Three Millennia of Southwestern North American Dustiness and Future Implications

Abstract

Two sediment records of dust deposition from Fish Lake, in southern Colorado, offer a new perspective on southwest United States (Southwest) aridity and dustiness over the last ~3000 years. Micro scanning X-ray fluorescence and grain size analysis provide separate measures of wind-deposited dust in the lake sediment. Together these new records confirm anomalous dustiness in the 19th and 20th centuries, associated with recent land disturbance, drought, and livestock grazing. Before significant anthropogenic influences, changes in drought frequency and aridity also generated atmospheric dust loading. Medieval times were associated with high levels of dustiness, coincident with widespread aridity. These records indicate the Southwest is naturally prone to dustiness. As global and regional temperatures rise and the Southwest shifts toward a more arid landscape, the Southwest will likely become dustier, driving negative impacts on snowpack and water availability, as well as human health.

Fig 1. Study site location and photograph.

The map shows Fish Lake (star) located in the South San Juan Mountains, Colorado, and the Colorado Plateau dominant dust source region highlighted in grey. The base map digital elevation model was obtained through the open source USGS National Map Viewer: http://viewer.nationalmap.gov/viewer/. The photograph of Fish Lake was taken in July 2009, looking to the North West down the small inlet stream. The outlet channel cuts through the rock bank on the left side of the photograph. Photograph credit Cody Routson.

Fig 2. Age depth chronology.

Age depth chronology based on ²¹⁰Pb dates on the upper 9 cm (light blue), and ¹⁴C dates (dark blue). The red line shows the best fit of 1000 age models and 1-sigma errors are shown in the dotted grey lines. The age depth model and figure were generated using the Bayesian age-depth modeling program BACON [36].

Fig 3. Grain Size.

Grain size distributions of dust off snow (red) and Fish Lake sediment (black). The distribution of sediment grain sizes is an average of 337 down-core sediment samples.

Fig 4. Down core particle size fractions.

The dust grain sizes in the sediment core were characterized as those smaller than or equal to 15.1μm (fine silt).

Fig 5. Dust versus bedrock geochemistry.

Histograms showing the frequency of elemental abundances characterized using μXRF counts. Counts increase to the right on the x-axis. Note the x-axis scale differs between plots. (A) Titanium has similar abundances in local bedrock and in windblown dust. (B) Potassium has slightly higher abundance in windblown dust. (C) Calcium has moderate to high abundance in windblown dust and much higher abundance in local bedrock. (D) Strontium has slightly higher abundance in local bedrock than windblown dust, but counts were too low to measure in the sediment reliably.

Fig 6. Elemental mixing model.

Elemental μXRF ratio/ratio end-member mixing model including titanium, potassium, and calcium, showing sediment (black) distributed between bedrock (blue) and windblown dust (red) end-members. Points in the mixing model represent individual μXRF measurements taken at hundreds of different locations on pulverized dust and bedrock samples and the entire length of the sediment core, measured at 25-micron resolution.

Fig 7. Elemental abundances.

Down core μXRF elemental abundances including Ti, K, and Ca counts, and Ti/Ca and K/Ca elemental ratios.

Fig 8. Comparison of Fish Lake dust records with regional drought indicators.

(A) Fish Lake grain size dust record with (B), μXRF dust record. (C) Composite Fish Lake dust record (combined particle size and μXRF estimations). Unsmoothed data are shown in grey, and the median age-depth ensemble is shown in black, with dark and light blue bands showing the associated 1 and 2 sigma error bands respectively. (D) Southwestern PDSI [48], smoothed with a 70-year cubic smoothing spline. (E) Dune deposition dates from the Great Sand Dunes National Park (diamonds) [24], and dune (squares) and loess (circles) activity dates from the Great Plains [26]. The y-axis on panel E has no units: the dates are distributed vertically so they don’t obscure one another. Vertical grey bars denote intervals of Roman and medieval aridity. Vertical blue bars show intervals of more stable Southwestern PDSI.