Isotope Evidence

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Early Cretaceous


Mid-Cretaceous



Late Cretaceous



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Sedimentological
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Isotope Evidence



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Isotope Evidence


Water, the compound making up the oceans of the Earth, is composed of the elements oxygen and hydrogen. These elements have several similar forms, known as isotopes. For example, the isotopes of oxygen are oxygen-16, oxygen-17 and oxygen-18, where the number refers to the number of protons and neutrons in the nucleus.

Fluorine, the element above oxygen in the periodic table, has 9 electrons in its outer shells and a nucleus of 8 protons and 9 neutrons.

It is not a problem if you do not understand this. What is important is to understand the measurable effects that these isotopes create.

Oxygen isotopes

In the hydrologic cycle, evaporation preferentially removes water with oxygen-16 (i.e. light oxygen). This oxygen isotope is therefore rich in the gas phase of water in clouds. Precipitation and runoff returns water with high oxygen-16 to the Earth's surface. During glacial epochs, this precipitated oxygen-16 is preferentially stored in polar icecaps and continental ice sheets. This leaves the oceans enriched in oxygen-18. In the opposite case, when the cliamte is very hot, icecaps do not exist, and oceans are no longer enriched in oxygen-18.

The stable isotopes of oxygen are used to reconstruct palaeoclimates. The abundance of oxygen-18 compared to oxygen-16 is displayed in a ratio of the the two isotopes. The relative value of this ratio is compared to a standard so that the climate change with respect to time can be measured.

The ratio of isotopes (signatures) can be recorded in the rocks that are forming at that time. For example, forams create their shell of calcium carbonate from the water and food they consume. When they die and their body sinks to the bottom of the ocean, this isotope signature is preserved in these shells. Sediment accumulates and eventually forms a rock which can become uplifted and be available for study.

However, not all forams precipitate shells in isotopic equilibrium with water. Isotope geologists have to be careful to select those that do.

The most continuous record of marine temperature variations for the Late Cretaceous has been construed from isotope analyses of well preserved forams in deep-sea sediments. Oxygen isotope ratios of well preserved marine calcareous fossils are indicative of the temperature of ancient ocean waters.

This approach is based on the fact that the difference in oxygen-18:oxygen-16 ratios between calcium carbonate and the water from which it precipitates is a function of temperature. The value for calcite increases as temperature decreases. The isotope diagram) displays this relationship and summarises the isotope evidence results.

Evidence from isotopic measurements of deep-sea deposits indicate that intermediate and deep waters in the oceans were 15 to 20 degrees warmer than now.

Evidence for change

Palaeotemperature trends from shallow-marine bivalves from northwest Europe are similar to those for the Pacific low-latitude surface waters (shown in the figure). This suggests that the decline in ocean temperatures over the Late Cretaceous was global.

further information on the subject.


Sr isotopes

The chemical similarity of strontium (Sr) and calcium (Ca) means that the calcium carbonate shells of forams contain high concentrations of Sr (up to 1000 parts per million). There is no fractionation of heavy isotopes so foram shells record the strontium isotope ratio of seawater. Therefore it is possible to use the sediment record to study the evolution of strontium isotopes in oceans.

There are two major inputs of Sr into oceans:
  • sediment carried to the sea by river water,
  • hydrothermal fluid from mid-ocean ridges.
Therefore, the change in the strontium isotope ratio in the sediment reflects changes in the amount of:
  • terrestrial erosion
    • (which is influenced by climate)
  • seafloor spreading rates
    • (which can affect climate through carbon dioxide release and sea level through the buoyancy of the sea floor).
 
 

| Intro | Early Cretaceous | Mid-Cretaceous | Late Cretaceous |

| Palaeontological Evidence | Sedimentological Evidence | Isotope Evidence |

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