CHEMICAL AND MINERALOGICAL ANALYSIS OF FE-RICH MICROSPHERULES OF UNKNOWN ORIGIN FROM CENTRAL NEW YORK SEDIMENTS

The presence of microspherules in sediment or rock beds, especially beds associated with an extinction event or rapid environmental change, is commonly interpreted as evidence for an impact event. It is important to understand, however, that microspherules can be formed from a wide variety of processes, including human industrial activity. Determination of microspherule origin is, therefore, very important, and can be difficult, especially in modern sediments. In an effort to determine both the origin and the relative abundances of spherules in Central New York, we collected soil samples from 13 different sites; 7 urban surface samples, 3 rural surface samples, and 3 from stratified sedimentary deposits. Spherules were hand-picked under a stereo zoom microscope and placed on conductive carbon tape for imaging and chemical analysis using a FEI scanning electron microscope equipped with an EDAX energy-dispersive X-ray spectrometer. Both internal and external spherule surfaces were examined using a Nicolet Almega XR Dispersive Raman spectrometer in order to more precisely determine the mineral phases present.

Though we found both metallic and glassy microspherules in most of our sediment samples, we focused this study on the metallic spherules due to their abundance, diversity, and the many similarities between the anthropogenic and cosmic varieties. The vast majority of the metallic spherules were found to be almost pure iron-oxide in composition. Mineralogically, they tend to consist of a thin coating of hematite with an inner core of magnetite. Surface morphologies are highly variable, with crystal shapes ranging from blocky to dendritic. While anthropogenic microspherules are morphologically and chemically similar to extraterrestrial microspherules, previous work by others has found two diagnostic features of extraterrestrial spherules to be a crust of magnetite and wüstite, and a core consisting of platinum group elements. Because our Fe-rich microspherules lack these features, we conclude that they are all of anthropogenic origin. The observed abundance of anthropogenic Fe-rich microspherules in both rural and urban surface sediments suggests that contamination of geological samples by modern processes needs to be carefully considered.