Earth Sciences 240A - Lecture 26 - Flood Basalts II

The Deccan Traps, India

Volume: 500,000-1M km3

Time: Original est. ~60-65 mya

Possible correlation with KT extinction

Deccan origin

Reunion Island hotspot; Late Cretaceous ‘cruise’ of India

Magnetic polarity of Traps

Reversals occurred ~ every 10,000 years toward end Cretaceous

~80% flows show ‘reverse’ polarity

Best estimate: Most erupted in less than 1 my

Dating Details

K-Ar technique: 40K to 40Ar: cheap, quick, inaccurate

Age range: 80-30 mya (useless)

Ar-Ar technique: Convert 39K to 39Ar: Use 40Ar / 39Ar: expensive, accurate

Age range: major pulse 64-68

70% less than 1 my

Ir Anomaly

Mapped/Analyzed evidence

KT horizon ejecta (high Ir) preserved inside soil layer in stratigraphy

15 times higher Ir than other soil horizons

2 times higher than highest basalt

Caldeira & Rampino 1990 research

Used Hawaiian Ir value

Need 20x actual Deccan volume to explain KT Ir anomaly only from eruption

Carbon budget

Eruption would produce high CO2; Global T should rise

Biological/limestone production: Initially should rise

Caldeira & Rampino 1990 research

Use v.v. high basalt CO2 content plus >double Deccan volume

Lower cal’d greenhouse CO2 than expected from current fossil fuel use

If all eruption in 10 years, could get +10oC rise

Cooling evident, not heating

Sulfur budget

Using the most S-rich basalt values known, would still need 2x Deccan volume to explain analyzed S at KT horizon

Fluorine budget

Suppose Laki eruption was ‘typical flood basalt’

So far, no abnormal F analyses in anything from 66 mya

But maybe ‘evidence’ has dissolved and disappeared into sea water (?)

The Siberian Traps

Extinction Event - review

Never since so many dramatic changes

Dating evidence: Between 100,000 -10,000 years from event to beginning of recovery

Evidence: China, South Africa

Siberian Traps

~ 1.5x106 km3 volume

~250 mya

Major eruptions probably in less than 600,000 years (certainly less than 1 my)

Early magma: deep mantle

Later magma: very complex

Mixed with lithosphere material (more like CRB than Deccan)

Short-term eruption effects (general)

Initial cooling

Dust and aerosols (explosive events)

Acid rain

Sulfuric and carbonic acids

Wildfires

Probably not more extensive than flow areas

Poisonous emissions

Fluorine, others?

Ozone depletion

Temporary destruction from O/CO2 change

Siberian Traps as ‘murderer’?

Very difficult to kill 90% of everything!

Same problems as with Deccan

My conclusion:

“Definitely a contributor to poor environment, but can’t be sole cause of catastrophe”

The Core-Mantle Boundary

ULVZ

Seismic waves tend to get ‘bogged down’ at the boundary (i.e. slowed)

1980 grad student work revived in 1992

Lowest 100 km of mantle may contain 5-30% liquid

Source of initial plumes material?

Next:

Mass extinction correlations with

Impacts

Plume eruptions

Others