Stable Isotopes as Climate Proxies Flashcards
What is the CET?
CET
• World’s longest instrumental record of climate (monthly temperature)
• Extends from 1659
• Combination of records for a roughly triangular area in the UK Midlands (between Bristol, Lancashire, and London)
• Describes the climate for an area about 0.005% of the Earth’s surface
• Describes the climate for about 0.0000008% of the history of the Earth
What is EPICA?
ice core, Antarctica:
• Ice core with oldest timescale
• Not snow, but glacial ice (snow -> névé -> firn -> ice)
• Preserves the isotopic values of snowfall
• 800,000 yrs continuous climate record
• Annual-subannual layers toward top
• Resolution decreases with depth due to compaction
• Preserves geochemistry of snowfall
o Deeper down = compacted + convoluted
o Deeper = chronologies get progressively worse
What is Deuterium?
• Deuterium is one of two stable isotopes of hydrogen with an atomic mass of 2 (instead of 1)
graphs show the ratio of hydrogen two to hydrogen one
• Shows a peak every 100,000 years – can be explained by regular forcing (eccentricity)
Basic Chemistry (a refresher hopefully)
- Nucleus is positively charged
- Balanced by negatively charged electron cloud
- Atoms are electrically neutral (no charge)
- But if not…Ion!
- Chemical properties controlled by electrons
- Atomic Number Z is number of protons
- Every element has a unique atomic number (Z)
- Number of electrons e = Z
- Mass number A = atomic number (Z) + number of neutrons (n)
What is Raleigh Distillation?
• As a vapour mass migrates to a region of lower temperature it condenses and equilibrium fractionation (between the vapour mass and the condensing phases within the cloud) preferentially removes heavy isotopes (18O and 2H) from the remaining rainwater.
o Water containing the heavier isotope (18O and 2H) has a lower vapour pressure so when the temperature falls, the heavier water molecules condense faster than the lighter ones
What effect does the Raleigh Distillation have on climate recording?
- The heavier isotopes (18O, 2H) always prefer to be in the least energetic state
- Therefore, the heavier isotopes are included preferentially in rainfall rather than water vapour, and ice rather than water
- The further the moisture mass (i.e., cloud) has moved the more fractionated it has become
- The lower the temperature, the more fractionation occurs
- Snow is particularly good at removing 18O
- The lower the latitude, the heavier the rain, the more 18O
Responsible for decreasing precipitation d18O & dD with increasing:
• Latitude - “” latitude effect”
• Distance inland - “continental effect” “rainout effect”
• Altitude - “altitude or orographic effect
What is SMOW?
- Standard Mean Ocean Water (SMOW) has 1 18O for every 499 16O
- 2005 ppm
- So: 18O/16O for SMOW = 1/499 = 0.002005
What is the delta 18O %o?
The ratio of Delta 18O/ Delta 16O of the sample minus that of the SMOW, divided by the ratio of the SMOW (a standard), multiplied by 1000
Change in rainfall d18O according to Rayleigh distillation:
- Difference between d18O of rainfall and snow is significant
- Most d18O is gone from vapour before it reaches Antarctica and therefore core site
- Colder temperatures mean less d18O make it to Antarctica
- More negative d18O = colder
• ‘Temperature Effect’ controls fractionation factor
-lower T = greater fractionation
• ‘Rainout Effect’ controls the amount of 18O left
• -more rainout, less 18O and more -tive ratios
• -Raleigh Distillation
d18O‰ = 0.69 Temp – 13.6
• Describes precipitation values around N Atlantic Based on the temperaturd18O‰ = 0.69 Temp – 13.6e and rainout
What is the Rainout effect?
decrease amount of rain towards centre of continent
What is the Global Meteoritic Water Line?
• Hydrogen isotopes (dD) are fractionated along with oxygen.
• They vary predictably and follow the Global Meteoritic Water Line (GMWL)
Shows a trend line which all water bodies fall on
GRIP (Greenland Icecore Project) versus NGRIP (north GRIP)
- Both cores match up very well with each other
- T can be estimated from d18O by using T-isotope equation (Dansgaard, 1964)
- T can be estimated from d18O by using T-isotope equation (Dansgaard, 1964)
- Match less well during glacial periods
How Temperature – d18O relationships vary?
1) From site to site
2) Depending on the original d18O of the air mass
3) Depending on air mass trajectory
4) Depending on who measures them
5) Depending on when measured
stable isotopes in rainwater are affected by?
1) ‘Temperature effect’
2) ‘Rainout effect’
3) ‘Amount effect’
4) Isotope ratio of source moisture
5) Latitude
6) Altitude
7) Distance inland (‘continental effect’) – greater during glaciation
