Mapping composition in dust-producing regions: Extending geochemical measurements over large scales with remote sensing
Abstract
Dust is a critical, yet understudied, component of geochemical cycles. Dust inputs to the ocean have been deemed especially significant in the iron cycle, for instance, since dust stimulates natural iron fertilization and thus potentially affects climate. Dust may also be important to the cycles of other elements, such as calcium (Ca). To determine the importance of dust relative to other geochemical inputs, some estimate must be made of the mass flux of Ca from dust-producing regions. However, understanding the spatial distribution of Ca within terrain known to generate dust has been problematic. Remote sensing may offer a critical perspective with measurement at landscape scales instead of extrapolation from a few point measurements, allowing for investigation closer to the scale at which dust is produced. This study investigates the elemental geochemistry of surface sediments and the distribution of non-silicate bound Ca in a large playa system (Black Rock Desert) in northwestern Nevada (USA). We used satellite-derived hyperspectral data, field-collected ground spectra, surface sediment samples, and Synthetic Aperture Radar (SAR) in our analysis. We collected approximately 50 samples and their spectra from the Black Rock Desert in July/August 2007. We measured the bulk and soluble Ca by ICP-OES and mineralogy by x-ray diffraction (XRD). Assuming all Ca in weak HCl (0.5 N) leaches is derived from calcite, we found that calcite concentrations in a subset of collected samples range from 5 to 15 wt. %. Acid-soluble calcite, which was generally 90 to 100 mol % of the total Ca in sediments (determined by lithium metaborate fusion), correlates well with the relative abundance of calcite determined by XRD analyses. Using hyperspectral measurements of surface reflectance, we found that acid soluble Ca concentration is correlated with the depth of a calcite absorption feature at 2335 nm. We then mapped this depth in satellite hyperspectral data from the Hyperion instrument to estimate calcite content of surface sediments at the playa scale. We calibrated absolute calcite composition by correlating depth of the calcite feature to calibration ground spectra, and ground spectra to acid soluble Ca fraction. We also found that in an areal mixture, calcite fit values range from 0 to 25% surface coverage, consistent with acid soluble Ca fractions. Additionally, the mineral content of playa surfaces may be useful as an indicator of processes responsible for generating variations in surface texture, in particular the precipitation of evaporite minerals related to the movement of water. We examined radar data from the ENVISAT Advanced Synthetic Aperture Radar instrument as a proxy for surface texture and relate surface textures in the Black Rock Desert to mineral composition. By elucidating some of the connections between the physical and chemical properties of playa sediments, this work serves as a foundation to predict the effects of changing climatic conditions on dust and to extend observations of modern systems to the geologic past and future.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMEP21A0567T
- Keywords:
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- 1029 GEOCHEMISTRY / Composition of aerosols and dust particles;
- 1631 GLOBAL CHANGE / Land/atmosphere interactions;
- 3311 ATMOSPHERIC PROCESSES / Clouds and aerosols