Animals & Snowball Earth

Question 1

Snowball Earth

Answer 1

(720-635 Ma)
- Evidence for glacial deposits in marine rocks, distributed across many parts of the world

Hypothesis: An Earth-system equivalent to “punctuated equilibrium”
- Tipping points
- Drastic change going into and out of snowball, driven by ice albedo

Question 2

Evidence for glacial deposits in marine rocks

Answer 2

• Some deposits → near paleo-equator
• Glacial deposits are often overlain by carbonates- implies warm climate
• Very low δ13C during glacial periods- implies low organic matter burial
• Evidence for ice (low latitude sites)- glacial sediment, contact between glacial sediments and cap carbonate

Question 3

Ice albedo feedback loop

Answer 3

Positive FL
Ice / Temperature / Surface reflectance (albedo)

Question 4

In a snowball

Answer 4

Nearly complete shutdown of silicate weathering
- Super cold
- Super dry (no liquid water)
- Rock is covered by ice

Very little organic matter burial
- Not hospitable to life
- Land and ocean is frozen over

Very little OM respiration- cold, dry, very little life

Volcanism

Question 5

Evidence supporting Snowball scenario

Answer 5

• Tube structures in cap carbonates- rapid formation; methane?
• Sharp contact between glacial sediments and cap carbonates
• Massive crystals- implies rapid formation

Return of BIFs (oxidized iron) after 1.5 b.y hiatus
- Ice barrier on ocean, physical separation between ocean and atmosphere- no oxygen penetration into ocean from atmosphere, no O2 production in ocean via photosynthesis

Question 6

Animals at 635 Ma

Answer 6

Long time between evolution of eukaryotes (~2 Ga; ~1.6 for multicellularity) and evolution of animals (~635 Ma)

Explosion of multicellular animal life ~580 Ma
- Ediacaran fauna (0-40 m.y before Cambrian)

Question 7

Correspondence between atmospheric O2 and Snowball Earth

Answer 7

1. Via enhanced organic matter burial- implies increased δ13C
   - CO2 + H2O ⇒ CH2O + O2
2. Evidence for lots of phosphorus (P) in ocean after snowball events
   - Linked to silicate weathering- lots of stuff entering ocean in aftermath of snowball, including dissolved P
   - P would stimulate life- would lead to higher rates of OM burial → would help draw down increased CO2 in aftermath of snowball

Question 8

Other evidence for rise in atmospheric O2 during snowball Earth

Answer 8

1. forg ramps up
2. Presence of sulfate (S6+O2-4) in ocean
   - Sulfide (S1-)- exists in absence of oxygen
   - If there is enough O2 in ocean, you will have more sulfate and less sulfide
3. Body size through time- linked to both O2 pulses
4. Most “basal” animals do not require O2 for their metabolism
   - Earliest forms of animals didn’t need O2