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.
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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 Montan˜ez, 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. REFERENCES CITED Buol, S.W., Hole, F.D., McCracken, R.J., and Southard, R.J., 1997, Soil genesis and classification (fourth edition): Ames, Iowa State University Press, 527 p. Busacca, A.J., 1989, Long Quaternary record in eastern Washington, U.S.A., interpreted from multiple buried paleosols in loess: Geoderma, v. 45, p. 105–122, doi: 10.1016/0016-7061 (89)90045-1. Coplen, T.B., and Kendall, C., 2000, Stable hydrogen and oxygen isotope ratios for selected sites of the U.S. Geological Survey’s NASQAN and benchmark surface-water networks: U.S. Geological Survey Open-File Report 00-160, 424 p. Evarts, R.C., and Swanson, D.A., 1994, Geologic transect across the Tertiary Cascade Range, southern Washington, in Swanson, D.A., and Haugerud, R.A., eds., Geologic field trips in the Pacific Northwest: Seattle, University of Washington Department of Geological Sciences, p. 2H1–2H31.
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