Paleoclimatology: Using Proxies and Ocean Sediment Cores to Reconstruct Climate

How Ocean Sediment Cores Record a History Of Climate

Paleoclimatology is the study of past climates. We cannot go back in time, but imprints created in the past, known as proxies, allow us to interpret and investigate paleoclimate. These indirect measurements are from natural sources. Useful climate proxies include organisms (e.g., diatoms, foraminifera and coral), ice cores, ancient pollen, tree rings, and sediment cores.

A combination of different types of proxy records can be used to reconstruct past climate. How far back the paleoclimate record extends depends on the type of proxy data used.
These records can then be combined with observations of Earth's modern climate and fed into computer models to infer past climates and predict future climates.

Part a: How Ocean Sediment Cores Record a History Of Climate

Deep sea sediment cores tell a fascinating story of past climates. Most of the information in these cores comes from microscopic fossils contained within the sediments. Foraminifera (or “forams”) and diatoms are single-celled, shelled organisms found in aquatic and marine environments. The “shells”, or tests, of both forams (CaCO3) and diatoms (SiO2) record evidence of past environmental conditions. When the organisms die, their remains sink to the bottom of the lake or ocean and are buried and preserved by the sediments. Scientists can then painstakingly extract these remains, or microfossils, from sediment cores and use them to infer information about past climates. For example, foram tests twist in different directions depending on the temperature of the water at the time of growth. By analysing the number of tests that twist each direction, scientists can determine the temperature of the surface water at the time that the forams grew. Population dynamics can also be used to infer past climate. For instance, the relative abundance of a particular species and the species composition at a certain site may be indicators of past environmental conditions.

Another powerful technique involves measuring the oxygen isotope ratios of the microfossils. Atoms of an element can have different numbers of neutrons in their nucleus. These are known as isotopes. For example, an oxygen-16 (16O) isotope has 8 neutrons and an oxygen-18 (18O) isotope has 10 neutrons, but they have the same number of protons.
Thus, 18O is slightly heavier than 16O. Both are found naturally in the atmosphere and in water, but the vast majority of all oxygen on Earth is 16O.

The ocean contains both H216O and H218O. Forams and diatoms use oxygen from the water to create their tests and are therefore good indicators (or proxies) of water chemistry at the time of shell formation. During glacial periods, the ocean becomes enriched in 18O (i.e., the 18O/16O ratio increases) and the shells of the organisms become enriched in 18O as well. During non-glacial (warmer) periods, the 18O/16O ratio is closer to 1 (i.e.,
“normal”) and this ratio is again reflected in the
chemistry of the microfossil shells.

The Use of Microfossils as Climate Proxies

Instructions: The age of ocean sediments from off the coast of California and the 18O:16O ratio of marine microfossils found within the sediments is provided. Answer the following questions.

1.

a)Oxygen occurs in three stable isotopes: oxygen-16 (16O), oxygen-17 (17O) and oxygen-18 (18O). Which isotope is heavier?

b)The oxygen in water (H2O) is usually 16O, but sometimes 18O. 17O is very rare. Water containing which oxygen isotope would evaporate first? Why?

c)Water containing which oxygen isotope would be preferentially removed first by precipitation? Why?

Source: http://oceanleadership.org

2.Organisms such as forams and diatoms use oxygen from the water as one component of their shells. As a result, they record the oxygen chemistry of the water when and where they lived. Table 1 shows the age of ocean sediments in the Santa Barbara sediment core and the 18O:16O ratio of marine microfossils found within the sediments. Plot the 18O:16O ratio as a function of sediment age and connect the points with straight lines.
a)Describe your graph.

 

b)Which ages or time periods had the highest 18O:16O ratios?

 

c)Which ages or time periods had the lowest 18O:16O ratios?

 

d)What might have been happening climatologically that could explain this pattern?

 

e)What should happen to the 18O:16O ratios as global average temperature continues to rise? Explain your prediction.

f)Explain how the 18O:16O ratio indicates the amount of ice found on land.

3.Add information to the following diagram in order to accurately summarize the oxygen isotope relationship during glacial (cooler) periods. Hint: Assume that the majority of the evaporation is occurring at the equator and that precipitation occurs as it moves to the pole. Also assume that the glaciers are located at the poles.

 

4.Add information to the following diagram in order to accurately summarize the oxygen isotope relationship during non-glacial (warmer) periods. Hint: Assume that the majority of the evaporation is occurring at the equator and that precipitation occurs as it moves to the pole.

 

Part B: 

 

As snow falls, the particles and compounds that are in the air at the time are captured along with it. In cold areas (e.g., the poles or high altitudes), snow accumulates from one year to the next without melting. As a result, a continuous record of the constituents contained in the snow is created.

 

Ice cores are collected by drilling down from the surface of an ice sheet. As scientists analyze snow from greater and greater depths, a history of the compounds in the air can be reconstructed. Debris-filled layers can be used to date the core by correlating them with known geological events, such as volcanic eruptions. Pollen trapped within the layers of ice and the stable isotope ratios of the ice itself can also be analyzed to infer paleoclimate.

Inferring Past Climate Using Oxygen Isotope Ratios

 

At deeper depths, snow turns into ice due to the weight of the snow above it. During this compaction process, small bubbles of air are trapped within the ice. This trapped air is further analyzed and provides information about the composition of the atmosphere at the time the ice formed.

 

Sediments accumulate much slower than snow, so resolving short-term changes is much more difficult with sediment cores than with ice cores. One major advantage of using sediment cores is that they can provide records that are much longer (several million years) than those of ice cores (several hundred thousand years). Consequently, the climate information provided by sediment cores and ice cores is complimentary.

 

When carbon dioxide (CO2) increases in the atmosphere, more of the Earth’s longwave radiation is absorbed and re-radiated by this greenhouse gas. This causes Earth’s surface temperatures to increase. Analysis of air bubbles in ice cores drilled from polar ice sheets (e.g. the Vostok Ice Core in Antarctica and the Greenland Ice Cores) shows that atmospheric carbon dioxide concentrations have varied throughout geological time. However, there is now deep consensus in the international scientific community that the present rate of global warming is a direct result of human emissions of CO2 and other greenhouse gases.

 

Methane (CH4) is a very powerful greenhouse gas that is also trapped in the air bubbles of ice cores. Since methane has a relatively short residence time in the atmosphere (around 12 years), variations can be seen in the amount produced from decade to decade. Sources of methane include rice paddies, permafrost thaw, and domestic livestock. More recently, oceanographers have discovered vast deposits of methane hydrates (frozen crystals of natural gas and water) contained within ocean sediments. Methane hydrates are extremely sensitive—even small increases in ocean temperature could cause massive amounts of methane to bubble out of the sediments, rise to the ocean surface and release into the atmosphere.

 

Instructions: Appendix D provides a time series of carbon dioxide (CO2) and methane (CH4) derived from Antarctic ice cores, and temperature derived from ocean sediment cores, for the past 420,000 years. Answer the following questions.

5.What do you notice about the CO2 and CH4 curves over the last 400,000 years? Give the approximate maximum and minimum values for CO2 and CH4 before the present day values.

6.The year 1998 was actually “present-day” when this graph was created. What are the present-day values for CO2 and CH4 today (i.e., within the last year)? How do they compare with the last 400,000 years? Find the difference between the present day and past maximum CO2 and CH4 concentrations. Hint: you will have to look up the current concentrations online. Try http://www.csiro.au/greenhouse-gases/

Using Sediment and Ice Cores to Study Paleoclimate

 

7.List at least four specific human activities that are contributing to the high present-day CO2 and CH4 concentrations?

8.Describe the temperature curve over the last 400,000 years. Include the maximum and minimum values as well as the frequency of the fluctuations. Note: Temperature in Appendix D is actually the temperature change from the present localtemperature.

 

9.Is there a correlation between the temperature curve and the CO2 and CH4 curves? Describe.

10.Prior to the extraction of the Vostok Ice Core, a lack of data resolution made it difficult for scientists to determine whether the temperature was changing the greenhouse gas concentrations, or vice versa. Give a plausible explanation for both scenarios (i.e., first explain how increasing greenhouse gases could cause increased temperatures, then explain how increasing temperatures could cause an increase in atmospheric greenhouse gas concentrations)

 

11.How many ice ages are recorded in this data set?

 

12.Ignoring human influences, what do you predict will happen in the next 100,000 years based on the pattern of the curves prior to present day?

 

13.How do the present day concentrations of CO2 and CH4 change your prediction? Explain.

 

14.Based on your answers above, how might this figure be misleading if it was simply presented without an accurate title or explanation?

 

5.There is almost no doubt in the scientific community that human activities (e.g., land use changes and greenhouse gas emissions) are contributing to the increase in global average temperatures. What other natural causes may also have some effect? Give at least two examples.

 

Part C:

 

Tree-ring analysis, or dendrochronology, has been used to reconstruct the climate over decades, centuries and even millennia. In fact, the search for proxy climatic data was the original application of tree rings. The field was founded in the early 1900s by Andrew Douglass—an American astronomer who had a strong interest in studying Earth’s climate. He observed that variations in tree ring width seemed to be controlled by the climatic conditions and he theorised that they could be used as proxy data to extend climate study back further than was previously possible.

 

Each year, a tree adds to its girth. In environments with distinct seasons, the new growth often forms a visibly distinct “tree ring”. Each tree ring consists of one spring layer (light- coloured) and one late-summer layer (dark-coloured). Tree growth depends on local conditions such summer temperature or winter and spring precipitation. Because the amount of water available to the tree varies from year to year, scientists can often use tree- ring patterns to reconstruct regional patterns of drought and climatic change. Most trees

 

grow more during wet, cold years than during hot, dry years. As a result, the tree’s rings are wider during the wet, cold years. In contrast, drought or a severe winter can cause narrower rings. The growth layers can also record evidence of floods, insect attacks, fires, and even earthquakes.

 

Instructions: Use your investigative skills to answer the following questions.

16.Trees in tropical regions are generally not used in dendrochronological analyses. Why?

17.Observe the cross-sections on display around the lab. What is the dark ring in Sample A from? What causes the blue colouration in Sample B?

18.In temperate zones, why does spring/summer climate matter more than thewinter climate in terms of ring width?

19.What are two advantages and two disadvantages of the dendrochronological method?

 

Tree-ring analysis often requires the comparison, or cross-dating, of two or more cores in order to confirm the climatic conditions of a given year. Cross-dating also allows the climate record to be extended into the past if the tree-ring cores are all taken from the same region. Large databases, such as the International Tree-Ring Data Bank, house several thousands of tree-ring chronologies from sites around the world. Today, most chronologies are determined using core samples that are extracted from the tree using a straw-sized borer.

Instructions: In Appendix E you are given core samples from 4 different trees that grew in the same region. Use the cores to answer the following questions.

 

20.Knowing that Sample 1 was cored in 1990, determine: the age of each tree, the year their growth began (age of pith) and the year they were cored (age of bark). You may wish to cut the cores out of the paper so you can visualize their alignment. Note: The pith and bark are not counted when determining the age of the tree.