Lecture 20

GS 206- LECTURE 20 Evolution and Extinction: Links to Climate Change Via Snowbal Earth! 1. Review of Lecture 19 material: evolution key events Sexual reproduction, oxygen, ozone layer 2. Paterns of extinction: 5 plus one more coming! 3. The Snowbal Earth hypothesis! Heat budget Freze fry cycle Links to major evolutionary events Radiative heat balance (at equilibium): HEAT ABSORBED = HEAT EMITTED R = radius of the Earth E s = solar irradiance a = planetary albedo f = effective infrared transmission factor (greenhouse effect) s = Stefan-Boltzman constant T s = surface temperature PLANETARY ALBEDO: The fraction of incoming radiation that is reflected back to space. [ sea water ~0.1; bare land ~0.3; sea ice ~0.6; fresh snow ~0.9 ] ICE-ALBEDO FEEDBACK: For any imposed cooling (or warming), the resulting higher (or) albedo will cause further cooling (or warming). Thus, ice advance is self-stabilizing. RUNAWAY ICE ALBEDO: If ice lines close to within ~30? of the equator, the ice albedo feedback becomes unstoppable and ice quickly covers the tropics. see Budyko, M.I., The effect of solar radiation variations on the climate of the Earth; TELLUS 21: 611-619 (1969). R 2 E s [ 1 ? a ] = 4 R 2 [ f s T s 4 ] cc V olcanoes emit CO 2 Rain w ashes CO 2 out of air Carbonic acid reacts with silicate roc ks Riv ers w ash cations and bicarbonate to ocean CaCO 3 and SiO 2 deposits as sediment on seafloor CaSiO 3 + CO 2 CaCO 3 + SiO 2 (w eather ing) (metamor phism) Sediment subducts to source of v olcanoes oceanic cr ust THE CARBON CYCLE mantle continental cr ust [ Processes lettered in b lue are absent in a sno wball Ear th ] - - - - - - - - km 10 5 0 -5 - - - - - - - - km 10 5 0 -5 - - - - - - - - km 10 5 0 -5 - - - - - - - - km 10 5 0 -5 albedo ~ 0.3 NAm Sib Pl Amaz Congo Ant Kl NAm Sib NEu Pl Amaz K WAf Congo SAs Aus Ant Kl NAm Sib NEu Pl Amaz K WAf Congo SAs Aus 2. 3. 4. 1. SNOWBALL FREEZE-FRY SCENARIO CO-- 2 ~ 1.0 CO-- 2 ~ 0.1 sand dunes marine shelf CO-- 2 ~ 100 CO-- 2 < 100 glacial deposits "cap" carbonate albedo ~ 0.6 albedo ~ 0.7 albedo ~ 0.4 Ant Kl Aus Ant Kl SAs NEu WAf K sea ice ~1.4 km ~0.4 km sea ice H 2 O low-latitude continent glacier clastic sediment CO 2 shallow troposphere deep troposphere (with clouds) NAm Sib NEu Pl Amaz K WAf Congo SAs Aus Cartoon of one complete 'snowball' episode, showing variations in planetary albedo, atmospheric carbon dioxide, surface temperature, tropospheric depth, precipitation, glacial extent, and sea ice thickness. Stage 1. incipient glaciation; 2. runaway ice- albedo (onset of 'snowball'); 3. end of 'snowball'; 4. transient 'hothouse' aftermath. 50 o C -50 o C 0 o C 50 o C -50 o C 0 o C 50 o C -50 o C 0 o C 50 o C -50 o C 0 o C equinox equinox LOW OBLIQUITY (23.5 degrees) HIGH OBLIQUITY (>54 degrees) solstice solstice solstice solstice equinox SUN SUN 23.5° 65° For obliquities > 54 degrees, mean annual temperatures in the tropics are lower than at the poles, but low-latitude glaciation is unlikely because of very high seasonality. . . . . . . . . . . . .. . . sea ice Fe 2 0 3 iron-formation O 2 O 2 Fe 3+ Fe 3+ Fe 2+ Fe 2+ Fe 2+ Fe 2+ iron-formation mid-ocean ridge hydrothermal vent 01.02.03.0 Age (billions of years before present) If O 2 is absent, iron is soluble as ferrous (Fe 2+ ) ion. 1 10 -2 10 -4 . Snowball earth: anoxic ocean Deglaciation: ocean ventilation If O 2 is present, iron is insoluble as ferric (Fe 3+ ) ion. dropstones (xPAL) atmospheric oxygen levels . Abundance of sedimentary iron-formation through time 1400 1200 1000 800 600 400 200 Neoproterozoic Mesoproterozoic Age (millions of years before present) 10 5 0 -5 -10 d 13 C carb ( o / oo VPDB ) 10 5 0 -5 -10 Phanerozoic 0 * * * * Secular v ar iation in carbon isotopic composition of shallo w mar ine carbonates o v er the last 1600 million y ears (adapted from Kaufman, 1997; Kah et al., 1999). (more organic burial) (less organic burial) ? Sno wball e v ents 0.8 0.6 0.4 0.2 0-200 400 600 800 1000200 Time (x1000 years) d 13 C (%) Instantaneous mass extinction 0.0 100% recovery 90% recovery 0% recovery Changes in carbon isotopic composition of shallow marine carbonates following an instaneous collapse in organic production. Loss of the dynamically maintained biological "pump" causes an immediate drop of 0.2% due to ocean mixing. Longer term trends depend on biological recovery. The "90% recovery" trend approximates the record of the K-T boundary mass extinction. Values approach 0% if no recovery occurs in 0.5 million years (e.g. "snowball" earth). COLLAPSE IN ORGANIC PRODUCTIVITY 600 500 550 80 60 40 20 0 Ar thropoda Nematoda Pr iapula Mollusca Annelida Platyhelminthes Br achiopoda Chordata Echinoder mata Cnidar ia P or if er a LA TE NEOPR O TER OZOIC CAMBRIAN orders classes SQANTUM GLACIATION MARINOAN GLACIATION A GE ( millions of y ears bef ore present ) cro wn g roups stem g roups FOSSILS 18 S rDNA sequence ph ylogen y 1 2 3 1 2 3 Ecdysoz oa Lophotrochoz oa Deuterostoma THE MET AZO AN EXPLOSION F aunal div ersity Protostome- deuterostome div ergence ? ?