Earth and Planetary Science Letters 448 (2016) 62–68

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Earth and Planetary Science Letters

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87 86 Lacustrine Sr/ Sr as a tracer to reconstruct Milankovitch forcing of the Eocene hydrologic cycle ∗ M’bark Baddouh, Stephen R. Meyers , Alan R. Carroll, Brian L. Beard, Clark M. Johnson

Department of Geoscience, University of Wisconsin-Madison, 1215 West Dayton Street, Madison, 53706, USA a r t i c l e i n f o a b s t r a c t

Article history: The Green River Formation (GRF) provides one of the premier paleoclimate archives of the Early Eocene Received 25 January 2016 Climatic Optimum (∼50 Ma), representing the apex of the early Cenozoic greenhouse climate. Rhythmic Received in revised form 3 May 2016 lake-level variability expressed in the GRF has inspired numerous hypotheses for the behavior of the Accepted 5 May 2016 Eocene hydrologic cycle, including its linkage to astronomical forcing, solar variability, and the El Niño Available online 24 May 2016 Southern Oscillation (ENSO). However, the lack of sufficient proxy data to document atmospheric water- Editor: H. Stoll mass transport and the geographic pattern of evaporation/precipitation/runoff has made it difficult to 87 86 Keywords: discriminate between different models for astronomical forcing. Variable Sr/ Sr ratios of bedrock strontium isotopes that encompass the GRF provide an opportunity to reconstruct the spatial expression of the Eocene Green River Formation hydrologic cycle and its linkage to lake level. Here Sr isotope data from the Wilkins Peak Member, a astronomical forcing rhythmic succession that has been demonstrated to record Milankovitch forcing of lake levels, indicate ENSO that high lake levels reflect an increased proportion of runoff from less radiogenic rocks west of the basin, eliminating a number of the existing astronomical-forcing hypotheses. The 87Sr/86Sr variability is consistent with a change in mean ENSO state, which is predicted by climate models to be linked to orbital-insolation. Thus, the 87Sr/86Sr data reveal a coupling of high frequency (ENSO) and low frequency (astronomical) climate variability, and also predict the existence of sizable astronomically-forced alpine snowpack during the last greenhouse climate. More broadly, this study demonstrates the utility of 87Sr/86Sr as a powerful tool for reconstructing the deep-time hydrologic cycle. © 2016 Elsevier B.V. All rights reserved.

1. Introduction (ENSO) to astronomical scale-climate variability (Sloan and Morrill, 1998; Huber and Caballero, 2003). Providing another perspec- The Eocene GRF contains rhythmic deposits that have been the tive, Lawrence et al. (2003) suggest that lacustrine depositional focus of cyclostratigraphic inquiry for almost a century (Bradley, processes could have been extremely sensitive to small regional 1929), and it represents one of the most well-constrained (ra- climate changes, rather than representing large magnitude climate dioisotopically) and rigorously-tested records of astronomical- change. Thus, while the GRF provides a remarkable window into forcing from the last greenhouse (Fischer and Roberts, 1991; Eocene paleoclimate – at a time when high quality paleoclimate Roehler, 1993; Machlus et al., 2008; Meyers, 2008; Smith et al., records from the marine realm are scarce (Zachos et al., 2001; 2010, 2014; Aswasereelert et al., 2013; Machlus et al., 2015). Pälike and Hilgen, 2008)– its relation to larger-scale global cli- Anumber of hypotheses have been offered to explain its astro- mate change during the last greenhouse remains uncertain. nomically-forced depositional cycles, including global changes in Establishing the link between the GRF record and global scale temperature and humidity (Bradley, 1929; Roehler, 1993), changes climate requires an understanding of the geographic expression of in local incident shortwave radiation (and thus evaporation) on climate variables such as evaporation and precipitation. Unfortu- the lake’s surface (Morrill et al., 2001), and changes in catchment nately, proxy-based studies have not yet been able to constrain or lake characteristics (Morrill et al., 2001). Modeling studies sup- the geographic footprint of the hydrologic cycle and the role of re- port a strong link between GRF deposition and high frequency gional moisture sources in controlling GRF lake levels. Oxygen iso- tope compositions offer one potential means for investigating such changes, based on differences in δ18O of atmospheric moisture * Corresponding author. Tel.: +1 608 890 2574. derived from different marine sources (Chamberlain et al., 2012). E-mail address: [email protected] (S.R. Meyers). This approach, however, is also subject to substantial uncertainties http://dx.doi.org/10.1016/j.epsl.2016.05.007 0012-821X/© 2016 Elsevier B.V. All rights reserved. M. Baddouh et al. / Earth and Planetary Science Letters 448 (2016) 62–68 63

that the lake rarely (if ever) rose above the relatively low-gradient basin floor to directly contact the bedrock of the basin-bounding uplifts. Continuously high salinities are supported by the pres- ence of Na-evaporite minerals and the complete absence of fish or other megafossils, and there is no evidence that the lake spilled into downstream basins. Individual lacustrine expansion– contraction cycles typically begin with interbedded carbonate-rich mudstone, calcareous sandstone, and intraclast conglomerate inter- preted to represent littoral facies deposited during transgression. Minor scours, desiccation cracks, wave ripples, and wavy bedding are all consistent with shallow water and intermittent subaerial exposure. These deposits grade upward into kerogen-rich, dark gray to brown, finely laminated, calcitic to dolomitic mudstone (oil shale), interpreted to represent sublittoral deposition (e.g., Carroll and Bohacs, 2001). Primary trona and halite are closely associated with the profundal facies near the basin depocenter, and diagenetic shortite crystals sometimes disrupt primary lamination. Finally, the kerogen-rich facies grade upward into gray-green carbonate-rich mudstone and siltstone facies that record gradual regression of the lake. Wavy lamination, mudcracks, brecciation, and displacive shortite crystals are common and reflect deposition in littoral to palustrine environments. Fig. 1. Simplified geological map of the Green River Basin region and location of the White Mountain Core #1 (WM-1) in Wyoming, including 87Sr/86Sr ratios of modern rivers and concentration-weighted averages for Precambrian 87Sr/86Sr reported by 2.2. Bedrock 87Sr/86Sr and lake water provenance Doebbert et al. (2014). Inferred Eocene rivers modified from Smith et al. (2014). 87 86 Note that lower Sr/ Sr ratios generally correspond to streams that drain ma- During deposition of the WPM the drainage catchment of Lake rine carbonate strata, and ratios greater than ∼0.710 are exclusively associated with Precambrian rocks. The Cathedral Bluffs Member (CBM) of the Wasatch Formation Gosiute was largely restricted to mountain ranges lying within consists primarily of Precambrian detritus, which should therefore be characterized 100–200 km of the GRB, based on paleocurrent and sedimentary by higher 87Sr/86Sr values, and was deposited by rivers that drained toward the provenance evidence (Smith et al., 2014). Previous studies have es- 87 86 WPM. Relatively low Sr/ Sr ratios near the Sierra Madre likely reflect local influ- tablished large geographic differences in bedrock 87Sr/86Sr (Beard ence of Cretaceous marine carbonate units within the Mancos Group. The Bridger and Johnson, 2000; Bataille and Bowen, 2012), which are reflected Basin lies between the Cordilleran Fold and Thrust Belt and the Rock Springs Arch. in the isotopic composition of modern rivers (Doebbert et al., 2014; Fig. 1). These differences are strongly bimodal. To the west of related to fractionation of meteoric δ18O along atmospheric trans- the GRB, the Cordilleran Fold and Thrust Belt (CFTB) contains sev- port pathways and during lake-surface evaporation, transpiration eral imbricated, Sr-rich marine carbonate intervals totaling several by plants, precipitation of authigenic mineral phases, and diage- hundred meters. Modern rivers draining this area have 87Sr/86Sr nesis (Talbot, 1990). In contrast, the 87Sr/86Sr ratio imparted on ratios of 0.70869 to 0.70917 (Doebbert et al., 2014). In contrast, source waters by bedrock geology does not experience significant ranges to the north, south, and east of the GRB are cored by highly- meteorologic or biologic fractionation (Capo et al., 1998), and con- radiogenic Precambrian rocks. Marine carbonate strata thin or dis- sequently, strontium isotope ratios can be used to directly trace the appear going eastward and were generally eroded from the range geographic source of runoff into a lake, based on the known com- crests prior to deposition of the WPM (Carroll et al., 2006), unroof- positions of rocks and sediment within a lake’s watershed (Placzek ing crystalline cores that shed broad aprons of alluvial, arkosic de- et al., 2011; Joordens et al., 2011). Changes in lake water 87Sr/86Sr tritus. Modern rivers that drain these cores typically have 87Sr/86Sr ratios may therefore be used to infer changes in runoff patterns, ratios of 0.7157 to 0.7432 (Doebbert et al., 2014). The overall struc- which in turn imply changes in the regional distribution of pre- ture and lithology of ranges bounding the GRB have changed little cipitation/runoff. Here we present 51 carbonate 87Sr/86Sr measure- since the Eocene; the strong east–west bimodality evident in mod- ments through 28 meters of rhythmic lake deposits of the Wilkins ern river water 87Sr/86Sr is also reflected in the isotopic compo- Peak Member (WPM), to help discriminate between competing cli- sition of older lacustrine carbonates. For example, 87Sr/86Sr ratios matic explanations for precessional-scale lake level fluctuations, in lacustrine carbonate in Utah associated with drainage from the and provide new constraints on the Eocene hydrologic cycle. CFTB average 0.7100 (Gierlowski-Kordesch et al., 2008), whereas capture of an eastern drainage by Eocene Lake Gosiute following 2. Geologic background WPM deposition increased carbonate 87Sr/86Sr ratios to 0.7146 in the Laney Member (Doebbert et al., 2014). Based on these observa- 2.1. Sedimentary facies of the Wilkins Peak Member tions, we infer that the present-day 87Sr/86Sr bimodality provides an approximate analogue during WPM deposition. The WPM was deposited in the Green River Basin (GRB) of southwest Wyoming (Fig. 1). It alternates between discrete in- 3. Materials and methods tervals of lacustrine carbonate- and evaporite-rich lithologies de- posited by Eocene Lake Gosiute, versus intervals of alluvial silici- This study is based on a 28 m interval of the uppermost WPM clastic strata (Fig. 2). Wilkins Peak Member lacustrine facies have recovered in the White Mountain #1 drill core (Fig. 1). The study previously been interpreted to record repetitive deepening and interval is bounded below by a composite alluvial bedset (the shallowing of a saline to hypersaline lake that at its maximum arkosic “I” bed; Culbertson, 1961), and above by the Laney Member covered much of the Bridger basin (Eugster and Hardie, 1975; of the GRF (Fig. 2). Carbonate Sr isotope ratios, Rb and Sr con- Smoot, 1983; Roehler, 1993; Bohacs et al., 2000; Pietras and Car- centrations, and percent carbonate (Supplementary Table S1) were roll, 2006; Smith et al., 2015). The maximum lake depth is un- measured on 100 mg aliquots of powder material obtained from known, but shoreline facies preserved within the basin suggest splits of the White Mountain #1 drill core. The analyzed powders 64 M. Baddouh et al. / Earth and Planetary Science Letters 448 (2016) 62–68

Fig. 2. Lithostratigraphy, Fischer Assay data (U.S. Geological Survey Oil Shale Assessment Team, 2008), and 87Sr/86Sr ratios for the upper Wilkins Peak Member in the White Mountain Core #1, plotted versus depth. U–Pb radioisotopic data come from Machlus et al. (2015), and include 2σ analytical uncertainties. Horizontal gray bars indicate visually-identified intervals of profundal mudstone, which generally correlate with 87Sr/86Sr minima, and express Milankovitch forcing (see Supplementary Information Fig. S2 for precession and obliquity bandpass filtered records). Small black rectangles on the right side of the figure indicate 20 ka increments, using a linear sedimentation model based on the two U–Pb ages.

come from 51 carbonate-bearing mudstone samples from the core, Table 1 collected by microdrilling within discrete areas (∼0.5 × 2cm) at Measured Rb–Sr isotope data for the carbonate and silicate fractions from the White intervals of 30 cm. Mountain Core #1. Carbonate fraction 3.1. Rb–Sr isotope geochemistry Sample ID Carbonate Rb Sr 87Rb/86Sr 87Sr/86Sr (%) (ppm) (ppm) 87Sr/86Sr ratios were analyzed using a VG Instruments Sector WM-371-3 13.10 1.4764 3211 0.00133 0.71291 54 multi collector thermal ionization mass spectrometer. The re- WM-381-35 23.50 1.9708 1461 0.00390 0.71208 WM-382-0 27.10 1.1261 1249 0.00261 0.71247 ported 87Sr/86Sr value is based on the average of 120 ratios with WM-418-0 17.39 1.2364 3118 0.00115 0.71378 88 −11 an Sr ion intensity of 3 × 10 A. Reported errors are the inter- WM-425-9 24.42 1.4316 914 0.00453 0.71317 nal 2-standard errors (2-SE) which is slightly less than the long- WM-435-9 36.89 0.4564 1843 0.00072 0.71168 term external error, which is defined as 2 standard deviations of WM-445-10 44.82 0.5195 1634 0.00092 0.71382 the mean (2-SD) based on analysis of the NIST SRM-987 Sr isotope Silicate fraction standard (0.710262 ± 0.000016; 2-SD; n = 66) that was analyzed Sample ID Rb Sr 87Rb/86Sr 87Sr/86Sr during the course of this study. (ppm) (ppm) To evaluate possible contamination of the measured carbon- WM-371-3 85.60 44.2 5.6029 0.71794 ate Rb–Sr isotope composition by the silicate fraction, the silicate WM-381-35 93.79 156.6 1.7340 0.71408 fraction was analyzed after acetate leaching and carbonate extrac- WM-382-0 56.98 72.2 2.2853 0.71973 tion from 7 samples that ranged in percent carbonate from 13 to WM-418-0 51.05 66.3 2.2321 0.72208 45% (Table 1; Fig. S1). For six of the seven silicate analyses, the WM-425-9 73.21 2133.7 0.0993 0.71269 WM-435-9 48.69 163.5 0.8623 0.71741 87Sr/86Sr ratio was higher than the corresponding carbonate, rang- WM-445-10 71.03 408.8 0.5030 0.71477 ing to a maximum of 0.7221 (Table 1, Fig. S1). The carbonate’s Rb concentration and its 87Rb/86Sr ratio can be sensitive indica- tors of possible contamination of the carbonate 87Sr/86Sr ratio, by material leached from the silicate fraction, which in these samples contribution of Sr from the silicate fraction is minimal, confirm- tends to be much more radiogenic than the carbonate material. ing the robustness of the methodology. See the Supplementary Based on the low Rb concentration and 87Rb/86Sr of the carbon- Information text for more details on the Sr-isotope analytical pro- ate, and the carbonate’s high Sr concentration, we conclude that cedures. M. Baddouh et al. / Earth and Planetary Science Letters 448 (2016) 62–68 65

Table 2 ∗ Evaluation of the correlation between oil yield (gpt) and 87Sr/86Sr data from the White Mountain Core #1. + Data sets compared Correlation method Correlation coefficient p-Value Oil yield vs. 87Sr/86Sr Pearson −0.4201 0.0027 Oil yield vs. 87Sr/86Sr Spearman rank −0.6017 < 0.001 Oil yield vs. 87Sr/86Sr – no outlier Pearson −0.6022 < 0.001 Oil yield vs. 87Sr/86Sr – no outlier Spearman rank −0.6689 < 0.001 Oil yield vs. 87Sr/86Sr first differences – no outlier Pearson −0.7048 < 0.001 Oil yield vs. 87Sr/86Sr first differences – no outlier Spearman rank −0.7768 < 0.001 ∗ Oil Yield has been resampled onto the Sr-isotope sampling grid, using piecewise linear interpolation. + p-Values are determined using 10,000 simulations with phase-randomized surrogates, following the approach of Ebisuzaki (1997).

3.2. Mineral identification via X-ray powder diffraction 4. Results and discussion

4.1. Evaluation of 87Sr/86Sr and lake water provenance during X-ray diffraction was performed for mineral phase identification deposition of the Wilkins Peak Member and semi-quantitative analysis of the core samples, using a Rigaku Rapid II diffractometer with a curved two-dimensional imaging Carbonate 87Sr/86Sr for the 51 samples analyzed here ranges plate. The percent clay and major mineralogy for each core sample from 0.71154 to 0.71504, averaging 0.71265 ± 0.00123 (2SD; Ta- as determined by X-ray diffraction are presented in Supplemen- ble S1). Average 87Rb/86Sr is 0.00137, indicating minimal contribu- tary Table S1. Samples are demonstrated to be calcitic to dolomitic tion from clay minerals to the measured Sr isotope compositions marlstone with varying amounts of organic carbon enrichment. (see Table 1 and Table S1). Several lines of evidence support the interpretation that 87Sr/86Sr ratios reflect either original lake wa- 3.3. Fischer Assay oil yield ter composition or penecontemporaneous diagenetic fluids. First, the relatively impermeable mudstone facies sampled in this study generally lack petrographic evidence for late diagenesis. Second, Fischer Assay (FA) oil yield, which measures the amount of liq- preservation of relatively large 87Sr/86Sr fluctuations at sub-meter uid hydrocarbon compounds released by retorting, has served as a scale throughout the interval demonstrates that pervasive resetting proxy for lake depth and organic carbon richness in previous sedi- of Sr isotope compositions during burial has not occurred. Third, mentologic and cyclostratigraphic analyses of the GRF (Fischer and recent mass-balance modeling of Lake Gosiute by Doebbert et al. Roberts, 1991; Roehler, 1993; Machlus et al., 2008; Meyers, 2008; (2014) suggests relatively short Sr residence times, on the order Smith et al., 2010). Each published Fischer Assay analysis (U.S. Geo- of 103–104 yrs or less. This result confirms that lacustrine carbon- logical Survey Oil Shale Assessment Team, 2008)used in this study ate was capable of preserving a high-resolution, primary record of represents a homogenized section of the White Mountain #1 core, changing water sources to the lake. Finally, WPM 87Sr/86Sr ratios typically ∼30 cm thick, and samples extend continuously to cover correlate closely with visually identified sedimentary facies and the entire study interval. The samples used for Sr isotope analysis with Fischer Assay oil yield (a proxy for organic carbon richness; correlate with the Fischer Assay samples, but are more discontin- Figs. 2 and 3; Table 2; U.S. Geological Survey Oil Shale Assessment uous and represent much smaller stratigraphic increments. Given Team, 2008), both of which record syndepositional lake-level fluc- the U–Pb linear sedimentation age model employed in this study tuations (Pietras and Carroll, 2006). 87 86 (from Machlus et al., 2015; Fig. 2), the ∼5mmthick samples used The WPM data show that less radiogenic Sr/ Sr ratios cor- for Sr-isotope analysis represent ∼60 yrs, whereas the 30 cm thick relate with sedimentary facies that reflect deeper lacustrine de- samples used for Fischer Assay represent ∼3.7 kyrs. position (Figs. 2 and 3; Table 2). Rhodes et al. (2002) reported similar results based on coarse resolution sampling through the GRF. In contrast, higher 87Sr/86Sr ratios occur within mudstone fa- 3.4. Statistical assessment of data correlation via ‘surrogateCor’ cies deposited in shallower water, which also have lower oil yields (Figs. 2 and 3; Table 2). We infer that these differences in Sr iso- tope composition primarily reflect changes in the provenance of To provide a quantitative evaluation of the correlation between runoff into the lake. An alternative explanation is that leaching/re- oil yield and 87Sr/86Sr ratio, we introduce a statistical approach working of Rb-rich clays from exposed lake plains caused higher that employs sample interpolation, and significance testing with 87Sr/86Sr ratios within corresponding lowstand lakes (Rhodes et phase-randomized surrogate data (Ebisuzaki, 1997). The oil yield al., 2002). There is, however, no stratigraphic evidence for incision = = data (n 78) was resampled on the Sr isotope sample grid (n within the study interval, or for advection of substantial detritus 51), using piecewise linear interpolation (the sparser sampling grid toward the basin depocenter at times of low lake levels. Further- was used to avoid over-interpolation). Correlation is evaluated us- more, we observe no significant correlation between XRD % clay ing both Pearson and Spearman Rank coefficients. The statistical and 87Sr/86Sr in the samples (Fig. 4), as would be expected if the significance of the resulting correlation coefficients are estimated radiogenic Sr isotope compositions reflected increased interaction via 10,000 Monte Carlo simulations using phase-randomized surro- with an exposed lake plain and recycling of clay. Even if leach- gates; the surrogates are subject to the same interpolation process, ing of clay within the exposed lake plain did contribute to higher and compensate for potential serial correlation of data (Ebisuzaki, carbonate 87Sr/86Sr during lowstand lakes – a time of limited wa- 1997)(Table 2). The first-difference series of each variable is also ter delivery to Lake Gosiute – this does not obviate the need for evaluated, to assess correlation in the magnitude of change be- an enhanced flux of source waters with low 87Sr/86Sr (west of the tween sequential stratigraphic samples rather than absolute mag- GRB) during lake highstands. nitude. All statistical analyses employ the Astrochron package for R It is also possible that the WPM contains detrital carbonate (Meyers, 2014; R Core Team, 2015), which includes a new function grains derived from older marine strata exposed within the water- (‘surrogateCor’) to perform the correlation assessment. shed of Lake Gosiute. However, previous studies indicate that such 66 M. Baddouh et al. / Earth and Planetary Science Letters 448 (2016) 62–68

Fig. 3. Cross-plots of (a) White Mountain Core #1 87Sr/86Sr ratios and resampled oil yield values (one outlier is identified as a filled symbol). (b) First difference of the White Mountain Core #1 87Sr/86Sr ratios and resampled oil yield values (excluding the outlier identified in panel “a”). The use of first differences permits evaluation of relative changes between sequential stratigraphic samples, rather than absolute values.

served in the WPM makes this interpretation robust. As a second order effect, variability in the 87Sr/86Sr composition of the specific lithologies contributing to the bimodal source regions over time is expected to contribute some degree of noise, and should be considered when attempting a quantitative reconstruction of the hydrologic cycle (e.g., via modeling; Doebbert et al., 2014).

4.2. Coupling of high frequency (ENSO) and low frequency (astronomical) climate variability during the Early Eocene Climatic Optimum

As described below, the observed 87Sr/86Sr record from the GRF is consistent with Milankovitch-forcing of mean ENSO state. High- resolution regional climate modeling using paleotopographic data Fig. 4. Plot of carbonate 87Sr/86Sr versus XRD percent clay from White Mountain Core #1. Note the lack of correlation between 87Sr/86Sr and percent clay, supporting (Sewall and Sloan, 2006)indicates that the GRF lakes occupied a the interpretation that carbonate 87Sr/86Sr is representative of water chemistry and relatively dry, low elevation intermontane area between the Rocky provenance. The p-value for the Pearson correlation coefficient is 0.5390, and the Mountain front ranges to the east and the Sierran arc and proto- p-value for the Spearman rank correlation coefficient is 0.7603. Cascade Mountains to the west (Smith et al., 2014). Precipitation on the Rocky Mountain front ranges was strongly influenced by a grains are absent or rare (Smoot, 1983; Pietras and Carroll, 2006; summer monsoon that originated within the Mississippi Embay- Murphy et al., 2014), and they were not observed microscopi- ment, whereas the high topography area immediately to the west cally in the micro-drilled samples used in the present study. Fur- and northwest of the GRB received precipitation via winter storms, thermore, detrital marine carbonate grains would be most likely producing perennial snowpack (Sewall and Sloan, 2006). The pres- to reach the basin-center position of the study core during lake ence of high-altitude snowpack in this region is further supported lowstands, carried across the exposed lake plain by streams. The by extremely depleted oxygen isotope values from aragonite mol- 87 86 Sr/ Sr composition of these grains should reflect less radiogenic lusk fossils and GRF lacustrine carbonates (Norris et al., 1996; values typical of sea water, within the range of 0.7070 to 0.7095 Dettman and Lohmann, 2000). Community Climate System Model (Bataille and Bowen, 2012). In stark contrast, lowstand carbon- (CCSM) results (Huber and Caballero, 2003)demonstrate a strong ate facies of the WPM are the most radiogenic reported in this coherence between reconstructed Eocene ENSO index and monthly study, thus the presence of detrital marine carbonate grains would temperature in the GRB drainage basin, suggesting that ENSO- strengthen rather than weaken our interpretation of pronounced driven intervals of enhanced warmth facilitated greater snowpack changes in lake water 87Sr/86Sr. meltwater delivery (with a low 87Sr/86Sr) to Lake Gosiute. A con- Finally, the close correlation between observed 87Sr/86Sr and nection between ENSO and lake level is further supported by oil yield (Figs. 2, 3) might suggest contamination of the carbon- varve thickness analysis of lacustrine sediments from Lake Gosiute, ate fraction by Sr associated with organic material that is leached which reveals periodicities consistent with reconstructed ENSO in- during sample digestion. The strontium concentration in kerogen dex (Ripepe et al., 1991; Huber and Caballero, 2003). and petroleum is low however, while both the Rb/Sr (concen- Milankovitch-forcing of mean ENSO state has been previously tration) ratio and 87Rb/86Sr ratio are high (e.g., Bonham, 1956; identified for the Quaternary, although there exists debate about Bing-Quan et al., 2001). Thus, the low Rb concentrations and the precise mechanism responsible for this forcing (Clements et 87Rb/86Sr measured in the carbonate fractions analyzed in this al., 1999; Timmermann et al., 2007; Leduc et al., 2009; Wolff study indicate that any contribution from organic material is min- et al., 2011; Salau et al., 2012; Koutavas and Joanides, 2012; imal. Sadekov et al., 2013; Carré et al., 2014; Staines-Urías et al., 2015; We therefore conclude that lake highstands occurred in re- Ford et al., 2015). Using the modeling results of Clements et al. sponse to increased rates of runoff from areas to the west of the (1999), a modern orbital configuration yields a 500 year aver- ◦ ◦ GRB, relative to runoff from the east, and that these changes in age NINO3 index of ∼0.4 C, with values ranging from −0.1 C ◦ the hydrologic cycle were a consequence of Milankovitch-forcing to 0.9 Cduring the past 150 ka. The Eocene CCSM ENSO simu- (see also Fig. S2). The large magnitude of the 87Sr/86Sr signal ob- lations (Huber and Caballero, 2003)also use a present day orbital M. Baddouh et al. / Earth and Planetary Science Letters 448 (2016) 62–68 67 configuration, representing a rather unusual condition, since the Acknowledgements amplitude envelope of precession is presently within a “node” due to the 405 ka and 2.4 myr amplitude modulation terms associ- Financial support was provided by the Donors of the Petroleum ated with eccentricity (additionally, obliquity is at an intermediate Research Fund of the American Chemical Society, the Center for Oil value; Laskar et al., 2004). Thus, the CCSM results (Huber and Shale Technology and Research (COSTAR), the Geoscience depart- Caballero, 2003)are expected to represent an intermediate ENSO ment at the University of Wisconsin-Madison, NSF-EAR 1422819 forcing, which should be substantially amplified or diminished at (ARC), NSF-EAR 1151438 (SRM), and NSF-ATM 0081852 (CMJ). The times, given the range of expected precession and obliquity am- authors thank the USGS Core Research Center (Denver, Colorado) plitudes. Of note, the modern orbital configuration used in Huber for providing core samples for this study. We are grateful for con- and Caballero (2003) does not elicit a strong coupling between structive reviews provided by three anonymous referees, which reconstructed ENSO index and precipitation in the GRF drainage improved the quality of this manuscript. basin (unlike temperature), but this does not preclude such a cou- Appendix A. Supplementary material pling during other configurations, which could serve to augment the snowpack melt contributions noted above. Supplementary material related to this article can be found on- In contrast to the ENSO-based model introduced in this study, line at http://dx.doi.org/10.1016/j.epsl.2016.05.007. the new strontium isotope data exclude the hypothesis that lake highstands occurred in response to strengthening summer mon- References soons and resultant transport of moisture from the Mississippi Embayment. According to high-resolution regional climate model- Aswasereelert, W., et al., 2013. Basin-scale cyclostratigraphy of the Green River For- ing (Sewall and Sloan, 2006), moisture derived from monsoonal mation, Wyoming. GSA Bull. 125, 216–228. Bataille, C.P., Bowen, G.J., 2012. Mapping 87Sr/86Sr variations in bedrock and water flow from the Mississippi Embayment would have mostly precip- for large scale provenance studies. Chem. Geol. 304–305, 39–52. itated on the front ranges east of Lake Gosiute, where it would Beard, B.L., Johnson, C.M., 2000. Strontium isotope composition of skeletal material have acquired relatively high 87Sr/86Sr via weathering of Precam- can determine the birth place and geographic mobility of humans and animals. brian rocks and associated detritus. The above observations are J. Forensic Sci. 45 (5), 1045–1061. Bing-Quan, Z., Jing-Lian, Z., Xiang-Lin, T., Xiang-Yang, C., Cai-Yuan, F., Ying, L., Ju- also inconsistent with explanations that rely solely on changing lo- Ying, L., 2001. Pb, Sr, and Nd isotopic features in organic matter from China 87 86 cal evaporation rates (Morrill et al., 2001), because Sr/ Sr is not and their implications for petroleum generation and migration. Geochim. Cos- significantly fractionated during evaporation/precipitation, or dur- mochim. Acta 65, 2555–2570. ing the formation of carbonates phases (Capo et al., 1998). Bohacs, K.M., Carroll, A.R., Neal, J.E., Mankiewicz, P.J., 2000. 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