Surface water chemistry in the main stem and source points of the Rio Icacos basin 7 (Luquillo Experimental Forest, Puerto Rico) was studied to investigate the factors 8 regulating spatial variability in major solutes in a rapidly weathering landscape. We 9 sampled along the main stem as well as at small source points at high elevation where 10 fresh bedrock is frequently exposed, and at low elevation in the floodplain/colluvial 11 plain of the main stem. Concentrations of silicon, alkalinity, and the sum of base 12 cations were lower at the source points than in the main stem, and were lowest in low- 13 elevation source points. Calcium and sodium were the dominant cations at all sampling 14 points after sea-salt correction, reflecting the weathering of plagioclase feldspar 15 throughout the basin. The partial pressure of carbon dioxide (pCO2) tended to be higher, 16 and HCO3
concentrations were lower, in the low-elevation source points than at other 17 positions in the landscape. When coupled with the relatively low concentrations of Si and 18 base cations, this suggests that the availability of primary reactive minerals, rather than 19 carbonic acid concentrations, limits weathering in these low-elevation sources. 20 Mechanical denudation appears to enhance chemical weathering rates not only by 21 refreshing reactive mineral surfaces but also by contributing carbon dioxide from the 22 decomposition of organic-rich material in landslides, which occur frequently. The spatial 23 variability of major solutes appears to depend primarily on the availability of fresh primary 24 reactive minerals, carbon dioxide concentrations, and hydrolysis conditions.