Controls on major solutes within the drainage network of a rapidly weathering tropical watershed

Abstract

[1] Surface water chemistry in the main stem and source points of the Rio Icacos basin (Luquillo Experimental Forest, Puerto Rico) was studied to investigate the factors regulating spatial variability in major solutes in a rapidly weathering landscape. We sampled along the main stem as well as at small source points at high elevation where fresh bedrock is frequently exposed, and at low elevation in the floodplain/colluvial plain of the main stem. Concentrations of silicon, alkalinity, and the sum of base cations were lower at the source points than in the main stem, and were lowest in lowelevation source points. Calcium and sodium were the dominant cations at all sampling points after sea-salt correction, reflecting the weathering of plagioclase feldspar throughout the basin. The partial pressure of carbon dioxide (pCO 2 ) tended to be higher, and HCO 3 À concentrations were lower, in the low-elevation source points than at other positions in the landscape. When coupled with the relatively low concentrations of Si and base cations, this suggests that the availability of primary reactive minerals, rather than carbonic acid concentrations, limits weathering in these low-elevation sources. Mechanical denudation appears to enhance chemical weathering rates not only by refreshing reactive mineral surfaces but also by contributing carbon dioxide from the decomposition of organic-rich material in landslides, which occur frequently. The spatial variability of major solutes appears to depend primarily on the availability of fresh primary reactive minerals, carbon dioxide concentrations, and hydrolysis conditions.

Chemistry of Supraglacial Ponds in the Debris-Covered Area of Lirung Glacier in Central Nepal Himalayas

“…We found sulfate concentration many times higher than in most of the largest river systems of the world and in the fastest weathering regimes of a silicate terrane on the earth surface (McDowell and Asbury 1994;Meybeck and Ragu 1997;Gaillardet et al 1999;Bhatt and McDowell 2007;Tartai et al 1998;Bhatt et al 2014a). Takeuchi et al (2012) found that various organisms such as insects, copepods, and algae live in ponds in the debris-covered area of this glacier.…”

“…*K) among the ponds within Lirung debris areas appeared lower than the calculated ratios from Langtang-Narayani river system in central Nepal Himalayas (unpublished data) and much lower than the calculated ratios from different positions of the Rio Icacos (which drains a tropical rain forest) that has the fastest documented weathering rates of any silicate terrain in the world (McDowell and Asbury 1994;White and Blum 1995;Bhatt and McDowell 2007). The wide variation in Si/ (*Na ?…”

“…Dissolved organic nitrogen (DON) was calculated by subtracting inorganic nitrogen from TDN. Marine aerosol contribution to different elements was corrected by using molar ratios of different elements as reported in earlier studies (Keene et al 1986;McDowell et al 1990;Millot et al 2002;Bhatt and McDowell 2007). Marine aerosol contribution to pond waters was corrected for Na ?…”

“…It is well-known that sea-salt-contributed aerosols impact major ions observed in watersheds (Stallard and Edmond 1981;McDowell et al 1990;Bhatt and McDowell 2007); therefore, we evaluated these impacts in our study area. We found that calcium and sodium are the least and the most impacted element by marine aerosols, respectively, and that the contribution of marine aerosols to the total dissolved load appears to be minor advocating the lithological control on dissolved matter as reported by earlier studies (Dalai et al 2002;Bhatt et al 2008Bhatt et al , 2009).…”

“…Debris may have played a great role in the release of solute from this catchment. Uplift and erosion, tectonic recycling of sediment, drives high denudation rate in many active mountain belts (Stallard 2000;Stallard and Edmond 1983;Gaillardet et al 1999;West et al 2005;Bhatt and McDowell 2007;Gabet 2007) together with climatic conditions of that basin (McDowell and Asbury 1994;White and Blum 1995;Hartmann 2009;Bookhagen 2010;Gabet et al 2010;Hirschmiller et al 2014). Air temperature and precipitation control glacier melting and hydrolysis conditions of glaciered basins, thus modulating dissolution rates of the minerals of such basins according to the seasons (Bhatt et al 2000.…”