2006 OCRT Meeting, Providence

Assessment of River Margin Air-Sea CO2 Fluxes

Steven E. Lohrenz, Wei-Jun Cai, Xiaogang Chen,

Merritt Tuel, and Feizhou Chen

IntroductionIntroduction

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

IntroductionIntroduction

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

IntroductionIntroduction

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

IntroductionIntroduction

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

IntroductionIntroduction

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

IntroductionIntroduction

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

OutlineOutline

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

ApproachApproach

ApproachApproach

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

ApproachApproach

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

June 2003

ApproachApproach

Apply algorithm to products retrieved from MODIS-Aqua imagery

ApproachApproach

Produce pCO2 map

June 2003

ApproachApproach

Additional cruises in August 2004 and October 2005

August 2004 October 2005

ApproachApproach

PCA algorithms

August 2004 (r2=0.858) October 2005 (r2=0.974)

ResultsResults

Aug 2004

August 2004

ResultsResults

ResultsResults

Aug 2004

October 2005

ResultsResults

ResultsResults

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

ResultsResults

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

ResultsResults

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

CruiseAir-Sea Flux (mmol m-2 d-1)

June 2003 -(2-4)

August 2004 6-10

October 2005 53-120

Challenges and Future DirectionsChallenges 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

Challenges and Future DirectionsChallenges 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

AcknowledgementsAcknowledgements

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