Feedbacks among weathering, erosion, and other biogeochemical processes make quantification and modeling of the Critical Zone (CZ) difficult. Mass-dependent fractionation of many elements can be used to fingerprint CZ material fluxes. Now researchers can use multiple isotopes (O, H, C, S, Ca, Mg, Sr, Li, B, Be, etc.) in the same setting to elucidate complex systems. In addition, metal stable isotopes (e.g. Cu, Zn, Cd, Hg, and Pb) can fingerprint anthropogenic influences. Still, there is a need to develop better approaches toward using multiple isotopes to compare biogeochemical interactions in environments of differing lithology and environmental characteristics.

Boron (B) Jérôme Gaillardet and Johanna Noireaux (Institut de Physique du Globe de Paris, France)

Boron has two isotopes, ¹⁰B and ¹¹B that are naturally present on Earth. The average ratio of ¹¹B/¹⁰B is about 4, however, subtle differences exist between water and rocks. Classically, we express the boron isotopes by the deviation of an international standard, in ‰.  Previous work by Lemarchand et al. (2001) exemplified that dissolved boron in rivers is 10‰ enriched in ¹¹B compared to the upper continental crust. Dissolved boron is therefore “ heavy” compared to crustal boron. The origin of this isotope discrimination is due to the preferential incorporation of ¹⁰B in the secondary phases formed during weathering and to the preferential uptake of boron by the vegetation. Clays and plants are “light” for boron. The project aims at analyzing boron isotopes in the different compartments of the SSHCZO, to explore the isotopic fractionation generated by water rock interactions within the watershed and to constrain the links between chemical weathering of shales and vegetation activity.

Boron will be measured by MC-ICPMS in Paris, after chemical extraction from geological or biological matrix.

Calcium (Ca) Matthew Fantle (Pennsylvania State University, USA) and Grit Steinhoefel

Lithium (Li) Maureen Fieneman (Pennsylvania State University, USA)

Carbon (C) Lixin Jin (University of Texas at El Paso, USA)

C concentrations and isotopic compositions (δ¹³C) of different reservoirs are studied in the Susquehanna Shale Hills Critical Zone Observatory, including organic matter in the soils and Rose Hill bedrock, carbonate minerals, CO₂ in the soil gas, and dissolved inorganic carbon (DIC) and DOC in the natural waters (soil water, ground water and stream). The goals are: 1) to evaluate the degradation of organic matter in controlling seasonal variations of soil CO₂ concentrations; 2) to determine the relative contribution of silicate and carbonate dissolution to overall stream DIC; 3) to identify the sources of stream DIC: soil CO₂, atmospheric CO₂ and/or carbonate C; and 4) to quantify the CO₂ budget over the whole watershed by determining the mineral weathering rates and organic matter decomposition rates. This case study will help understand C fractionation in the surface and subsurface and develop C isotopes as an environmental tracer. More importantly, it will allow us to evaluate the roles of shale in global CO₂ cycles.

Magnesium (Mg) Lin Ma (University of Texas at El Paso, USA), F. Zhen

The behavior of Mg ions and their transport during biogeochemical cycling at Earth’s surface is closely linked to long-term changes in Earth’s climate. Mg stable isotopic ratios have great potential as a natural tracer to elucidate weathering processes and biogeochemical pathways in surficial environments. Here, we investigated the Mg isotope fractionation during shale weathering under a temperate climate in northeastern USA. Significant fractionation of Mg isotopes during shale weathering is clearly observed at Shale Hills: δ²⁶Mg values of stream and soil pore waters are about ~0.5 ‰ to 1 ‰ lighter than the bedrock. Most soil samples show distinctly lighter Mg isotope compositions compared to the bedrock. We suggest that the accumulation of light Mg isotopes in soils at Shale Hills is due to i) mobilization of isotopically light Mg solute during clay dissolution and re-precipitation as vermiculite or HIV; and ii) loss of isotopically heavy particulate Mg. Our study provides the first field evidence that changes in clay mineralogy affect the Mg isotope compositions of residual bulk soils. This example also demonstrates a new and important mechanism – loss of isotopically distinct fine particles from clay-rich systems -- that could drive the Mg isotope compositions of weathering residuals in an opposite direction as normally expected from silicate weathering, i.e. to lighter Mg values instead of heavier values.

Source: Ma et al. (in review), Mg isotope fractionation during shale weathering in the Shale Hills Critical Zone Observatory: Accumulation of light Mg isotopes in soils by clay mineral transformation, Chemical Geology.

Lead (Pb) and Cadmium (Cd) Lin Ma (University of Texas at El Paso, USA)

Human activities have significantly increased the metal emission to the atmosphere, and these metals are subsequently deposited back to Earth’s surface, disrupting their natural biogeochemical cycles. Elevated concentrations of heavy metals have been documented in water and at the land surface on a global scale. The Shale Hills CZO is located in a small temperate watershed in Pennsylvania. This watershed experienced relatively minimal human disturbances in the past. However, detailed soil characterization (Herndon et al., 2011) has revealed the presence of unusually high levels of Mn, Pb and Cd in surface soils due to atmospheric deposition. The metal contamination can be attributed to several emission sources including: 1) iron production in the 19^th century in Pennsylvania; 2) steel plants and coal-burning power plants; and 3) contributions from gasoline. Contribution from iron production was highlighted as particularly important. That interesting study invoked more questions regarding the historical metal loadings at Shale Hills and northeastern USA which we investigate in this study. Specifically, we sought to understand whether the iron production also led to enrichment of metals such as Pb and Cd in surface soils and, if so, whether Pb and Cd isotopes could be used to determine the local and/or regional sources of the metal loadings, including whether the metal signatures are derived from the early industrial revolution period or modern industrial activities.

Silicon (Si) Friedhelm von Blanckenburg (Department 3 “Geodynamics”, Postdam)

Strontium (Sr), Germanium/Silicon (Ge/Si)  Louis Derry (Cornell University, USA)

²¹⁰Pb and ¹³⁷Cs Diana Karwan (University of Minnesota, USA)

Oxygen (δ¹⁸O) and Deturium (δ²H) Christopher Duffy (The Pennsylvania State University, USA) and Evan Thomas (The Pennsylvania State University, USA)

Beryllium (¹⁰Be) Paul Bierman (The University of Vermont, USA) and Nicole West (The Pennsylvania State University, USA)

Iron (δ⁵⁶Fe) Tiffany Yesavage (The Pennsylvania State University, USA) and Susan Brantley (The Pennsylvania State University, USA)

Copper (Cu) Ryan Mathur (Juniata College, USA)