Holocene chronostratigraphy and paleoclimate implications of dune fields across southern Utah

Sand dunes are threshold systems capable of recording past changes in effective moisture and aridity, and sensitive to sustained, decadal-scale drought. Records of past drought can provide a reference to past climate variability and expected extremes in an area. Such records provide data for climate modeling refinement, and regional and community adaptive planning. We use optically stimulated luminescence (OSL) dating and radiocarbon ages from charcoal to investigate eolian activity in two dune fields in southern Utah, located 250 km apart in the central Colorado Plateau. Dune forms in this semi-arid region have been mapped and auger samples obtained from mapped units, typically to depths of 3.5 m with two OSL samples from each core. With OSL ages, chronostratigraphic reconstructions are based on grain size analysis, geochemistry, organic and magnetic properties, and identification of paleosols (representing periods of dune stability). Kanab dune field, located 13 km northwest of Kanab, Utah, covers approximately 12.5 km2 and is oriented WSW with active and stabilized parabolic dune forms and sand sheets. Initial OSL results from the windward (presumed older) end of the dune field have identified at least four periods of dune activation between 8 and 0.7 ka. San Rafael Desert dune field, located approximately 25 km southwest of Green River, Utah is composed of multiple dune fields encompassing a greater area (\u3e2500 km2). Linear sand ridges, oriented NNE dominate here, with barchanoid and parabolic dunes present. Preliminary OSL ages identify at least five periods of dune formation between 13 and 0.5 ka. We interpret coeval dune activity the dune fields to represent persistent (at least decadal scale) regional droughts. Chronologically distinct patterns of dune activity are interpreted to reflect local disturbances impacting vegetative cover. Further analyses will refine chronologies of both sites for regional comparison and with existing Holocene climate records

Holocene Chronostratigraphy of Dune Fields in Southern Utah: Geomorphic Record of Past Aridity in the Central Colorado Plateau

The southwestern United States is characterized by dry climate, and droughts are common. The region is currently in an extreme drought that began in 2000 CE and has lasted longer than any previous drought in at least 500 years. Models predict greater future climate extremes under human-caused climate change. Understanding of the natural range of climate variability is important to put these changes in context. Sedimentary archives of past sand dune activity can help extend the available instrumental observations (last century) and tree-ring records (last millennium). Sand dunes are landforms that are sensitive to aridity and decreased vegetation cover. They can reactivate and migrate downwind during periods of aridity leaving behind a sediment record of past dune field activity. Research goals were to reconstruct past dune activity the Kanab and San Rafael dune fields in southern Utah. This was accomplished by mapping the dune forms and sampling for age control and sediment character. Modern wind data were compared to the orientation of the older dune forms to determine if wind directions have changed. The Kanab dune field, in southwestern Utah, was found to have been active five times over the last 10,000 yr. The data indicate 1000 yr-long periods of activity separated by similar duration periods of stability. The San Rafael dune field, in east central Utah contained records of seven episodes of eolian activity from late Pleistocene (~17,000 years ago) to the present. Dunes are active today in this dune field with thinner dune deposits and sparser vegetation suggesting greater wind erosion than in the Kanab dune field. Comparing records between the two dune fields indicates three time periods when they were both actively migrating, suggesting regional aridity ~9,500-7,500, 2000-1500 and 1000-500 years ago. Expanding this comparison to existing sand dune records across the Colorado Plateau suggests at least these three periods, and as many as five periods of regional aridity, may have occurred in the last 10,000 years. Wind and geochemical data indicate similar wind and source sediments have been active over the history of the dune fields

Sampling Methods for Luminescence Dating of Subsurface Deposits from Cores

Study of subsurface deposits often requires coring or drilling to obtain samples for sedimentologic and geochemical analysis. Geochronology is a critical piece of information for stratigraphic correlation and rate calculations. Increasingly, luminescence dating is applied to sediment cores to obtain depositional ages. This paper provides examples and discussion of guidelines for sampling sediment core for luminescence dating. Preferred protocols are dependent on the extraction method, sedimentology, core integrity, and storage conditions. The methods discussed include subsampling of sediment in opaque core-liners, cores without liners, previously open (split) cores, bucket auger samples, and cuttings, under red lighting conditions. Two important factors for luminescence sampling of sediment core relate to the integrity of the natural luminescence signal and the representation of the dose rate environment. The equivalent dose sample should remain light-safe such that the burial dose is not reset (zeroed) by light exposure. The sediment sampled for dose rate analyses must accurately represent all units within at least 15 cm above and below the equivalent dose sample. Where lithologic changes occur, units should be sampled individually for dose rate determination. Sediment core extraction methods vary from portable, hand-operated devices to large truck- or vessel-mounted drill rigs. We provide recommendations for luminescence sampling approaches from subsurface coring technologies and downhole samplers that span shallow to deep sample depths

Reply to Simon and Reed: Independent and Converging Results Rule Out Historic Disturbance and Confirm Age Constraints for Barrier Canyon Rock Art

We welcome this further discussion of our results on the age of the Great Gallery rock art in the Canyonlands of Utah. The comment by Simon and Reed (1) focuses on just one of the three components of our study (2), which is presented in greater technical detail in ref. 3 and is surely our best-constrained and least-surprising result: the dating of a rock-fall that removed some of the art and thus provides a minimum age. Simon and Reed (1) point out that the Great Gallery panel is not pristine and relate the sordid human history of visitation and possible disturbance to the site. Indeed, being aware of this during our research, one of our initial hypotheses was that the rock fall may be historic. Despite the possibility of recent disturbance to some of the talus boulders, our results document that the rock fall occurred ∼900 y ago, and for the boulder we sampled a scenario of historic disturbance and exposure such as postulated by Simon and Reed (1) can be ruled out

Sampling Methods for Luminescence Dating of Subsurface Deposits from Cores

Study of subsurface deposits often requires coring or drilling to obtain samples for sedimentologic and geochemical analysis. Geochronology is a critical piece of information for stratigraphic correlation and rate calculations. Increasingly, luminescence dating is applied to sediment cores to obtain depositional ages. This paper provides examples and discussion of guidelines for sampling sediment core for luminescence dating. Preferred protocols are dependent on the extraction method, sedimentology, core integrity, and storage conditions. The methods discussed include subsampling of sediment in opaque core-liners, cores without liners, previously open (split) cores, bucket auger samples, and cuttings, under red lighting conditions. Two important factors for luminescence sampling of sediment core relate to the integrity of the natural luminescence signal and the representation of the dose rate environment. The equivalent dose sample should remain light-safe such that the burial dose is not reset (zeroed) by light exposure. The sediment sampled for dose rate analyses must accurately represent all units within at least 15 cm above and below the equivalent dose sample. Where lithologic changes occur, units should be sampled individually for dose rate determination. Sediment core extraction methods vary from portable, hand-operated devices to large truck- or vessel-mounted drill rigs. We provide recommendations for luminescence sampling approaches from subsurface coring technologies and downhole samplers that span shallow to deep sample depths