System Earth 12a

Long-term climate regulation

Misha Velthuis

m.velthuis@uva.nl

Tue 19 Nov 2024

Today

Small adjustments

Long-term climate regulation: proof and protagonists

Four glaciations:

1 Huronian

2 Neoproterozoic

3 Permo-carboniferous

4 Quaternary

Two adjustments to our model

Carbon in the mantle

Silicate weathering

A total of 0.17+0.17+0.03=0.37 GtC moves into the Oceanic HCO₃^- + CO₃^2-.

Precipitation - sea-floor dissolution = 0.20 GtC.

So the remaining 0.17 GtC (0.37-0.20=0.17) must "be moved" by the "chemical equilibration" to Oceanic CO₂ (H₂CO₃).

This fits with what we know about carbonate weathering + carbonate precipitation: for every mole of C that is taken from the atmosphere in carbonate weathering (in this case 0.17 GtC), one mole of C is released again to the atmosphere in carbonate precipitation (also 0.17 GtC). (See reactions below).

But it does not fit with what we know about the silicate weathering: of every two moles of carbon that are taken down from the atmosphere in silicate weathering, one mole is released again to the atmosphere in carbonate precipitation. The other mole of C is taken down in marine carbonate sediments.

You would therefore expect half of the C that is taken down from the atmosphere in silicate weathering to return to the atmosphere (through the flow "Chemical equilibration") and half of it to be taken down to "marine carbonate sediments".

That is currently not the case: all of the 0.03 GtC that is taken down from the atmosphere in silicate weathering is taken down to marine carbonate sediments.

To "make the model fit", you could replace the 0.03 GtC of silicate weathering with 0.06 and then assume that the chemical equilibration shows a flow of 0.20 GtC (0.17 + 0.03). The net flow towards marine carbonate sediments would remain 0.17+0.03=0.20.

Long-term climate regulation

Clear usage of terms

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

Some signs that it has been very cold …

… on unlikely places

Some more (indirect) evidence

Main suspects (protagonists)

Enhanced greenhouse effect

Carbonate-silicate geochemical cycle

(Self-destructive) life

Huronian ice age

How did it start?

Faint young sun

The sun used to be less bright

Nuclear fusion in the sun: four hydrogen nuclei → one helium nucleus
Contraction → higher temperature
Higher temperature → higher rate of nuclear fusion
Result: over time the Sun's luminosity increases

At 30% lower luminosity we would expect a frozen planet. But it cannot haven been frozen.

When the Sun formed 4.6 billion years ago, it was 30% less luminous than it is today
Still: for about 3.7 billion years: life, and life needs liquid water
Apparently there was liquid water

Most probable solution:

A stable equilibrium, on the basis of:

Methane production
Organic haze

See graph:

Relation 1 (linear):

Higher temperature: more methane production by methanogens (microorganisms that produce methane as a metabolic byproduct in hypoxic (low oxygen) conditions).

Relation 2: (parabolic):

below optimum: more CH₄ → stronger greenhouse effect
above optimum: more CH₄ → more organic haze (absorption of incoming radiation and re-emission into space)

Then life invents photosynthesis…

About image:

The upper red and lower green lines represent the range of the estimates.
The stages are: stage 1 (3.85–2.45Gyr ago (Ga)), stage 2 (2.45–1.85Ga), stage 3 (1.85–0.85Ga), Stage 4 (0.85–0.54Ga )and stage 5 (0.54Ga–present)

Correction Tue 19 Nov 15:30h:

As you can read here, photosynthesis actually appeared earlier:

3.4 billion years ago:
- Non-oxygenic photosynthesis: bacteria absorbed near-infrared rather than visible light and produced sulfur or sulfate compounds rather than oxygen.
2.7 billion years ago:
- Oxygenic photosynthesis: cyanobacteria were the first oxygen producers. They absorb visible light using a mix of pigments: phycobilins, carotenoids and several forms of chlorophyll.

Still, the "Great Oxidation Event" only started around 2.46 billion years ago.

Why did it take oxygenic photosynthesizers 200 million years to raise the oxygen levels in the atmosphere? This might have to do with …

… the fact that there were som oxygen sinks that first needed to be "filled" before oxygen could accumulate in the atmosphere (think BIF).
… the fact that cyanobacteria only developed the ability to fixate nitrogen from the atmosphere (a key limiting nutrient) around 2.5 billion years ago.

As I already noted in my little blog post it was this combination: the evolution of oxygenic photosynthesis and the evolution of nitrogen fixation, coming together in cyaonobacteria, that ultimately caused the oxygen levels in the atmosphere to go up.

Banded iron formations

No oxygen = high concentratiosn of soluble ferrous iron (Fe(II)) in oceans.

Appearance of oxygen = insoluable ferric iron (Fe(III)) settles on ocean floor.

Quote from Bjornerud, M. (2018). Timefulness: How thinking like a geologist can help save the world. (p. 103):

Steel tycoon (and later, philanthropist) Andrew Carnegie — richer in his day than Bill Gates, Sam Walton, and Warren Buffett combined — a massed his fortune through the labor of thousands who toiled in his mills, but he actually owed everything to the work of ancient microbes. Carnegie’s steel, and indeed almost all the steel ever produced in the world, was made with iron from a type of rock that is, in a sense, extinct. Most rock types—for example, the basalts that midocean ridge volcanoes exude, or sandstones composed of the granular remains of other rocks — are more or less timeless in the sense that they form on Earth today in the same way they have for billions of years. But the unimaginatively named sedimentary rocks called “iron formations” accumulated during only a specific interval in Earth’s history and record a one-time revolution in the planet’s surface chemistry in the Early Proterozoic Era, between about 2.5 billion and 1.8 billion years ago. In particular, these densest of rocks testify to changes in the air — the transition from a surface environment with no free oxygen (O2) to a brave new world created by the rise of oxygen-emitting photosynthetic microorganisms like bluegreen algae, or cyanobacteria (whose modern descendants are often called, with less than due respect, “pond scum”).

Surprising connections

Main result (for us today):

How did we get out again?

Carbonate-silicate geochemical cycle to the rescue

Neoproterozoic glaciation

How did it start?

Snowball earth

How did we get out?

Carbonate-silicate geochemical cycle to the rescue

Permo-carboniferous glaciation

Can you find it?

How did it start?

The outcome

How did we get out?

Quaternary glaciation

Can you find it?

How did it start?

How are we going to get out of it?

But first…

Let's have a better look at this glaciation of ours: Pleistocene glaciation.