I' I 

EMISSION OF METHRNE AND OTHER TRACE GASES 

FROM THE AMAZON VARZEA 

Final Report 

Grant # NAGW-711 

University of Washington # 62-7518 

Principal Investigators: 

Jeffrey E. Richey 

Allan H. Devol 

Quaternary Research Center 

and 

School of Oceanography 

University of Washington 

Seattle, Wa 98195 

Period of Grant: 

January 1, 1985 - December 31, 1986 

i (EASA-CR-177042) EBI55lCb CF EE'IHANE A80 ~67-1a926 

CILBEB ?RACE GASES FECE THE BFAZCE VABZEA 

Vinal Resort, 1 Jan, 19E5 - Dee- 1986 (iiashiaqton Uziv-) 6s F CSCL 130 Unclas 

G3/45 43442 
1 .O SUMMARY OF RESULTS 

The Amazon River is bordered by an extensive floodplain, or 

"varzea." The varzear an area of 100,000 km" in Brazil aloner 

extends from the bed of the Amazon landward to the slope of the 

terra firme, and consists of fertile alluvial soils with gallery 

forests, grasslands, lakes (Lagos), flooded fields and swamps, 

and intermediate channels ("paranas"). It is periodically 

inundated by the annual 10 m rise and fall of the main channel; 

such that during high water much of the land will be 6-8 m deep. 

At this point the lakes become deep enough to stratify, on a time 

scale of weeks. As the river falls, land is re-exposed and lake 

levels drop to 1-2 m. 

As part of the ABLEIAGE-IIa experiment (July-August 1985, 

early falling water), we proposed to measure the distributions 

and fluxes of methane and other trace gases from the various 

Amazon floodplain environments. These were determined during both 

a large scale, quasi-synoptic survey along a 2000-km reach of the 

Amazon river and an intensive local study (by J. Melack, R. 

Harriss et al.) covering a six-week period. The environments 

studied included the major rivers, connecting channels (paranas), 

floating macrophyte beds, flooded forests? open lakes and 

recently wetted soils. The results are summarized below; see the 

attached manuscripts for detail. 

The results from both the large scale and intensive studies 

were in general agreement. Measured rates of methane emission 

averaged about 300 mg m-'-E d-"' but with considerable variance, 

and were comparable to or higher than previously reported 
emissions from similar temperate zone environments. In 

general, areas covered by floating macrophytes showed the highest 

emissions. Individual "hotspots" had among the highest rates ever 

observed, over 10 g m-'-" d"'"'.. Data taken during the CAMREX 

program (University of Washington, NSF-funded), which covered a 

three year period, suggest that there are significant spatial and 

temporal trends in dissolved methane concentration (fluxes were 

calculated, not measured directly), but the magnitude of the flux 

calculated for other times of the year is probably comparable to 

the AGE-IIa measurements. 

The high methane emissions appear to result because about 

50% of the organic matter fixed on the floodplain (either 

terrestrial or aquatic) that is oxidized in the water i5 

decomposed anaerobically via methanogenesis. A redox 

gradient exists, with flowing waters being the most oxidized and 

still varzea waters being the most reduced. Ebullition accounted 

for 70-90% of the measured flux. Measured fluxes of methane to 

the atmosphere appear to be significantly correlated with surface 

water dissolved methane concentrations. Diffusive fluxes appear 

to be directly related to the dissipation of turbulent 

kinetic energy in the upper mixed layer; mixing rates were 

obtained from measurements of thermal structure and meterological 

conditions. From AGE-IIa data, the del 1.z3C of CH, dissolved in 

anaerobic bottom waters and in bubbles released from the 

sediments is considerably lighter upstream9 -65 o/oo, than 

downstreamr -45 o/oo, with the bubbles being about 5 o/oo lighter 

than the dissolved CH,. in the water column. 
Extrapolation from the AGE-IIa measurements to the entire 

Amazon basin and adjacent regions would yield methane emissions 

to the troposphere of 10-20 Tg/y. It is important to note that 

the AGE-IIa fluxes were obtained during one time period, and that 

extrapolation to a full year is tentative. However, if this 

extrapolation is reasonable, the estimate represents a 

significant fraction of the total global source of between 200 

and 550 Tg/y (Seiler 1984, Khalil and Rasmussen 1983, and 

others). 

Our results to date, which are based on sampling limited in 

time and space, are very provocative, but not conclusive. Based 

on these results9 we pose the following hypotheses concerning the 

overall magnitude and dynamics of methane flux in the Amazon: 

Hypothesis 1. Mean Methane Emissions to the Troposphere 

The dominant source of CrC., to the Amazon troposphere is the C1~.-.Arrl IruuupAair: * (varzea!. The mear! ann==! flnx is cy? the nrrl~r cllf 300 

mg m”-= d--x. This flux is the integral of considerable variance 

in mechanisms of emission and spatial and temporal distributions: 

fa) Temporally, high emission rates are expected during 

rising waters, due to the entrainment and decomposition of 

terrestrially-derived plant material, whereas during falling 

water rates are expected to be high due to the die off and decay 

of the floating macrophyte vegetation, reduced hydrostatic 

pressure and release of accumulated gas. 

(b) Spatiallyr two regions should dominate the flux; upriver 

in the Japura confluence zone and downriver near Obidos. The Rio 

Japura is the region of the strongest redox gradients from 

mainstem to the floodplain, especially during high water periods. 

Downriver, lakes are larger, rounderr and more oxygenated due to 

greater wind mixing. Although emissions per unit area from open 

waters in this area are probably less than upriver, the more 

extensive macrophyte cover may result in high emissions from this 

area also. 

(c) The del composition of the CH, emitted to the 

atmosphere upriver is lighter than the global average, while that 

emitted downriver is comparable. 
Hypothesis 2. CH+ and CO, Source Dynamics 

particular sequence of organic matter oxidation that is 

established as a function of carbon fixed in the water and in the 

bordering terrestrial zone and degree and timing of inundation 

and physical mixing: 

High CH,, production in the varzea is a function of a 

(a) Net carbon oxidation on the floodplain is on the order 

of 1700 mg m'-2 d'-". approximately 50% of this oxidation is 

anaerobic, as methanogenesis, with about 20% of the carbon lost 

to the atmosphere as CH+. 

(b) Although ebullition appears to be the major pathway of 

transport to the atmosphere9 the relative importance of diffusion 

may increase during rising water and, especially9 during periods 

of lake overturn and venting. 

(c) The dissolved CH.,. is isotopically heavier than bubble 

CH+ because dissolved CH,. is the residual after CHr. oxidation 

(which fractionates towards heavier carbon). The downstream 

trend is due both to a greater percentage of C-4 (isotopically 

heavy) plant material in the sediments downriver and to increased 

rates of CH24 oxidation that result from greater physical mixing 

and, hence9 greater oxygen input in the downriver lakes. 
11. MANUSCRIPTS SUPPORTED BY THIS GRANT 

Richey, J.E., A.H. Devoly S.C. Wofsy, R.Victoria, and M.N.G. 

Ribeiro. (in review). Oxidation and reduction rates for organic 

carbon in the Amazon mainstream, tributaries, and floodplainl 

inferred from distributions of dissolved gases. Limnol. Oceanogr. 

Devol, A.H., J.E. Richey? W.A. Clark, S. King, and L.A. 

Martinelli. (in review). Methane emissions from the Amazon 

floodplain. J. Geophys. Res. 

Quay, P.D.3 S. King, D. Wilbur, S. Wofsy, andJ. Richey. (in 

prep. 9 soon-to-be-submitted). nrrC/1zY2 of atmospheric Cor:;> in the 

Amazon Basin: forest and river sources. J. Geophys. Res. 

[not included].