PTYS 214 – Spring2011  Homework #5 DUE in class TODAY  Homework #6 available for download on the website DUE on Tuesday, March 1 (yes, next class!)  Study Guide for Midterm available for download on the website  No office hours for Pierazzo today  Reminder: Extra Credit Presentations (up to 10pts) Deadline: Thursday, Mar. 3 ( must have selected a paper )  Useful Reading: class website  “Reading Material” Announcements

Earth’s Climate Earth's climate has changed 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

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) pre- complex 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)

Evidence for glaciations 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 Brooks Range, Northern Alaska ( from Skinner & Porter, The Blue Planet )

Evidence for glaciations Diamictite — a rock containing unconsolidated smaller fragments Tillite — a diamictite produced by burial of glacial till Whitefish Falls, Ontario, Canada

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

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

Extreme Glaciations: Snowball Earth Low-latitude glaciations are inferred at ~0.63, 0.72, and possibly 2.3 billions years ago from paleomagnetic data and 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

Important Limitations to Investigations of Early Earth The farther back in time you go, the more difficult is to find clear and abundant evidence 1)Less and less crust of older age is available for investigation (plate tectonics) 2)Age determination of rocks becomes less clear (higher uncertainty) 3)Many environmental and climatic characteristics can only be inferred (e.g., atmospheric composition, land extension, Earth magnetic field, etc.)

How do we know about low- latitude glaciations million years ago?

How do we know about the distribution of continents over 600 Myr ago? From the record of Earth’s ancient magnetic field recorded by magnetic minerals in the old rocks (limited by the amount of rocks older than 600 Myr ago)

Paleolatitudes from Magnetic Data Courtesy of Adam Maloof Polar Equatorial Earth’s Magnetic Field

Late Precambrian (600Ma) Geography (PALEOMAP Project, Scotese) Hyde et al., Nature, 2000 * glacial deposits

Explaining Extreme Glaciation Episodes  High obliquity hypothesis –Earth’s spin axis was tilted at a high angle (> 54 o to its orbital plane (currently 23.5 o ) –Pretty much abandoned  “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 –Glaciers formed on high mountain tops and flowed down to the seashore faster than they could melt

Original Snowball Earth Model Because of an extended cold spell, oceans start freezing Higher reflectivity causes further cooling, ending in snowball Earth CO 2 cycle in oceans stops; CO 2 outgassed by volcanoes builds up CO 2 cycle restarts, pulling CO 2 back into oceans, reducing greenhouse effect to normal Strong greenhouse effects melts snowball Earth, results in a hothouse Earth Strong Carbonate-Silicate cycle km-thick ice cap

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) 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  It is very difficult to get out of it!

Slushball Earth Model Consequence of some of the problems associated with the Snowball Earth hypothesis Advantages  Much easier to get out of it  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

Why does not every glaciation result in Snowball? Runaway Ice age … but… IR flux/surface temperature feedback! - - TsTs Snow and Ice Cover Planetary Albedo TsTs Outgoing IR flux -

What Causes Snowball Glaciations? Small glaciers do not cause Snowball Collapse!  Ice albedo feedback takes over when the polar caps reach some critical latitude (near 30 o ) What was the cause(s) for the sea-ice to extend down to 30 o ? 1.Trigger had to be abrupt, strong and long-lasting 2.Need to reduce the concentrations of greenhouse gases (CO 2 and/or CH 4 ) or decrease the Solar constant somehow There is no consensus on the trigger!

Trigger 1: Land Distribution (most popular theory)  Continents concentrated at low latitudes  Intense silicate weathering  atmospheric CO 2 goes down  Weathering continues even when ice advances Problem: The continental distribution is difficult to establish that far back in time, especially when plates seem to be moving quite fast!

Late Precambrian Geography (PALEOMAP Project, Scotese) Hyde et al., Nature, 2000 * glacial deposits

Trigger 2: Atmospheric Methane Mid-Proterozoic CH 4 levels were high prior to glaciation (methanogenesis >> respiration) As photosynthesis increased, oxygen level increases Rise of oxygen in the ocean limited methanogenesis Oxygen reacts with methane to form carbon dioxide (weaker greenhouse than methane) Atmospheric greenhouse decreased Trigger for glaciation

There is no evidence that oxygen levels increased before glaciation (Pavlov et al., 2003) Sulfate formation occurs in the presence of oxygen

Trigger 3: Extraterrestrial  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  Antigreenhouse effect (small size dust is transparent to IR and reflects visible) and severe glaciations Problems:  Too quick to be recorded in the geologic record  Not clear why limited to only two major events  Lots of dust in the Solar System would cause instabilities in planetary orbits!

Original idea:  Volcanic CO 2 buildup Other suggestions:  Large releases of methane from subsurface  Large atmospheric injection of water and sea salts from a big impact Still highly debated! How can the Earth recover from a Snowball Glaciation?

Recovering from a Snowball Earth Atmospheric CO 2  Volcanic CO 2 builds up in the atmosphere until the greenhouse effect becomes big enough to melt the ice  The meltback is very quick (a few thousand years)  Surface temperatures climb briefly to o C  CO 2 is rapidly removed by silicate (and carbonate) weathering, forming cap carbonates For the hard snowball Earth hypothesis, it would require huge amounts of CO 2 in the atmosphere! More realistic for a slushball Earth hypothesis

Quiz Time !