The Cenozoic

Question 1

Mesozoic Climate recap

Answer 1

• 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.

Question 2

Cenezoic Overview

Answer 2

• 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

Question 3

The Epochs of the Cenozoic

Answer 3

• 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)

Question 4

Climate Change over the Cenozoic

Answer 4

Cretaceous extremely warm

Cenozoic basically exhibits a steady cooling trend starting with K/T event

Question 5

Eocene Paleogeography

Answer 5

• 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

Question 6

Palaeocene/Eocene Thermal Maximum

Answer 6

• 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?)

Question 7

PETM Clathrate hypothesis

Answer 7

• 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

Question 8

post PETM climate

Answer 8

• 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?

Question 9

Shutting off the thermostat

Answer 9

• 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’

Question 10

What happened after the Early Eocene?

Answer 10

General cooling until Holocene

Question 11

BLAG hypothesis

Answer 11

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

Question 12

Uplift/Silicate Weathering Hypothesis

Answer 12

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

Question 13

Chemical Weathering Rates:

Answer 13

• 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

Question 14

BLAG or Uplift Weathering?

Answer 14

• 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?

Question 15

Cooling post-Eocene

Answer 15

Hypothesis:

1. BLAG hypothesis: decrease in mid-ocean spreading rates reduced CO2 outgassing
2. Tectonic uplift: cooling from ~50 Ma BP may be due to rise of Himalaya and accelerated silicate weathering - drawing down CO2
3. 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

Question 16

Oi-1 Event

Answer 16

32-33 Ma
(Antarctica East ice Sheet forms)

• Isolation of Antarctica?
• Passing of some critical atmospheric CO2 threshold?
• Orbital Ice-Climate Feedbacks

Question 17

Ocean Heat transport:

Answer 17

• 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

Question 18

Opening of Drake Passage:

Answer 18

• 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

Question 19

Isthmus of Panama:

Answer 19

• 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

Question 20

Ocean Heat transfer paradox

Answer 20

• 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

Question 21

Burial of organic material as driver for Cenozoic climate change?

Answer 21

• 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

Question 22

Weathering of organic carbon in rock:

Answer 22

• 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?

Question 23

Late Cenozoic Ice Age

Answer 23

• 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?

Question 24

Causes of Ice Ages:

Answer 24

• 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 cap appears ~2.7 Ma BP, due to: closure of Isthmus of Panama changed oceanic circulation
• Once ice caps formed, controlled by Croll-Milankovitch cycles