GEOCHEMICAL ANALYSIS OF SOIL DEVELOPED ON THE MARCELLUS SHALE IN CENTRAL PENNSYLVANIA

In order to determine the effects of weathering and the mobility of elements within soils overlying black shales, we conducted a geochemical study of soil developed on the Marcellus Shale. This study provides a basis for understanding the pedogenic processes that have occurred on top of the Marcellus Shale. Our site is entirely underlain by Marcellus Shale, wooded, undeveloped, and is near a local topographic maximum. The characteristics of the site minimize the possibility of external effects on the chemical composition of the soils.

We collected sixteen soil samples that have developed on the Marcellus Shale and one sample of parent material. We used a hand auger to drill a core through 193cm of soil to the bedrock and we sampled soils at various depths along the core. We collected the parent material from the bedrock interface as rock fragments at the base of the soil. For analysis, the soils were dried and sieved to ≤ 75μm.

Bulk chemistry of the samples was determined by lithium metaborate fusion followed by ICP-AES analysis. Using this data, we normalized the elemental proportions to Ti and constructed tau plots to graphically demonstrate the enrichment or depletion of cations compared to the parent material as a function of depth. In general, the oxides are depleted relative to parent material near the surface, and show a trend of enrichment closer to the bedrock.

We determined the mineralogy of the soils by XRD and scanning electron microscopy with energy dispersive spectroscopy. XRD patterns show that the dominant mineralogy in the samples consists of quartz, illite, montmorillonite, muscovite, and biotite. Mineralogical variety increases in soils closer to the bedrock, including phases of todorokite and trona. SEM with EDS analysis of one soil from 104cm depth reflected the mineralogy reported by the XRD and revealed the presence of chlorine and sulfur salts.

Geochemical and physical data from this study show that weathering has an increased effect on soils near the top of the profile and that those effects decrease with increased proximity to the bedrock.

In the future, we will continue exploratory research with this site to determine if Cu isotopes alter with depth through the soil profile. We have developed a method for the extraction of copper from the soil matrix and preliminary data reveal a trend in the fractionation of copper isotopes within the soil profile.