Developed by Thomas (Tom) C. Lindsay Ph.D. © 2007 for UNST 253 Global
Environmental Change; Department of Geology, Portland State University, PO Box
751, Portland, OR 97207-0751
Over the next few weeks, you will use the mentor section to investigate climate over the last 500,000 years using “proxy” data from ice and sediment cores. You will be using the same data that Earth scientists use every day (these are some famous data sets). The data are all publicly available at data repositories like NOAA’s Paleoclimatology website http://www.ngdc.noaa.gov/paleo/paleo.html
. Funding agencies such as the National Science
Foundation require their scientists to archive data at public-access sites after the research has been completed. This ensures that the data will always be available and can be shared by many investigators, even us. (Note: Last week’s Reconstructed Temperature data was from this repository. bjs)
A climate proxy is an environmental variable, such as the relative proportion of Oxygen isotopes in seawater or the CO
2
content of the atmosphere, that varies according to changes in
Earth’s climate. 18
O/
16
O ratios change as water is transferred from the ocean to the ice sheets and back again. CO
2
variation is more complicated by the many feedbacks in Earth’s climate system, but in general, larger atmospheric CO
2
content accompanies warmer climates.
The data you will need for these investigations have been arranged in two Excel workbooks. You will use spreadsheets in the workbooks to make graphs of changes in the measured quantities over time and to interpret the data in terms of climate change. You will be directed toward fairly simple analyses but if you’d like to try something more involved, go for it.
Also available to assist in your interpretation of those data, are the results of celestial mechanics calculations that describe how the insolation (a measure of solar radiation energy received on a given surface area in a given time ) received at the top of Earth’s atmosphere changes over time.
This assignment is written in sections that serve as a guide through the data sets.
You will be working with the same data and this assignment guide for several weeks. At the end of the document you will find numbered questions that are intended to assist you in studying these data sets. You should write answers the questions using complete sentences and short paragraphs. Where the question asks for a plot from your data analysis, print the graph, number
1

it, and refer to that number in your answer. Your completed set of answers is due in the lecture section on March 5th .
Background
Oxygen Isotopes
Oxygen has three stable isotopes,
18
O,
17
O, and
16
O. The numbers (18, 17, and 16) signify the number of neutrons bound within the nucleus of the oxygen atom. The isotopes all have the same number of protons in their nuclei – that’s what makes them oxygen. The different numbers of neutrons give the isotopes slightly different atomic weights,
18
O is the heaviest and
16
O is the lightest. When oxygen is removed from a reservoir, the relative amount of each isotope removed – the isotopic ratio – depends on the temperature of the reservoir and the original ratios of the isotopes available. Of particular interest to paleoclimatologists are the oxygen atoms within water (H
2
O) and within calcium carbonate (CaCO
3
).
Water molecules containing the light
16
O isotope evaporate more easily than molecules with the heavy
18
O isotope. Thus, clouds are relatively enriched in
16
O compared to ocean water.
When the evaporated water precipitates back into the ocean, there is no change in the oxygen isotopic ratio in the ocean. When the evaporated water precipitates on land there is a net transport of
16
O from the ocean to the land, leaving the ocean relatively enriched in
18
O. If the cloud vapor precipitates as snow on an ice sheet, it may remain there for many thousands of years.
Oxygen isotopes are measured as ratios between heavy and light atoms. We can guess, however, that isotopic ratios on their own would be of little value because ratios in source reservoir change as land ice waxes and wanes. The remedy to this problem is to report oxygen isotope values relative to a standard water sample. The formula used is:
The ratio has units of ‰ (which means parts per thousand and is usually said “per mille”) is written as δ
18
O and pronounced “delta-18-O” or “delta-O-18.” More negative δ
18
O means more
18
O depletion.
2

Two factors, in addition to the amount of
18
O or
16
O available, affect the oxygen isotope ratio in the snow that falls on an ice sheet. One factor is temperature. At colder temperatures, relatively more
18
O is precipitated. The second factor is elevation, or distance from the original reservoir source. When rain or snow begins to fall from a cloud, the heavier
18
O is preferentially removed. Thus, as the cloud moves away from its source (say the ocean) it, and the rain or snow it produces, become relatively enriched in the lighter isotope,
16
O.
Oxygen also accumulates in CaCO
3
as microscopic marine animals called foraminifera,
(“forams”) grow and secrete their own shells. Oxygen isotope ratios in foram shells depend on the isotopic ratio and the temperature of the water in which they live(d). In cooler water, relatively more
18
O is used. When the forams die, their shells sink to the seafloor. Over time, the layers of sea-floor sediments, containing near-surface (planktonic) and deeper-dwelling
(benthic) forams, become a chronicle of changes in the ocean water. Harold Urey and Toshiko
Mayeda at the University of Chicago in 1947 demonstrated mathematically that the isotopic fractionation (a separation technique) during CaCO
3
crystallization was temperature-dependent.
In the 1950’s, Ceseare Emiliani, a student and then research associate in that same laboratory, demonstrated that oxygen isotope ratios in sea-floor sediments could be used to infer past ocean temperature and ice sheet volume. In so doing, he founded the science of paleoceanography.
The
18
O /
16
O ratios in ocean sediments record two variables: the volume of ice on land and the temperature of the ocean. One way to decipher the isotopic record is to make an assumption about the relative importance of the two effects. Another method is to study planktonic and benthic forams separately and assume that deep sea temperature changes very little over long time scales (100s of thousands of years). The latter approach was used to generate the long ocean δ
18
O you will use in the mentor section. A more positive δ
18
O indicates a relatively
18
O -enriched ocean. What does that mean in terms of the volume of ice on land?
The sea-surface record is a little more difficult to decipher than the deep-sea record because the planktonic foraminifera record changes in both isotopic content and temperature of the ocean. In this case, all the different species of foraminifera present in each time interval
(sediment layer in the core) are identified and counted in order to produce a profile of the foraminiferal population living during each interval (these are called assemblages). Because different species prefer to live in different temperature waters, the planktonic assemblages can be used as a proxy for sea-surface temperature.
3

Hydrogen Isotopes
Hydrogen has two stable isotopes: the common
1
H and a heavier
2
H that is usually called
Deuterium (D). Hydrogen isotopes area also fractionated with evaporation and precipitation in a process that is strongly temperature-dependent. Most of the water on Earth is H
2
16 O (99.77%).
Two other isotopic combinations, H
2
18
O and H
D
16
O, produce the isotopic variations that are of interest in ice cores.
Insolation and Earth’s Orbital Cycles
Three aspects of Earth’s orbit about our sun change over time. Eccentricity, the elongation of Earth’s elliptical orbit changes slowly, in 100,000–year cycles. Obliquity, the tilt of Earth’s axis relative to its orbital plane, varies in 41,000–year cycles. Precession, the
“wobble” of Earth’s rotation axis, varies in approximately 21,000–year cycles. Together, these orbital cycles generate cycles in the amount of solar radiation arriving at any location on Earth’s surface.
Mathematical models of Earth’s orbit are used to compute the variation of insolation over time. June and December insolation calculated at 1000-year intervals for 7 latitudes are included in the Excel workbooks you will use with this assignment. The latitudes are 90N, 60N, 30N, 0,
30S, 60S, and 90S.
The insolation data are from Berger, A., 1992, Orbital Variations and
Insolation Database. IGBP PAGES/World Data Center-A for Paleoclimatology Data
Contribution Series # 92-007. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.
Antarctic Ice Cores
Long records of local temperature (variation from the present, computed using δ
Deuterium), atmospheric CO
2
, atmospheric CH
4
, and dust flux are available from ice cores drilled at the Russian station Vostok, on the eastern Antarctic plateau (78¡ 30_ S, 105¡ E).
The Vostok data are arranged for you in an Excel workbook named paleoclimate_timeseries_V.xls
4

The data are made available to us by Petit, J.R., et al., 2001, Vostok Ice Core Data for
420,000 Years, IGBP PAGES/World Data Center for Paleoclimatology Data Contribution Series
#2001-076. NOAA/NGDC Paleoclimatology Program, Boulder CO, USA.
A good scientific paper from which you can learn more about the data is Petit J.R., Jouzel
J., Raynaud D., Barkov N.I., Barnola J.M., Basile I., Bender M., Chappellaz J., Davis J.,
Delaygue G., Delmotte M., Kotlyakov V.M., Legrand M., Lipenkov V., Lorius C., Pépin L., Ritz
C., Saltzman E., Stievenard M., 1999, Climate and Atmospheric History of the Past 420,000 years from the Vostok Ice Core, Antarctica, Nature, 399, pp.429-436.
Deep-Sea Sediment Cores
The discovery of δ
18
O variations in foraminifera was fundamental to development of the
Quaternary climate chronology. The deep-sea δ
18
O record is used to identify times of relatively more and relatively less ice on land while the planktonic record is used to study regional variations in climate. We will examine data from three ocean sediment core records and one
“stacked” record that represents the global ocean. The data derive from the SPECMAP project, an extensive deep-sea drilling and interpretation project conducted in the 1980’s. These data are still in use today and scientists still argue about conclusions drawn from them. They are archived by the National Geophysical Data Center, part of the National Oceanic and
Atmospheric Administration. A good reference for them is Oceanic Response to Orbital Forcing in the Late Quaternary: Observational and Experimental Strategies, 1989, Imbrie,J., A.
McIntyre, and A. C. Mix. in 'Climate and Geosciences, A Challenge for Science and Society in the 21st Century', A. Berger, S. H. Schneider, and J.-C. Duplessy, eds., D. Reidel Publishing
Company
The SPECMAP data are arranged for you in an Excel workbook named paleoclimate_timeseries_SP.xls. The global time series of ocean δ 18O are a composite of data from many sites. Sea-surface temperatures are from cores at three locations: core V27-116 52¡ 50_ N 30¡ 20_ W water depth = 3202 m core V30-49 18¡ 26_ N 21¡ 5_ W water depth = 3093 m core RC11-120 43¡ 31_ S 79¡ 52_ E water depth = 3913 m
The “warm” and “cold” temperatures recorded in these data sets are interpretations based on the foraminifera assemblages (groups of species) found at different depths within the cores. The
5

warm value represents the estimated summertime temperature and the cold value represents the wintertime temperature inferred for each foraminiferal assemblage, according to a wellestablished transfer function.
Open the two workbooks in Excel and familiarize yourself with the data contained in each worksheet. Individual worksheet labels report their contents. Within each worksheet, the column headings indicate the data type and measurement units. The time represented by each measurement is always presented in thousands of years but may sometimes also be presented in years. You will notice that different records are of different lengths. Why do you suppose this is? For questions in which you are asked to plot data, be sure that the figures you produce could be understood by anybody who looks at them. Label your axes. Include a caption for each graph. Please type all responses in 12 point font with 1” margins.
Question 1 . How does the
18
O content of the ocean change as glaciers and ice sheets grow?
How does the ratio between
18
O and
16
O change as glaciers and ice sheets grow?
Question 2.
How do you suppose scientists developed the “transfer functions” to interpret foraminiferal assemblages in terms of sea-surface temperature?
Question 3. Why are June and December insolation curves particularly useful in climate analysis?
Question 4. Find the three deep-sea sediment core sites on a map of the world. In which oceans are they located?
Question 5. Using the SPECMAP workbook, construct two plots that are proxies for global land ice volume over time, one over the last 400 thousand years and one over the last 800 thousand years. Print the 400 thousand year plot. a. Is there a change in the signal over time? b . Is it a symmetric or asymmetric cycle? c. What is the frequency of the dominant cycle? d . On the 400 thousand year plot you printed, identify the last glacial maximum and the last interglacial.
Question 6. Using the Vostok workbook, construct a plot of temperature variation at Vostok station over the last 420 thousand years. a. What is the frequency of the dominant (largest) temperature cycle? b . Sketch the shape of the temperature cycle—is it a symmetric or asymmetric cycle?
6

c. Compare temperature variation during the last 20,000 years with variation over the time span from 140,000 years ago to 120,000 years ago.
How are the trends similar and how are they different?
Question 7. Using the Vostok workbook, construct the following plots of insolation over time:
• summer insolation at 60N and 60S
• summer insolation at 60N and at the equator
• summer insolation at 60N and winter insolation at 60N a. Are northern and southern hemisphere insolation cycles in phase or out of phase? b. Construct plots to compare summer insolation at 60N and 60S with the air temperature at Vostok. The time axis should be from 0 to 420 thousand years. Print your plots.
Changes in which hemisphere seem most closely related to changes in air temperature at
Vostok? Explain your answer. c. How well do changes in temperature correspond to the insolation cycle in the hemisphere you identified in part b ?
Question 8. Using the Vostok workbook, construct a plot to compare the variation of atmospheric CO
2
content and and changes in temperature over the last 420 thousand years. a. Does one climate signal lead the other or are the changes in phase? Do you think changes in atmospheric CO
2
drive Earth’s glacial cycle or does the evidence suggest the opposite? Explain your reasoning. b. Make a print of your graph and identify the last glacial maximum and the last interglacial.
Question 9. Using the Vostok workbook, construct a plot to compare the variation of atmospheric CO
2
and CH
4
. Does one of these atmospheric gasses lead the other?
Question 10. Describe the relationship between the Vostok temperature record and the global land ice proxy record. Does one lead the other or do they appear to change synchronously?
Question 11. In our classroom discussion of the present-day climate, we recognized that the tilt of Earth’s spin axis gives rise to distinct changes in solar radiation that become more pronounced with distance from the equator. The concept of seasonality is often used to describe such annual variations. a. How could you construct a graph that represents seasonality at the three ocean sediment core sites? b. Print your seasonality plot for core V27-116. Identify the time of the last glacial maximum and the peak of the last interglacial on the plot. c. Compare seasonality at the North Atlantic site with seasonality at the Indian Ocean site. Are the variations similar?
Question 12. Write a short essay (up to 2 pages) describing how global climate has varied over the last 240,000 years, according to your analysis of the ocean sediment and ice core proxy records.
7

8