In the weathering zone, chemical reactions break down minerals. These reactions release solutes and so represent the starting point for the biological and geochemical cycling of most elements. For the elements with two or more isotopes, isotope fractionation (the slight bias towards one isotope over another for a given process) means that as the elements cycle through plants, soils and rivers, the different phases develop characteristic isotope compositions.
For many elements, particularly silicon and lithium, the available data suggests that most of the fractionation is caused by the formation of new clay minerals. Simple mass balance approaches therefore require that both the residual dissolved element as well as the newly formed clay should hold valuable information about the how much of the initially weathered material ends up in the new solid, or stays dissolved. This is a key index of the style of weathering, also known as weathering congruency, and is related to the efficiency and rate at which chemical weathering reactions take place. This means there’s a huge potential for records of silicon and lithium isotope variation in the past to tell us about how weathering has varied in response to climate (or has perhaps even driven climate).
The challenge is to ‘calibrate’ this isotopic fingerprint to the style of weathering, and perhaps even further to the weathering rate. By combining modern data generated from carefully selected sites with new sedimentary records, this project hopes to begin to address this challenge. Two initial periods of particular interest include the transition from the last ice-age to the warm stability of the current interglacial, and a period of brief but intense warming 56 million years ago termed the Paleocene-Eocene Thermal Maximum (PETM).
Central to this project is the development of precise and accurate stable isotope data using the MC-ICP-MS facilities at the Vega-center of the Swedish Museum of Natural History and at GFZ-Potsdam.