2008 Joint Meeting of The Geological Society of America, Soil Science Society of America, American Society of Agronomy, Crop Science Society of America, Gulf Coast Association of Geological Societies with the Gulf Coast Section of SEPM

Paper No. 10
Presentation Time: 10:50 AM

Impact of Urbanization on CO2 Fluxes and Sources from Houston-Area Watersheds


ZENG, Fanwei and MASIELLO, Carrie A., Earth Science, Rice University, 6100 Main Street, Houston, TX 77005, fwzeng@rice.edu

Rivers are generally supersaturated with CO2, implying a role as a conduit of soil CO2 to the atmosphere. Knowledge of the fluxes and sources of CO2 evaded from rivers is essential to regional carbon budgets. In this study, we directly measured partial pressure of CO2 (pCO2) for two nearby subtropical rivers, Buffalo Bayou and Spring Creek in southeast Texas, from June 2007 to May 2008. Buffalo Bayou's watershed is almost entirely contained within the city of Houston, while Spring Creek is not yet urbanized. There is no significant difference in pCO2 between the two rivers. The overall average pCO2 is 3589 ± 1464 μatm, and mean CO2 evasion rate is 6.84 Mg C ha-1 y-1, comparable to the Amazon.

Carbon isotopic signatures of riverine dissolved inorganic carbon (DIC) point to different CO2 sources for the two rivers. DIC in Spring Creek is generally young and more d13C-depleted, indicating that a young carbon pool is cycling rapidly and sustaining the high CO2 concentration in Spring Creek. For Buffalo Bayou, DIC is always depleted in Δ14C and more enriched in d13C, suggesting an input of DIC from carbonate dissolution. This is also supported by a local soil survey, which shows the presence of soil carbonates in the Buffalo Bayou watershed.

There are two possible sources of soil carbonates in the Buffalo Bayou watershed: naturally precipitated pedogenic CaCO3, and CaCO3 from shells used to make roads in East Texas in the early 20th century. In the first scenario, soil carbonates are likely in steady state and thus act only to shift the isotopic signature of respired riverine CO2. In the second scenario, shells from road building are both a net source of CO2 to the atmosphere and are altering the C isotopic signature of Houston-area respiration.