System Earth 12b

Quaternary glaciation

Misha Velthuis

m.velthuis@uva.nl

Wed 20 (not the 12th 🙃) Nov 2024

Today

Quaternary glaciation

Glacials and interglacials

Driving force

Reinforcing feedback loops

Addendum: Younger Dryas

Some corrections

In the news

In Eos, the news magazine of the American Geophysical Union:

https://eos.org/articles/a-close-asteroid-encounter-may-have-once-given-earth-a-ring

"For that to happen, an asteroid would have had to pass within Earth’s Roche limit, a theoretical boundary within which Earth’s gravity overcomes an object’s internal strength and shreds it to pieces."

"The models showed that 466 million years ago, all of the craters were concentrated within 30° of the equator. Plenty of crater-preserving crust was outside that narrow strip, so it’s not likely that nonequatorial Ordovician craters escaped notice"

One hypothesis for the "brief (and poorly understood) spike of glaciation during the Late Ordovician" as Kump et al. describe it, is that this "may have shaded Earth from the Sun's rays and triggering significant cooling" (Wiki).

Common ground in terms of terms

	cold	warm
long-term	ice age/ glaciation	greenhouse period
short-term	glacial (period)	interglacial (period)

Last Tuesday: glaciations

Quaternary: 2.58 Ma - present

Glacials and interglacials

(Careful: the units of the time axis change over time.)

Driving force: Milankovic cycles

Earth's orbit around the sun

Step 1

Step 2

Step 3

Glacials start when the NH summer temperature is relatively low.

When is the NH summer relatively cold?

When the eccentricity is large
When the NH summer falls on the aphelion
When the obliquity is low

Without eccentricity, there is no perihelion/aphelion. Only when there is high eccentricity, the precession-driven timing of the NH summer (aphelion or not) starts to really matter.

Right now:

NH summer at aphelion …

But:

Eccentricity:

Range: 0-0.06 (not 0.6 as I say in the video!)
Current value: 0.017 (relatively low)
(Eccentricity is distance of sun to center of ellipse divided by half of the distance of the main axis of the ellipse (not the whole distance, as I suggest in the video))

Obliquity:

Range: 22.5°-24.5°
Current value: 23.5° (in the middle)

Future:

Earth’s axis is currently tilted 23.4 degrees, or about half way between its extremes, and this angle is very slowly decreasing in a cycle that spans about 41,000 years. It will be at its lowest angle in 10,000 year.
Earth’s eccentricity is very slowly decreasing and is approaching its least elliptic (most circular).

link

Connection with data?

Only four major glacials?

Oxygen isotopes

Kump et al.:

"The colder the water, the greater the tendency for minerals to incorporate ¹⁸O, and thus the larger the ratio of ¹⁸O to ¹⁶O in CaCO₃."
"Evaporation transfers both H₂¹⁶O and H₂¹⁸O from the ocean to the atmosphere, but there is a preferential release of H₂¹⁶O to the vapor phase."
"Moreover, water-vapor molecules containing ¹⁸O tend to condense more readily than do those containing ¹⁶O… Snow falling onto ice caps has had its ¹⁶O considerably enriched."

So as it gets colder …

The tendency for CaCO3 to incorporate ¹⁸O increases.
And the oceans become richer in ¹⁸O as more ¹⁶O is captured in continental glaciers

So colder temperatures → higher δ ¹⁸O in calcite.

Maximum age: ± 200 million years.

Extra background info:

This is how you actually calculate the δ¹⁸O:

Fourier analysis of two signals

Fourier analysis (decomposing a signal into its constituent frequencies):

Top row: "insolation (W/m²) at 65°N in June" over time.
Bottom row: δ¹⁸O over time

Kump et al.: "The direct forcing, that is, the average annual insolation change, in the eccentricity band is very small (some 10% of that in the other bands),yet the climate response is the largest of the three bands. The importance of eccentricity is evidently more indirect. Eccentricity modulates the insolation changes associated with the precessional band, as can be seen in the envelope of variation for precession in Figure 14-8."

Not clear to me: for precession, both the insolation signal (top left) and the isotope signal (bottom left) still show a composite signal: there is the 26,000 years periodicity, but also a 100,000 year periodicity on top of that. I would expect the Fourier analysis to filter out (pick up) this 100,000 periodicity signal in both, but it appears to only do so for the isotope data (bottom right). Why isn't it also picked up in the insolation data (top right)?

Nonetheless, the question remains: the insolation alone cannot explain the fluctuations in δ¹⁸O. What processes could strengthen the impact of the insolation fluctuations?

The story told by CO₂ in air bubbles

CO₂ as glacial booster?

Potential positive feedback loops

Step 1: Stronger biological pump

biological pump.png

Step 2a: shelf nutrient hypothesis

Step 2b: Iron fertilization hypothesis

Coral reef hypothesis

Carbonate weathering, carbonate precipitation

Weathering, precipitation and the sea

As the sea level rise, coral reefs can accumulate: if carbonate weathering remains the same and precipitation increases, the net effect will be a release of CO2 to the atmosphere.

As the sea level drops (because it gets colder), coral reefs are exposed to the atmosphere: carbonate weathering increases, while precipitation decreases: the net effect will be a draw down of CO2 (which would make it even colder).

Note that on a long timescale carbonate weathering + carbonate precipitation does not affect atmospheric CO2.

On a short timescale, in contrast, an increase of weathering and a slow down of precipitation can increase the fraction of C stored as bicarbonate ion at the expense of (the C in) atmospheric CO2.

By the way:

The reefs that were growing and shrinking during the Quarternary glaciation were of course very different from their much older predecessors from the Devionian (behind Lea).

Coral reef hypothesis

Overview

CO2 driven feedback loops reinforcing fluctuation in insolation

Option 1

Stronger biological pump

because

Shelf nutrient hypothesis

Option 2

Stronger biological pump

because

Iron fertilization hypothesis

Option 3

Coral reef hypothesis

System Earth 12b

Quaternary glaciation

Misha Velthuis

m.velthuis@uva.nl

Wed 20 (not the 12th 🙃) Nov 2024

Today

Some corrections

In the news

Common ground in terms of terms

Last Tuesday: glaciations

Quaternary: 2.58 Ma - present

Glacials and interglacials

Driving force: Milankovic cycles

Earth's orbit around the sun

Step 1

Step 2

Step 3

When is the NH summer relatively cold?

Connection with data?

Only four major glacials?

Oxygen isotopes

Fourier analysis of two signals

The story told by CO2 in air bubbles

CO2 as glacial booster?

Potential positive feedback loops

Step 1: Stronger biological pump

Step 2a: shelf nutrient hypothesis

Step 2b: Iron fertilization hypothesis

Coral reef hypothesis

Carbonate weathering, carbonate precipitation

Weathering, precipitation and the sea

Coral reef hypothesis

Overview

CO2 driven feedback loops reinforcing fluctuation in insolation

Option 1

Option 2

Option 3

Addendum

20,000 years ago it started to get warmer

But then…

Catastrophic drainage of lake Agassiz

Fresh water undermining thermohaline circulation

Or maybe…

What happened around 13000 years ago?

Laacher See eruption!

The story told by CO₂ in air bubbles

CO₂ as glacial booster?