Earth History Exam

what does the accessible rock record look like through time?

decreases exponentially from today to 4.5 Ga

sedimentary rocks

• gives us a sense of time

• fossils

• what situation and context

• chemistry

igneous rocks

• timing

• dates

• volatiles

metamorphic rocks

• reconstruct plate tectonics

uniformitarianism

• the modern processes that happen today can explain what we see in a geologic record

  • accretion

  • people and stuff

  • oxygen

methane clathrate

-methane locked in ice

-potent GHG, released to environment

bioturbation

when animals burrow in sediment they mix it and ruin nice sedimentation. 

how does oxygen affect earth history

driving planetary cooling and glaciation, altering weathering processes and ocean chemistry, and enabling the formation of the ozone layer and more complex ecosystems by facilitating new biochemical pathways for energy

horizontal gene transfer

bacteria can pick up genes from the environment: 16srRnA/18srRNA

monophyly

came from a single source

paraphyly

came from different loci/no single source

morphology leads to 5 kingdoms

-Plants

-Animals

-Fungi

-Protists

-Monera

3 domains 

-Eucarya

-Archaea

-Bacteria

phylogenetic tree of the order of LUCA, bacteria, archaea, and eukaryotes

LUCA first, then bacteria, then archaea, then eukaryotes

which domain is the largest?

The domain Eukarya is the largest in terms of the number of classified species, containing the kingdoms Animalia, Plantae, Fungi, and Protista

eukarya branched off from who

asgard archaea

archaea cellular membrane

have a monolayer, isoprenoid, esther linkage

bacteria/eukarya cellular membrane

bilayer, straight chains, esther linkage

Lipids

the longest living half-life, best for earth history

LUCA

LUCA, the Last Universal Common Ancestor, was a complex single-celled organism from which all current life on Earth descends, likely existing around 4.2 billion years ago, shortly after the planet's formation

tree of life vs origin of life

LUCA may not have been the origin of all life. We don’t exactly know when LUCA existed.

time frame of LUCA existence

3.5-4.1 Ga

sequence alignment

sequences in amino acid align when related

lipid biodiversity and earth history

lipid biodiversity is critical in earth history. it is geologically stable and can tell us what an organism is and how it metabolized (oxic vs anoxic). lipids can be in carbon, oil, which we can analyze readily.

Carbon reservoirs

Ocean, earths crust, atmosphere

Ocean C reservoir

dissolved inorganic carbon

• H₂CO₃—>CO₂+HCO₃^-+CO₃^2-

Earth’s crust

-CaCO_3, limestones, dolomite ((Ca,Mg)(CO₃)₂)

-oil, natural gas, coal as organic matter

Atmosphere

-CO₂, CH₄ 

photosynthesis

6CO2+6H2O+light energy→C6H12O6+6O2

respiration

C6H12O6+6O2→6CO2+6H2O. The process by which organisms convert glucose and oxygen into carbon dioxide, water, and energy.

Carbon matrix entering the ocean DIC

C enters as CO2 —> ocean DIC —> Oxygen, reduced organic carbon, oxidized carbonate

most prevalent isotope of carbon 

Carbon 12. 

¹³C/¹²C=

the ratio of stable carbon isotopes used in various scientific applications, indicating biochemical processes and environmental conditions.

interpreting delta 13C values

more positive delta 13 C is more 13C enriched material

more negative delta 13 C is more 12C enriched material

Ocean Carbon System and Fractionation

• Carbon enters the ocean mainly as CO₂ or bicarbonate.

• Biological processes (photosynthesis, respiration) and carbonate precipitation alter isotopic composition.

• The isotopic composition of carbon leaving the ocean differs from what enters it, depending on how much goes into:

  • Carbonates (δ¹³C_carb) — heavier, more ¹³C.

  • Organics (δ¹³C_org) — lighter, more ¹²C.

carbon Mass Balance Equation

δ13C_in = (1 - f_org) δ13C_carb + f_org δ13C_org

definition of f_org

f_org = F_org / (F_org + F_carb)

mass balance equation definitions

• forg​ = fraction of carbon buried as organic matter

• Forg​ = flux of organic carbon burial

• Fcarb = flux of carbonate carbon burial

sulfate reduction

2H + SO4 + 2(CH2O) → H2S + 2CO2 + 2H2O

sulfate oxidation

H2S + 2O2 —> 2H + SO4

Explain in words how carbon and sulfur cycles regulate (and are regulated by) atmospheric oxygen. 

When Oxygen is abundant, it tends to favor pathways that are oxidative. When oxygen is heavily present, processes like aerobic respiration and sulfate oxidation are more prominent. For sulfur, sulfide does not accumulate in high oxygen scenarios, and instead it is quickly oxidized to sulfate. When there is low oxygen, then processes like aerobic respiration cannot occur, and fermentation occurs instead. For sulfur, when oxygen is scarce sulfide builds up. When organic carbon ('CH2O') is buried rather than respired, this results in a net O2 release into the environment.

Sulfur matrix entering the ocean DIS

34S enters in as SO4, sulfate reduction converts into pyrite and then carbonates and siliclastics come out. sulfate oxidation can cause carbonates and evaporates like gypsum.

sulfur isotopes

34S/32S relationship. more negative means less 34S.

steady state sulfur cycling eq

δ34S_in = (1 - f_py) δ34S_carb + f_py δ34S_py

f_py definition

f_py = F_py / (F_py + F_carb)

definition of steady sulfur cycle equation

• δ34S_in = isotopic composition of sulfur entering the ocean

• δ34S_carb = δ34S of sulfate (often recorded in evaporites or carbonates)

• δ34S_py = δ34S of pyrite (reduced sulfur)

• f_py = fraction of sulfur buried as pyrite

• F_py and F_carb = fluxes of pyrite and sulfate burial, respectively

32S and its prominence

32S is more easy to break apart from sulfate to H2S. this easier breaking makes it more abundant.

sulfur fractionation (epsilon)

for sulfur, the faster the cell metabolizes, the smaller the fractionation rate

carbon and sulfur cycles coupling

anti correlation. when carbon isotopes are heavy, sulfur isotopes are lighter. 

oxygen isotopes

18O/16O. lighter oxygen isotopes want to be in the atmosphere, heavier isotopes want to be in ice or in a solid form.

what happens to ocean delta 18O when we accumulate precipitation on land?

during glacial times, more ice is on land, oxygen composition in sea water gets heavier. interglacial: less ice —→ higher delta 18O. glaciation: more ice ——> lower delta 18O.

relationship of H2O and CaCO3

H₂O + CO₂ ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ ⇌ 2H⁺ + CO₃²⁻ ⇌ CaCO₃ (solid)

• Water (H₂O) dissolves CO₂ to form carbonic acid (H₂CO₃).

• This reacts and breaks down into bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻) ions.

• Calcium ions (Ca²⁺) in water then combine with carbonate (CO₃²⁻) to form calcium carbonate (CaCO₃).

• All these species exchange oxygen atoms, creating isotopic equilibrium between CaCO₃ and the surrounding H₂O.

• When isotopic equilibrium is reached, CaCO₃ records the δ¹⁸O of the water it formed in.

• Because this fractionation depends on temperature, scientists can use δ¹⁸O in carbonates to reconstruct past climates and ocean conditions (paleotemperature proxy).

Oxygen Isotopes and Temperature Relationship

• At higher temperatures, CaCO₃ incorporates less ¹⁸O, so δ¹⁸O becomes lower (more negative).

• Conversely, when temperature decreases, δ¹⁸O of carbonates increases (more ¹⁸O retained).

Ice volume effect and oxygen isotopes

When more ice forms on land:

• Ice traps ¹⁶O (light oxygen) in glaciers.

• The ocean becomes enriched in ¹⁸O.

• Marine carbonates (CaCO₃) forming in that ocean record higher δ¹⁸O values.

When ice melts:

because ¹⁶O returns to the ocean, making seawater lighter again.

Over the course of the last glacial-interglacial cycle, δ¹⁸O_carb in tropical marine carbonates changed by 2.0‰. Was it more enriched (higher δ¹⁸O_carb) during the glacial or interglacial period? Would the change in δ¹⁸O_carb be of greater or lesser magnitude than that in coeval seawater δ¹⁸O_H2O? Why?

During glacial periods, oceans become enriched in 18�(carb) as 16� is trapped in ice. Carbonates record this, so 18�(Carb) is higher in glacial periods. For the second question, tropical waters generally tend to cool down during glacial periods, which means there is additional 18� in carbonates during this time. This means that the 18�(carb) change is greater than the change in 18�(H2O) alone. 

You have access to a lab that can analyze gene expression, one that can analyze DNA sequences, and one that can analyze organic biomarkers (i.e., cellular lipids). Briefly explain the different types of information each can provide with regard to phylogeny, type of metabolism, and metabolic activity. What techniques (and why) are most useful for modern environmental samples? And for ancient samples?  

Analyzing gene expression tells us which genes are actively being used at a specific moment. It gives us information on metabolic activity, and functional states of the living community. However the limitation here is that this technique only works in living or recently living cells since RNA decays quickly. This technique is most useful for modern samples. DNA sequences tell us who is present in phylogeny and taxonomy. DNA can provide us with potential metabolic capabilities. The limitation here is that DNA also degrades relatively quickly, and ancient DNA is relatively rare. Organic biomarkers like lipids, sterols, and hopanes, are much more stable. They can tell us about broad metabolic types like if something was a methanogen, etc. This type of analysis is best for ancient samples, and is useful when DNA/RNA is gone.

δ¹⁸O_carb

When carbonate minerals precipitate, they take oxygen from water (H₂O). carbonates precipitate more readily in hot water.

δ¹⁸O_seawater

This measures the isotopic composition of the ocean water (or ancient seawater if inferred).

What major taxonomic change did Woese et al. propose in 1990?

They introduced the three-domain system — Bacteria, Archaea, and Eucarya — above the level of kingdom, based on molecular sequence data (rRNA)

Why did Woese et al. propose adding a new rank called domain?

Because molecular data (especially ribosomal RNA) showed fundamental differences between Bacteria, Archaea, and Eukarya that were too deep to fit within the old “kingdom” system.

What molecular evidence revealed the existence of Archaea as distinct from Bacteria?

rRNA sequence comparisons — 16S and 18S ribosomal RNA structures — showed unique base pair patterns and loops that divided life into three molecularly distinct lineages.

What was wrong with the five-kingdom and two-empire (prokaryote/eukaryote) systems?

• The five-kingdom model (Monera, Protista, Plantae, Fungi, Animalia) didn’t reflect evolutionary relationships — it lumped all prokaryotes together.

• The prokaryote/eukaryote dichotomy was cytological, not phylogenetic — “prokaryotes” were defined by lack of features, not shared ancestry.

• Molecular data showed Bacteria and Archaea are as unrelated to each other as they are to eukaryotes.

How does the three-domain system connect to early Earth and LUCA?

It identifies the Last Universal Common Ancestor (LUCA) as predating the split of the three domains, meaning that life diversified very early (3.5–4 Ga).

What two kingdoms did Woese et al. recognize within Archaea?

• Euryarchaeota: Methanogens, halophiles, sulfate reducers.

• Crenarchaeota: Thermophiles, likely resembling the ancestral archaeal phenotype.

What did Schopf et al. (2002) claim using Raman spectroscopy?

They used laser–Raman imagery to correlate morphology and chemical composition in 3.5 Ga Apex chert filaments, arguing this proves biogenic kerogenous cell walls

Why did Schopf use Raman spectroscopy on Apex fossils?

It’s a non-destructive technique that detects the carbon structure (D and G bands), allowing scientists to confirm kerogenous (organic) composition

What features did Schopf interpret as proof of life?

Filamentous, cylindrical, and cellular morphologies with kerogenous carbon, transverse cell walls, and tapered ends

Where and when are the fossils from?

Apex chert, Warrawoona Group, Western Australia — ~3.465 billion years old

What does Schopf’s study imply for early life?

If valid, these fossils show that complex microbial ecosystems existed by 3.5 Ga

What did Brasier et al. (2002) argue against Schopf’s findings?

They re-examined the same Apex chert samples and found the “microfossils” were abiotic artifacts — graphite-rich filaments formed by hydrothermal and metamorphic processes

How did Brasier reinterpret the rock context of the Apex chert?

Instead of being sedimentary and biogenic, the Apex chert was a hydrothermal vein breccia cutting volcanic basalt — meaning the filaments formed in metamorphic or hydrothermal fluids

How could “fossil-like” filaments form abiotically?

Graphitic carbon and silica spherulites can produce pseudoseptate, branching, filament-like textures during hydrothermal silica crystallization

What isotopic and mineral data supported the abiotic interpretation?

δ¹³C values (-30‰) overlapped with abiotic carbon fractionation; presence of native metals (Fe, Ni, Cu, Zn) and sulfates implied formation at 250–350°C in hydrothermal systems.

What flaw did Brasier find in Schopf’s analysis?

Raman spectra confirmed carbon but did not prove biological origin — since both the “fossils” and surrounding matrix shared identical graphitic signals.

What did Brasier et al. say about standards of biogenicity?

all abiotic explanations must be ruled out first. Ancient “microfossils” must be contextually, chemically, and texturally consistent with biology.

oxygen isotopes and temperature diagram.

• δ¹⁸Oₛw reflects how much ¹⁶O is locked in ice sheets.

• Once the ice is completely melted (in a HOT climate), there’s no more ice left to melt — so δ¹⁸Oₛw can’t decrease further.

• Even after δ¹⁸Oₛw stops changing (because the ice is gone), temperature keeps rising, so δ¹⁸O_carb continues to decrease.

when did mass independent fractionation occur?

in the Archaen, before the GOE

when does mass dependent fractionation occur?

currently

why did MIF occur when the earth’s atmosphere was anoxic?

it occurred because there was no ozone layer so photons can reach down to where sulfur is in the atmosphere to break it apart using photochemical reactions.

sulfur mass dependent fractionation graph 

δ³³S and δ³⁴S values are proportional which means that isotopes are fractioning in a mass dependent way. 

sulfur mass independent fractionation graph

no oxygen, UV driven photochemistry. peak occurs at 2.4 Ga. 

when Δ³³S=0,

it indicates mass dependent fractionation is dominant.

when Δ³³S=/=0,

it signifies mass independent fractionation.

y and x axis of mass dependent fractionation of sulfur

represent δ³³S and δ³⁴S values, respectively, slope= 0.5

y and x axis of mass independent fractionation of sulfur

Age on x axis, Δ³³S on y axis

Δ³³S=

δ³³S - 0.5δ³⁴S

sulfur isotopes through mass dependent fractionation graph.

plotted with time (Ga) vs δ³⁴S. shows how when oxygen increases, we get oxidative weathering, and sulfate delivery increases, so sulfide can readily be converted into pyrite. 

why does more oxygen lead to glaciation?

increase in weathering, takes CO2 out of atm and decreases temperature. when ozone levels increase, temperature decreases. when CH4 decreases, temperature decreases.

redbeds

are sedimentary rocks that indicate oxidation of iron due to exposure to oxygen.

what other geologic indicators show atmospheric O2 was low prior to 2.4 Ga?

stromatolites increase, redbeds decrease, banded iron formations disappearence, paleosols had low Fe on the soil surface before 2.4 Ga. reason why is during oxygenation, Fe minerals stay in place while other minerals dissolve and leave.

ferruginous

Fe²⁺ ferrous , Fe³⁺ ferric

euxinic

refers to anoxic, sulfidic waters characterized by the presence of hydrogen sulfide, often associated with low oxygen conditions.

redox condition in euxinia

Fe³⁺ is never soluble. Fe²⁺ is mobile. as you move down the slope, total amount of Fe increase relative to Al. As oxygen levels decrease, Fe2+ becomes more prevalent, while the solubility of Fe3+ diminishes, leading to increased total iron relative to aluminum.

transition to a sulfidic ocean graph

shows Total Fe, Fe_HR / Fe_T, Fe_py / Fe_HR, δ³⁴S (‰) 

Total Fe (wt%)

The highest Fe values occur in the lower part (Fe-rich zone) — meaning those rocks formed when iron was abundant, likely due to precipitation from an anoxic, Fe²⁺-rich ocean (typical of Banded Iron Formations).

Fe_HR / Fe_T

• “Fe_HR” = highly reactive Fe (the fraction of iron that can easily react with sulfur to form pyrite, such as Fe in oxides, hydroxides, or carbonates).

• “Fe_T” = total Fe.

• So this ratio tells how much of the total iron is reactive.

Interpretation:

• Low Fe_HR/Fe_T (<0.38) → oxic marine environment (less reactive Fe).

• High Fe_HR/Fe_T (>0.38) → anoxic environment (more reactive Fe).

In the graph:

• Below the tuffaceous horizon, Fe_HR/Fe_T is relatively high → anoxic iron-rich conditions.

• Above it, values become lower and fluctuate → the water column may have had more oxygen or partial redox stratification (transitional conditions).

Fe_py / Fe_HR

• “Fe_py” = Fe bound in pyrite (FeS₂).

• This ratio shows how much of the reactive Fe got converted into pyrite.

• High Fe_py/Fe_HR (>0.7) → sulfidic (euxinic) water (lots of hydrogen sulfide available).

• Low Fe_py/Fe_HR → iron was reactive but didn’t meet much sulfide (ferruginous rather than euxinic).

In the figure:

• In the Fe-rich zone, Fe_py/Fe_HR is very low → little sulfide; mostly ferruginous.

• In the transitional zone, the ratio rises — sulfide begins to appear.

• In the upper sulfidic zone, Fe_py/Fe_HR is near 1 — strongly euxinic conditions.

δ³⁴S (‰) on sulphidic ocean graph

• This measures the isotopic composition of sulfur in pyrite.

• δ³⁴S = heavy vs. light sulfur isotopes.

  • Low (negative) δ³⁴S → open-system microbial sulfate reduction (light sulfur).

  • High (positive) δ³⁴S → more closed system or intense sulfate limitation (heavy sulfur).

In the graph:

• Below the tuffaceous horizon, δ³⁴S is low to moderate (–10 to 0‰) → open, iron-rich and low-sulfide system.

• Upward into the sulfidic zone, δ³⁴S shoots to very positive values (+15 to +30‰) → restricted, sulfate-limited environment — euxinic, sulfur-rich basin.

Around 1.8 Ga, oceans were stratified:

• Surface: mildly oxygenated.

• Mid-depth: euxinic (rich in H₂S).

• Deep waters: ferruginous (Fe²⁺-rich, no oxygen or sulfide).

Molybdenum Isotopes

• Under oxic conditions, Mo exists as soluble MoO₄²⁻; under euxinic conditions, it precipitates as MoS₂.

• δ⁹⁸/⁹⁵Mo becomes more positive as oceans become more oxygenated (fractionation increases).