history of our planet week 5.2

causes of PETM, hominid and human evolution

Paleocene-eocene thermal maximum temperature change

+5 degrees C global warming within 20 Kyr

PETM - Volcanic trigger hypothesis:

A large flood basalt intrusion from Iceland hotspot began to open the north Atlantic 56.1Ma with splitting of the continents 55.5Ma

5-10 million km3 of basalts deposited in ~1myr intruded  into fossil-fuel-rich sediments

This released co2, and thermogenic methane

Thermogenic methane is less 13^C depleted than biogenic methane/ hydrates hence more of it would be required to explain the δ^13C excursion

PETM - Methane hydrates:

Where the rest of the carbon needed for this 5*C increase in temp

Gas molecules trapped inside a frozen water lattice

]Sufficient methane trapped that they can be ignited

They form in the pore spaces in sediments

They occur in a layer in ocean sediments stabilised by high pressure (the large amount of sediment pushing down – will hold the gas in the lattic) and cold temperatures

Form at continental shelfs

PETM - Methane hydrate destabilisation:

They can be destabilised by: - function of warming then destabilising of the sediments which was keeping it under pressure

Increasing temperature of overlying water

Depressurisation by lowering sea level,

(destabilise)  slope failure

Hypothesis for PETM:

Warming of intermediate depth ocean waters destabilised mathen hydrate

Outgassing of methane added to warming causing more methane to be released

= a positive feedback

Total of 1500-4500 GtC of CH4 released in quite short of time

PETM - Champagne cork effect:

PETM - Timescale of recovery:

Methane is rapidly oxidised to CO2 in the atmosphere, warming the climate

It takes >100 kyr for ocean to be replenished with alkalinity from silicate weathering, drawing down CO2 and cooling the climate

PETM - Gas hydrates today:

Estimates of total reservoir range from:

1000 GtC (milkov, 2004, ESR) to 10,000 GtC (kvenvolden, 1998)

1 GtC = 1 PgC = 10^15 gC

if this carbon were to be released it would seriously accelerate global climate change

Lessons from the PETM:

The amount of carbon released at the PETM is comparable to known fossil fueld reserves today and gave at least 5C long term warming

Models suggest that destabilisation of methane hydrates cold significantly amplify long term climate change

It will take at least 100Kyr for the carbon cycle and the climate to recover

Most anthropogenic emissions have been since the 1990s

Cenozoic climate and co2:

Peak warmth ~ 50 Ma in Early Eocene then cooling begins

Possible co2 decline ?

Abrupt growth of Antarctic ice sheet growth 34 Ma at Eocene – Oligocene boundary

Possible co2 peak ?

No clear cooling in Oligocene, But rapid co2 decline

Miocene climatic optimum ~ 15 Ma before further cooling

stable, low co2 since 24 Ma

Himalayan uplift hypothesis

Raymo and ruddiman 1992:

Uplift of the Himalaya/ Tibetan plateau caused increased rates of weathering

This decreased atmospheric co2 thus cools the planet

This should have caused a negative feedback response

Lower co2 and temp reduce silicate weathering until it again matches degassing

What stabillised Miocene CO2:

If co2 drops too low then the majority of (‘C3’) plants cannot photosynthesise

At 200-250ppm co2 during the Miocene upland vegetation was near co2 starvation

Pagani et al (2009) argues this limited the effect of plants on weathering, holding co2 stable

Great rift valley:

Savannah habitat

Divergence from chimpanzees ~6 Ma

Up to 5 genera:

Ardipithecus 5.8-4.4 Ma

Australopithecus 3.9-3 Ma

Praeanthropus?

Paranthropus ~2 ma

Homo ~ 2ma

Rise of grasslands: Characteristics:

  -33% of vegetated land

Low to moderate rainfall

]High Si weathering

Promote glazing and fires

Rise of grasslands: 3 stages of spread:

Eocene/ oligocene 33 Ma: Desert grasslands

Early Miocene 17 Ma: Ungulates; horse, antelope

Late Miocene 7 Ma: C4 grasses

Stone tools, homo habilis 2.5Ma

Literally ‘handy man’

Better classified as Australopithecus habilis

Prey for large cats

The first tool makers used for scavenging

Stones moved < 13 km range of an individual

Indicates planning but not group activity