?
(1) These cyanobacteria had to spread globally before having global effects.
(2) The initial O2 produced went into oxidizing the highly reduced compounds readily avaible in Earth's atmosphere and oceans before it could build up in the atmosphere as pure O2. (For example, it was reacting with things like reduced Fe-sulfides to form oxidized Fe-oxides before it could remain stable as O2)
How did Snowball Earth potentially help the evolution of multicellular life?
Higher water viscosity due to the low temperatures of the ocean may have made multicellularity much more advantageous during snowball earth, essentially forcing its evolution.
We measure the carbon isotope signature of the fossil fuels we are burning and how that's affecting the atmosphere's carbon isotope signature to better understand the magnitude of our human impact.
Fossil fuels have a very light stable carbon isotope ratio (C-12 rich, C-13 poor) and a negligible radioactive carbon isotope, (C-14) percentage.
Snowball Earth began at ~716 Ma.
You are an isotope geochemist measuring stable carbon isotope ratios in ancient Precambrian limestone. What do you expect to see right before this Snowball period (~750 Ma), compared to during (~700 Ma)?
Positive C isotope excursion (spike) just before (high rates of productivity => more Corg burial)
Negative C isotope excursion (dip) during (lower biological productivity => less Corg burial)
Why did the evolution of O2 respiration allow for larger, more complex, multi-celled organisms?
O2 respiration yields more energy than any other metabolic pathway.
(earlier, smaller microbial organisms used compounds other than O2, like SO42-, or Fe/Mn compounds to metabolize and did not gain as much energy from those pathways)
There are many lines of evidence present in the rock record for the GOE.
Name 3 pieces of evidence for the GOE.
1. Banded Iron Formations
2. Timing of Cyanobacterial spread & collapse
3. Timing of reduced vs red beds
4. Sulfur and Carbon isotope excursions
5. Other trace metal signatures that indicate O2 rise
6. Glacial evidence in rock record directly following GOE
Extra 400 points to the team that can tell me how the sulfur isotope record indicates the timing of the GOE.
List 4 processes that caused (and/or exaccerbated) the NOE & global glaciation (Snowball Earth event)?
1. Spread of newly evolved algal species (more photosynthetic uptake of carbon).
2. Increased rate of weathering due to breakup of Rodinia supercontinent
3. Volcanic eurptions that released ash & sulfur compounds into the stratosphere blocking sunlight
4. Lower intensity of solar radiation at the time
5. Albedo feedback of global ice cover
Based on what we learned about chemical toxicity in the Precambrian, how might modern climate change cause chemical toxicity in the ocean? (aka: What compound might spread due to current climate changes?)
Hydrogen sulfide (H2S) will spread due to ocean anoxia (lack of oxygen allows spread of anaerobic microbes that produce sulfude).
H2S is toxic to animal life.