1. Introduction Rivers are generally supersaturated with CO2

1-3. The rate of CO2 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 ocean4-5. Most of the excess CO2 in rivers originates from terrestrial organic matter1,6. Land use, therefore, is likely an important control on the amount and sources or turnover times of riverine CO2. In this study, we directly measured partial pressure of dissolved CO2 (pCO2), and carbon isotopic signatures (Δ14C and δ13C) 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 CO2 source to the atmosphere and the potential impact of urbanization on the amount and sources of evaded CO2, both of which are poorly known.

The Effects of Land Use on Riverine CO2 Isotopic Signatures in the US Gulf Coast

F.W. Zeng, C.A. Masiello

Department of Earth Science, Rice University, Houston, TX 77005

fwzeng@rice.edu, masiello@rice.edu

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 14C-POC analysis.

5. Conclusions(1) Subtropical rivers: maybe a large CO2 source to the atmosphere

(i) Both urbanized and undeveloped rivers studied are highly supersaturated in CO

2 with respect to the atmosphere; mean CO2 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) pCO2 and CO2 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 CO2 outgassing from the urbanized river

(i) Respiration of young organic matter sustains CO2 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.

References1. Mayorga, E., et al. (2005) Nature, 436, 538-541. 2. Raymond, P.A., et al. (1997) Estuaries, 20, 381-390.3. Yao, G., et al. (2007) Sci. of the Total Environ., 376, 255-266.

4. Richey, J.E., et al. (2002) Nature, 416, 617-620. 5. Rasera, M.F.F.L., et al. (2008) Earth Interactions, 12(6), 1-16.6. Cole, J.J. and Caraco, N.F. (2001) Mar. Freshwater Res., 52, 101-110.

3. Methods3.1 Estimating river CO2 emission fluxes We directly measured partial pressure of dissolved CO2 (pCO2) in surface water and the ambient air, and calculated the fluxes of CO2 emission as follows 2:

kCO2: gas exchange coefficient for CO2; pCO2,w and pCO2,a: pCO2 in surface water and the overlying air, respectively; Kh: Henry’s law constant; 3.2 Tracking sources of riverine CO2

Riverine DIC samples were sent to the National Ocean Sciences Accelerator Mass Spectrometry Facility (NOSAMS) in Woods Hole for 13C and 14C analysis; We analyzed 14C of riverine POC at the Keck Carbon Cycle AMS laboratory of the University of California, Irvine.

))(( ,2,22 hawCO KpCOpCOkFlux

4.2 Different riverine CO2 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 pCO2 for both rivers: 37961843 atm (Amazon: 4350±1900 atm4);

pCO2: the undeveloped river (4810±1979 atm) > the urbanized river (3107±1379

atm) (p<0.001);

CO2 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 CO2

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 CO2

Young CO2 in the undeveloped river: respiration of young OM;

Old POC may account for only a small fraction of the old CO2 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 CO2 in the urbanized river.

Likely source of more 14C-enriched (young) and 13C-depleted DIC in Spring Creek: young organic matter (OM) respiration;

Potential sources of more 14C-depleted (old) and 13C-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 CO2 is consistent with distribution of shell roads 8;

Pedogenic carbonate: present in the Vertisols in the Beaumont Formation 9.

River Land use Climate Geologic setting

Buffalo Bayou 80% urbanizedMAT: 21°C

MAP: 100-180cm

Mainly Beaumont Formation (clay, silt and

sand, no carbonate bedrock)

Spring Creek

Dominantly forested, some agriculture and residential use

MAT: 21°C

MAP: 100-180cm

Mainly Willis Formation (clay, silt and sand, no

carbonate bedrock)

Downtown Houston

Fig. 1 Study area

Fig. 2 Monthly pCO2 for Buffalo Bayou

and Spring Creek. Dash line is the

mean pCO2 of the atmosphere,

409 atm.

-30

-25

-20

-15

-10

-5

0

5

-500.0 -400.0 -300.0 -200.0 -100.0 0.0 100.0 200.0 300.0

14C-DIC (‰)

13

C-D

IC (

‰)

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.

-350.0

-250.0

-150.0

-50.0

50.0

150.0

Jun-07 Aug-07 Oct-07 Dec-07 Feb-08 Apr-08 Jun-08 Jul-08 Sep-08

Month

14

C (

‰)

Spring Creek

Buffalo Bayou

POCDIC

Fig. 4 14C of POC and DIC for Buffalo Bayou and Spring Creek

Shell roads

Fig. 5 Shell road distribution compiled from Doran, 1965

7. Raymond, P.A., et al. (2004) Mar. Chem., 92, 353-366.8. Doran, E. (1965) Geogr. Rev., 55(2), 223-240.9. Nordt, L., et al. (2006) The Journal of Geology, 114, 501-510.