GEOG5426 Stable isotopes

November 24

Brief (15-minute) summaries of project topics.

(1) What are the most important features of the modern climate in your region?

(2) What proxies are available in your region, over the time interval specified? How are they related to climate? and

(3) How different were past climates from modern conditions? Why is that important?

S TA B L E I S O T O P E S I N P A L E O C L I M AT O L O G Y

Isotopes are elements with the same number of protons but varying numbers of neutrons.

Stable isotopes are either not radioactive or decay so slowly that their half-lives cannot be measured.

Stable isotopes have the same chemical properties but don’t have the same atomic mass.

O16 O18

Stable isotopes of oxygen

O1699.76%

O180.2%

O170.04%

Photograph: David SifrySMOW standard mean ocean water

Evaporation Condensation

Fractionation

H2O18

H2O18H2O18

H2O18

H2O18

H2O16H2O16

H2O16

H2O16

H2O16

H2O18

H2O18H2O18

H2O18

H2O18

H2O16H2O16

H2O16

H2O16

H2O16

H2O16

H2O16

H2O16

more O18“enriched”

more O18“enriched”

less O18“depleted”

more O18“enriched”

more O18

“enriched”

more O18

“enriched”

less O18“depleted”

“heavy”

“heavy”

“light”

O18

O16

O18

O16

small

O18

O16

small

BIG

O18

O16SMOW

O18

O16sample

“more positive” δO18 = “heavy” = “enriched”for example, ocean water

“more negative” δO18 = “light” = “depleted”for example, glacial ice

Source: Jouzel et al., Journal of Geophysical Research, 1994

Source: Jouzel et al., Journal of Geophysical Research, 1994

“light”

“heavy”

paleothermometer

Source: Bowen and Revenaugh, Water Resources Research, 2003δO18 in precipitation

Source: Bowen and Wilkinson, Geology, 2002

δO18 in precipitation

The “amount effect”: The greater the amount of rainfall, the lower the δO18 of the rainfall.

(i.e., more negative)

O18

O18 O18

O18O16

O16O16

O16O16

O18

O18 O18

O18O16

O16O16

O16O16

O16

O16

“more negative” δO18 = “light” = “depleted”

O18

O18 O18

O18O16 O16

O16

O16

O16

O18

O18 O18O18O16

O16

O16

O16

“more positive” δO18 = “heavy” = “enriched”

Source: Shanahan et al., Science, 2009

less O18

more negative“depleted”

more O18

more positive“enriched”