Early-diagenetic methane from various tropical freshwater sediments: molecular and carbon isotope variations in one dial cycle

Annales S o c ieta tis G e o lo g o m m P o lon iae (1997), vol. 67: 93-101.

EARLY-DIAGENETIC METHANE FROM VARIOUS TROPICAL FRESHWATER SEDIMENTS: MOLECULAR AND CARBON

ISOTOPE VARIATIONS IN ONE DIAL CYCLE

Mariusz Orion JĘDRYSEK1, Stanisław HAŁAS2, Eitaro WADA3, Suporn BOONPRAKUP4, Shingo UEDA5, Pisoot VIJARNSORN4 & Yasuo TAKAI6

1 L a b o r a t o r y o f I s o to p e G e o lo g y a n d B io g e o c h e m is tr y , U n iv e r s ity o f W r o cła w , C y b u ls k ie g o 3 0 , 5 0 - 2 0 5 W r o c la w , P o la n d M a s s S p e c tr o m e tr y L a b o r a to r y , M a r ia C u r ie -S k lo d o w s k a U n iv e r s ity , 2 0 - 0 3 1 L u b lin , P o la n d

^ C e n te r f o r E c o lo g i c a l R e s e a r c h , K y o to U n iv e r s ity , S h im o s a k a m o to 4 -1 -2 3 , O tsu , S h ig a 5 2 0 - 0 1 , J a p a n 4 D e p a r tm e n t o f L a n d D e v e lo p m e n t, S o il S u r v e y a n d C la s s if ic a tio n D iv is io n , B a n g k h en , B a n g k o k 1 0 9 0 0 , T h a ila n d

5 N a tio n a l I n s titu te f o r R e s o u r c e s a n d E n v iro n m e n t, T su ku ba, O n o g a w a 16, I b a r a k i 3 0 5 , J a p a n 6 N o d a i R e s e a r c h I n stitu te , T o k yo U n iv e r s ity o f A g r ic u ltu r e , S e ta g a y a , T o k y o 154, J a p a n .

Jędrysek, M. O., Halas, S., Wada, E., Boonprakup, S., Ueda, S., Vijarnsorn, P. &Takai, Y., 1997. Early-diagenetic methane from various tropical freshwater sediments: molecular and carbon isotope variations in one dial cycle.

Ann. Soc. G eol. P olon ., 67: 93-101.

Abstract: In the tropical climate o f Southern Thailand, on September 5 and 6, 1994. dial emissions o f CO2 and CH4, and dial variations in bubble CO2 and CH4 concentrations and the 13C /I2C ratio in bubble methane were observed in shallow freshwater sediments o f a high sulphate concentration pit, a high turbulence canal, and in a pond with a high density o f emerged macrophytes. Measurements show that 5 I3CH4 values are lowest in the night, and highest in the daytime, ranging from a minimum of-63.8%o at 02:20 (canal) to a maximum of-47.6% o at 6:45 (pit). High 5 5 I3CH4 values o f daytime methane occur when the concentration o f CO2 decreases and acetic acid fermentation becomes relatively more important. Minimum 5 I3CH4 values are apparently caused by an enhanced CO2-H2 methanogenesis pathway. Irregular diurnal variation o f the 5 13CH4 value in the pond was caused by oxidation and ventillation o f sediments by macrophytes and the different lithologies o f the sediments in separate sampling stations. Generally, 5 I3CH4 values in the pit and the pond were about 10%o higher as compared to values from the canal, suggesting an active acetic acid pathway and oxidation effect in the former two environments.

Likewise, the lowest production rate o f methane and the lowest 5 I3CH4 values in the canal suggest an enhanced CO2 reduction pathway. The maximal 5 I3CH4 value accompanied by the highest bubble CO2 concentration is caused by strong oxidation o f methane when the production rate o f methane in early morning is lowest.

Abstrakt: Badania dotyczyły dobowych zmian w emisji CO2 i CH4 z powierzchni wody oraz stężenia CO2 i CH4 a także stosunków izotopowych l3C /l2C w bąblach gazowych uwalnianych z osadów słodkowodnych klimatu tropikalnego. Opróbowania przeprowadzono w Południowej Tajlandii, 5 i 6 września 1994, na obszarach plytko- wodnych: wkopu wypełnionego wodą o wysokim stężeniu siarczanu, kanału melioracyjnego o silnej turbulencji wody i stawu gęsto zarośniętego roślinami ukorzenionymi. Stwierdzono, że wartość 5 3CH4 bąbli gazowych z osadów jest najniższa nocą a najwyższa w ciągu dnia, wahając się w zakresie od -63.8%o (godz. 02:20. kanał) do -47.6%o (godz. 6:45 wkop). Wysokie wartości S IjCH4 występują kiedy stężenie CO2 obniża się powodując proporcjonalnie większy udział fermentacji octowej w produkcji metanu. N iższe wartości 5 ,3CH4 są wynikiem wzmożonej metanogenezy opartej na redukcji CO2 przez H2. Nieregularne wahania 5 I3CH4 w osadach stawu są najprawdopodobniej wynikiem intensywnego, z różnym nasileniem, utleniania metanu, wentylacji osadu przez system korzeniowy roślin i różną litologię w różnych miejscach poboru prób. Generalnie, wartość 5 13CH4 była około 10%o wyższa w osadach wkopu i stawu niż w osadach kanału, co było najprawdopodobniej wynikiem niskiego udziału fermentacji octowej w osadach kanału oraz rezultatem intensywnego utlenieniem w osadach wkopu i stawu. Najwyższa wartość 8 13CH4, której towarzyszy najwyższe stężenie CO2 w osadzie, jest najpraw­

dopodobniej wynikiem silnego utleniania metanu nad ranem (najniższa produkcja metanu).

Key words: freshwater sediments, diagenesis, methane, carbon dioxide, carbon isotopes, greenhouse effect.

M an uscript re c e ic e d 2 2 A p ril 1996, a c c e p te d 3 0 Janu ary 1997.

94

INTRODUCTION

Early d iagen esis o f organic matter and sim ultaneous em issio n o f m ethane are crucial factors govern in g sub­

sequent generation o f hydrocarbons in later stages o f dia­

g en esis and burial o f th e sed im en ts. T hus, inform ation on the m ech an ism s o f early-d iagen etic m eth an ogen esis and on the isotop e signature o f eb u llitin g (natural bubbling from sed im en ts) m ethane appear to b e very important for hydro­

carbon exploration. H o w ev er, such inform ation is currently lim ited o n ly to the d ay-tim e m eth an ogen esis. On the other hand, m ethane em ission from w etlan d s to the atm osphere is o f concern e sp ecia lly in relation to the greenhouse effect (C raig & C hou, 1982; K hail & R asm ussen, 1983; C icerone, 1987; W ada, 1990). Inform ation on the exact isotop e co m ­ p osition o f m ethane from different em itters is crucial to the calculation o f the iso to p e m ass balance o f greenhouse gases.

Freshw ater sed im en ts are con sid ered to be the largest global em itter o f m ethane and the m ain factors govern in g isotope com p osition and production rate o f such m ethane are is o ­ top e com p osition o f m ethane precursors, tem perature, m ech an ism o f m eth an ogen esis and m ethane consum ption.

M ost inform ation on m eth an ogen esis in natural conditions has resulted from d ay-tim e sam p lin g o f early-diagenetic bacterial m ethane em itted from w etlands, paddy field s, landfills, w aste areas, etc. H o w ev er, it has been found re­

cen tly that m echanism s o f m eth an ogen esis in tem perate c li­

mate freshw ater sy stem s m ay vary w ith diurnal c y c le (Jędrysek e t a l., 1993; Jędrysek, 1994, 1995). T hus, one m ay infer that current k n o w led g e o f the m echanism o f m eth an ogen esis in natural freshw ater conditions is not e x ­ ten siv e en ou gh , and iso to p e valu es c o llected to the present day are not representative o f nearly h a lf o f the m ethane em itted from sedim ents. T he aim o f this paper is to sh ow that in a variety o f d ifferent conditions diurnal variation in stable carbon isotope ratios in m ethane from tropical fresh­

w ater sed im en ts m ay b e different due to diurnal variations in the m ech an ism s o f m eth an ogen esis and the oxidation e f­

fect (or m ech an ism o f oxid ation ).

GEOLOGICAL SETTING

O bservations on diurnal variation in m ethanogenesis in sed im en ts w ere m ade in the artificial aquifers (pit, canal and pond) in the experim ental field s o f the Pikum thong D e v e l­

opm ent R o y a l Study C enter (P D R S C ), N arathiw at Prov­

ince, Southern Thailand (F ig. 1). T his part o f Thailand is dom inantly characterised b y three m ain structural features, dom inated by granitoide batholiths, arrayed in N W -S E di­

rection. T he Eastern batholith co n sists o f m edium to coarse­

grained porphyritic b iotite granite, adam ellite and grano- diorite o f T riassic and Jurassic age. T he other tw o batholiths are dom inated by porphyritic biotite granite, granodiorite, h om b len d e-ad am ellite and fine-grained m u scovite tourm a­

line granite, form ed in C retaceous and Tertiary. M oreover, in the southern portion o f the area under consideration, adja­

cen t to M a la y sia border, so m e sedim entary seq u en ce co m ­ parable to Kanchanaburi F orm ation has also been observed.

It co n sists m ainly o f sandstones, shales, cherts, m atatuffs,

schists, quartzites, black shales and m u d ston es w ith lim e­

stone len ses. Quaternary d ep osits co v erin g the w h o le area are dom inantly peats and peat so ils, or sandy s o ils acid ified by marine sulphate (V ijam sorn, 1996; V ija m so m & L ieng- sakul, 1987).

FIELD OBSERVATIONS

Sam pling w as carried out on Septem ber 5 and S ep tem ­ ber 6, 1994. The pit w as ca. 50 m 2 in area, 2 m deep and free from any m acrophytes. The w ater in it w a s transparent, in so far that the bottom (2m depth) co u ld be seen , and lo w in su s­

pended and d isso lv ed organic matter. The pit w a s dug in su l­

phate-rich so il and the w ater is rich in sulphate. T he canal is a stream isolated from the so il b y an im p erm eable cem en t cover, ca. 0 .5 m deep and about 1.5 m w id e, and w a s free from any m acrophytes. T h ick n ess o f sed im en ts w as about 10 cm . W ater in it w as h ig h ly turbide (m axim u m flo w v e ­ lo city w as about 0.8 m /s) and w a s fairly rich in suspended so il material. H ow ever, it w a s transparent in so far as the sh a llo w depth 0.5 m had not a llo w e d one to ju d g e the trans­

parency lim it. T he pit and canal sed im en ts w ere very h o ­ m ogen ou s and sim ilar in each station. T he pond w as c. a. 0.1

EARLY DIAGENETIC METHANE FROM FRESHWATER SEDIMENTS

95

2

km in area, several m etres deep and rich in d isso lv ed or­

gan ic matter. Water in it sh o w ed very poor transparency (the bottom b e lo w the depth o f 0.1 5 m cou ld not be seen ). A bout 80% o f total surface o f the pond w as covered by lea v es o f em erged m acrophytes and several trees and bushes grow w ithin 2 - 3 m o f the sam p lin g stations. Sedim ents in the pond (cla y , sand, m ud to organic gyttia) w ere different at each sam p lin g station and the density o f em erged and sub­

m erged m acrophytes varied substantially - all o f w hich could result in variability o f the oxidation state in the sed i­

m ents.

SAMPLING

Carbon d ioxid e-an d -m eth an e-con tain in g gas m ixtures (bubbling and d iffu sin g from sedim ents and water) w ere c o llected in floating p lastic cham bers, for gas chrom atogra­

phy analysis. The v o lu m e o f a sin gle cham ber w as about 2 dm 3 and the co llectin g surface w as about 3 50 cm 2. Each cham ber w a s initially fille d w ith am bient atm osphere. The bottom o f the cham bers w ere op en ed to the underlying w ater surface and the tem perature in the cham bers w as not controlled. T h ese cham bers w ere situated in the sam e sam ­ p ling stations (over the sam e sed im en ts) throughout the du­

ration o f the observations and the first sam pling w as several hours after the cham bers w ere put on the w ater surface.

A bout 5 m l o f co llected gas, from the cham bers w as trans­

ferred by gas-tight syrin ge to bottles initially filled w ith IN HC1 solu tion (bubbled w ith H e to elim inate C O2 and C H4) and sealed w ith a butyl rubber cap and an alum inium seal.

S o m e solu tion rem ained in the bottles. B ottles w ith sam ples w ere stored in a refrigerator j|3—4°C ) in an inverted position.

B u b b le m ethane for * 'C /I2C an alysis w as obtained from subm erged sed im en ts by agitation and trapped in an in­

verted funnel about 2 0 cm in diam eter. The bubbles pro­

duced w ere introduced into a g la ss bottle filled w ith the aquifer water. For all sam pling, gases w ere co llected from 0 to about 15 cm depth in the sedim ents. Water depths over the sed im en ts from w h ich the gas w as sam pled w ere 2 .0, 0.5 and 0.7 m in the pit, canal and the pond resp ectively. The sam p lin g w as done at different stations, 0.5, 0.5 and 1.0 m from the banks o f the pit, canal and pond resp ectively. The stations a lo n g the banks w ere about 2 m apart. The sam pling tim e o f bubble gases w a s usually no longer than 2 m inutes.

S o m e w ater rem ained in the b ottles. T he bottles w ere sealed w ith a butyl rubber cap and an alum inium seal and the sam ­ p les w ere im m ed iately treated w ith H g C h solution. The b ot­

tles w ere a lw a y s in inverted p osition in order to prevent dif­

fusion. B o ttles w ith sam p les w ere transported to Poland by air-plane as a personal baggage, to prevent substantial vari­

ation o f pressure or tem perature and reorientation o f bottles.

B ottles w ith sam ples w ere stored in a refrigerator (3^ 1°C ) in an inverted position. R epeated analysis (after about 6 m onths) proved that the length o f the period over w hich sam ples w ere held before they w ere analysed did not influ­

en ce the results. M oreover, it w as proven p reviou sly that sam ples c o llected at the sam e tim e from the sam e depth o f water, from the sam e depths w ithin sedim ents ( 0 - 2 5 cm ) and from reasonably sim ilar sedim ents but at different sam ­

p ling stations, gave the sam e results w ithin an alytical error (Jędrysek e t a l., 1994; Jędrysek, 1994, 1995).

ANALYTICAL TECHNIQUES

C O2 and C H4 from floatin g collectors w ere determ ined in the laboratory u sin g gas chrom atograph eq u ip p ed w ith a F U V detector (H N U system M od el 3 2 1 ) se t up at D epart­

m ent o f Land D ev elo p m en t in B angkok, T hailand, and the bubble gases w ere analysed b y T C D gas chrom atograph 5 04 in the Laboratory o f Isotope G e o lo g y and B io g eo ch em istry , U n iversity o f W rocław , in Poland.

U sin g m olecular sie v e s, a d ry-ice-eth an ol m ixture, and liquid nitrogen, the m ethane in the sam ple g a ses w as cryo- g en ica lly purified under vacuum from other hydrogen-and- carbon containing gases. The m ethane w a s then passed through a copper o x id e furnace ( 8 5 0 - 9 0 0 ° C ) tw ic e . The products obtained, H2O and C O2, w ere separated cryogen i- cally. Isotope preparation w as carried out in the Laboratory o f Isotope G e o lo g y and B io g eo ch em istr y , U n iv ersity o f W rocław , in Poland. Carbon iso to p e a n alyses w ere m ade on a m od ified M I-1305 m ass spectrom eter w ith a hom e-m ade inlet (H ałas, 1979) and detection sy stem s (H ałas & Sko- rzyński,, 1980) at the M ass Spectrom etry Laboratory, Maria C u rie-S k łod ow sk a U n iversity, L ublin^Poland. P recision is 0.05%o, and ratios are exp ressed as 8 " C v a lu es relative to the P D B standard, using a m ass spectrom etric com parison o f w orking C O2 gas with C O2 prepared from N B S 19 and N B S 22 standards. The reproducibility o f iso to p e prepara­

tion w as ± 0 .0 5 to ±0.2%o.

RESULTS

Carbon dioxide and methane from floating chamber collectors

The observation tim e, usin g floatin g cham ber c o lle c ­ tors, w as probably o f too short duration to reveal clear diur­

nal variation in em ission o f m ethane and carbon d ioxid e from water. The results o f an alysis o f C O2 and C H4 c o n cen ­ trations in cham ber collectors are sh ow n on F ig. 2 and Table 1. In the pit, a large increase in concentration o f m ethane w as observed - from an initial valu e about 4 5 ppm one hour after setting up the g a s-c o lle c tin g cham ber to about 7 5 0 ppm after about 2 4 .5 hours. N o constant rate o f the increase o f concentration o f m ethane su g g ests that one bu b b lin g event occurred b etw een 14:00 and 18:00 (Fig. 1). In the case o f the canal the increase w as less rem arkable, from about 4 20 ppm after about 4 hour cham ber to about 4 8 0 ppm in the end o f the experim ent - after about 21 hours. In the ca se o f the pond, the initial value after 4 .5 h w a s ca. 2 0 0 ppm (m ore than after the sam e tim e in pit) and after about 25 hours o f co llectin g , the concentration o f m ethane w a s about 6 0 0 ppm (less than after the sam e tim e in the pit). A sim ilar, progres­

siv e increase in carbon d ioxid e concentration w a s observed in cham bers floatin g on the w ater o f the pit, th e canal and the pond. C oncentrations o f C O2 in the cham bers varied b e­

tw een 2773 to 11245 ppm. A rem arkable d ecrease in the

96

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Fig. 2. CO2 and CH4 concentrations in chamber collector float­

ing on water surface o f pit, canal and pond. See captions for Table 1 for details

C O2 to a m inim um concentration w as observed ju st before sunset. C oin cid en tal d ecrease both in C O2 and C H4 co n cen ­ trations w a s observed e x c lu s iv e ly in the pond collectors ju st after sunrise (F ig. 1). D ifferen ces in the C O2/C H4 ratio, ap­

parently resulted from d ifferen ces in the production rate o f C O2 and C H4 and oxid ation rate, in the pit, the canal and the p ond are sh o w n clearly in F ig. 3. In this figure, tw o sam ple points from the pit (1 1 :0 0 and 14:00) do not fall on the best fit line p a ssin g through the rem aining pit data.

Bubble carbon dioxide and methane

T he concentration o f bubble carbon d ioxid e and m eth­

ane varied from 0 (w ater in the bottle w as not saturated w ith respect to C O2) to 21.14% and from 3 1 .7 4 to 75.71% , re­

sp ectiv ely . In general, the concentrations o f bubble C O2 and C H4 varied indep en d en tly (F ig. 4 ). D ifferen ces in bubble C O2 and C H4 concentrations are clearly different b etw een pit and p ond (Fig. 5). In the case o f canal, the w ater in the gas-con tain in g bottle w a s not saturated w ith respect to bub­

ble C O2, so no C O2 o ver w ater saturation lim it w as o b ­ served.

T he 8 1:,C o f bubble m ethane ranged from - 6 4 to —47%o (F ig. 6, T ab le 1) and the canal m ethane w as 13C -depleted w ith resp ect to pit and pon d m ethane b y about 10%o. H ow - ever, dial variation in the 5 C H13 4 value in the pit and canal

Table 1

C O2 and C H4 concentrations in cham ber co llecto rs float­

ing on w ater surface and bubble C O2 and C H4 concentra- tions and 5 C o f bubble m ethane from sed im en ts o f pit, 13 canal and pond in the experim ental field s o f P ikum thong D ev elo p m en t R oyal Study C enter, N arathiw at, Southern Thailand). Sam pling w as carried ou t on S eptem ber 05 and

Septem ber 0 6, 1994. b.s - b e lo w saturation (due to low concentration o f bubble C O2, w ater rem aining in the b ot­

tles containing gas sam ples w as n ot saturated w ith respect to C O2) - hen ce canal data are not plotted on resp ective

Figures; n.s. - not sam pled; lost - sam p le lost during transport

Sampling Gases in collectors Bubble gases SampI

stat.

Date 1994

Time h:m

c h4 ppm

CO2 ppm

Time h:m

c h4

% C 0 2

% 5 l3C

c h4

%0 Pit 5.09 11:00 48 7025 11:01 72.33 10.11 -51.8 Pit 5.09 14:00 87 4362 14:01 70.25 5.36 -51.5 Pit 5.09 18:00 535 2773 18:10 63.06 5.35 -55.0 Pit 5.09 22:05 611 6938 lost lost lost lost Pit 6.09 2:00 665 8196 2:10 58.26 8.87 -55.6 Pit 6.09 6:35 707 9056 6:45 66.73 21.14 -47.6 Pit 6.09 10:35 739 9044 10:45 75.71 12.98 -52.0 Canal 6.09 n.s. n.s. n.s. 14:10 62.84 b.s. -58.5 Canal 5.09 18:20 430 3200 18:21 58.08 b.s. -58.1 Canal 5.09 22:20 455 3902 22:21 48.89 b.s. -58.5 Canal 6.09 2:20 467 4555 2:21 56.05 b.s. -63.8 Canal 6.09 6:55 503 5107 7:00 72.22 b.s. -56.6 Canal 6.09 10:55 497 4775 11:00 64.65 b.s. -55.3 Pond 5.09 n.s. n.s. n.s. 11:30 49.88 4.41 -49.3 Pond 5.09 14:30 208 3929 14:31 45.4 3.88 -50.7 Pond 5.09 18:25 399 6793 18:30 36.17 3.21 -50.2 Pond 5.09 22:35 483 6448 22:40 44.06 3.66 -49.1 Pond 6.09 2:20 588 8281 2:35 31.74 3.36 -53.4 Pond 6.09 7:05 520 7481 7:10 38.46 3.55 -54.4 Pond 6.09 11:15 664 11245 11:20 36.46 3.51 -53.6

10°/oo low er during night than in the daytim e. M ethane is o ­ tope data do not sh ow regular diurnal variation in the pond.

The highest 5 13C value (—47.6%o) in bubble m ethane and h igh est C O2 concentration w a s found in the sam e bubble sam ple taken in the early m orning in the pit.

Sim ilarly, plots 5 13CH4 versu s C O2 concentration (Fig.

7) and 5 1_1CH4 versus C H4 concentration (F ig. 8) clearly discrim inate betw een three different C O2-C H4 sy stem s o f the pit, canal and the pond resp ectiv ely .

w as observed. The bubble m ethane S 13C value w as nearly

EARLY DIAGENETIC METHANE FROM FRESHWATER SEDIMENTS

97

D I S C U S S I O N

In natural freshwater sy stem s, ferm entation o f acetate (Barker, 193 6 ) and reduction o f carbon d ioxid e (Takai,

1970) are the dom inant m eth an ogen ic pathw ays. Other pathw ays (Zindler & B rock , 1978ab; W eim er & Z eikus,

1978; Patterson & H esp ell, 1979; O rem land e t a l., 1982) w h ich co u ld be con sid ered as a third group o f pathw ays, are n eg lig ib le from the iso to p ic point o f v ie w (L o v le y & K lug, 1983). O n the basis o f tracer experim ents w ith an oxic m a-

methane in collectors [ppm]

Fig. 3. Correlation o f the CCb and CH4 concentrations emitted from water surface o f pit, canal and pond into floating chamber collector. See captions for Table 1 for details

10:0014:0018:0022:00 2:00 6:00 10:0014:00

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Fig. 4. Bubble CO2 and CH4 concentrations in sediment pore- gases o f pit, canal and pond. See captions for Table 1 for details

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rine sedim ents, Blair & Carter (1 9 9 2 ) estim ated the 5 13C valu es o f endm em ber C H4 to be —43%o from acetate and -6 5 % o from C O2. Sim ilarly, b ased upon freshw ater paddy so il incubation experim ents, the endm em ber S 13C o f m eth­

ane produced from acetate d issim ila tio n is -36% o and the 5 I3C value o f C H4 from C O2/H2 w as estim ated to be - 7 7 to -60%o (Su gim oto & W ada, 1993). T herefore, w e m ay sug­

g est that sm aller 8 13CH4 v a lu es w ere lik e ly to be the result o f an enhanced activity o f the C 0 2 -red u cin g m eth an ogen e­

sis pathw ay. On the other hand, higher v a lu es m ay be caused by increased im portance o f the acetic acid pathw ay as a result o f reduction o f the l2C0 2 p o o l siz e and increase in primary production.

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Fig. 6. Dial 5 13C variations in bubble CH4 from sediments in pit, canal and pond. See captions for Table 1 for details

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-64 -60 -56 -52 -48

ô 13C (bubble methane) [%o]

-44

Fig. 7. Correlation o f bubble CCb concentrations and 5 13CH4 o f pit. canal and pond. See captions for Table 1 for details

80 -,

7 0 -

(Uc

TO

<D E Q) 50 - x>

XI D

X I

40 -

30

02:21

Bubble methane Southern Thailand

07:00

+ ^11:01_14:01

14:10

+

18:21

+

11:00

"^18:10

07:10

X 11:20

*

02:35

¥

11:30

*

14:31

x 3?A07K

18:30

i 1 i 1 r ~

-64 -60 -56 -52 -48

5,3C [%o]

-44

Fig. 8. Correlation o f the bubble CH4 and 5 ' CH4 in pit, canal and pond. See captions for Table 1 for details

A tm osp h eric carbon d io x id e and its 8 C value range 13 from ca. 3 5 0 ppm at n oon to ca. 5 50 ppm during the night, and from ca. -7%o in the early ev en in g to ca. —13%o during the night, resp ectiv ely (Szaran, 1990). T he 8 l3C 0 2 variation is driven directly by ph otosyn th etic and respiration a ctiv i­

ties. T he pattern o f 10%o dium al variation in S l3CH4 in Thailand sh o w s a sig n ifica n t co in cid en ce w ith the 813C 0 2 change in the atm osphere and the 8I3C change in m ethane from tem perate clim ate freshw ater sedim ents (Jędrysek, 1994, 1995). T he decrease in the C O2 concentration in the pit cham ber to the m inim al valu e 2 7 7 3 ppm observed after­

n oon ju st before sunset w a s probably caused by sign ifican t absorption o f C O2 from the atm osphere (F ig. 2). H ow ever, absorption o f the iso to p ica lly h ea v iest atm ospheric C O2 in the d aytim e, and release o f iso to p ica lly lighter C O2 at night, m ay not be a p lau sib le driving m echanism for the dium al variation in S ''C H4 i f transport o f C O2 betw een the atm os­

phere and the m ethane production locu s in the sedim ents is probably too slow . H o w ev er, w h en the concentration o f CO2 in w ater decreases, th e absorption o f C O2 from se d i­

m ents (perhaps even iso to p ic a lly heavier due to diffusion isotop e e ffe c t) cou ld be substantial enough to decrease its concentration in porew aters, and con seq u en tly increase 5 13CH4 valu e due to en h anced m eth an ogen esis v ia acetic acid ferm entation. L ik ew ise, photosynthetic-respiration ac­

tivities in w ater m ay in flu en ce isotope ratios in DIC. In­

creased prim ary production during daytim e should have con su m ed iso to p ica lly lighter C O2, leavin g heavier C O2 in the residual C O2 p ool. T his p rogressive increase in S13C0 2 m ay increase the S I3C v a lu e o f the m ethane produced dur­

in g the d aytim e in the surficial regions o f sedim ents. In gen ­ eral, atm ospheric C O2 o f equatorial regions is depleted in

l3C iso to p e com pared to tem perate clim ate C O2 in atm os­

phere. T herefore, as it m ay b e ob served here, m ethane from tropical freshw ater sed im en ts is in general m ore depleted in h eavy carbon isotop e than the tem perate clim ate earlydiage- netic m ethane.

L ik ew ise, during d ay-tim e, hydrogen is produced in

substantial am ounts in aquatic sy stem and, sim ilarly, C O2 concentration decreases and its 8 13C valu e in creases due to p h otosynthetic activity. T hus, afternoon m eth a n o g en esis via the C O2/H2 reduction pathw ay in the upperm ost horizon s o f sedim ents m a^ b eco m e less active, resulting in a subsequent increase in 8 C H4 (F ig. 6). In fact, concentration o f bubble C O2 is lo w e st in the afternoon and ev en in g . T his fact, in conjunction w ith the step by step (1 0 :4 5 , 11:00 and 14:00) decrease in pit bubble C O2 concentration, acco m p a n ied by slight increase in S 13CH4 (F ig. 7), supports the th esis that the daytim e d eficit o f C O2 m ay increase the S I3C H4.

A nother factor increasing th e S13C H4 v a lu e co u ld be b io lo g ica l oxidation o f m ethane lea v in g behind iso to p ica lly h eavier m o lecu les. M oreover, oxid ation o f acetate (and probably o f its precursors) is assu m ed to be the first order factor controlling, acetate concentration and m ay result in a substantial enrichm ent in h ea v y isotop es in the rem aining acetic acid and, co n seq u en tly, in m ethane. D uring this proc­

ess, the rem aining acetic acid or/and m ethane can sig n ifi­

cantly shift the hydrogen and carbon iso to p ic ratios p o si­

tiv e ly in the n e w ly produced and residual m ethane (Barker

& Fritz, 1981; C olem an e t a l., 1981). T his p ro cess strongly m odulates m ethane flu xes in aquatic sy stem s (K in g, 1990);

h ence, the m ore p o sitiv e d ay-tim e 8 13C H4 v a lu es w ere ob ­ served. On the other hand, m ethane p roductivity sh ou ld d e­

crease and the concentration o f carbon d io x id e in bubble m ethane sh ou ld increase as a co n seq u en ce o f m ethane o x i­

dation and/or that o f the m ethane precursor’s. In general, this is not the ca se, h ow ever. V isu a lly ju d g ed high produc­

tivity during day-tim e m ay su g g est that the acetic pathw ay b eco m es m ore a ctive as a co n seq u en ce o f increasing pri­

m ary production and im m ediate acetate ferm entation. H o w ­ ever, one pit sam ple (6:45; T able 1) is excep tion al. In this sam ple, the h igh est bubble C O2 concentration (21.14% , 6:45 on Fig. 4, 5 and 6 ) cou p led w ith the h igh est 8 13C H4 value (-47.6% o, 6:45 on Fig. 6 and 7) m ay su g g est sig n ifi­

cant oxidation o f m ethane. T he oxid ation e ffe c t is n ot re­

flected , how ever, in the m ethane concentration (F ig. 4 and

EARLY DIAGENETIC METHANE FROM FRESHWATER SEDIMENTS

99

5) probably b ecau se o n ly several per cen t o f m ethane w as o x id ised . T hus, the h igh concentration o f C O2 probably re­

sulted from the cum ulation o f C O2 in sedim ents and w ater due to respiration during th e night. On the other hand, the h igh est C O2 concentration and the h igh est 513CH4 value is not resulted b y oxidation o f any sign ifican t portion o f acetic acid, b ecau se this p rocess sh ou ld decrease the acetic acid p athw ay and con seq u en tly increase the C O2 pathw ay. Fi­

nally, oxid ation o f acetic acid m ay substantially decrease the production rate o f m ethane but presum ably have a lesser in flu en ce on S l3C H4 valu e. T his one sam ple does not, o f course, p rove that such oxid ation is a regular pattern in the early m orning in oxidant-rich aquifers sim ilar to the pit w ith its enhanced concentration o f sulphate in water. W hen

0 — 9

SO4 is present, acetate is u sed b y S 0 4' reducers. The fact that pit bub b les sh o w the h igh est carbon d ioxid e content and high 5 l3CH 4 v a lu es co u ld not be resulted b y enhanced a ce­

tic ferm entation, but rather supports the th esis on strong o x i­

dation effects in the pit sed im en ts. M ore intense, acetic acid ferm entation caused b y higher primary production in the h igher w ater colum n o f the pit ( 2 m ) com pared w ith the ca­

nal and pond (0 .5 and 0 .7 m ) is probably n eg lig ib le becau se the pit w ater w as ex trem ely poor in livin g organic matter e.g. algae. D uring ferm entation o f fresh organic matter, its rem aining part b eco m es iso to p ic a lly enriched. Therefore, organic m atter sinking in the m orning m ay b eco m e sig n ifi­

cantly enriched in h eavy carbon isotope. On the other hand, a d ecrease in ô13C H4 v a lu es w ith increasing depth o f about

—l% o /-l m o f w ater co lu m n (Jędrysek e t a l., 1994, 1996) su g g est that i f the pit and canal bubble sam ples w ere from the sam e depth, the d ifferen ce in carbon isotop e ratios w o u ld be higher than that ob served (F ig. 6) - m ethane from sh a llo w er w ater should be 8 13C -enriched.

T he canal m ethane sh o w ed lo w 513C, presum ably due to lo w a ctivity in prim ary production (potential precursor o f acetic acid ). C on seq u en tly, m ethane w as produced m o stly by reduction o f C O2. T he pond m ethane sh ow ed no regular­

ity in dium al variation, partly b ecau se o f in h om ogen eity o f the sed im en ts that contained the bubble C O2 and C H4. Fur­

therm ore, high 513CH4 v a lu es accom p an ied b y the lo w est concentration o f bubble carbon d io x id e (Fig. 7) and m eth­

ane (Fig. 8) w a s probably cau sed b y high d en sity o f roots o f m acrophytes em erging and veg eta tin g on the bank o f the pond. T his w o u ld prom ote ven tilation o f the sedim ents w ith air and result in a decrease in C O2 and C H4 and sim ultane­

o u sly in sig n ifica n t 13C enrichm ent o f the residual m ethane.

M oreover, light penetration into the pond w ater w a s e x ­ trem ely p oor b ecau se o f v ery lim ited transparency and high abundance o f m acrophytes. T his lim ited prim ary production in w ater co u ld be, perhaps, the reason for the sign ifican t shift in p h ase o f the m in im al 8 13C H4 value from night to early m orn in g and the m axim al 513CH4 valu e from the tim e o f sunset to early night.

C O N C L U S I O N S

1. 13 8 C H4 valu es are lo w e st in the night and h igh est in the daytim e. H igh v a lu es representing daytim e m ethane o c ­ cur w h en the C O2 p ool d ecreases and acetic acid ferm enta­

tion b eco m es relatively m ore im portant. M in im u m valu es are apparently cau sed by an en h an ced C O2-H2 m ethano- g en esis pathw ay.

2. The sca le and shape o f diu m al em issio n s o f C O2 and C H4, and dium al variations in bubble C O1 - 1 | 9 2 and C H4 co n cen - tration and in the C l C ratio in bubble m ethane, strongly depend on p h y sica l, ch em ical and b io lo g ic a l con d ition s, in­

clu d in g vegetation.

3. O xidation m ay p lay crucial role in 8 13CH4 v a lu es e s ­ p ecia lly w h en the production rate o f m ethane is low .

4. R em arkable 13C -d ep letion o f m ethane from tropical freshw ater sedim ents m ay b e im portant factor w h ich could help to discrim inate, origin o f hydrocarbon dep osits. M ore studies w o u ld be required at this p oin t in the near future.

Acknowledgements

Great thanks are due to Dr. Ch.Sittibusl, the director o f PDRSC, Narathiwat, for his help while rendering sampling base and laboratory accessible and supply o f a motorbike for day sam­

pling and a car with a driver during night sampling. Thanks are also due to Mr. Batchong for driving and his presence during night sampling (cobra hunting time). Authors are indebted to K.

Sokołowski for gas chromatography analysis o f bubble gases and to A. Wójcik for sample preparation and mass spectrometrical analysis. Deep thanks to Dr. P. Kennan (University College Dub­

lin), two anonymous reviewers, Prof. Z. Wilk and Prof. W. Naręb- ski for their critical reading this manuscript. This study was supported by Grant-in-Aid for the Creative Basic Research Stud­

ies, Ministry o f Education, Science and Culture (Japan), entitled

“Studies on Global Environmental Changes with Special Refer­

ence to Asia and Pacific Regions” and State Committee for Scien­

tific Research 0370/P-2/92/03 (Poland), 2022/W / ING/96-10 and I017/S/ING/96-IX. This work was also partly supported by the Nomura Gakugei Foundation, Japan, and International Atomic En­

ergy Agency.

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Streszczenie

W CZESNODIAGENETYCZNY METAN Z RÓŻNYCH OSADÓW SŁODKOW ODNYCH KLIMATU TROPIKALNEGO: ZASTOSOW ANIE

CHROM ATOGRAFII GAZOW EJ I ANALIZY STOSUNKÓW IZOTOPOW YCH W OBSERW ACJACH CYKLU DOBOW EGO

M a r iu s z O r io n J ę d r y s e k , S ta n is ła w H a ła s , E ita r o W ada, S u p o rn B o o n p r a k u p , S h in g o U e d a , P is o o t V ija r n s o r n &

Y a su o T a k a i

Głównym produktem wczesnodiagenetycznego rozkładu ma­

terii organicznej w osadach anoksycznych jest metan. Powstaje on w wyniku redukcji dwutlenku węgla (Takai, 1970) i fermentacji octowej (Berker, 1936). Inne drogi wczesnodiagenetycznej meta- nogenezy nie są istotne z punktu widzenia badań izotopowych (Lovley & Klug, 1983). Metan powstały w wyniku redukcji CO2 jest silnie zubożony w izotop 13C w stosunku do metanu powsta­

łego z CH3COOH (Blair & Carter, 1992; Sugimoto & Wada, 1994). Proces utleniania metanu prowadzi do wzbogacenia pozos­

tałego metanu w izotop l3C (Barker & Fritz, 1981; Coleman et al., 1981).

Metan jest jednym z najważniejszych gazów efektu cieplar­

nianego a wzrost jego stężenia w atmosferze jest obecnie około 10- cio krotnie szybszy niż wzrost stężenia innych gazów efektu cie­

plarnianego (np. Craig & Chou, 1982; Khail & Rasmussen, 1983;

Cicerone, 1987; Steele et al., 1987; Wada, 1990). Szacuje się, że w około 45% metanu atmosferycznego pochodzi z obszarów kon­

tynentalnych okresowo lub stale pokrytych w odą (pola ryżowe, bagna, jeziora). Izotopowy bilans mas umożliwia znacznie dokład­

niejsze, niż inne obecnie znane metody, oszacowanie udziału po­

szczególnych źródeł metanu w bilansie atmosfery Ziemi. Wyniki takich obliczeń mogą w przyszłości istotnie wpłynąć na decyzje w polityce międzynarodowej poprzez wymuszanie na niektórych krajach zmniejszenia emisji metanu do atmosfery. Jednakże, aby dokonać takich obliczeń, konieczne jest zebranie informacji o cza- sowo-przestrzennej zmienności składu izotopowego metanu po­

chodzącego z różnych źródeł oraz poznanie przyczyn tych zmien­

ności. Wykazane wcześniej, silne zubożenie w ciężki izotop węgla metanu wczesnodiagenetycznego powstałego w nocy, w stosunku do metanu powstałego w dzień, pozwala zasugerować, że przyj­

mowany 45% udział metanu wczesnodiagenetycznego w ogólnym bilansie metanu atmosfery, jest prawdopodobnie o około połowę mniejszy (Jędrysek, 1995).

Jednocześnie, proces utleniania metanu sprzyja powstawaniu węglanów wczesnodiagentycznych silnie zubożonych w izotop 13C. W ten sposób powstaje, w profilach osadów, zapis izotopowy o warunkach panujących w zbiorniku. Ma to duże znaczenie we współczesnych interpretacjach paleośrodowiskowych. Poznanie mechanizmów rządzących zm iennością składu izotopowego me­

tanu powstającego w wyniku procesów wczesnodiagenetycznych ma więc istotne znaczenie dla izotopowego bilansu mas gazów efektu cieplarnianego oraz dla badań paleośrodowiskowych.

Ponadto, zaobserwowane niższe wartości S I3C metanu z osadów

EARLY DIAGENETIC METHANE FROM FRESHWATER SEDIMENTS

101

klimatu tropikalnego niż metanu z osadów klimatu umiarkowa­

nego mogłoby być dobrym wskaźnikiem do badań nad warunkami powstawania i pochodzeniem złóż ropy i gazu.

Przedstawione w tej pracy wyniki badań dotyczyły trzech różnych typów źródeł metanu wczesnodiagenetycznego powsta­

łego w klimacie tropikalnym. Obserwacje i opróbowanie przepro­

wadzono 5 i 6 września 1994, na polach eksperymentalnych Kró­

lewskiego Centrum Rozwoju Badań Pikumthong w okolicach Narathiwat, Pd. Tajlandia (6° szer. geogr. Pn.). W odstępach ok. 4 godzin pobierano próby gazów zbieranych do specjalnie w tym celu skonstruowanych pływających kolektorów (początkowo w y­

pełnionych powietrzem), oraz gazów, których ucieczkę z osadu wymuszano. Wykonano analizy chromatograficzne stężenia CO2 i CH4 w atmosferze zawartej w kolektorach i w gazach z osadów oraz analizy izotopowe 813C metanu z osadów (5 3CH<i).

Badane gazy pochodziły z:

- osadów wkopu - sztucznego zbiornika wykopanego w gle­

bie o wysokim stężeniu siarczanu (obecność SO4- powoduje ut­

lenianie metanu i zapobiega jego powstawaniu w szczególności na drodze fermentacji octowej),

- osadów strumienia o silnie natlenionej wodzie płynącej (obecność tlenu w wodzie utrudnia także procesy redukcyjne w osadzie),

- osadów stawu silnie zarośniętego roślinami (rośliny ukorze­

nione powodują wentylację osadu i w wyniku respiracji dostar­

czają CO2),

Zaobserwowano, niezbyt regularne ale wyraźne wahania

dobowe w mierzonych parametrach. Wartość 8 13CH4 była naj­

niższa w nocy, a najwyższa w dzień: minimum -63.8%o o godz.

2:20 (kanał) a maksimum -47.6%o o godz. 6:45 (wkop). W oparciu o te dane (Tab. 1) oraz zależności przedstawione na Fig. 2 -8 w y­

sunięto przypuszczenie, że wysokie dzienne wartości 5I3CH4 są związane ze spadkiem stężenia CO2 (dyfuzja do kolumny wodnej w wyniku zwiększonej asymilacji fotosyntetycznej planktonu) i wzrostem aktywności procesu fermentacji octowej w wyniku roz­

kładu świeżo obumarłego planktonu. Minimalne wartości 5 13CH4 związane są z relatywnie w ysoką aktywnością procesu metano- genezy w wyniku reakcji CO2-H2, co jest następstwem braku foto­

syntezy i wzrostu koncentracji CO2 w wyniku respiracji. Niere- gularności zmian dobowych są związane najprawdopodobniej z procesami utleniania, niehomogenicznością osadów (różny osad w różnych punktach opróbowania warunkuje produkcję i dyfuzję metanu co wywołuje także efekt izotopowy), zróżnicowaniem stopnia ukorzenienia, niepowtarzalnością sposobu wymuszania ebullicji (ucieczki gazów z osadu). Zaobserwowano, że generalnie wartość 5 13CH4 wkopu i stawu była o około I0%o w yższa niż war­

tość Sl3CH4 strumienia (Ryc. 6). Można to tłumaczyć brakiem fer­

mentacji octowej w osadach strumienia oraz silniejszym utlenia­

niem metanu w osadach wkopu i stawu. Ponadto, fakt zanotowania najniższych wartości 5 13CH4 kanału sugeruje intensywną redukcję CO2 w jego osadach. Najwyższa wartość 5 13CH4 towarzysząca najwyższej koncentracji CO2 w gazach osadów wkopu (Fig. 7, godz. 06:45) pozwala wnioskować o zaawansowanym procesie ut­

leniania (związanego z redukcją siarczanu).