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Orographic Fractionation
A
18 O/ 16 O gradually lower (lighter)
storm direction &
moisture advection
*
*
continental landmass
small isotopic
difference
B
18 O/ 16 O rapidly lower (lighter)
orographic precipitation
rain shadow
storm direction &
moisture advection
*
*
rising topography
large isotopic
difference
Fig. 7.29 Isotopic fractionation of precipitation and the effects of topography.
A. In the absence of a mountain range, the oxygen 18 O/ 16 O ratio gradually decreases inland with greater distance from
the moisture source. B. In the presence of topography, isotopic fractionation is strongly altitudinally dependent as a
result of orographic precipitation. The increased fractionation also causes a stronger gradient in isotopic ratios across
the mountain at the same altitude. Stars mark sites that would display weak and strong contrasts in isotopic ratios
before and after the mountain range grew, respectively. Importantly, the isotopic ratio at the downwind site reflects
not its paleoaltitude, but rather the height of the mountain passes over which the moisture-bearing winds passed.
compare the composition on opposing sides of
the range from sites that pre-date and post-date
growth of the range, the isotopic difference
between the sites should increase significantly
as the range rises and increases the fractionation
(Fig. 7.29). Because the temperature at the time
of precipitation also affects isotopic fractionation,
climate change can still affect the isotopic
composition of rainfall and needs to be
considered. Nonetheless, recent isotopic studies
from the South Island of New Zealand clearly
demonstrate a strongly increased fractionation
from west to east as the Southern Alps rose
into  the path of storms driven by the prevail-
ing  westerly winds during the past 5 Myr
(Chamberlain et  al. , 1999). If many sites
representing many time intervals could be
sampled, these diverse data could place some
robust timing constraints on the topographic
growth of the range.
Over the past two decades, numerous studies
have exploited 18 O/ 16 O ratios of authigenic clays,
soil carbonates, and volcanic glasses to estimate
paleoaltitudes (Cassel et  al. , 2009; Garzione
et  al. , 2000; Mulch et  al. , 2006; Rowley and
Garzione, 2007). Most such estimates do not
account for the fact that growing mountains or
plateaus are likely to induce regional climate
change in which the distribution of rainfall, the
average surface temperatures, and even the
major storm tracks and moisture sources, can
change (Ehlers and Poulsen, 2009). Each of
these changes could add significant uncertainty
to paleoaltitude reconstructions based on
18 O/ 16 O ratios. Some of these issues can be
avoided with a new isotopic method, termed
the  “clumped-isotope thermometer,” that uses
the bonding characteristics of 13 C and 18 O to
determine the paleotemperature (
±
°
C) of
formation of soil carbonates (Eiler, 2007; Ghosh
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