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1 Geological Society of America Bulletin Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity time scale: Paleomagnetism of radioisotopically dated tuffs from Laramide foreland basins Kaori Tsukui and William C. Clyde Geological Society of America Bulletin published online 24 February 2012; doi: /B alerting services Subscribe click to receive free alerts when new articles cite this article click to subscribe to Geological Society of America Bulletin Permission request click to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. Individual scientists are hereby granted permission, without fees or further requests to GSA, to use a single figure, a single table, and/or a brief paragraph of text in subsequent works and to make unlimited copies of items in GSA's journals for noncommercial use in classrooms to further education and science. This file may not be posted to any Web site, but authors may post the abstracts only of their articles on their own or their organization's Web site providing the posting includes a reference to the article's full citation. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Notes Advance online articles have been peer reviewed and accepted for publication but have not yet appeared in the paper journal (edited, typeset versions may be posted when available prior to final publication). Advance online articles are citable and establish publication priority; they are indexed by GeoRef from initial publication. Citations to Advance online articles must include the digital object identifier (DOIs) and date of initial publication. Copyright 2012 Geological Society of America

2 Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity time scale: Paleomagnetism of radioisotopically dated tuffs from Laramide foreland basins Kaori Tsukui and William C. Clyde Department of Earth Sciences, University of New Hampshire, Durham, New Hampshire 03824, USA ABSTRACT Age calibration of the early to middle Eocene geomagnetic polarity time scale remains highly uncertain due to conflicting magnetostratigraphic, radioisotopic, and astro chronologic results. In this study, new paleomagnetic polarity determinations of 29 ash-fall tuffs preserved in strata of five Laramide foreland basins were used in conjunction with previously published Ar/ 39 Ar ages from the same tuffs to evaluate eight different calibration models for the early to middle Eocene part of the geomagnetic polarity time scale. Reliable paleomagnetic information was recovered from 23 tuffs, of which 17 showed normal polarity and six showed reversed polarity. After comparison of the models with the paleomagnetic and radioisotopic data from the tuffs and an array of independent chronostratigraphic observations, the new Willwood model is herein selected as the best alternative to the current geomagnetic polarity time scale calibration for the early to middle Eocene. Three important implications are apparent in our proposed model. First, the early Eocene is shortened by 0.6 m.y., and the middle Eocene is lengthened by 0.8 m.y. compared with the 2004 geomagnetic polarity time scale. Also, the early Eocene climatic optimum is estimated to have lasted from 52.9 to 50.7 Ma, ~1 m.y. longer than previously suggested, and overlapping in time with the inferred age of the Wasatchian-Bridgerian faunal transition. Our new model agrees with a previous astronomical model when it is tied to the oldest proposed age for the Paleocene-Eocene Thermal Maximum at Ma. Present address: Division of Geochemistry, Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York 10964, USA, and Richard Gilder Graduate School at the American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024, USA; ktsukui@ldeo.columbia.edu INTRODUCTION The geomagnetic polarity time scale plays an integral role in interpreting geologic records ranging from biologic evolution and climate change to seafloor spreading, making the precision of its calibration a matter of fundamental importance to our understanding of Earth history (e.g., DeMets et al., 1994). However, for the pre-neogene geomagnetic polarity time scale, where theoretical uncertainties in the orbital calculations do not permit reliable astronomical calibration of magnetic polarity reversals, numerical calibration has been achieved mainly via interpolation among a limited number of radioisotopic age determinations that are correlated to marine magnetic anomalies, and an assumption of smoothly varying seafloor spreading (Cande and Kent, 1992; Ogg and Smith, 2004; Laskar et al., 2004). However, calibration of intervening magnetic chrons using this interpolation method is highly sensitive to the number and accuracy of the tie points used, making the interpolated segments poorly defined in absolute time if insufficient numbers of tie points are used. Furthermore, the assumption about the rates of seafloor spreading, on which the accuracy of the geomagnetic polarity time scale calibration relies, is not routinely tested by empirical data. A more precise and reliable time scale is needed to correlate geologic and paleoclimatologic records that are being recovered at the temporal scale of astronomical forcing (Lourens et al., 2005; Zachos et al., 2005; Sexton et al., 2011). The early Eocene is a relatively poorly calibrated interval of the geomagnetic polarity time scale. In the most recent time scale (Ogg and Smith, 2004, hereafter GOS2004), the ~22-m.y.-long Eocene segment of the geomagnetic polarity time scale is calibrated by applying a cubic spline function through only five tie points that are spaced on average every ~4 5 m.y. In the previous two time scales by Cande and Kent (1995, hereafter CK95) and Ogg and Smith (2004), radioisotopic ages of the tie points were obtained with different fluence monitor standards, and thus the accuracy of the resultant time scale was compromised. Furthermore, there is as much as a 1 m.y. difference in the age of the C20r-C21n chron boundary and ~1.2 m.y. difference in the duration of the early Eocene between the two time scales. Given that the geomagnetic polarity time scale serves as a global reference to which radioisotopic, magnetostratigraphic, and biostratigraphic data are correlated, these types of uncertainties in the geomagnetic polarity time scale calibration can quickly propagate and affect studies that rely on it for chronostratigraphic purposes (Machlus et al., 2004). Some of the most complete sedimentary records from the early and middle Eocene are preserved in the Laramide basins of southwestern Wyoming, northeastern Utah, and northwestern Colorado (Bradley, 1964; Roehler, 1992a; Murphey, 2001). Within the last decade, a suite of ash-fall deposits of volcanic origin from these basins has been dated using high-precision Ar/ 39 Ar geochronology, resulting in several different efforts to calibrate this interval of the geomagnetic polarity time scale (Murphey et al., 1999; Wing et al., 2000; Machlus et al., 2004; Murphey and Evanoff, 2007; Smith et al., 2003, 2006, 2008a). Additionally, Westerhold and Röhl (2009) developed the first astronomically calibrated age model for the early to middle Eocene (base of C21r to base of C24n) based on marine sedimentary records from the western Atlantic (Demerara Rise, Ocean Drilling Program [ODP] Leg 207, Site 1258). Although the astronomical age model is not calibrated in absolute time because it is beyond 42 Ma, the limit of precise astronomical solutions, it provides an independent basis for determining the durations of chrons and forces reevaluation of the current calibration scheme for the Eocene part of the geomagnetic polarity time scale. One way to assess the accuracy of the proposed calibration models is to determine the GSA Bulletin; Month/Month 2012; v. 1xx; no. X/X; p. 1 16; doi: /B ; 9 figures; 2 tables; Data Repository item For permission to copy, contact editing@geosociety.org 2012 Geological Society of America 1

3 Tsukui and Clyde 100 km N Wind River Basin HD WL Wind River Uplift TB Granite Mtns Uplift CP Figure 1. Geologic map of the study area showing sampling locations of tuffs and lines of cross sections. See Table 1 for abbreviations of tuffs. The base map is after Burchfiel (1993). Sa A Fossil Basin K i A' A B LC Uinta Uplift Greater Green River Basin Bridger Basin CB HF BE SCM 6 L M C F B R S G Rock Springs Uplift A SB Great Divide Basin B Washakie Basin Sand Wash Basin WY CO 42 N C St O Fa Cu W Bl C C' Y N Uinta Basin Piceance Creek Basin UT CO MT ID UT WY CO Early Paleogene Cretaceous X Cambrian-Jurassic clastic Precambrian cored uplifts Sample locality X' Lines of cross sections 38 N 112 W 110 W 108 W paleomagnetic polarity of radioisotopically dated tuffs and compare the observed polarity to that predicted by each age model. McIntosh et al. (1992) used a similar approach on a sequence of ignimbrites from the southwestern United States to provide four possible calibrations for the late Eocene to Oligocene. In this study, we determined the magnetic polarity of 29 tuffs from the Greater Green River, Wind River, Uinta, Fossil, and Piceance Creek Basins (Fig. 1; see Table DR1 for detailed description of the sampled tuffs 1 ). According to the Ar/ 39 Ar geochronology of Smith et al. (2008a), these tuffs were sampled on average 1 GSA Data Repository item , three additional figures and five additional tables that contain sample descriptions, measurements, and details of our method of age model evaluation, is available at or by request to editing@geosociety.org. every ~0.36 m.y., and thus they allow us to test the competing calibration models at a temporal resolution much finer than that at which the Eocene geomagnetic polarity time scale is currently calibrated. An improved calibration of the early to middle Eocene time scale has important implications for studies that rely on temporal synchroneity of geologic data for understanding causal mechanisms. For instance, a coupling 2 Geological Society of America Bulletin, Month/Month 2012

4 Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity time scale Figure 2. Representative fence diagrams for Fossil Basin (A-A ), Greater Green River Basin (B-B ), and Uinta Basin (C-C ) along lines of cross sections shown in Figure 1. Approximate stratigraphic positions of sampled tuffs are shown relative to major lithologic boundaries (solid line) and marker beds (broken line, b Buff marker bed, m Mahogany marker bed, h Horse Bench sandstone). Dotted lines show general interbasinal correlation. For global positioning system (GPS) coordinates and abbreviations of the tuffs, see Table 1. Lithologic units are colored according to lake type assign ment of Smith et al. (2008a). Abbreviations of lithologic units are as follows: Tfbh Bulldog Hollow Member of the Fowkes Formation; Tfs Sillem Member of the Fowkes Formation; Twb Stratigraphic thickness (m) Tga Tgf Fossil Basin K Tfbh Tfs Sa Twb Bullpen Member of the Wasatch Formation; Tga Angelo Member of the Green River Formation; Tgf Fossil Butte Member of the Green River Formation; Twm main body of the Wasatch Formation; Twk Washakie Formation; Tb Bridger Formation; Tgl Laney Member of the Green River Formation; Tgw Wilkins Peak Member of the Green River Formation; Tgt Tipton Member of the Green River Formation; Twc Cathedral Bluffs Tongue of the Wasatch Formation; Tu Uinta Formation; Tgsl sandstone and limestone facies of the Green River Formation; Tgs saline facies of the Green River Formation; Tgu upper member of the Green River Formation; Tgtr transitional interval of the Green River Formation; Tg main body of the Green River Formation. Lithostratigraphic data are adapted from Buchheim (1994); Oriel and Tracey (1970; profile A); Culbertson (1961); Roehler (1992a, 1992b); Murphey and Evanoff (2007; profile B); Remy (1992); Dane (1954); Bryant et al. (1989); Prothero (1996; profile C). Bridger Basin Rock Springs Uplift Washakie Basin Twm Twm Twm A A B B C C Volcaniclastic Alluvial b BE HF Tb LC CB C F Tgt SCM 6 L M TB G B S R Tgl Tgw Twk CP A SB Twc Fluvial-lacustrine Fluctuating profundal O Fa h m Indian Canyon Tgsl St Tgs Tgu Tgtr Tg Interbedded evaporites Evaporative Gate Canyon Cu Bl W Tu between the early Eocene climatic optimum, which represents the long-term Cenozoic peak in warmth (Zachos et al., 2001), and the Wasatchian-Bridgerian North American Land Mammal Age (NALMA; Wood et al., 1941) faunal turnover has been suggested based on their co-occurrences in chron C23r (Clyde et al., 2001; Woodburne et al., 2009). However, other studies dispute the result (Smith et al., 2003, 2004; Clyde et al., 2004), and the underlying mechanism for this coupling remains elusive. A resolution to these questions will require an improved chronostratigraphy for the Greater Green River Basin and a more reliable geomagnetic polarity time scale to make precise correlations between deep-sea climatic proxies and terrestrial biotic records. Finally, a refined geomagnetic polarity time scale can help reconstruct the history of seafloor spreading rates and thus test the assumption of smoothly and continuously varying seafloor spreading on which the precision of the geomagnetic polarity time scale has traditionally relied. Geologic Setting The ash-fall tuffs collected for this study were deposited in fluvial and lacustrine environments after being transported from the Absaroka and Challis volcanic fields of northwestern Wyoming and Idaho via winds or rivers (Surdam and Stanley, 1980; Chetel et al., 2011). In general, the tuff-bearing formations record geomorphic and hydrologic changes resulting from the interplay between regional tectonics and climate (Fig. 2; Pietras et al., 2003). The Wasatch and Wind River Formations contain mostly fluvial facies (e.g., red paleosol mudstones and channel sandstones), whereas the Green River Formation is dominated by lacustrine facies (e.g., laminated shales interbedded with thin sandstones and limestones). The Bridger Formation and its lateral correlatives, the Fowkes Formation in Fossil Basin and the Washakie Formation in Washakie Basin, are composed of fine-grained volcaniclastic deposits with laterally extensive limestone and sandstone marker beds. On fresh surfaces, these water-laid tuffs are usually no more than 30 cm thick and vary in color from white to light gray to orange in outcrop. Grain size of the tuffs varies from very fine sand to clay, and textural differences correlate with different depositional settings (see GSA Data Repository Table DR1 [see footnote 1]). For instance, the lacustrine tuffs (e.g., K-spar and Sixth tuffs) are relatively fine grained and are more distinctive in outcrop, having sharp upper and lower contacts with the surrounding mudstone or limestone units. They also typically lack the evidence of postdepositional reworking that is otherwise common in the fluvial tuffs (e.g., Basal Bridger E tuff), which are relatively coarser grained, have more diffuse contacts with the facies above and below, and often contain pumice clasts or fragments. Signs of postdepositional zeolitization caused by saline and alkaline lake water are common in the matrix of the lacustrine tuffs (Ratterman and Surdam, 1981). A grading-upward pattern of the biotite crystals, which were abundant in many tuffs, indicates that the tuffs were deposited in single events, thus ensuring that the paleomagnetic data from Geological Society of America Bulletin, Month/Month

5 Tsukui and Clyde these tuffs record an instantaneous polarity at the time of deposition. Previous studies (e.g., Reynolds, 1979; Hayashida et al., 1996; Iwaki and Hayashida, 2003) have shown that waterlaid ash-fall tuffs typically yield early acquired detrital remanent magnetizations, probably of detrital origin, that record paleomagnetic field directions acquired in a relatively short time period. METHODS Paleomagnetic Sampling Paleomagnetic samples were collected from 29 previously dated ash-fall tuffs in five Laramide Basins (Figs. 1 and 2; Smith et al., 2003, 2006, 2008a, 2010; Murphey et al., 1999; Machlus et al., 2004; Murphey and Evanoff, 2007). In the field, tuffs were located with the help of colleagues (see Acknowledgments) or by global positioning system (GPS) coordinates and rock descriptions provided in the literature (Table DR1 [see footnote 1]). Effort was made to sample from the exact same locations that had been sampled for Ar/ 39 Ar analysis in the previous studies (Smith et al., 2003, 2008a). At each site, weathered materials were removed to expose a fresh surface for sampling. Five to ten separately oriented samples were collected as 2.5-cm-diameter cylinders with a portable gas-powered core drill (85 samples total) or as oriented hand samples (43 samples total), which were later cut into 8 cm 3 samples with a saw. Whenever possible, the finest-grained parts of a tuff were sampled for paleomagnetic analysis because they may contain single-domain grains, which are more likely to preserve reliable characteristic remanent magnetizations (ChRM; Butler, 1992). However, Smith et al. (2003, 2008a) sampled the coarsest fraction of each deposit (i.e., base) in order to collect the largest possible crystals for radioisotopic dating. In one case (Boar tuff), the tuff bed was too friable for paleomagnetic sampling, so samples were collected from a siliciclastic layer immediately (~20 cm) above. Also, where stratigraphic integrity of the tuffs was unclear (e.g., due to suspected slumping as in the case of the White Lignitic and Wavy tuffs), samples were collected from multiple locations within the same bed to average out the possible effect of postdepositional disturbance. Laboratory and Data Selection Procedures The paleomagnetic samples were demagnetized by alternating field (AF) and/or thermal methods between 2.5 mt and 100 mt and/or 25 C and 690 C, respectively. Natural remanent magnetization (NRM) was measured on an HSM 2 SQUID-based spinner magnetometer inside a three-dimensional, direct current (DC) coil, low-field cage at the Paleomagnetism Laboratory at the University of New Hampshire. Characteristic components of NRM were calculated by least squares analysis through the high-coercivity or high-unblocking-temperature demagnetization paths that trended toward the origin (Kirschvink, 1980). The maximum angular deviation (MAD) of the ChRM was calculated for each sample, and at least three samples with MADs less than 20 were used to determine a statistically robust site mean polarity. Those samples with MADs above 20 were rejected from further analyses. Sites that passed Watson s test for randomness at the 95% significance level at N = 4 with directional precision parameter (k) of >10 (<10) were classified as alpha (beta) sites (Watson, 1956). Alpha 95 (α 95 ) expresses within-site scatter as the 95% cone of confidence around the estimated mean direction. The relatively liberal data quality criteria on MAD and precision parameter were chosen because the objective of the study is to determine polarity for specific tuff horizons, rather than a precise paleopole. For the samples collected from beds with measurable dips, tectonic correction was performed to the measured declinations and inclinations. Virtual geomagnetic poles (VGP) were calculated for the alpha and beta sites, and VGP latitude was used to infer magnetic polarity of sites, assuming that the ChRM is primary in origin. To investigate the magnetic mineralogy, we conducted isothermal remanent magnetization (IRM) acquisition experiments on a suite of samples cut into 1 cm 3 cubes. Using a ASC IM10 impulse magnet, samples were subjected to a magnetic field of different intensities (0.12 T, 0.4 T, and 1.1 T) along three orthogonal axes, isolating low-, medium-, and highcoercivity fractions in the x, y, and z directions, respectively (Lowrie, 1990). Subsequently, the acquired IRM was thermally demagnetized at steps of 25, 50, 75, 100, 125, 150, 200, 250, 300, 0, 500, 5, and 580 C. The three-axis IRM experiment shows the ferromagnetic mineral content of a sample using the characteristic coercivities and temperature-dependent properties of different magnetic minerals. Bulk susceptibility was measured on selected samples to further facilitate interpretation of the magnetic mineralogy of the tuffs. PALEOMAGNETIC RESULTS Statistics for all paleomagnetic sites examined in this study are summarized in Table 1. NRM intensities are highly variable, ranging over three orders of magnitude between ma/m and ma/m before demagnetization (Table DR2 [see footnote 1]). Neither intensity nor grain size appears to control demagnetization behavior or the quality of the isolation of a ChRM. Tuffs deposited in fluvial environments (Wind River, Bridger, and Fowkes Formations) generally have higher NRM intensities than those deposited in lacustrine settings (Green River Formation). The fluvial tuffs also have higher magnetic susceptibility compared with lacustrine tuffs, likely due to higher concentrations of volcanigenic ferro/ferrimagnetic minerals (Dunlop and Özdemir, 1997; Table DR3 [see footnote 1]). After initial pilot studies were carried out to determine the most effective demagnetization protocol for each tuff, 73 out of 128 samples were demagnetized using thermal demagnetization, and the remaining 55 samples were demagnetized with AF demagnetization. In some cases, both methods were combined to resolve a ChRM. Demagnetization revealed either one or two NRM components in every sample. Most reversed polarity samples exhibited overprint components, which were unblocked by 0 C or randomized by mt (Fig. 3). The overprint components show highly variable directions, making it difficult to infer the cause or timing of the secondary magnetization acquisition. After removal of the low-temperature or lowcoercivity component, ChRM was progressively demagnetized up to 690 C or 100 mt (Table DR2 [see footnote 1]). Five samples that had best-fit lines to the ChRM with MADs above 20 were rejected from further analyses. The average MAD of the ChRM for the remaining 123 samples was 8.16, and the ChRM from those samples was used to estimate site mean directions. Of the 23 sites that passed Watson s test for randomness at the 95% significance level, six sites were characterized by reversed polarity with ChRM showing S-SE declinations and moderate to steep negative inclinations (mean of Dec tec = 159, Inc tec = 62 [Dec tec declination in tectonic coordinates, Inc tec inclination in tectonic coordinates], α 95 = 25.3 ). The remaining 17 sites showed normal polarity with N-NW directions and moderate to steep positive inclinations (mean of Dec tec = 1.6, Inc tec = 56, α 95 = 7.8). Eighteen sites with k > 10 are referred to as alpha sites; five sites with k < 10 are referred to as beta sites. The beta sites are characterized by anomalous site mean declinations between 7 and 59 for normal polarity sites and 56.9 for the one reversed polarity site. Inclinations of their ChRM, however, are largely consistent between samples within a site and therefore are included in the calculation of site VGPs, but we caution that those polarity 4 Geological Society of America Bulletin, Month/Month 2012

6 Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity time scale Site* Formation Name Abbreviation Lat ( ) Long ( ) TABLE 1. SUMMARY SITE STATISTICS Strike ( ) In geographic coordinates In tectonic coordinates Dip Declination ( ) N ( ) Inclination ( ) Declination ( ) Inclination ( ) R # k** BR0501 α Bridger Henrys Fork HF N W β BR0502 Green River Grey G N W BR0503 α Green River Sand Butte SB N W BR0504 Green River Analcite A N W α BR0505 Green River Sixth 6 N W α BR0506 Green River Layered L N W BR0507 BR0508 α Green River Main M N W α Bridger Church Butte CB N W BR0509 Green River Scheggs S N W β BR0510 Green River Firehole F N W BR0511 BR0512 BR0513 CP0646 α Green River C Bed C N W α Bridger Basal Bridger E BE N W α Bridger Leavitt Creek LC N W CP N W α Bridger Continental Peak KT0701 Wagon Bed White Lignitic WL N W KT0702 Bridger Tabernacle TB N W Butte α KT0703 Bridger Tabernacle TB N W Butte KT0704 KT0705 KT0706 KT0707 KT0708 α Green River Rife R N W α Green River Boar B N W α Green River K-spar K N W α Fowkes Sage Sa N W SCM N W α Bridger Sage Creek Mtn Pumice β KT0709 Green River Fat Fa N W β KT0710 Green River Oily O N W β KT0711 Green River Strawberry St N W α KT0712 Green River Blind Canyon Bl N W KT0713 Green River Wavy W N W KT0714 Green River Curly Cu N W α KT0715 Green River Yellow Y N W KT0716 Wind River Halfway Draw HD N W *α and β refer to alpha and beta sites, respectively (see text). Sites without α or β designation did not satisfy the Watson s test (Watson, 1956) and were excluded from polarity determination. Strike and dip are measured using right-hand rule (dip to the right of strike). N is number of samples analyzed for paleomagnetic polarity determination. # R is resultant vector. **k is precision parameter. α95 is cone of 95% confidence about estimated mean direction. Latvgp and Long vgp refer to latitude and longitude of the site virtual geomagnetic pole (VGP) where positive is north and east. α 95 ( ) Lat vgp ( ) Long vgp ( ) Geological Society of America Bulletin, Month/Month

7 Tsukui and Clyde A BR0506F C KT0706D 525 C N, Up 50 mt 10 mt W 100 mt 50 mt 10 mt MAD = 1.6 N, Up 0 C B KT0703D 300 C 130 C S W, Up 560 C 660 C 560 C MAD = 1.9 Our paleomagnetic results show that ash-fall deposits in general can reliably record the ancient geomagnetic field via the acquisition of a detrital remanent magnetization and can typically provide paleomagnetic polarity data even when the surrounding carbonate and siliciclastic rocks are not well suited for paleomagnetic analysis (Strangway and McMahon, 1973; Reynolds, 1979; Sheriff and Shive, 1982; Hayashida et al., 1996; Iwaki and Hayashida, 2003). The sites for which results could not be statistically distinguished from random may have been handicapped by possible postmagnetization slumping (e.g., Wavy and White Lignitic tuffs) or severe postdepositional chemical alteration (likely in the Analcite, Curly, and Wavy tuffs). Step-wise thermal demagnetization of three-component IRM indicates a dominance of magnetite in some tuffs (e.g., Henrys Fork and Grey tuffs) and a dominance of hematite in others (e.g., Basal Bridger E and Leavitt Creek tuffs; Fig. 5). These interpretations of the IRM results are consistent with the laboratory unblocking temperatures of the ChRM of the respective tuffs (Table DR2 [see footnote 1]). W 0 C MAD = C Figure 3. Vector end-point diagrams (Zijderveld, 1967) for three representative samples with different textures and from different depositional environments. Open (filled) squares show vector end points in the vertical (horizontal) plane. (A) BR0506F (Layered tuff) is a laminated tuff deposited in a lacustrine setting. Alternating field (AF) demagnetization isolated a single component for which characteristic remanent magnetization (ChRM) was demagnetized by 100 mt with a maximum angular deviation (MAD) of 1.6. It is of normal polarity. (B) KT0703D (Tabernacle Butte tuff) was deposited along with pumice clasts in a fluvial setting. Thermal demagnetization isolated a single component for which ChRM was demagnetized by 660 C with a MAD of 1.9. It is of reversed polarity. (C) KT0706D (K-spar tuff) is a homogeneous crystalline tuff deposited in a lacustrine setting. Thermal demagnetization revealed two components. An overprint component was removed by 0 C, leaving the characteristic component that was demagnetized by 525 C with a MAD of 7.7. The ChRM is of reversed polarity. Final demagnetization step and two intermediate steps are shown. Tics represent intensity increments of 0.5 ma/m. determinations are of lesser quality than those for the alpha sites. These sites could record transitional polarities or may have been affected by vertical-axis rota tions. The 18 alpha sites are of both normal and reversed polarity and pass a reversal test at the 95% confidence level (Tauxe, 1998). The mean direction (dec/inc) when all reversed sites are inverted is /57 (α 95 = 5.5, k =.23, N = 18), which is statistically indistinguishable from the expected early Eocene direction for southwestern Wyoming (349 /61 ; Fig. 4; Diehl et al., 1983). EVALUATION OF AGE MODELS Evaluation of the Input Data Paleomagnetic Data of the Tuffs The reliability of our paleomagnetic results is demonstrated by the passage of a reversal test based on 23 sites, the correspondence between the mean direction for all 18 alpha sites and the expected direction for the early Eocene, and the lack of complications in magnetic mineralogy, as indicated by IRM experiments. However, postdepositional diagenesis, which has been shown to have affected carbonate and siliciclastic deposits of the lacustrine Green River Formation, could potentially have obscured the primary ChRM in some of the tuffs (Strangway and McMahon, 1973; Sheriff and Shive, 1982). In addition, some of the beta sites with similar inclinations but variable declinations may record short-term field behavior. Although the possibility of unrecognized overprints cannot be refuted due to a lack of opportunity to perform a fold test in the subhorizontal strata that characterize the study area, the criteria described here attest to the reliability of the polarity determination of the tuffs. Ar/ 39 Ar Data of the Tuffs from Previous Studies The Ar/ 39 Ar geochronologic data reported by Smith et al. (2008a, 2010), together with our paleomagnetic results, comprise the input 6 Geological Society of America Bulletin, Month/Month 2012

8 Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity time scale A North Dec = 173, Inc = 56 k = 28, α 95 = 15 North Dec = 354, Inc = 57 k = 44, α 95 = 6 Figure 4. Equal-area projections of mean characteristic remanent magnetization defined by progressive demagnetization directions of 18 alpha sites in tectonic coordinates. Circles show a 95% cone of confidence around the estimated mean direction. Filled (open) symbols lie on the lower (upper) hemisphere of the projection. Statistical parameters (directional precision parameter [k] and α 95 ) are also shown. When the reversed sites are inverted, the mean direction (dec/inc) for all the alpha sites is /57, which is very close to the expected direction for the early Eocene based on the Eocene reference pole for North America (349 /61 ; Diehl et al., 1983). BR0501A C BR0512B data used to evaluate competing age models for the geomagnetic polarity time scale. Smith et al. (2008a) determined Ar/ 39 Ar ages of 29 tuff beds (22 ash beds and 3 volcaniclastic sand beds) based on 2234 analyses of sanidine and biotite crystals using laser incremental heating experiments and/or laser fusion experiments on single- and multiple-crystal aliquots. The types of Ar/ 39 Ar analyses used include (in order of accu racy): (1) sanidine single-crystal laser fusion, (2) biotite single-crystal step heating, (3) sanidine multiple-crystal laser fusion, (4) biotite single-crystal laser fusion, and (5) biotite multiple-crystal analyses. These age determinations are consistent with the stratigraphic order of the tuff beds and thus are deemed reliable to the first degree. Those ages with no distinct outliers due to contamination by xenocrysts or without indications of Ar* loss are most preferred. Contamination by xenocrysts is readily identifiable using single-crystal fusion analyses because of their significantly older ages. In the case of the Sixth tuff, in which <10% contamination by xenocrystic or detrital grains was found, those analyses derived via only concordant incremental heating of individual biotite crystals Magnetization (ma/m) B Magnetization (ma/m) BR0502A D BR0513I Figure 5. Step-wise thermal demagnetization of acquired isothermal remanent magnetization for selected samples. The samples were demagnetized in three orthogonal axes after fields of 0.12 T, 0.4 T, and 1.1 T were applied to the x (diamonds), y (squares), and z (triangles) axes, respectively. Observed unblocking temperatures indicate the presence of magnetite in (A) BR0501A (Henrys Fork tuff) and (B) BR0502A (Grey tuffs) and hematite in (C) BR0512B (Basal Bridger E tuff) and (D) BR0513I (Leavitt Creek tuff) Temperature ( C) Temperature ( C) Geological Society of America Bulletin, Month/Month

9 Tsukui and Clyde were used to infer the depositional age (Min et al., 2001; Smith et al., 2006, 2008a). Incremental heating experiments on multiple-crystal sanidine aliquots yielded internally concordant plateau ages that were consistent with fusion ages, thus demonstrating the absence of Ar* loss in these samples. With the exception of the Yellow and Strawberry tuffs, only 3.7% of all sanidine analyses were excluded from all age estimates. In the absence of sanidine, biotite was analyzed instead. Laser incremental heating experiments and electron probe microanalyses were performed on euhedral biotite crystals to determine the presence of alteration-related Ar* loss and 39 Ar K recoil induced during irradiation. Discordant age spectra were correlated with the presence of K-depleted alteration phases and resulted in considerable age scatter (Smith et al., 2008b). For high-precision age determination, age spectra that were concordant and reproducible were used with the weighted mean as the best estimate of the eruptive age. All Ar/ 39 Ar ages were calculated relative to the Taylor Creek rhyolite (28.34 ± 0.28 Ma; Renne et al., 1998) in Smith et al. (2008a) but were recalculated to the astronomically calibrated age of Ma for the Fish Canyon sanidine standard (FCs K08 ; Kuiper et al., 2008) in Smith et al. (2010). By intercalibrating all of the Ar/ 39 Ar ages to FCs K08, it is possible to make precise comparison with astrochronology and ages derived by other methods such as U-Pb chronometer, because the FCs K08 reduces the absolute uncertainty from ~2.5% to less than 0.25% and it eliminates the ~1% discrepancy between radioisotopic and astronomical dating. This is an important point because the age models under consideration in this study are based on astrochronology as well as calibration points that were obtained under different calibrations. In the following discussion, to allow intercomparison of ages obtained by Ar/ 39 Ar dating using different fluence monitor standards and astronomically determined ages, we used the FCs K08 -calibrated Ar/ 39 Ar ages as reported in Smith et al. (2010; see their Supplement table DR2) with a 2σ fully propagated uncertainty in all cases. A summary of all of the input data used to evaluate the age models is available in Table DR4 (see footnote 1). Previous Age Models Since the early compilation of the seafloor magnetic anomaly pattern by Cande and Kent (1992), eight calibration models have been proposed for the early to middle Eocene part of the geomagnetic polarity time scale. These efforts not only reflect growing interest in Eocene time (e.g., Jaramillo et al., 2010; Sexton et al., 2011) but also underscore uncertainties in this part of the time scale. Although one of the goals of the geomagnetic polarity time scale is to provide an age estimate for magnetic chron boundaries, the radioisotopic age determinations of the tie points and width estimates of the marine magnetic anomalies, which are two major pillars in the construction of the geomagnetic polarity time scale, have inherent analytical and measure ment uncertainties. The resultant calibration of the chrons thus has built-in uncertainties. Such uncertainties are not always quantified or acknowledged, but it is crucial to recognize them and try to further improve the accuracy and precision of the time scale using additional empirical data from deposits that yield both paleomagnetic polarity and radioisotopic data. The age models we evaluate in this study include the two most recent geomagnetic polarity time scales (CK95, GOS2004; Cande and Kent, 1995; Ogg and Smith, 2004), models B, C, and D of Machlus et al. (2004), models by Smith et al. (2008a, 2010), an extended version by Wing et al. (2000), and the astronomical models of Westerhold and Röhl (2009) (Table DR5; Fig. DR1 [see footnote 1]). Geomagnetic Polarity Time Scales CK95 is a revised version of the time scale by Cande and Kent (1992), and it incorporates changes that arose as a result of a new age estimate for the Cretaceous-Paleogene (K-Pg) boundary (65 Ma rather than 66 Ma) and an astronomical age estimate for the base of chron C3n.4n (5.23 Ma). Its Eocene part is calibrated by fitting a cubic spline through three tie points: 33.7 Ma at the Eocene-Oligocene (E-O) boundary, 46.8 Ma in the middle Eocene, and 55.0 Ma at the Paleocene-Eocene (P-E) boundary. However, as pointed out by Machlus et al. (2004), there are important uncertainties on the age estimates of the two older tie points. Conflicting K-Ar ages (45.8 ± 0.5 Ma and 46.8 ± 0.5 Ma) have been proposed for the middle Eocene tie point (Bryan and Duncan, 1983), and the age estimate for the early Eocene tie point is questionable because of an unconformity of unknown duration that separates the dated tuff and the tie point (Aubry et al., 1996). GOS2004 uses the same method of interpolation as CK95 but is based on five calibration points, three of which are new additions since CK95 (however, note that a new Ar/ 39 Ar age was used for the tie point at C21n.33; sensu Ogg and Smith, 2004). Models by Machlus et al. (2004) Machlus et al. (2004) proposed three alternative models (models B, C, and D) for the interval between chrons C15 and C29 based on different calibration points, but in all cases, they used a natural cubic spline fit. In model B, the two older tie points of CK95 were replaced with an unpublished age (45.6 Ma) of Swisher and Montanari (in Berggren et al., 1995) and an interpolated age for the Paleocene-Eocene boundary (55.3 Ma; Wing et al., 2000). In model C, the two older tie points of CK95 were replaced with a single-crystal biotite Ar/ 39 Ar age for the Sixth tuff (48.8 Ma; Machlus et al., 2004) and the aforementioned Paleocene- Eocene boundary age. In model D, the two older tie points of CK95 were replaced by the age for the Sixth tuff only. In these models, the Cretaceous-Paleogene and Eocene-Oligocene boundaries were left unchanged as in CK95, resulting in a time scale where the early Eocene was lengthened at the expense of the Paleocene and middle Eocene. Model by Smith et al. (2008a, 2010) Smith et al. (2008a, 2010) recalibrated the interval, C24 through C20 based on Ar/ 39 Ar ages of ash beds, existing magnetostratigraphic data, NALMA biostratigraphy from the Bighorn Basin, Wind River Basin, Greater Green River Basin, Uinta Basin, Devil s Graveyard Formation in Texas, and Absaroka volcanic province, as well as marine biostratigraphy from San Diego region (see table 4 in Smith et al., 2008a). However, uncertainties remain in the correlation of some of the tuffs to local magnetostratigraphic records (e.g., Layered tuff, Sixth tuff, and Continental Peak tuff). This model implies the presence of several shortduration polarity chrons that are not shown in the original marine magnetic anomaly records of Cande and Kent (1992). Smith et al. (2008a) attributed those to tiny wiggles described in Cande and Kent (1992). Model by Wing et al. (2000) In Wing et al. (2000), a Ar/ 39 Ar sanidine age from the bentonitic tuff at the base of chron C24n.1n (also referred to as Willwood Ash ) was used in place of the calibration point at the Paleocene-Eocene boundary of CK95 (Wing et al., 1991; Tauxe et al., 1994). Chrons between C29 and C22 were calibrated by linear interpolation between the bentonitic tuff and two calibration points of CK95 at the Cretaceous-Paleogene boundary and in the middle Eocene. We have extrapolated the original calibration to chron C20n to accommodate some of the younger tuffs collected from the Uinta Basin in this study. This new version, which includes the extrapolated segment, is referred to as the Willwood model in the following discussion. 8 Geological Society of America Bulletin, Month/Month 2012

10 Fine-tuning the calibration of the early to middle Eocene geomagnetic polarity time scale Models by Westerhold and Röhl (2009) Westerhold and Röhl (2009) used a fundamentally different approach than those previously discussed, which relied on radioisotopic ages of discrete tuff horizons. Instead, the Westerhold and Röhl (2009) models are based on orbital tuning and cycle counting of Fe intensity data from ODP Leg 207, Site 1258, and they provide estimates of chron durations for the interval between chrons C20 and C24. In the tuned model, the cycles are tuned to the stable 5-k.y.-long eccentricity cycle, whereas the cycle-counted model is based on cycle counting assuming 21 k.y. and 95 k.y. for the precession and short eccentricity cycles, respectively. The accuracy of an astronomical time scale based on orbital cyclicities depends on the accuracy of the orbital solution used (although it is presently not available for the Paleogene), the stratigraphic completeness of climatic proxy records, proper correlation between the proxy records and the orbital solution, the accuracy of magnetostratigraphic data for the section in which the proxy records are identified and correlated, and finally, the assumed value of sediment accumulation rates between two astronomically tuned calibration points. Uncertainties in the Westerhold and Röhl (2009) geochemical data set include: potential ambiguities in magnetostratigraphy of ODP 1258 due to near-equatorial paleolatitudes and weak magnetization; potential unrecognized faults in Hole 1258A; and ambiguities in construction of the composite section (Westerhold and Röhl, 2009). In contrast to the models based on Ar/ 39 Ar age determinations, testing the Westerhold and Röhl (2009) models requires us to first anchor them to a reference point, as they are floating age models due to the uncertain age of the Paleocene-Eocene Thermal Maximum and uncertainties in orbital solutions for this interval (Laskar et al., 2004; Westerhold et al., 2007, 2008). To calibrate their models in absolute time, we used three astronomically proposed ages for the top of chron C24r as anchor points (53.53 Ma, Ma, and Ma according to option 1, option 2, and option 3, respectively; Westerhold et al., 2007). Because Westerhold and Röhl (2009) used two methods to calibrate the observed cycles (tuning and cycle counting), and each of them is anchored to three proposed age estimates for the top of chron C24r, in total, six variations are considered for evaluation in this study (these models will be referred to as T-1 to T-3 and CC-1 to CC-3, where T and CC stand for tuning and cycle counting, respectively, and the number following the hyphen corresponds to either option 1, 2, or 3). Tuffs that are younger than the younger limit of a particular age model are not considered in such cases. Figure DR1 summarizes all of the calibration models considered in this study, showing different ways in which chrons are scaled according to different tie points and interpolation methods used (see footnote 1). Method of Age Model Evaluation The paleomagnetic polarity results from the 23 alpha and beta sites were considered in conjunction with the most recently published Ar/ 39 Ar ages of the tuffs (Smith et al., 2008a, 2010) to evaluate geomagnetic polarity time scale age models for the interval between ca. 53 and 44 Ma. For the purpose of evaluating these age models, the Ar/ 39 Ar and paleomagnetic polarity data are assumed reliable (see previous section on Evaluation of the Input Data for uncertainties of the polarity and Ar/ 39 Ar data). Because every age model predicts a polarity for the Ar/ 39 Ar age range of each tuff defined by a 2σ fully propagated uncertainty, we determined the number of tuffs that have concordant predicted and measured polarities within a framework of a particular age model. The tuffs for which Ar/ 39 Ar age ranges fall within a single chron in a particular age model are referred to as category I tuffs and are included in the calculation of the index of agreement (see following). However, Ar/ 39 Ar age ranges of some tuffs in any given age model are likely to span chron boundaries by chance, preventing an unequivocal assessment of the congruence between the predicted and observed polarity. These tuffs are classified as category II and are excluded from the evaluation of the particular age model because they are equivocal. Note that the category assignment is age model specific, and thus any given tuff may be classified Index of agreement CK GOS Model B 0.56 Model C 0.50 Model D (Machlus et al., 2004) 0.60 Willwood model as category I according to one age model but as category II according to another. The tuffs with relatively large uncertainties in Ar/ 39 Ar age determinations are weighted less in this method of evaluation because they are more likely to be classified as category II (e.g., Yellow and Strawberry tuffs). An index of agreement (IA = number of category I tuffs with concordant measured and predicted polarity divided by the total number of category I tuffs within a particular age model) was calculated for each model and used to define the model with maximum congruence between expected and measured polarity. Because the number of category I tuffs (i.e., denominator) varies among the models, the index of agreement provides a semiquantitative means to measure the effective explanatory power of each model with an expectation that a perfectly congruent calibration model would have an IA of 1. Results of Age Model Evaluation Calculated IAs range from 0.29 to 0.60, with the highest value recorded for the Willwood model, followed by model C and T-3 (IA = 0.56; Fig. 6; Table DR5 [see footnote 1]). No age model has an IA close to 1, and we interpret this result to indicate that further modifications to the models are necessary to reconcile the paleomagnetic polarity data with the radioisotopic data. Because the use of IA was not satisfactory in defining one best model, the three models with the highest IA values (Willwood, model C, and T-3) are further discussed in the following. It is worth noting that the same test was performed using the Ar/ 39 Ar age estimates considering only the 2σ analytical uncertainty (as opposed to fully propagated uncertainty), 0. Smith et al. (2010) 0.44 T CC T CC T-3 (Westerhold and Röhl, 2009) Figure 6. Index of agreement (IA) for all the calibration models evaluated in this study. See text for derivation of IA and abbreviations of the model names CC-3 Geological Society of America Bulletin, Month/Month

11 Tsukui and Clyde but the IA values did not change significantly, implying that the match between the model and the input data is not controlled by the size of uncertainty associated with each Ar/ 39 Ar age. Further adjustments were made to the three models by shifting the chron ages in order to force at least a minimum overlap between expected and measured polarity for all category I tuffs. Chron boundaries nearest to discordant category I tuffs (those with unmatched expected and measured polarities) were shifted to either the top or bottom of the Ar/ 39 Ar age range of the tuffs so that predicted and measured polarities matched at least minimally within the age range determined for the tuffs. The ages of either the top or base of the discordant category I tuffs toward which the chron bound aries were shifted were then tentatively used as best approximate age estimates for the polarity boundaries. For example, four chron boundaries (C20r-C20n, C21n-C20r, C22n-C21r, and C22r-C22n) in the Willwood model were incre mentally shifted to the top of the Strawberry tuff age range at Ma, base of the Oily tuff age range at Ma, base of the Church Butte tuff age range at Ma, and top of the Main tuff age range at Ma, respectively (Fig. 7). Hereafter, the three adjusted models will be referred to as new Willwood, new model C, and new T-3 to differentiate them from the original versions (Figs. DR2 and DR3; Table DR5, see the last three columns [see footnote 1]). DISCUSSION Comparison with Chronostratigraphic Data The three modified models (new Willwood, new model C, and new T-3) were evaluated in a regional chronostratigraphic context to determine which one integrates existing chronostratigraphic constraints in the most coherent way. To do so, we used bio- and magnetostratigraphic data from sections in the Bighorn Basin (Clyde et al., 1994), Greater Green River Basin (Clyde et al., 1997, 2001), and Uinta Basin (Prothero, 1996). These bio- and magnetostratigraphic rec ords were placed within each of the three calibration frameworks in exactly the same manner as they were correlated to the geomagnetic polarity time scale in the original studies (Fig. 7; Figs. DR2 and DR3 [see footnote 1]). These magnetostratigraphic sections were chosen because they had been correlated to the NALMA biostratigraphy, which is important for evaluating the veracity of the age models because the biostratigraphy is established with respect to magnetostratigraphy for a given area, independent of geomagnetic polarity time scale calibration in absolute time. Comparison of the three models with these independent chronostratigraphic data favors the new Willwood model over the new model C and the new T-3 model, although the new T-3 model is very similar to the older half of the new Willwood model. The chronostratigraphic data used to support the new Willwood model are summarized below and in Table 2: (1) Walsh (1996) and Prothero (1996) showed that the Bridgerian-Uintan NALMA boundary should lie in chron C21n. The Sage Creek Mountain pumice from the Bridger E lithology approximates the position of the boundary because the Bridger E faunal assemblage that contains a mix of Bridgerian and Uintan fauna is found ~7 m above the base of the Bridger E lithology (Evanoff et al., 1994; Robinson et al., 2004; Murphey and Walsh, 2007). The new Willwood model places the Sage Creek Mountain pumice in chron C21n. (2) The Tabernacle Butte locality yields a Br-3 fauna (the youngest subage of the Bridgerian NALMA) according to West (1973). The new Willwood and new T-3 models place the Tabernacle Butte tuff in chron C21r. This placement is consistent with the bio- and magnetostratigraphic records of Clyde et al. (2001), which place Br-1b in C22n, and records from Prothero (1996), which place the Bridgerian- Uintan boundary in chron C21n. (3) The Church Butte tuff is exposed at the fossiliferous Grizzly Buttes locality, which has been associated with a Br-2 fauna (upper Blacksforkian subage; Alexander and Burger, 2001). The Br-2 fauna was found in chron C21r in the Delmar Formation in California (Robinson et al., 2004). The Delmar Formation has been correlated directly to marine biostratigraphy and thus has a reliable chron assignment. The new Willwood and new T-3 models place the Church Butte tuff in chron C22n. This correlation is also consistent with that of Clyde et al. (2001), who placed the underlying Br-1b in chron C22n. (4) Prothero (1996) proposed a polarity stratigraphy from Indian Canyon of the Uinta Basin that spans from the Horse Bench Sandstone marker bed to the limestone and sandstone facies of the Green River Formation (Fig. 2). The Fat and Oily tuffs can be correlated to his section based on their meter levels with respect to the Horse Bench Sandstone and the saline facies limestone-sandstone facies transition. The new Willwood model places the Fat tuff within chron C21n, and this correlation is in agreement with Prothero (1996). His magnetostratigraphic correlation places the Oily tuff in chron C19r, which conflicts with the polarity determination of this study. (5) The Continental Peak tuff is exposed in the South Pass section in a stratigraphic interval that is interpreted to be Bridger B (or uppermost Bridger A; Zeller and Stephens, 1969; Gunnell, 2006, personal commun.). The new Willwood and new T-3 models place the tuff in chron C21r; this is consistent with the magneto stratigraphy of Clyde et al. (2001) from the same South Pass section, which correlates the lower part of the Bridger Formation with chron C22n. This contrasts with the new model C because it correlates the Continental Peak tuff with chron C23n.1r, which has been associated with the older Wasatch Formation in the magneto stratigraphy of Clyde et al. (2001). (6) The Willwood Ash is a bed of particular interest for calibration of the geomagnetic polarity time scale because it is located at the base of chron C24n.1n and is used as one of the five calibration points in GOS2004 and in the original age model by Wing et al. (2000) (Tauxe et al., 1994; Ogg and Smith, 2004). It was dated at ca Ma by Wing et al. (1991) but has been recently redated and recalibrated to 52.9 ± 0.18 Ma (2σ fully propagated uncertainty) relative to the FCs K08 by Smith et al. (2004, 2010). The new Willwood model is expected to meet these magnetostratigraphic and radioisotopic criteria, since these data were used in building the calibration model by Wing et al. (2000) from which the Willwood model was derived. However, the new model C and new T-3 model place the base of chron C24n.1n outside the error of the Willwood Ash. (7) The tuff just below the Alamo Creek basalt in the Lower Member of the Devil s Graveyard Formation lies just below rocks of reversed polarity and above the Junction faunas, which contain a mixture of the Bridgerian-Uintan faunas (Prothero and Emry, 1996; Walton, 1992). Smith et al. (2010) recalibrated the age of this tuff to ± 0.14 Ma using the FCs K08. Based on its Ar/ 39 Ar age, the new Willwood model places the tuff in chron C21n, i.e., the same chron the Bridgerian-Uintan boundary is predicted to lie by Prothero (1996). In contrary, the new model C places the tuff in chron C22n, which is correlated to Br-1b by Clyde et al. (2001). (8) The Mission Valley Ash bed has been correlated to Ui-3 and chron C20n (Prothero and Emry, 1996; Walsh et al., 1996; Robinson et al., 2004). Smith et al. (2010) recalibrated the age of this tuff to ± 0.49 Ma using the FCs K08. The new Willwood model places the tuff in chron C20n based on its Ar/ 39 Ar age, consistent with the bio- and magnetostratigraphic records of Prothero (1996). The new model C places the tuff in chron C21n instead. (9) The Blue Point Ash (Hiza, 1999) lies near the base of chron C21r (Sundell et al., 1984; 10 Geological Society of America Bulletin, Month/Month 2012