Assessment of River Margin Air-Sea CO2 Fluxes

1
Assessment of River Margin Air-Sea CO2 Fluxes

• Steven E. Lohrenz, Wei-Jun Cai, Xiaogang Chen,
• Merritt Tuel, and Feizhou Chen

2
Introduction

• A major objective of the U.S. Global Change
  Research Program Climate Change Science Program
  Strategic Plan and the North American Carbon
  Program is the application of satellite ocean
  color to characterize the spatial variability of
  air-sea CO2 flux in the oceans adjacent to the
  North American continent

3
Introduction

• Despite their relatively small surface area,
  ocean margins may have a significant impact on
  global biogeochemical cycles, and potentially, in
  the global air-sea fluxes of CO2

4
Introduction

• Margin ecosystems receive inputs of terrestrial
  organic and mineral matter and exhibit intense
  geochemical and biological processing of carbon
  and other elements
• Nowhere is this more evident that in regions
  influenced by large rivers

5
Introduction

• Recent studies (Lohrenz and Cai, 2006) in the
  northern Gulf of Mexico and elsewhere (Tsunogai
  et al., 1999 Ternon et al., 2000 Cai, 2003)
  demonstrate that enhanced biological production
  in large river plumes may lower surface pCO2
  levels such that these regions may exhibit a net
  surface influx of atmospheric CO2
• But characterizations have not been conducted
  over representative seasonal and discharge
  conditions

6
Introduction

• Systems are likely to differ depending on the
  both biological, chemical, and meteorological
  properties
• Quantification of the contributions of
  river-influenced margins to regional CO2 fluxes
  is difficult due to the high degree of spatial
  and temporal variability in these regions

7
Introduction

• Satellite-based regional approaches (e.g.,
  Lefevre et al., 2002 Ono et al., 2003 Olsen et
  al., 2004 Lohrenz and Cai, 2006) can be used to
  extend the spatial and temporal coverage for
  broad scale assessments of pCO2 distributions and
  air-sea fluxes of CO2
• The primary objective of our research is to apply
  these approaches to the characterization of pCO2
  and air-sea fluxes of CO2 in the river-influenced
  margin of the northern Gulf of Mexico

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Outline

• Brief description of approach as applied to the
  Mississippi River outflow region
• Summary of initial findings
• June 2003
• August 2004
• October 2005
• Challenges and future directions

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Approach
10
Approach

• In situ measurements of surface pCO2 and
  environmental variables (T, S, chlorophyll)
  conducted

11
Approach

• Algorithm based on empirical relationships of in
  situ measurements of surface pCO2 to
  environmental variables (T, S, chlorophyll)
• June 2003

12
Approach

• Apply algorithm to products retrieved from
  MODIS-Aqua imagery

13
Approach

• Produce pCO2 map

14
Approach

• Additional cruises in August 2004 and October
  2005
• August 2004 October 2005

15
Approach

• PCA algorithms
• August 2004 (r20.858) October 2005
  (r20.974)

16
Results

• Aug 2004

August 2004
17
Results
18
Results

• Aug 2004

October 2005
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Results
20
Results

• Air-sea flux calculations
• CO2 fluxes across the air-sea interface were
  calculated by the one-dimensional thin-film
  model
• CO2 Flux k?pCO2(w) pCO2(a)
• where k is the gas transfer velocity and has the
  largest uncertainty in the flux calculation ? is
  the solubility of CO2 at given temperature and
  salinity

21
Results

• Air-sea flux calculations
• Various sets of the gas transfer velocity, k, vs.
  wind speed relationships were used to provide a
  range of values to bracket the gas flux
  including
• Liss and Merlivat, 1986 Wanninkhof and McGillis,
  1999 Nightingale et al., 2000 McGillis et al.,
  2001

22
Results

• There was net uptake of CO2 in June, but a net
  release in August and October
• October 2005 followed two major storm events
  accompanied by coastal flooding strong winds

Cruise Air-Sea Flux (mmol m-2 d-1)
June 2003 -(2-4)
August 2004 6-10
October 2005 53-120
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Challenges and Future Directions

• Challenges
• Variability in conditions in coastal waters
  limits the generality of algorithms across
  temporal and spatial ranges
• Remedy is to provide ample in situ data,
  including ship-based surveys and moored
  time-series
• Couple ship-based operations with radiometric
  measurements to provide complementary data set

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Challenges and Future Directions

• Future Directions
• Continued characterization of Mississippi River
  system to capture range of seasonal and discharge
  conditions
• Comparison of other river systems and shelf
  environments
• Relate patterns to biogeochemical rates (primary
  production, net ecosystem metabolism)
• Couple observations with ecosystem models to
  provide more informed understanding of carbon
  cycle controlling mechanisms

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Acknowledgements

• Funding for this work was provided by NASA
  (NNG05GD22G and NNS04AB84H)