Oceans, Coasts and Climate Change
Tectonics, oceans and climate change: The opening andclosing of ocean gateways – effects on climate and oceancirculation
This component of the course aims to introduce the links between tectonics, ocean circulation and climate change. You will have three 2-hour practical classes to work on this exercise (Week 7, 8, 9).You are expected to complete and submit an essay by the end of Week 9 (Friday 29thApril 23:59pm). Essay format is outlined on page 8. Remember that the concepts covered in this practical, as well as all practicals, are examinable.
A late submission of the essay will incur a 5% penalty for every 24 hours after the submission deadline. Late penalties can be waived, or alternative arrangements made, in cases of Special Consideration approved by the Faculty of Science. Please read ALL instructions and ask the lecturer or tutor if you have questions. Essays must be submitted in pdf format to Canvas. The assessment for this Practical is worth 15%of your final mark for this unit.
Climate change has (and will continue to have) a profound impact on our lives in a multitude of ways. The scientific community is quite confident that present-day global climate change is instigated by anthropogenic greenhouse gas emissions into the atmosphere (IPCC, 2013). Nevertheless, global climate has never been stagnant; it has undergone considerable fluctuation since Earth’s inception ~4.6 billion years ago, alternating between “icehouse” periods (where global temperatures favour the formation of continental ice sheets) and “hothouse” periods (where global temperatures are so high that no glaciers form on Earth whatsoever). In order to contextualise the rapid rate of change we are witnessingtoday, we need to understandhow, why, and to what degreeearth’s climatehas changed in the deepgeologicpast.To approach this problem, geologists and geophysicists examine the geologic record (i.e. the “rock record”).
The world’s ocean sediments provide an excellent catalogue of Earth’s majorenvironmental changes. Sediments contain a wealth of information if you know how to read them; apart from recording Earth’s temperature through time (Riebeek 2005), ocean sediments record the ecosystems that were living in the oceans at the time, they can tell you where ancient river systems spilled into the sea, and they reveal where vast continental glaciers locked up fresh water and subsequently melted away. The oceans, covering the lowest points on Earth’s surface, act as excellent traps for these sediments. As a consequence,
however, these areas are often highly inaccessible. Over the past several decades, the scientific community has come to understand the importance of the ocean sediment record in piecing together Earth’s climatic history. In 1968, about a year before Neil Armstrong first stepped onto the moon, the Deep Sea Drilling Project (DSDP), later to be renamed the ODP and then IODP (International Ocean Drilling Project), was created, affording geologists from all over the world the ability to venture out on massive research vessels to collect and analyse deep sea sediments. (see https://www.iodp.org/expeditions/expeditions- schedule)
Because of these drilling expeditions, we have learned that the arrangement ofcontinents and ocean basins can play a dominant role in determining long-termoceanic circulation, sea level and climate. The surface of the planet has been shaped and re-shaped by plate tectonics, where continents break apart to form new ocean basins at the expense of older ocean basins that are destroyed at subduction zones. The process of ocean basin formation is part of the Wilson Cycle, named after the Canadian geologist J. Tuzo Wilson. Subsequent lectures will cover Plate Tectonics in more detail. For this exercise, the important thing to keep in mind is that continents move around, and consequentlythat oceanic gateways open and close through time, affecting the large-scaleoceaniccirculationthatgoverns globalclimate.
The establishment of a continuous circum-Antarctic oceanic current during the Eocene- Oligocene transition (~34 Ma) is thought to have been one of the most profound shifts of oceanic circulation that was principally governed by plate tectonics (Fig. 1). At around the same time, vast inland ice sheets formed on the Antarctic continent, which caused global sea level to fall. Though the establishment of a deep-water oceanic gateway between Tasmania and Antarctica by ~34 Ma is consistent with the timing of Antarctica glaciation, it can be argued that the Drake Passage (between South American and Antarctica) only opened at 22 2 Ma (Barker and Thomas, 2004), suggesting that the development of the Antarctic Circumpolar Current (ACC) may be more complex than previously thought. Paleoceanographers continue to debate the regional climatic consequences of the ACC’s formation; initially, some argued that the gradual establishment of the ACC led to the thermal isolation of Antarctica, instigating glaciation on the continent. Others argue that the emergence of this ocean gateway played a lesser or minimal role in the onset of Antarctic glaciation.
The evidence for and the processes that govern plate tectonics will be covered in later classes. However, to begin with we will examine the relative motion of the southern continents since 100 Ma (the abbreviation Ma for mega-annum describes the point in time that is so many millions of years before the present) and how the changing geography of the continents has affected the development of the Southern Ocean and the Antarctic Circumpolar Current.
You are provided with a reconstruction of the South Polar continents made by the Earthbyte research group here at Sydney University (see paper by Wright et al., 2020). In this paper we have used combined geological and geophysical data to create maps of the age distribution of the ocean floor through time, which you can view interactively using a web browser on the GPlates Portal. The age of the ocean floor is of interest to us because it has a straightforward connection to water depth.
You can view (and download) a reconstruction of the water depth, derived from the age of the crust, in the form of an animation (mov format) in 1 million year intervals on Canvas, which renders the evolution of the Southern Ocean as a continuous process. The sequence highlights that Australia’s separation from Antarctica was the last stage in the extended breakup of Gondwanaland.
- Review the plate tectonic evolution of the southern ocean using the resources we have pointed you to (usethehyperlinksabove)
- Use GeoMapAppto find and describe evidence in the sedimentary record for the onset of the Antarctic Circum-Polar Current
- Summarise the key observational evidence for how the Southern Ocean Basins and gateways have developed during the Cainozoic, in the context of published papers on the topic. This will become the basis for your essay.
Use Google Scholar or the Library website to find relevant articles. You need to have read and understood the following papers to completing this assignment. These papers should be included in your reference list.
- Barker, P.F., and Thomas, E., 2004, Origin, signature and palaeoclimatic influence of the Antarctic Circumpolar Current: Earth-Science Reviews, v. 66, no. 1, p. 143- 162.
- Bijl, P. K., Schouten, S., Sluijs, A., Reichart, G.-J., Zachos, J. C., and Brinkhuis, H., 2009, Early Palaeogene temperature evolution of the southwest Pacific Ocean: Nature, v. 461, no. 7265, p. 776-779.
- Exon, N., Kennett, J., Malone, M., Brinkhuis, H., Chaproniere, G., Ennyu, A., Fothergill, P., Fuller, M., Grauert, M., and Hill, P., 2002, Drilling reveals climatic consequences of Tasmanian gateway opening: Eos,TransactionsAmericanGeophysicalUnion, v. 83, no. 23, p. 253-259.
- Scher, H. D., Whittaker, J. M., Williams, S. E., Latimer, J. C., Kordesch, W. E., & Delaney, M. L., 2015, Onset of Antarctic Circumpolar Current 30 million years ago as Tasmanian Gateway aligned with westerlies. Nature, 523 (7562), 580.
- Zachos, J., Pagani, M., Sloan, L., Thomas, E., and Billups, K., 2001, Trends, Rhythms, and Aberrations in Global Climate 65 Ma to present: Science, v. 292, no. 5517, p. 686-693.
In this part of the exercise you will use GeoMapAppto access the Ocean Floor Drilling portal to find evidence of the Eocene-Oligocene transition in the Southern Ocean. You will need to make maps and graphs, as well as extract core logs and photographs, for use in your essay. Note:GeoMapAppisfreelyavailablefordownloadhere:http://www.geomapapp.org/
- Open GeoMapAppfrom the Start Menu (or download the application to your personal computer, install the program, and open it).
- It will download the Java-based application (which can take a few minutes), and will launch automatically. Select the default Mercator projection, and click Agree at the prompt to open the program.
- Zoom to the region that includes the ocean floor between mainland Australia and Antarctica (Figure 2) and activate the Ocean Floor Drilling portal in GeoMapApp by clicking on Portals > Ocean Floor Drilling in the top menu:
Let it load and do not touch the app! (Patience) A number of grey points will appear
in the oceans. Each represents one or more drill cores collected by the Ocean Drilling Program (ODP), Deep Sea Drilling Program (DSDP) or the Integrated Ocean Drilling Program (IODP – now International Ocean Discovery Program).
- Accessdrillcoredataforaparticularsitebyclickingonthatpoint,startingwith Leg 189 Site 1172 (shown in red below). This is one of the key sites described by Exon et al. (2002). Activate the DSDP – ODP – IODP DRILL HOLES window (under the Window menu) and click on the “View down-core measurements for selected cores” button (red zig-zag line icon). The graphical display can show various measurements as a function of depth in the sediments.
- Explore the different measurements that are recorded for site 1172-A, which is described by Exon et al. (2002). Check out the carbonate content in particular and any other measurements that show strong variation near the Eocene-Oligocene boundary. Such measurements are important in developing an interpretation of how climate or oceanic circulation has changed through time.
- For each hole, the graphs will display the depth(meters below sea floor, mbsf), the core number and the geological age. The Eocene (56-34 Ma) and the Oligocene (34-23 Ma) are two of the major divisions of the geological time-scale in the present Cainozoic Era. These ages are defined in terms of micro-fossil
assemblages and calibrated using geochronological techniques.
- Access the core photographs for site 1172-A. The small black squares in the column next to the age scale are supposed to link to photographs of the cores, but since this does not work in some cases, the photos can also be accessed directly from the Core Photo web interface at:
Enter the leg (189), Site (1172) and Hole (A), and click Submit Request. You will see a list of photos. Use your GeoMapAppgraph and the depths to find photographs of the cores near/at the Eocene-Oligocene boundary. The next column of small black squares on the GeoMapApp graph links to core logs, which may be useful in interpreting what you see in the photos.
- Explore the data held for other drill sites mentioned by Exon et al. (2002). At least look at sites 1168, 1170, 1171and any others that might be interesting. Can you find any evidence of whether the changes seen at the Eocene-Oligocene transition at site 1172 are local to the East Tasman Plateau, or of greater regional extent? You will find that many cores do not go deep enough to sample the Eocene-Oligocene transition (at around 34 Ma),while other cores may have missing sections due to poor recovery from the drill hole, and older sites generally have fewer measurement types available.
Based on your study of the essential reference papers, and your own investigation of the IODP data using GeoMapApp you need to summarise observational evidence for howand whenthe Southern Ocean developed around Antarctica, and the globaland regionalchanges that are associated with this process. This report must be in your own words (and, after submitting it in week 9, together with the rest of this assessment, the TurnItIn system will check that you are not using someone else’s text), but to provide some focus, please address the following points:
- Give a brief (and illustrative) synopsis of the tectonic events that led to the establishment of the modern Antarctic Circumpolar Current (ACC).
- According to the scientific literature, how does the presence of the ACC affect the pole- to-equator thermal gradient?
- What evidence for the establishment of the Tasman Gateway, a necessary precursor for the modern ACC, can you see from the ocean drilling results available via GeoMapApp?
- You must include material from Site1172on the East Tasman Plateau.
- You must include your own figures and maps made using GeoMapApp.
- If you choose to include the core photographs, make sure you annotate them appropriately, and provide a meaningful figure caption.
Figures.Figures should be embedded in the text and cited within the text by Figure number. Each figure should have a meaningful and succinct caption. You are limited to 6 displayitems (figures/tables) totalfor this part of the assignment, where at least 2 items should be your own original figures produced with GeoMapApp. Other display items provided to help support your answers should come from reputable sources and should be properly cited after the caption (e.g., from Smith et al., 2017).
References: Any references you use must be properly cited throughout your answers, and you must use the Harvard citation style. We expect to see at least a couple of additional relevant references based on your independent search. We encourage you to use EndNote or a similar bibliography manager to help manage your citations.
- The primary rule in scientific writing is that it should be concise and accurate, and contain enough information to be understood by the audience. If a sentence includes words that don’t add any useful meaning, delete them. If the meaning is unclear, what additional context is needed?
- The second rule is: proofread before submission – check what you have written; does the text really say what you mean it to say? And will it be understood?
- Avoid passive voice (e.g., “the following measurements were made”). Better to say who made the measurements.
- Keep the grammar simple, i.e., follow the basic pattern of: subject – verb – object, with qualifying clauses as appropriate.
- Be quantitative rather than qualitative where possible. Physical quantities have units, make sure they get attached to the numbers (preferably use a non-breaking space).
- Read published journal articles to get a better idea of the language style that is expected. It will help you write better figure captions as well.
- Follow a single referencing style in your document – start using EndNote (it is available free from the Library website).
- Use the Library search engine to find relevant papers. Also, Google Scholar is your friend – it even has a link to directly import citations into EndNote.
- Make sure your figures have meaningful figure captions and are referred to in the text.
Your essay must be typed, 1.5 line spacing, on A4 paper with 2.5 cm margins all around. Font type must be 12 pt Times New Roman and essay text must be no more than 4 single-sided pages (excluding references, figures and tables). Pages must be numbered and those over the limit will not be read/marked. Remember, an important aim of this exercise is for you to learn how to write concisely and effectively. Any figures or tables that you choose to include must be attached to the end of the report and must have succinct and meaningful captions. You are limited to 6 display items (figures/tables) for this essay so make them count! Figures and tables should be referred to in the text in the appropriate manner (i.e “see Figure 1” or “In Figure 2….”). All material that is not your own must be appropriately cited (see examples in this document). This essay is worth 15%ofyourfinalmark.
Figure 2. Long-term eustatic (global) sea level curve from Haq et al. (1987) with respect to present day.