Download Oxygen isotope evidence for the late Cenozoic development of an

Oxygen isotope evidence for the late Cenozoic development of an
orographic rain shadow in eastern Washington, USA
Akinori Takeuchi 

Peter B. Larson 
Department of Geology, Washington State University, P.O. Box 642812, Pullman, Washington 991642812, USA
ABSTRACT
Topographic development of the southern Washington Cascade Range and its influence
on regional climate on the leeward side of the range for the past 15.6 m.y. are evaluated,
using oxygen isotope ratios (d18O) of ancient meteoric water recorded in authigenic smectites. The d18O values of authigenic smectites from paleosols and altered tuffs on the east
side of the range exhibit a temporal and continuous decrease of ;3‰–4‰ from 15.6 Ma
to the present. Taking into account a regional temperature change in eastern Washington
since the middle Miocene, the calculated d18O values of regional meteoric water show a
negative shift of ;3.5‰–4.5‰ over the same interval. Such a decrease is similar to the
change in the d18O values of modern precipitation from the coastal side to a region downwind of the Washington Cascades. This negative shift of the calculated d18O values on
the east side of the range is best explained by the development of a rain shadow due to
the tectonic rock uplift of the Cascades during the late Cenozoic. Based on the empirically
calculated relationship between change in elevation and change in the d18O value of precipitation, paleorelief of the southern Washington Cascades since 15.6 Ma is estimated to
be ;1.2–1.7 km.
Keywords: stable isotopes, authigenic minerals, climate change, Cascade Range, tectonics.
INTRODUCTION
According to Ruddiman and Kutzbach
(1989), topographic and tectonic uplift of
mountain belts could be intimately linked to
regional and global climate change. Large
mountain belts act as orographic barriers over
which air masses rise, and the rain-shadow effect often creates arid to semiarid regions on
the lee side of the mountain belts, where precipitation is significantly low and isotopically
light. Thus, coupled with a modern climate
pattern in a rain-shadow region, changes in the
oxygen isotopic composition (d18O) of meteoric water recorded in authigenic and pedogenic minerals over time can be used to infer
both timing and magnitude of development of
the rain shadow associated with the timing
and magnitude of development of a topographic barrier to precipitation. Moreover, a
stratigraphic d18O record of the authigenic
minerals in a rain shadow has been recently
used to estimate a quantitative relationship be-
Figure 1. Shaded relief map of Pacific Northwest and sampling locations in eastern Washington. All sample locations are in present-day rain-shadow region (see GSA Data Repository [see text footnote one] supplementary information for full details of sample locations).
tween a change in paleoelevation of a mountain belt and an isotopic shift in precipitation
(Page Chamberlain and Poage, 2000).
As a result of the present-day topography
of the southern Washington Cascades and a
part of the northern Oregon Cascades, a significant arid to semiarid climate extends to the
east side of the range. We studied the d18O
values of smectites separated from paleosols,
weathered tuffs, and soils in the rain-shadow
region to evaluate the late Cenozoic development of this rain shadow. In this paper, we
discuss (1) causes of the measured d18O variations in the authigenic minerals, (2) the magnitude of the net elevation change of this part
of the Cascades for the past 15.6 m.y., and (3)
the temporal relationship between topographic
development of this part of the Cascade Range
and regional climate change in eastern
Washington.
MODERN CLIMATE OF
WASHINGTON
The present-day Washington Cascades are
nearly parallel to the coastline of the western
United States and transverse to the prevailing
direction of moist air moving inland from over
the Pacific Ocean, and form a climatic barrier
separating the state into western and eastern
Washington (Fig. 1). A mild and humid climate predominates in western Washington,
where average annual precipitation is ;1800
mm, ranging from ;900 mm in the Puget
Sound Basin to ;4500 mm in the Olympic
Mountains (Kimbrough et al., 2001). In eastern Washington, a warmer and drier climate
during summer prevails, whereas a colder and
drier climate during winter prevails. During
most of a year, the Cascade Range acts as an
orographic barrier to the easterly movement of
the cool summer and mild winter humid air
masses from the Pacific Ocean. Mean annual
precipitation in eastern Washington is only
180–500 mm (Kimbrough et al., 2001) where
sagebrush and grasses grow, and irrigation is
required for many crops. The stable isotopic
compositions of surface water in eastern
Washington are distinctly lower by 4‰–5‰
in d18O and ;40‰ in the hydrogen isotopic
composition in comparison to those in western
Washington, and they indicate a rain-shadow
effect of the present-day Cascades (Coplen
and Kendall, 2000).
q 2005 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
Geology; April 2005; v. 33; no. 4; p. 313–316; doi: 10.1130/G21335.1; 3 figures; Data Repository item 2005052.
313
LATE CENOZOIC PALEOCLIMATE
RECORD IN EASTERN WASHINGTON
Many geological and paleofloral studies
support the late Cenozoic climate change in
eastern Washington associated with an increase in relief of the Washington Cascades.
Efforts to reconstruct the paleoclimate record
in eastern Washington have focused mostly on
the paleofloral evidence of the middle Tertiary
terrestrial sediments adjacent to the Washington Cascades. Biostratigraphic studies of the
Eocene–Oligocene nonmarine sedimentary
rocks on the east side of the central Washington Cascades show that the units represent
coastal fluvial and swamp sediments, and are
depositionally time equivalent to the Eocene
Puget Group preserved on the west side of the
range (Newman, 1981). This Eocene–
Oligocene succession suggests that the topography of the central Washington Cascades was
not prominent at this time. Also, abundant botanical fossils in the Ellensburg Formation, including the Ginkgo Petrified Forest in the
middle Miocene Vantage Member, indicate
that a more humid and temperate climate extended to the east side of the range during the
Neogene (Prakash and Barghoorn, 1961;
Wolfe, 1994).
Pedological and hydrological studies provide the Quaternary paleoclimate record in
eastern Washington. Pedogenic carbonate concentrations in the Palouse loess for the past 2
m.y. show that Quaternary soils have been developed under arid to semiarid climatic conditions (Busacca, 1989). The progressive decrease of d18O values in groundwater in a 1.5
km vertical section beneath the Hanford Reservation, within the most arid part of Washington State, has been interpreted to be the
consequence of mixing more depleted d18O
values of shallower groundwater with more
isotopically enriched deeper groundwater
(Hearn et al., 1989). This may suggest that the
d18O values of regional meteoric water recharging the aquifers have been gradually depleted owing to the uplift of the Cascade
Range during the late Cenozoic.
SAMPLING STRATEGY AND
METHODS
A number of paleosols and weathered tuffs
have been recognized in the middle to late
Miocene Ellensburg Formation (Smith, 1988),
in the late Miocene to middle Pliocene Ringold Formation (Lindsey, 1996), and in the
Quaternary sedimentary deposits (Busacca,
1989) throughout the Columbia Plateau (Fig.
1). In addition, significant oxygen isotopic
fractionation of soil water by evapotranspiration, observed in the uppermost part of soils
(Hsieh et al., 1998), dictated that all the bulk
paleosols and soils be collected from at least
50 cm below their overlying contacts.
314
Ages of the authigenic minerals are estimated by applying a linear interpolation of
age against meter level of the dated overlying
and underlying Columbia River Basalt Group
flow units (McKee et al., 1977; Long and
Duncan, 1982), of the dated several tuffs in
the Ellensburg Formation (Smith, 1988), and
of the estimated sedimentation ages in the
Ringold Formation (Lindsey, 1996). It was
suggested that authigenic clay minerals in
weathered tuffs formed soon after deposition
(Page Chamberlain and Poage, 2000). Conversely, the rate of soil formation in the past
as well as the rate of authigenic mineral formation within paleosols are unknown. The estimated rates of modern soil formation (hundreds to tens of thousands of years) are rapid
in the geologic time scale (Buol et al., 1997).
Therefore, the depositional ages of the sedimentary interbeds are assumed to be the formation ages of the authigenic minerals in
paleosols.
It has been shown that smectite is resistant
to subsequent oxygen isotopic exchange under
low-temperature conditions and tends to retain
its isotopic composition after it formed (Yeh
and Epstein, 1978). Therefore, here we only
discuss the d18O values of the purified smectite samples within authigenic clay minerals.
Authigenic smectites were isolated from bulk
paleosols, weathered tuffs, and soils preserved
in the stratigraphic sequence in eastern Washington for the past 15.6 m.y. using physical
and chemical treatments (see GSA Data Repository1 for analytical details). The purity of
the smectite samples was checked with X-raydiffraction techniques (Moore and Reynolds,
1997). The d18O values of the purified smectites were measured with a Finnigan Delta S
isotope ratio mass spectrometer.
The equilibrium oxygen isotope fractionation (Sheppard and Gilg, 1996) between
smectite and water was used to calculate the
d18O values of ancient meteoric water. Temperatures for the isotope fractionation calculations are assumed to be 11 8C for the postMiocene and 13 8C for the pre-Pliocene
samples. A temperature of 11 8C is a mean
annual temperature (MAT) within the modern
rain-shadow area in eastern Washington (Busacca, 1989), and the pre-Pliocene MAT (13
8C) in eastern Washington was estimated by
the degree of chemical weathering of the interbedded paleosols in the middle Miocene
Columbia River Basalt Group and fossil
leaves in the middle to late Miocene Ellensburg Formation (Wolfe, 1994; Sheldon, 2003).
1GSA Data Repository item 2005052, Table DR1
(sampling locations and oxygen isotope data), analytical details, and additional references cited, is
available online at www.geosociety.org/pubs/ft2005.
htm, or on request from [email protected] or
Documents Secretary, GSA, P.O. Box 9140, Boulder,
CO 80301-9140, USA.
RESULTS
The measured d18O values of the authigenic
smectites and the calculated d18O values of
regional meteoric water since the middle Miocene, plotted against their estimated ages
(Figs. 2 and 3), exhibit a temporal and continuous decrease of ;3‰–4‰ and ;3.5‰–
4.5‰ with decreasing age, respectively. The
middle Miocene d18O values of meteoric water are estimated to be higher in 18O than the
average d18O values of modern surface water
in eastern Washington and to be closer to the
d18O values of modern surface water on the
west side of the range, whereas the calculated
post-Miocene d18O values of meteoric water
are more depleted than the average modern
surface water in eastern Washington. The calculated d18O values of meteoric water from
the youngest smectite samples are nearly identical to the d18O values of modern shallow
groundwater within an arid part in eastern
Washington (Hearn et al., 1989).
CAUSES OF OXYGEN ISOTOPE SHIFT
In order to understand a possible link between the observed d18O trend of the authigenic smectites and topographic development
of the southern Washington Cascades, the possible parameters that control d18O variation in
precipitation must be understood. The controlling parameters include regional temperature change as a result of the late Cenozoic
global climate change and change in the d18O
values of source water for precipitation as a
result of change in regional atmospheric circulation patterns.
Because of the strong spatial correlation between modern d18O values in precipitation
and regional MAT (Rozanski et al., 1993), a
d18O shift of authigenic minerals is frequently
interpreted as a result of a regional temperature change related to a global climate change.
However, the regional temperature decrease in
eastern Washington from the middle Miocene
would have caused the d18O values of authigenic smectites to increase by ;0.7‰ rather
than decrease. Thus, the observed d18O trend
of authigenic smectites cannot be simply interpreted as a function of regional temperature
change. The regional temperature decrease of
2 8C in the late Cenozoic in eastern Washington would have caused the d18O values of authigenic smectites to increase by ;0.4‰
(Sheppard and Gilg, 1996), and over the same
period the d18O values of precipitation in the
study area would have decreased by ;21.2‰
as a result of the regional temperature change
(Rozanski et al., 1993). The d18O values of
seawater have increased by ;1.5‰ since the
middle Miocene (Zachos et al., 2001).
In some cases, changes in atmospheric circulation patterns associated with changes in
the sources of precipitation can primarily conGEOLOGY, April 2005
Figure 2. d18O values of authigenic smectites in eastern Washington
plotted against estimated ages of samples (see GSA Data Repository; see text footnote one). Uncertainties of estimated ages and
d18O values are smaller than size of symbols. VSMOW—Vienna standard mean ocean water.
trol the d18O record in authigenic and pedogenic minerals (e.g., Tabor and Montañez,
2002). However, at least since the middle Cenozoic, the long-term atmospheric circulation
pattern in the Pacific Northwest has not
changed significantly, as inferred from the
northeasterly depositional pattern of the late
Eocene to early Miocene air-fall and ash-flow
tuffs in the John Day Formation in northcentral Oregon (Robinson et al., 1990) as well
as that of the Quaternary loess deposits in
eastern Washington (Busacca, 1989). In addition, the observed d18O trend of authigenic
smectites cannot be explained by changes in
precipitation sources as a result of the late Cenozoic southwestward drift of the North
American continent. According to the traces
of the Yellowstone hotspot in the Snake River
Plain in Idaho, the North American continent
has drifted ;38 to the south in the past 16 m.y.
(Pierce and Morgan, 1992). In general, the oxygen isotopic composition of precipitation in
a lower latitudinal region is more enriched in
18O than that in a higher latitudinal region
(Rozanski et al., 1993).
It is likely that the change of d18O values
of authigenic smectites observed in this study
is the result of increasing aridity in eastern
Washington created by the rain-shadow effect
of the rising southern Washington Cascades.
The magnitude of the estimated oxygen isotopic depletion in the meteoric water over time
(3.5‰–4.5‰) is similar to that of the modern
oxygen isotopic depletion (4‰–5‰) from the
coastal side to the present-day rain-shadow region. In addition, several uplift episodes of
this part of the Cascades between the middle
Miocene and the early Pliocene have been
proposed (Hammond, 1979; Evarts and SwanGEOLOGY, April 2005
Figure 3. Calculated d18O values of meteoric water in eastern Washington plotted against their estimated ages. Timing of central and
northern Washington Cascades uplift was suggested by apatite (UTh)/He ages (Reiners et al., 2002). d18O values of modern surface
water for both regions are from Coplen and Kendall (2000).
VSMOW—Vienna standard mean ocean water.
son, 1994). Thus, increasing topography of the
southern Washington Cascades during the late
Cenozoic has influenced the regional precipitation patterns associated with regional climate change in eastern Washington.
TOPOGRAPHIC DEVELOPMENT OF
THE SOUTHERN WASHINGTON
CASCADES
Reiners et al. (2002) used apatite (U-Th)/
He ages of igneous and metamorphic rocks in
the central and northern Washington Cascades
to suggest a period of rapid exhumation, presumably caused by uplift, between 8 and 12
Ma (Fig. 3). However, the thermochronometric technique can provide only an indirect estimation of rock and surface uplift from evidence of timing and rates of cooling, and it
also yields no direct information on the elevation history of mountain belts. In this study,
an oxygen isotopic lapse rate (20.28‰/100
m), which is a near-linear empirical relationship between relief of mountain ranges and
changes in the d18O value of precipitation
along their latitudinal transects (Poage and
Page Chamberlain, 2001), was applied to estimate the paleorelief of the southern Washington Cascades for the past 15.6 m.y.
Taking the previously suggested error of the
effects of temperature changes on the d18O
values of authigenic minerals under nearsurface conditions into consideration (Poage
and Page Chamberlain, 2001), the calculated
3.5‰–4.5‰ oxygen isotopic decrease in the
meteoric water recorded in the authigenic
smectites corresponds to an increased elevation in the southern Washington Cascades of
;1.2–1.7 km in the past 15.6 m.y. The estimated paleorelief of the southern Washington
Cascades in this study is almost identical with
the change in elevation of the contact (;1.4–
1.7 km) between the two youngest magnetostratigraphic units of the Columbia River Basalt Group Grand Ronde Formation (ca. 16–
17 Ma) from the Columbia Plateau to the
southern Washington Cascades (Swanson,
1997). This suggests that the accumulation of
Pliocene to Quaternary extrusive igneous
rocks, the High Cascade Group, has been negligible for the topographic development of the
southern Washington Cascades. In addition,
considering the current mean elevations of the
Columbia Plateau (;400 m) and of the southern Washington Cascades (;2,000 m), current
relief of the southern Washington Cascades
was mostly created in the past 15.6 m.y.
Based on our data set, it is impossible to
decide whether a rapid or a gradual uplift of
the southern Washington Cascades would genetically relate to the suggested late Miocene
uplift of the central and northern Washington
Cascades. However, other geological evidence
supports the late Miocene uplift of the southern Washington Cascades. Unlike the crystalline rocks that dominate the central and northern Washington Cascades, the bedrock of the
southern Washington Cascades is dominated
by Oligocene to early Miocene volcanic and
intrusive rocks and the High Cascade Group
(Smith et al., 1988). The middle to late Miocene unconformity in the southern Washington Cascades and the large volume of the late
Miocene volcaniclastic rocks deposited adjacent to them in eastern Washington (Smith,
1988) are related to the significant erosional
processes possibly induced by the late Miocene uplift of the southern Washington Cascades. Also, an extensive structural deforma315
tion of the middle to late Miocene rocks
observed in the Yakima fold belt in eastern
Washington adjacent to the southern Washington Cascades indicates a strong post–middle
Miocene tectonism in this region (Reidel et
al., 1994).
CONCLUSIONS
The d18O values of authigenic smectites
separated from paleosols and weathered tuffs
preserved in eastern Washington from 15.6
Ma to the present record regional climate and
tectonic changes. Three major conclusions of
this study are as follows: (1) the d18O values
of the authigenic smectites and the calculated
d18O values of ancient meteoric water in eastern Washington record a temporal and continuous decrease of ;3‰–4‰ and ;3.5‰–
4.5‰, respectively, (2) this hydrologic change
is related to the development of a rain shadow
as a result of increasing topography of the
southern Washington Cascades, and (3) net elevation of the southern Washington Cascades
for the past 15.6 m.y. is estimated to have risen by ;1.2–1.7 km. This suggests that relief
of the current southern Washington Cascades
has mostly developed by tectonic rock uplift.
The d18O values of authigenic smectites from
paleosols and altered tuffs demonstrate a direct relationship between mountain building
of the southern Washington Cascades and the
late Cenozoic regional climate change in eastern Washington.
ACKNOWLEDGMENTS
We are particularly grateful to Alan Busacca,
Charles Knaack, Mary Fauci, and Carey Gazis for
their assistance. We thank also Peter Reiners, C.
Page Chamberlain, Michael Poage, and an anonymous reviewer for helpful comments on the earlier
manuscript. This study was supported by National
Science Foundation grant EAR-0087318 and a Geological Society of America Graduate Student Research Grant.
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Manuscript received 9 November 2004
Revised manuscript received 3 December 2004
Manuscript accepted 3 December 2004
Printed in USA
GEOLOGY, April 2005