Temporal variations in continental weathering processes: Insights from Li and Mg isotopes

• Main Author: Fries, David, Mike
• Format: Thesis
• Language: English
• Published: University of Southampton 2018
• Subjects: Greenland
• Online Access: https://eprints.soton.ac.uk/427039/; https://eprints.soton.ac.uk/427039/1/Fries_David_PhD_Thesis_Nov_2018.pdf

Continental weathering delivers elements to the oceans via rivers, is responsible for soil and landscape formation, and regulates the concentration of CO2 in the atmosphere. There are multiple controls on rates of continental weathering that can vary in both space and time. This study utilises the lithium (Li) and magnesium (Mg) isotope composition (δ7Li and δ26Mg) of weathering products to better understand the hydrological controls on weathering processes in different environmental settings. The effects of glacial weathering processes on the isotope signatures of Li and Mg delivered to the ocean are assessed by analysis of fjord waters in Greenland at times of high and low ice melt. These data demonstrate that glacial weathering is an insignificant source of both Li and Mg even when ice melt is high. The influence of melt water input on δ7Li and δ26Mg can only be detected at very high freshwater/seawater ratios (>5). Although weathering rates in highly weathered tropical river catchments are typically considered to be very low, analyses of weathering products from the Quiock Creek catchment in Guadeloupe show that weathering processes can respond to short-lived rain events. The δ7Li composition of the Creek decreases after a storm event because of increased input of Li from soil solutions input that have relatively low δ7Li; however, there is little change in δ26Mg after rain events because most of the Mg in the Creek is supplied by throughfall. There are also significant differences in the δ7Li value of the River Amazon between the high and low water stages. δ7Li values are 0.7 to 8.7‰ higher in the wet season, which is attributed to increased input of Li stored in floodplains. However, input of Mg from floodplains has little effect on δ26Mg.