Isotopes

One of the many ways in which paleoclimatologists know past climate and ocean conditions is by using the chemical makeup of rock and fossil specimens. Remember that chemical elements are composed of some number of protons, neutrons, and electrons. Elements have a charged balance (neither positive or negative) because they have an equal number of electrons and protons. However, various chemical reactions in nature will cause elements to either gain or lose electrons, and the elements become positively or negatively charged. When this happens, the elements become ions. Positive and negative ions will attract each to form solids, some liquids, and some gases. When a solid dissolves in water, the positive and negative ions break apart and dissociate through the water. Most rocks and fossil-hard parts are made of ionic compounds.

For example table salt, sodium chloride, will dissolve in water forming the positively-charged sodium ion and negatively-charged chloride ion. This forms an aqueous (a water based) solution:

In the above equation, the (s) indicates a solid material (table salt), whereas the (aq) indicates that these ions are dissolved in an aqueous solution.

Two isotopes of oxygen. P indicates the number of protons; N indicates the number of neutrons.

Chemical elements are found in different versions, called isotopes. Isotopes are elements that contain the same amount of protons, but differ in the number of neutrons in their nuclei. For example, there are three isotopes of the element oxygen (O): Oxygen 16, 17, and 18. Each isotope of oxygen contains 8 protons, but differs in the number of neutrons.  An isotope number is a shorthand representation of its mass.  Because protons and neutrons are roughly equal in mass, an isotope’s number is equal to the sum of its protons and neutrons.  Therefore, oxygen 16 has 8 protons and 8 neutrons, oxygen 17 has 8 protons and 9 neutrons, and oxygen 18 has 8 protons and 10 neutrons.

There are two main types of isotopes that geoscientists use to interpret the ancient Earth: stable and unstable isotopes. An unstable isotope experiences radioactive decay, where the element will lose energy over time. Several radioactive isotopes occur naturally, and not all are bad or cause harm to humans. However, paleoclimatologists do not commonly work with these unstable isotopes. Instead, we use stable isotopes that are not undergoing radioactive decay.

Two of the most common stable isotopes that are used by geoscientists are those of carbon (C) and oxygen (O). Although there are several types of stable isotopes, we will mainly talk about carbon and oxygen obtained from planktic and benthic foraminifera, as these are very common in paleoclimatology (especially to study our oceans), but will also briefly touch on other proxies used for isotope analyses.

How are carbon and oxygen isotopes obtained?

Adriane pointing out a type of stalactite called ‘cave curtains’ while caving in western Ireland. These were formed by dissolved ions carried by groundwater into the cave, where they created new rock formations.

Paleoclimatologists obtain carbon and oxygen isotopes from calcite, a common variety of calcium carbonate, with the chemical formula CaCO3. In this formula, there are three elements: calcium (Ca), carbon (C), and three oxygen atoms (O). Calcite and calcium carbonate are common on the Earth and in the oceans, and can take several forms. Here we will talk briefly about the most common types of calcite used for isotope analysis.

Calcite is a component in many sedimentary rocks. When a sedimentary rock is composed dominantly of calcium carbonate, geoscientists call it a limestone. Limestone rocks are easy to erode compared metamorphic and igneous rocks. Calcium carbonate dissolves when exposed to acids. Because rainwater is slightly acidic, prolonged exposure to rain will chemically erode away limestone rock formations (or even a limestone statue for that matter).

A speleothem that is being sampled by a microdrill.

When this occurs, the dissolved ions from limestone are then carried by water into the soil, where they can eventually find their way to caves. Here, the limestone ions have space to drip into the cave and form new limestone formations in the form of stalactites and stalagmites (commonly referred to as speleothems). To analyze stable isotopes of carbon and oxygen from speleothems, they are cut out of a cave and taken to a lab, where they are sawed in half and polished. A microdrill is then used to drill tiny samples from defined intervals along the speleothem for isotope analysis.

Calcite is also used by marine organisms to build their shells and hard parts. Invertebrate animals (those lacking a backbone) have been using dissolved calcite ions to build their shells since at least the Cambrian (~550 million years ago). Common fossil groups that utilize calcite include brachiopods, trilobites, and ancient echinoderms, such as blastoids. Some extant (still living) animals, like sea urchins and oysters also build their skeletons from calcite. In addition, some protists, such as planktic and benthic foraminifera, use calcite to build their tests. Calcite-producing organisms record the values of carbon and oxygen in their shells, and can be analyzed for carbon and oxygen isotopes.

In rocks of Paleozoic age, scientists commonly obtain oxygen isotopes from another type of fossil: conodonts. These small, tooth-like fossils are all that remain of ancient eel-like organisms that represent some of the earliest chordates. Conodonts are commonly found in limestone rocks as these creatures swam in the seas in which the limestone was deposited. Unlike the calcareous brachiopods and trilobites that they lived among, conodont teeth are made of apatite, or calcium phosphate, with the chemical formula Ca3O8P2. These scientists can analyze conodonts to obtain oxygen isotopes.

Scientists can also use limestone samples taken directly from an outcrop to analyze isotopes of carbon and oxygen.  Obtaining these bulk carbonate samples of limestone typically involves finding a suitable outcrop of limestone, hammering away some chunks at defined intervals, and taking the samples back to the lab to analyze.

How are carbon and oxygen isotopes measured?

A mass spectrometer. The red arrow is pointing to the carousel, where samples are placed.

Once the appropriate material (limestone samples, speleothems, or fossils) is collected for isotope analyses, a small sample is put into a mass spectrometer to measure the amounts of carbon and oxygen isotopes within each sample. Each sample is loaded into a vial, and all the vials are then put into a carousel (see image at left, with red arrow pointing to sample carousel). Approximately three drops of acid are put into the vials to dissolve the sample, creating a gas that contains the ions to be measured. Ions are very reactive, so the measurements within the mass spectrometer take place within a vacuum. There are several different types of mass spectrometers, but one of the common ways to measure isotopes is to manipulate them by magnets and electric fields, and shoot them down a bent tube.

Because isotopes of elements differ in weight due to additional neutrons (for example, oxygen with 18 neutrons is heavier than an oxygen molecule with 16 neutrons), they will deflect at different angles in the tube. The degree to which the ions/atoms are deflected by a magnet is how heavy they are. A heavier ion/atom/molecule is harder for the magnet to deflect, so it will only turn slightly, while a lighter i/a/m has less inertia and is easier to turn.

Thus, lighter molecules are deflected more than heavier ones. This information is sent to a computer, which gives the researcher data on the amount of each isotope in every sample.

For a more detailed account of how mass spectrometry works, click here. For a video demonstration on how ions are deflected within a mass spectrometer, click here.

To learn how paleoclimatologists interpret carbon and oxygen isotopes, continue to the ‘Carbon & Oxygen Isotopes’ page!

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