Holocene chemostratigraphy of spring sediments in Range Creek Canyon,Utah,USA

Abstract

This study builds off the paleoclimatic reconstruction created by Hart et al. that used a multiproxy study to examine the role of moisture in the exodus of the Fremont from Range Creek Canyon in the 12th century. For this research, elemental ratios, weathering indices, and pollen data from two wetland spring sediment cores were used to compare with the existing Hart et al. paleoclimatic reconstruction (Objective 1). Elemental ratios and pollen data proved to be effective proxies for precipitation fluctuation, with the ratio of Pinus to Juniperus pollen representing effective moisture and increasing with the intensity of chemical weathering. Elemental data were additionally used to identify crypto tephra in the cores to validate Range Creek Canyon’s existing chronology (Objective 2). The XRF analysis of the sediment cores constrained the chronology of environmental change in the canyon by identifying the elemental signature of the Mazama eruption (7627 ± 150 cal. year BP). The concentration of Al, Y, and Ti were 50 times higher in this layer than elsewhere in the core, indicating a sudden depositional event, such as a volcanic eruption. Based on the multiproxy data and confirmed chronology, the Fremont entered the canyon during a period of elevated precipitation lasting until 600 AD. Precipitation levels remained steady until 1200 AD, after which precipitation levels decreased, causing drought conditions that coincide with the Fremont’s departure from Range Creek Canyon.

Keywords

Fremont paleoclimatology paleolimnology pollen tephra XRF

Introduction

For over a decade, anthropologists working at the University of Utah Range Creek Canyon field station have worked to identify factors driving the departure of the Fremont from Range Creek Canyon in the 12th century (Kloor, 2007; Rittenour et al., 2015). The Fremont were an indigenous pre-Columbian culture of hunters and farmers who lived between 200 AD and 1350 AD (Metcalfe, 2008; Rittenour et al., 2015). Fremont artifacts have been found in Utah, Idaho, Wyoming, Colorado, and Nevada (Barlow, 2002). Common characteristics of Fremont sites include representation of humans in prehistoric art, thin gray ceramics, baskets in a single-rod-and-bundle style, and moccasins derived from deer or sheep (Barlow, 2002). One of the largest sites of Fremont occupation is in Range Creek Canyon, located in southeastern Utah (Morris, 2010). Fremont occupation of Range Creek Canyon extended from 500 AD to 1350 AD, with peak occupation between 1000 and 1200 AD (Green, 2008; Spangler, 2004). Range Creek Canyon was privately owned from 1844 AD to 2003 when it was purchased by the University of Utah. The long history of private ownership allowed the canyon to avoid much of the degradation typical of archeological sites in Utah. Fremont sites found in the canyon consist of petroglyphs, pit houses, and stone granaries built into canyon walls (Metcalfe, 2008; Morris, 2010; Rittenour et al., 2015). No Fremont artifacts from Range Creek Canyon are dated past the 12th century, indicating the Fremont left the canyon after only centuries of occupation.

A leading theory behind the Fremont’s departure is that an environmental change made Range Creek Canyon no longer suitable for habitation. Modern Range Creek Canyon is a semiarid environment (Hart et al., 2021). A paleoclimatic reconstruction created by Hart et al. (2021) using pollen data indicated that the level of summer precipitation in Range Creek Canyon during early Fremont occupation was higher than modern summer precipitation levels. However, summer precipitation rates steadily decreased throughout mid to late Fremont occupation. Reduced moisture during the growing season may have limited the ability of the Fremont to irrigate crops, influencing their decision to leave the canyon.

Refining the paleoclimatic reconstruction created by Hart et al. (2021) with additional proxy would strengthen the hypothesis that the Fremont left Range Creek Canyon due to environmental change. For semiarid settings such as Range Creek Canyon, the extent of chemical weathering provides a useful climate proxy. Chemical weathering is dependent on meteoric precipitation, with increased precipitation resulting in more intense chemical weathering of clastic sediments (Nesbitt and Young, 1989). Creating a stratigraphic profile of chemical weathering throughout a Range Creek Canyon sediment core would provide further definition of the paleoclimatic record of the canyon.

Chronological control is another crucial element of paleoenvironmental reconstruction. Obtaining chronological control of Range Creek Canyon sediment cores has been challenging due to carbon contamination from local geological deposits (Rittenour et al., 2015). The paleoclimatic reconstruction created by Hart et al. (2021) utilized radiocarbon analyses on pollen concentrates to create an approximately 8000-year chronology. Collecting additional chronological data on Range Creek Canyon cores would allow for validation of the existing age model. Studies such as Balascio et al. (2015) have used elemental analysis to detect tephra from volcanic eruptions in sediment cores. Examining elemental data from Range Creek Canyon sediment cores to identify known tephra deposits would validate the existing ~8000-year chronology and provide further insight as to when the Fremont left Range Creek Canyon.

Objectives

The objectives of this study are to validate and compare with the paleoclimate reconstruction created by Hart et al. (2021) through a multiproxy analysis. This study will endeavor to:

Compare with the paleoclimatic reconstruction made by Hart et al. (2021) by creating a stratigraphic profile of the extent of chemical weathering. X-ray fluorescence (XRF) data were used to create a time series of the Chemical Index of Alteration (CIA) and the ratio of Al/Si to quantitatively show fluctuations in chemical weathering (Nesbitt and Young, 1982; Roy et al., 2008).

Validate the pollen-based time series created by Hart et al. (2021) by identifying crypto tephra, or tephra that are invisible to the naked eye, from the Mount Mazama eruption, dated at 7627 ± 150 cal. year BP (Davies, 2015; Zdanowicz et al., 1999).

Site description

Range Creek Canyon is a canyon on the Colorado Plateau near Price, UT. The Books Cliffs border the canyon to the south and east (Figure 1). The canyon is north-northwest trending with flat-lying bedrock (Rittenour et al., 2015). Elevations within the canyon range from 1290 to 3084 m. Range Creek Canyon is composed of two geologic formations that span the Late Paleocene to the Early Eocene (Nieminski and Johnson, 2014; Pitman et al., 1986). The lower unit, the Flagstaff Formation, is a fossiliferous lacustrine limestone. Overlying the Flagstaff Formation is the Colton Formation, a mix of interbedded quartzose sandstone and mudstone (Marcantel and Weiss, 1968). The erosional differences between the sandstone and mudstone of the Colton Formation created the steep canyon outcrops the Fremont utilized for granary placement (Metcalfe, 2008; Rittenour et al., 2015; Towner et al., 2009). Modern Range Creek Canyon has a semiarid climate, and a mean annual precipitation of 328.4 mm (Bares, 2014; Rittenour et al., 2015). Peak precipitation is derived from spring and late summer snowmelt, with the remaining precipitation from summer convective storms originating in the Gulf of Mexico (Bares, 2014).

Figure 1.

Overview map. Top right: location of Range Creek Canyon in Utah. Bottom right: a photo from the extraction of BSB14A from Billy Slope Bog. Left: map of known tephra locations for the Mazama eruption. The estimated extent of the Mazama eruption is pictured in gray, and sites where tephra has been located are represented by small black dots. The locations of Crater Lake (CL) and Range Creek Canyon (RCC) are symbolized by large black dots. Crater Lake is 1,092 km from Range Creek Canyon. Modified from Sarna-Wojicicki and Davis (1991).

The study site, Billy Slope Bog, is a wetland spring in Range Creek Canyon located at the base of the eastern canyon wall (110°12′51.893″W, 39°25′41.701″N). The site is 1862 m asl, and is above the elevation of Range Creek. The elevation change from the canyon wall to the lower alluvial valley creates a break in the slope of the water table where groundwater discharges directly to the surface at Billy Slope Bog. The groundwater seepage in Billy Slope Bog has caused a low-discharge perennial flow in the study area (Rittenour et al., 2015) which provided a depositional area with an undisturbed sediment record. The constant perennial flow in the Billy Slope Bog has promoted the growth of wetland vegetation as well as a mix of grasses (Poaceae) and sagebrush (Artemisia). Two modern forest communities reside around Billy Slope Bog. Higher elevations along the canyon walls contain Douglas-fir (Pseudotsuga menziesii), single-leaf pinyon (Pinus monophyla), and juniper (both Juniperus osteosperma and Juniperus scopulorum, referred to in this paper collectively as “Juniperus”). The lower elevation forest around Range Creek is a gallery forest populated by boxelder maple (Acer negundo) and narrowleaf cottonwood (Populus angustifolia) (Hart et al., 2021).

Methods

Fieldwork

In June 2009, a 5.10 m spring sediment core, BSB09B, was collected from Billy Slope Bog by researchers from the Records of Environment and Disturbance (RED) Lab at the University of Utah. The core was removed from the spring using a modified Livingstone piston corer. Core sediments demonstrated intact stratigraphy. The core was brought to the RED Lab, where it was stored, and refrigerated at 33°F (0.56°C). A second sediment core, BSB14A, was collected in October 2014. The 4.84 m core was extracted with a vibracorer and stored at the RED Lab. In June 2015, BSB14A was cut in half with a radial saw, and photographed. Core photos are included as Supplemental Figure 1. Color and lithology were recorded at this time.

Previous work by Hart et al. (2021)

Pollen samples for BSB09B were processed at 1 cm intervals for the first 1.3 m of the core by Hart et al. (2021). The samples were 1 cc in volume and processed using methods established by Faegri and Iverson (1989). Further details on pollen processing as well as identification can be found in Hart et al. (2021). The resulting pollen data were described in terms of ratios, which compares single, or multiple taxon of pollen to either another taxon, or a group of taxa. The ratio used in this paper is Pinus to Juniperus (P:J) (Hart et al., 2021). Pinus pollen represents dense forests in a moist climate, while Juniperus pollen is found in dry environments. The ratio, therefore, serves as a proxy for estimating effective moisture. Pollen data were also described in terms of total pollen influx (TPI, grains/cm2/yr), which is a measure of the entirety of pollen entering the catchment, and serves as a proxy for the density of vegetation and effective moisture.

Three additional pollen samples were taken from BSB09B at 1.47, 2.99, and 4.73 m by Hart et al. (2021) to create an age-depth model. The pollen samples were sent to the University of Georgia’s Center for Applied Isotope Studies for radiocarbon dating. The radiocarbon dates were then used by Hart et al. (2021) to generate an age-depth model using a smooth-spline interpolation in CLAM, a modeling package in R (Blaauw, 2010) (Figure 2). Further information regarding the age-depth model of BSB09B can be found in Hart et al. (2021). BSB14A was related to the BSB09B age-depth model by comparing the elemental stratigraphy of the two cores and lining up similar elemental trends.

Figure 2.

Age-depth model for Billy Slope Bog, Utah. Model created by Hart et al. (2021) using samples from BSB09B. The model was based on existing AMS and carbon isotope data. The top of the y-axis at zero represents the surface of Billy Slope Bog. Additional information can be found in Hart et al. (2021).

Elemental analysis for the chemostratigraphic profile (Objective 1)

In July 2015 elemental analysis of BSB14A was performed using a portable Bruker Tracer III-SD series pXRF spectrometer mounted on an MCS-100E Automatic 1 m Core Scanner, which over 12 h scanned every 2 mm of a 1 m core section. To fit the 4.84 m of BSB14A onto the core scanner, the core was cut into 1 m sections. Etnom Ultra-Polyester film was placed over the core to prevent drying and potential contamination of the pXRF spectrometer. BSB14A was first scanned at 40 keV and 30 µA, with a dwell time of 60 s and an added 1 mil Ti/12 mil Al filter to focus the results between Al and Ti on the periodic table. A second scan with no filter scanned all samples at 15 keV and 25 µA in August 2015. Unlike the previous trial, these trials incorporated a helium gas purge to replace the argon-rich air surrounding the x-ray tube and detector. This allowed the direct analysis of trace elements without the contaminating effect of the surrounding air. XRF photon scans were then translated into parts-per-million and weight percent data using a mudrock calibration created by Dr. Bruce Kaiser in May 2015 (B. Kaiser, personal communication). The calibration was created by scanning 40 mudrock samples with a known elemental composition on the spectrometer twice, once with the 1 mil Ti/12 mil Al filter and once with helium and no filter. Each scan was for a total of 180 s. Using the Excel macro S1CalProcess, a best-fit model was applied to these samples of known composition, resulting in a calibration for 1 mil Ti/12 mil Al filter scans and a calibration for helium scans.

Once the data were collected, element ratios were calculated. Element ratios were used to quantify fluctuations in chemical weathering, inferred to be from changes in precipitation, into the study catchment (Metcalfe et al., 2010; Roy et al., 2012). This interpretation assumes the ability to link the composition of elements derived from weathering to the surrounding bedrock. During physical, and chemical weathering, more soluble cations and, oxyanions such as Ca⁺², Na⁺, and K⁺ are leached, leaving the sediment relatively enriched in less soluble elements such as Al⁻³ and Ti⁻ (Nesbitt and Young, 1982; Roy et al., 2012; Taylor and McLennan, 1985). Therefore, in a sediment core, an increase in Al and Ti and a decrease in Ca, Na, or K would coincide with an increase in weathering and may indicate an increase in precipitation (Metcalfe et al., 2010). The elemental indices used to generate the chemostratigraphic profile of Range Creek Canyon were the Chemical Index of Alteration (CIA) and the ratio of Al/Si.

The CIA uses the equation CIA = (Al₂O₃/Al₂O₃ + CaO* + Na₂O + K₂O₃)*100 (Nesbitt and Young, 1982) and measures the degree of chemical weathering in clastic sediments. The CIA includes both immobile and mobile elements and is therefore ideal for analyzing detrital elements. The CIA assumes that the dominant process of chemical weathering is the degradation of feldspars into clay minerals. If there is largely unaltered feldspar, physical weathering is the dominant force behind erosion (Nesbitt and Young, 1982). The degree of chemical weathering in an area is dependent on both temperature and precipitation (Nesbitt and Young, 1989).

The viability of using the CIA as a paleoclimatic indicator has been called into question by multiple studies (Li and Yang, 2010; Wang et al., 2020; Xiao et al., 2010). In particular, Goldberg and Humayun (2010) state that the use of the CIA as a paleoclimatic indicator (referred to as a paleo-humidity in the study) can be limited by the presence of carbonate-rich sediments (with carbonate-rich meaning sediments with >30% carbonate content), post-depositional K additions, and by inheritance of clays from sedimentary rocks in the source area. The provenance of the sediments in BSB09B and BSB14A is the Colton Formation, a mix of interbedded quartzose sandstone and mudstone. As such, the sediments are not considered carbonate-rich, and have not undergone metasomatism, metamorphism, or diagenetic illitization to create post-depositional K additions. In addition, as the sediment provenance is consistent for both cores, changes in overall weathering patterns due to carbonate sourcing was not considered to be an issue that would affect the results of the XRF scanning. However, to adequately assess the level of clay content within the core sediments, a grain size analysis was performed on BSB14A. See Section 2.4 for details of the grain size analysis methodology.

In addition to the CIA, the element ratio Al/Si was also utilized to characterize the intensity of weathering in the study area, with increases in Al/Si corresponding to an increase in weathering (Drever, 2005). Precipitation changes alter the transport rate of weathered materials from their origin to a catchment. For example, increased rainfall can cause enhanced physical weathering that erodes, and transports chemically altered minerals that have accumulated on the landscape during a relatively dry period. Al is an insoluble hydrolyzate and is less susceptible to removal via weathering. Therefore, higher Al/Si indicates a period of intense weathering.

Grain size analysis

A grain size analysis was performed on BSB14A in 2015 by Dr. Richard Langford of the University of Texas in El Paso. About 1 g samples were taken from each centimeter of BSB14A, weighed, then placed into whirl pack bags. Samples were then placed in a beaker containing hydrogen peroxide and set on a hot plate until all organics burned off, indicated by a lack of foaming. Samples were then put through a 1000 µm sieve, washed with distilled water, and then analyzed with a Malvern Mastersizer 2000 Laser Diffraction Particle Size Analyzer.

Mount mazama cryptotephra identification (objective 2)

In October 2015, a tephra sample representative of the Mount Mazama eruption (dated at 7627 ± 150 cal. year BP in Zdanowicz et al. (1999) was taken near Jukebox Cave, Wendover (40.757148, −114.011614) and scanned six times with a Bruker Tracer III-SD series pXRF spectrometer. Three of the scans were performed with a 1 mil Ti/12 mil Al filter at 40.00 keV and 30.00 µA. The other three were scanned with a helium purge at 15.00 keV and 25.00 µA. The results of each scan type were averaged. Based on research from Balascio et al. (2015), the element examined as rhyolitic tephra markers were Ti, Mn, and Si. In Balascio et al. (2015), Al was also used as a potential identifier as it is common in rhyolite but was not detected in the rhyolitic tephra samples examined. While Balascio et al. (2015) used a detection time of 20 s, this study used a detection time of 60 s per sample, theorizing that the longer detection time would yield more accurate results.

Based on the Hart et al. (2021) chronology, evidence of the Mount Mazama eruption was hypothesized to be located at 4.445 m in BSB14A. From this core depth, a 1 cm sample was taken and placed on a smear slide. Silicone oil was added to the slide to identify glass shards, and then examined under a microscope, yielding sediments that were hypothesized to be glass shards. Five sediment samples were removed from the core, one representing the target area, and the other four were removed at 5 cm increments around the target area (4.540–4.448, 4.448–4.444, 4.444–4.440, and 4.440–4.390 m). The sediment samples were dried, disaggregated with a mortar, and pestle, and sieved to capture the 60–120 µm and the >120 µm fractions. Each sample fraction was rinsed until all runoff water was clear, after which it was treated with a dilute (10%) nitric acid in a sonic bath to eliminate carbonates. The sample was also treated with dilute (5%) hydrofluoric acid for 30 s in a sonic bath to clean clays off the glass shards. Finally, the sample was rinsed, dried in a low-temperature oven, and examined under a petrographic microscope.

Results/discussion

Chemostratigraphic climate reconstruction (Objective 1)

The first objective of this study was to use elemental analysis to compare with the pollen-based paleoclimatic reconstruction made by Hart et al. (2021) by creating a stratigraphic profile of the extent of chemical weathering. Proxies analyzed include the CIA and the ratio of Al/Si.

CIA values in BSB14A ranged from 4 to 74 (Figure 3a). When interpreting the CIA, values below 50 indicated fresh rock with no chemical weathering, values below 60 indicated low chemical weathering, and values between 60 and 80 indicated moderate chemical weathering (Nesbitt and Young, 1982; Roy et al., 2008; Figure 3). Over the ~8000-year period of deposition, 53% of the CIA values for BSB14A were below 50, and 89% of CIA values were below 60. The low CIA values indicate that sediments in Billy Slope Bog largely underwent little to no chemical weathering prior to deposition. Instead, physical weathering and internal organic production were the primary processes for sediment influx into Billy Slope Bog. The main period when the CIA indicates moderate or greater levels of chemical weathering is from ~2000 to 1250 cal. year BP (0–1250 CE). Figure 4 compares the elemental data with the pollen data from Hart et al. (2021). During the wet period identified from the elemental data (~2000–1250 cal. year BP) the pollen data also show the highest TPI. TPI represents the productivity or density of vegetation and is associated with wet conditions. In addition, the Pinus:Juniperus ratio also records wet conditions at this time, with relatively more Pinus present. A comparison of the CIA data with the pollen index data from Hart et al. (2021) shows that both records agree that ~2000–1250 cal. year BP was a long, wet period in Range Creek Canyon. The wetter-than-average conditions during this period overlap with early Fremont occupation of Range Creek Canyon, which peaked at ~1050 cal. year BP. It is possible that this wet period led to conditions that would have encouraged settlement in the canyon.

Figure 3.

Chemical Index of Alteration (CIA) and Al/Si time series for BBS14A. In Figure 3a (left), values below 45 on the CIA indicate fresh rock (yellow), CIA values between 45 and 60 indicate periods of low chemical weathering (blue), and CIA concentrations above 60 indicate periods of moderate chemical weathering (green). Shading has been added to better show the transition between fresh rock, low chemical weathering, and moderate chemical weathering. Figure 3b (right) demonstrates the changes in weathering intensity over time.

Figure 4.

Comparison of weathering indices and element ratios to pollen-based climate reconstructions. Stratigraphy of the chemical index of alteration ratio (CIA) and Al/Si ratio from the top of sediment core BSB14A to core depth 1.6 m (−50–2500 cal. year BP). Pollen analysis results from BSB09B are also displayed, including total pollen influx, and the ratio of Pinus: Juniperus.

In the Al/Si ratio (Figure 3b), higher values indicate more aluminum, representing a higher intensity of both physical and chemical weathering. The Al/Si ratio in BSB14A was higher than average from 4190 to 1170 cal. year BP and lower than average from 6990 to 4190 cal. year BP.

The majority of the CIA values are below the moderate threshold for chemical weathering activity (<60) (Figures 3a and 4). The low CIA values indicate that Range Creek Canyon had an arid climate with little chemical weathering for most of its history except for the period between ~2000 and 1250 cal. year BP. Low moisture would limit chemical weathering, leaving physical weathering as the dominant form of weathering in the catchment. However, in addition to the period ~2000–1250 cal. year BP, chemical weathering rates rose above 60 during two other periods, indicating periods of increased chemical weathering. At 6120–6047, and 7221–7092 cal. year BP, the CIA rises past 60. Both the increase in the CIA at 6120–6047 and 7221–7092 cal. year BP are very short. They are accompanied by a rise in Al/Si, representing an overall increase in weathering intensity. The rise in Al/Si coincides with a period of higher CIA at ~2000–1250 cal. year BP. All three instances indicate a shift toward a wetter climate and an increase in precipitation during these periods, consistent with the interpretation from Hart et al. (2021).

Grain size analysis results

The results of the grain size analysis indicate that the majority of BSB14A is composed of sand (Figure 5). The exception is between ~2100 and 1250 cal. year BP, where the dominant sediment size in BSB14A’s composition is silt. The sedimentological record of BSB14A further supports the conclusion that Range Creek Canyon had an arid climate with little chemical weathering for most of its history, with the exception of the period between ~2000 and 1250 cal. year BP. Between ~2000 and 1250 cal. year BP, chemical weathering increased due to a wetter climate, causing the corresponding shift from sand as the dominant grain size in BSB14A to silt during this period. The clay content of the core was minimal, with the majority of the core being under 10% clay. As such, the inheritance of clays from sedimentary rocks in the source area was not considered to have substantially affected the results of this analysis.

Figure 5.

Grain size fluctuation in BSB14A. The results of a grain size analysis performed by Dr. Richard Langford of the University of Texas in El Paso in 2015. From left to right, the results of the grain size analysis are shown in mean grain size (left), the weight percent of clay, silt and sand (right) throughout BSB14A.

Elemental identification of the mazama cryptotephra (Objective 2)

In Balascio et al. (2015), Mn, Si, Ti, and Al were chosen as elements representative of rhyolitic tephra. Based on Balascio et al. (2015), these elements were examined in BSB14A and the Mazama tephra standards. Mn and Si displayed no visible spikes in the lower half of BSB14A to indicate the presence of tephra (Figure 6). However, Ti displayed its largest concentration spike at 4.44 m, rising to 0.23 weight percent. The average Ti concentration was 0.16 weight percent, therefore the event at 4.44 m was 70% higher than average. In the same area, Al had an even larger spike at 4.39 m, reaching 6.00% concentration, while the average was 3.04%, 51% higher than average. Similar to the event at 4.39 m in BSB14A, in the scanned Mazama ash, the average Al percentage was also 6.00% (Table 1). Additionally, an extreme rise in Y concentrations was observed at 4.428 m. where Y ppm were 56.1 ppm, 10 times the average concentration of Y in the rest of BSB14A. Using Ti, Al, and Y, a target area was identified, and a sample was removed to identify the potential volcanic glass. The results were inconclusive, so five sediment samples were taken from the target area, and 5 cm intervals around the target area. All samples were examined under a petrographic microscope for volcanic glass shards. None were found.

Table 1.

Mazama ash elemental concentrations. Major and trace element concentrations for a Mazama Ash tephra sample on loan from the University of Utah volcanology lab.

Element	Value
Al (wt.%)	6.08
Si (wt.%)	29.72
S (wt.%)	0.54
K (wt.%)	2.41
Ca (wt.%)	0.65
Fe (wt.%)	1.65
Ti (wt.%)	0.26
Y (ppm)	15.95
Co (ppm)	9.05
Ba (ppm)	909.22
V (ppm)	47.15
Cr (ppm)	59.34
Mn (ppm)	638.39
Ni (ppm)	62.4
Cu (ppm)	140.48
Zn (ppm)	249.32

Figure 6.

Stratigraphies of Al, Ti, Y, Mn, and Si in BSB14A. Al, Ti, Y, Mn, and Si were hypothesized to be accurate indicators for the Mazama eruption. The gray area is where elemental spikes in several of the stratigraphies indicate Mazama Ash is presumed to be located.

Chronology of range creek canyon

Comparing the tephra spikes of Al, Y, and Ti within BSB14A to the age model created by Hart et al. (2021), the elements are generally located in the core section interpreted to be representative of Mazama ash. The Mazama eruption is dated at 7627 ± 150 cal. year BP (Zdanowicz et al., 1999). The Ti spike in BSB14A occurs at 7896 cal. year BP, the Al spike at 7760 cal. year BP and the Y spike at 7948 cal. year BP (Figure 6). Given some allowance for errors in the age model, these three elements are well within the range and timeframe to be considered representative of Mazama ash. The Y spike strongly indicates the presence of Mazama ash at this location. There are no substantial sources of Y in Range Creek Canyon. However, at 7948 cal. year BP, Y is 50 times higher than from any other depth of the core. Mn and Si, on the other hand, while recommended for tephra identification in Balascio et al. (2015) were not effective markers in this core. A contributing factor is that Billy Slope Bog is a silica-rich area. Therefore, the tephra signal for Si was difficult to distinguish against the background of heavy silica composition, making Si a poor element for identifying tephra in Range Creek Canyon.

While elemental data suggest the Mazama ash was identified, no volcanic glass shards were found with petrographic observation. Therefore, the site of Mazama ash in BSB14A cannot be unambiguously confirmed by this study. However, there are multiple reasons why glass shards might not have been found. The eruption occurred 1088 km from Range Creek Canyon, making any potential glass extremely small. Since deposition, the small fragments may have deteriorated into their elemental states. Therefore, it is not unreasonable the spikes in Al, Ti, and especially Y, signify the presence of Mazama ash. The lack of volcanic glass shards accompanying the tephra can be explained by degradation through weathering. In this case, the increase in these elements would be the only sign of the ash presence. The presence of Y likely confirms the location of the Mazama ash, which also validates the age-depth model of Billy Slope Bog.

Conclusion

The purpose of this study was to validate and further compare the climatic history of Range Creek Canyon using a proxy of allochthonous input. Elemental ratios were used as proxies for precipitation-induced erosion and weathering to further compare with the paleoenvironment during the Fremont occupation described using sedimentary pollen. Elemental data from a wetland spring sediment core were also used to validate the age model created by Hart et al. (2021).

Fluctuations in the CIA confirm the conclusions made in Hart et al. (2021) that Range Creek Canyon has been largely arid for the past 8000 years. The exception to the low rate of moisture availability in the canyon occurred from ~2000 to 1250 cal. year BP, where an increase in the intensity of chemical weathering indicates a corresponding increase in moisture availability that aligns with the wet period identified using pollen data from Hart et al. (2021). The increased moisture availability in Range Creek Canyon overlaps with the arrival of the Fremont in 1350 cal. year BP. Moisture levels in Range Creek Canyon steadily decreased after ~1200 cal. year BP, as evidenced by decreased CIA values. Therefore, the climate that was in place when the Fremont arrived in Range Creek deteriorated over their period of occupancy. The drought that started ~1200 cal. year BP escalated in intensity over time, potentially leading to their departure from the canyon.

The results of the cryptotephra analysis show a sharp increase in the elemental concentrations of Al, Y, and Ti between BSB14A core depth of 4.39–4.44 m. In particular, the concentration of Y at 4.428 m, an element that does not occur naturally in Range Creek Canyon, were 50 times higher at this core depth as compared to the rest of the core. Based on the age model created by Hart et al. (2021), this core depth corresponds to approximately 7700 cal. year BP, the same timeframe as the 7627 ± 150 cal. year BP Mazama volcanic eruption. The spikes in elemental concentrations at this core depth are thus Mazama cryptotephra, validating the accuracy of the Hart et al. (2021) age model. Physical volcanic glass shards were not identified in BSB14A. Future research will replicate the cryptotephra identification methodology outlined in this study in subsequent cores at the RED Lab to find physical volcanic glass shards to validate the use of portable XRF for cryptotephra identification.

Supplemental Material

sj-pdf-1-hol-10.1177_09596836231169987 – Supplemental material for Holocene chemostratigraphy of spring sediments in Range Creek Canyon, Utah, USA

Supplemental material, sj-pdf-1-hol-10.1177_09596836231169987 for Holocene chemostratigraphy of spring sediments in Range Creek Canyon, Utah, USA by Danielle Ward, Andrea Brunelle and Brenda B Bowen in The Holocene

Supplemental Material

sj-pdf-2-hol-10.1177_09596836231169987 – Supplemental material for Holocene chemostratigraphy of spring sediments in Range Creek Canyon, Utah, USA

Supplemental material, sj-pdf-2-hol-10.1177_09596836231169987 for Holocene chemostratigraphy of spring sediments in Range Creek Canyon, Utah, USA by Danielle Ward, Andrea Brunelle and Brenda B Bowen in The Holocene

Footnotes

Acknowledgements

We thank B. Kaiser for technical support during the XRF measurements and for developing a tailored calibration dataset, B. Nash (University of Utah) for supplying tephra samples, R. Langford for performing the grain size analysis, and the staff at Range Creek Canyon for helping with fieldwork.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the University of Utah Geography Department, Global Change and Sustainability Center, which provided a research grant.

ORCID iDs

Danielle Ward

Andrea Brunelle

Supplemental material

Supplemental material for this article is available online.

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