1. Introduction Rivers are generally supersaturated with CO The rate of CO 2 evasion from rivers to the atmosphere is on an order of 1 Gt C per year globally, comparable to annual river total organic carbon (TOC) or dissolved inorganic carbon (DIC) export to the ocean 4-5. Most of the excess CO 2 in rivers originates from terrestrial organic matter 1,6. Land use, therefore, is likely an important control on the amount and sources or turnover times of riverine CO 2. In this study, we directly measured partial pressure of dissolved CO 2 (pCO 2 ), and carbon isotopic signatures (Δ 14 C and δ 13 C) of DIC and particulate organic carbon (POC) in two subtropical North American rivers, one entirely urbanized and the other almost undeveloped. Our goal is to evaluate the role of subtropical rivers as a CO 2 source to the atmosphere and the potential impact of urbanization on the amount and sources of evaded CO 2, both of which are poorly known. The Effects of Land Use on Riverine CO 2 Isotopic Signatures in the US Gulf Coast F.W. Zeng, C.A. Masiello Department of Earth Science, Rice University, Houston, TX Acknowledgements This work was financed by the Texas Water Resources Institute through a grant program supported by the U.S. Geological Survey and the National Institutes for Water Research. We acknowledge the generous support of Hans O. and Suze Jahns. We acknowledge Dr. Xinfeng Shi, Dr. Yanlu Ma, Dr. William Hockaday, Shuaiping Ge, Kaijian Liu, Jianping Chen, Yongbo Zhai, Jianping Huang, LaQuanti Calligan, Krystle Hodge, Xuan Guo, Baoshan Wang, Yan Chen, Li Zhang, Yan Zhou, Qinglian Chen, Wei Chen, Xinling Wang and Lacey Pyle for their help with sample collection. We also acknowledge people at the Keck Carbon Cycle AMS laboratory of the University of Californina, Irvine for their valuable assistance in  14 C-POC analysis. 5. Conclusions (1) Subtropical rivers: maybe a large CO 2 source to the atmosphere (i) Both urbanized and undeveloped rivers studied are highly supersaturated in CO 2 with respect to the atmosphere; mean CO 2 emission flux for the two rivers is 7.3 Mg C ha -1 y -1, close to the Amazon and much higher than northeast US rivers (1,4,6). (ii) pCO 2 and CO 2 emission flux are higher in the undeveloped river than the urbanized river, probably due to higher carbon load from the forests and shallower water depth; (2) Old CO 2 outgassing from the urbanized river (i) Respiration of young organic matter sustains CO 2 supersaturation in the undeveloped river; (ii) The urbanized river is releasing much older carbon to the atmosphere than the undeveloped river. This old carbon may mainly come from carbonate dissolution, with a small contribution of old organic matter respiration. Potential sources of carbonate are shells and limestone gravels used as road construction material, and pedogenic carbonate. References 1. Mayorga, E., et al. (2005) Nature, 436, Raymond, P.A., et al. (1997) Estuaries, 20, Yao, G., et al. (2007) Sci. of the Total Environ., 376, Richey, J.E., et al. (2002) Nature, 416, Rasera, M.F.F.L., et al. (2008) Earth Interactions, 12(6), Cole, J.J. and Caraco, N.F. (2001) Mar. Freshwater Res., 52, Methods 3.1 Estimating river CO 2 emission fluxes We directly measured partial pressure of dissolved CO 2 (pCO 2 ) in surface water and the ambient air, and calculated the fluxes of CO 2 emission as follows 2 :  k CO2 : gas exchange coefficient for CO 2 ;  pCO 2,w and pCO 2,a : pCO 2 in surface water and the overlying air, respectively;  K h : Henry’s law constant; 3.2 Tracking sources of riverine CO 2  Riverine DIC samples were sent to the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) in Woods Hole for  13 C and  14 C analysis;  We analyzed  14 C of riverine POC at the Keck Carbon Cycle AMS laboratory of the University of California, Irvine. 4.2 Different riverine CO 2 sources between the urbanized and undeveloped rivers 4. Results 2. Study area: Houston metropolitan area  Seasonal pattern: high in summer and fall, low in winter and spring;  Mean pCO 2 for both rivers: 3796  1843  atm (Amazon: 4350±1900  atm 4 );  pCO 2 : the undeveloped river (4810±1979  atm) > the urbanized river (3107±1379  atm) (p<0.001);  CO 2 emission flux: the undeveloped river (9.05±4.02 Mg C ha -1 y -1 ) > the urbanized river (5.53±2.78 Mg C ha -1 y -1 ) (p<0.001). 4.1 Our humid subtropical rivers are highly supersaturated with CO 2 NACP Fig. 4. Carbon isotopic signatures of DIC for Buffalo Bayou and Spring Creek in comparison with previous studies (1,7). 4.3 Carbonate: an important source of old CO 2  Young CO 2 in the undeveloped river: respiration of young OM;  Old POC may account for only a small fraction of the old CO 2 in the urbanized river: DIC concentration is 4 to >100 times (generally ≥10 times) higher than POC concentration;  Carbonate maybe an important source of old CO 2 in the urbanized river.  Likely source of more  14 C-enriched (young) and  13 C-depleted DIC in Spring Creek: young organic matter (OM) respiration;  Potential sources of more  14 C-depleted (old) and  13 C-enriched DIC in Buffalo Bayou: carbonate dissolution and/or old OM respiration. 4.4 Possible carbonate sources in the watershed of the urbanized river  Shells and limestone gravels used in road construction: distribution of old riverine CO 2 is consistent with distribution of shell roads 8 ;  Pedogenic carbonate: present in the Vertisols in the Beaumont Formation 9. RiverLand useClimateGeologic setting Buffalo Bayou80% urbanized MAT: 21°C MAP: cm Mainly Beaumont Formation (clay, silt and sand, no carbonate bedrock) Spring Creek Dominantly forested, some agriculture and residential use MAT: 21°C MAP: cm Mainly Willis Formation (clay, silt and sand, no carbonate bedrock) Downtown Houston Fig. 1 Study area Fig. 2 Monthly pCO 2 for Buffalo Bayou and Spring Creek. Dash line is the mean pCO 2 of the atmosphere, 409  atm  14 C-DIC (‰)  13 C-DIC (‰) Buffalo Bayou Spring Creek. Atmosphere Carbonate Weathering Old OM Respiration Young OM Respiration H P A Y A: Amazon (carbonate free) Y: York River P: Parker River H: Hudson River Fig. 3 Isotopic signatures of DIC in Bufffalo Bayou and Spring Creek, as well as all DIC sources. MAT: mean annual temperature; MAP: mean annual precipitation. Spring Creek Buffalo Bayou POC DIC Fig. 4  14 C of POC and DIC for Buffalo Bayou and Spring Creek Shell roads Fig. 5 Shell road distribution compiled from Doran, Raymond, P.A., et al. (2004) Mar. Chem., 92, Doran, E. (1965) Geogr. Rev., 55(2), Nordt, L., et al. (2006) The Journal of Geology, 114,