Date of Award
Fall 12-20-2024
Level of Access Assigned by Author
Open-Access Thesis
Degree Name
Master of Science (MS)
Department
Earth Sciences
Advisor
Amanda A. Olsen
Second Committee Member
Alicia Cruz-Uribe
Third Committee Member
Dawn Cardace
Abstract
Serpentinite weathering environments represent unique natural systems at the intersection of the lithosphere, biosphere, hydrosphere, and atmosphere. The unusual chemistry of serpentinites and serpentine soils, which is characterized by low levels of key plant nutrients and elevated concentrations of phytotoxic trace and heavy metals, distinguish these systems from the rocks that form most of the continental crust. Identifying the minerals in serpentine soils that host trace metals is critical for understanding their fate and transport throughout the surrounding environment. Sequential chemical extractions have been used to assess changes in the mobility and bioavailability of trace metals in serpentinite weathering profiles. However, differences in laboratory protocols make it difficult to compare results from one study to the next, limiting our understanding of how factors such as climate affect trace metal bioavailability in serpentine soils. To address this gap, we applied a sequential chemical extraction procedure previously used in the study of a temperate serpentinite weathering profile to soils collected from the Río Cupeyes, a tropical weathering environment in southwestern Puerto Rico.
The top 1.1 m of the serpentinite weathering profile was studied. Bulk chemistry of soils was measured using a combination of XRF and ICP-MS. Bulk mineralogy of rocks and soils was determined with PXRD, which identified montmorillonite, vermiculite, kaolinite, and goethite as important precipitated phases in the profile. Primary and accessory minerals in thin sections of weathered clasts were characterized using BSE imaging and EDS, providing insights into the mineral origins of trace metals during the early stages of soil development. We also attempted to visualize chemical weathering of clasts using high resolution LA-ICP-MS element mapping. These chemical and mineralogical analyses demonstrated that rocks and soils at the Río Cupeyes are pervasively weathered.
The four-step sequential chemical extraction procedure described by Caillaud et al. (2009) was used to estimate bioavailable levels of trace metals in soils. ICP-OES analysis of obtained liquid fractions showed that most of the Ni (51.2 – 77.4%) and Cr (83.0 – 95.5%) in soils remained within the crystal structure of less reactive minerals (e.g., silicates and primary spinels), whereas Mn and Co tended to reside in more labile amorphous Fe-Mn (oxyhydr)oxides. Comparing our results to similar studies of temperate serpentine soils, we noted modest increases in the mobility of all four trace metals and determined that increases in bioavailability would coincide with conditions lowering the redox potential (Eh) of soils.
Recommended Citation
Sternberg, Francis, "Trace Metals Speciation in a Tropical Serpentine Soil Profile: Estimating Bioavailability with Sequential Extractions" (2024). Electronic Theses and Dissertations. 4099.
https://digitalcommons.library.umaine.edu/etd/4099
