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Tropical Circulation Intensification and Tectonic Extension Recorded by Neogene Terrestrial D18o Records of the Western United States

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Tropical Circulation Intensification and Tectonic Extension Recorded by Neogene Terrestrial D18o Records of the Western United States Tropical circulation intensification and tectonic extension recorded by Neogene terrestrial d18O records of the western United States Ran Feng1,2, Christopher J. Poulsen1, Martin Werner3 1Department of Earth and Environmental Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA 2National Center for Atmospheric Research, Boulder, Colorado 80303, USA 3Alfred Wegener Institute for Polar and Marine Research (AWI) Bussestraße 24, D-27570 Bremerhaven, Germany ABSTRACT Pacific Coast Ranges and Sierra Nevada, through progressive rainout Terrestrial water isotope records preserve a history of hydrologi- of the heavy isotopologues (e.g., Mulch, 2016). Elevation changes (in cal cycling that is influenced by past climate and surface topography. time and space) are calculated by scaling proxy d18O and dD differences d18O and dD records from authigenic minerals of the western United by modern observed precipitation isotope lapse rates (e.g., Poage and States display a long-term increase during the Neogene in the vicinity Chamberlain, 2002). of the Sierra Nevada and the central Rocky Mountains (Rockies), but d18O values of early Cenozoic (55–28 Ma) records are strongly nega- a smaller increase or decrease in the northern Great Basin. Interpre- tive within the Great Basin, a signal that has been interpreted to reflect the tations of these isotopic trends require quantitative estimates of the presence of a high plateau of 3–4 km elevation across this region (Horton influence of climatic and environmental changes on d18O and dD of soil et al., 2004; Horton and Chamberlain, 2006; Mix et al., 2011; Feng et al., water. Here we use a coupled atmosphere-land model with water-iso- 2013; Mulch et al., 2015). Differences in proxy d18O between the Great topologue tracking capabilities, ECHAM5-JSBACH-wiso, to simulate Basin and the adjacent northern Sierra Nevada (Mulch, 2006; Henry and precipitation and d18O responses to elevation-independent changes in Faulds, 2010; Cassel et al., 2014) and central Rocky Mountains (Rock- Neogene geography, equator to pole temperature gradient (EPGRAD), ies) (Sjostrom et al., 2006) are small, suggesting that these two mountain grassland expansion, and tropical Pacific sea surface temperatures. ranges were ramps of the high plateau. In this scenario, subsequent Basin 18 18 Both precipitation and soil water d O (d Osw) respond strongly to and Range extension within the Great Basin lowered the landscape to Neogene strengthening of the EPGRAD, but weakly to other forcings. its modern elevations (~1.5–2 km), while the northern Sierra Nevada An increase in EPGRAD leads to significant drying and18 O enrich- and central Rockies remained relatively high. Great Basin extension is ment (3‰–5‰) of soil water over the northern Sierra Nevada and supported by structural evidence of widespread extensional deformation central Rockies as a result of Hadley circulation strengthening and throughout this region (Dickinson, 2006) and by the presence of thin crust enhanced coastal subtropical subsidence. These large-scale circulation within the Basin and Range province in comparison to the surrounding changes reduce inland moisture transport from the Pacific Ocean and Sierra Nevada and central Rockies (Chulick and Mooney, 2002). 18 18 18 Gulf of Mexico. Our simulated d Osw responses could explain 50%– Although d O changes (proxy dD values are converted to d O values; 100% of the proxy d18O increases over the Sierra Nevada and central see Table DR3 in the GSA Data Repository1) in Neogene (since ca. 23 Ma) Rockies, suggesting that climate change rather than surface subsid- records from the Great Basin are broadly consistent with the inferred ence may have been the dominant climate signal in d18O records in extension of this region (e.g., Horton et al., 2004), records from areas these regions. On the contrary, d18O responses to climate changes are surrounding mountains of Sierra Nevada (Horton and Chamberlain, 2006) small in the Great Basin, indicating that the observed d18O increase and the central Rockies (Fan et al., 2014) suggest a Neogene elevation over this region was likely a direct response to surface subsidence history that is inconsistent with the Eocene reconstructions and structural with elevation losses of 1–1.5 km. Adding this elevation loss to current evidence of Neogene extension. These records exhibit a d18O increase of Great Basin elevations reveals the former existence of a uniformly up to 8‰, a change that is substantially greater than the increase in the high plateau extending from the Sierra Nevada to the central Rock- Great Basin (e.g., Horton et al., 2004) and, if interpreted solely in terms ies prior to Neogene extension. This revised elevation history brings of paleoelevation, would signify greater elevation loss in these regions Neogene d18O and dD paleoaltimetry of the western United States in than across the Great Basin. One explanation for the seemingly incom- accordance with independent lines of structural evidence and early patible elevation histories is that Neogene d18O records were influenced Cenozoic elevation reconstructions. by factors other than elevation. Mix et al. (2013) and Chamberlain et al. (2014) proposed that western North American d18O increased as a result INTRODUCTION of changing hydrological balance due to Neogene replacement of forests Cenozoic mountain-building events, most notably the Sevier and by grasslands. However, reconstructions of phytoliths in the fossil record Laramide orogenies and subsequent Basin and Range extension, shaped indicate that grassland expansion in the western U.S. likely occurred prior western North America, creating the numerous mountain ranges and to the Neogene, in the late Oligocene–early Miocene (Strömberg, 2011). extensive topographic relief that mark the surface today. Paleoaltimetry Environmental changes other than elevation and grassland expansion estimates of past surface elevations have been widely derived from geo- may also have contributed to the d18O increase. The Neogene climate logical proxies, assuming that the proxies preserve an accurate signal that underwent a major thermal restructuring that included a strengthening translates quantitatively to elevation. In western North America, stable of the zonal sea surface temperature (SST) gradient by 2–6 °C primarily oxygen and hydrogen compositions (d18O and dD) of terrestrial sediments and organic material are frequently used as proxies for the isotopic com- 1 GSA Data Repository item 2016330, additional methods, discussions, Figures positions of ancient surface and soil waters, which display strong negative DR1–DR8, and Tables DR1–DR4, is available online at http://www.geosociety .org correlations with elevation in many regions, including the present-day /pubs /ft2016.htm or on request from [email protected]. GEOLOGY, November 2016; v. 44; no. 11; p. 1–4 | Data Repository item 2016330 | doi:10.1130/G38212.1 | Published online XX Month 2016 GEOLOGY© 2016 Geological | Volume Society 44 | ofNumber America. 11 For | www.gsapubs.orgpermission to copy, contact [email protected]. 1 across the tropical Pacific (e.g., Zhang et al., 2013) and an increase in seasonal dry soil conditions (Fig. DR4). Furthermore, Neogene proxy 18 the meridional SST gradient by 5–8 °C (Goldner et al., 2014) associated d Om changes (with variable record lengths) are standardized into changes with drawdown of atmospheric CO2 and the expansion of high-latitude over 10 m.y. intervals by scaling coefficients of least square regressions 18 glaciation (Raymo, 1994). Concomitantly, hydrological conditions across of individual time series of proxy d Om (Table DR3; Fig. DR5). western North America underwent a marked transition from a moist state during the early Neogene to its present-day semiarid state. Evidence from CLIMATE MODEL RESULTS fossil leaves (Pound et al., 2012), soil chemistry (Retallack et al., 2002), Annual mean temperatures across continental North America (14°– and herbivore mammal hyposdonty (Eronen et al., 2012) indicate a large 56°N, 47°–141°W) differ by -3.1, -0.54, -0.7, and -0.14 °C between reduction in precipitation of 102 cm/yr across western North America. NG-EPGRAD, NG-ELNINO, NG-GEO, NG-GRASS, and the CNTL. Enhanced surface evaporation and reduced soil moisture associated with Correspondingly, the coldest month mean temperature varies by -5.7, 18 18 this regional aridification event may have increased soil waterd O (d Osw) -0.45, 0.24, and 0.93 °C and the regional annual moisture budget (pre- (e.g., Poage and Chamberlain, 2002; Horton and Chamberlain, 2006; Fan cipitation minus evaporation) changes by -74, 0.22, -36, and 0.01 mm/ et al., 2014; Chamberlain et al., 2014). The cause of regional aridification yr. Collectively, an increase in the EPGRAD leads to the strongest dry- is not known with certainty, but may be linked to large-scale Neogene ing and cooling, greater annual temperature range, and best match with climate change. Previous studies have demonstrated that variations in Neogene precipitation changes recorded by proxies across the western 18 atmospheric CO2 (Poulsen and Jeffery, 2011) and tropical SST distribu- United States (Fig. DR6). Increasing EPGRAD leads to d O changes of 18 tion (Winnick et al., 2013) can lead to substantial changes in hydrological ~2‰–5‰ (Fig. 1A, shown for d Oc), about twice the magnitude of the cycling and the stable isotopic composition of continental precipitation. The objective of this study is to identify the influence of climate change on soil
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