On the previous Isotope page, you learned a bit about what isotopes are, how they are obtained, and how the isotopes of certain elements are measured. Here, we’ll elaborate on how to read carbon and oxygen isotope data and how the values are often interpreted by paleoclimatologists.

Reading Isotope Data

Measurements of carbon and oxygen isotope values of a sample obtained using a mass spectrometer are compared to a sample of known isotopic values, called a reference standard. The resultant isotopic signature of a sample is expressed using a delta (δ) followed by the isotope number and the symbol of the element being measured. Oxygen isotope measurements are read as δ¹⁸O, or delta oxygen eighteen, and carbon is read as δ¹³C, or delta carbon thirteen. These values are expressed as per mil (‰). The definitions of  δ¹³C and δ¹⁸O are as follows:

Thus, delta values of carbon and oxygen can be either positive or negative.

Interpreting Carbon and Oxygen Isotopes

Because both carbon and oxygen isotopes are measured simultaneously from one sample, the data are usually interpreted together. Oxygen and carbon isotope data from samples that are plotted, either against age or depth, are called ‘curves’ by scientists. There are several factors that influence carbon and oxygen curves, but below, we will focus on carbon and isotope curves obtained from planktic and benthic foraminifera, and a few of the ways these curves can be interpreted. Carbon isotopes are a bit more complex, so we have only included a few of the ways they can be interpreted. As a reminder, planktic foraminifera live near the surface of the ocean (in the mixed layer or the upper thermocline), and benthic foraminifera live at the ocean bottom, with some species living within the sediment on the seafloor.

Oxygen Isotopes

Evaporation and precipitation are two factors that most influence the ratio of heavy (oxygen 18; O¹⁸) to light (O¹⁶) oxygen in the oceans. When seawater evaporates, O¹⁶ is preferentially uptaken because it is lighter, while the heavier O¹⁸ is left behind. When water vapor condenses, the heavier oxygen leaves first, as precipitation, before the lighter oxygen.

During different times in Earth’s history, the oceans had more O¹⁸ relative to O¹⁶. We’ll briefly discuss how these oxygen isotope values are interpreted by paleoclimatologists.

Increased Oxygen 18 Values
Water evaporated from the ocean surface is enriched in O¹⁶ (it has a lighter or more negative δ¹⁸O signal). When this water precipitates out as snow at the poles, it becomes trapped on land, compacting over time to create ice. When Earth is cool enough year-round that ice caps are permanent, the ice on Earth’s surface is enriched in O¹⁶ relative to O¹⁸. This happens as more and more of the lighter O¹⁶ is locked up in ice (the water has a more positive δ¹⁸O signal). During colder times in Earth’s history, the oxygen isotope value extracted from the shells (tests) of benthic and planktic foraminifera shells have heavier, or more positive, δ¹⁸O values. The foraminifera will preferentially create their tests out of the lighter O¹⁶ unless it is not available and during these cold intervals, O¹⁸ is more abundant. Warmer intervals are indicated by lighter, or more negative, δ¹⁸O values.

Decreased Oxygen 18 Values
When the Earth begins to warm, ice caps begin to melt back, and if the Earth warms further, there are hardly any permanent ice sheets at the poles. When this happens, the O¹⁶-enriched ice returns to the ocean as water. A huge influx of water enriched with O¹⁶ creates ocean waters that then become diluted with respect to O¹⁸. On the chart on the right, warmer times are indicated by more negative (or ‘lighter’) O¹⁸ values (more negative δ¹⁸O), indicating a reduction in the amount of ice on the Earth.

The above scenario applies to the global oxygen values of the oceans. Isotopic values of oxygen can also be measured from different regions and compared for the same time in Earth’s history. Regionally negative δ¹⁸O signals (more O¹⁶) can indicate that an area experienced increased rainfall.

Benthic Foraminifera
Because the ocean bottom water is more homogeneous, or well-mixed, compared to the surface ocean water, benthic foraminifera record ‘global’ isotope values. The figure at the right show the δ¹⁸O values from benthic foraminifera for the last 10 million years. Because we are geoscientists, we read time from the oldest to the youngest, just like you read English from the left to right. Thus, to interpret what is happening through time, begin at the bottom of the curve and ‘read’ up, towards younger dates. Notice at the bottom of the curve, values are in the red color with more negative values. This indicates that at this time in Earth’s history, the Miocene, the bottom ocean temperature was much warmer, and there was much less ice at the poles. As you move up the curve, the values fluctuate quite a bit, but notice how the curve, in general, trends to the right in the blue area. This means the values are becoming more positive, which indicates that there are more O¹⁸ molecules in the water, which is interpreted to mean the Earth was cooling down during this time, and there was more ice at the poles.

Planktic Foraminifera
Oxygen curves from planktic foraminifera record a more local signal than benthic foraminifera. This is because the surface ocean is very dynamic (it’s always changing) compared to the deep ocean. In addition, it is not as homogeneous (well-mixed) as the bottom of the water column. Curves from planktic foraminifera can be interpreted the same as benthic foraminifera in general, but the effects of evaporation and precipitation can also affect the isotope signal. Evaporation causes a decrease of O¹⁶ ions and thus an increase of O¹⁸ in the water column, which causes the curve to move towards more positive δ¹⁸O values. Precipitation increases the amount of O¹⁶ ions in the water column, which causes the curve to move towards more negative δ¹⁸O values.

Carbon Isotopes

Carbon isotopes obtained from the shells of marine organisms are strongly influenced by photosynthesis, respiration, and upwelling of ocean waters, the process by which older, more C¹²-rich waters are brought from the bottom ocean to the surface. Therefore, when a planktic foraminifera builds its shell in an upwelling area, the δ¹³C signal will be more negative. There are two main isotopes of carbon that we are concerned with in this website: carbon 13 (C¹³) and carbon 12 (C¹²). Just like oxygen isotopes, the carbon isotope with more neutrons (C¹³) is heavier, whereas C¹² is lighter.

Throughout Earth’s history, the amount of C¹² and C¹³ in the atmosphere and in the oceans has changed, which makes working with carbon isotopes a bit trickier than oxygen. Thus, we’ll just briefly discuss some of the ways carbon curves can be interpreted without bogging you down with too many technical terms and background information (although if you are interested in learning more about this, please contact us!).

Increased Carbon 13 Values
Increased, or heavier, δ¹³C values, generally indicate increased productivity in the ocean. Photosynthesizing organisms, such as algae and plankton, preferentially uptake C¹² during photosynthesis, which leaves more C¹³ in the water column with which marine organisms build their shells. When there is a lot of photosynthesis happening in the water column geoscientists refer to this as a time of increased productivity. When there are times of increased productivity (lots of things growing and photosynthesizing in the water column), this usually means there is increased burial of carbon, or sediment accumulation on the seafloor (because there’s lots of things pooping and dying and falling down to the seafloor).

An increase in δ¹³C values can also indicate that erosion from land (the terrestrial realm) is decreased. Soil tends to have a more negative δ¹³C value because it contains remains of dead plants (that are generally made of C¹² ions). When this soil washes into the ocean, it puts more C¹² in the waters.

Decreased Carbon 13 Values
Alternatively, when there is very little photosynthesis occurring in the water column, there are more C¹² molecules in solution, and thus more are incorporated into the shells of marine organisms. This can also correlate with times of decreased burial, because there are not as many organisms living in the water column (less poop and fewer dead organisms falling to the seafloor).

δ¹³C values can also become more negative due to increased erosion from land into the ocean (see above paragraph). CO₂ from volcanoes tends to also be very enriched in C¹², so times in Earth’s history that are characterized by intense volcanism can create more negative δ¹³C values.

Benthic Foraminifera 
Carbon curves created from benthic foraminifer shells tell geoscientists something about the bottom water conditions in the ocean. Respiration, or the decay of organic matter (remember the poop and dead organisms we talked about earlier?) tends to release more C¹² into the bottom waters. As water masses move along the seafloor, they pick up this C¹² signature, which gets incorporated into the benthic foraminifer’s shell. Thus, a benthic foraminifer’s δ¹³C value can tell us about the age of the bottom waters (older bottom waters that have picked up more C¹²= more negative δ¹³C values; younger bottom waters= more positive δ¹³C values). The δ¹³C curve at left is a global stack (lots of carbon curves from all over the world were plotted together) from benthic foraminifera. Again, we read the figure from the bottom (oldest) to the top (youngest). Notice that δ¹³C values, in general, from 10-0 million years become more negative. This signal is in part from the bottom ocean waters becoming older as the modern-day deep-water ocean circulation began (youngest waters sink in the Arctic and travel along the bottom of the water column into the Pacific Ocean).

Benthic foraminifera carbon values also tell us something about sea level through time, as related to weathering. When sea level is high, this generally leads to less weathering, or less transport of soils (which, remember, are enriched in C¹²), into the oceans, and thus is recorded as a more positive δ¹³C signal. Low sea level generally correlates to higher global erosion rates, and thus a more negative δ¹³C signal. On the chart at left, notice that during 10-6.5 million years ago, δ¹³C values are generally more positive. As you move into the modern towards 0 million years, the values are more negative. If you look back at the oxygen isotope chart, you’ll notice that these relatively negative carbon values correlate with times of cooler climate and more ice. When there is more ice, this leads to a drop in sea level. Thus, the more negative values on the carbon isotope curve are partly due to a sea level fall from increased ice sheets at the poles, which led to increased weathering.

Planktic Foraminifera
Similar to oxygen isotopes, carbon isotopes obtained from planktic foraminifera are generally interpreted as a more local signal, as the surface layer in the ocean is not as homogeneous (well-mixed) as the bottom waters. Carbon isotope values in planktic foraminifera shells are mainly affected by photosynthesis and upwelling. When more organisms are photosynthesizing in the upper water column (increased productivity), this leads to more C¹³ ions available in the water for foraminifera to build their shells (because the C¹² ions are being used for photosynthesis). When there is little to no photosynthesis happening in the water column, more C¹² ions are available for the foraminifera to build into their shells, causing the δ¹³C signal to become more negative.

To learn more about oxygen and carbon isotopes, visit the following sites:

• Harvard University Isotope Analysis
• University of Oregon Oxygen Isotopes in Paleoclimate Studies
• NASA Paleoclimatology: the Oxygen Balance