Lecture 11 The Climate History of Earth
To recap: The Oxygen Paradigm: - C-cycle has evolved radically over time - Prior to 2.2 Ga, anaerobic prokaryotes dominated; oxygenic photosynthesis existed but overall O2 levels were low because of too many strong sinks (such as the mantle) - GOE, and rise of prominence of aerobes; dominance of aerobic autotrophs o O2 in atmosphere remained low (due to sulfidic ocean?) o Second oxidation event caused “reorganization” and allowed atmospheric O2 levels to rise to near PAL (present atmospheric levels)
In order to get O2 into the atmosphere, you need to have a place for the carbon that is also produced to go, to interact with the oxidation sinks, so that the O2 created alongside it doesn’t go to those sinks.
Half life of a sedimentary rock is about 300 million years. Mantle turns over in billions of years. Shorter timescales for carbon in the atmosphere and ocean (shortest for atmosphere; atmosphere contains the least carbon).
Much argument over whether “Snowball Earth” (the idea that the Earth was completely covered in ice at some point or multiple points) is true or not.
5 periods in Earth’s history when we’ve had ice at the poles, otherwise it’s been very warm; we’re currently in a lower climate extreme!
Sun’s luminosity changes over a billion-year timescale. Continental drift: 100s of millions of years. Orogeny (tectonics): 10^7… composition of the atmosphere: shortest timescale.
Given the luminosity of the Sun in the past, Earth should have been frozen until about 2 Ga; but it wasn’t. - Energy emitted by the Earth needs to equal the energy absorbed, or you’ll get temperature change. - Important surfaces for reflectivity (albedo): cloud and snow cover. - But neither albedo nor geothermal heat flux changes could have kept the Earth from freezing - Larger greenhouse effect could counteract these
Greenhouse gases: most effective: water, CO2, N2O, and methane. Keeling curve (from observation station on Mauna Loa): shows that atmospheric levels of CO2 have been rising steadily since 1956. (Zigzags in the line are due to seasonal effects.)
What’s behind these climate extremes? - “geochemical” carbon cycle – chemical weathering (carbonate, silicate) o Components of weathering are transported into the ocean (via rivers); this is balanced by precipitation out of the ocean (CaCO3 and SiO2 in diatoms) - Increased atmospheric CO2 Æ greenhouse effect Æ surface temp and rainfall Æ silicate weathering rate (which inhibits additional atmospheric CO2)
Carbon Isotopic Excursions - massive perturbations in the carbon cycle are associated with rocks that showed evidence of glaciation. (Keep in mind this is a correlation, not causation necessarily; that’s unknown).
Evidence for 3-4 glaciation events in the Neoproterozoic period. (In any one rock, you can only ever find solid evidence for 2; no single rock structures show evidence for all 3- 4; that theory is from cumulative evidence).
Geological evidence for glaciers: tillites, glacial striations, and dropstones. - Tillites: packed pebbles, sand and mud; remnants of moraines (rocks at the edges of glaciers created by melting of material off of glaciers from what the glacier has picked up as it’s moved). - Glacial striations: scratches from rocks dragged by moving ice - Dropstones: icebergs carry rocks into ocean; ice melts and rocks fall to the bottom of the ocean and are buried in the mud. “Poorly-sorted sediment.” - Anomalous iron formations: evidence that places in the ocean must have been anoxic (covered in ice), reduced iron, for a long period of time
Earth’s landmass(es) likely used to be located at the equator (or, what was the equator at the time) – and so were glaciers! (On continents, at sea level). Strong evidence for this. There are very few glaciers at the equator today, and they’ll probably be gone soon (e.g. Kilamanjaro). - Former equator: paleomagnetism: determine paleolatitude from remnant magnetism in sediments (inclination of sedimentation doesn’t line up with current magnetic field) Equators were colder than the poles; to make this possible, the Earth’s spin axis would have had to be very oblique (so that the poles were facing the sun, not the Equator). Since the moon stabilizes the Earth’s spin axis, you would need major impacts to shift it; but there is no evidence for such impacts.
So, Snowball Earth hypothesis: 1. Breakup of equatorial supercontinent 2. enhanced weathering from rainfall 3. drawdown atmospheric CO2 Æ global cooling 4. runaway albedo effect: cannot get out of this system of icing over 5. global glaciation for ~10 Myr - sea ice is ~1000 m thick but geothermal heat flux keeps oceans liquid
Breaking out of the Snowball Volcanic outgassing of CO2 over ~10^6 yr may have increased greenhouse effect sufficiently to melt back the ice.
Runaway carbonate precipitation event during hothouse