Deep time: Lectures 9-13 Flashcards
(30 cards)
What era is the rise of plants associated with? (The other eras within the associated eon that are not related are the Mesozoic and Cenozoic)
Within the Phanerozoic eon, the PALEOZOIC era. This consists of Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian periods (542-250 Ma)
What was the planet like before plants?
- Atmosphere - less O2, CO2 around 15x present atmosphere level, 7 degrees warmer
- Oceans more anoxic
- Land uninhabited except for few anthropods (sandstone tracks 550 Ma), most life in water
What is the name of the tree used to plot out plant evolution?
Phylogenetic Tree
- 1st photosynthesizing cyanobacteria engulfed by eukaryotes (2 Ga) creating first eukaryotic algae
- Formed 1st multi-cellular sea-weeds and algae (water-bound)
- 1st land-plants non-vascular bryophytes (e.g. liverwort, moss)
What were the earliest land plants? In what period?
Bryophytes: non-vascular plants in the Ordivician period, ~470 Ma
What evidence is there for the first land plants?
Cryptospores:
- Sudden change to new kind of these spores
Trilete spores
- Dated at 445 Ma, ‘Y’ marking
- Hardy spores, groups of 4 connected spores = resistance to dessication (extreme dryness)
What types of plants followed non-vascular plants? In what period? What evidence?
Mid-Silurian = vascular plants (425 Ma)
- Fossils from 425 Ma
- Structure implies tubing; maintenance of internal water pressure (homiohydric); remaining upright
- No leaves or roots
Late-Silurian = leaves and roots
- Fossils 420 Ma, larger 30cm plants
- V small leaves (microphylls), roots
- Like modern-day clubmosses
- Evidence of giant fungi (prototaxites) 420-370 Ma (multiple C isotopes, multiple food sources)
What plants followed vascular plants? In what period and using what evidence?
Early-Devonian = Rynier Chert
- Exceedingly well-preserved ecosystem 410 Ma (Aberdeen)
- Complex; plant-fungi symbiosis, rock weathering, nutrient recycling
Mid-Devonian = first trees
- ~385 Ma, up to 8m high (Wattieza stump, Gilboa, 1870s)
- Lacked proper leaves
Late-Devonian - first forests
- Archaeopteris ~375 Ma
- Proper leaves, large 1.5m diameter trunks, 10m high
- Source of organic carbon; enhanced weathering greatly
What were the consequences of plant evolution?
Amplification of weathering/factors
- Bcos they accelerate rate at which nutrients released (P has no gaseous form; rock breaking function)
- By a factor of 10 (even earliest non-vascular mosses 2-9)
Global
- Phosphorus leak to oceans; increasing ocean productivity –> organic C burial –> O2 increase
- Less CO2 = cooling
CO2 & climate
- Models; CO2 nearly constant at 16x PAL without plants
- Plants HALVE CO2 to 8x PAL (allowing glaciation threshold to be met)
- Earlier glaciations not explained by CO2 model predictions (too high)
Glaciations:
- 300 Ma (Permo-Carboniferous) = by first forests
- 450 Ma (Ordivician) = by first plants
What were the constraints on oxygen?
Combustion sensitivity - less energy for ignition when O high. If lower than 15%, fires cannot start!
Lower limit = 15-17% (charcoal record 400 Ma)
Upper limit = 25-30% (abundant forests 350 Ma, fires can’t have been too frequent otherwise trees wouldn’t survive)
By what mechanism/process did O increase and become regulated in the ocean? What does this point to?
Ocean-based regulator:
- BEFORE PLANTS; system would counter a O drop
- Anoxic waters promote P recycling –> increased productivity (cyanobacteria) –> CO2 burial
- For every P, 250 C buried!
Plants double source of O to atmosphere = above 17% –> more fires
Points towards land-based regulator:
- Veg limited by fires so O decreases
- Photo-respiration may occur if O too high (Rubisco favouring O instead of CO2)
- C-P ratios; fires redistribute P when biomass burned, washed into oceans BUT ocean material results in less burial than land = less burial = less O2
Describe oxygen level research on the Carboniferous-Permian (360-250 Ma)
Swamp environments (sinking continents) = coal deposits (dead matter, organic C)
- 30-35% O peak predicted
- V high O levels = giant insects (Meganeura dragonfly)
- 10x size today, but 20-20% O too small
- Other explanation: no larger competitors, filling ecological niche later occupied by flying reptiles/birds
Describe oxygen level research on the Mesozoic (250-65 Ma)
- Dinosaurs so large because of high O levels?
- Some predict low O at 12%
- Recent studies = abundant charcoal throughout Mesozoic (espec. Jurassic period) - models wrong!
- O couldn’t drop below 15%
What role did O play in animal evolution? Did it enable intelligent life?
Rising O triggered evolution of placental mammals?
BUT abundant charcoal suggests O actually higher than present, declined during Cenezoic
Brain function suffers at around 15% Oxygen (one settlement 11%)
- Charcoal record = many fires = O not below 17% so…
NO, INTELLIGENT LIFE NOT HELD BACK BY LACK OF O
Name the 5 key extinctions. What are the two main ones?
- End Ordovician
- Late Devonian
- End Permian = BIGGY
- End Triassic
- End Cretaceous = SECOND BIGGY
Describe the End Permian extinction - evidence and aftermath.
250 Ma
- 61% families extinct
- Evidnece: plant mutagnesis (mutated plants, exposure to UV from ozone depletion)
- Loss of forests for 5Myr in records
- Lystrosaurus shovel lizard survived!!
Aftermath:
- Oscilation of C cycle and ocean anoxia for 5 Myrs
- ~10 Myr recovery of ecosystems
- ~100 Myr recovery of global biodiveristy
- Shift in marine ecological state: increased mobile animals and predators
- Mammals and flowering gymnosperms replaced reptiles and non-flowering
What are the proposed causes of the P-Tr extinction?
Impact? - REJECTED
- Shocked quartz, buried crater, fullerenes (extraterrestrial isotopes)
- Not reproducable, lack of iridium
Volvanic? -CLEAR
- Siberian Traps LIP - 2 Mkm3 of lava = 1.6 M km2 covered, 3000m deep
- V short timescale, 60kyrs
- Release of CO2, melting of org. matter in crust = CH4 and CO2 (13-C depleted material)
- Temp rise of 10 degrees
Ocean acidification?
- More C dissolved, carbonic acid, H dissolves
- CaCO3 shells inhibited; shells lacking from record
Anoxia?
- Lack of larger fossils suggests lack of O
- Euxinic waters = rise of hydrogen sulphide
- Why? Fast basalt weathering = more P = increased O demand
- Temp rise reduces oxygen solubility = less O dissolved in ocean
Describe the K-P extinction - evidence and aftermath.
60% genera extinct, rapid
Unusual selectivity e.g. large dinos extinct, not smaller burrowing ones
- Evergreen suffered more than higher-lat deciduous veg
Aftermath:
- Origination rates rapidly increase; empty ecological niches filled
- Diversity of mammals rose; espec. size since dinos gone
LAND DIVERSITY (mammals) OVERTAKES MARINE FOR 1ST TIME IN PHANEROZOIC
What are potential causes of the K-P extinction?
Impact event (CONVINCING)
- Ir-rich sediment
- 180km wide crater dated 65 Ma, Mexico
Volcanic (LIKELY)
- Deccan Trap LIP; 800 kyr basalt outpouring; 365k km3
- Increased planet vulnerability (warming)
Strangeglove ocean
- Species loss of 90% for marine phytoplankton
- Acidification? Impact/eruption aerosols blocked out Sun, prohibiting photosynthesis…
Bio pump collapse
- C transfer to ocean stops; suggested by dorp in C isotopic ratio
- 40-60% drop in CaO within ocean sediments
- 2-3x increase in atmospheric CO2, combined with eruptoins = dramatic CO2 rise and warming
Describe the Cenozoic Era and what is the PETM?
Divided into 2 periods = Tertiary and Quaternary
Tertiary split into Paleocene (65-55 Ma) and Eocene (55-34 Ma)
Spike in temp between these two eras = Paleo-Eocene Thermal Maximum (PETM) = +5 degrees warming within 20 kyrs
How has the PEMT been reconstructed?
C and O isotopes…
- Suddent warmth in Cenozoic record identified through 13-C enriched oceans and 13-C depleted land biomass = large injection of C (and much burial)
- Later cooling trend linked to declining CO2
- Fossils (foraminifera) CaCO3 shells = 16-O in shells indicates warming temps
(at warmer temps, oceans have more 16-O bcos of melted ice and high energy evaporation of 18-O. In colder periods, 16-O is preferentially evaporated and later precipitated to form ice, so oceans are 18-O enriched)
What has the PEMT been linked to?
Diversification of mammals:
- Mammals 150g-1kg at end of Mesozoic
- Not bcos of O rise (charcoal) = Ecological explanation = first orders
1. Even-toed ungulates (e.g. pigs)
2. Odd-toed ungulates (e.g. horses)
3. Primates (e.g. lemurs, apes…us)
What evidence is there for the PEMT?
Ocean acidification:
- Mass CO2 injection = carbonic acid
- Less CaCO3 shells (dissolved in acid)
- Would require ~2000 GtC (100k yr recovery)
C cycle perturbation:
- 2 negative shifts in 13-C 1k yrs long, 20k yrs apart
- Suggests large release of 13-C depleted C from org matter or CH4
Source of much 12-C?
How has the PEMT been explained - the driving mechanism?
Volcanic trigger:
- Released CO2 and thermogenic methane (org C from crust cooked into gasses, out gassed)
- Possible bcos of opening N Atlantic 56.1 Ma
- 5-10 M km3 basalts deposited
Methane hydrates:
- Trapped CH4 gas in frozen water lattice
- Form under high pressure and cold temps
- PETM = warming of surface ocean = destabilisation og methane hydrates = outgassing of methane –> positive feedback
- Sediment avalanches decreasing pressures = more CH4 escape
= 1,500-4,500 GtC of CH4 released!
Describe the PEMT recovery. What can be learnt from this?
Potential for methane hydrates to be distabilised in the present (1000-10,000 GtC)
Humanity burning so much C, could reach similar levels of PETM so need to be careful
Especaially bcos of ~100,000 recovery from ~1000 GtC
