Conclusion

δ¹⁸O values immediately prior and after the MECO interval in central Montana (Figs. 2 and 4). The rapid post-MECO increase in pedogenic CaCO3 content further points to enhanced levels of aridity, which is in good agreement with (a) regional paleobotanical evidence indicating cooling and drying (climatically and orographically induced) during the middle Eocene (e.g., DeVore and Pigg, 2010), and (b) post-MECO mid-latitude cooling and aridification in the northern hemisphere (Bosboom et al., 2014).

47 Acknowledgements

Acknowledgements

AM and KM acknowledge support through the LOEWE funding program (Landes-Offensive zur Entwicklung wissenschaftlich-ökonomischer Exzellenz) of Hesse's Ministry of Higher Education, Research, and the Arts. CPC, SAG and AM acknowledge support through NSF EAR 1019648 and JF through DFG grant FI-948/4-1. We further thank J. Caves (Stanford) and T. Schwartz (Stanford) for valuable field support and S.

Hofmann (Frankfurt) for technical assistance.

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Figures

Fig. 1. (A) Overview map of the western United States. Star marks the location of Fig.

1b. (B) Geological map of SW Montana, USA, with sampling locality of the Upper Dell Beds, southern Sage Creek Basin. (C) Dell Beds at the Kay Draw locality. The sections are composed of a series of stacked paleosols. The section exposes the disconformable contact between the Dell Beds and the overlying Oligocene Cook Ranch member (Tabrum et al. 1996). (D) Composite section of the Dell Beds. Dots mark carbonate samples (orange for ∆₄₇ samples), stars denote the samples for U-Pb dating.

53 Figures

Fig. 2. Stratigraphic, stable isotope, and clumped isotope record of the Dell Beds, Montana. (A) Clumped isotope (∆₄₇) temperatures of pedogenic mudstones (squares) and nodules (circles) in [°C] with 2σ standard errors. n denotes the number of replicate measurements for each sample. (B) Carbonate content of pedogenic mudstone in [%]. (C) Oxygen [‰, VSMOW] isotope record. Colored symbols represent average values from individual measurements (grey). Stars and grey lines mark the position of U-Pb geochronology samples.

Fig. 3. Tera-Wasserburg diagrams of paleosol samples. (A) ²³⁸U/²⁰⁶Pb – ²⁰⁷Pb/²⁰⁶Pb plot of sample 11-KM-124 (at 14.6 m of section) yields an isochron age of 39.5 ±1.4 Ma with a MSWD (mean squared weighted deviates) of 0.89 and (B) ²³⁸U/²⁰⁶Pb – ²⁰⁷Pb/²⁰⁶Pb plot of sample 11-KM-122 (at 12.9 m of section) yields an isochron age of 40.1 ±0.8 Ma with a MSWD of 1.15. All uncertainties are reported at the 2σ level.

55 Figures

Fig. 4. Comparison of temperature estimates and δ¹⁸O values of the Middle Eocene Climatic Optimum in marine (A, B; adapted from Sluijs et al., 2013) and terrestrial records (C, D; this study). (A) U^K37 (blue) and TEX86 (purple) sea surface temperatures (SST) reflect 3-point running average (Bijl et al., 2010). (B) Benthic foraminifera δ¹⁸O data and corresponding ice-free temperatures from Southern Ocean ODP sites 738 (orange) and 748 (red), adapted from Bohaty et al. (2009). (C) Clumped isotope (∆₄₇) paleotemperatures of the Dell Beds with 2σ standard error. The terrestrial warming event is characterized by a transient increase in temperatures followed by a profound temperature drop after peak warming, which is similar to the first-order characteristics of the marine δ¹⁸O record. Our dating results (white dots) place the temperature excursion slightly after the marine MECO, however within error U-Pb geochronology permits the Dell Beds to cover the same time interval as the marine MECO records (see text for details). (D) δ¹⁸O values of soil water in [‰, VSMOW], calculated from carbonate δ¹⁸O values considering ∆₄₇ temperatures and calcite-water oxygen isotope fractionation (Kim and O’Neil, 1997).

57

Chapter 3

Rapid change in high-elevation precipitation patterns of western North America during the Middle Eocene Climatic Optimum (MECO)

Mulch, A., C.P. Chamberlain, M.A. Cosca, C. Teyssier, K. Methner, M.T. Hren, and S.A.

Graham

Published in: American Journal of Science, 2015, 315, 317-336, doi:10.2475/04.2015.02

Abstract

We present Eocene terrestrial oxygen (δ¹⁸O), carbon (δ¹³C), and strontium (⁸⁷Sr/⁸⁶Sr) isotope lake and paleosol records from two high elevation sites on the western North American plateau. These records represent terrestrial stable isotope evidence of the Middle Eocene Climatic Optimum (MECO) an enigmatic and rapid global warming event that interrupted protracted Eocene global cooling at around 40.0 Ma. Revised stratigraphy based on 15 ⁴⁰Ar/³⁹Ar ages from air-fall and water-lain ash in the Elko basin (Nevada) places evaporative lake conditions followed by rapid (<150 ka) freshening and a large (14-15 ‰) negative shift in δ¹⁸O of lake water between 40.2 Ma and 39.4 Ma, right at the climax of MECO. Prior to MECO high δ¹⁸O and δ¹³C values in well-laminated organic-rich shales are consistent with high evaporation-to-precipitation ratios, lack of lake overturn, persistent lake stratification, and moderate to high lake salinity. However, a step-wise large magnitude (>20 ‰) decrease in δ¹³C values already during the pre-MECO cooling phase, requires that in contrast to pre-MECO climate conditions, MECO temperature seasonality in western North America was sufficiently strong to (re-)establish regular lake overturn. The rate and magnitude of the rapid negative lacustrine δ¹⁸O shift during MECO in the Elko lake basin is inconsistent with a simple scenario of regional surface uplift affecting decrease of δ¹⁸O in precipitation and associated lake freshening.

We consider the overall negative shift in δ¹⁸O of non-evaporatively ¹⁸O-enriched lake waters (-9.3 ±1.8 ‰) between the 42 to 43 Ma Elko Formation and the ca. 38 Ma base of the Indian Well Formation to be composed of a ca. 3 to 4 ‰ decrease in δ¹⁸O of lake carbonate as a consequence of post-MECO cooling and an additional ca. 5 to 6 ‰ decrease in δ¹⁸O of riverine lake input as a response to 43 to 38 Ma surface uplift. When combined with isotope-enabled global circulation model results, the Elko lake data further suggest that once critical elevations were attained a change in upstream moisture transport including strengthening of monsoonal summer rainfall on the (south-)eastern flanks of the Cordillera and a larger fraction of air parcel trajectories to the (eastern) lee of the Cordilleran highlands that had passed over the (western) continental interior were responsible for the rapid middle Eocene decrease in δ¹⁸O of precipitation in the central Cordilleran hinterland.

1. Introduction

A series of short-term (10⁴-10⁵ a) hyperthermal events during the Early to Middle Eocene (e.g., Zachos et al., 2001, 2006; Galeotti et al., 2010) were characterized by high atmospheric CO2 concentrations and a 3 to 6 °C warming of surface and deep oceans (for example Lourens et al., 2005; Bohaty et al., 2009; Bijl et al., 2010). Lowering of carbon isotopic compositions and associated pH of the oceans (Zachos et al., 2005) and shoaling of the carbonate compensation depth suggests rapid release of low-δ¹³C methane (Dickens et al., 1997) or high atmospheric pCO2 (Bohaty et al., 2009; Bijl et al., 2010) as potential feedbacks and triggers in the ocean-atmosphere system, yet corresponding continental paleoclimate records are still sparse and consequences of such rapid warming for terrestrial ecosystems pretty elusive.

Another first-order control on global atmospheric circulation is the topography of the world’s largest mountain ranges, especially for the mid-latitudes where the transience of equator-pole temperature gradients directly influences zonal moisture and heat transport.

Model experiments highlight the role of western North American elevations on northern hemisphere atmospheric circulation and the resulting climatic teleconnections (e.g., Ruddiman and Kutzbach, 1989, 1990; Ruddiman and Prell, 1997; Seager et al., 2002).

Stable isotope paleoaltimetry in the North American Cordillera (e.g., Horton et al., 2004;

Mulch et al., 2007; Mix et al., 2011; Chamberlain et al., 2012; Lechler et al., 2013; Cassel et al., 2014; Fan et al., 2014) points to Eocene north-to-south reorganization of surface topography culminating in the assembly of a 3 to 4 km Eocene highland, whose construction was assisted by magmatism and mid-crustal extension, collectively calling for the convective removal of mantle lithosphere during the Early to Middle Eocene (Mix et al., 2011; Chamberlain et al., 2012). At the same time a transient global warming event, the ~40.0 Ma Middle Eocene Climatic Optimum (MECO) interrupted a period of protracted Eocene cooling (Bohaty and Zachos, 2003; Bohaty et al., 2009). Despite the importance of understanding such aberrations on global climate and resulting atmospheric circulation patterns we lack reliable terrestrial climate records from hyperthermal events when major reorganization of the global hydrological cycle and enhanced seasonality characterized continental precipitation patterns (e.g., Giusberti et al., 2007; Nicolo et al., 2007).

Here, we present lacustrine and pedogenic terrestrial stable isotope records from the western North American plateau region covering the MECO interval (ca. 42 – 38 Ma) when the surrounding landscapes experienced a major phase of volcanism and reorganization of surface elevations (Brooks et al., 1995; Horton et al., 2004; Henry, 2008; Mix et al., 2011; Chamberlain et al., 2012).

59

Im Dokument Recovering Eocene paleotopography and paleoclimate of the North American Cordillera through integrated stable isotope and clumped isotope analyses (Seite 60-73)