Controls on the origin and cycling of riverine dissolved inorganic carbon in the Brazos River, Texas Fan-Wei Zeng • Caroline A. Masiello • William C. Hockaday Received: 22 January 2010 / Accepted: 21 June 2010 Ó Springer Science+Business Media B.V. 2010 Abstract Rivers draining watersheds that include carbonate bedrock or organic matter (OM)-rich sedi- mentary rocks frequently have 14 C-depleted dissolved inorganic carbon (DIC) relative to rivers draining carbonate- and OM-free watersheds, due to dissolution of carbonate and/or decomposition of ancient OM. However, our results from a subtropical river, the Brazos River in Texas, USA, show that in this watershed human activities appear to dominate basin lithology in controlling the origin and metabolism of DIC. The middle Brazos flows through limestone and coal-bearing bedrock, but DIC isotope data suggest no limestone dissolution or respiration of ancient OM, and instead reflect efficient air–water CO 2 exchange, degradation of relatively young OM and photosynthe- sis in the river as a result of river damming and urban treated wastewater input. The lower Brazos drains only small areas of carbonate and coal-bearing bedrock, but DIC isotope data suggest the strong influence of carbonate dissolution, with a potentially minor contri- bution from decomposition of old soil organic matter (SOM). Oyster shells and crushed carbonate minerals used in road construction are likely sources of carbonate in the lower Brazos, in addition to natural marl and pedogenic carbonate. Additionally, the generally low pCO 2 and high DIC concentration in the Brazos River lead to a low CO 2 outgassing:DIC export ratio, distinguishing the Brazos River from other rivers. Keywords Carbon cycle  Dissolved inorganic carbon  Lithology  pCO 2  River system Introduction Conservative estimates show that globally, rivers receive at least 1 Gt C year -1 (in both organic and inorganic forms) from land (Cole et al. 2007; Meybeck 1993), equivalent to half of the net oceanic uptake of anthropogenic CO 2 (Sarmiento and Sund- quist 1992). Of the terrestrial carbon transported by river systems, at least 0.35 Gt year -1 is released to the atmosphere as CO 2 , and about 0.7 Gt year -1 is exported to the ocean (Cole et al. 2007). One important control on the source and fate of terrestrial carbon in river systems is basin lithology, as reflected in the isotope values of riverine dissolved inorganic carbon (DIC = dissolved CO 2 ? HCO 3 - ? CO 3 2- ). Black shales rich in organic matter (OM) in the northeast US provide rivers with very old organic carbon, and decomposition of this ancient terrestrial organic carbon may have contributed to the low D 14 C values of the DIC (-69 to -6%) and to evasion of CO 2 F.-W. Zeng (&)  C. A. Masiello  W. C. Hockaday Department of Earth Science, Rice University, P.O. Box 1892, Houston, TX 77251-1892, USA e-mail: fwzeng@rice.edu; fwzeng@gmail.com W. C. Hockaday Geology Department, Baylor University, One Bear Place #97354, Waco, TX 76798, USA 123 Biogeochemistry DOI 10.1007/s10533-010-9501-y