Download Lecture 37 - Cornell Geological Sciences

Stable Isotopes in
Paleoclimatology
Lecture 37
Water-Carbonate
Fractionation
• Urey calculated the
temperature
dependence of the
water-carbonate δ18O
fractionation and
pointed out it could be
used as a
paleothermometer by
solving for T:
• T (˚C) = 16.9-4.2∆+0.13∆2
o where ∆ is the difference
between calcite and the water it
precipitated from.
• He then had his students
perform experiments to
verify predictions.
Quaternary
•
•
•
Urey’s student, Cesar Emilliani,
analyzed δ18O in forams from
a variety of deep-sea cores
and reported 15 glacial
cycles in the last 600,000
years in his 1955 dissertation.
Subsequent work greatly
refined this record, leading to
a standard δ18O curve in the
late 1970’s.
Emilliani had noticed the
cyclicity in the curves and
concluded that
Milankovitich’s theory of
climate change was correct:
it was caused by changes in
the Earth’s orbit and rotation.
18
δ O
Record
Deducing Temperature
Change
•
Two factors result in change in
δ18O:
o
o
•
In order to determine temperature
changes, one must know how the
isotopic composition of water
changed.
o
o
•
Temperature dependence of the fractionation
factor - carbonate will be heavier at lower T.
Storage of isotopically light water on continents
as glaciers. Consequently, seawater, and also
carbonates, will be heavier during glacial
periods.
Deep water temperature changes less, so
benthic forams provide some control on this.
Ice volumes can be determined from sealevel
change (subsequently constrained by dating
coral reefs with U-Th).
In addition, of course, it is
necessary to accurately date
strata in the cores.
o
Has evolved from extrapolating 14C dates and
magnetostratigraphy to more sophisticated
approaches like U-Th and 10Be, etc.
Milankovitch Theory
• Earth’s orbit and rotation
vary regularly in 3 ways:
o
o
o
The obliquity of the rotational axis
relative to the orbital plane.
Eccentricity of the orbit
Precession: the direction the Earth’s
rotational axis points at perigee and
apogee of orbit.
• These factors influence the
distribution of solar energy
(insolation) in time and
space over the course of a
year, but do not change
global annual insolation.
• ‘Milankovitich parameters’
are well determined from
astronomical observations
(have been known for a
very long time).
•
•
•
•
Imbrie, Hayes and others
model
Imbrie and colleagues (1976,
1985) applied Fourier analysis
to the standardized δ18O curve
(CLIMAP project) to deduce
the primary frequencies
(dividing into two parts, <400ka
and >400ka).
They then build a model where
each Milankovitch frequency
influenced climate with a
different phase and gain.
The model accounted for r2 =
0.77 of the observed variance
in δ18O.
This kind of model has, of
course, been greatly
subsequently enhanced with
better data, GCM’s, ocean
circulation models, etc.
The Antarctic Ice Record
•
•
•
•
Much subsequent
paleoclimate effort has
focused on δD in ice cores
from Antarctica and
Greenland.
The Vostok core from
Antarctica went back 400 ka.
Subsequent work shifted to
the EPICA core which went
back >800 ka.
Complications in
interpretation arise here too
because of changes in δD of
the oceans and changes in
atmospheric circulation result
in complex relationship
between T and δD, but
temperatures can be worked
out.
Overall, agreement between
the marine and Antarctic
records is excellent, but shows
some differences between
Antarctic and global climate
change.
Greenland Ice Record
• Ice records from
Greenland are not as
long, but provide finer
details of the last glacial
cycle.
o Greenland is ‘ground zero’ of
glaciation.
• They reveal extremely
variable climate in the last
ice age -DansgaardOeschager events - likely
related to iceberg events
documented in deep-sea
cores.
Feedback Factors
•
•
•
Milankovitch variations
provide only a weak climate
signal that has been
apparently greatly amplified
in the Quaternary by
feedback factors.
June insolation at 60˚N
appears to be the key
sensitivity.
Feedbacks include:
o
o
o
•
Albedo
Shift of CO2 from atmosphere to
oceans with consequent change in
greenhouse effect
Changes in ocean circulation,
particularly with delivery of heat to the
North Atlantic (ground zero for
continental ice sheets).
The role of CO2 is well
documented by CO2
concentrations in bubbles in
Antarctic ice.
Figure 12.45
The Next Ice Age?
From Marcott et al. (2013) Science, 339: 1198
Soil Paleoclimate Proxies
• Hydrogen and Oxygen
isotopes in soil clays
reflect (with
fractionation), the
isotopic composition of
meteoric water.
• This allows
reconstruction of
paleoprecipitation
patterns - Cretaceous
precipitation in N.
America in this figure.
Pedogenic Carbonate
• δ18O in pedogenic
carbonate also reflects
composition of
meteoric water (with
fractionation).
• In Pakistan, δ18O in
paleosol carbonates
record the evolution of
the monsoons.