Ch 6 The Benthic Foraminiferal Oxygen Isotope Record of Cenozoic Climate Change

Transcription

1 INSTRUCTOR GUIDE Chapter 6 The Benthic Foraminiferal Oxygen Isotope Record of Cenozoic Climate Change SUMMARY This chapter explores one of the most widely accepted and scientifically cited lines of evidence for global climate change, the 65 Ma-long composite stable oxygen isotope record derived from benthic foraminifera. In Part 6.1, you will familiarize yourself with some of the graphic elements of the oxygen isotope record and make some initial observations. In Part 6.2, a primer on stable isotope geochemistry, you will unravel the implications of the physical states of water and the movement of water through the hydrologic cycle in the distribution of the two most common stable isotopes of oxygen. In Part 6.3, you will examine the biogeochemical connections that allow scientists to extract such a detailed record of long-term oceanic, continental, and atmospheric change. In Part 6.4, you will integrate and apply your knowledge from Parts by making a detailed examination of the Cenozoic benthic foraminiferal δ 18 O record by identifying patterns and changes in trends and by interpreting what the patterns, trends and changes indicate, thereby recognizing some of the most significant climatic events that have occurred over the past 65 Ma. FIGURE 6.1. Hydrologic cycle reservoirs. (a) Iceland Vatnajökull ice cap meltwater and runoff into North Atlantic Ocean, (b) evaporative haze over Lake Superior, and (c) rain and fog at Höfn, Iceland. Photos courtesy of Megan Jones Page 1 of 24

2 Goal: to understand and interpret the composite stable oxygen isotope record derived from benthic foraminifera, which is among the most widely accepted and scientifically cited lines of evidence for past global climate change. Objectives: After completing this exercise your students should be able to: 1. Explain or show (e.g., label a diagram, work though logic scenarios) how oxygen isotopes in water molecules are fractionated as the water travels though the hydrologic cycle. 2. Explain how foraminifera (and other carbonate-shelled marine organisms) record the isotopic composition and the temperature of sea water in which they live, and are therefore a biogenchemical proxy of climate change. 3. Explain how O 16 /O 18 ratios in samples are measured, and calculate δ 18 O given measured, and standard values. 4. Make observations and paleoclimate (ice volume and temperature) interpretations of marine δ 18 O data for the Cenozoic. 5. Distinguish between slow, gradual change and abrupt and rapid change in climate based on δ 18 O data. 6. Apply their understanding of marine stable oxygen isotopes to interpret marine δ 18 O data from other time periods. I. How Can I Use All or Parts of this Exercise in my Class? (based on Project 2061 instructional materials design.) Part 1 Part 2 Part 3 Part 4 Title (of each part) How much class time will I need? (per part) Can this be done independently (i.e., as homework) Introduction Stable Isotope Geochem A Biogeochem Proxy Patterns Trends Implications min min min min Yes, but needs instructor wrap-up Yes, but needs instructor wrap-up Yes, but needs instructor wrap-up Yes, but needs instructor wrap-up What content will students be introduced to in this exercise? Application of microfossils Exploratory research/nature of science Focused questions/nature of science Where do we learn about earth history (e.g., land vs. sea vs. ice) Awareness of deep time and subdivisions of geologic time Use of proxies Concepts of atom, stable isotopes and fractionation Measuring stable isotopes and use of δ 18 O notation How do we know what we do about earth history (e.g., climate change) Environmental responses Ocean-atmosphere-biospherecryosphere system interactions and feedbacks Page 2 of 24

3 Climate change can be gradual Climate change can be abrupt Climate change can be cyclic Glacial-interglacial cycles What types of transportable skills will students practice in this exercise? Make observations Read and interpret graphs Recognize patterns and trends Form questions and hypotheses Synthesize and integrate data drawing broad conclusions x Oral communication Analysis and evaluation of authentic data and hypotheses Critical thinking What general prerequisite knowledge & skills are required? 1. geologic time scale 2. relative & radiometric dating 3. science process 4. concept of proxies 1. atoms, elements, compounds 2. hydrologic cycle 3. science process 1. atoms, elements, compounds 2. hydrologic cycle 3. science process 4. concept of proxies 1. geologic time scale 2. relative & radiometric dating 3. science process What Anchor Exercises (or Parts of Exercises) should be done prior to this to guide student interpretation & reasoning? What other resources or materials do I need? (e.g., internet access to show on-line video; access to maps, colored pencils) 1. Intro. To Paleoclimate Records (Ch. 1) 2. Seafloor Sediments (Ch. 2) 3. Microfossils (Ch. 3) 4. CO 2 as Climate Regulator (Ch. 5) 1. colored pencils 1. Intro. Paleoclim Records (Ch. 1) 2. Seafloor Sediment (Ch. 2) 3.Mcrofoss ils (Ch. 3) 4. CO 2 as Climate Regulator (Ch. 5) 1. internet access to view animation 1. Intro. Paleoclim Records (Ch. 1) 2.Seafloor Sediment (Ch. 2) 3.Mcrofossils (Ch. 3) 4. CO 2 as Climate Regulator (Ch. 5) 1. Intro. Paleoclim Records (Ch. 1) 2. Seafloor Sediments (Ch. 2) 3. Microfossils (Ch. 3) 4. CO 2 as Climate Regulator (Ch. 5) 1. colored pencils Page 3 of 24

4 What student misconception does this exercise address? What forms of data are used in this? (e.g., graphs, tables, photos, maps) What geographic locations are these datasets from? How can I use this exercise to identify my students prior knowledge (i.e., student misconceptions, commonly held beliefs)? How can I encouraging students to reflect on what they have learned in this exercise? [Formative Assessment] How can I assess student learning after they complete all or part of the exercise? [Summative Assessment] Where can I go to for more information on the science in this exercise? 1. Climate change does not occur 2. It is hard to understand the science of how we know about earth history graphs, photos, schematic diagrams 1. Elements come in only one form 2. It is hard to understand the science of how we know about earth history photos, schematic diagrams 1. Climate change does not occur 2. It is hard to understand the science of how we know about earth history photos, schematic diagrams 1. Climate change does not occur 2. It is hard to understand the science of how we know about earth history graphs Global Global Global Global Part 1 of this exercise module is designed as an initial inquiry aimed at drawing out student beliefs and prior knowledge. In addition, Parts 2-4 often lead with tasks or questions that can further identify student prior knowledge. Exercise Part can be concluded by asking: On note card (with or without name) to turn in, answer: What did you find most interesting/helpful in the exercise we did above? Does what we did model scientific practice? If so, how and if not, why not? What did you learn that was new for you or a reinforcement of what you already knew? See suggestions in Summative Assessment section below. See the supplemental materials and reference sections below. II. Annotated Student Worksheets (i.e., the ANSWER KEY) This section includes the annotated copy of the student worksheets with answers for each part of this chapter. This instructor guide contains the same sections as the student book chapter, but also includes additional information such as: useful tips, discussion points, notes on places where students might get stuck, what specific points students should come away with from an exercise so as to be prepared for further work, as well as ideas and/or material for mini-lectures. Page 4 of 24

5 Chapter 6 The Benthic Foraminiferal Oxygen Isotope Record of Cenozoic Climate Change Part 6.1. Introduction One of the most valuable proxies of past climate change is the benthic foraminiferal stable oxygen isotope record (Figure 6.2). Fundamentals of chemistry, biology, geology, and physics are all important in understanding the nature of this type of data and how it is useful as a paleoclimate proxy. We will explore it in detail in this chapter and revisit it again in Chapters 7, 10, and 11 as it applies to particular climate patterns and events. FIGURE million year composite record of marine benthic foraminiferal stable oxygen isotope values (δ 18 O) from Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP) sediment cores (modified from Zachos et al., 2008, which is an update of Zachos et al., 2001). Note that the blue arrows and the red circle on this diagram are annotations added specifically for work you will do in Part 6.4, and can be ignored for your work in Part 6.1. Start by becoming familiar with the data shown in Figure 6.2 above. Page 5 of 24

6 1 What are the variable and its units for the x-axis? δ 18 O or staple oxygen isotope values; = per mil or parts per thousand 2 What are the variable and its units for the y-axis? Age or Time; Ma = millions of years 3 There are thousands of individual data points on this graph. Using a colored pencil, draw a line on the graph representing the mean values of the data as they change over time. Note this is not a straight line, but a curved line representing your best visual estimate of the running mean. See line in graph to right. 4 Make a list of your initial observations about changes displayed in the data (Figure 6.2). Below are several of the most common observations (though not inclusive) that students will come up with: - Lots of zigzagging back and forth of the data - More data points in the upper half of the curve - Overall general increasing trend in δ 18 O from about 52 million years ago to the present - Lowest δ 18 O occurs at Paleocene-Eocene boundary and at about 52 million years ago - δ 18 O values range from a low of ~ -0.2 to a high of ~ 5 - Some changes in trend of the δ 18 O data are gradual whereas some changes in trend of the δ 18 O data are quite rapid or abrupt. To wrap up this part it is a good idea to get students to either share their observations with one another or even better to have instructor ask for observations from the class as a whole and list some on the board. This way the instructor can make sure students are clear about the following points: 1. What the variables and their units are for each axis. 2. That there is a quite a lot of data here a place for discussion of what more data might mean with respect to significance and/or reliability of the curve they drew. 3. What the meaning is of the average or mean line that they drew through all the data points. 4. Though at this point the delta 18 O notation (δ 18 O) is unfamiliar to them it will not be by the end of the exercise. 5. To recognize that there is a lot of change in the distribution of the data points and hopefully, the idea is that they get curious about why there is so much change and wonder what is causing it! Page 6 of 24

7 The following two paragraphs are a brief summary of why stable oxygen isotopes are such an important type of data for understanding climate change as well as a number of research areas in which they are important. The following would be useful as a minilecture after the completion of Part 6.1 and as a lead in to Part 6.2 by describing the relevance of understanding their use. Alternatively, you may wish to wait and do this lecture at the end of Part 6.3, so that students can construct more of their own understanding of fundamentals of marine oxygen isotopes first. One of the reasons these geochemical data are valuable in paleoclimate research is that stable oxygen isotopes records can be obtained not only from deep sea sediments, but from ice, cave deposits, and modern and ancient coral reefs. By having oxygen isotope records from these various sources scientists can connect and correlate records globally. In addition, since stable oxygen isotopes can be measured from water in the modern hydrologic cycle (e.g., ground water, ocean water, and atmospheric water vapor), these records of past oxygen isotope changes can be compared to modern conditions. By having a common type of data such as this, we can more easily make direct comparisons between the marine and terrestrial environments, as well as across large geographic distances. This enables scientists to be able to recognize and distinguish global climatic events from more local climate variations. An especially interesting and useful aspect of the biogeochemistry of marine oxygen isotopes is that the organisms shells from which we get the isotope data provide scientists with information on where in the water column the particular organism lived. For example, different species of planktic foraminifera live in different depth horizons. Variations in oxygen isotope data from species that live in different parts of the water column can provide climate scientists with information on how various physical aspects (e.g., stratification) of the water column have changed over time. Stratification of the water column has a strong connection to climate because it is greatly influenced by forming and melting of glacial ice (e.g., the input of fresh meltwater lowers the density of the surface waters and can thereby reduce overturn or deep water formation). In addition, oxygen isotope records derived from benthic foram shells (those that live on, in or near seafloor) can be compared with those of planktic forams to see how conditions have changed between the sea surface to the deep sea over time. Part 6.2. Stable Isotope Geochemistry Atomic Nuclei All atoms contain a nucleus (protons plus neutrons) surrounded by an electron cloud. The groups of blue and white spheres (Figure 6.3) represent generalizations of the atomic nuclei of two isotopes of the element oxygen. Figure 6.3. Generalization of the atomic nuclei of two different oxygen isotopes. From: Page 7 of 24

8 1 How are the two nuclei similar and how are they different? From just looking at the images above the following similarities and differences can be recognized: Similarities Both contain blue & white spheres Both are nuclei of oxygen Both nuclei have same number of blue spheres Differences Nuclei on right ( 18 O) has more white spheres than blue spheres Nuclei on left ( 18 O) has same number of both blue and white spheres Nuclei on right ( 18 O) looks bigger than nuclei on left ( 16 O) 2 Go to the link in the caption for Figure 6.3 to rotate (view all sides) of the atomic nuclei via animation. (a) What do the blue spheres represent? Protons (b) What do the white spheres represent? Neutrons 3 Based on your prior knowledge, what do we call nuclei that all have the same number of protons? Elements 4 Based on your prior knowledge, what do we call nuclei that have the same number of protons, but have different numbers of neutrons? Isotopes 5 Write a new caption for Figure 6.3. Use your answers from Questions 1 5 to help you construct an accurate, descriptive, and concise caption in the space below. Some variation on statements below: Nuclei of two oxygen atoms showing one, right ( 18 O) with more neutrons (white spheres) and thus, larger, than the nucleus on the left ( 16 O). Two isotopes of oxygen, 18 O and 16 O, showing that 18 O nucleus has more neutrons than 16 O nucleus which accounts for its higher atomic mass. INSTRUCTOR NOTE: Start by having some students share their figure captions with the class either on board or read aloud. This allows the instructor to: (a) quickly recognize misconceptions and clarify muddy points, (b) review understanding of basic chemistry terminology/concepts (e.g. atoms, elements, isotopes, molecules) and (c) segue into thinking about the implications of isotopes that is introduced in question # 6. Below are listed the critical points that students should come away with from questions The nucleus of an atom has protons & neutrons which make up the majority of an atom s atomic mass (# protons + # neutrons). The number of protons is an element s Page 8 of 24

9 identity you might ask students how many protons oxygen has? As a way to check for understanding. 2. Different isotopes of the same element have more or less neutrons than the isotope shown in the periodic table; which shows the most abundant isotope found in nature. 3. Isotopes with higher atomic mass are often slightly larger than others. 4. Any oxygen containing compound is likely to have a combination of both isotopes ( 16 O, 18 O) in it. 5. When working with an element that has several isotopes, scientists typically compare the amounts of the isotopes to each other in a ratio or fractional form. For example, amount of 18 O to 16 O as 18 O/ 16 O. Physical Properties and Processes 6 How would water (H 2 O) containing 16 O be similar to water that contains 18 O and how would it be different. Similarities Both types of water would have the number of oxygen atoms Both types of water would still be found in three physical states solid, liquid and vapor (gas) Differences The water molecules with 18 O same would be a bit larger than molecules with 16 O. The water molecules with 18 O would be heavier than those with 16 O HYDROLOGIC CYCLE AND ISOTOPIC FRACTIONATION Water molecules change physical state (liquid, gas, solid) and move from reservoir to reservoir (e.g., ocean, atmosphere, glacial ice, Figure 6.1) over long distances via the hydrologic cycle (Figure 6.4). The differences in mass between water molecules that contain 18 O and water molecules that contain 16 O are large enough for physical and biological processes to fractionate, or partially separate water containing 18 O from water containing 16 O. Processes that move water between reservoirs via the hydrologic cycle will result in isotopic fractionation. Such fractionation has been demonstrated by measuring the relative abundance of 18 O and 16 O in modern water samples from different reservoirs of the hydrologic cycle at different locations. You will have the opportunity to work through examples of this fractionation effect in Questions Read the box above on isotopic fractionation. (a) Which would tend to evaporate preferentially from a glass of water: a water molecule containing 16 O (H 2 16 O) or a water molecule containing 18 O (H 2 18 O)? 16 O (H 2 16 O) Page 9 of 24

10 (b) Why? Some variation on the answers below: (1) Because water with 16 O (H 2 16 O) is more abundant in nature. (2) Because the mass difference between water with 18 O (H 2 18 O) and water with 16 O (H 2 16 O) is great enough to make it more difficult (yet not impossible) to evaporate 18 O (H 2 18 O) water into its vapor state. It is much easier to evaporate 16 O (H 2 16 O) water, so water vapor generally contains more 16 O than 18 O; which is an important take home message. FIGURE 6.4. Schematic diagram showing subtropical to polar water movement between ocean, atmosphere, and continental ice sheet reservoirs under present day conditions (heavy black line) and under conditions of lowered sea level and increased ice sheet volume (dashed blue lines). Diagram drawn by Megan Jones. Page 10 of 24

11 Refer to Figure 6.4 (above) as you complete Questions Select the correct oxygen isotopes for the multi-part scenario described below by circling them. (Note: annotating Figure 6.4 may help you reason through the scenarios and determine your answers.) (a) When water evaporates from the ocean in the tropics or subtropics, the water vapor is enriched in (has more) 16 O 18 O relative to the original water. When the resulting precipitation falls, the remaining water vapor is enriched in 16 O 18 O. (b) Water vapor is transferred from the tropics to the poles through the hydrologic cycle. When this water vapor condenses and precipitates as snow on land in the polar regions and later recrystallizes as ice, this 16 O 18 O enriched water becomes locked up in the ice. Because of this transfer, the oceans become relatively depleted in 16 O 18 O (i.e., have less 16 O 18 O than they did before evaporation removed the water vapor) and therefore are relatively enriched in 16 O 18 O. (c) If the precipitation falls as rain, and is NOT locked up in ice, but is returned to the ocean, then the ocean does NOT become enriched in 16 O 18 O. (d) Hence, during times of continental ice sheet formation (glacial periods) the oceans are relatively enriched in 16 O 18 O and during warmer times with reduced or no ice sheet formation (inter-glacial periods), the oceans are relatively enriched in 16 O 18 O. 9 Demonstrate your understanding of the isotopic fractionation effect on oxygen isotopes in the hydrologic cycle by filling in the blanks below with increase or decrease as appropriate. (a) As the global volume of ice sheets increases, the relative abundance of 16 O in the ocean will decrease and the relative abundance of 18 O in the ocean will increase. (b) As the global volume of ice sheets decreases, the relative abundance of 16 O in the ocean will increase and the relative abundance of 18 O in the ocean will decrease. Page 11 of 24

12 Part 6.3. A Biogeochemical Proxy Application of Important Marine Microfossils One of the main types of microscopic life in the ocean that has a mineralized shell of calcium carbonate (CaCO 3 ) are the foraminifera (Chapters 2 and 3). FIGURE 6.5. Representative mix of microfossil shells of benthic and planktic foraminifera. Scale bar = 100 µm (= 0.1 mm). Scanning electron microscope images by Mark Leckie. 1 Microfossil shells of some representative and important foraminiferal species are shown in Figure 6.5. The top two rows show planktic species and the bottom two rows show benthic species. Planktic foraminiferal species live in the near surface ocean, (i.e., upper 200 m of water column). Page 12 of 24

13 In contrast, benthic foraminiferal species live on the seafloor, either just above (epifaunal) or within (infaunal) the sediments. Make a list of your observations about foraminifera based on the species shown in Figure 6.5. Some observations that students may describe are listed below: (list not inclusive!) -many species are globular or puffy looking -many species have a rounded shape to them -some species have spines on them -most have several sections (chambers) to them -some have openings in their center whereas others have openings at their shell tops -some species shells are rectangular or lines -some species shells look spiralled or coiled -some shells have small holes or pores in them, whereas other species shells appear to be smooth or without holes or bumps on them -some shells look like sections are stacked on top of one another Note: Top two rows are planktic species and bottom two rows are benthic species. Instructor might want to have an all class discussion about what observations the students have made distinguish between the two groups. A BIOGEOCHEMICAL PALEOCLIMATE PROY Both benthic and planktic foraminifera (Figure 6.5) construct (precipitate from seawater) their CaCO 3 shells either in isotopic equilibrium or in a consistent isotopic disequilibria with the seawater in which they live (Figure 6.6). Such disequilibria vital effect maybe due to the influence of respiratory CO 2, but appear unrelated to environmental conditions such as depth or temperature (Faure, 1986), and can be corrected for. In the case of data in Figure 6.2, the genus-specific vital effects were corrected for by adjusting the δ 18 O values by for Cibicidoides and +0.4 for Nuttalides (Barker et al., 1999). The isotopic composition of seawater (see Part 6.2) is an important factor affecting the isotopic composition of the shells of the foraminifera, but so is the temperature of the seawater in which they live. This is because the isotopic fractionation is temperature dependent. Thus, the seawater temperature creates an additional fractionation effect that influences the isotopic composition of the shell. As a result of this temperature fractionation effect, in isotopically identical waters, calcite (CaCO 3 ) shells precipitated in cooler water temperatures are enriched with 18 O relative to 16 O, whereas calcite precipitated in warmer water temperatures is depleted of 18 O relative to 16 O. Therefore, both the isotopic composition of seawater and the temperature of seawater affect the isotopic composition of the foraminiferal shells. Based on these principles, the foraminifera (or other marine organisms that form calcareous shells) serve as proxies for the chemical and isotopic composition of the waters in which they live. Their shells can accumulate on the seafloor after death (Figure 6.7), over time becoming part of the sedimentary record. Thus, we can use the isotopic signature in their shells to reconstruct the oceanographic and climatic history through time. Read the box above on biogeochemical paleoclimate proxies and examine Figures 6.2, and to answer Questions 2 5. Page 13 of 24

14 FIGURE 6.6. Schematic diagram from subtropic to polar latitudes showing where benthic (B) and planktic (P) foraminifers live and the location of deep and bottom water formation under today s sea level and ice volume conditions. Heavy arrows indicate locations of deep and bottom water formation and flow. The fossil planktic foraminifera shown is the species Globigerinoides ruber. The fossil benthic foraminifera shown is the species Planulina wuellerstorfi. Diagram drawn by Megan Jones, microfossil images from Mark Leckie. FIGURE 6.7. Schematic diagram illustrating the accumulation of benthic and planktic foraminifera shells on the seafloor. Note that the size of the foraminifera shells is highly exaggerated in comparison to the water depth in these diagrams. Diagram drawn by Megan Jones. 2 Foraminifera use Ca 2+ and CO 3 2 ions dissolved in seawater to make their CaCO 3 shells. Think about the composition of the microfossil shells of foraminifera. How might benthic foraminifera be related to the stable isotope record you examined in Figure 6.2? The oxygen isotope measurements come from the shells of the benthic foraminifera. Instructor might want to hint to students to read the figure caption for 6.1. Page 14 of 24

15 3 How would the stable oxygen isotope composition of foraminifera shells change if the local temperature of the seawater in which they live became colder? The isotopic composition of the foraminifera shells living in the seawater that became colder would become enriched with respect to 18 O due to the cooler water temperature. 4 How would the stable oxygen isotope composition of foraminifera shells change if there was an increase in global ice volume? The isotopic composition of the foraminifera shells living in the seawater at the time of increased global ice volume would become enriched with respect to 18 O. This is due to the oceans becoming more enriched with 18 O because the ice sheets are enriched with 16 O, thus leaving more 18 O in the seawater at a time of increased global ice volume. 5 An increase in global ice volume and a decrease in seawater temperature are both indications of climatic cooling. Write a statement predicting how climatic cooling would affect stable oxygen isotopes of marine biogenic calcite. Anything along the lines of: Climatic cooling, which could involve either or both a decrease in seawater temperature and an increase in global ice volume, would result in the biogenic calcite forming in the oceans to be enriched in 18 O and depleted in 16 O. Thus it is important to note that it is difficult and often impossible to tease out whether the seawater temperature has decreased or the global volume of ice has increased based on stable oxygen isotope data alone! We address this in part 6.4. MEASURING STABLE ISOTOPES [CONTINUES ON NET PAGE] The relative abundance of stable oxygen isotopes in foraminifera shells is measured using a mass spectrometer. Typically, the calcite (CaCO 3 ) is dissolved in acid to liberate CO 2 gas. The CO 2 gas is ionized in a vacuum and sent through a strong magnetic field in a curved tube. The trajectories of the ions bend (i.e., are deflected) depending on their masses; lighter ions are deflected more than heavier ions. By putting sensors at different positions, or by changing the strength of the magnetic field, the relative abundance of ions with different masses (i.e., different isotopes) can be measured. The typical way of recording the measured isotopic mass differences is in a ratio format. The measured ratios of 18 O/ 16 O in a sample are compared to the isotopic ratios in a known standard, as shown in the equation below which calculates the difference between the ratios in the sample and the standard. The use of a common standard in all geochemical labs around the world helps ensure consistency among the data produced from different mass spectrometers (Wright, 1999). δ 18 O (in 0 /00 or per mil) = ( 18 O/ 16 O) sample ( 18 O/ 16 O)standard x 1000 ( 18 O/ 16 O)standard These delta (δ) values are reported in units of parts per thousand ( ), or per mil (in contrast to units of parts per hundred, or percent). Page 15 of 24

16 A positive or increasing value per mil (or ) indicates that more 18 O than 16 O is present in the marine microfossil sample than is present in the standard. This is commonly referred to as being a high or increase in δ 18 O value and is interpreted as reflecting a cooler climate. In contrast, a negative or decreasing value per mil (or ) indicates less 18 O than 16 O is present in the marine microfossil sample than is present in the standard. This is commonly referred to as being a low or decrease in δ 18 O value and is interpreted as reflecting a warmer climate. Read about how scientists measure stable oxygen isotopes in the box (above) and complete Questions 6 8: 6 What would the δ 18 O value be if the ratio of 18 O/ 16 O in a sample is the same as the ratio of 18 O/ 16 O in the standard? Zero 7 Would the δ 18 O value be a positive or negative number if the ratio of 18 O/ 16 O in a sample is greater than the ratio of 18 O/ 16 O in the standard? Positive 8 Would the δ 18 O value be considered high or low if the ratio of 18 O/ 16 O in a sample is less than the ratio of 18 O/ 16 O in the standard? Low Part 6.4. Patterns, Trends and Implications for Cenozoic Climate With background knowledge gained from Parts , you will now focus on identifying patterns and the trends in the Cenozoic benthic foraminiferal oxygen isotope record (Figure 6.2). 1 Working independently, examine and annotate the oxygen isotope record (Figure 6.2) as follows: Starting from the bottom of the figure, work your way up and mark any trends in the data with arrows (see example in Figure 6.2: blue arrows = trend); mark the position of any changes in trends with a circle (see example in Figure 6.2: red circle = change in trend). Un-annotated Graph for above question Number 1. See next page for examples of very detailed interpretation and a more generalized interpretation. There will be quite a varying degree in the level of detail that each person uses in describing their observations. Chances are instructors will get Page 16 of 24

17 both versions of the annotated graphs. This can lead into a brief discussion on different people s approaches to things in general, such as lumpers vs. splitters and/or big picture thinkers vs. more fine-scale, detailed-oriented people. Very detailed interpretation showing A less detailed, more generalized 18 changes in trend (red circles) of interpretation of the data. δ 18 O over the last 65 Ma. Page 17 of 24

18 2 In Table 6.1 (Column 1), record the observations of the trends and changes in trends in the δ 18 O record that you recognized as you completed question 1. Leave Column 2 blank for now! Always, start from the bottom and work your way up epoch by epoch. Use descriptive terminology (e.g., high, low, constant, abrupt increase, gradual decrease, etc.) and realize that an epoch may be marked by more than one trend or change in trend. Remind students to start at the bottom and work up as they did when they annotated the graph! See notes after completed table 6.1 for helpful suggestions. Completed table below answers question 2 and 4. TABLE 6.1. Observations & Interpretations of the Cenozoic Benthic Foraminiferal Stable Oxygen Isotope Record. Epoch Pleistocene Pliocene Miocene Column 1 Observations of the Cenozoic Benthic Foraminiferal δ 18 O record The increasing trend in δ 18 O values continues to the present with only a brief negative (decreasing) excursion at ~ 1.6 Ma. Note that the range of δ 18 O values through this increasing trend is ~ 3.1 to 4.9 and δ 18 O reaches its highest level in Cenozoic at the present time. An abrupt and brief (~ 500,000 yrs) positive (increasing) excursion from Miocene-Pliocene boundary to ~ 5.5 Ma, at which time another abrupt and brief (~ 700,000 yrs) excursion (negative, decreasing) occurs until ~ 4.8 Ma. From ~ 4.8 Ma to ~ 2.2 Ma there is little change in δ 18 O values; the range of δ 18 O values is ~ 2.8 to ~ 3.1. At 2.2 Ma there is a gradual increasing trend in δ 18 O values to the Pliocene-Pleistocene boundary with the range of δ 18 O values is through this increasing trend is ~ 3.9 to ~ 4.3. Abrupt but brief negative (decreasing) δ 18 O excursion from the Oligocene-Miocene boundary to ~ 22 Ma. From ~ 22 Ma to ~ 17 Ma δ 18 O values remain rather constant with a range of 1.8 to 2.8. From 17 Ma to 15 Ma δ 18 O decreases with two rapid and abrupt δ 18 O shifts occur, one negative (decreasing) and one positive (increasing) then ending at 15 Ma with a Miocene δ 18 O minimum of ~ 1.4. From 15 Ma to 14 Ma δ 18 O increases. From 14 Ma to 9 Ma the rate of δ 18 O increase increases (note the slope change) and throughout this generally increasing trend there are several rapid and abrupt δ 18 O positive (increasing) and negative (decreasing) excursions. At 9 Ma δ 18 O begins a slight decreasing trend until 7 Ma. From 7 Ma to Miocene-Pliocene boundary δ 18 O increases to the Miocene δ 18 O maximum of ~ 3.5, then an abrupt and brief δ 18 O decrease occurs right at the Miocene-Pliocene. Column 2 Interpretations of the Cenozoic Benthic Foraminiferal δ 18 O record In general, cooling (or increasing ice volume) continues to the present time. With the exception of a potential abrupt and brief (100, ,000 yrs) warming (or decreasing ice volume) occurring at ~ 1.6 Ma. Cooling (or increasing ice volume) into Pliocene begins and in general continues into the Pleistocene. There is an abrupt and brief warming (or decreasing ice volume) occurring at ~ 4.8 Ma. At ~ 2.2 Ma longterm cooling throughout the Pliocene and Pleistocene begins. A brief warming (or decreasing ice volume) occurs from the Oligocene into the Miocene to ~ 22 Ma. Little climate change occurs from 22 Ma to ~ 17 Ma. There is a general warming trend with two abrupt but brief climate changes, one warming (or decreasing ice volume) and one cooling (or increasing ice volume) occurring from ~ 17 Ma to ~ 15 Ma; ending with the Miocene thermal maximum at 15 Ma.. At ~ 15 Ma, a large-scale cooling trend (or increasing ice volume) begins and continues through Miocene-Pliocene boundary. From ~ 14 Ma to 9 Ma the rate of cooling (or ice volume increase) increases. From 9 Ma to 7 Ma there is a brief warming, then back to a cooling trend to the Miocene cooling maximum at 6 Ma. From 6 Ma to the Miocene-Pliocene boundary there is an abrupt and brief warming episode. Page 18 of 24

19 Epoch Oligocene Eocene Paleocene Column 1 Observations of the Cenozoic Benthic Foraminiferal δ 18 O record A shift to decreasing δ 18 O values, from the Eocene- Oligocene boundary to ~ 31 Ma occurs, from then, several rapid excursions, both positive (increasing) and negative (decreasing) occur until ~ 27.5 Ma, where, at the Oligocene δ 18 O maximum of ~ 3, a gradually decreasing trend in δ 18 O begins and continues until 24.5 Ma where an abrupt positive (increasing) excursion occurs with gradually increasing δ 18 O values to ~ 23 Ma the Oligocene-Miocene Boundary. From the lowest δ 18 O value (~ -0.2 ) in the Cenozoic at Paleocene-Eocene boundary (~ 55 Ma) there is an abrupt increase in δ 18 O to positive values ( ), then no change until 54 Ma at which point there is a decreasing trend in δ 18 O approaching negative values at ~ 51 Ma. Then begins a long, gradual trend in increasing δ 18 O back to positive values (~ 1.3 ) at ~ 44.5 Ma. From ~ 44.5 Ma to ~ 43.5 Ma is a brief (~ 1 Ma) decrease in δ 18 O, then from 43.5 Ma 42.1 Ma a brief (~ 1.4 Ma) increase δ 18 O occurs, at which time (42.1 Ma) an abrupt (600,000 yrs) and large (1.6 to 0.6 ) negative (decreasing) excursion occurs. From Ma to ~ 40.8 Ma there is a brief increase in δ 18 O, then an abrupt (700,000 yrs) negative (decreasing) excursion occurs. Starting at 40.1 Ma, a long (~ 6 Ma), gradual trend of increasing δ 18 O occurs ending at 36 Ma. From 36 Ma to 34 Ma a brief (2 Ma) period of decreasing δ 18 O occurs, and then an abrupt (500,000 yrs) increase in δ 18 O occurs up to and at the Eocene-Oligocene boundary. Gradual increasing δ 18 O trend from base of Paleocene (~65 Ma) to ~63 Ma, then abrupt shift to gradual decreasing δ 18 O trend to 61 Ma then gradual increasing trend in δ 18 O to ~58 Ma where it peaks in the Paleocene, then back to gradual decreasing δ 18 O trend to ~55 Ma Paleocene-Eocene boundary where an abrupt decrease occurs to reach the lowest δ 18 O value of ~ -0.2 in the Cenozoic is found. Column 2 Interpretations of the Cenozoic Benthic Foraminiferal δ 18 O record From the Eocene-Oligocene boundary abrupt cooling (or increasing ice volume) event, a brief warming (or decreasing ice volume) at ~ 31 Ma. Between ~ 31 Ma and 27.5 Ma, the Oligocene maximum cooling, several climatic fluctuations occurred. Starting at ~ 27.5 Ma, a gradual warming (or decreasing ice volume) occurs with a thermal max at 24.5 Ma. After which, the climate cools (or increasing ice volume) until the Oligocene-Miocene boundary Climate warming (or decreasing ice volume) from Paleocene into Eocene with a warming maximum (or decreasing ice volume) almost equivalent to the PETM occurring at ~ 51 Ma. Then mostly an overall trend of climate cooling (or increasing ice volume) through remainder of Eocene punctuated with 4-5 abrupt warming or decreasing ice volume events, ending with a cooling maximum at the Eocene- Oligocene boundary Climate cooling (either decreasing seawater temperature or increasing global ice volume or both) to ~ 63 Ma, then abrupt climate warming (or increasing seawater temperature or decreasing global ice volume or both) to ~ 61 Ma then cooling again until at ~58 Ma, then warming again to a thermal maximum at the Paleocene-Eocene boundary (PETM). Additional Notes: The observations described in column one (above) were made from the more detailed, annotated graph previously shown. Some students may not have recognized all of the δ 18 O shifts, whereas others will and maybe then some (lumpers vs. splitters). However, all students should recognize major changes and trends. Page 19 of 24

20 A key point in this part of the exercise is for the students to start to distinguish between slow, gradual change and abrupt and rapid change, so as students are working the instructor might want to reiterate the need to use the descriptive terminology outlined in the above instructions. 3 (a) Share/compare your written observations for each epoch with the person next to you or those at your lab group. (b) How similar or different are your observations for each epoch compared with those of your classmates? Explain with an example. For the most part the observed trends and/or changes in trends should be the same. What may vary will be the level of detail regarding what students consider to be a change. Some might include every wiggle whereas as others may only consider larger deviations to be changes in trends. There will be, again, quite a varying degree in the level of detail that each person uses in describing their observations. This can lead into a brief discussion on different people s approaches to things in general, such as lumpers vs. splitters and/or big picture thinkers vs. more fine-scale, detailed-oriented people. For example, most students will probably identify the change in trend to a very low δ 18 O value that occurs at about 51 million years whereas some students may disregard the extreme low δ 18 O value that occurs at the Paleocene-Eocene boundary which turns out to be an important climatic event (PETM). This is a place where a more directed mini-discussion from instructor may be needed about the nature of data and observations: Idea here again is for students to recognize that data is, something tangible, measurable, recordable and, ideally, permanent. It can be evaluated over and over again, in light of new technology, additional new data and/or news ways of thinking. Thus, the general patterns and trends recognized/identified in them should be the same no matter what scientist examines the data. Where some variation might occur, again, is in the level of detail change considered relevant by the one making observations. However, the overall take away message for students here is that in general, the data one is working with now, could be the same today, tomorrow, and 50 years from now and still useful. We will probably have more data in the future, but the old data can still be evaluated in light of the new data. 4 Working independently and referring as needed to Parts 6.2 and 6.3, interpret your observations (from Column 1 of Table 6.1) of the δ 18 O record (Figure 6.2). Record your interpretations on the table below. As you did for Question 2, start at the bottom of Table 6.1, read your observations for one epoch, then write your interpretations for that epoch in Column 2. Continue working your way up the table epoch by epoch. Use descriptive terminology (e.g., abrupt warming, gradual increase in ice volume, etc.). Remind students to start at the bottom and work up as they did when they annotated the graph! Page 20 of 24

21 The interpretations described in column two (below) were made directly from the observations in column one. So the extent of students the students interpretations will vary based on the scale of their observations. Again, however, all students should recognize major changes and trends and should include both climatic cooling/warming and global ice volume increase/decrease since we cannot distinguish between the two based on the Zachos curve. Here the key point described above in the observation section becomes important because now we are talking about changes in climate being slow and gradual or abrupt and rapid; implications that may have not occurred to most students before. So, again, as students are working the instructor might want to reiterate the need to use the descriptive terminology outlined in the above instructions. See completed table above for answer. 5 (a) Share/compare your written interpretations for each epoch with the person next to you or those at your lab group. (b) How similar or different are your interpretations for each epoch compared with those of your classmates? Explain with an example. Students should probably answer a little of both. Since we are using oxygen isotope data alone we are not able to distinguish between increasing global ice volume and decreasing seawater temperature. But in general lower and/or decreasing values of δ 18 O indicate warming climates and higher and/or increasing values of δ 18 O indicate cooling climates. 6 Recall (Part 6.3) that changes in global ice volume, as well as changes in local seawater temperature, influence benthic foraminifera stable oxygen isotope (δ 18 O) values. However, if no continental ice sheets were present, the variation in δ 18 O would solely reflect changes in temperature. (a) Would we know from the δ 18 O data alone whether continental ice sheets were present in the past? Explain. No. Oxygen isotopes alone cannot reveal whether the climatic change is due to temperature change of seawater or global ice volume change or both. (b) What other sources of data might you use to determine whether continental ice sheets were present in the past? -land-based data on glacial sediment distributions and/or the presence of accurately dated glacial landforms -marine sedimentary data which indicate ice rafting such as dropstones or IRD layers -pollen records that show changes in plant communities on adjacent landmasses (c) Look back at Chapter 5, Figure 5.9 and the related discussion on Greenhouse and Icehouse times. Based on this information, when did Cenozoic ice sheets first form? ~ 36 my, with short-lived, ephemeral ice sheets, with full scale and permanent ice sheet development on Antarctica by ~ 32 my. Page 21 of 24

22 (d) What does this imply about the Cenozoic changes in δ 18 O (Figure 6.2) prior to this time? The answer to (c) above implies that changes in δ 18 O occurring before ~ 36 my are reflecting changes in temperature only, no change in global ice volume is represented by the changes. (e) What were the early Cenozoic δ 18 O changes driven by? These early changes could have been driven by one or more of the following factors: (1) increases/decreases in rates of seafloor spreading and/or volcanism, (2) changes in the latitudinal positions of continents, which may disrupt global ocean circulation and subsequent hear transfer across the globe, (3) perturbations in the global carbon cycle created by changes in rates of uplift, chemical weathering and/or the biological cycling carbon (e.g., burial of and/or erosion and release of organic carbon and CO 2 ), increases/decreases in productivity). 7 The epoch boundaries on the geologic timescale were largely defined over 200 years ago based on the fossil record of relatively large fossils (e.g., clams, snails) deposited in shallow marine sediments that were subsequently exposed on land. How do the changes in benthic foraminiferal δ 18 O compare to the already defined epoch boundaries? At most of the epoch boundaries there is an abrupt change in the trend of δ 18 O. 8 Summarize your thoughts about, and understanding of, the isotopic evidence that indicates how the Earth s climate has changed over the past 65 Ma. This question will provide the instructor with a sense of where students understanding lies at the close of this exercise. These data and other stable isotope data are used in many of the upcoming exercises so a solid understanding of how to interpret them in critical. The students success in future exercises depends upon the foundation that they build here. The key points that students must come away understanding are: 1. What the δ 18 O values indicate about climate 2. What the δ 18 O values can mean in terms of seawater temperatures and ice volumes 3. That scientists do not use data in isolation 4. That the data used in this exercise benthic foraminifera provide isotopic information on sea surface temperatures in polar regions since the deep and bottom waters where benthic foraminifera live come from those regions 5. That the earth s climate has changed numerous times over the past 65 million years 6. That some of the climatic changes that occurred happened very rapidly or were abrupt and some climate changes occurred gradually over rather long periods of time Hopefully, students are now starting to think about implications of abrupt climate change, as well as climate change in general. And that they will start asking themselves or the instructor such things as: over what time frame do these abrupt changes occur? Why do they occur? What was the earth like during these times? Is this happening today, now? Page 22 of 24

23 III. Summative Assessment: The nature of this exercise lends itself to be easily replicated by using various types of graphic records appropriate to the course and having the students perform the same tasks. Also, an instructor could take any curve and identify parts of it with labels A-E and then ask the students a series of questions about the curve or its meaning and have them use locations A-E on the curve as their answer choices for a multiple choice test. Finally, on the below are several multiple choice questions related to an earlier version (Zachos et al. 2001) of the δ 18 O record used in this activity so you can get a sense of how to design your own activity using other graphic records. Figure. Composite marine benthic foraminifera δ 18 O record 1. What would cause a δ 18 O increase in benthic foraminifera shells? a. Formation of continental ice sheets b. A magnetic polarity reversal c. A cooling of the deep sea d. A warming of the deep sea e. Both A and C 2. If modern global warming continues how would you expect values of δ 18 O in the deep sea to change? a. Increase b. Decrease c. Stay the same. Page 23 of 24

24 3. Based on the global δ 18 O record (Fig. ) what is the general trend of climate change since the start of the Paleocene? a. Gradual warming b. Gradual cooling c. Abrupt warming d. Abrupt cooling e. Gradual trends punctuated by abrupt events of warming or cooling 4. Give the ages of two abrupt changes in climate you can identify on Figure. IV. Supplemental Materials: The following two websites are excellent for an introduction to paleoclimatology and the use of oxygen isotopes as a proxy: The url is to the United States Geologic Survey Resources on Isotopes which provides information on the fundamentals of stable isotopes geochemistry (e.g., definitions and terminology, basic principles, standards) as well as some additional references: The link below provides access to many additional resources on paleoclimate proxies: The url below will take you to the introductory page for a climate change and ice ages Calspace Course that covers a variety of related topics, including the use of oxygen isotope records: V. References: Barker, P., Barrett, P., Cooper, A. and Huybrechts, P. 1999, Antarctic glacial history from numerical models and continental margin sediments, Palaeogeography, Palaeoclimatology, Palaeoecology, 150, pp Faure, G., 1986, Principles of Isotope Geology, Wiley, New York, 589, p. ISBN Wright, J.D., 1999, Global climate change in marine stable isotope records. In Quaternary Geochronology: Applications in Quaternary Geology and Paleoseismology. Noller, J.S., et al. (eds), U.S. Nuclear Regulatory Commission, NUREG/CR 5562, Zachos, J., et al., 2001, Trends, rhythms, and aberrations in global climate change 65 Ma to present. Science, 292, Zachos, J., et al., An early Cenozoic perspective on greenhouse warming and carbon-cycle dynamics. Nature, 451, Page 24 of 24