The Cenozoic Flashcards
Mesozoic Climate recap
- Pangea breaks apart.
- Massive flood basalts form associated with the breakup of Pangea and Gondwana.
- Rapid seafloor spreading releasing additional mantle-derived CO2 into the atmosphere.
- Increased seafloor spreading rate increases size of mid-ocean ridges.
- Sea level rose - flooding vast continental areas
- The Earth was very warm (mean T 10 to 20 degrees warmer than today)
- Forests spread to latitudes currently covered by tundra or ice.
- No polar ice caps.
Cenezoic Overview
- Starts just after K/T impact
- High resolution data, particularly for Quaternary
- Lasts 65 million years
- The Cenozoic is divided into two sub main periods: The Tertiary and the Quaternary
- Only about 1.5% of geological time
- But we know much more about it than previous eras
The Epochs of the Cenozoic
• Fossils description based on relative ratios of living vs extinct species
• Proposed by Charles Lyell in 1833
Holocene (Q)
Pleistocene (Q)
Pliocene (T)
Miocene (T)
Oligocene (T)
Palaeocene (T)
Climate Change over the Cenozoic
Cretaceous extremely warm
Cenozoic basically exhibits a steady cooling trend starting with K/T event
Eocene Paleogeography
• Very similar to today
• Differences
o India not yet collided with Asia
In Pacific around the equator
o Open water between Pacific and Caribbean
o Australia not fully separated from Antarctic
Palaeocene/Eocene Thermal Maximum
• 5 to 8C warming at all latitudes
• Arctic surface water increased from 18 to 23C
• Warming in both surface and deep oceans
o Wouldn’t expect that today – why are they heating at the same time?
• Warmth lasted less than 100 ka
• Oxygen isotopes indicate a global warming event similar to or greater than 21st Century forecasts (+5°C)
• Carbon isotopes indicate a rapid influx of lightweight carbon (methane release? carbon dioxide?)
PETM Clathrate hypothesis
- Methane trapped in clathrate (ice-like substance)
- Ice ‘cage’ structure
- Found in the pore space of oceanic sediment found along continental margins
- Stability depends on temperature of bottom and intermediate water
- Methane clathrates disassociation is temperature driven
- Lower temp of water, the more stable
- When destabilised, the methane is released
• Hypothesis states that bottom water temperature increase causes the catastrophic release of methane along continental margins.
o Crossed a margin
• Methane is a strong greenhouse gas with a very low d13C, and results in atmospheric warming
• Explains sudden decrease of d13C values apparent in various proxies at glacial terminations
• Explains global warming found at PETM
post PETM climate
- Even after PETM the Eocene (~55-35 Ma BP) was a very warm period.
- Eocene (and Cretaceous!) polar warmth is difficult to explain:
- If it’s due to enhanced greenhouse effect, why aren’t the tropics warmer?
- If fluxes are in any sense diffusive, how can more heat be
- transported across a smaller gradient?
- Not diffusive - advective!
- Emanuel (2001): Are hurricanes more frequent in warmer environment? Do these cause the necessary changes in advective heat transport?
- Ocean currents?
Shutting off the thermostat
• Middle Eocene warming due to the long-term reduction in the negative feedback silicate weathering system
• Pangea breaks apart
o No new rocks exposed, less and less fresh frock exposed after weathering
o Silica feedback system turned off – ‘thermostat off’
What happened after the Early Eocene?
General cooling until Holocene
BLAG hypothesis
Potential explanation for post early Eocene cooling
• Depends on global seafloor spreading rates
• 55-15 mya general decrease in spreading
• 15 mya to today spreading increased
• Consistent with record prior to 15 mya
• Inconsistent with record from 15 mya to present
• Cannot alone explain cooling
Uplift/Silicate Weathering Hypothesis
Potential explanation for post early Eocene cooling
1) Unusual amounts of high elevation
2) Active mountain building
3) Evidence of increased erosion in sedimentary record
Strontium Isotopes:
• Curve reflects relative contributions of Sr to the ocean
– Continental weathering
– Hydrothermal activity along mid-oceanic ridges
• General decrease in Early Phanerozoic due to increasing activity along mid- ocean ridges
• 87Sr/86Sr increase could be due to:
1) Increase in chemical weathering
2) Rock type being weathered is more radiogenic
• No change in rate of chemical weathering
3) Decrease in seafloor spreading
Chemical Weathering Rates:
• India begins to collide with Asia around 50 Ma BP
• Resulting Tibetan-Himalayan complex very large and at very high elevation
• High elevations receive lots of rainfall
• Heavy rains produce high suspended and dissolved sediment load
• Abundant CO2 drawdown
• Summer monsoons hit Himalayas = massive amounts of weathering
Mountain building event:
• Himalayan orogeny
o Has a pronounced effect
Cooling
BLAG or Uplift Weathering?
• No “proof” of either hypothesis exists:
– BLAG explains well cooling from 55-15 mya
– Uplift weathering supported by conditions in Tibetan- Himalayan Complex
• Would a combination of the two hypotheses best explain global cooling over last 55 my?
Cooling post-Eocene
Hypothesis:
- BLAG hypothesis: decrease in mid-ocean spreading rates reduced CO2 outgassing
- Tectonic uplift: cooling from ~50 Ma BP may be due to rise of Himalaya and accelerated silicate weathering - drawing down CO2
- Subtle continental drift and sea level changes altering ocean circulation, changing polar heat transport
Global Surface cooling:
• Temperatures dropped by about 8-13 oC near the Eocene- Oligocene boundary, as indicated by isotope data from brachiopods from New Zealand. (Oi-1 Event)
• Antarctic sea ice began to form by 38 ma BP.
• Greenhouse conditions were replaced by icehouse conditions
Oi-1 Event
32-33 Ma
(Antarctica East ice Sheet forms)
- Isolation of Antarctica?
- Passing of some critical atmospheric CO2 threshold?
- Orbital Ice-Climate Feedbacks
Ocean Heat transport:
• Opening or closing of critical gateways
– Narrow passages linking major ocean basins
– Change heat and salt balance
– Two critical oceanic gateway changes
1) Opening of Drake Passage: produces the Antarctic Circumpolar Current
2) Formation of the Isthmus of Panama: stopped equatorial flow between Atlantic and Pacific
Opening of Drake Passage:
• The gap between South America and Antarctica opened 25-20 ma BP allowing initiation of Antarctic Circumpolar Current (ACC)
– Strongest ocean current
– Prior to opening, flow from north kept Antarctica warm
– Onset of ACC accelerated glaciation on Antarctica
• Drake Passage opened 25-20 mya
• Eastern Antarctic Ice Sheet began forming around 35 ma BP (Oi-1 Event)
• Western Antarctic Ice Sheet began forming around 13 ma BP
Isthmus of Panama:
• Closure within last 10 mya
– Complete closure 4 mya
– N. America glaciations 2.7 mya
• Caribbean warms
• Gulf Stream moves warm, saline water north
• Increases ocean evaporation and consequently precipitation on land
• Redirection of west flowing warm saline water into Gulf Stream
• Stops return flow of low salinity water into Atlantic from Pacific
• Increases salinity of Gulf Stream
• Less sea ice -> more moisture -> glacier growth
Ocean Heat transfer paradox
• Illustrates fundamental disagreement
– Stopping poleward flow enhanced glaciations (Antarctic)
• Colder = glaciers grew
– Starting poleward flow enhanced glaciations (Arctic)
• Warmer = glaciers grew
• Likely related to complex feedbacks involving altitude and albedo
Burial of organic material as driver for Cenozoic climate change?
• Changes in the rate of burial of organic matter affect atmospheric CO2
• Rate of burial of marine organic matter sensitive to:
– Changes in rates of production
• Nutrient supply
– Change in upwelling
– Change in delivery of nutrients from land
– Changes in mode of preservation
• Bottom water oxygenation
• Organic carbon-rich sediments deposited along California coast 13 ma BP
• Coincided with global cooling
• Strong winds enhanced upwelling
• – Termed the ‘Monterey Hypothesis’
• Timing of maximum organic carbon burial occurs 3 Ma after maximum cooling
Weathering of organic carbon in rock:
• Organic carbon from bedrock weathered and oxidised
o Black shales
• Does weathering of organic-rich rocks produce CO2?
• Glaciers accelerate rock C oxidisation, releasing CO2
• Negative feedback to cooling (and warming)?
• Absence of this feedback in
• Neoproterozoic?
Late Cenozoic Ice Age
• Plio-Pleistocene Glaciations 30% of Earth’s surface covered by glaciers
o Canada but not Alaska because too much sea ice and not enough moisture reaches it
• The Late Pliocene and Pleistocene experienced strong, rapid, climatic fluctuations.
• Characterized by glacial expansions separated by warmer interglacial intervals.
• There may have been dozens (if not hundreds) of glacial advances over the past 3 million years (roughly every 41 ka before 700 ka BP, every 100 ka after).
• Perhaps controlled by Croll-Milankovitch Astronomical forcing?
Causes of Ice Ages:
- Himalayan uplift started ~50 Ma BP
- Atmospheric CO2 reduced
- Onset of monsoons – abundant rain
- East Antarctic Ice Sheet formed around 35 Ma BP
- West Antarctic Ice Sheet at ~13 Ma BP
- Arctic
ice capappears ~2.7 Ma BP, due to: closure of Isthmus of Panama changed oceanic circulation - Once ice caps formed, controlled by Croll-Milankovitch cycles
