The Dynamic Earth

The Dynamic Earth

 
Oxygen Isotope Ratios
The Carbon Cycle - Geological Timescales
Strontium Isotope Ratios
The Milankovitch Cycles
The Carbonate Compensation Depth

 

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Oxygen Isotope Ratios

 
d18O is a comparison of ratios, like the d13C values.
How is oxygen fractionated?
Physical processes:
Evaporation favours H216O (the lighter isotope).
Precipitation condenses H218O preferentially (the heavier isotope).  Therefore atmospheric water vapour is enriched in 16O.
Ice caps remove 16O-enriched water from the ocean/atmosphere cycle.  Therefore during times of extensive glaciation, the oceans are depleted in 16O and enriched in 18O.  Since glacial episodes correspond to periods of low sea level, measuring d18O in seawater will tell us about relative sea levels.
Biological processes:
Formanifera and other marine organisms fractionate oxygen in the formation of carbonate skeletons.
How is information about the oxygen isotope ratio in seawater preserved?
Carbonate skeletons secreted by formanifera reflect the 18O/16O ratio in seawater.
18O/16O ratios in carbonate skeletons are greater in cold than in warm water. So  d 18O values of carbonate sediments preserve information about deep ocean temperatures (and, by extension, about surface temperatures), and about surface ocean temperatures at high latitudes.
Different formaniferan species fractionate oxygen differently.  Therefore d18O values in carbonate sediments are determined by a complex interplay of the species concerned, water temperature, and the 18O/16O ratio of the seawater. Nevertheless,  d18O values are used to infer deep ocean temperatures and sea levels for the Pleistocene.

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The Carbon Cycle on Geological Timescales

Key Facts and Frequently Asked Questions

Equations affecting the carbon cycle
Carbonate weathering

(TDE Equation 3.6a)

Weathering of carbonate minerals takes 1C from the atmosphere and 1C from the carbonate mineral to form 2 molecules of bicarbonate in solution.  Precipitation of calcium carbonate from solution simply reverses the first reaction, and returns 1C to the atmosphere.  Overall the cycle is in balance, with no net loss of C from the atmosphere.
Silicate weathering (TDE Equation 3.6b)

(TDE Equation 3.6a)

Weathering of silicate minerals removes 2C from the atmosphere to form 2 molecules of bicarbonate in solution.  Precipitation of calcium carbonate from solution is not the reverse of the first reaction.  Precipitation restores 1C to the atmosphere, leaving a net loss of 1C from the atmosphere.  Hence silicate weathering removes CO2 from the atmosphere.

Note that weathering of calcium silicates also draws down CO2, as described in Origin of Earth and Life, p. 76:

where the carbonic acid on the left is formed by reaction of atmospheric CO2 with water.
Organic carbon burial (OEL Equation 1.4, aerobic photosynthesis)

(OEL Equation 4.7, respiration)

Burial of organic carbon blocks the respiration reaction in the cycle above, causing a net drawdown of CO2 and allowing O2 to accumulate in the atmosphere.
Summary

What causes the net loss of C from the atmosphere?

Net loss of C from the atmosphere arises from the fact that, whereas the formation and weathering of carbonate rocks (CaCO3) is a cycle in balance, silicate rocks are formed by geological processes not involving atmospheric carbon. Silicate weathering removes 2C from the atmosphere, and precipitation of carbonates using HCO3 (aq) released by silicate weathering returns only 1C to the atmosphere.

In effect, the formation and weathering of silicates links processes in the mantle and crust to processes affecting the atmosphere.

In conclusion:

Weathering & precipitation of carbonate rocks results in no net gain or loss of CO2 from the atmosphere.

Weathering of silicates and precipitation of carbonates results in a net loss of CO2 from the atmosphere.  

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Strontium Isotope Ratios

  1. The sources of 87Sr and 86Sr in the Earth.  86Sr is a primordial constituent of the Earth, acquired at the time of accretion from the Primitive Solar Nebula. 87Sr is entirely derived from the radioactive decay of 87Rb, which is a primordial constituent of the Earth.
  1. Why the 87Sr/86Sr ratio in any rock increases over time. 86Sr is a stable isotope and the amount does not change, whereas the radioactive decay of 87Rb adds to the amount of 87Sr.
  1. Why the 87Sr/86Sr ratio in continental crust is higher than that in oceanic crust, and the ratio in oceanic crust is higher than that in the upper mantle.  87Rb is an incompatible element. Partial melting which generates basaltic magma at mid-ocean ridges depletes the upper mantle of 87Rb and enriches it in the oceanic crust. Likewise, partial melting at subduction zones depletes 87Rb in oceanic crust and enriches it in continental crust. So crustal rocks are enriched in 87Rb, which is the source of 87Sr.
  1. Why the 87Sr/86Sr ratio in oceanic hydrothermal fluids is lower than that in rivers. Hydrothermal systems in the oceans sample basaltic oceanic crust, which, because it is the first distillation of the upper mantle by partial melting, has a lower 87Sr/86Sr ratio than the continental crust being sampled by rivers.
  1. With reference to Figure 6.20, how can 87Sr/86Sr ratios be determined for the geological past.  By measuring the ratio in marine carbonate rocks.
  1. What the ratios tell us about palaeoweathering.  They show changes in the input of 87Sr to the oceans. This is argued (TDE p. 205) to have been caused by changes in the rate of continental weathering, which in turn may be a result of uplift of the Himalayas and the Tibetan Plateau. The evidence from 87Sr/86Sr ratios must be supplemented by independent evidence of other kinds (e.g. rates of ocean floor production).

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The Milankovitch Cycles

Facts and Frequently Asked Questions

There are three orbital cycles which are thought to influence climate by changing the incidence and distribution of solar radiation on the Earth, viz.:

 
Eccentricity of the Earth’s orbit. Periodicity of 110,000 years. Affects the total amount of solar radiation reaching the Earth. An elliptical orbit exaggerates the seasons in the hemisphere in which winter occurs at aphelion and summer at perihelion.
Change in the angle of tilt of the Earth’s axis (21.8° - 24.4° ). Periodicity of 40,000 years. Changes the effective latitude of the Tropics. Higher tilt gives warmer summers & colder winters.
Precession of the equinoxes. Periodicity of 22,000 years. Solstices and equinoxes move clockwise around the orbit. Changes the position in the Earth’s orbit at which the seasons occur.

The three cycles operate independently of each other. They combine to produce variations in the intensity of the seasons and the amount of solar radiation reaching the Earth. They are the principal forcing factors controlling the glacial/interglacial cycles during the Pleistocene Ice Age.
Frequently Asked Questions Answers
How far back in geological time can these cycles be traced? About 1.6 Ma, into the Pleistocene.
Did the Milankovitch cycles operate in the distant geological past? Probably. There is little direct evidence. Previous ice ages probably experienced similar climate variations to that of the Pleistocene, though the rock record does not resolve such small time cycles for the distant geological past.
Do the Milankovitch cycles operate only during ice ages?   No. Their effects are most obvious during the Pleistocene Ice Age because the advance and retreat of ice sheets leaves a strong signal.
In what ways have the signals of orbital forcing during the Pleistocene been detected?
TDE Figure 5.14 - Variations in sea level and the amount of ice in Northern Hemisphere ice caps, to 600,000 yrs.
TDE Figure 5.15 - Global sea level record to 1.6 Ma.
TDE Figures 6.22 & 6.23 - Cyclical changes in the Monsoons, especially as recorded in sediment records in the Arabian Sea (% CaCO3, % terrigenous sediment, total sediment accumulation, d 18O variations in biogenic carbonates).

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The Carbonate Compensation Depth

Facts and Frequently Asked Questions

Frequently Asked Questions Answers
How is the Carbonate Compensation Depth (CCD) defined? The CCD is the depth below which <20% of the sediment buried on the sea-bed consists of carbonate material.
Is the CCD everywhere and always the same? No. The CCD is determined by the concentration of dissolved carbonate [CO32-] in ocean waters and the acidity of ocean waters. High levels of dissolved carbonate will depress the CCD; high acidity will tend to raise it.
What processes will affect the CCD?
High organic productivity in surface waters will depress the CCD, by increasing the amount of carbonate material sinking through the water column, thereby raising the concentration of dissolved carbonate.
A high concentration of dissolved CO2 will increase acidity, tending to raise the CCD.
In what circumstances will marine limestones be deposited? Where the sea bed is above the CCD.
What happened concerning carbonate deposition during the Cretaceous (Ref Fig. 5.23, p. 173)?
Before 100 Ma, shallow water carbonate factories caused the deposition of limestones in shelf seas.
In deeper water, carbonaceous rocks (e.g. the Gault Clay) were deposited in anoxic conditions
Increased hydrothermal activity (superplumes) raised CO2 concentrations in the atmosphere and oceans, contributing to global warming and to anoxia in the oceans
At about 100 Ma, deep water carbonate factories become established with the evolution of coccolithophores.
CCD is depressed by high surface productivity in deep water, leading to deposition of more carbonate rocks. Much more efficient sink for CO2.
Anoxic conditions give way to oxic conditions in deep water, which further depresses the CCD
Is the Chalk a manifestation of the deep sea carbonate factory? Yes and No! The Chalk was deposited in shallow shelf seas, not deep sea basins. But it is composed of coccoliths, the calcareous skeletons of the same planktonic plants which were responsible for the deep sea carbonate factory – they flourished in the photic zone, regardless of the water depth beneath them!

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