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Geology; March 2009; v. 37; no. 3; p. 255-258; DOI: 10.1130/G25195A.1
© 2009 Geological Society of America
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Climate control on Quaternary coal fires and landscape evolution, Powder River basin, Wyoming and Montana

Catherine A. Riihimaki1,*, Peter W. Reiners2 and Edward L. Heffern3

1Biology Department, Drew University, Madison, New Jersey 07940, USA
2Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA
3U.S. Bureau of Land Management, 5353 Yellowstone Road, Cheyenne, Wyoming 82009, USA

Correspondence: *E-mail: criihimaki{at}drew.edu.


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 POWDER RIVER BASIN
 METHODOLOGY
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES CITED
 
Late Cenozoic stream incision and basin excavation have strongly influenced the modern Rocky Mountain landscape, but constraints on the timing and rates of erosion are limited. The geology of the Powder River basin provides an unusually good opportunity to address spatial and temporal patterns of stream incision. Numerous coal seams in the Paleocene Fort Union and Eocene Wasatch Formations within the basin have burned during late Cenozoic incision, as coal was exposed to dry and oxygen-rich near-surface conditions. The topography of this region is dominated by hills capped with clinker, sedimentary rocks metamorphosed by burning of underlying coal beds. We use (U-Th)/He ages of clinker to determine times of relatively rapid erosion, with the assumption that coal must be near Earth's surface to burn. Ages of 55 in situ samples range from 0.007 to 1.1 Ma. Clinker preferentially formed during times in which eccentricity of the Earth's orbit was high, times that typically but not always correlate with interglacial periods. Our data therefore suggest that rates of landscape evolution in this region are affected by climate fluctuations. Because the clinker ages correlate better with eccentricity time series than with an oxygen isotope record of global ice volume, we hypothesize that variations in solar insolation modulated by eccentricity have a larger impact on rates of landscape evolution in this region than do glacial-interglacial cycles.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
POWDER RIVER BASIN
 METHODOLOGY
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES CITED
 
Climate may be the dominant factor affecting landscape evolution during the late Cenozoic (Molnar and England, 1990), but erosional landscapes that have dominated most continents since ca. 4 Ma (Zhang et al., 2001) tend to lack geomorphic features that can be precisely and accurately dated, and that can be readily interpreted in the context of climate controls on landscape evolution. The modern Rocky Mountain landscape has developed through extensive stream incision since the Pliocene (McMillan et al., 2006), but constraints on the timing and rates of erosion are limited. Volcanic ash deposits in river terraces provide snapshots of former landscapes (Dethier, 2001), but these layers are rare and become degraded through time. Cosmogenic exposure ages of terrace gravels tend to have large error bars, particularly for early to mid-Quaternary terraces, because episodic erosion and deposition of loess confound interpretation of cosmogenic nuclide concentrations (Hancock et al., 1999). Nevertheless, rates of Quaternary stream incision in the central Rocky Mountains have been described as climatically controlled, with highest incision rates during interglacial periods and strath terrace formation from lateral migration of streams during glacial periods (e.g., Hancock and Anderson, 2002).

The Powder River basin, Wyoming and Montana (Fig. 1), provides an opportunity to collect a high density of landform ages that can test the impact of Quaternary climate fluctuations on landscape evolution. Late Cenozoic stream incision and hillslope erosion have exhumed Paleogene coal layers, which burn when exposed to near-surface conditions, through spontaneous combustion, lightning strikes, or heating from surface wildfires (Coates and Heffern, 1999; Heffern and Coates, 2004; Lyman and Volkmer, 2001). Coal layers that are still below the water table or are buried deeply enough to be poorly ventilated do not burn, and modern burn fronts are typically within ~30 m of the surface (Heffern et al., 2007). Coal burning bakes overlying strata into a metamorphic rock called clinker, which is relatively resistant to erosion and typically forms escarpments, plateaus, and terraces. If the ages of clinker formation, and thus coal burning, can be constrained, they can be used to understand the frequency of coal fires and the rates and patterns of erosional landscape evolution. Dating with precision necessary for geomorphic applications requires thermochronometric techniques that record the timing of heating by coal fires and are insensitive to protolith or detrital ages. Fission-track age determinations on 38 zircon (ZrSiO4) samples from the Powder River basin (Coates and Heffern, 1999; Coates and Naeser, 1984; Heffern et al., 1993) and one from the Williston basin demonstrate regional variability of clinker ages, but the uncertainties on the ages are large (typical 2{sigma} of 30%–60%).


Figure 01
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  		Figure 1. Zircon He dating sample locations on shaded relief map of Powder River basin. Symbols for sample sites are colored based on zircon He age of local clinker (m.a.s.l.—meters above sea level).

 
Radiogenic growth of He from decay of U and Th in zircon grains provides a thermochronometric dating system (zircon He dating) that records ages since cooling below temperatures at which He diffusivity out of zircon becomes very slow. Both laboratory and field experiments suggest that in most cases, the zircon (U-Th)/He system has a nominal closure temperature of ~180 °C (Reiners, 2005; Reiners et al., 2004; Stockli, 2005), though this may vary for zircons with extremely high or low extents of radiation damage (Farley, 2007; Nasdala et al., 2004; Reiners, 2005). Zircon He dating provides a useful measure of the age of clinker formation and coal burning because detrital zircons in clinker are completely reset (by He loss) if they are sufficiently close to (typically within meters of) the coalbed. Other thermochronometers could alternatively be used, but zircon (U-Th)/He dating is preferred because in most cases zircon grains are abundant in the sandstones, they have U-Th concentrations high enough to yield sufficiently precise ages, and their He ages are completely reset in most thick clinker units. The precise kinetics of He diffusion are not critical for this study because incomplete resetting is easily detected in these rocks as irreproducible old ages. In this application, analytical precision of most zircon He ages is ~4% (2{sigma}), but observed reproducibility is typically ~8%–10%, which is nonetheless much better than that achievable through zircon fission-track dating of samples this young.

Here we present 55 zircon He ages from the Powder River basin, representing the greatest density of Quaternary landform ages in the central Rocky Mountains. The data represent a novel application of the (U-Th)/He radioisotopic method, which is more commonly used to assess long-term exhumation rates of landscapes. The precision of the ages allows quantitative assessments of the relationship between coal fires and paleoclimate conditions. Our results indicate that coal fires, and hence landscape evolution in the Powder River basin, are strongly affected by climate. Moreover, we find that direct solar forcing modulated by variations in the eccentricity of Earth's orbit may be more important than glacial-interglacial cycles on the evolution of this landscape.


   POWDER RIVER BASIN
TOP
 ABSTRACT
 INTRODUCTION
 POWDER RIVER BASIN
METHODOLOGY
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES CITED
 
The Powder River basin (Fig. 1) is a structural syncline and topographic lowland between the Bighorn Range to the west, the Black Hills to the east, and the Hartville uplift, Laramie Mountains, and Casper arch to the south. Near-surface lithologies comprise Late Cretaceous–Paleogene sediments eroded from adjacent highlands and deposited in the basin during the Laramide orogeny (Dickinson et al., 1988). Low-grade (subbituminous) coal is common in the upper Paleocene and lower Eocene sections throughout the basin (the Fort Union and Wasatch Formations, respectively). Several relatively thick coal seams (commonly 20 m) are present in this stratigraphic interval; the thickest (to 40 m) and most laterally continuous is associated with the Wyodak-Anderson and Knobloch coal zones (Averitt, 1975; Flores, 1999), which actually comprise many different coal beds that split, merge, and pinch out over a variety of scales (Goolsby and Finley, 2000). Above these zones are the Felix and Lake DeSmet coal zones, important both as coal resources and clinker-forming units.

Through the late Cenozoic, the Powder, Tongue, and Cheyenne River drainage networks have partially exhumed the basin, with river incision of 500–1000 m into the sedimentary strata (McMillan et al., 2006). Much of the basin's topographic relief is controlled by the distribution of clinker (Coates and Heffern, 1999). Most of the southern parts of the basin are characterized by broad rolling hills and flat-topped buttes capped by clinker, with relief typically <~200 m. Large (~200 m) clinker-capped escarpments are present in some areas, such as the Rochelle Hills on the eastern side of the basin, where the Wyodak-Anderson coal reaches the surface and a large clinker escarpment has formed. In the central and northern parts of the basin, a stratigraphy of generally flat-lying clinker beds creates flights of terraces that ascend to clinker-capped plateaus or ridges, with local relief of ~200–400 m.


   METHODOLOGY
TOP
 ABSTRACT
 INTRODUCTION
 POWDER RIVER BASIN
 METHODOLOGY
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES CITED
 
We collected 71 samples from clinker outcrops during field seasons from 2003 to 2006. Baked sandstones and siltstones provided the most suitable sources for zircon He dating. Our sample collection focused on the western, northeastern, and southeastern parts of the basin, because coal layers are near the surface in these areas and are sufficiently thick to have created clinker through hot coal fires. In the central and southernmost parts of the basin, thin or steeply dipping coal beds have not produced significant clinker. Our sampling goal was to collect a geographically and geomorphologically diverse array of samples. Therefore we collected samples from a large portion of clinker outcrops in the basin (Fig. 1), and across a wide variety of geomorphic settings, including hillsides, terrace and plateau surfaces, and banks of ephemeral streams. We conducted sample transects along several streams, whereas other samples are from areas far (>10 km) from local base level.

Dated zircon crystals were selected from mineral separates prepared by standard techniques (see the GSA Data Repository1), and analyses were performed at Yale University and the University of Arizona. Weighted mean ages of 55 distinct clinker locations were determined from multiple (or in a few cases, single) aliquots from each location. Seven locations yielded partially reset and uninterpretable ages, and nine yielded no datable zircon crystals.

We statistically compare our clinker ages to paleoclimate records using a series of t-tests to assess whether coal fires occurred during particular climate regimes. Although there are numerous field studies constraining the timing of late Quaternary glacial advances in the central Rocky Mountains (Chadwick et al., 1997; Clark and Bartlein, 1995; Richmond, 1986), there are no high-resolution paleo-climate reconstructions for the region over the full duration of our data set, especially beyond 300 ka. Therefore, we compare clinker ages to two oceanic records as proxies for Quaternary climate conditions in the Powder River basin: a global benthic oxygen isotope stack showing global variations in ice volume (Lisiecki and Raymo, 2005), and a sea-surface temperature reconstruction for the eastern tropical Pacific Ocean using the Uk37 alkenone methodology (Lawrence et al., 2006). We also compare clinker ages to time series of Earth's orbital parameters that affect the seasonal and annual amounts of solar insolation (Berger and Loutre, 1991). In each t-test, we evaluate the average paleoclimate-proxy values when clinker formed and the average paleoclimate conditions since 1.15 Ma. A significant difference in these averages, as expressed in the variable t, would indicate that coal fires preferentially occurred during particular climate regimes. To account for uncertainty of the clinker ages, we use a bootstrap algorithm at 100,000 iterations to establish mean and standard deviations of t for each time series, given normal distributions of clinker ages about each nominal age.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 POWDER RIVER BASIN
 METHODOLOGY
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES CITED
 
Zircon He ages of in situ clinker outcrops range from 0.007 to 1.113 Ma (Fig. 2). The oldest ages are in the northern part of the basin, but otherwise there is no clear spatial pattern at a basin-wide scale (Fig. 1). At local (<~10 km) scales, older ages are generally found at higher elevations, reflecting increasing relief in the landscape, but spatial age patterns are often more complex. Younger clinker ages are more common than older ones (e.g., 50% of the samples are younger than 0.2 Ma), likely because long-term stream incision and lateral backwasting have eroded older outcrops. Periodic gaps in the probability density function of clinker ages (Fig. 2A) cannot be explained by a simple preservational bias to younger ages, but instead mimic fluctuations in the {delta}18O (Fig. 2B) and eccentricity records (Fig. 2C).


Figure 02
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  		Figure 2. Probability density function of clinker zircon He ages (black). A: With histogram of ages (gray). B: With time series of {delta}18O (gray). C: With eccentricity (gray). Distribution of clinker ages can be explained by combination of preservational bias toward modern and climate controls on the timing of coal fires. Clinker ages preferentially occur during times of low {delta}18O (low ice volume) and high eccentricity. Statistical tests and the absence of clinker ages during interglacial period at 0.4 Ma (shaded box) suggest that direct solar forcing may be more important to landscape evolution in the Powder River basin than are glacial-interglacial fluctuations.

 
Results of t-tests (Table 1) indicate a strong correlation between times of clinker formation and particular climate conditions. Times of high eccentricity provide the greatest correlation with the clinker ages (p = 0.002), followed by high sea-surface temperatures in the eastern equatorial Pacific (p = 0.031) and low benthic {delta}18O (i.e., low global ice volume) (p = 0.146). Because of uncertainty in individual clinker ages, t-values are close to zero for obliquity or precession, which, together with eccentricity, produce higher frequency variations in solar insolation. However, the sign (positive or negative) of each t-value is consistent with the hypothesis that most coal fires occurred during times of intense summer insolation, low global ice volume, and warm Pacific Ocean temperatures, with only a probability of 0.05 that the collective t-value signs are coincidental.


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  		TABLE 1. t-TEST ANALYSIS OF CLIMATE–COAL FIRE CORRELATION

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 POWDER RIVER BASIN
 METHODOLOGY
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES CITED
 
These results indicate a robust relationship between climate and landscape evolution, because clinker ages record coal burning that can only occur when fresh coal is exhumed to shallow depths by erosion, and because clinker formation has a strong effect on the spatial patterns of subsequent erosion. If the primary control on clinker formation is the exhumation of coal by stream incision, then our clinker ages directly record sensitivity of stream incision to climate change. If instead the primary control on clinker formation is the abundance of ignition sources (i.e., lightning strikes or wildfires) or the desiccation of coal seams, then our clinker ages instead record an indirect relationship between climate and landscape evolution, with climate affecting the frequency of coal fires, which in turn decreases the erodibility of clinker layers and slows local erosion rates.

The best statistical correlation between paleo-climate time series and the clinker ages comes from variations in solar forcing recorded in the eccentricity time series (Berger and Loutre, 1991), not from glacial-interglacial cycles reflected by the benthic {delta}18O record (Lisiecki and Raymo, 2005). This is counter to the interpretation that the presence or absence of ice in the headwaters of streams in the central Rocky Mountains dictates the rates and patterns of landscape evolution in basins downstream (Chadwick et al., 1997; Hancock and Anderson, 2002). In particular, the absence of clinker ages ca. 0.4 Ma contrasts with the {delta}18O record (Fig. 2B), because this interglacial period was comparable to or greater in magnitude than other interglacial periods since 1 Ma. However, this was a time period of unusually low eccentricity compared to other cycles (Fig. 2C). It is possible that this gap is the result of incomplete sampling in the basin, but we note that 20% of our ages are older than this time interval. Moreover, preservation of the oldest three ages in the data set can perhaps be explained by the unusually high eccentricity values ca. 1 Ma, which may have generated frequent coal fires that are now preserved in only a few uneroded sites. We also note that some coal fires appear to have occurred during peak glacial times more recently than 0.4 Ma, such as ca. 0.15 Ma, but that these tend to correspond with eccentricity values that are moderately high.

With this data set alone we cannot uniquely determine the mechanism by which climate affects the frequency of coal fires in the Powder River basin. The correlation of clinker formation with high eccentricity indicates that high seasonality may be the most important forcing for landscape evolution since 1 Ma in the Powder River basin. Glacial-interglacial cycles may be less important in this area given the distance (generally >100 km) of the sampled locations from the glaciated headwaters of streams in the Bighorn Mountains. We hypothesize that coal fires in the region preferentially occur during hot summers at times of high eccentricity (when the relative importance of precession is greatest) through three possible mechanisms. First, warmer summers might increase the frequency or intensity of surface wildfires that serve as ignition sources for the coal, through either drier conditions or more lightning associated with greater storminess. This hypothesis can be tested with improved records of Quaternary paleowildfire. Second, the depth of the groundwater table may vary depending on the aridity. Third, stream incision and hillslope erosion, which exhume coal beds to depths where they burn, may be higher during times of high seasonality and/or decreased glaciation in stream headwaters.

A preferential occurrence of natural coal fires during warm periods may be a heretofore-unrecognized feedback on the climate system through greenhouse gas emissions from the fires. Based on the thickness and abundance of clinker in the basin and erosion of clinker in the geologic past, the total amount of coal that has naturally burned in the basin is >43 x 109 t over the Quaternary, 1–2 orders of magnitude more than the amount of coal that has been mined from the region (Heffern and Coates, 2004). While we do not expect that the amount of coal burned in the Powder River basin alone is sufficient to significantly change greenhouse gas concentrations in the atmosphere, if other coal-rich areas undergo similar patterns of landscape evolution, natural burning of coal and peat swamps worldwide during relatively warm climates may create a small positive feedback on the climate system. Other regions in which natural coal fires have been recognized include Russia, Australia, and broader regions of western North America (Grapes, 2006). Further research on clinker ages in these regions may resolve the broader relationship between natural coal fires and Quaternary climate change.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 POWDER RIVER BASIN
 METHODOLOGY
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES CITED
 
Our middle to late Quaternary zircon He ages of clinker are consistent with a hypothesis that coal fires occur during times of high seasonality, possibly intense summers in particular, in the Powder River basin. The occurrence of coal fires in the region is associated with landscape evolution, with stream incision exposing coal to near-surface conditions where it can burn, and with metamorphism caused by coal fires changing the erodibility of the local bedrock. Therefore, our data provide direct evidence of the importance of climate in recent landscape evolution of the central Rocky Mountains foreland.

Additional statistical correlations between solar forcing and landscape evolution require suites of landform ages at sufficiently high precision to resolve eccentricity, obliquity, and/or precession cycles. Low-temperature thermo-chronology in certain geologic settings provides the ability to acquire numerous ages at greater precision than traditional dating methods allow. Similar studies of natural coal fires in broader coal-rich regions should improve our understanding of the connections between surface processes and climate.


   ACKNOWLEDGMENTS
 
Analytical assistance from Stefan Nicolescu, Chris Earnest, and James Holden is appreciated. This work was supported by National Science Foundation grant EAR-0518754. Field work was done with the assistance of Zoe Ruge, Camille Jones, Jessica Scheick, Kaia Davis, Jason Whiteman, William Reiners, and Paul F. Gore. Justin Revenaugh and Charles Lawrence suggested the statistical algorithms. We thank Bodo Bookhagen and Kurt Frankel for helpful reviews.


   FOOTNOTES
 
GSA Data Repository item 2009066, supplemental data for clinker ages and locations, and details of laboratory and statistical methodologies, is available online at www.geosociety.org/pubs/ft2009.htm, or on request from editing{at}geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301, USA. Back


   REFERENCES CITED
TOP
 ABSTRACT
 INTRODUCTION
 POWDER RIVER BASIN
 METHODOLOGY
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES CITED
 

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Received for publication 27 June 2008

Revised manuscript received 27 October 2008

Manuscript accepted 31 October 2008





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