Evolving Life and the Earth

Figure 2.3 and the Late Proterozoic Ice Ages

The Gaia Hypothesis and S269

Figure 2.3 and the Late Proterozoic Ice Ages

In the table below the questions and suggested answers refer to Figure 2.3:

Question

Suggested Answer

What does the high overall d¹³C_carb in the Late Proterozoic possibly indicate?

High d¹³C_carb points to intense organic carbon burial.

Is there a link between high d¹³C_carb and the breakup of Rodinia?

Possibly. Extensive rifting implies a high rate of sea-floor spreading, which implies the outgassing of large amounts of reducing compounds from hydrothermal vents, which implies the drawdown of O₂, which would tend to increase ocean anoxia. Anoxic oceans would increase the rate of organic carbon burial.

Is there a link between the sharp dips in d¹³C_carb and the ice ages?

Glaciations cause eustatic marine regressions, which increase the erosion and weathering of organic carbon-bearing sediments on former continental shelves (remember, no land plants!), which raises the concentration of ¹²C in the oceans, and so causes d¹³C_carb to fall.

What is the significance of the Banded Iron Formations (BIFs)?

BIF formation implies a renewed supply of reducing Fe compounds from mid-ocean ridge hydrothermal vents, consistent with other lines of evidence pointing to increased rifting.

Is the ⁸⁷Sr/⁸⁶Sr curve consistent with other evidence about mid-ocean ridge activity and rates of weathering?

Broadly, yes. (1) Low values throughout the Late Proterozoic point to high output of ⁸⁶Sr from the mantle; (2) strong dip between 900 & 800 Ma coincides with BIF formation and breakup of Rodinia; (3) relative high in the Late Proterozoic might reflect increased continental weathering while hydrothermal vent activity also still remained high; (4) rise in the Vendian to a high in the Cambrian points to reduced hydrothermal vent activity relative to increased continental weathering.

What do the phosphorites represent?

Phosphorites are formed when limestones in shallow seas are replaced by phosphates, such as apatite, Ca₅[F, Cl, OH]₃(PO₄)₃, when phosphate-rich, deep ocean cold currents well up to the surface. The phosphate is ultimately derived from high productivity in surface waters. Upwelling cold currents are consistent with an ice-house regime in the Late Proterozoic.]

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The Gaia Hypothesis and S269

James Lovelock expounded his idea of the Earth as Gaia, "a kind of living organism ... able to regulate its climate and composition so as to be comfortable for the organisms that inhabit it" in his book Gaia, A New Look at Life on Earth, published in 1974. Here are a few more quotations from the book:
Referring to the Earth: "A stable planet made of unstable parts."

"The entire surface of the Earth including life is a superorganism..."

"Gaia is the superorganism composed of all life tightly coupled with the air, the oceans, and the surface rocks."

The Gaia hypothesis has gained widespread public acceptance. This acceptance has sometimes been founded on a misunderstanding of what Lovelock meant. There are broadly two versions of the Gaia hypothesis: what might be called a "hard" Gaia hypothesis, and a "soft" Gaia hypothesis. These two versions may be summarized as follows:

Hard Gaia hypothesis:
The Earth is a living thing. There is evident design in the way the Earth has maintained equable conditions for life throughout geological history.
This is a popular view, but the idea that the Earth was designed to support life or that it is in any sense alive is not scientific.

Soft Gaia hypothesis:
Gaia is a "superorganism", which is not the same as being a living thing. This is what Lovelock meant. There is no design or intention in its working, but rather a dynamic steady state involving the Earth System.
This version of the Gaia hypothesis is closer to the view of the Earth System proposed by S269.

Analogy between Gaia and biological evolution:
Evolution is an undirected result of change induced by natural selection acting upon random mutations. In the same way, the maintenance of favourable conditions for life on the Earth can be thought of as an emergent steady state brought about by the interactions of the biosphere with the systems of the physical planet, including the atmosphere, oceans and crust, and even the mantle and core.

How friendly to life is the Earth?
Here are a few things you might like to think about:

Not all feedbacks are negative.

The Earth is a planet, behaving as planets do – generating heat through radioactive decay in its crust and mantle, and venting that heat to space.

Geological events can affect life, e.g. the output of reducing compounds at mid-ocean ridges during the Proterozoic; superplume events as in the Cretaceous.

The Permian mass extinction was probably "home grown", resulting from disequilibria in the Earth System and/or random effects of geological and biological events, such as the formation of Pangea (causing a reduction in mid-ocean ridge activity and a eustatic fall in sea levels) or alteration of the carbon cycle by the evolution and spread of land plants. It was probably not the result of an external influence such as the impact of a giant meteorite.

S269 says that the Earth System is a "complex, chaotic system, lurching between temporarily equilibrating states."

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Question	Suggested Answer
What does the high overall d¹³C_carb in the Late Proterozoic possibly indicate?	High d¹³C_carb points to intense organic carbon burial.
Is there a link between high d¹³C_carb and the breakup of Rodinia?	Possibly. Extensive rifting implies a high rate of sea-floor spreading, which implies the outgassing of large amounts of reducing compounds from hydrothermal vents, which implies the drawdown of O₂, which would tend to increase ocean anoxia. Anoxic oceans would increase the rate of organic carbon burial.
Is there a link between the sharp dips in d¹³C_carb and the ice ages?	Glaciations cause eustatic marine regressions, which increase the erosion and weathering of organic carbon-bearing sediments on former continental shelves (remember, no land plants!), which raises the concentration of ¹²C in the oceans, and so causes d¹³C_carb to fall.
What is the significance of the Banded Iron Formations (BIFs)?	BIF formation implies a renewed supply of reducing Fe compounds from mid-ocean ridge hydrothermal vents, consistent with other lines of evidence pointing to increased rifting.
Is the ⁸⁷Sr/⁸⁶Sr curve consistent with other evidence about mid-ocean ridge activity and rates of weathering?	Broadly, yes. (1) Low values throughout the Late Proterozoic point to high output of ⁸⁶Sr from the mantle; (2) strong dip between 900 & 800 Ma coincides with BIF formation and breakup of Rodinia; (3) relative high in the Late Proterozoic might reflect increased continental weathering while hydrothermal vent activity also still remained high; (4) rise in the Vendian to a high in the Cambrian points to reduced hydrothermal vent activity relative to increased continental weathering.
What do the phosphorites represent?	Phosphorites are formed when limestones in shallow seas are replaced by phosphates, such as apatite, Ca₅[F, Cl, OH]₃(PO₄)₃, when phosphate-rich, deep ocean cold currents well up to the surface. The phosphate is ultimately derived from high productivity in surface waters. Upwelling cold currents are consistent with an ice-house regime in the Late Proterozoic.]