Transcript Magma Supply Vs Magma Plumbing

OCEAN/ESS 410
Class 19. Paleoceanography
William Wilcock
Learning Goals
• Understand how 18O is defined
• Understand why 18O decreases with
decreasing temperature in ice sheets
• Understand what causes 18O in foraminifera
to vary and how it can be used to infer past
climate.
Paleoclimate
• Ice cores
123,000 years Greenland, 800,000 years Antarctica
Temperature & air bubbles
Sub annual resolution
• Tree Rings
Continuous for a few thousand years (older with radiocarbon
dating)
• Corals
Continuous for a few hundred years (older with dating)
• Sediments
>100 Million years
but not in 1 core and preservation of fossils effectively limits it
to significantly less.
Time resolution of sediment record
• Typical deep sea sedimentation rates
– 0.1 to 3 cm / 103 yr
• Bioturbation in most settings
– 3-10 cm
• Resolution
– 103 to 105 years
– Changes over shorter term cannot be resolved in
a sediment core
Dating Sediments
• Absolute
– Radiometric (14-C, 230-Th/U, K-Ar)
• Relative from cross-correlation
– Paleomagnetic
– Fossil record
– Lithology
• Time on a rubber band
Bainbridge (Sector) Mass
Spectrometer
1. Create Ions
2. Accelerate Ions
3. Select Ions based
on velocity (electric
and magnetic
forces cancel out
for selected
velocity)
4. 2nd magnetic field
separates ions
based on
charge/mass ratio
5. Detector
Equations for Mass Spectrometer
• Velocity selection stage
– Electrostatic force
• FE = qE
Where q is charge and E is electric field
– Magnetic force
• FB1 = qvB
• Where v is velocity and B1 is magnetic field
– Selection (no bending) when FE = FB1 or v=E/B1
• Charge to mass ratio separation
– Acceleration from magnetic field
• FB2 = ma = qvB2 or a=qvB2/m
– Centripetal force
• a = v2/r = qvB2/m or r = mv/(qB2)
• r increase with mass of ion
Oxygen Isotopes
Stable Isotopes
99.759% 16O
0.037% 17O
0.204% 18O
The lighter isotopes is preferentially incorporated into
vapor, slightly more so at lower temperatures, and the
heavier isotope is preferentially incorporated into rain.
é ( 18 O / 16 O)
ù
d 18 O = ê 18 16 sample -1ú ´1000
ê ( O / O)
ú
ë
û
standard
Standard = SMOW (Standard Mean Ocean Water)
Water vapor in equilibrium with SMOW had 18O = -9 to -11‰
d18O of precipitation – Latitude
Dependence
• Most evaporation
occurs at low latitudes
and most precipitation
at high latitudes.
• Vapor forming with the
equator with d18O = 9‰ will always
precipitate rain with
more 18O and the
remaining vapor will
get progressivly
lighter in 18O as it
moves to higher
latitudes.
• This process is known
as fractionation
18O in ice cores
• Modern Ice Averages 18OSMOW = -25‰ but it
depends on location
• During Ice Ages it was colder and therefore
ice is lighter (18O more negative)
18O, ‰
Hydrogen Fractionation Also Occurs
Antarctic Ice Core Records
Temperature comes from Oxygen and Hydrogen isotopes
Oxygen Isotopes Foraminifera
Calcium
Carbonate
skeletons for
Foraminifera form
with a 18O value
that is offset from
water by an
amount
dependent on
temperature
(some variation
between species)
Average 18O record of foraminifera in sediments
• Ice Volume (18O of oceans increases when more
isotopically light ice is locked up on the continents) - ⅔ of
variation (calibrate with deep sea foraminifera)
• Temperature - ⅓ of variation
Effect of Ice Volume on d18O
18O of
present day
surface waters
Isotope record of ice ages
Lisiecki and Raymo stack of 18O in deep water benthic
foraminifera in 57 cores
Fourier Transform
Fourier Transform Versus Time
• 100 kyr
eccentricity
period
important now.
• 41 kyr obliquity
important in
the past