INSTRUCTOR GUIDE Interpreting Antarctic Sediment Cores: A Record. of Dynamic Neogene Climate

Transcription

1 INSTRUCTOR GUIDE Chapter 12 Interpreting Antarctic Sediment Cores: A Record of Dynamic Neogene Climate SUMMARY This set of investigations focuses on the use of sedimentary facies (lithologies interpreted to record particular depositional environments) to interpret paleoenvironmental and paleoclimatic changes in Neogene sediment cores from the Antarctic margin. Particular attention will be given to characteristics of settings close to the ice (ice-proximal) and far from the ice (ice-distal) in high-latitude settings. In Part 12.1, you will build your knowledge of polar sediment lithologies and the corresponding facies through conceptual diagrams, geological reasoning, and use of core images and core logs (a graphical summary of the sediments). In Part 12.2, the core log for the entire 1285 m ANDRILL 1-B core is presented. You will characterize each of the key lithostratigraphic subdivisions and use your knowledge of depositional facies to write a brief history of the Neogene climatic and environmental conditions in the Ross Sea region. In Part 12.3, you will use your core log reading skills and facies knowledge to evaluate patterns in the Pliocene sediments from ANDRILL 1-B. You will correlate quantitatively patterns in your dataset with cycles in insolation, influenced by changes in the Earth s orbit during the Pliocene. FIGURE ANDRILL Scientists discussing a part of the ANDRILL-1B core in the Core Lab, McMurdo Station Season 2006/7. Courtesy of the ANDRILL Program. Page 1 of 41

2 Goal: to assess the nature of Antarctic climate change during the Neogene. Objectives: After completing this exercise your students should be able to: 1. Recognize and differentiate among the different primary sedimentary facies proximal to and distal from ice in high-latitude settings and connect each facies to a particular depositional environment. 2. Apply their understanding of facies and the lithologic log of the ANDRILL 1-B core to write a brief history of Neogene climate and environmental conditions in the Ross Sea region of Antarctica. 3. Qualitatively and quantitatively describe the patterns of deposition in the Pliocene, and correlate these to orbitally-driven cycles of insolation change. I.How Can I Use All or Parts of this Exercise in my Class? (based on Project 2061 instructional materials design) Title (of each part) How much class time will I need? (per part) Part 12.1 Part 12.2 Part 12.3 What Sediment Facies are common on the Antarctic Margin? 40 to 60 mins + (depends on amount of discussion, extra material used, student experience, or mini-lectures given) ANDRILL 1B The Big Picture 30 to 120 mins (depends on amount of discussion and extra material used, or mini-lectures given, and student experience) Pliocene Sedimentary Patterns in the ANDRILL 1-B Core 40 to 80 mins (depends on student level, amount of discussion and student comfort with math and rate calculations) Can this be done independently (i.e., as homework)? Yes. Would need follow-up discussion in class Yes. Would need follow-up discussion in class Yes. Would need follow-up discussion in class. Might need preparatory review of rate calculations. What content will students be introduced to in this exercise? Science as human endeavor Judgement, decision-making, problem-solving x x x Science as an evolving process / Nature of Science New Research builds on x x x previous research Unexpected discoveries x x x Exploratory research vs. x focused questions Research enabled by technology (technology change through time) x x Page 2 of 41

3 Earth History Archives (nature of the sedimentary record) How do you know about x x x earth history? Types of archives outcrops vs. cores Where do you go to learn x x x about earth history? Land vs. sea vs. ice Geographic awareness x x x Awareness of deep time x x Marine sediments (distribution x x x & controls on distribution) Stratigraphic Principles Relative dating x x x Subdivisions of geologic x time Unconformities, hiatuses, x x x missing records Climate Change Glacial-interglacial cycles x x x Climate change can be x x gradual Greenhouse - Icehouse x Climate change can be x abrupt Regional to global scales x x of change Ocean-atmosphere-bios phere-cryosphere system x x x interac- tions/feedbacks High latitude climate x x x change sensitivity What types of transportable skills will students practice in this exercise? Make observations (describe x x x what you see) Recognize trends x x x (abrupt vs. gradual vs. patterns) Plot data map, graph, x pictoral form Form Questions x Interpret graphs, diagrams, x x x photos, tables Make hypotheses or x x x predictions Test a hypothesis x x x Critical reading & analysis x Synthesize/integrate & x x x draw broad conclusions Page 3 of 41

4 Math Integration Perform calculations x x (rates, averages, unit conversions) & develop quantitative skills Communication Written communication x x Making persuasive, well x x x supported arguments Uncertainty in Science Identifying assumptions x x x & ambiguity Levels & types of uncertainty x x (quantitative vs. qualitative) Significance/evaluation x x x of uncertainties & ambiguity What general prerequisite knowledge & skills are required? None required, but prior exposure to the following topics would be helpful: 1. Antarctic Geography 2. Nature of the Cryosphere 3. Nature of sediment cores 4. General stratigraphic principles 5.General geologic time scale 1.Ability to summarize / write in clear written english 2. Ability to read stratigraphic columns 3. Basic understanding of the concept of different depositional environments 4. Ability to make simple estimates and determine approximate percentages 1. Ability to follow instructions 2. Basic knowledge of what a sed core is 3. Ability to read stratigraphic columns 4. Basic math skills (rate calculations) 5. Knowledge of long-term orbital variations of eccentricity, obliquity, and precession (see Climate Rhythms module) What Anchor Exercises (or Parts of Exercises) should be done prior to this to guide student interpretation & reasoning? 1.Intro to Cores exercises 2.Cenozoic Overview exercises 3.Seafloor Sediments exercises 4. Past Antarctic Climates exercises Part 1 of this exercise 2.Intro to Cores exercises 3.Cenozoic Overview exercises 4.Seafloor Sediments exercises 5. Past Antarctic Climates exercises 1. Parts 1 and 2 of this exercise; 2.Intro to Cores exercises 3.Cenozoic Overview exercises 4.Seafloor Sediments exercises 5. Past Antarctic Climates exercises 6. Climate Rhythms exercises Page 4 of 41

5 What other resources 1.World map or or materials do I globe; Map of Antarctica or the Arctic need? (e.g., internet access to show 2.Internet connection and data pro- on-line video; access to maps, colored jector for viewing pencils) videos, and access to supplementary materials 3.Document camera or over head projector for discussion of figures 4. You may want to print out/laminate images in Figure 3 for use in class 1.World map or globe; Map of Antarctica or the Arctic 2.Internet connection and data projector for viewing videos, and access to supplementary materials 3.Document camera or over head projector for discussion of figures 4. Calculators 5. Highlighters help students select abundant facies in Table 2 (Question 5) 6. Hand-lens or magnifying glass to help read fine print in Figure 1 or print out of enlarged versions 1.World map or globe; Map of Antarctica or the Arctic 2. Document camera or over head projector for discussion of figures 3. Calculators 4. Maps / figures from Past Antarctic Climates exercises for use in discussions 5. Earth-Sun-Moon model if students need assistance with seasons and obliquity, eccentricity and precession. 6. Symbols used for sediment portrayal in sediment core logs What student misconception does this exercise address? 1.All sediment is the same 2. Nothing can live in cold water (diatoms) 3.Diatoms are plants (they are not plants) 4. The depositional environment at one fixed locality cannot change over time (they DO change over time) 5. There is no order or reason for the sedimentary sequences that we see. 6. It is all in Antarctica, so it won t tell us anything useful about climate change because it has always been cold there 1.There really isn t much one can do with a a sediment core from the seafloor 2. Only igneous rocks provide paleomagnetic data 3. The information is too complex to make any interpretations 4. Microscopes are used only for biology 1.Thre is no real reason for changes in seafloor sediment 2. All cores of a fixed length represent an equal amount of time (i.e. rate of sedimentation does not matter) 3. Repeated patterns in sedimentation are just chance Page 5 of 41

6 What forms of data 3-D sketch, cross are used in this? (e.g., section, written graphs, tables, photos, descriptions, core maps) images, core logs What geographic locations Antarctica are these datasets from? How can I use this exercise to identify my students prior knowledge (i.e., student misconceptions, commonly held beliefs)? Instructor grading of exercises checks on student understanding of: Sedimentary environments, order of depositional events (& Walthers Law); sedimentological terminology and reasoning; ability to predict and develop hypotheses and test them; how to apply knowledge of depositional environments in core interpretation Videos, core logs, tables Antarctica (Ross Sea region) Instructor grading of exercises checks on student understanding of: Data that can be collected from a sediment core; estimate percentages; develop simple reasoned interpretations Core logs, thicknesses, age data Antarctica (Ross Sea region) Instructor grading of exercises checks on student understanding of: Reading core logs; complete guided rate calculations; make connections (orbital cycles) How can I encourage 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.Ask students: what they found interesting/useful? 2.Ask students: what was new? 3.Ask students: what questions it makes them want to ask? See suggestions in Summative Assessment section at end of See the supplemental materials and reference sections at end of Page 6 of 41

7 What is the Context This could be used as a final review & capstone activity in an for use of these exercises? upper-level geoscience course that incorporates sedimentology or introductory geoscience course, or as an introductory review in an climate change. Part 1 could be used to introduce students to sedimentary facies and Walthers Law; Part 1 could be a stand-alone exercise. Part 2 could be used as a stand-alone exercise for students already familiar with glaciomarine depositional settings, but it does require the understanding that is built in Part 1. Part 3 is designed to follow Part 2, but it could (if adapted) be a stand-alone exercise IF students are adequately prepared. ALL of these exercises benefit enormously from construction of knowledge in the Past Antarctic Climates Ch 11 exercise, although they do not depend on it. 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 Chapter 12. This instructor guide contains the same sections as in 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. Part What Sediment Facies are Common on the Antarctic Margin? Figures 12.2 & 12.3 illustrate the variety of depositional environments and sediment types possible at the margin of a glacially influenced land mass, such as in the Ross Sea region, Antarctica. Figure 12.2 shows outlet glaciers from the East Antarctic Ice Sheet (EAIS, upper left) cutting through the rocks of the Trans Antarctic Mountains to the Ross Sea (lower right). Note that Nothofagus trees would have only been present during a period of temperate or warmer conditions. In Figure 12.3 the glacier (stippled) is flowing from right to left, into Antarctic Ocean waters. The different sediment lithologies are labeled (e.g., diamict). Page 7 of 41

8 A A B A C B A B D C B C D FIGURE General paleoenvironmental setting for sedimentation along the flanks of the Trans Antarctic Mountains in late Oligocene time. From Hambrey et al., Nothofagus refers to southern beeches (Genus Nothofagus, Family Nothofagaceae) which are native to temperate oceanic to tropical regions in the southern hemisphere. FIGURE Cross section showing a conceptual model for growth and decay of a tidewater glacier in Lambert Graben Fjord, Lambert Glacier Region, Antarctica. From Hambrey and McKelvey, The vertical scale is in meters and the horizontal scale is in kilometers. Letters A D are explained in Question 1. 1 Match locations A D in the conceptual model (Figure 12.3) to locations within the paleonvironmental setting (Figure 12.2). Do this by writing letters A D on Figure In the following table, explain why you placed each letter where you did. If this exercise is done in class you could start by asking the students to compare and contrast the two figures. You may need to provide guidance in reading the captions, and emphasize the importance of reading figure captions and legends, and review the difference between a cross-section and a 3-D visualization, as well as conceptual models. Students may need reminding that the white Page 8 of 41

9 represents rocks sticking up above the pale blue ice, and that the sea is gray. For information on the role of the grounding line go to or Conway (1999) or Anderson (2007). The Lambert Glacier region is not the closest comparison to the Ross Sea Region, but these figures are the best diagrams available in the published literature. Some of the selections simply involve matching words for environmental settings others involve 3-D thinking. For more detailed discussion of Glacial Marine depositional settings refer to the texts of Martini et al. (2001), Menzies (2002), Benn and Evans (1998), and Bennett and Glasser (1996). Location A. On the surface of, and beneath, grounded outlet glacier Your Reasoning Location (see Figure 12.2): Beneath the ice; below (down-ice of) the crevasses that mark the transition from the East Antarctice Ice Sheet (EAIS) the valleys occupied by the outlet glaciers. The location needs to be up-ice from the grounding line (which can be treated as the position of the coastal plain beneath the ice). Reasoning: If the ice melts, any sediment on the ice surface would be deposited where the ice was. The location is far enough back from the front of the ice to be above the maximum reach of seawater under the ice. Arguments for how far up-ice the location could be are complex relatively little is known about how much sediment is carried and deposited by the EAIS. B. Region where a braided river, or subglacial meltwater, enters ocean waters Location (see Figure 12.2): Either where it says braided river OR beneath the ice close to (above or below) the grounding line, which in this case can be approximated as the position of the coastal plain beneath the outlet glacier. Reasoning: Sub-glacial and pro-glacial streams may carry large volumes of sediment. These streams may transition to braided streams away from the ice front. An important alternative is the environment beneath the ice both above and below the grounding line; sub-glacial water can transport and organize glacially deposited sediment and sediment that is melting out of the overlying ice. C. Marine region characterized by icebergs Location: (see Figure 12.2): Anywhere beneath icebergs Reasoning: This location is beneath current icebergs. Note that Figure 12.2 is a snapshot for a fixed moment in time; icebergs will move away front the ice front until they melt; under appropriate conditions they may also move parallel to the coast this means that there will be a zone in front of the ice in the ocean where icebergs may be present; this zone will essentially run approximately parallel to a coastline that is marked by outlet glaciers and ice shelves. D. Marine region below wave base and seaward of melting icebergs Location: (see Figure 12.2): Anywhere in the bottom right of Figure 12.2 to the right of the dashed line marked below wave base Reasoning: This location is seaward of the line that marks wave base, and is also seaward of any of the icebergs. This zone will broadly parallel a coastline marked by outlet glaciers and offshore icebergs. The following section introduces several important lithologies that are diagnostic of ice-distal to ice-proximal depositional settings in polar regions. Note that these lithologies are more specific than the general marine lithologies (e.g., siliceous ooze, glaciomarine) of the global ocean that were introduced in Chapter 2. Answer Questions 2 5, based on: The information on sediment type in the boxes below Figures 12.2 & 12.3 The short videos 5:Telling Time (2007) and 6:Cenozoic Global Climate (2007) from Page 9 of 41

10 The following four questions focus on only four of sediment types deposited near and at glaciated continental margins. These are the end-member sediment types used here for building the sediment core interpretations. For images of these settings go to: SEDIMENT TYPE: DIATOMITE Diatoms are an important group of single-celled free-floating photosynthetic protists (i.e., phytoplankton) that precipitate an opaline silica shell. They are particularly important in areas of open water around Antarctica. Some species of diatoms thrive in the ocean waters under ice shelves. Sediment made up of diatoms is called diatom ooze or siliceous ooze if it is unlithified, and diatomite if it is lithified. These sediments are typically a pale yellowish brown or greenish grayish yellow in color and may be bedded or laminated; sometimes they are bioturbated. 2 If you are interpreting the history of depositional conditions on the continental margin of Antarctica, what important piece of environmental information about a location would you gain from the presence of diatom-rich sediments at that site? For images and information on diatoms and diatomite to supplement this information go to: and The presence of diatom-rich sediments indicates open-water conditions, with the water being clear enough (free of suspended sediment) for diatoms to photosynthesize, and subsequently settle to the seafloor. Conversely, this indicates that the immediate vicinity was not covered by ice, or beneath a sediment plume. SEDIMENT TYPE: SEDIMENT CONTAINING ICEBERG-RAFTED DEBRIS When a glacier terminates in the ocean, icebergs can break off (or calve ) from the front of that glacier or ice shelf, and drift out to sea. The icebergs carry sediment particles that were eroded by and embedded in the glacier as it moved across the land. The particles range from clay- to gravel-sized. As the iceberg melts, the sediment particles are released and settle on the seafloor. The most noticeable particles deposited by this process are the larger gravel-sized particles, which are called iceberg-rafted debris (IRD); these particles are particularly noticeable as isolated large grains within finer grained sediment (terrigenous sands, silts, and muds). The finer grained sediment containing the IRD may also contain a low concentration of diatoms, particularly at locations further from the end of the glacier as open-marine conditions become more dominant. 3. If you are interpreting the history of depositional conditions on the continental margin of Antarctica, what important piece of environmental information about a location would you gain from the presence of a mud that contains a few percent of diatoms, as well as isolated gravel-sized (or larger) grains? The presence of diatoms indicates that the region is in the vicinity of open water, which allows diatoms to photosynthesize. They subsequently die naturally and fall to the seafloor. The presence of mud indicates fall-out of suspended sediment that could be derived from a sediment plume or associated with turbidity currents carrying material winnowed from diamictites at the submarine exits of subglacial streams. The presence of isolated gravel grains tells us that icebergs carrying sediment Page 10 of 41

11 were in the process of melting and releasing coarser particles for sedimentation on the seafloor below. This setting is transitional between permanently open ocean and ice-shelf, and indicates the presence of icebergs. SEDIMENT TYPE: DIAMICTITE A diamictite is a deposit of poorly sorted clastic sediments in other words, it contains a mixture of particles ranging in size from very small ( clay ) to very large ( gravel ). In addition, diamictites contain little or no internal pattern of layering (i.e., they are unstratified or poorly stratified). Diamictites can be deposited by several processes, including glacial activity and landslides. A diamictite that was deposited directly from glacial ice is called till. Most tills are deposited beneath a glacier, rather than along the glacier s sides. 4 If you are interpreting the history of depositional conditions on the continental margin of Antarctica, what important environmental information about a location would you gain from the presence of a till at that location? The presence of till indicates that the immediate region was beneath glacial ice.the glacial ice could be on land or below sea level. Page 11 of 41

12 SEDIMENT TYPE: WELL-SORTED SANDS AND/OR GRAVELS In some places at the base of a glacier, large quantities of meltwater form subglacial streams. Where these subglacial streams flow out into the ocean from the glacier s front, the flowing water can remove the smaller sediment particles from the glacial till. This process of removing the smaller particles is called winnowing and leaves a deposit of well-sorted sands and/or gravels close to the end of the glacier. The smaller particles are carried away from the glacier front as clouds of muddy water (also known as sediment plumes ) and are deposited at more distant locations where turbidity current deposits, IRD, and diatoms may be present. 5 If you are interpreting the history of environmental conditions on the continental margin of Antarctica, what important environmental information about a location would you gain from the presence of well-sorted sands and gravels at that location? The presence of well-sorted sands and gravels indicates the reworking of a sedimentary deposit by subglacial water. The water removes the finer particles (and transports them away from the submarine exit of the subglacial stream), and leaves behind coarser particles that are better sorted (and also typically more rounded). The important environmental information is that moving water deposited /organized the sands and gravels. In glaciomarine settings this takes place where glacial till is available for reworking, near the submarine exits of subglacial streams, typically in the vicinity of the grounding line. The sediment outflows produced during this process may generate turbidity currents, as well as rising towards the ocean surface, producing the sediment plumes. 6 In the table below, name and describe the sediment type (e.g., diamictite) expected for each depositional environment listed. Use the information in Figures 12.2 & 12.3, the lithologic information in the text boxes, and your answers to Questions 1 5. Succinctly explain the environmental processes that produce those sediment characteristics. Depositional Environment Open ocean, beyond iceberg influence Sediment Type and Description Diatomite / siliceous ooze Fine-grained terrigenous sediment Processes Producing the Lithologies Diatoms can photosynthesize in clear, open water Some fine-grained sediment of terrigenous origin may reach the site through fall-out from suspension Open ocean, within iceberg influence Isolated larger grains (or clasts) within finer-grained sediment Terrigenous sands, silts and muds Some diatoms Some clear open water, allows limited diatom photosynthesis Melting icebergs carry coarse grained sediment, and drop it when they melt Some sediment of terrigenous origin may reach the site through turbidity currents or fall-out from suspension Page 12 of 41

13 Glacial front, near the exit of Well-sorted sands and/or a subglacial stream gravels. Submarine glacial front near the exit of a subglacial stream Submarine deposits Subglacial (i.e., underneath the glacier) Till - Poorly sorted clastic sediments containing a mixture of particles ranging in size from very small ( clay ) to very large ( gravel ). Unstratified or poorly stratified. Ice does not sort or organize material Ice includes fine particles that result from abrasion of rock material as well as larger, angular particles 7 Images of four core intervals are shown in Figure Compare them with your summary of sediment characteristics (Question 6) and the information presented in the boxes describing Sediment types. In the table on the next page match each of the sediment images/descriptions to one of the depositional environments you described in Question 6, explain your reasoning and ask question(s) about features you observe that do not seem to fit the model. Students may need additional links to images of glacial and glaciomarine sediments to help them answer this question. Students typically need to be reminded to draw on the knowledge they have built in part 1 of Ch.12. Page 13 of 41

14 FIGURE Four core intervals from ANDRILL 1-B. Photos courtesy of the ANDRILL Program. Page 14 of 41

15 Sediment Core ANDRILL 1-B mbsf Sediment Name & Depositional Environment Name: Diatomite Environment: Open ocean with no suspended sediment where diatoms can photosynthesize prior to dying and falling to the ocean floor. Reasoning & Questions Some students will wonder whether this is fine-grained terrigenous material or wonder why they can t see the diatoms. Remind them about the size of diatoms. The diatomites are often disrupted when they are over-ridden by advancing ice; this accounts for some of the disruption. ANDRILL 1-B mbsf Name: Silt, diatomite, and isolated large clasts ( dropstones ) Environment: Area of iceberg rafting, close to open ocean with clear water; some fall-out of terrigineous sediment transported from basin Some students will want to know which way is up use the draping of sediment over the IRD clast to answer this question. ANDRILL 1-B mbsf Name: Conglomerate (well-sorted) Environment: Submarine deposits, formed near the submarine exit of a subglacial stream, at or near the grounding line Students may have difficulty distinguishing between this and the Diamictite. Show them how the clasts in the diamictite are matric supported vs. the clast support in this section of the core. ANDRILL 1-B mbsf Name: Diamictite (Till) Environment: Sub-glacial deposition Note that diamictites are highly variable in their grain size. Some diamictites contain dominantly cobble- and boulder-sized clasts, whereas others contain granuleto pebble-sized clasts. Page 15 of 41

16 SEDIMENTARY FACIES When sedimentologists describe a core or outcrop, they identify distinctive sediment lithologies based on a suite of objective observable properties, such as grain size, sorting, composition, and/or color (see Chapter 2). When lithologies are interpreted in terms of their location within a complex three-dimensional conceptual model of the region s depositional setting, lithologic facies are defined. The term facies can also be used in conjunction with the interpreted environment e.g. ice-proximal facies or ice-distal facies. Each of the sediment types you described and interpreted is one of the facies recognized by sedimentologists as they describe and interpret sediment cores from the Antarctic margin. Because the environment that exists at one location can change through time, the facies being deposited at that location can also change. In other words, depositional environments (and the facies deposited in those environments) migrate laterally as glacial ice advances and retreats across the area. Over time, such changes produce a vertical stack of different sedimentary facies; this is known as Walther s Law. At this point you could do a mini-lecture or short review of Walther s Law. A variety of activities or exercises on Walther s Law are available at Alternatively (and probably more effectively) it could be presented after the students have essentially used it to answer question Examine Figure 12.5, and imagine that you are stuck underneath the glacier at the point labeled A. Over time, as the glacier retreats toward the south (the right side of the diagram), all of the other depositional settings shown in Figure 12.5 will also shift toward the south. When the glacier has retreated furthest to the south, open-marine conditions extend to A. Demonstrate your understanding of Walther s Law (see box on Sedimentary Facies) by making a list of the sedimentary facies you would expect to be deposited at location A in this scenario. Be sure to put the oldest deposit at the bottom and more recent deposits at the top. FIGURE Cross section showing part of conceptual model for growth and decay of tidewater glacier in Lambert Graben Fjord, Lambert Glacier Region, Antarctica. Scales are in meters (vertical) and kilometers (horizontal). Vertical scale is in meters above or below sea level. Horizontal scale is in kilometers. This figure is nearly identical to Figure From Hambrey & McKelvey, Page 16 of 41

17 Youngest deposit: Diatomite (Diatom Ooze) Silt, diatomite, and isolated large clasts ( dropstones ) Conglomerate, Well-sorted Sands and Gravels Oldest Deposit: Diamictite or Glacial Till Students may need reminding that they can just use the four end-member sedimentary facies they have already examined. If this is being done in class, this could segue into a discussion on the nature of the transition between each facies. Many students don t understand (or struggle with) the fact that tills / diamictites are deposited underneath advancing ice they think that the till represents material pushed in front of the ice. This may need to be reviewed with them. 9 Use the core logging sheet on the next page to draw a simple stratigraphic column that shows the vertical stack of sediments you would expect to be deposited at A as the glacier retreats landward (i.e. to the south) and then advances seaward (i.e. to the north). Note that you will not be able to fill in the columns labeled color, sketch, and depth, so leave these columns blank. In the comments section (a) label each facies in the vertical sequence, and (b) indicate the depositional environment present at A when that facies was being deposited. For help with development of your predictive model go to the simulation at The model available at the ANDRILL website provides students with assistance in understanding how facies changes take place. You can also access it through the ANDRILL Education page ( make sure you scroll down!) The Core logging sheet used here is part of a core-logging sheet available at The full logging sheet reminds core loggers that outcrops and cores can be logged using this sheet. It is important that students realize that seafloor cores provide the same data that are available from land cores, and that both land and seafloor cores are used when outcrop data is not available. One can also log data when measuring a section at an outcrop. No instructions regarding the vertical scale have been given to the students. This is to make them think about how thick the sedimentary sequence is likely to be. Based on this cross section (Figure 12.5) alone they could infer that it might be as thick as about 500m (based on the thickness of the sediments shown there), although it could be very significantly thinner, dependant on sedimentation rates and climate regimes. Many of the sequences preserved in the Andrill core are 5 m 20 m thick; some of them include diatomite portions that are as much as 80 m thick (Krissek et al., 2007). You could direct students to select the vertical scale as they think is appropriate, and have a discussion about scale selection following completion of this part of the exercise, or you could direct them to present it as a 15 m sequence. It is convenient to get them to draw a 15 m sequence because the ensuing question (question 9) asks the students to compare their sequence with one of the ANDRILL motifs, which is ~ 13 m thick. Students may need some review of core-logging and use of patterns. Page 17 of 41

18 Stratigraphic column/core log showing predicted sedimentary sequence produced at site A (Figure 12.5) during ice retreat. Page 18 of 41

19 SEQUENCE MOTIFS Sedimentologists working on the Antarctic continental margin have recognized several typical sedimentary sequences that develop during the advance and retreat of the ice. One example of a typical sequence, called a sequence motif by the ANDRILL sedimentologists, is shown in Figure Note that the phrase ice-proximal is used to mean sediments deposited closer to the ice and ice-distal to mean sediments deposited further away from the ice. FIGURE Example of a sequence motif from ANDRILL core 1-B, mbsf. Green is diamictite, brown is conglomerate or breccias, gray is terrigenous siltstone and sandstone. The columns on the right of the graphic log show the main sedimentary features by symbols, facies number, and a glacial proximity graph. In the glacial proximity column, the width of the dark bar indicates ice proximity (wider = more proximal). From Krissek et al., Page 19 of 41

20 10 How does the vertical sequence you proposed for A in Question 9 compare with the vertical sequence shown in Figure 12.6? What is similar between the two? What is different? What explanations can you propose for any differences? The ideal or typical student core log/strat column answer shown here includes all the sediments discussed in questions 2-5; from the bottom this is: diamictite; [conglomerate]; sands and gravels; terrigenous sands, silts, and muds with some IRD and minor diatoms; diatomite; terrigenous sands, silts, and muds with some IRD and minor diatoms; sands and gravels; [conglomerate]; and diamictite. Overall similarities are: Both have diamictite at the top and base Both have finer-grained sediment in the middle In comparison to the student section, Motif 3 is different because it: Lacks any diatomite (instead it contains fine-grained terrigenous material) Includes well-developed interstratified mudstone and sandstone as part of the initial retreat facies above the conglomerate. Includes diamictites with sandstones and siltstones as part of the retreat facies Lacks a transition back to diamictite that includes sands and gravels and/or conglomerate This could be explained by an increased amount of terrigenous material being supplied to the area represented by motif 3; the turbid water could prevent diatoms from photosynthesizing. The interbedded diamictites and sandstones/siltstones in motif 3 could represent mass-flow deposits from the submarine ice-front. The lack of an ice-advance facies underlying the upper diamictite could represent erosion associated with the advancing ice. Well-sorted sands and gravels may be absent because the exits of the subglacial streams do not occur at every position along the ice front or grounding line; as a consequence there is a real chance in whether the meltwater influence is recorded at any particular location during each ice advance and retreat. Note that Krissek et al. (2007) have presented 3 sequence motifs that represent ice retreat and advance in ANDRILL 1-B. It is possible that variations in the motifs reflect retreats and advances under conditions that change from polar to sub-polar. 11 In the next section (Part 11.2), you are going to be presented with the entire 1285 m core log for ANDRILL 1-B. Recall that a core log is a graphical summary of the sediments recovered from one location. How could you use the information on depositional environments, facies, and sequence motifs that you learned in this investigation to aid in your interpretation of the 1285 m core log for ANDRILL 1-B? List your strategies. This is designed either for use as a discussion question or an essay question. The key points are: Identify which of the four key sediment types are present Look for large-scale patterns similar to those predicted and observed in questions 9 and 10 Identify possible major breaks, changes, or transitions in the core Look at a smaller scale for the patterns similar to those in motif 3 or the sequence drawn by the student Page 20 of 41

21 Part ANDRILL 1-B The BIG Picture ABOUT ANDRILL ANDRILL (ANtarctic geological DRILLing) is an international program involving scientists, students, engineers, technicians, drillers, and educators from the USA, New Zealand, Italy, and Germany. ANDRILL s goal is to drill and recover sediment cores from the ocean floor beneath the Antarctic ice shelf and sea-ice, where the most complete sedimentary records of Antarctica s glacial, climatic, and environmental history for the past 65 million years are most likely to be found. In order to drill and recover these cores, a new drilling and coring system was developed that could be placed on top of the ice shelf and sea-ice. Two ANDRILL projects have been completed successfully: the McMurdo Ice Shelf Project (MIS) in late 2006 and the Southern McMurdo Sound Project (SMS) in late Studies of the cores recovered during these projects are presently underway and will continue for many years to come. Preparatory work on Antarctic Geography and selection of the MIS and SMS core sites was included in exercises in the Past Antarctic Climates module. The documentary produced by WGBH and Nova is centred around the ANDRILL Core, and includes excellent field and lab shots, as well as good diagrams and explanations. It is available at: It is well-worth showing in class or assigning for students to watch. This investigation will focus on sediments recovered during the MIS Project. As an introduction to ANDRILL operations and sediment core description, watch the short ANDRILL video journals listed below (downloadable from The first two videos provide background information; if students have not completed the exercises in the Past Antarctic Climates Module, it would be very beneficial to watch them. Students that have completed those exercises will already have seen them. Videos 7-12 summarize that datasets that are obtained on-ice. 4: Selecting Where to Drill (2007) 5 mins 50 sec - Provides sense of Antarctica and McMurdo setting, and selection of a drillsite. 6: The Drill Rig(2006) 5 mins 18 sec - Introduces drill site personnel, and drill rig operation 7: Physical Properties & Logging (2006) 6 mins 52 sec Introduces concept of Discipline Teams, and reviews work undertaken at the actual drill site (vs. at the Lab in McMurdo). Explains initial work done by core technicians, evaluation of fractures or breaks either associated with faults or with that have developed as a consequence of drilling. Scanning of core to produce 2-D image of the 3-D core surface. Concluding with a review of the datasets collected in the physical properties Lab (Porosity, Density, Electric Resistivity, Magnetism, and Sonic Velocity). 8: Core Curation (2006) 6 mins 1 sec Commences with Thanksgiving on ice, then proceeds to explain process of splitting the core note that the example is a diatomite or a diatom-bearing mudstone. Digital images of the split core surface are obtained, and an XRF core-scanner is used to document the geochemistry at regular intervals; color reflectance data are also collected with a spectrophotometer. Page 21 of 41

22 9: Sedimentology Team (2006) 6 mins 52 seconds The core seen in this video is for the most part comprised of terrigenous sediments with some diamictite and some IRD. The sedimentology team records the physical characteristics of the sediment core these include the size of particles, the presence of layering, the color of the particles and layers. This data is recorded using PSICAT. They also make smear slides at regular intervals, which allow for microscope analysis of the component particles. 10: Paleontology (2006) 6 mins 34 secs The paleontology team samples the core for diatoms, foraminifera, macrofossils, and reworked material. 11: Petrology (2006) 7 mins 18 secs The petrology team collects core data on the composition, quantity, size and distribution of clasts. They also examine the volcanic clasts for information on age and composition, examine the sediment porewater geochemistry, and complete thin-section analysis of selected sediment and clasts. 12: Paleomagnetism (2006) 7 mins 3 secs Determination of the paleomagnetic character of selected samples. 1 Use the information in the video journals to write a paragraph that summarizes the types of observation and data that are collected from the sediment cores before they are shipped to the Antarctic Core Repository in the United States. Core data that are retrieved on ice during the core characterization phase of the research are retrieved at the Drill site and in Crary Lab at McMurdo Station. Specific data types are listed here next to the video that provides the information.the data collected is summarized under each video title above. LITHOSTRATIGRAPHIC UNITS (LSUs) Figure 12.7 (next page) is a graphic summary (i.e., core log) of the sediments recovered in the 1285-m long ANDRILL MIS core (officially named AND-1B ). In this graphic summary, glacial tills are shown in green, mudstones (generally containing IRD) are shown in gray, well-sorted sands and gravel are shown in brown, and diatom-rich sediments ( diatomites ) are shown in yellow. Volcanic rocks and sediments rich in volcanic material are shown in orange, but will not be considered further here. As the core was described, the sedimentologists identified lithostratigraphic units (LSUs). The LSUs are intervals of the core that are either: Dominated by a single sediment type (i.e., facies) or Exhibit a relatively consistent pattern of interbedding of two or more sediment types (i.e., two or more facies). The LSUs are numbered in increasing order down-core and some LSUs are further subdivided (e.g., LSU 2.1, 2.2, 2.3) to highlight more subtle differences in the relative abundance of sediment types. 2 In order to examine the differences between these LSUs, use Figure 12.7 to complete the table (located after Figure 12.7) by estimating the contribution of each sedimentary facies (i.e., diatomite, mudstone, till, and sand and gravel) to each LSU. To make each estimate, first identify the LSU number from the blue/white column on the left-hand side; then sum the thicknesses of the occurrences of each of the four facies within that LSU and divide by the total vertical thickness of that LSU. Page 22 of 41

23 Pliocene (note this label was accidently omitted from the student version) FIGURE Core log for ANDRILL 1-B (ANDRILL MIS ), LSU numbers shown in blue/white column on the left. The wavy lines represent breaks or gaps in the sedimentary sequence caused by erosion by grounded ice. These gaps in the record are called unconformities. Green = diamictite, yellow = diatomite, brown = conglomerate or breccias, grey = terrigenous siltstone and sandstone, orange = volcaniclastic sediment. From Krissek et al., Page 23 of 41

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25 In this part of the exercise the students have a chance to see how good they are at doing visual estimations. You may want to get the students to challenge each other to do preliminary visual estimations, and then to do calculated percentage determinations. Note: LSU 5 is not considered here because it is almost entirely volcaniclastic sediments. Students may be confused as to how to deal with the missing section near the top of the core. It should not be counted only count the thickness of the material retrieved. LSU % Diatomite % Mudstone & Siltstone % Sand & Gravel (& conglomerate) % Till (Diamictite) 1 ~60 m 0/60 0% Range 0% 5/60 8% Range 5-15% 0/60 0% Range 0% 55/60 92% Range 75-98% 2 ~40 m 5/40 12% Range 5-20% 10/40 24% Range 15-35% 7.5/40 19% Range 12%-25% 17.5/40 44% Range 35-55% m 67.5/ % Range 20-35% 50/ % Range 14-28% 20/ % Range 5-15% 100/ % Range 35-55% m 110/ % Range 45-65% 15/ % Range 2-10% 37.5/ % Range 15-25% 40/ % Range 15-25% m 0/305 0% 0% 220/305 72% Range 62-82% 10/305 3% Range 0%-10% 75/305 25% Range 15-35% m 0/154 0% 0% 10/154 6% Range 4-10% 0/154 0% 0% 145/154 94% Range 85-98% 3 The presence (100%) or absence (0%) of diatomite (open marine) compared with the till (sub-glacial) is clearly a key criterion for identifying the LSUs. However, abundance variations of as little as 5 10% in one of the four facies can help distinguish between LSUs. Based on your estimates in Question 2, are there significant differences in facies abundances in the LSUs identified by ANDRILL scientists? What are the significant differences? Page 25 of 41

26 Generally, yes, there are significant differences in facies abundances. Some students will be concerned as to exactly why the LSU boundaries were placed at a specific location, especially the boundaries between LSU 2 & 3, or 3 & 4. Students can generally appreciate that there is a visual difference between the LSU s, but often they cannot see the reason for detailed aspects of the boundary location. The easiest explanation is that the sedimentologists logging the core used some of their observations to identify a fundamental break; it could be in grain composition, overall color, matrix, cement, etc. Differences see percentages in Table 4 Use your answer from Question 2 and Figure 12.7 to summarize the major compositional changes that take place as one moves up-core from LSU 6.4 to LSU 1. LSU 6.4 is dominated by three thick (5-30m thick) diamictites that are separated by two ~5m thick siltstone/sandstone horizons. The upper diamictite has a slightly sandy matrix. The overlying LSU (6.3, 6.2, and 6.1) is dominated by terrigenous sediment (sandstone, siltstone and some mudstones, with some (5-40 m-thick) diamictites; these units probably include IRD. LSU 6.3, 6.4 & 6.5 grades upward into a volcaniclastic LSU. The volcaniclastic LSU is overlain by LSU 4, which is distinct in that it is dominated by thick diatomites (up to 60 m thick), with some mud, silt, and sand, as well as thinner (5-15 m thick) diamictites. Overall, LSU 4 is diatomite-rich near the base, and diatomite-rich near the top with a 20m-interval of varied sediment types between the diatomites. LSU 4 is overlain by LSU 3, which is characterized by repeated intervals of diamictite, diatomite, and terrigenous material (sandstone/siltstone/mudstone). LSU 2 is distinguished from LSU 3 by its lack of diatomite, and the presence of volcaniclastic sediment along with diamictite and terrigenous sediment. The uppermost LSU 1is almost entirely dominated by diamictite. As you learned in Part 12.1, the environmental conditions that existed at a location during times in the past can be interpreted from the characteristics of the sedimentary facies deposited during those times. All students have to do is pick the two highest percentages for each LSU from the table they constructed in Question 2, and then refer back to part 12.1 of this Chapter where they learned about sedimentary facies in glaciomarine settings. 5 In order to develop a general environmental history for the ANDRILL-1B site, use your data from Question 2 (Part 12.2) and notes from Part 12.1 of this chapter to complete the table below. Page 26 of 41

27 LSU Most abundant facies 2nd most abundant facies Most common environment 2nd most common environment LSU 1 Till (Diamictite) Mudstone Sandstone IRD Subglacial Ocean with icebergs and sediment plume LSU 2 Till (Diamictite) Mudstone Sandstone IRD Subglacial Ocean with icebergs and sediment plume LSU 3 Till (Diamictite) Diatomite Subglacial Open Ocean LSU 4 Diatomite Till (Diamictite) & Sand /Gravel Conglomerate Open Ocean Subglacial & Submarine exit of subglacial stream LSU Mudstone Sandstone IRD Till (Diamictite) Ocean with icebergs and sediment plume Subglacial LSU 6.4 Till (Diamictite) Mudstone Sandstone IRD Subglacial Ocean with icebergs and sediment plume 6 Use the data from the table you have just completed to write a history of climatic and environmental conditions at the site of ANDRILL-1B, from the time of deposition of LSU 6.4 to the time of deposition of LSU 1. Describe the environments present during deposition of each LSU, the stability or variability of conditions, and whether you would classify that interval as a time dominated by ice (glacials), a time dominated by the absence of ice (interglacials), or a time of repeated glacial interglacial cycles. The key points in this summary are: LSU 6.4: This interval is dominated by tills deposited during glacial advances (glacials?). At least two and possibly more retreats (interglacials?) are indicated through the presence of sandy intervals. This suggests that the area was never open ocean, although it is possible that ice readvances removed finer-grained sediment and diatomites. The sandy intervals probably represent subglacial Page 27 of 41

28 mass flow deposits. LSU 6.1, 6.2, and 6.3: This LSU is characterized by significant percentage of terrigenous sediments (and IRD) especially muds and silts, which are interbedded with diamictites. In contrast to LSU 4 (above the volcaniclastic sediment) this unit lacks any diatomite. This may indicate that open ocean conditions were never reached, or that a very high influx of terrigenous sediment prevented diatoms from thriving. Whilst this period may record retreat of the ice, the sediment record suggests that it does not record as marked a warming as LSU 3, and definitely not as much as indicated by LSU 4. LSU 4: Sediments in this LSU provide evidence for the most long-lived periods of open water (interglacials), with the most long-lived periods being at the base and the top. Between these interglacials there is evidence for at least six ice advances (glacial periods?); there is relatively limited diatomite preserved between the diamictites, suggesting that only rarely were open water conditions achieved between these advances although it is possible that diatomites were removed during periods of ice readvance. LSU 3: This unit is characterized by a repeated diamictites, terrigenous sediments and diatomites, None of the diatomites are as thick as those of LSU 4, which suggests that periods of ice retreat were limited but open ocean conditions did exist between periods of ice advance. This unit represents a transition from the essentially interglacial period of LSU 4, and the glacial of LSU 1; LSU 2 is a further step in that transition that lacks any diatomites. LSU 2: This unit is characterized by relatively thin muds, silts, sands and diamictites, as well as some volcaniclastic material. Noteably absent are diatomites and thick diamictite sequences. This unit probably records a transition from an overall warmer period (LSU 3) to the cooler period of LSU 1. LSU 1: This unit is similar to that of LSU 6.4, it is dominated by till indicating glacial conditions, although the presence of multiple unconformities suggests that much of the sequence has been eroded, it may record far more than the two separate periods of ice advance. Page 28 of 41

29 Part Pliocene Sedimentary Patterns in the ANDRILL 1-B Core Sedimentary sequences that show some kind of repetition or pattern are generally referred to as being cyclic. The cyclicity may take place over a regularly repeated time interval. This time interval is referred to as the periodicity of the cycles (see Chapter 8). In this investigation we will analyze the mbsf interval of the 1285-m long sediment core recovered by the ANDRILL McMurdo Ice Shelf project in 2006/07 (Figure 12.8). Facies patterns in this part of the sedimentary sequence will be evaluated qualitatively and quantitatively to identify possible climate cycles. In Figure 12.8 the narrow yellow intervals are diatomites and the wider intervals in green, brown, and gray are terrigenous sediments that contain common to abundant gravel-sized clasts. These terrigenous sediments can be generalized in this investigation as diamictites (or tills). Note that for Question 1 (below), you should consider the dominant lithologic facies in each interval. Sedimentologists use the term facies or lithology somewhat interchangeably for distinctive packets of sediment. Recognition of each sedimentary lithology or sediment type is based on a suite of objective, observable properties such as grain size, sorting, composition, and color (see Part 12.2). 1 Starting from 227 mbsf and working upwards to 150 mbsf, list the vertical succession of lithologic facies (either diamictite or diatomite) in the table below. Start the list at the bottom of the table, so the oldest intervals are at the bottom and the youngest at the top. List the core interval, the thickness of each interval, and the lithologic facies. Core Interval (in mbsf) Interval thickness Lithologies/Facies mbsf mbsf mbsf mbsf mbsf mbsf mbsf mbsf mbsf mbsf mbsf mbsf mbsf 2.0 m 8.0 m 5.0 m 4.5 m 5.0 m 6.5 m 2.5 m 2.0 m 5.0 m 11.0 m 10.0 m 13.0 m 2.5 m Diamictite Diatomite Diamictite Diatomite Diamictite Diatomite Diamictite Diatomite Diamictite Diatomite Diamictite Diatomite Diamictite Page 29 of 41

30 This graphic log was produced using PSICAT, the Paleontological Stratigraphic Interval Construction and Analysis Tool, which is a stand-alone Java based graphical editing tool for creating and viewing stratigraphic column diagrams from drill cores and outcrops. It is customized to the task of working with stratigraphic columns and captures data digitally as you draw and edit the diagram. It is free and can be downloaded from: When selecting the boundaries the dominant color should be considered. In intervals that include an additional sediment type (e.g. yellow for diatomite at mbsf), the sediment type shown by the narrower band of color is a minor component in the matrix of the coarser-grained lithology. Students have greatest difficulty selecting the intervals between 181 and 160 mbsf. Some students will note the missing section just above 160 mbsf; the boundary can be placed at the top of bottom of the missing section it does not make any difference. FIGURE Graphical log of ANDRILL Core 1-B showing interval from mbsf. Yellow intervals are diatomites. Green, brown, and gray intervals are terrigenous sediments. The scale at the top of the column gives grain size. Adapted from Naish et al., Page 30 of 41