1. Introduction

1
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_at_rice.edu,
masiello_at_rice.edu
NACP
1. Introduction Rivers are generally
supersaturated with CO21-3. 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 (?14C and
d13C) 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.
4. Results
4.3 Carbonate an important source of old
CO2
4.1 Our humid subtropical rivers are highly
supersaturated with CO2
2. Study area Houston metropolitan
area

• 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 gt100 times (generally 10
  times) higher than POC concentration
• Carbonate maybe an important source of old CO2
  in the urbanized river.

• Seasonal pattern high in summer and fall, low
  in winter and spring
• Mean pCO2 for both rivers 37961843 matm
  (Amazon 43501900 matm4)
• pCO2 the undeveloped river (48101979 matm) gt
  the urbanized river (31071379 matm) (plt0.001)
• CO2 emission flux the undeveloped river
  (9.054.02 Mg C ha-1 y-1) gt the urbanized river
  (5.532.78 Mg C ha-1 y-1) (plt0.001).

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.

4.2 Different riverine CO2 sources between the
urbanized and undeveloped rivers

Fig. 4. Carbon isotopic signatures of DIC for
Buffalo Bayou and Spring Creek in comparison with
previous studies(1,7).
River Land use Climate Geologic setting
Buffalo Bayou 80 urbanized MAT 21C MAP 100-180cm Mainly Beaumont Formation (clay, silt and sand, no carbonate bedrock)
Spring Creek Dominantly forested, some agriculture and residential use MAT 21C MAP 100-180cm Mainly Willis Formation (clay, silt and sand, no carbonate bedrock)

• 5. Conclusions
• (1) Subtropical rivers maybe a large CO2 source
  to the atmosphere
• (i) Both urbanized and undeveloped rivers
  studied are highly supersaturated in CO2 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.

MAT mean annual temperature MAP mean annual
precipitation.

• Likely source of more D14C-enriched (young) and
  d13C-depleted DIC in Spring Creek young organic
  matter (OM) respiration
• Potential sources of more D14C-depleted (old)
  and d13C-enriched DIC in Buffalo Bayou carbonate
  dissolution and/or old OM respiration.

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 D14C-POC analysis.

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