PTYS 214 Spring Announcements. Get exam from Kyle!

Transcription

1 PTYS 214 Spring 2018 Announcements Get exam from Kyle! 1

2 Midterm #3 Total Students: 24 Class Average: 78 Low: 32 High: 100 If you have questions see one of us! 2

3 All exams Top 2 exams 3

4 Previously Feedbacks loops Stable / Unstable Climate feedbacks Venus -- Runaway greenhouse effect Mars -- Loss of H2O to space; condensation of CO2 Habitable zone vs. time Continuous habitable zone Conditions for liquid water on surface 4

5 Earth s Climate Earth's climate has varied throughout its history, from glacial periods (or "ice ages") where ice covered significant portions of the Earth to interglacial periods where ice retreated to the poles or melted entirely Ice Age ~530 Myr ~300 Myr ~145 Myr 5

6 Ice Ages Geological periods of long-term reduction in the temperature of Earth s surface and atmosphere, resulting in an expansion of polar ice sheets, continental ice sheets and alpine glaciers Earth s major ice ages: Huronian: Myr ago (not well defined) precomplex life Snowball Earth: Myr ago, in two episodes, maybe the most severe of all Andean-Saharan: Myr ago, minor Karoo ice age: Myr ago Current ice age: Myr, alternating between glacial and interglacial periods (we now are in an interglacial period) How do we know about past glaciations? 6

7 Evidence for glaciations Brooks Range, Northern Alaska (from Skinner & Porter, The Blue Planet) Glacial till pieces of rock picked up by glaciers as they move across the landscape Moraine piles of glacial till deposited at the terminus terminal moraine) or sides (lateral moraine) of a glacier 7

8 Evidence for glaciations Whitefish Falls, Ontario, Canada Tillite a diamictite produced by burial of glacial till 8

9 Evidence for glaciations Shackleton Glacier, Transantarctic Mtns., Antarctica Striations parallel scratchings on rock surfaces caused by the passage of glaciers (bearing rocks) 9

10 Evidence for glaciations Dropstones: Isolated rocks found in smoothly laminated marine sediments that are interpreted as having fallen from melting icebergs Shackleton Glacier, Transantarctic Mtns., Antarctica 10

11 Extreme Glaciations: Snowball Earth Low-latitude glaciations are inferred at ~0.63, 0.72, and possibly 2.3 billion years ago from evidence of possible glacial sedimentary deposits at tropical latitudes The most investigated events are the Neoproterozoic Snowball events: Sturtian glaciaton (715 Myr ago) Marinoan glaciation (635 Myr ago) Currently an active area of research, the hypothesis is still under scrutiny 11

12 What's so difficult about investigating the geologic record of the early Earth?

13 Important Limitations to Investigations of Early Earth The further back in time you go, the more difficult is to find clear and abundant evidence Less and less crust of older age is available for investigation (plate tectonics) Age determination of rocks becomes less clear (larger absolute uncertainties) Many environmental and climatic characteristics can only be inferred (e.g., atmospheric composition, land extension, Earth magnetic field, etc.) 13

14 How do we infer low-latitude glaciations million years ago? 14

15 15

16 16

17 17

18 18

19 What's wrong with this? 19

20 What's wrong with this? Plate Tectonics! 20

21 Earth s Magnetic Field Paleolatitudes from Magnetic Data Polar Equatorial 21

22 22

23 Explaining Extreme Glaciation Episodes Snowball Earth model The Earth was completely ice-covered (thick oceanic ice) for as long as tens of millions of years Major decline of photosynthetic life (extinction?) Slushball Earth model The tropical oceans remained ice-free Photosynthetic life could survive in equatorial ocean 23

24 Why doesn't every glaciation result in a Snowball? - Ts + - Planetary Albedo Snow and Ice Cover Runaway glaciation + 24

25 Why doesn't every glaciation result in Snowball? - Ts + - Planetary Albedo + Ts - - Snow and Ice Cover Runaway Ice age + but Outgoing IR flux IR flux/surface temperature feedback! 25

26 What Causes Snowball Glaciations? Small glaciers do not cause Snowball runaway! Ice-albedo feedback takes over when the polar caps reach some critical latitude (near 30o) What was the cause(s) for the sea ice to extend down to 30o? Trigger had to be abrupt, strong and long-lasting Need to reduce the concentrations of greenhouse gases (CO2 and/or CH4) or decrease the Solar input somehow There is no consensus on the trigger! 26

27 Trigger 1: Land Distribution (most popular theory) Continents concentrated at low latitudes - Continents have higher albedo than ocean Less solar radiation absorbed - Intense silicate weathering atmospheric CO2 goes down Problem: The continental distribution is difficult to establish that far back in time, especially as plates seem to be moving quite fast! 27

28 Trigger 2: Atmospheric Methane Early Cambrian CH4 levels were high prior to glaciation (methanogenesis >> respiration) (e.g., CO2 + 4 H2 CH4 + 2H2O) As photosynthesis increased, oxygen level increased Oxygen reacts with methane to form carbon dioxide (weaker greenhouse than methane) Atmospheric greenhouse decreased Trigger for glaciation 28

29 Trigger 3: Interstellar Dust The solar system passed through a Giant Molecular Cloud, an area with increased H and dust abundance Direct influx of interstellar dust into the Earth atmosphere Anti-greenhouse effect (small size dust is transparent to IR and reflects visible) and severe glaciations Problems: No clear evidence in the geologic record Lots of dust in the Solar System would cause instabilities in planetary orbits! 29

30 Trigger 4: Orbital Cycles Variations in the Earth's orbit and axis cause changes in the intensity and distribution of Solar flux. Strong correlation with glacial / interglacial periods within ice ages. Problems: Correlation with major ice ages not apparent. 30

31 How can the Earth recover from a Snowball Glaciation? Original idea: Volcanic CO2 buildup Other suggestions: Large releases of methane from subsurface Large atmospheric injection of water and sea salts from a large impact Still highly debated! 31

32 Recovering from a Snowball Earth Atmospheric CO2 Volcanic CO2 builds up in the atmosphere (HOW?) until the greenhouse effect becomes strong enough to melt the ice The melt-back is very quick (a few thousand years) Surface temperatures climb briefly to 50-60oC ( oF) CO2 is rapidly removed by silicate (and carbonate) weathering, forming cap carbonates For the snowball Earth hypothesis, it would require huge amounts of CO2 in the atmosphere! More realistic for a slushball Earth hypothesis 32

33 Snowball Earth Model Advantages Explains the presence of thick cap carbonates (i.e., restart of the carbonate/silicate cycle) Accounts for the reappearance of Banded Iron Formations (cycles of high-low oxygen content in the oceans) as atmosphere regains contact w/ iron-saturated ocean Problems Presence of km-thick ice everywhere poses significant problems for survival of photosynthetic organisms Contrast with evidence of photosynthesis and thus ice free regions at low latitudes (e.g., 12C/13C; next lecture) It is very difficult to get out of it! 33

34 Slushball Earth Model Consequence of some of the problems associated with the Snowball Earth hypothesis Advantages Much easier to get out of Accounts for the evidence of tropical ice free ocean regions Easier to justify survival of photosynthetic organisms (although reduced in number) Problems The initial conditions used for models are rather unconstrained due to incomplete data 34

35 Snow/Slushball Earth Model: Synopsis Because of an extended cold spell, oceans start freezing Higher reflectivity causes further cooling, ending in snowball Earth km-thick ice CO2 cycle in oceans stops; CO2 outgassed by volcanoes builds up Strong greenhouse effects melts snowball CO2 cycle Earth, results in a hothouse Earth restarts, pulling CO2 back into oceans, reducing greenhouse effect to normal Strong Carbonate-Silicate cycle 35

36 Problems with Snowball Hypothesis? 36

37 Problems with Snowball Hypothesis? Magnetic polar wander? True polar wander? Obliquity variations? 37

38 Homework Homework #13 available shortly on the web site 38