Glaciation of Antarctica: the Oi1 event
10.1: Initial Evidence
1.
a. What is the nature of the δ18O signature that indicates cooling of the deep ocean and/or growth of large ice sheets? Specifically, is cooling/ice sheet growth indicated by more positive/greater values of δ18O or by more negative/ lesser values?
Cooling/ice sheet growth is indicated by more positive/greater values of d18O.
b. Give a brief explanation of why the ratio of 18O/16O in a benthic foram shell changes as a function of cooling and/or increasing ice volume.
The ratio of 18O/16Oin a bethic foram shell will change because forams will always prefer to use the 16O because it is lighter when there is a lack of O16 then the foram will use O18. The ratio is all related to the temperatures of the surrounding area.
2. Based on the δ18O data in Figure 10.2, what intervals of the Cenozoic are likely to be times of significant continental-scale glaciation? Make a list; give the approximate age (or age range) and the portion of the epoch (e.g., early, middle, or late Miocene) during which glaciation occurred.
Early Eocene, early-mid miocene
3. What evidence in the rock record (e.g., fossils, types of sedimentary deposits) would support the δ18O-based interpretation that glaciation occurred at these times?
We would find fossil records of plants and animals that became extinct because of glaciation. There are also ice rafted debris in the marine sediments.
4. Where in the ocean would you look for geological evidence of glaciation?
Near the continental ice sheets where there are fresh water dumps because you will have more ice rafted debris following the currents.
5.
a. What is the age of the positive isotope excursion? What is the age of the IRD concentration?
b. What are the implications of these findings?
10.2: Evidence for Global Change
1. 1 (a)
a. Compare the tropical δ18O data (Figure 10.6) with the polar δ18O data (Figure 10.4). What similarities exist between these two records?
Polar and tropical δ18O data in the upper Eocene to the lower Oligocene was lower than that in the upper Oligocene.
b. What do these similarities suggest about the geographic extent of environ- mental or climatic change at this time?
There was a gradient change between the tropical and Polar Regions as well as an overall decrease in temperatures.
2. What is the approximate duration of the Oi1 event as marked by the positive oxygen isotope excursion in Figure 10.6?
From 33.8 ma to 34.2 ma
3. What happens to the carbonate content (reported as %CaCO3 and/or CaCO3 mass accumulation rates, MARs) in the sediments across the Eocene– Oligocene transition and Oi1 event at ODP Site 1218? (Hint: think like a geologist; you’re reconstructing a geological story so answer the question in chronological order, from older to younger in the core or in the data.)
Late Eocene there are low concentrations of CaCO3 with a steep increase of about 80% where the levels stay consistent throughout the early Oligocene.
4. Propose a hypothesis to account for the observed change in carbonate content at ODP Site 1218 (Figure 10.6).
There could have been an increase in seafloor spreading, increased volcanoe eruptions, or an increase in rock weathering that could have caused such a large increase in carbonate.
5. Examine the core photos in Figure 10.7. Note that sediment color reflects the type of sediment (lithology) and carbonate content: lightest sediments are nannofossil ooze, darkest sediments are radiolarian ooze. Nannofossils are composed of carbonate, whereas radiolarians have shells made of silica. Make a list of observations about these cores from different sites in tropical Pacific.
The sites all start off from the bottom to the top with extremely dark sediments and they slowly make a gradual transition to the lighter nonnofossil carbonate.
How are the sediments at these sites similar; how are they different?
The site 1217 there is no transition to lighter sediments. Site 1221 has the second least prominent transition. The most prominent transition is at site 1218 and 1219.
6. Propose a hypothesis about a possible cause of the change from radiolarian ooze to nannofossil ooze at the ODP Leg 199 sites (Figure 10.7). Refer to the box above about the CCD, as well as Chapter 2 Seafloor Sediments.
There is an increase in carbonate in earlier years, which is the cause for more nannofossil ooze. The reasons I hypothesis would be the same as the reason for the increase in carbonate.
7. Examine the core photos in Figure 10.8. Make a list of observations. How are the sediments at these sites similar; how are they different?
The deeper the sediment core the darker the sediment is. Below the CCD we definitely see the nannofossil ooze become much more prominent for all the cores. Core 1263 already has a large amount of nannofossil ooze before the CCD and is also the least deep.
8. A lithologic change from clay or clay-bearing nannofossil ooze to nannofossil ooze occurs across the E–O transition and the Oi1 event at these Walvis Ridge sites. This lithologic change is associated with changes in the physical properties of the sediment, including decreasing magnetic susceptibility (MS) and gamma ray attenuation (GRA), and a lightening of the sediment color (L*). Collectively these data all indicate an upcore decrease in the relative proportion of clay to calcium carbonate (CaCO3; nannofossils) in the sediment. What could this compositional change suggest about climate conditions, and why? (Hint: The box concerning the CCD may help formulate some ideas.)
The composition change would suggest that the climate conditions during this time where
9. Based on Figure 10.10, describe how the lithology on the South Tasman Rise changed during Eocene–Oligocene time:
The souther tasman rise went from organic bearing, nannofossil bearing, silty claystone for most of the Eocene during the transition to the Oligocene where there it was organic and glauconite bearing silty claystone. During the early Miocene is where we clayey nannofossil ooze chalk to nannofossil ooze bearing forams and silica.
10. Tasmania and Antarctica separated during the time that these sediments were deposited. Describe how the lithostratigraphy in these cores tell the story of the break-up of Tasmania and Antarctica.
These sediment cores shows the transition from organic materials to nannofossil ooze.
11. Propose a hypothesis about how the break-up of Tasmania and Antarctica may be related to the Oi1 event and the glaciation of Antarctica.
The break up of Tasmania and Antarctica may have been related to the Oi1 event and the glaciation of Antarctica because it changed the circulation of the oceans which is a powerful mechanism for heat transport thorough out the world.
12. Use the data in Figure 10.12 to describe the changes in microfossil assemblages (i.e., the relative abundances of calcareous nannofossil and siliceous microfossils) in Hole 699A (Figure 10.11).
During the latest years in world history from the Paleocene to the Eocene there are no evidence of diatoms but there is a large abundance of calcerous nannofossils and planktonic formainefers. The calcerous nannofossils and planktonic formainefers reamian steady until the Oligocene and hardly show up past the Miocene. The planktonic foraminefers become largely abundant in the early oliogocene to the present time.
13. Does the change in the dominant type of microfossil through the Eocene– Oligocene transition appear to be abrupt or gradual?
It seems so be more abrupt that gradual .
14. How can the changing microfossil assemblages be used to infer changes in ocean circulation or climate? Recall from Chapter 3 that both calcareous nan- nofossils and diatoms (a siliceous microfossil) are phytoplankton (i.e., primary producers).
The dominant type of microfossil can explain the temperatures and salinity of the location at which they are found.
15. Propose a hypothesis about how this change in type of phytoplankton may be related to the Oi1 event and the glaciation of Antarctica:
The change in phytoplankton can show the amount of productivity in an ocean based on the conditions.
16. What are the dominant lithologies (i.e., rock and sediment types) in the Late Cretaceous, Paleocene, and Eocene (Figure 10.13)?
Age / Dominat LithologiesEocene
Paleocene
Cretaceous
17. What are the dominant lithologies in the Oligocene, Miocene, Pliocene, and Quaternary?
a. Based on the data presented in Figure 10.13, what can you infer about changes in climate or changes in the ocean environment during the Cenozoic?
b. Propose a hypothesis about how this change may be related to the Oi1 event and the glaciation of Antarctica:
19. Examine Figure 10.14. What type of sediment does diatom ooze–siliceous ooze replace at Site 689? When (at what age) did the change in lithology begin?
20. Does the change in lithology appear to be gradual or abrupt?
21. How does the Maud Rise lithologic record near Antarctica (Figure 10.14) compare with the Southern Ocean sites north of the Scotia Arc (ODP Leg 114 Sites 699 and 701; Figure 10.13)? Refer to map (Figure 10.11).
22. Examine Figure 10.15. Based on the New Jersey margin record of sea level change, how much did sea level change during the Eocene–Oligocene transition?
23. What is the likely cause of this change in sea level?
24. Refer back to Figure 10.6. What is the magnitude of the Oi1 δ18O positive excursion, in permil (i.e., use the ‰ scale on the graph)?
25. δ18O of deep-sea benthic forams is controlled primarily by temperature and ice volume. If an increase in ice volume produced a 10 meter drop in global sea level and resulted in a 0.1‰ increase in δ18O, how much of the Oi1 excur- sion was due to changing ice volume (and therefore sea level) at the Eocene– Oligocene transition? Show your calculation.
26. How does this estimate compare with the New Jersey margin record of sea level change (Figure 10.15)?
27.
a. If a 1°C decrease in temperature produces a 0.25‰ increase in δ18O, how much may deep-sea temperatures have changed during the Oi1 event? Show your work.
b. Based on your response to Questions 25–27(a), do you interpret the posi- tive oxygen isotope excursion of the Oi1 event to represent a purely ice volume increase (sea level fall) signal or a mix of ice volume and temperature decrease? Explain.
28. Examine Figure 10.16. The authors of this study attribute the extinctions and biotic turnover of planktic foraminifers during the Eocene–Oligiocene transi- tion to a major ecological disturbance. Speculate about ecological changes that may have affected near-surface dwelling plankton.
29. The global extinction of planktic forams occurs in two steps as observed in Tanzania (Figure 10.16) and elsewhere. Are there other data from this exercise (i.e., previous data plots in Part 10.2) suggesting that environmental changes across the Eocene–Oligocene transition may have occurred in two steps?
30. This marine record comes from a drill core taken on the tectonically stable coastal plain (i.e., on shore) of Tanzania. What does this imply about global sea level during the late Eocene?
31. Use Table 10.1 to summarize the key observational data from each of the deep-sea and land-based records presented above. List any questions that you have about what you have seen in the data.
32. Use the data summarized in Table 10.1 to identify possible connections (i.e., cause and effect, timing) among the following:
Glaciation:
Ocean circulation:
Global cooling:
Productivity:
Sea level:
Biotic evolution and extinction:
33. Write a paragraph in which you hypothesize about the cause and effect of glaciation on Antarctica. Be sure to support your hypothesis with evidence from this exercise.
Part 10.3. Mountain Building, Weathering, CO2 and Climate
1.
a. Based on the reactions above, explain how the chemical weathering proc- esses described above can affect climate.
b. If the rate of chemical weathering were to increase, what type of climate response would you predict?
2. If the rate of chemical weathering were to increase, what response might we expect to see in seafloor sediments?
3. Read the box above on strontium isotopes and continental weathering and then answer the following questions:
(a) As 87Sr/86Sr values have risen over the past 40 million years, do you predict that atmospheric CO2 values have risen or fallen?
(b) What impact might the chemical weathering-induced change in CO2 have had on global climate over the past 40 million years?
4. Summarize the relationship between the strontium isotope composition of seawater, the chemical weathering of continental rocks, and atmospheric CO2.
5. What other processes, besides chemical weathering of continental rocks, might affect atmospheric CO2 concentrations, and therefore, global climate? (Hint: think back to the long-term carbon cycle introduced in Chapter 5, Part 5.4.)
6. Make a list of possible causal mechanisms for the Oi1 event (i.e., the rapid glaciation of Antarctica as evidenced by the positive excursion of δ18O values in planktic and benthic forams). Explain how each would lead to the growth of continental-scale ice sheets.
7. Positive feedback amplifies climate change, while negative feedback dimin- ishes climate change (see Chapter 5, Part 5.2).
(a) What feedbacks associated with the global environmental changes of the Eocene–Oligocene transition (e.g., CO2, climate, glaciation, sea level, weather- ing, productivity, ocean circulation) may have contributed to the rapid glacia- tion of Antarctica approximately 33.7Ma? A sketch may help.
(b) Could multiple feedbacks be involved? Explain.
8. Based on the many ocean–climate system relationships you learned about in this exercise (Parts 10.1, 10.2, and 10.3), suggest at least two different proc- esses that may have caused CO2 to fall during the Eocene–Oligocene transi- tion. List your ideas and compare with others.
9. Read the box below about alternative hypotheses for the glaciation of Antarc- tica and examine Figures 10.19 and 10.20. Do any of the data we have con- sidered thus far in this exercise (Parts 1, 2, and 3) support either of these alternative hypotheses? If so, how?
Part 10.4. Legacy of the Oi1 Event: The Development of the Psychrosphere
1. 1 In Figure 10.21, circle the Oi1 event as recorded by foram data in the North Pacific and the Southern Ocean. How are these data similar and how are they different?