PRESENT-DAY SHALE WEATHERING RATES ACROSS A LATITUDINAL CLIMOSEQUENCE

Soil represents a central constituent of the Critical Zone, but the rate at which soil forms by weathering is not well quantified, especially as a function of climate. This lack of knowledge complicates predictions of the effects of global climate change on soils humans depend on for food. Bulk soil and rock geochemical analyses are commonly used to calculate weathering rates but the duration of weathering is often poorly constrained; direct weathering measurements on shorter time scales could help reduce this uncertainty. To investigate the role of climate in shale weathering, a transect of sites on shale lithology with varying mean annual temperature and precipitation was established in the northern hemisphere as part of the Susquehanna Shale Hills Critical Zone Observatory (SSHO). Present-day weathering rates were obtained by burying small shale fragments, approximately 2 cm by 1 cm, in soil pit walls at multiple depths. After two years, the shale chips were exhumed, washed and changes in mass were measured. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) techniques were employed to determine surface chemistry and physical alteration of the recovered shale. Exhumed shale chips showed mass loss after two years ranging from roughly 10 mg in Wales to 21 mg in Puerto Rico. Weathering rates were significantly different between Wales and Puerto Rico at 2.9 ± 0.9 m Ma^-1 and 11.2 ± 3.7 m Ma^-1, respectively. These results are similar to weathering rates obtained in a previous two year study comparing shale buried in Wales and Malaysia using a similar approach. Average weathering rates of shale buried in Virginia, Tennesse and Alabama were similar to rates measured in Puerto Rico and were not significantly different (8.6 – 10.6 m Ma^-1). We noted, however, that many of the shale chips in Puerto Rico retained soil particles even after washing, therefore mass loss at this site could be even greater. Evidence of physical and chemical changes, especially the loss of Na and Mg, from shale chip surfaces was greater at warmer and wetter study sites. These data can be compared to long-term weathering rates obtained from bulk soil geochemistry across the same transect of sites. Quantification of weathering rates on multiple time scales may allow society to forecast how ongoing changes in climate will impact soil production.