Sediment geochemistry and mineralogy from a glacial terrain river system in southwest Iceland
Résumé
The weathering of basalt has implications for the global carbon cycle on Earth as well as for understanding sedimentary processes on other terrestrial bodies dominated by a basaltic crust (e.g., Mars), but yet studies in mafic terrains are limited in comparison to their felsic counterpart. Our work details the compositional transformations resulting from the sedimentation process of first-cycle sediments generated in a basalt dominated watershed of southwest Iceland. By sampling multiple sites along the Hvítá S river transport pathway and analyzing the sedimentology, geochemistry, and mineralogy, this work provides a geologically integrated approach to understanding a fluvial source-to-sink system in mafic terrains. Environmental conditions such as climate, hydrology, and transport distance all influence the overall sediment composition, and in response, chemical weathering, physical abrasion, sorting, transport, and mixing are all sedimentary processes unraveled in the geochemical and mineralogical relationships of individual grain size bins. Chemical weathering initiates in the upper reaches of the watershed with the alteration of primary mafic phases to form secondary weathering products. As sediment continues to move downstream, the entire sediment suite becomes more altered, and the extent of chemical weathering is intensified, as evidenced by a higher abundance of clay minerals and glassy material. Fluvial sorting separates detritus by particle size and differentiates them by compositional gradients in geochemistry and mineralogy. As grain size decreases, mafic minerals become less abundant and the finer grain sizes are preferentially enriched in immature weathering minerals and mineraloids (e.g., smectite clays and X-ray amorphous phases). This grain size trend is also correlated with elemental fractionation, illustrated most notably by the clay size fraction (<2 μm) hosting the most altered material in river sediments. However, we note that the environmental conditions of Iceland (e.g., a cold climate and glacial reworking) result in limited element mobility when compared to more temperate climates around the world. Therefore, we suggest that the clay size fraction of river sediments provide a valuable target for understanding the intensity of weathering in these systems, particularly in a colder climate where mineralogical transformations are not always accompanied by a high degree of elemental loss. In addition to weathering and sorting, trace element abundances suggest the mixing of sediments from varying provenances. While the Hvítá S watershed is dominated by basalt, even minor amounts of evolved volcanics (e.g., andesites) contribute to the overall sediment composition. Overall, this work fills a knowledge gap in sedimentation processes in basaltic terrains on Earth, while additionally providing a valuable terrestrial analog for ancient fluvial-deltaic environments preserved in the sedimentary rock record of Mars (e.g., Gale Crater and Jezero Crater).
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