Understanding Climate Change Part 7 - Oxygen Signals and Paleoclimates

Oxygen Signals and Paleoclimates

(Reading #7 for my course on Climate Change, Alan Holyoak, PhD)

Note: This is the last reading in this series.  This set of 7 readings is designed to help college general education students gain the the foundational background they need to understand the contents of the book "The Climate Crisis" by Archer and Rahmstorf, which I use as a course textbook.  

Daily Objectives

1.     Be able describe the differences between heavy and light oxygen isotopes. 

2.     Be able to explain why ice cores from polar glaciers contain meaningful climate records.

3.     Be able to explain why speleotherms, wood, and coral contain meaningful climate records.

4.     Be able to explain why paleoclimatologists probably get excited when they find skeletons of foraminiferans in their sediment samples.

5.     Be able to comment on the general pattern of global temperature change over billions, millions, and hundreds of thousands of years.

Introduction

            A way tool to gain insight into current patterns of climate change is to learn as much as possible about Earth’s climate history.  This field of study is called paleoclimatology.   Paleoclimatologists collect data from as many sources as possible to help them develop a picture of what Earth’s climate was like in the past.  These observations range from glacial erratics and glacial striations that you have already learned about to oxygen and carbon isotope ratios in ice, water, and sediments, among other things.  In this reading you will learn about the data that paleoclimatologists collect to investigate Earth’s climate history.  One important thing to keep in mind is that the farther we look back in time, the greater the range of uncertainty there is in the data.

Climate Clues

            Just like a detective, paleoclimatologists are good observers and creative thinkers.  This helps them identify and make sense of the climate clues that exist in Earth’s historical record.  You already learned how boulders and rocks that seemed out of place and strange scratches on rock faces led to Louis Aggasiz’s 1837 Ice Age Theory.  Similarly, other good observers discovered other ways to tease more clues about Earth’s climate history from sources such as ice, wood, ocean sediments, and even stalagmites. 

Climate and oxygen isotopes

            All isotopes of an element have the same number of protons, but different numbers of neutrons.  This difference in neutron number gives each isotope a unique atomic mass.  There are three isotopes of oxygen, ¹⁶O, ¹⁷O, and ¹⁸O.  These isotopes are not radioactive so their global concentrations are stable.  Their characteristics are listed in Table 1.

Table 1. Characteristics of isotopes of oxygen

Oxygen Isotope	Number of Protons	Number of Neutrons	Proportion of all oxygen atoms
Oxygen-16 (16O)	8	8	99.76%
Oxygen-17 (17O)	8	9	0.04%
Oxygen-18 (18O)	8	10	0.20%

            Paleoclimatologists are interested in finding light oxygen (¹⁶O), and heavy oxygen (¹⁸O) in compounds in samples they collect.  They use the ratio between ¹⁸O and ¹⁶O atoms in oxygen-containing compounds as proxy data for temperatures of past time periods.  Fortunately these kinds of compounds are found in ice, sediments, fossils, and other long-lived substances.  The heavy to light oxygen ratio is calibrated against a standard of heavy to light oxygen isotopes in seawater 200-500 meters deep in our ocean today.  The ratio between light and heavy oxygen-containing compounds at this depth correlates extremely well with average surface seawater temperature.  It therefore stands to reason that whenever we find oxygen ratios similar to those in today’ oceans and associated seawater temperatures, the same oxygen-ratio/seawater temperature relationship should have existed in oceans throughout history.

            The vast majority of water molecules are made with light oxygen, but the rest contain heavy oxygen (Table 1).  Scientists discovered that the rates of evaporation and condensation of water with high and heavy oxygen are not identical; water with light oxygen evaporates slightly more readily than water with heavy oxygen, and water with heavy oxygen tends to condense and fall as precipitation before most of the water vapor that is made of light oxygen (Fig. 1). 

Figure 1. Relationship between temperature and heavy oxygen (¹⁸O) concentration in precipitation.  These data show the percent divergence from the standard ¹⁶O:¹⁸O ratio in the ocean.  Negative values mean there is less ¹⁸O present than in the ocean standard. (Image: NASA)

            When air cools water vapor condenses and falls as precipitation. ¹⁸O has a greater mass than ¹⁶O and water made with heavy oxygen therefore has a greater mass, and as subject to a greater gravitational force than something that has a lower mass.  As a result, water made with ¹⁸O falls more readily than water made with ¹⁶O.  Water vapor left in the atmosphere at this point is partially ¹⁸O-depleted.  This depletion happens faster when temperatures are low than when they are high.  Therefore, when the Earth is in a cool phase most of the ¹⁸O-water precipitates out before water vapor reaches the poles, leaving mostly ¹⁶O water to fall as snow to form ice layers there.  Conversely, when the Earth is in a warm phase ¹⁸O-water stays in the air longer, and more ¹⁸O reaches the poles than when the Earth is cool.  This produces a ¹⁶O to ¹⁸O ratio in polar ice that contains elevated levels of ¹⁸O when the Earth is warm (Fig. 2).  Scientists have measured the ratios of oxygen isotopes in ice layers from polar ice caps to produce accurate records of climate change going back as far as the ice record.  These data currently go back 800,000 years. Fortunately, we can look even farther back using other kinds of data, because oxygen isotope ratios go only so far back using ice alone. 

Figure 2. Water rich in both heavy and light oxygen evaporates at the equator, but as air moves away from the equator it cools, and heavy oxygen water falls as rain at a faster rate than light oxygen water.  During an ice age polar ice there will be significantly less ¹⁸O in the ice than when the Earth is in a warm phase.  (Image: NASA)

Before we look at other things that contain oxygen ratio data, however, there is something else of interest that oxygen ratios in water can tell us.  These ratios can tell us whether a particular time period was wet or dry.  This is the case because most of the heavy oxygen tends to condense first and fall as precipitation over oceans, leaving mostly light oxygen water to move onshore to fall as rain over continents.  So, if sediments in a region of ocean have increased amounts of ¹⁶O in it, this is almost certainly the result of freshwater runoff from continents.  When this is observed we conclude that Earth was experiencing a wet climate. 

Diversity of Oxygen Records

Ice was pointed out in the previous section as an extremely important source of climate data.  Why is this the case?  Scientists drill cores of ice from polar glaciers (e.g., Antarctica and Greenland) and collect date-specific data from water and other materials trapped in ice (Fig. 3).  The cores from this drilling show distinctive layers that are produced annually (Fig. 4 & 5).  Scientists take samples from each layer, analyze the ice for oxygen isotope ratios and other materials trapped in the ice, and can thereby determine the temperature of the Earth when the ice was formed.

Figure 3. Drilling ice cores from polar ice caps.  (Image courtesy of NASA.)

Figure 4. This photograph was taken when scientists in Antarctica dug a trench, but left a thin wall of snow between the two halves of the trench.  Light illuminating the wall clearly shows the annual layering of snow, which eventually gets compacted into ice layers. The stuffed animal is included for scale. (Image courtesy of NASA.)

Figure 5. These images show ice cores from different depths within a polar ice cap.  The upper image clearly shows layers of ice in the exposed side of an ice sheet.  The lower image shows layers of ice from ice cores, and that they can be quite distinctive, depending on their depth and age, as well as anything else that is trapped in the ice. (Images: NASA.)

            Oxygen isotopes are found in more materials than just ice.  It is also stored in wood, shells, bone, coral, and some kinds of rocks.  One extremely important source of temperature data is stored in the microscopic skeletons of tiny organisms called foraminiferans.  These small organisms are related to amoeba, but they produce calcium carbonate (CaCO₃) shells.  The shell of each species of foraminifera has its own unique shape and sculpturing (Fig. 6).  This means that whenever a scientist spots foraminiferan shells, they can know what species of foraminiferan they are looking at, and whether those foraminifera were warm-water or cold-water species.  Looking for the presence of warm and cold-water species, and the oxygen isotope ratios in their shells in sediment layers provide important clues about paleoclimates.  This is one of the reasons why climatologists drill sediment cores as well as ice cores (Fig. 7).

Figure 6. Scanning electron photograph of shells of foraminiferans. 

Figure 7. Drilling ships like the one above are used to collect sediment cores for analysis.  Scientists slice the sediment cores, do chemical analyses of each layer, and look for foraminifera and other evidence of climate change. (Image: NASA.)

            Paleoclimate records also exist in stalagmites that are formed in caves and caverns (Fig. 8).  These kinds of climate data are called speleotherms.  How can speleotherms contain meaningful climate information?  Structures in caves are largely isolated from the surface and do not experience processes of erosion like rocks on the surface do, so once materials are deposited there they remain there indefinitely. 

Figure 8.  Giant stalagmites in Carlsbad Caverns, New Mexico. (Photo: Wikimedia Commons.)

Stalagmites are produced by water that trickles through the soil and rock layers above the cave.  As water moves through the soil some materials, like CaCO₃ dissolve in it.  When water drips into caves and then evaporates, the CaCO₃ is left behind.  A new layer of CaCO₃ is added to a stalagmite each year, so if you slice through a stalagmite and polish the cut edge the individual layers are visible (Fig. 9).  Just like tree rings, the rings in speleotherms are wider during wet years and narrower during dry years, and samples can be taken from each layer for analysis.  But, how can we know the ages of the layers of a stalagmite?  Again, think back to FDSCI 101 and your discussions on radiometric dating.  That works here too!

            As indicated in the text below Fig. 9, Uranium-Thorium radiometric dating can be carried out on a sample from each layer of a speleotherm.  Uranium readily dissolves in water and is deposited along with CaCO₃ then the water evaporates.  Thorium, however, does not dissolve as readily in water, so all Thorium in a layer is the result of Uranium decay, and can be used to calculate an accurate age for each layer.  Pretty slick, huh?

Figure 9. This is a photograph of a cross-section cut through a small stalagmite.  Each band represents one year of deposition of chemicals from water dripping from above.  Samples from each section can be used to measure oxygen isotope ratios and to carry out radiometric dating, as indicated in the text below the photo.  (Image courtesy of NASA.)  

            Climate is, of course, more than just temperature.  It is also includes the pattern of precipitation area experiences.  Figure 10 shows the relative precipitation record for the region around Carlsbad Cavern, NM, based on speleotherm data.

 Figure 10.  A record of stalagmite ring thicknesses from Carlsbad Caverns, NM.  The ring thickness provides a record of relative amounts of rainfall in that area over the past 450 years. It is typical for any natural system to exhibit variability around the overall trend. (Image courtesy of NASA.)

            Scientists can also use tree core data to decipher recent climate changes.  As mentioned about in relation to speleotherms, trees in temperate and Polar Regions produce a pair of rings of wood each year.  In years when conditions promote abundant growth trees produce a thick tree ring.  In years when conditions limit growth trees produce a narrow ring (Fig. 11).  By producing overlapping tree ring records scientists have produced climate records extending back thousands of years.  Bristlecone pines are among the known longest living trees, and a climate record using their tree rings has been produced that goes back over 5,000 years (Fig. 12).

Figure 11. This photograph shows part of the cross section of a trunk from a tree.  The tree rings vary in width depending on local climate conditions; wide rings indicate favorable growth years, and narrow rings indicate years in which growth was limited. (Image: NASA)

Figure 12. Precipitation record from overlapping Bristlecone Pine tree ring data (blue line) compared to the average precipitation from the 20^th Century (tan line).  Scientists apply knowledge about the relationship between precipitation, temperature, soil quality, and other factors and tree ring thicknesses in living trees to reach conclusions about past climate conditions.  (Image: NASA)

While speleotherms and tree rings are useful for reconstructing terrestrial climate conditions, they do not help us understand how climate changes affect conditions in the ocean.  Fortunately, while trees produce annual tree rings, corals also produce records of annual growth that is recorded in their CaCO₃ (Fig. 13).  The CaCO₃ secreted by corals occurs in the upper levels of the ocean, so we can analyze each layer of coral skeletons to discover the ¹⁶O and ¹⁸O ratios to see how climates changed in the tropics during recent history (Fig. 14).

Figure 13. This x-ray of a cross section through a coral colony’s skeleton exposes the annual layers of growth.  The heavy and light oxygen ratios in the CaCO₃ that makes up each layer provides a signal that can be used to generate a climate record extending back as far as the age of the colony.  Some colonies are known to have lived well over 1000 years. (Image courtesy of NASA.)

Figure 14.  Oxygen isotope ratios were used to generate a temperature profile for ~1890-1950 (lower graph).  These data are compared to periodic ENSO (El Nino Southern Oscillation) oscillations that occur in the tropical Pacific Ocean to see if correlations exist (upper graph).  Red areas represent temperatures above the long-term average, and blue areas show cooler temperatures.  Dark gray vertical bars represent strong El Nino events, and light gray bars represent weak El Nino events. (Image: NASA.)

Climate Patterns in Deep Time

            Paleoclimatologists use data from sediment cores, ice cores, speleotherms, tree rings, coral growth layers, and other data, to reconstruct Earth’s climate history.  Several groups of climatologists, working independently, generated climate models using available data to provide estimates of past climates throughout Earth’s history.  While these models represent the best science on this topic, the farther we go back in time, the less precise the scenarios tend to be. Earth history with only a relative indication of temperature extremes.  The reconstruction of Earth’s climate history is shown in Fig. 15.  This figure is based on sea level change, proxy data of CO₂ concentrations in the atmosphere, and other proxy data from the geologic record.  Figure 16 shows a model of CO₂ levels from 400 Ma (million years ago) compared to proxy temperature data.

Figure 15. This figure shows a deep time reconstruction of Earth climate history.  Earth has oscillated between cold and warm periods throughout its history.  Cold periods are indicated in blue and warm periods in…peach J? Observe that the time scale on the left side of the figure is not linear.  It is roughly logarithmic, with older time periods being allocated less space than recent time periods.  This is appropriate since we have much better data for recent time periods than for more ancient times, but if one is unaware of this it can give someone a skewed view of earth’s climate history.  (Image from WW Norton, adapted from Kump et al. 1999.)

Figure 16. Model of paleo-CO₂ levels, comparing the model to other predictors of climate.  (Image from Archer and Rahmstorf.)

            Climate reconstructions from deep geologic time are helpful in showing general trends, but if we hope to identify current climate trends and make sense of what is happening now, we must compare what is happening today with more recent climate histories.  For example, by using oxygen isotope data from ocean sediments we can generate a relatively precise record of climate from 65 Ma.  This reconstruction is shown in Fig. 17.  Many climatologists who have studied the Paleocene-Eocene Thermal Maximum (PETM) have suggested that evidence of events before and during this thermal maximum around 55 Ma are similar in some respects to trends we are experiencing today.

Figure 17. Reconstruction of past climate using evidence from the deep ocean, including oxygen isotope ratios and other evidence from sediment cores. (Image from Archer and Rahmstorf.)

            As shown in Fig. 17, Earth has, with a few ups and downs in global temperature, undergone gradual cooling over the past 65Ma.  As we look at progressively shorter time intervals we see that over the 400,000 years that Earth has experienced a temperature equilibrium including oscillations between cool and warm periods, i.e., glacial and interglacial periods. 

Figure 18. The past 400,000 years of Earth’s temperature variability.  Four prolonged glacial periods are indicated in blue, and warm interglacial periods are shown in red. (Image adapted by ARH from a figure courtesy of NASA.)

            The bottom line when it comes to Earth’s climate history is that climate has changed in the past, it’s changing today, and it will continue to change in the future.  This conclusion is evidenced by the fact that we have seen periods of Earth’s history where it was probably a lot like a giant ice ball, and other times when it was like a giant hot house.  Geologically speaking, recently Earth has experienced a more or less stable climate cycle of warm and cool periods.  So, when we begin to consider the current state of the climate, this recent trend is the most meaningful baseline we have for comparison to what we see today.  Deviations from Earth’s recent past stable climate conditions deserve to be investigated.  Those are, of course, topics for other class discussions, and we will address them later in the semester. 

Source material

Archer, D., and S. Rahmstorf. 2010. The Climate Crisis. Cambridge University Press.

Riebeck, H. 2005. Paleoclimatology. NASA Earth Observatory Program. http://earthobservatory.nasa.gov/Features/Paleoclimatology/