QUANTIFYING SHALE WEATHERING RATES AS A FUNCTION OF CLIMATE

Weathering is an important process for landscape evolution in the Critical Zone but rates are not well quantified, especially as a function of climate. A transect of sites with varying mean annual temperature and precipitation has been established in the northern hemisphere as part of the Susquehanna Shale Hills Critical Zone Observatory (SSHO) to investigate the influence of climate on shale weathering. The transect consists of end members in Wales and Puerto Rico as well as sites in the Appalachian Mountains, including New York, Pennsylvania, Virginia, Tennessee and Alabama. All sites are underlain by shale, the dominant lithology amongst sedimentary rocks on Earth and an important parent material weathering to form soil. Weathering rates across these sites were determined using several different approaches. At the coldest and most northerly site in Wales, the only site with over 25 years of weekly stream and precipitation chemistry available, a catchment mass balance approach was used to calculate a shale weathering rate of 7.8 m Ma^-1. At all transect sites, weathering rates were calculated for ridgetop and slope topographic positions using soil geochemical profiles. On ridgetops, where water flow is largely vertical through the soil profile, the extent of chemical depletion at the soil surface increases from north to south. We observe a temperature dependence of Na loss, a proxy for feldspar weathering, across the transect with an apparent activation energy of 117 kJ mol^-1. Feldspar weathering on ridgetops progresses from kinetically limited (Wales to Alabama) to transport limited in Puerto Rico. In the case of Mg loss, a proxy for chlorite weathering, we also observe a temperature dependence, with an apparent activation energy of 60 kJ mol^-1. On slopes, soils show chemical depletion similar to that of the ridgetop profiles but soils are shallow (~70 cm) and thickness does not vary much along the transect. In contrast, the depth of ridgetop soils increases from shallow soils (~ 30 cm) in Wales and Pennsylvania to increasingly deep soils to the south (632 cm in Puerto Rico). These observations can be compared to geochemical models of regolith weathering to better predict the impact of climate change on soil formation and landscape evolution.