Principles of Isotope Fractionation

Mass-dependent isotope fractionation

The underlying physico-chemical principle is "mass dependent isotope fractionation". The smaller the relative difference between isotopes, the larger is the relative isotope shift. This shift is induced when an element is transferred from a compartment to another which corresponds to a change in its chemical binding state. Lighter isotopes react faster (kinetic isotope effect) or enter weaker bonds (equilibrium isotope effect). The relative fractionation is largest for lithium (largest relative mass differences) and smallest for the heavy isotopes (e.g. molybdenum, uranium, smallest relative mass difference).

Isotopes in Weathering

In weathering, rock is converted into soil. When soil is eroded, its products are found in solids and solutes transported by rivers. For lithium, the light isotope 6Li is partitioned preferentially into the clays that form, while the heavy 7Li moves preferentially into river dissolved Li.

Isotope fractionation through weathering processes

Characteristic 7Li/6Li isotope patterns are found in global rivers: light 6Li is preferentially partitioned into clays, hence its 7Li/6Li ratio is low. Heavy 7Li remains in the fluid, hence its isotope ratios 7Li/6Li are high. Shown are 7Li/6Li ratios in permil differenve to a reference material.

Novel stable isotopes in the Critical Zone: the links between erosion, soil production and saprolite weathering

Soils, that cover 94 % of the continental surface of our planet, are produced by the chemical reactions that transform parent bedrock into saprolite at the weathering front (weathering) and destroyed by the physical removal of solid material at their top (erosion, Fig. 1). The delicate balance between weathering and erosion sets the thickness of soil, this resource that sequesters large amounts of atmospheric CO2 by weathering rock and growing biomass, thereby stabilizing the Earth’s climate at a habitable level.

 

Fig. 1: Soil dynamics: (A) schematic of the weathering zone and the processes producing and destroying soils. Bedrock is converted into saprolite (weathered rock) by chemical weathering, which results in a flux of dissolved elements leaving the systems in water. Soil is mobile, and is formed by physical mixing of weathered material by e.g. tree throw or animal burrowing. Soil is removed at its top by physical erosion. Modified from ref. 2. The rate of weathering increases with landscape erosion rates (Fig. 2), suggesting that physical erosion continuously rejuvenates the landscape leading to the supply of fresh weatherable minerals from below to the surface weathering zone; and that chemical weathering reactions weaken rock so that physical erosion is sustained. In addition, a potential feedback enabling the soil balance manifests itself in a relationship between soil thickness and soil production. This feedback is thought to operate through bioturbation, freeze-thaw cycles, and access of water and air to the weathering front. However, we still poorly understand the dynamics of soil and the pathways followed by chemical elements in the so-called Critical zone, which extends down from the weathering front up to the top of the vegetation canopy.

 

Fig. 2: Top: Landscapes typical of our three sampling sites. Bottom: relationship between chemical weathering rates and physical erosion rates across a wide range of settings, as measured by cosmogenic nuclides and chemical mass balance, and river loads.

When a chemical element is involved in chemical reactions or physical transfer between compartments, the slight mass difference between its isotopes leads to minute shifts in isotope relative abundances (stable isotope ratios). These shifts are called isotope fractionation, are „mass-dependent“, and lead to isotopic variations between geological / biological materials. Measuring isotope ratios in the compartments of the Critical Zone offer the opportunity to identify processes and to quantify the corresponding rates of the massive biogeochemical transformations occurring at the Earth Surface. The recent advent of multicollector-inductively-coupled mass spectrometry (MC-ICP-MS) allows detecting small shifts in isotope ratios (down to < 0.1 parts per thousand) of virtually all metal and metalloid elements that are an integral part of the matter transfers in the Critical Zone.

Our section at GFZ is developing chemical and mass spectrometric procedures that enable us to precisely measure a whole array of these novel isotope ratios (7Li/6Li, 26Mg/24Mg, 30Si/28Si, 56Fe/54Fe, 88Sr/86Sr) in the compartments of the Critical Zone: parent rock, saprolite, soil, soil water, higher plants, river water, river sediment. Each of the corresponding chemical elements displays a distinct behaviour at the Earth Surface, allowing for a better understanding of the large variety of relevant processes such as dissolution of primary minerals, precipitation of secondary minerals, or uptake by plants.

Our approach is based on the study of three well-constrained selected field sites, that have been chosen for their diversity in tectono-geomorphic character, but which are all of granitoid parent material and all featuring eroding hillslopes in non-depositional settings: the slowly eroding Highlands of Sri Lanka1, a moderately eroding mountain upland in Sierra Nevada, California2 and a rapidly uplifting alpine mountain belt in the Swiss Central Alps3. Together these landscapes provide a wide range of denudation rates ranging from 2 to 2100 mm ky-1 (~5 to 5600 tons km-2 y-1) with which we can effectively study the patterns of erosion–weathering coupling (Fig. 2). At these sites, we selected soil profiles along recently exposed roadcuts, along which we sampled surface soils, through saprolite, to bedrock. We also extracted pore waters from soils and saprolites, main types of vegetation, and collect stream water.

With the aim of quantifying and understanding the seasonal variability in weathering processes and fluxes, time series of water samples are currently being acquired in Sri Lanka (stream waters, collaboration with Dr. T. Hewawasam, Sarabagamuwa University of Sri Lanka) and in Sierra Nevada (soil and stream waters, collaboration with the Southern Sierras Critical Zone Observatory – Matt Meadows, University of California at Merced, with the Forest Service, US Department of Agriculture, Fresno – Dr. Carolyn Hunsaker, and Dr. Jean Dixon, University of Santa Barbara, California).

References:

1 von Blanckenburg F., Hewawasam T. and Kubik P.: Cosmogenic nuclides evidence for low weathering and denudation in the wet, tropical highlands of Sri Lanka, J. Geophys. Res. – Earth Surf., 109:F03008, 2004.

2 Dixon J.L., Heimsath A.M. and Amundson R.: The critical role of climate and saprolite weathering in landscape evolution, Earth Surf. Proc. Landforms 34:1507-1521, 2009.

3 Norton K.P. and von Blanckenburg F.: Silicate weathering of soil-mantled slopes in an active Alpine landscape, Geochim. Cosmochim. Acta 74:5243-5258 , 2010.