Uranium-series and strontium isotope ratios in soils from Basse-Terre Island, French Guadeloupe: Insights for rapid soil formation and impact of dust in a tropical volcanic setting
Chemical weathering of silicate rocks generates soils that contain mineral nutrients, sustain ecosystems, modify water chemistry in the hydrosphere, and regulate the global carbon cycle. Understanding the timescales of soil formation and sources of soil mineral nutrients has great implications for securing the future of agriculture and food production. However, few tools are currently available to directly quantify the rates of chemical weathering and timescales of soil formation. Uranium-series isotopes fractionate during chemical weathering and the activity ratios of (234U/ 238U), (230Th/238U), (238U/ 232Th) and (230Th/232Th) have great potential to constrain the rates and timescales of chemical weathering and soil formation in soil profiles. In addition, U-series techniques have been used in conjunction with strontium (Sr) isotopes to trace source materials in open systems, provided that the end-member reservoirs have distinct isotope signatures. By combining U-series and Sr isotope analysis in soil profiles, it is possible to quantify rates of chemical weathering and soil formation as well as to identify trace sources of mineral nutrients. The objectives of this study are 1) to identify key processes such as chemical weathering vs. atmospheric deposition that affect soil formation and development and 2) to quantify chemical weathering rates in volcanic soil profiles under a tropical climate. The study site is situated in Basse-Terre Island of French Guadeloupe in the Lesser Antilles volcanic arc. Thick soil/regolith profiles with rock clasts are developed due to rapid and intensive weathering of andesitic parent materials. Shallow soils are highly depleted with respect to mineral nutrients and atmospheric contributions to soil mineral nutrients have been shown to be important for the volcanic island. In this study, I focused on three deep soil cores with depths ranging from 8 to 12 m from the Bras-David, Moustique Petit-Bourg, and Deshaies watersheds that are distributed across the large precipitation gradient of the island, and water samples from the rivers adjacent to these soil sites. Soil and water samples were collected from field trips to Basse-Terre in 2014 and from archived soil samples collected in July 2007. Analyses for major element concentrations, U-series, and Sr isotopic ratios in bulk soils, sequential extraction fractions, and river water were performed at The University of Texas at El Paso. Results indicate that chemical weathering reactions are integral to the conversion of andesitic bedrock into thick accumulations of depleted soils since mobile elements such as calcium, magnesium, and strontium have undergone intensive chemical weathering at the deep soil profiles when compared to samples representing the parent bedrock collected from outcrops. Surface processes, such as dust addition, impact soil development, as evidenced by increase of mobile elements near the surface. This observation is further confirmed by the isotopic analysis, where U-series and Sr ratios increase at the surface with signatures similar to the Saharan dust end-member. Sequential extraction procedures reveal the release of mobile elements near the surface during the first leaching phase, with U and Sr isotope signatures similar to marine aerosols. Constraining soil profile-scale weathering rates of andesite were achieved by solving a set of differential equations on U-series mass balances in the profile, thus yielding a duration of chemical weathering range from 300 to 400 Kyr, and an average weathering rate of 30 m/Ma in the weathering profile. Because of ongoing investigations of chemical weathering rates across a range of scales of observation on Basse-Terre Island, the results from the three soil profiles located in the Deshaies, Moustique Petit-Bourg and Bras David watersheds can be directly compared with local clast and watershed scale observations. The sensitivity of calculated chemical weathering rates varies according to the scale of observation. The collected soil profiles did not reach the unweathered bedrock, therefore the deep regolith samples are composed of highly weathered clay minerals. Weathering clasts exhibit a slower weathering rate (0.3m/Ma) due to its small scale of surface area, while high solute fluxes transported by rivers in the Bras David watershed yield much faster chemical weathering rates (300m/Ma), therefore, the transformation of bedrock to regolith occurs at much greater depth.
Pereyra, Yvette, "Uranium-series and strontium isotope ratios in soils from Basse-Terre Island, French Guadeloupe: Insights for rapid soil formation and impact of dust in a tropical volcanic setting" (2016). ETD Collection for University of Texas, El Paso. AAI10118237.