Paleoclimate Proxies and Earth’s Climate History

Posted on May 16, 2024 in Geology

L15: Proxies of Paleoclimate I

Learning Objectives:

  • Explain at least one way that understanding past climate helps our present understanding.
  • Define “paleoproxy.”
  • Explain the principles behind the four paleoproxies presented.

a. Why Look to the Past?

Studying past climate helps us understand the patterns of climate change better.

b. What is a Proxy?

A proxy is something that, in theory, closely tracks something else we cannot measure directly. For example, grades are a proxy for learning.

c. Paleoproxies

Paleoproxies (paleo means “old/ancient”) are features preserved in rocks that correlate to things like temperature, ice extent, etc., and help us understand Earth’s history better.

d. Sedimentary Rock Relationships

  • Law of Original Horizontality: Rock layers are laid down horizontally.
  • Law of Superposition: Layers toward the bottom of a rock sequence are older than those above them.

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e. Paleoclimate Proxies

  • Depositional environment records of climate zones
  • Pollen (花粉) records
  • Tree rings
  • Animal and plant forms
  • Isotopes

L16: Proxies of Paleoclimate II

Learning Objectives:

  • Define what an isotope is.
  • Explain what isotope fractionation is.
  • Explain delta notation.

a. Isotopes

Atomic Weight (A) = #protons + #neutrons. If Atomic Number = protons ≠ neutrons, then it’s an isotope.

b. Isotope Fractionation

Isotope fractionation refers to the differences in the relative abundance of different isotopes of an element between two reservoirs.

c. Evaporation and Condensation

  • Evaporation (liquid to gas): Light isotopes are favored.
  • Condensation (gas to liquid): Heavy isotopes are favored.

d. Isotope Escape

Light isotopes escape more easily than heavy isotopes. For example, H216O escapes more easily than H218O.

e. Delta Notation

ΔXsample = {(Rsample – Rstandard) / Rstandard} * 1000 (P.S. R stands for the ratio of isotopes)

f. Delta Values and Temperature

  • Lower Delta: Less of the heavy isotope = colder = depleted
  • Higher Delta: More of the heavy isotope = warmer = enriched

g. Isotope Terminology

Enriched — Depleted / Heavier — Lighter / Higher — Lower / Positive — Negative

L17: Proxies of Paleoclimate III

Learning Objectives:

  • Understand how temperature correlates with oxygen isotopes when measured from H2O versus when measured from CaCO3.
  • Explain why the negative atmospheric carbon isotope excursion over the past 100 years can be explained by fossil fuel use.

a. Oxygen Isotopes in H2O

Present oxygen isotopes measured from H2O are correlated to temperature.

b. Hydrogen Isotopes in H2O

Hydrogen isotopes in H2O fractionate the same way that oxygen does and can also be used to track temperature.

c. Organic Carbonate (CaCO3)

Marine organisms use oxygen from the water to build their shells (CaCO3) and thus reflect changes in the ocean reservoir.

d. Glaciers and Isotopes

When more water is in glaciers, the remaining water in oceans is enriched in the heavy isotopes.

e. Atmospheric CO2 and Isotopes

When atmospheric CO2 concentrations are low/high, enrichment of the heavy isotope in that pool is high/low (the same is true for ocean water).

f. Biological Tissues and Carbon Isotopes

Biological tissues prefer light isotopes of carbon (negative).

g. Fossil Fuels and Carbon Isotopes

Burning fossil fuels releases light carbon into the atmosphere.

L18: Earth Climate History

Learning Objectives:

  • Know how to read a geologic time scale.
  • Be able to explain how the planet escaped the “Snowball Earth” Event.
  • Know what the relationship between the direction and rate of climate change are with mass extinction events.

a. Divisions of the Geologic Time

Eon / Era / Period / Epoch / Age — Phanerozoic / Cenozoic / Quaternary / Holocene / Recent

b. Hadean Eon (4-4.54 Ga)

  • More atmosphere than today
  • Mostly Carbon Dioxide, Nitrogen, water, and methane (甲烷)

c. Archean Eon (2.5-4 Ga)

  • Planet continues to cool
  • Ocean well-established
  • Life appears for the first time

d. Proterozoic Eon (2.5-0.541 Ga)

O2 produced by photosynthetic algae but consumed by reaction with iron in ocean water, so very little free oxygen accumulated.

e. The Great Oxidation Event (~2.5-2 Ga BP)

Oxygen began to accumulate in the atmosphere.

f. Snowball Earth

  • Huronian (2.1-2.4 Ga)
  • Sturtian (660-717 Ma)
  • Marinoan (635-640 Ma)

g. Escaping Snowball Earth

The planet escaped Snowball Earth due to the shutdown of lithosphere carbon sinks while lithosphere carbon emissions continued.

h. Phanerozoic Eon (0-541 Ma)

  • Overall low variability in temperature and CO2 compared to earlier eons.
  • Earth’s climate becomes self-buffering.
  • LIFE INCREASINGLY INFLUENCES THE CLIMATE

i. Plants and Climate

  • Plants invaded land, causing decreased albedo.
  • Increased silicate weathering.

j. Life Timeline

  • 3700 Ma: Life arises, chemotrophs may produce methane.
  • 3200-3500 Ma: Oxygenic photosynthesis arises, consumes methane and CO2 while building up O2.
  • 2000-2500 Ma: Great Oxygenation Event.
  • 541-635 Ma: Substrate revolution leading to a more active carbon cycle.
  • 460-491 Ma: Great Ordovician Biodiversity Event, colonizing water volume.
  • 390-420 Ma: Invasion and establishment of land plants.
  • 100-145 Ma: Flowering plants evolve and take over land, facilitating soil insertion.
  • 30-45 Ma: Evolution and expansion of grassland.
  • 0 Ma: Human industrialization and fossil fuel release.

k. Mass Extinctions & Climate Change

Both warming and cooling have led to mass extinctions (>50% species extinct).

L19: Ordovician (奥陶纪)

Learning Objectives:

  • Know the direction of climate change for each of the three events (Late Ordovician, End-Permian, End-Cretaceous).
  • Know which climate change event was the most deadly to life.

a. The Big Five Mass Extinctions

  1. Late Ordovician (443-445 mya, 68% species extinct)
  2. Late Devonian (369-375 mya, 38% species extinct)
  3. End-Permian (252 mya, 81% species extinct)
  4. Late Triassic (201 mya, ~38% species extinct)
  5. End-Cretaceous (66 mya, 64% species extinct)

b. Hirnantian Mass Extinction

  • Pulse 1 (glaciation – 445.2 Ma): Extinction of species that were adapted for warm-water, low O2 habitats as glaciers rapidly grew.
  • Pulse 2 (deglaciation – 443.8 Ma): Extinction of Hirnantia fauna as sea level rose and anoxic conditions became widespread again.