Controls on the δ13CDIC and alkalinity budget of a flashy subtropical stream (Manoa River, Hawaii)

Abstract Hawaiian streams are flashy in nature because watersheds are small and steep and receive intense and unevenly distributed rainfall. As a result, stream chemistry is characterized by considerable spatiotemporal variability. To examine how rainfall and streamflow affect the solute content of the Manoa River in Hawaii, time-series geochemical data collected during 17 sampling campaigns in spring-fall of 2010 were evaluated in a coupled δ 13 C DIC /major ion inversion model. Spatially, the stream is characterized by a distinct shift from a low HCO 3 (43 mg/L), low pCO 2 (3760 ppmv) and heavy δ 13 C DIC (−6.5‰) fingerprint in the upper reaches to a high HCO 3 (91 mg/L), high pCO 2 (8961 ppmv) and light δ 13 C DIC (−11.7‰) signature in the lowlands. These trends are attributed to (1) progressive weathering of exposed aluminosilicates, and (2) downstream enrichment in CO 2 from organic matter decay in the soil zone. Solute (i.e., nitrate) yields from nitric acid weathering are generally low ( 13 C DIC and rainfall rates in the mid-stream section of the river which is consistent with an atmospheric CO 2 dilution effect during high rainfall events. This dilution effect needs to be accounted for to reliably describe the role of volcanic island river systems in global assessments of silicate weathering and CO 2 degassing.