T165 Using Buried Soils to Reconstruct Past Climates: Opportunities and Considerations (Posters)

Tuesday, 6 November 2012
9:00 AM - 6:00 PM
Hall B

IDENTIFYING AND QUANTIFYING GOETHITE AND HEMATITE IN ATMOSPHERIC DUST AND SOURCE SEDIMENTS BY MAGNETIC METHODS

Paper number: T165-5
Program: Technical Sessions
Time: 9:00 AM – 6:00 PM

Authors

Abstract

Ferric oxide minerals in atmospheric dust are important components of dust because they can affect phytoplankton productivity in the oceans, influence atmospheric temperatures, and reduce snow albedo. In particular, hematite (α-Fe2O3) and goethite (α-FeOOH) are strong absorbers of sunlight in the visible and near-infrared parts of the solar spectrum, and even in low abundances (< few wt%) can influence the radiative properties of mineral dust aerosols. For example, dust loading on snowpack in the San Juan Mountains (southwest Colorado) and the Wasatch Mountains (northern Utah) can decrease albedo and advance the timing and rate of spring and summer snowmelt. However, the iron mineralogy of dust is poorly characterized and one goal of our research is use magnetic methods to provide better quantitative estimates of amounts and particle sizes of hematite and goethite in dust which can then be use in radiative transfer models. Magnetic measurements are advantageous because they are non-destructive, can be run on bulk dust samples, and can detect low abundances of target minerals ( < 1 wt%) as well as small grain sizes (<100 nm) which are common in mineral dust aerosols.

In this study, we evaluated several different magnetic methods, all involving variable temperatures (20 -400 K) and high magnetic fields (0-5 T), to identify the relative contributions of goethite and hematite in dust and dust sources from various locations in the US, Australia, and Africa. Our study include samples of dust-on-snow from the San Juan and Wasatch Mountains , dust from eastern Australia (Lake Cowal, Orange, Hornsby, Sydney), dust sources from southwest USA (Nellis, NV; Chinle Valley, AZ; Little Colorado River Dust Corridor, AZ; and Milford Flats, UT), and the Kalahari Desert (southern Africa).

Our preliminary findings indicate that all analyzed samples contain goethite and hematite in various proportions and most contain magnetite in trace amounts (<0.5 wt%). At 300 K, the ratio of goethite to hematite magnetizations (G/H) is highest in the Australian (>0.5) and Nellis samples (~0.3) and low in the Kalahari (<0.17), Milford Flats ( ~0.1) and some Chinle Valley (~0.07) samples. The partition between hematite and goethite is important to know to improve models for the radiative impact of ferric oxide minerals in mineral dust aerosols.