Molecules to Mountains: A Multi-Proxy Investigation Into Ancient Climate and Topography of the Pacific Northwest, USA

Abstract

We characterize the topographic evolution of the Pacific Northwest, United States, during the Cenozoic. New paleosol carbonate stable isotope (δ18O) results from central Oregon are presented, along with published proxy data, including fossil teeth, smectites, and carbonate concretions. We interpret a polygenetic history of Cascade Mountain topographic uplift along-strike, characterized by: 1) Steady uplift of the Washington Cascades through the Cenozoic due long-term arc rotation and shortening against a Canadian buttress, and 2) Uplift of the Oregon Cascades to similar-to-modern elevations by the late Oligocene, followed by topographic stagnation as extension developed into the Neogene. Since the Miocene, meteoric water δ18O values have decreased in Oregon, possibly due to emergence of the Coast Range and westward migration of the coastline. Spatial variability in isotopic change throughout the Pacific Northwest suggests that secular global climate change is not the primary forcing mechanism behind isotopic trends, though Milankovitch cycles may be partly responsible for relatively short-term variation.

Drier Winters Drove Cenozoic Open Habitat Expansion in North America

“…However, this hypothesis of Cascades uplift as the OHT driver remains speculative. The uplift history of the Cascades is debated (Bershaw et al, 2019;Kohn & Fremd, 2007;McLean & Bershaw, 2021;Pesek et al, 2020;Reiners et al, 2002;Takeuchi et al, 2010), and while tectonic forcing can explain many aspects of our results, more isotope data (especially clay data) with robust age constraints are needed to rigorously test any links. Improved constraints on the spatial extent of the OHT would also help address the possibility that it was driven by Cascades uplift.…”

“…We also eliminate Quaternary data (the last 2.6 Myr) to avoid confounding signals with glacial‐interglacial cyclicity (especially the Laurentide ice sheet) which are known to have changed both mean annual precipitation and the pattern of atmospheric circulation across the western U.S (Amundson et al., 1996; Badgley & Finarelli, 2013; Oster et al., 2015; Oerter et al., 2016). Second, following the original authors' interpretations, we eliminate data that (a) do not reflect a primary meteoric signal ( n = 61; 2.5% of all data, (this study; Chamberlain et al., 2012; Horton et al., 2004; McLean & Bershaw, 2021)); (b) were updated in a later publication ( n = 5; 0.2%, (Kent‐Corson et al., 2006)); or (c) record a transient isotope excursion such as a climate event that may not reflect long‐term, background conditions ( n = 6; 0.2%, (Methner, Mulch, et al., 2016)). Of the samples determined to not reflect primary meteoric conditions, most ( n = 56) are from the Muddy Creek Basin data presented in this study where we sampled from strata with interlayered gypsum (indicative of strong evaporation) and δ 18 O values are generally high (although the evaporative designation of samples depends solely on the presence of gypsum).…”

“…Other mechanisms that can alter Δδ 18 O clay−carb are unlikely to explain our results. For example, an increase in winter δ 18 O could explain a shift toward higher clay δ 18 O in the west and central domains, but global cooling and Cascades uplift since the OHT (Bershaw et al, 2019;Kohn & Fremd, 2007;McLean & Bershaw, 2021;Pesek et al, 2020;Reiners et al, 2002;Takeuchi et al, 2010;Westerhold et al, 2020;Zachos et al, 2001) should, all else being equal, decrease winter δ 18 O. Colder clay formation temperatures are also consistent with increasing clay δ 18 O and Δδ 18 O clay−carb , but any cooling would likely occur in the winter and summer, causing the same signal in soil carbonate δ 18 O. The OHT itself might decrease soil CO 2 by decreasing productivity, which can affect carbonate δ 18 O by changing the seasonal timing of carbonate formation.…”

“…We compile 2,065 existing clay ( n = 235) and carbonate ( n = 1,830) δ 18 O measurements and present 173 new measurements (33 clay; 140 carbonate) spanning 162 sites across WCNA to assess precipitation seasonality on a regional scale (Amundson et al., 1996; Chamberlain et al., 2012; Fan et al., 2014, 2018; Fox & Koch, 2003, 2004; Horton et al., 2004; Kent‐Corson et al., 2006, 2010; Kukla et al., 2021; McLean & Bershaw, 2021; Methner, 2015; Methner, Fiebig, et al., 2016; Mix & Chamberlain, 2014; Mix et al., 2013; Mulch et al., 2015; Mullin, 2010; Retallack, Wynn, & Fremd, 2004; Schwartz, 2015; Schwartz et al., 2019; Sjostrom et al., 2006; Takeuchi, 2007; Takeuchi & Larson, 2005; Takeuchi et al., 2010). These data reveal a diverging trend in clay and carbonate δ 18 O across the OHT.…”

“…Each sub-domain records the ∼3‰ increase in clay δ 18 O across the OHT(Kukla et al, 2021). The carbonate data in the west come from the same sub-domain, except for one data point(McLean & Bershaw, 2021) that we remove because it is insufficient to analyze trends through time.2.6. The Difference in Clay andCarbonate δ 18 O (Δδ 18 O clay−carb ) 2.6.1.…”