286 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 56

Aquatic Sources and Sinks of CO2 and CH4 in the Polar Regions

I. P. SEMILETOV Paci®c Oceanological Institute, Vladivostok, Russia

(Manuscript received 2 September 1997, in ®nal form 15 June 1998)

ABSTRACT

The highest concentration and greatest seasonal amplitudes of atmospheric CO 2 and CH4 occur at 60Њ±70ЊN,

outside the 30Њ±60ЊN band where the main sources of anthropogenic CO2 and CH4 are located, indicating that the northern environment is a source of these gases. Based on the author's onshore and offshore arctic experimental results and literature data, an attempt was made to identify the main northern sources and sinks for atmospheric

CH4 and CO2. The CH4 ef¯ux from limnic environments in the north plays a signi®cant role in the CH 4 regional

budget, whereas the role of the adjacent arctic adjacent seas in regional CH 4 emission is small. This agrees with

the aircraft data, which show a 10%±15% increase of CH4 over land when aircraft ¯y southward from the Arctic

Basin. Offshore permafrost might add some CH4 into the atmosphere, although the preliminary data are not suf®cient to estimate the effect. Evolution of the northern lakes might be considered as an important component of the climatic system. All-season data obtained in the delta system of the Lena River and typical northern lakes

show that the freshwaters are supersaturated by CO2 with a drastic increase in the CO2 value during wintertime.

The arctic and antarctic CO2 data presented here may be used to develop understanding of the processes

controlling CO2 ¯ux in the polar seas. It is shown that Arctic seas are a sink for atmospheric CO 2, though

supersaturation by CO2 is obtained in areas in¯uenced by riverine output and in coastal sites. The pCO 2 difference between the surface of the Southern Ocean and atmosphere observed in the austral autumn shows that the area

east of 7ЊW might be considered a source of CO2 into the atmosphere, whereas the area west of 7ЊW is a net

sink of CO2. This is corroborated with literature data that indicate an overestimate of the role of antarctic waters

as a sink for atmospheric CO2.

1. Introduction (Rassmussen and Khalil 1984; Steele et al. 1987; Ree- burgh and Crill 1996). Data from the present interna- Carbon dioxide is the most abundant and most im- tional network of atmospheric CO2 monitoring sites, portant greenhouse gas (other than water vapor) in the located almost exclusively in oceanic areas, cannot be atmosphere. The maximum annual average concentra- used to resolve longitudinal gradients. Thus identi®ca- tion of atmospheric CO2 is located within 60Њ±80ЊN tion of the important source±sink areas is dif®cult (Tans (Tucker et al. 1986; Conway et al. 1994), outside the et al. 1990; Conway et al. 1994; IGAC 1994), especially 30Њ±60ЊN band where the main sources of industrial in the Arctic, where only three North American stations CO2 are located (Rotty 1983). Annually about 0.1 ϫ for CO monitoring have been established. At present 15 2 10 g C of anthropogenic CO2 is emitted within the the global carbon cycle cannot be balanced to better 60Њ±70ЊN latitudinal band. The equilibrium greenhouse than about 25% of anthropogenic CO2 emissions. In warming associated with increase of CH4 and chemical order to bring the atmospheric budget CO2 and CH4 into feedback (through change of stratospheric water vapor) balance to the desired closure, improved measurements relative to warming during deglaciation is about 30% of CO must be combined with measurements of CH of the warning due to CO (Chappellaz et al. 1990). 2 4 2 and with improved oceanic and terrestrial measurements Currently, the rate of increase warming due to CH is 4 of both. Because the CO and CH interhemispheric about 38% of the CO warming effect (Craig and Chun 2 4 2 gradients and seasonal amplitudes show that the north- 1982). The maximum of atmospheric CH is also located 4 ern environment is a major contributor to the Northern over the Arctic and subarctic; the value of CH over 4 Hemisphere CO and CH maxima and seasonal vari- Greenland exceeds that over Antarctic by 8%±10% 2 4 ations (Rassmussen and Khalil 1984; Tucker et al. 1986; Cicerone and Oremland 1988; Nisbet 1989; Tans et al. 1990; Fung et al. 1991; Quay et al. 1991; Reeburgh et al. 1994), the role of Arctic seas and terrestrial ecosys- Corresponding author address: Dr. I. P.Semiletov, Arctic Regional Center, Paci®c Oceanological Institute, 43, Baltiyskaya Street, Vla- tems as sources and sinks of these greenhouse gases divistok 690041, Russia. should be evaluated. E-mail: [email protected] The interhemispheric gradient of atmospheric CO2

᭧ 1999 American Meteorological Society

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was ϳ1 ␮atm in the 1960s and is ϳ3.0 ␮atm now (Fung sider here a northern source of atmospheric CH4 (meth- 1993). This change in the interhemispheric gradient ane hydrate, organics buried in the permafrost, and nat- might be determined by an increase of the Arctic and ural gas) as a CO2 source also. subarctic natural source of CO2 and/or an increase of a Anaerobic oxidation of organic matter in the season- sink in the Southern Ocean. The recent data (1989/90± ally thawed layer of onshore permafrost (Bartlett et al.

96), based on year-round measurements of CO2 ef¯ux 1992; Reeburgh et al. 1994) and peat bogs (Matthews from the typical northern soil in Siberia, show that only and Fung 1987; Nisbet 1989) may be an important wintertime soil respiration could provide a net source northern source of CH4. Different estimates show a of CO2 into the atmosphere up to 1 Gt C-CO2 and more large range of summertime ef¯ux of CH4 from the north- (Zimov et al. 1993; Zimov et al. 1996). This agrees with ern environment. This uncertainty in the ef¯ux of CH4 the evaluation of Ciais et al. (1995a), which shows that is related to inhomogenious time and spatial distribu- arctic ecosystems north of 65ЊN are a net source of CO2 tions of CH4 ¯ux, and to limited experimental ground- of 1±1.2 Gt. The CO2 concentration that was measured level data (Reeburgh et al. 1994). In general, it was beneath the snow in winter in Siberia was similar to stated that CH4 might be produced in the northern eco- that measured in arctic tundra in Alaska (Kelley et al. systems only during summertime (100 days), and total

1968; Coyne and Kelley 1974) and temperate alpine evaluation of the northern source of CH4 was based on (Sommerfeld et al. 1993), suggesting also that the north- this assumption (Matthews and Fung 1987; Fung et al. ern ecosystems could exhibit large winter CO2 ef¯uxes. 1991; Reeburgh et al. 1994). However, the dramatic The winter CO2 emission from the northern soils would autumn surge in CH4 levels in the northern atmosphere increase the average CO2 concentration in air over the (Steele et al. 1987; Quay et al. 1991) requires us to Arctic by approximately 7±16 ppm in different years search for other regional sources of CH4. Early inves- (Zimov et al. 1993). Such a value is close to 50%±100% tigations of CH4 emission from a small lake in north- of the CO2 seasonal amplitude observed in this area. It western Ontario show that lake overturning might be a was found also that the range of CO2 ¯ux change is in signi®cant factor in the ultimate evasion of methane to good agreement with the varied double peak or near the atmosphere (Rudd and Hamilton 1978). double peak in atmospheric CO2 wintertime distribution Other arctic sources of CH4 include methane gas hy- (Semiletov et al. 1993) that was recorded by monitoring drates and natural gas losses, especially in Siberia. One stations such as Alert, Mould Bay, Point Barrow, and of the largest potential sources of CH4 emission to the Sable Island (Wong et al. 1984). Some measurements atmosphere is natural gas hydrates (Makagon 1982); the of summer CO2 ¯ux to the atmosphere were made in shelf and continental slope reservoir is estimated to be the 1990s in northern Siberia (Zimov et al. 1993; Zimov roughly 6 ϫ 1018 g (or 6000 Gt), and the onshore per- et al. 1996) and in Alaska (Oechel et al. 1993). Oechel mafrost reservoir is about 16 ϫ 1015 to 32 ϫ 1015 g. et al. (1993) estimate that arctic tundra might contribute Usually, onshore CH4 is stable in hydrate in permafrost 0.19 Gt C-CO2 during summer, though long-term data at depths of 100 m and more, and offshore CH4 hydrate obtained in northeastern Siberia show a higher value of is potentially stable at shallow levels in the sea¯oor, in

CO2 emission (Zimov et al. 1996). The conclusion of water depths in excess of about 250±300 m. At present, these works is that the large pools of carbon accumu- the Alaskan records show a warming of permafrost in lated in the north have became vulnerable to decay, and this century (Lachenbruch and Marshall 1986), and no carbon is released from soils mainly in CO2 form. It is experimental data indicate a temperature increase of on- important that the main input of CO2 into the atmo- shore permafrost in Siberia and Canada. Temperature sphere be determined by ef¯ux in winter when respi- pro®les in the offshore permafrost are episodic or lim- ration of biota adopted for cold soil environment is not ited in timescale (Fartishev 1993; Romanovskiy et al. balanced by photosynthesis. In spite of the small area 1997); that is, long-term representative temperature data of the tundra and northern taiga soils (ϳ7.3 ϫ 106 km2 are absent today. From an oceanographic point of view, and 2.1 ϫ 106 km2, respectively), their carbon contents the marine permafrost hydrate is stable at present, be- are among the highest on earth and combined represent cause usually the shelf bottom water has a temperature an inventory of 455 Gt (Post et al. 1982; Gorham 1991), below 0ЊC (Treshnikov and Salnikov 1985). Thus, no about 60% of the total atmospheric CO2 burden. Be- experimental data indicate an unstable hydrate environ- cause the large pool of soil carbon in the north becomes ment, but it would be vulnerable if the permafrost is vulnerable to decay by an increase in the depth of the warming. seasonal thaw, the carbon released as CO2 might further A crude estimate of the natural gas losses to the air speed the warming. Based on recent data we can con- from gas processing and transport is about 1.5 ϫ 1013 sider the northern soil as a main regional contributor in gofCH4 (about 2.5% of gas production) emitted an- budget of atmospheric CO2, though the Arctic Ocean nually in northern latitudes (Cicerone and Oremland might also play a signi®cant role (Kelley and Gosink 1988). This is probably not the principal cause of the

1988). marked seasonal behavior of in northern CH4; an annual Because CO2 is the end product of CH4 oxidation in midsummer minimum in CH4 is followed by rapid au- the atmosphere (Cicerone and Oremland 1988) we con- tumn rise (Steele et al. 1987; Quay et al. 1991). Analyses

Unauthenticated | Downloaded 09/25/21 10:31 AM UTC 288 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 56 of ice core samples also suggest a strong net northern the northern Paci®c and Bering Sea, and entered the source of CH4 back to preindustrial times (Rasmussen hard ice ®eld in the Western Chukchi Sea. Amderma and Khalil 1984; Chappellaz et al. 1993). The Tropics, reached its westernmost point at 162ЊE near the mouth too, are a strong source of atmospheric CH4, but there of the Kolyma River. Surface water was measured at 23 the production of OH, which destroys CH4, is also high. locations. In late September 1994, the same measure- In contrast, the northern source is less balanced by sinks. ments were made from the research and supply vessel

The net effect is a buildup of CH4 in the Arctic with Mikhail Somov, of the Arctic and Antarctic Research strong southward transport (Blake and Rowland 1988; Institute (AARI), Saint Petersburg, Russia. Surface wa- Nisbet 1989). Both these processes should lead to low ter was taken and measured from 30 points in the Lap- summer and autumn CH4 levels in the northern tropo- tevs Sea. TCO2 was measured by the stripping gas chro- sphere; the implication of the CH4 rise during the north- matography (GC) technique using catalysis conversion ern autumn is that there are seasonal sources of CH4 in of CO2 to CH4 in a hydrogen stream (Weiss and Craig the north. In contrast, Southern Hemisphere data show 1973; Weiss 1981; Semiletov 1992); CH4 and CO2 were much smaller variations, implying that no comparable separated on a chromatographic column packed by Po- late summer±autumn source exists there (Fraser et al. ropac T (0.3 cm ϫ 200 cm, 40±60 mesh) and installed 1984). in front of the Ni convertor (methanator). The precision Here we present our results for the carbonate system of this procedure was about Ϯ1%. Carbon dioxide pres- study in the Arctic seas and in freshwater ecosystems sure (pCO2, ␮atm) was computed from TCO2 and pH of Siberia. The role of the northern lakes and offshore (NBS scale) according to a scheme presented in permafrost in formation of CH 4 maximum in atmo- UNESCO (1987) and based on equations and constants sphere is discussed. Based on our recent results and advocated by UNESCO (1987). Speci®cally, the equa- literature data we attempt to identify the main source tions used were recommended by Millero (1978) for and sinks for greenhouse gases that are re¯ected in the temperature and salinity dependence of the apparent dis- spatial and temporal variations of CO2 and CH4 con- sociation constants for carbonic acid in seawater as de- centration patterns in the atmosphere over the Arctic. termined by Meerbach et al. (1973). Measurements of Also, we present some Antarctic data that might be used pH were carried out immediately after sampling at 25Њ to develop an understanding of the processes controlling Ϯ 0.1ЊC with precision Ϯ0.01. Total uncertainty in

CO2 ¯ux in the polar seas. The estimation of CO2 and pCO2 values was about Ϯ10 ␮atm due to experimental CH4 ¯uxes is not the primary concern of this paper; errors in carbonate parameters as well in equilibrium rather, it is to gain insight into the mechanism for for- constants. Comparison of our calculation with the recent mation of CH4 and CO2 maxima over the Arctic through scheme and constants advocated by Millero (1995) identi®cation of processes that produce the maxima. shows the results to be similar. The dissolved CH4 was Because the interaction between the atmosphere and measured by static headspace GC with using a ¯ame surface water with respect to the direction of CO2 (CH4) ionization detector. The dissolved gas concentrations transport can be evaluated in terms of the partial pres- were calculated using the main static headspace equa- sure gradient across the air±water interface, we present tion (Vitenberg and Ioffe 1982): CL ϭ CG(VG/VL ϩ G), here the difference between surface water and atmo- where CL and CG are the concentrations of a gas in the sphere (⌬pCO2); (Ϫ) shows undersaturation of the sur- liquid phase (L) and in the equilibrated headspace (G), face water (sink), (ϩ) shows supersaturation (source). VG and VL are volumes in the gas (G) and liquid (L) Results from these studies may be used to partition the phases of the closed headspace system, and G is a gas contribution from the sea and terrestrial biosphere as partitioning coef®cient. The total precision of this tech-

CO2 (and CH4) source or sink of the natural carbon nique was about Ϯ1%±3%. The GC methods were pre- cycle (IPCC 1992; Ciais et al. 1995a; Ciais et al. 1995b). sented and discussed in detail recently (Semiletov 1993; Semiletov et al. 1996a). The surveyed areas are pre- sented in Fig. 1. The data obtained in fall 1994 are 2. Study regions and measurement presented in Table 1 and Fig. 2a. In September 1995, a. Arctic we measured pH25, TCO2, and dissolved CH4 on board Mikhail Somov again (Fig. 1): 67 surface samples were 1) SEAS taken in the Kara Sea and Laptevs Sea. The results are Until very recently, marine chemistry studies in the presented in Table 1. In September 1996, we measured polar seas were mainly performed in summer. Little or pH25 (SWS scale) and total alkalinity (TA) on board no constituent data were collected during or after au- Alpha Helix (HX 194), U.S. National Science Foun- tumn convection because of operational problems. dation, in the Chukchi Sea. The expedition was funded

In fall 1994, the concurrent pH25 (at temperature by the U.S. National Science Foundation. In this cruise 25ЊC) and total CO2 (TCO2) and dissolved CH4 were we measured pH at 25ЊϮ0.1ЊC with ORION 8103 measured on the commercial icebreaker Amderma of the Ross electrode in SWS scale, using tris-buffer prepared Far Eastern Shipping Company, Vladivostok, Russia. by the prescription of Goyet and Dickson (DOE 1994). Amderma left Vladivostok in early September, crossed The precision was Ϯ0.002 pH unit. Total alkalinity data

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FIG. 1. Map showing location of the study sites (Tiksi area and Chersky area), expedition along the Lena River Stream (´´´´)inAugust± September 1995, and cruise tracks of Amderma in September±October 1994 (± - - - ±), Mikhail Somov in September±October 1994 ( ) and in September 1995 (- - -), Alpha Helix in September 1996 (ϫϫϫ), and DUNAY in September 1997 (/ / /).

were obtained by direct indicator titration in an open 2) LAKES AND RIVERS cell using a 665 Dosimat system with a precision of Surface water is a signi®cant part of the landscape of 0.1%. Comparison of TA determination by direct titra- the Arctic coastal plain, composing up to 50%±80% of tion in open cell (Bruyevich's method) and potentio- the land area. Here we consider the typical northern metric titration in closed cell (Edmond's method) shows lakes of Siberia. In general, the northern lakes are ther- differences within 1% (Rogachev et al. 1996). About mokarst or thaw by origin (Hopkins and Kidd 1988). 700 samples were analyzed in the Chukchi Sea east of Thaw lakes, lakes that result from surface collapse 170ЊE to map pCO2 values computed from TA and pH caused by the thawing of ice (rich permafrost), are im- from surface to bottom. Values of pCO2 were calculated portant and conspicuous features of Arctic and subarctic given TA, pH, T, S, following a scheme and constants lowland landscapes in both tundra and taiga regions. advocated by Millero (1995). Distribution of pCO2 in Thaw lake sediments are underlain by zones of thawed the surface water of the Chukchi Sea during autumn permafrost, called taliks. The depth of taliks increased convection is presented in Fig. 2b. The experimental with the age of thaw lakes and changed from 100 to 102 technique and preliminary data are described in Semi- m (Chekhovskiy and Shamanova 1976). For instance, letov et al. (1998). In late November 1996, we took thaw lakes aged about a few thousand years might cover water samples from beneath the ice to the bottom in the a layer of thawed permafrost with a depth of ϳ100± mouth of the Lena River and adjacent part of the Laptevs 200 m or more; that is, the vast organic reservoirs im- Sea. Forty samples were taken and measured to map- mobilized in permafrost became available for anaerobic dissolved CH4 (Fig. 3) and pCO2 values (Table 1). This destruction by way of a lake evolution. Note that thaw- work was done mainly to evaluate the ef¯ux of CH4 ing of the permafrost under lakes might be a mechanism from offshore permafrost in a shallow shelf. that is able to involve methane buried in hydrate res- In September 1997 we investigated dynamics of the ervoir into the present biogeochemical cycling. In spring

CO2 system and dissolved CH4 in coastal waters of the 1992, the study of carbon emission from the northern Laptevs Sea from the hydrographic vessel Dunay of the lakes was started at the lower Kolyma River, about 80± Tiksi Hydrobase. The study area is presented in Fig. 1 100 km from the east Siberian Sea of the Arctic Ocean. and some data are presented in Table 1. The study area is phytogeographically a transition of

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TABLE 1. Range of variability of pCO2 and dissolved CH4 in northern Asia and adjacent seas. Range of variability

Ϫ1 Sampling site pCO2, ␮atm CH4, ␮ML Month, year Lakes of the Kolyma lowland, northern taiga±subarctic tun- dra removed about 150±200 km from coast of the east Si- berian Sea 400±2000 4±360 all seasons 1992±1994 Region of the Olutorsky Cape, Bering Sea ϳ355 Ͻ0.015 September 1994 NW part of the Bering Sea ϳ395 Ͻ0.015 September 1994 Bering Straits ϳ380 Ͻ0.015 Sep±Oct 1994 S, SW part of the Chuckchi Sea 295±466 Ͻ0.015 September 1994 Long Straits 381±490 Ͻ0.015 September 1994 SW part of the east Siberian Sea 623±844 Ͻ0.015 Sep±Oct 1994 Sandbar of the Kolyma River, east Siberian Sea 716±1779 trace September 1994 NW edge of the Kotelny Island (Novosibirsky Islands), Lap- tevs Sea 215±280 Ͻ0.015 Sep±Oct 1994 NE part of the Laptevs Sea 216±311 Ͻ0.015 Sep±Oct 1994 Sannikov Straits, the Laptevs Sea 245±267 Ͻ0.015 September 1994 SE part of the Laptevs Sea 215±267 Ͻ0.015 Sep±Oct 1994 Coastal water of the Tiksi Bay, the Laptevs Sea 609±706 Ͻ0.015 Sep±Oct 1994 Neelovsky Gulf, SE part of the Lena River delta, the Laptevs Sea 829±929 0.05±0.07 Sep±Oct 1994 SW part of the Laptevs Sea 261±475 0.015 Sep±Oct 1994 Lakes of the Yakutian lowland, Bykovsky peninsula, coast of the Laptevs Sea 400±1500 0.06±0.4 all seasons 1994±1995 Coastal water of Tiksi Bay 501±5919 Ͻ0.015 November 1994±May 1995 SW part of the Laptevs Sea near the Lena River delta 987±1987 Ͻ0.015 November 1994±May 1995 Vilkitsky Strait 253±278 Ͻ0.015 September 1995 Kara Sea 273±405 Ͻ0.015 September 1995 Karskie Vorota Strait 307±322 Ͻ0.015 September 1996 Chukchi Sea; near 72ЊN, ice edge 150±200 Ð September 1996 NE Chukchi Sea 250±300 Ð September 1996 SE Chukchi Sea 300±400 Ð September 1996 Bering Strait 300±450 Ð September 1996 NE Bering Sea 350±400 Ð September 1996 Coastal waters of the Beaufort Sea, east of Barrow Canyon 150±500 Ð September 1996 Tiksi Bay 508±622 trace±0.077 November 1996 Shallow water near Bykovsky peninsula 539±723 trace±20.0 November 1996 Buor-Haya Guba 339±665 trace±0.05 September 1997 Yansky Gulf 338±633 Ͻ0.015 September 1997 Dm. Laptev Strait 416±580 Ͻ0.015 September 1997 Near Dunay Island (north of the Delta Lena) 415±467 Ͻ0.015 September 1997 main northern landscapes (Zimov et al. 1993): forest± harsh environment of the high Arctic. The ®eld surveys tundra; tall shrub community on ¯oodplain; alpine tun- were conducted at the Bykovsky Peninsula, the delta of dra; southern Khalerchin tundra, which is a mosaic of the Lena River, and the north slope of the Verkhoyansky typical tundra (sedge±dwarf-scrub polygonal mires) and Mountain Ridge (Primorsky Kryazh). Since fall 1994 southern tundra (low-shrub±sedge, tussock±dwarf- the estuary of the Lena River has also been surveyed. shrub mire) with patches of fall shrub vegetation; This study is based at the Polar Geocosmophysical Ob- marshy thermokarst depressions; and grass±shrub ¯ood- servatory located near Tiksi, on the coast of the Laptevs plains. The main study was not very far from the tim- Sea (Fig. 1). In summer (late August±early September berline. Within this area containing a range of hills and 1995) the CO2 research was conducted along the Lena mountains, ice complex (edoma), alases (depression River from Yakutsk to the Laptevs Sea (Tiksi) (Table with gentle slopes and ¯at bottom), ¯oodplain, and a 2). The expedition's tracks are presented in Fig. 1. number of lakes of both alluvial and edomic origin are More then 1000 samples were taken during all sea- presented. The typical lakes were presented in a sample. sons, though the main study was conducted between fall

This part of the joint study was based at the northeastern and early summer. The range of variability of pCO2 and science station located near Chersky (Zelenny Mys); see CH4 in the surface waters of north Asia and the Arctic Fig. 1. Sea is presented in Table 1. Because the dynamics of In an effort to improve our understanding of northern the carbonate system and dissolved methane is usually aquatic ecosystems as a source of atmospheric CH4 and investigated during summertime (Kling et al. 1992; Ree- CO2, in fall 1994 we extended the study area from the burgh et al. 1994), we focused this paper on our data Kolyma River lowland to the polar semidesert area near obtained in the time between autumn and spring. Note the Lena River mouth, where arctic lakes are in the most that arctic summer is different on land and sea; ``land

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FIG. 2. Distribution of ⌬pCO2 (␮atm) between the surface water and atmosphere in the (a) Laptevs Sea, and (b) Chukchi Sea, (Ϫ) shows

invasion of atmospheric CO2,(ϩ) the evasion of atmospheric CO2.

FIG. 3. Distribution of dissolved CH4 (nM) in the subice in the Lena River±Laptevs Sea system, late November 1996.

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TABLE 2. Variability of pCO2 and pH-TCO2 along the Lena River TABLE 2. (Continued) stream from Yakutsk to Tiksi: August±September 1995. TCO2, pCO2, Ϫ1 TCO2, pCO2, Sampling site pH25 mM L ␮atm Sampling site pH mM LϪ1 ␮atm 25 Neelov Bay: Near Sobo-Sise Is- ϳ40 km downstream from Ya- land 7.82 1.04 822 kutsk 7.94 0.92 657 ϳ200 m near Bykovsky Mys 7.76 1.14 965 ϳ110 km downstream from Ya- Laptevs Sea: ϳ1.5 km from By- kutsk 7.90 0.75 587 kovsky Mys 7.83 0.96 708 ϳ180 km downstream from Ya- Laptevs Sea: Near Razdelniy kutsk 7.79 0.78 784 Mys 7.81 0.96 727 ϳ5 km upstream from mouth of Laptevs Sea: Near Muostakh the Aldan 7.78 0.64 658 Mys 7.81 1.16 889 ϳ15 km downstream from Tiksi Bay: ϳ2 km from Muos- mouth of the Aldan 7.67 0.66 775 takh Mys 7.78 1.36 1112 ϳ25 km downstream from Tiksi Bay: Near Muostakh Mys- mouth of the Aldan 7.57 0.66 972 Kosistiy Mys 7.78 1.25 989 ϳ20 km downstream from San- Tiksi Bay: ϳ7±8 km from Tiksi gar 7.66 0.66 824 Town 7.75 1.38 1171 2 km upstream from Kisi Bely- Tiksi Bay: ϳ2 km from Tiksi aga Channel 7.68 0.65 776 Town 7.79 1.32 1016 ϳ10 km upstream from mouth of the Vilyui 7.67 0.63 769 ϳ7 km downstream from mouth of the Vilyui 7.78 0.82 811 summer'' months (no snow covering) are June, July, ϳ55 km downstream from August, and early September; ``sea summer'' (main sea mouth of the Vilyui 7.81 0.75 679 ϳ140 km downstream from aquatory is free from ice) is from late July until late mouth of the Vilyui 7.84 0.84 724 September. Note that it is not possible to obtain reliable ϳ220 km downstream from oceanographic data in the real high Arctic between Oc- mouth of the Vilyui 7.90 1.00 761 tober and early July without the very expensive use of ϳ300 km downstream from mouth of the Vilyui 7.94 1.07 764 icebreakers or organization of drifted stations. ϳ355 km downstream from Information about the change in atmospheric CO2 and mouth of the Vilyui 7.96 0.99 649 CH4 is obtained from the National Oceanic and At- ϳ3 km downstream from mouth mospheric Administration/CMDL, Commonwealth Sci- of the Vilyui 7.95 1.01 677 enti®c and Industrial Research Organisation, and other ϳ107 km downstream from Zhi- gansk 7.95 1.02 684 networks, and a number of World Meteorological Or- ϳ265 km downstream from Zhi- ganization (GAW) and IGAC Activities (GLOCARB, gansk 8.01 0.95 522 BIBEX, TRADEX, HESS, etc.) that are summarized in ϳ200 km downstream from Sik- Steele et al. (1987), Blake and Rowland (1988), Cice- tyah 7.95 1.06 669 ϳ50 km upstream from Kyu rone and Oremland (1988), Tans et al. (1990), Quay et Syur, timberline 7.86 0.92 713 al. (1991), Fung et al. (1991), Ciais et al. (1995a), Ciais ϳ1 km downstream from Tit- et al. (1995b), Conway et al. (1994), Prinn (1994), Ree- Ary 7.80 0.93 828 burgh and Crill (1996), and other literature. ϳ1 km upstream from Stolb Is- The study area includes most northern landscapes, land 7.81 0.93 809 Near Stolb Island: The begin- making it representative of the variety found in the Arc- ning of the Lena River delta 7.83 0.90 733 tic. Mys Boyarintseva 7.86 0.84 638 Bykovsky Channel: Chay-Ary Island 7.85 1.03 801 b. Southern Ocean Bykovsky Channel: ϳ20 km up- stream from Lena±Norden- The atmospheric gradient constrains the combined sheld 7.86 0.99 737 uptake by the Southern Ocean gyres and Antarctic wa- Bykovsky Channel: ϳ1 km up- ters to be from 0.6 to 1.4 Gt of C per year (Tans et al. stream from Lena±Norden- sheld 7.87 0.99 706 1990). An analysis of NOAA/CMDL Global Air Sam- ϳ300 m upstream from Lena± pling Network shows that a large sink of CO2 must be Nordensheld 7.75 1.01 968 between 30Њ and 60ЊS (Conway et al. 1994), though ϳ30 km upstream from Bykov- there are not enough oceanographic data to understand sky Mys 7.85 0.95 724 process of CO exchange in this region. According to Gerasimovsky Channel: Inlet 7.82 0.93 747 2 Gerasimovsky Channel 7.87 0.98 713 Ciais et al. (1995b), the latitude band south of the Ant- Neelov Bay: Outlet from Ispola- arctic convergence might be a small sink of atmospheric

tova Channel 7.78 0.94 824 CO2 (in 1992) or source of CO2 into the atmosphere (in 1993). To understand distributions of sources and sinks additional oceanographic data are needed. For this rea-

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FIG. 4. Distribution of ⌬pCO2 (␮atm) between the surface water and atmosphere in the Southern Ocean, February±March 1989.

son, we have investigated the distribution of pCO2 in advection by warm water of the North Atlantic through the surface waters south of 40ЊS in the Atlantic and the Fram Strait (Aagaard and Carmack 1989; Broecker Indian sector of the Southern Ocean. Note that until the 1997). The Arctic Ocean receives about ϳ10% of the late 1980s only seven oceanographic stations collected global river runoff but composes only ϳ5% of the area carbonate data in the entire Indian Ocean south of 30ЊS and ϳ1.5% of the volume of the global ocean. In fact, (Chen 1988). the combined freshwater in¯ow (ϳ3500 km3 yrϪ1)is

Concurrent measurements of pH25 (NBS scale) and determined by the great north Asian rivers, Yenisey (602 3 Ϫ1 3 Ϫ1 3 Ϫ1 TCO2 were made on board the research vessel Professor km yr ), Lena (513 km yr ), and Ob (451 km yr ), Vize of AARI during austral summer±autumn (Febru- and additional runoff (estimated at 1670 km3 yrϪ1, when ary±March) 1989. After a stay in Montevideo, Uruguay, referenced to salinity of 34.8 psu) enters indirectly Professor Vize twice crossed the subantarctic conver- through the Bering Strait (Antonov and Morozova 1957; gence (SC), Antarctic convergence (AC), and Antarctic Aagaard and Carmack 1989). Clearly, any attempt to divergence (AD), and reached its southernmost point understand the effect of the Arctic Ocean on global within the ice ®eld at the Antarctic Circle near the Mir- change or the effect of global change on the Arctic niy observatory in the Davis Sea. The experimental Ocean requires a thorough knowledge of the riverine technique is the same as used in the Arctic research and in¯uence on hydrochemistry of the arctic adjacent seas, described in Weiss and Craig (1973) and Semiletov et especially on the carbonate system of water and dis- al. (1995). The study area of this expedition and dif- solved and solid substances. Investigations for the ference of pCO2 between ocean and atmosphere are MacKenzie and Yukon (whose discharge enters the Arc- shown in Fig. 4. The calculated ¯ux of CO2 (F) in the tic via the Bering Strait in¯ow) are currently under de- atmosphere±ocean system is presented in Fig. 5. velopment and ongoing data are available. Otherwise, the Siberian rivers, especially the Lena River, which contributes most of the freshwater input to the Amer- 3. Results and discussion asian basin, are poorly investigated. a. Arctic

1) ARCTIC SEAS (i) Inorganic carbon system The Arctic Ocean is the most sensitive link in the Here we present our recent results that were obtained climatic system; only a thin layer of freshwater beneath mainly in the Laptevs Sea, which is in¯uenced strongly the ice prevents the ice cover from melting due to heat by in¯ow of the Lena River (Fig. 2a). Also we brie¯y

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Ϫ2 Ϫ1 FIG. 5. Distribution of CO2 ¯uxes (F, mM m day ) between the surface water and atmosphere in the Southern Ocean, February±March 1989.

consider the CO2 data obtained in the Chukchi Sea (Fig. dynamics of the carbonate system in the Chukchi Sea 2b). are presented in detail elsewhere (Semiletov et al. 1998).

The shallow Chukchi Sea with a typical depth of The ⌬pCO2 data presented in Fig. 2a show that the about 40 m is a controlling area for the ¯uxes of the aquatory of the Laptevs Sea was also undersaturated freshwater, nutrients, and carbon into the Arctic from strongly by CO2 relative to the atmosphere, though the the North Paci®c. Late September and October mark the area adjacent to the delta of the Lena River is super- transition time between summer and winter seasons in saturated signi®cantly by CO2. Our year-round study of the Arctic. During this time, the magnitude of river dis- the CO2 system in the Lena River±Laptevs Sea system charge is decreased signi®cantly, about 2- to 3-fold for shows that the riverine waters are a source of CO2 for the Yukon and Mackenzie Rivers (Coachman et al. the atmosphere during all seasons (Semiletov et al. 1975) and 10- to 20-fold for the Siberian Rivers (An- 1996b). Likewise, a long-term hydrochemical investi- tonov and Morozova 1957) and the regional thermo- gation of AARI demonstrates that the spatial distribution haline regime is switched from summer estuarine en- of dissolved oxygen is in opposition to pCO2 variability vironment to a winter ``reverse estuary'' out¯ow of obtained in our research (Rusanov et al. 1979). The high high-density saline waters (Aagaard and Carmack value of pCO2 agrees with the low concentration of 1989). Distribution of ⌬pCO2 (Fig. 2b) between the dissolved oxygen. Measured concentrations of dissolved surface waters and atmosphere shows that the aquatory CO2 and oxygen are a result of the interaction between is strongly undersaturated by atmospheric CO2, which different physical and biological processes as cooling± should result in absorption of CO2 from the atmosphere. warming, photosynthesis±respiration, and conservative Supersaturation by CO2 was found in a small area near mixing of different waters (Park et al. 1974; Skirrow Cape Barrow, which might be governed by coastal up- 1975; Weiss et al. 1982; Bordovsky and Ivanenkov welling. The increase in the magnitude ⌬pCO2 (Fig. 2b) 1985; Chen 1988). In different seasons and latitudes from the Bering Strait toward the ice edge (ϳ72ЊN) is physical or biological processes might dominate the dis- controlled mainly by temperature. Evaluation of the tributions of pCO2 and dissolved oxygen in surface wa- temperature effect on the carbonate system shows (Mil- ter. lero 1995) that the observed temperature decrease of Other data show that the sea aquatory in¯uenced the surface water ϳ6Њ±7ЊC should cause a decrease in strongly by the riverine input is also two to three times equilibrated value of pCO2 of about 100 ␮atm, which or more supersaturated by CO2 (Table 2). For instance, is similar to observations (Fig. 2b). Our results for the the value of pCO2 near the sandbar of the Kolyma River,

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the east Siberian Sea was 716±1779 ␮atm in fall 1994 with our data. Recent Japanese pCO2 data obtained in (Amderma cruise). Our experimental data obtained in the Greenland Sea (August 1993; April and May 1994) fall 1995 on board R/V Mikhail Somov also shows that and in the Barents Sea (June 1995; July and August not far from the mouth of the Ob and Yenisey, the sur- 1996), presented at the BASIS Conference (February face waters of the Kara Sea were higher in comparison 1998, Saint Petersburg) by H. Ito also indicate signi®- with an aquatory moved offshore (Table 2). This is cor- cant undersaturation of the surface layer by CO2 with roborated by the data of Kelley (1970), which were a decrease of pCO2 value toward the ice edge (down to obtained in the Kara Sea during the late summer of 1967. 200 ␮atm). It is probable that relatively high photosyn-

Existing data show that the surface pCO2 in the Arctic thesis near the outlet of the riverine waters plays a neg- Sea varies mainly in the range 150±500 ␮atm. It is ligible role in a local budget of atmospheric CO2, be- affected by changes in water temperature, photosynthe- cause during all seasons the delta area is a source of sis of marine plants, CO2-rich river runoff, biochemical CO2 into the atmosphere. Likewise, the riverine input oxidation, coastal upwelling, upward divergence of deep of inorganic carbon might play a signi®cant role in the water by cyclonic gyres, freezing processes, changes in hydrochemical regime of halocline waters in the Arctic depth of the surface mixed layer, and by changes in basin. For instance, our data (Semiletov et al. 1996b), advection of water determined by variability in the at- corrected with the most recent data (September 1998), mospheric circulation. From Figs. 2a,b and Table 1, we show that only the Lena River brings about 3±4 Tg C- can see that the whole aquatory of the Arctic Sea, even CO2 in the subice water annually. Our recent measure- during and after convection and freeze-up, seems to be ments show that the Lena River brings also about 6 Tg Ϫ1 a sink of atmospheric CO2 with undersaturation up to Cyr in the form of dissolved organic matter that might 40%±60%. Supersaturation by CO2 is obtained only in be oxidized in CO2 form. the restricted areas near mouths of the Siberian rivers There are currently few observations for the Arctic and in the coastal sites (Table 1), related mainly to up- and subarctic seas during ice-covered periods, which welling of waters more enriched by CO2. Our recent might be 10±11 months per year. Therefore we can only data obtained in September 1998 and 1997 indicate that speculate about the net CO2 ¯ux between atmosphere summertime coastal retreatment (from 2 to 40 m yr Ϫ1) and arctic waters. Because young arctic ice is permeable might be an important source of terrestrial organics into for gas ¯ux (Gosink et al. 1976) yet there are no rep- the shelf waters. And oxidation of these organics could resentative experimental data for the transfer rate of CO2 be a cause of supersaturation of bottom water by CO2 across the ice cover, we attempt here to answer quali- (up to 2000 ␮atm). We did not calculate the value of tatively the following question: is the Arctic Shelf a

CO2 ¯ux here because to date there are no representative source or sink for atmospheric CO2? In general, the data for the transfer velocity of CO2 during and after arctic measurements show that surface water near the freeze-up time. wintertime marginal ice zone is undersaturated by CO2 Our CO2 data agree well with literature data obtained because of cooling, and because biological respiration in different seasons (Kelly 1970; Park et al. 1974; Cod- is negligible. Photosynthesis might decrease the pCO2 Ϫ ispoti et al. 1982; Chen 1993). Previous studies of (and TCO2) value, because C-CO2 and C-HCO3 (bi- ⌬pCO2 between the atmosphere and the surface waters carbonate ion) are utilized during photosynthesis (Smith of the North Atlantic Ocean and the Barents and Kara and Sashaug 1990). For this reason, unusually high ox- Seas show that the aquatory of the Kara, Barents, and ygen concentrations (sometimes greater than 150% of Norwegian Seas during the late summer of 1967 was saturation values) observed in near-freezing waters undersaturated by CO2 regard to the atmosphere in range (Codispoti and Richards 1971) should indicate the ex- of ⌬pCO2 from 100 to 150 ␮atm (Kelly 1970). Like- istence of low values of pCO2. Our data and data in the wise, the surface water near the mouths of the Ob and literature show that cooling of water might be consid-

Yenisey Rivers were supersaturated in CO2 with respect ered a process that decreases the pCO2 distribution in to air. Carbonate data obtained in September±October the polar seas. Weiss et al. (1982), based on observa- 1993 near the mouths of the Ob and Yenisei also indicate tions, demonstrate that in the low latitudes and other supersaturation by CO2 from 10 to 200 ␮atm (Mak- oligotrophic zones temperature might greatly in¯uence kaveev 1994). As the surface water cools in its ¯ow the distribution of pCO2 in the surface waters. In winter northward, the CO2 pressure is decreased in a similar (February±March) 1983 Gosink (1983) found that the manner as observed for the Chukchi Sea in Fig. 2b. surface water of the Barents, Norwegian, and Greenland

Because the uptake of CO2 by photosynthesis is high Seas were undersaturated in CO2 with respect to air, and only in the delta waters of the Siberian rivers (in the ⌬pCO2 varied from Ϫ20 to Ϫ50 ␮atm. The CO2 system Lena River, up to 0.1±0.3 g C per mϪ2 per day, integral data obtained by Chen (1985, 1993) in February±March value) and decreases 10±100 times toward the shelf edge indicate undersaturation near and beneath ice across the (Sorokin and Sorokin 1996), we can assume that the marginal ice zone of the central and southeastern Bering general decrease in CO2 saturation from Arctic coast Sea Shelf, that is, the greatest sea shelf in the subarctic. toward the north is determined mainly by cooling, as The undersaturation was found from the surface to a was stated previously by Kelley (1970) in agreement depth of about 75 m. In this research the biological

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factor is negligible, and undersaturation of CO2 is due the late winter±spring bloom and early winter±fall con- to cooling of the eastern shelf water. Carbonate data of vection and freeze-up show that the Arctic shelf waters

Codispoti et al. (1982) obtained before and during the are a sink rather than a source of CO2 into the atmo- spring bloom in a similar area show that before the sphere. Due to the lack of data for the Arctic Basin and bloom the surface water of the eastern shelf is near kinetics for CO2 transfer across the ice cover, at present equilibrium with air by CO2. Near the shelf edge the we cannot evaluate the overall role of the Arctic Ocean water was supersaturated by CO2, probably due to up- in regional CO2 cycling. Based on data in the literature welling of the deep water (Paci®c in origin) more en- (Kelley 1970; Park et al. 1974; Bordovsky and Ivanen- riched by nutrients and CO2 (Park et al. 1974). Thus, a kov 1985; Smith and Sakshaug 1990; Codispoti and limited dataset shows that the shelf waters of the Arctic Richards 1971) we can assume that during summertime and subarctic seas are mainly a sink of atmospheric CO2 aquatory of the Arctic Sea is mainly undersaturated by during wintertime. Alhtough there are no available data CO2 and supersaturated by O2 due to photosynthesis. for the CO2 system beneath ice in the Arctic seas located between 90ЊE and 170ЊW, we can consider an oxygen (ii) Dissolved methane dataset obtained by AARI from the drifted ice stations

NP1±NP21 during 1948±73 (Rusanov et al. 1979). The Our fall (1994, 1995, 1997) study of CH4 in the sur- mean average data show that the subice water in the face water of the Arctic Sea from 70ЊE to 170ЊW shows

Amerasian sector (Canadian Basin) are supersaturated that the concentration of CH4 is usually less than 0.015 by dissolved oxygen up to 5%, though the aquatory, ␮M(␮M ϭ 10Ϫ6 moles per liter) (i.e., analytical zero which is in¯uenced strongly by riverine input, is en- in our GC measurement). Because of interference from riched by organic and TCO2 and undersaturated by dis- the ship's electrical system, it would not have been pos- solved oxygen about 5%±10%. The rise in oxygen su- sible to detect lower concentrations of CH4 in the sam- persaturation beneath the ice might be governed by mix- ples. Because equilibrated concentration of dissolved ing with more warm water, not by biochemical processes CH4 (Weisenburg and Guinasso 1979) in arctic air±wa- (Chen 1988). The most recent research obtained on ter system ranges from 0.003 ␮M to 0.004 ␮M with board icebreaker Des Grosielliers in winter 1997±98 temperatures of 10ЊC and Ϫ1ЊC, respectively (for con- shows that under the in¯uence of global warming, the stant salinity ϭ 20½), we can assume that uncertainty salinity in the upper 50-m water column in the Amer- in our measurements is limited by possible supersatu- asian sector (between 74Њ and 77ЊN) decreased signif- ration or undersaturation by a factor of 5 in summer icantly in comparison with previous measurements (10 and a factor of 4 in winter. In comparison with CH4 and 20 yr ago), and the water beneath the ice was su- lake data presented in Table 1, which corresponded to persaturated by oxygen during polar night when the bi- a supersaturation factor range between 102 and 105,we ological factor is negligible (I. Melnikov 1998, personal consider the possible CH4 ef¯ux from arctic seas as communication). Using the anticorrelation in biogeo- ``small,'' though this assumption is crude and needs chemical cycling of CO2 and O2 in¯uenced strongly by additional data. Note that our assumption agrees with a temperature factor (Bordovsky and Ivanenkov 1985) we 10%±15% increase of tropospheric CH4 over land (Kel- assume that the surface water enriched by oxygen within ley and Gosink 1988; Harriss et al. 1992). a stable temperature regime should be undersaturated To evaluate the role of shallow offshore permafrost by CO2. In this case, following Treshnikov and Salnikov in late September 1994 we have sampled water above (1985) we consider a temperature regime beneath the the subwater ice complex (near the Bykovskiy penin- ice as a relatively stable in comparison with the open sula) enriched by organics. The CH4 concentration was aquatory. Previously, Kelley (1970) compared the sur- detected at the level 0.030±0.050 ␮M (Semiletov et al. face water oxygen concentrations to the pCO2 values 1996b), which is similar to the range of CH4 measure- and found mutual variations with temperature. Other- ments in Canadian Beaufort Shelf waters (MacDonald wise, we can assume that the shelf water in¯uenced by and Thomas 1991). riverine input should be supersaturated by CO2, because The recent (November 1996) wintertime measure- the riverine waters are enriched by dissolved organic ment of CH4 in the shallow waters above the offshore carbon that oxidized to CO2. This agrees with our ex- permafrost near the Lena River Delta (Bykovskiy pen- perimental data: during fall the open (fall 1995) and ice- insula) shows that the bottom sediment/thaw permafrost covered (fall 1994) aquatory of the Laptevs Sea is a might be a source of CH4 in the atmosphere, because sink for atmospheric CO2, except the water adjacent to the subice water is enriched signi®cantly by CH4 (up the Lena delta and some coastal sites (see Fig. 2a and to 103 times) relative to the atmosphere. The vertical

Tables 1 and 2). Data for pCO2 distributions near the distribution of dissolved CH4 shows that the main max- delta of the Ob and Yenisei Rivers also demonstrate imum is located just beneath the ice, not in the bottom supersaturation by CO2 (from ϩ30 ␮atm to ϩ140 ␮atm) layer. The distribution of CH4 in the subice layer is in late summer and autumn (Kelley 1970; Makkaveev presented in Fig. 3. Because the bottom is the source

1994). of CH4, it is evident that transfer of CH4 toward the Thus, the available wintertime data obtained during surface is determined mainly by ebullition. The same

Unauthenticated | Downloaded 09/25/21 10:31 AM UTC 15 JANUARY 1999 SEMILETOV 297 vertical distribution with a subice maximum was ob- ni®cantly higher in comparison with the summertime tained in the thaw lakes of the Kolyma lowland during Alaska data (North Slope region). The corresponding or after a cyclonic system crossed the study area (Sem- diffusive ¯uxes into the atmosphere might be signi®- iletov et al. 1996a). It is interesting that we did not ®nd cantly higher, although the ice covering might resist gas dissolved CH4 in such high concentration in summer- transfer from the water body to the atmosphere, and time in the same place. We assume that the following wintertime ef¯ux of CH4 (and CO2) is going through factors might in¯uence an increase in CH4 concentration ``koshkas'' (see below) or ice trenches and ice over¯ow beneath the ice. First, since the sea ice cover signi®- water (Semiletov et al. 1994). The data obtained in the cantly decreases the transfer rate of any gas into the Kolyma lowland were taken mainly in the area adjacent atmosphere (Gosink et al. 1976), we can consider the to the timberline. Most of the data are representative for ice cover a barrier trapping the methane below. Second, the vast region of the northern taiga, woodland tundra, the bacterial activity for methane oxidation is reduced and low Arctic tundra. To obtain the data in the high strongly due to the drop of temperature to 0ЊC and less. Arctic we did the same study in the most severe con- Third, it might be important that summertime warming ditions of the high Arctic tundra, in the Tiksi area (see reaches the greatest depths of the active layer during Fig. 1). We found that in fall the content of CH4 in the wintertime; that is, the temperature of the deep layer of thaw lakes located in the high Arctic ranged from 0.06 the bottom sediment and thaw permafrost is increased to 0.4 ␮M in the subice layer and from 0.07 to 2.2 ␮M during fall and early winter. Increase of the sediment in the bottom layer of lakes. These values are about 102 temperature dominates in production of CH4 (Reeburgh lower than in the Kolyma lowland (Table 2). It is prob- et al. 1994), which was detected in the northern lakes able that the lower production of CH4 is related to the in early wintertime (Zimov et al. 1997b). Note that the difference in the thermic regime and environment for range of variability of the CH4 concentration in the ther- formation of the ice covering. Also, strong winds are mokarst lakes of the Bykovsky peninsula (onshore per- typical for the coastal arctic zone and usually snow is mafrost) during wintertime 1994/95 (Semiletov et al. blown up from the ice surface, that is, the thermoiso- 1996a; Semiletov et al. 1996b) is similar to the range lation effect from the snow covering is absent here, of CH4 variability obtained over shallow offshore per- whereas the snow thickness usually ranged between 0.2 mafrost (Fig. 3) that is the same by origin (Pleistocene and 0.4 m on the lakes of the Kolyma lowland. There- ice complex) with onshore permafrost. An anomalous fore, at the Tiksi area the temperature in the water body high value of CH4 is observed at point 17 (Fig. 3), where drops to 0.1Њ±0.3ЊC in late October when the thickness CH4 concentration beneath ice is 20 ␮M. We explain of the ice is about 0.4 m, whereas in the Kolyma lowland this by an episodic increase in CH4 ebullition from this these values vary ϳ1.0Њ±2.4ЊC and ϳ0.2 m, respec- site due to a previous drop in the atmospheric pressure, tively. In late wintertime the ice thickness at the Tiksi which might induce the CH4 ebullition (Semiletov et al. area reaches 2.2±2.3 m, whereas at the Chersky area 1996a). Such a correlation was obtained previously in only 1.3±1.5 m. The CH4 production decreases in the the temperate latitudes at Mirror Lake, New Hampshire upper layer of limnic sediment in the Tiksi area. Like- (Mattson and Likens 1990). wise, the thaw zone underneath the water body of the lakes is shallower here [about 15±30 m, by Kunitskiy (1989)] than in the more mild arctic±subarctic environ- 2) ARCTIC LAKES AND RIVERS ment (Chekhovskiy and Shamanova 1976), such as the Usually, levels of dissolved methane in the northern Kolyma lowland near and south of the timberline where lakes and rivers were measured during summer (Bartlett through taliks might exist. For this reason, in the high et al. 1992; Kling et al. 1992; Reeburgh et al. 1994). It Arctic CH4 is produced mainly in the younger surface was found that in summer the Alaskan arctic lakes are sediments. Indeed, CH4 sampled from the North Slope a source of CH4 for the atmosphere that is similar in region of Alaska was only 200 years old (Martens et al. magnitude to the CH4 source from wet soils. Our all- 1992). In contrast, in two lakes located near the treeline season study for dissolved CH4 dynamics and ef¯ux into in the Kolyma Lowland about half of the current annual the atmosphere shows that the northern lakes play a methangenesis is fueled by Pleistocene C (Zimov et al. signi®cant role in the regional CH4 budget during all 1997b), which is involved in the modern biochemical seasons (Semiletov et al. 1994; Semiletov et al. 1996a; cycling through deep thaw of permafrost underneath the Semiletov et al. 1996b). lakes.

Here we discuss mainly the data for the pCO2 and In order to examine factors controlling methane and CH4 distributions obtained from fall until springtime, CO2 concentrations in various lake depths in different because the behavior of both gases in the summer season seasons, we have measured the vertical distribution of is described in detail in the literature (Bartlett et al. methane in different seasons in the typical northern 1992; Kling et al. 1992; Reeburgh et al. 1994). Our lakes. The vertical pro®les show that the dissolved meth- wintertime data show the vast range of CH4 values in ane concentrations were usually elevated drastically to- the subice layer of water, from 4 to 360 ␮M in the ward the bottom. Usually the CH4 concentrations in Kolyma River lowland. These values of CH4 are sig- bottom water increased during the wintertime CH4 ac-

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FIG. 6. Dynamics of the vertical distribution of dissolved (a) CH4 and (b) pCO2 in the typical thaw lake (the Kolyma lowland) during springtime 1994.

cumulation period from fall (October) to late spring lett et al. (1992) found that in summer the surface CH4 (May). Furthermore, the bottom layer, which contained concentrations differed signi®cantly between large and the highest CH4 concentration, became thicker during small lakes. Methane concentrations in large lakes wintertime. In general, the dynamics of methane con- ranged from 0.01 to 0.31 ␮M. Concentrations in smaller centration pro®les is corroborated by that in Canadian lakes were higher and ranged from 0.21 to 10.4 ␮M. lakes (Rudd and Hamilton 1978). We found that the Calculated diffusive ¯uxes from open water varied with Ϫ2 nitrogen content is near-equilibrated with air and rela- the lake's size. The large lakes emitted 3.8 mg CH4 m tively high oxygen concentrations were observed in the dayϪ1, and small lakes emitted an average of 77 mg mϪ2 water column under ice. For example, until 18 January dayϪ1. Thus summertime diffusive methane ¯uxes from 1993 the oxygen concentrations dropped to the mean the thaw small lakes and ponds are nearly 20 times value about 2.5±3.0 ml LϪ1 in the thaw lakes with a higher than those from large thaw lakes and are ap- typical depth of 3±10 m (the Kolyma lowland). proximately equal to one-half of ¯uxes from wet mead-

The springtime dynamics of dissolved CH4 and pCO2 ow sites. show (Figs. 6a,b) that during the period of observations, Emissions from lakes by sporadic bubbling from the the distribution of pCO2 between 1- and 6-m depth was sediment are dif®cult to quantify, since measurement quasi-uniform, whereas CH4 in the bottom water de- depends upon different factors, such as sedimentary en- creased with time. Note that dynamics of the CH4 pro®le vironments, organics content, evolution of thaw lakes, might be considered to be controlled by partial up- and the amount and distribution of ground ice in the welling of bottom water due to springtime overturning. substrate, which depend upon local deposition and ther- It correlates with the existence of subice maximum of mal history. To improve the study of factors controlling

CH4 obtained on 29 May 1994 in the thaw lake near lake bubble composition we surveyed many different Chersky in the Kolyma lowland. thaw lakes located from Dyvanny Yar to the mouth of The methane pro®les observed in the lake suggest the Kolyma River (Fig. 1). We sampled visible bubbles two questions: what type of transfer provides the re- at the surface using a hand funnel. The bubble sizes corded CH4 distribution, and what amount of CH4 might were varied within 0.2±0.6 cm in diameter at water sur- escape to the atmosphere due to hydrodynamics? face. Usually the methane content was no less than The methane diffusive ¯ux might be equal to or ex- 43%±85% by volume of ebullient gases (Semiletov et ceed the bubble ¯ux in freshwater lakes (Cicerone and al. 1996a). It was found that most intensive ¯uxes of Oremland 1988). As was shown in Semiletov et al. methane-rich gas bubbles were obtained along the lake

(1996a), the formation of the near-bottom CH4 maxima edge environment, particularly near steep banks of ice might be determined by complete dissolution of small complex or ``edoms'' (the northern hills contained great sediment bubbles with a radius of about 0.01 cm. Bart- quantities of ground ice, about 40%±80% of the volume

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during the period of air pressure drop the net CH4 ¯ux to the atmosphere is increased signi®cantly. Such a cor- relation was obtained previously at Mirror Lake, New Hampshire (Mattson and Likens 1990). A similar phe- nomenon has been known to colliery ventilation engi- neers in the United Kingdom for more than 250 years

(McQuaid and Mercer 1991). We found that the net CH4 emission to the atmosphere is provided mainly by the large bubbles that rise quickly and dissolve in the water column in small amounts. In this case methane oxidation

in the water column is negligible. Likewise, the CH4 enrichment of a water body is provided mainly by the small bubbles that disappear when rising from the sed- iment (Semiletov et al. 1996). Note that low air pressure events increase ebullition in all seasons. In fall 1992 and spring 1993 the main transport of

CH4 through the ice covering in the Kolyma lowland was realized via the holes or koshkas (local name). These holes or koshkas are produced by relatively con- stant gas ebullition from each site. The size of the kosh- kas ranges from 100 to 102 cm in diameter (Semiletov et al. 1994), which is determined by gas ebullition rates from sediment. Usually a gas ebullition ¯ux obtained Ϫ2 Ϫ1 FIG. 7. Methane ebullition rates (mM m day ) and atmospheric Ϫ1 pressure P (mbar) in the typical thaw lake (the Kolyma lowland). at each site ranged from 5 to 10 cc min . Sometimes the sporadic large bubble ebullition ranged from 40 to 70 cc minϪ1. When the ice thickness is increased, a of perennially frozen eolian sand, silt, and peat rich in koshka begins to collect gas bubbles from a larger area organic matter). like a funnel, because risen bubbles are rolled along the The winter data obtained in the Kolyma lowland show ice±water interface toward the koshka. For example, in the high CH4 concentrations in the surface layer under mid-October, when mean ice thickness in the Chersky ice. The subice maximal CH4 value was obtained in area (Kolyma lowland) is about 15±20 cm, the koshka January 1993, after drastic CH4 ebullition related to a of about 10 cm in diameter collected different bubbles low air pressure event. All sites, including shallow-wa- injected from a distance of about 2±3 m; that is, one ter sites, show a remarkable synchrony in the bubble ``typical'' koshka collects risen bubbles from an area of release correlated with changes in local air pressure. We about 12±30 m2. A positive feedback mechanism is at assume that the subice CH4 maxima is associated with work here. Usually in the Kolyma lowland the largest rising CH4 bubbles accumulated partly under the ice koshkas exist until the coldest months (January±Feb- surface. It was observed visually through October when ruary). Periodically water above ice is formed when the ice surface was not covered completely by snow. In settling occurs due to an accumulation of snow on the winter 1995/96 the subice maxima in the dissolved CH4 ice covering. We assume that accumulated gas arrives distribution induced by ebullition in the lakes of the in the atmosphere with water above ice. This event is southern Alaska was detected also by A. Phelps (1996, found usually one or two times per winter. It should be personal communication). noted that the ice settling is found usually close to the Figure 7 shows methane ebullition rates at the site in lake's edge where the snow accumulation is more ef- typical thaw lake with depth 10 m (Kolyma lowland) fective. Simultaneously, in the central parts of lakes the obtained during two weeks synchronously with air pres- large-scale ice fracturing takes place and headspace gas sure (P) changes. During summertime the CH4 ebulli- under ice is liberated into the atmosphere. In the Tiksi tion from shallower sites was larger, probably due to area (coast of the Laptevs Sea) we found that the kosh- enhancement of CH4 production causing signi®cant kas are closed in early or mid-October (1994), because warming of the sediment surface. Low air pressure strong winds blow snow from the ice surface, which events associated with storm systems appeared to induce increases the rate of freeze-up signi®cantly. Also, in the ebullition, whereas high pressure inhibited ebullition. Tiksi area in mid-October we observed synchronous

Sometimes the CH4 ebullition attained value of tens of large fracturing of ice covering in the different lakes Ϫ2 mM CH4 m per day, when the air pressure dropped due to supertension of ice during quick freeze-up. This to 740 torr. The proportion of the methane in the bubbles is accompanied by the ventilation of the under-ice head- was changed. The CH4 portion was increased simulta- space, which also favors the CH4 and CO2 emission via neously with the total gas ebullition (probably due to diffusion transfer. However, our observations show that enhancement in bubble size and upward velocity). Thus, during the coldest months the number of active koshkas

Unauthenticated | Downloaded 09/25/21 10:31 AM UTC 300 JOURNAL OF THE ATMOSPHERIC SCIENCES VOLUME 56 is decreased signi®cantly, even in the relatively mild methane is not destroyed in the troposphere, it is pos- subarctic environment. Consequently, the total CH4 sible to estimate the magnitudes of the CH4 ¯ux, which (CO2) ¯ux to the atmosphere is decreased also. This is adequate to explain the interhemispheric CH4 gra- Ϫ2 correlates with the monitoring data for atmospheric CH4 dient. An air column of 1 m with a CH4 concentration (Quay et al. 1991). of 1.8 ppmv would contain ϳ10 g CH4. Then approx- Ϫ2 In the Lena River delta the concentration of dissolved imately 0.8 g ´ m of CH4 would arrive in the atmo- CH4 was usually too small to detect (Ͻ0.015 ␮M) dur- sphere for an observed increase of the CH4 concentra- ing summertime, but during the ice cover season the tion of ϳ150 ppbv (Steele et al. 1987; Quay et al. 1991). value of CH4 varied from trace up to 0.1 ␮Minthe Following Fung et al. (1991) we assume that the north- Bykovsky Channel and Neelovsky Gulf (Fig. 3) and in ern wetland area between 50Њ and 70ЊNisϳ2.65 ϫ 1012 the Olenek Channel (Semiletov et al. 1996b); at seaside m2 and the total area of the tundra and northern taiga 12 2 near delta concentration of CH4 reached 20 ␮M (Fig. is about 9.4 ϫ 10 m . Then the wetlands cover ϳ25% 3). In summer the riverine and lake water was usually of the total area north of 50ЊN. Taking into account that supersaturated by CO2 two±three times; the supersatu- in winter the soil source of CH4 is absent, the limnic ration increased during winter signi®cantly (up to 10± unfrozen sediment should produce more than 3.2 g CH4 20 times). Our measurement shows that the Lena River mϪ2 to produce the observed interhemispheric gradient. water is supersaturated by CO2 from the middle stream Our current observations show that this value is not (Yakutsk) downstream to the delta (Table 2). This result unreasonable. For instance, the cross movement of a agrees with pCO2 data obtained in the deltas of the Ob cyclone with an air pressure drop to 740 torr increases and Yenisey Rivers in September±October 1993 (Mak- the mean value of CH4 ebullition from background val- Ϫ2 Ϫ1 kaveev 1994). ues ranging between ϳ15±50 mg CH4 m day to 0.2± Ϫ2 Ϫ1 The wintertime increase of pCO2 in the lakes might 0.3 g CH4 m day (Semiletov et al. 1994; Semiletov be up to 10±30 times (Semiletov et al. 1996a; Semiletov et al. 1996a). Hence, the cross movement of cyclonic et al. 1996b), which is similar to the increase obtained systems might play the role of triggering the CH4 ef¯ux by Coyne and Kelly (1974) near Barrow, Alaska. from lakes. Also, the CH4 emission due to the spring and fall overturn, when bottom waters enriched by CH4 rise to the surface might be superimposed on CH eb- 3) EVALUATION OF CH EMISSION FROM THE 4 4 ullition. For instance, if about 1 m3 of typical bottom ARCTIC LAKES water with CH4 concentration of ϳ8mgCH4 per liter Ϫ2 Our direct measurement of CH4 ebullition shows that reaches the surface, then ϳ8gCH4 m might escape the CH4 ebullition rate might be increased drastically from the lake to the atmosphere. Here we assume that when atmospheric pressure drops (Fig. 7). Sometimes water overturning is a ``quick'' process, which precludes the CH4 ebullition achieved values of tens of mM CH4 consumption of CH4 by the methane oxidants. Our ini- per day near or south of the timberline. Is this CH4 tial observations show that such a quick process of over- emission signi®cant for the regional CH4 budget or not? turning takes place in aquatory of lakes with typical And how much CH4 might be emitted into the atmo- depths less then 3±5 m. It was shown that large quan- sphere from the northern lakes by other processes? tities of CO2 might escape into the atmosphere due to Taking into account that during polar night the supply diffusity, especially during overturning. Indeed, in of the hydroxyl radical is reduced dramatically in a com- spring and fall surface waters of lakes are supersaturated paratively closed arctic air mass (Vowinckel and Orvig signi®cantly by CO2 and CH4 (Rudd and Hamilton 1970; Cicerone and Oremland 1988; Nisbet 1989), we 1978; Semiletov et al. 1996a; Semiletov et al. 1996b), assume that in winter the CH4 emission from lakes via in agreement with the timing of the rise in atmospheric holes and cracks in ice is a net source to the atmosphere. methane over the Arctic (Steele et al. 1987; Quay et al.

We consider here the arctic atmosphere as a compara- 1991). Concentrations of dissolved CH4 and CO2 ob- tively closed arctic air mass, that is, a very simple ap- tained in the high Arctic (Tiksi area) are lower by about proach, but the existence of the maxima of CH4 and two orders of magnitude in comparison with low Arctic/ CO2 over the Arctic demonstrates that the main sources subarctic (Cherskiy area) but are 10±100 times more of CH4 and CO2 are at northern areas rather than mid- than values equilibrated with air. Note that the range of latitudes. Midlatitudes play only an episodic role in the pCO2 values of lakes located in the Tiksi area is similar increase of CH4 and CO2 through northward transport to that near Barrow, Alaska (Coyne and Kelley 1974), events (Worthy et al. 1994). The recent latitudinal and and the North Slope region of Alaska (Kling et al. 1992). seasonal variations of the atmospheric methane show a Measurements of CH4 ¯ux from the Kolyma lowland very broad maximum in winter and minimum in mid- lakes (Zimov et al. 1997a) show that the annual ¯ux is Ϫ2 summer (Fung et al. 1991; Quay et al. 1991; Reeburgh about 11 g CH4 m , of which 76% is released during and Crill 1996). Thus, the maximum interhemispheric ice cover season (October±May). This might be enough gradient is obtained during Northern Hemisphere winter. to maintain the interhemispheric gradient (see text If we assume that during winter there is no gas exchange above). The total winter ef¯ux from Siberian lakes ac- Ϫ1 between the northern atmosphere and midlatitudes and counts for 2.5 Tg CH4 yr (Zimov et al. 1997a). This

Unauthenticated | Downloaded 09/25/21 10:31 AM UTC 15 JANUARY 1999 SEMILETOV 301 is a lower evaluation, because it does not contain the 4) EVALUATION OF THE ROLE OF THE NORTHERN

CH4 emission produced by fall (and spring) overturning LAKES IN THE PAST that was detected in atmosphere over Barrow, Alaska, as a strong signal (Quay et al. 1991). If we use the result It has been shown that the concentration of methane in the Greenland ice core is about 10% (Ϯ4%) higher of our simplistic evaluation of CH4 emission due to autumn lake overturning, the total atmospheric input that in the Antarctic ice cores over the Holocene (Ras- mussen and Khalil 1984). At present, the NOAA/CMDL from the Siberian lakes will be about 4±5 Tg CH4 per winter. This somewhat crude estimate should be cor- network shows that the annually averaged methane con- centration is highest at Barrow (71ЊN), which is about rected for the northward decrease in CH4 production by limnic sediment and underlying thaw permafrost once 8% above that of the South Pole (90ЊS) (Steele et al. more data are obtained. At any rate, this crude estimate 1987; Blake and Rowland 1988; Fung et al. 1991). Com- based on our year-round data shows that the northern parison of historical data obtained in the works of Jouzel lakes may play a signi®cant role in formation of the et al. (1993) and Chappellaz et al. (1993) shows that atmospheric methane maximum over the Arctic. This the interhemispheric gradient is decreased from ϳ10% assertion is supported by the space±time changes in the in warm epochs to practically negligible value during 13 glacial epochs. Then it might be assumed that during ␦ C of atmospheric CH4 (Quay et al. 1991) that show a seasonal cycle with minimum ␦13C values in fall and global warming the north is a source of CH4 to atmo- maximum values in the summer (increased income of sphere (Semiletov et al. 1994). What is the signi®cance of such global changes? ``isotopically heavy'' CH4 from the midlatitudes), with the strongest season amplitude in the north. A meridi- The joint analysis of the data obtained from the new onal gradient shows that the lowest values of ␦13Cin Greenland Ice Core Project ice core (Jouzel et al. 1993) and measurements from Vostok (Antarctica) (Chappel- atmospheric CH4 are also in the north. Thus, the most biogenic and isotopically light methane is obtained in laz et al. 1993; Semiletov et al. 1994) shows that during the north during fall±winter, indicating a northern source two main glacial±interglacial transitions, the CH4 con- centration increased from ϳ350±360 ppbv to ϳ650± of biogenic CH4. The fall increase in atmospheric con- centration of biogenic CH might be associated with the 750 ppbv. Hence, it could imply that ϳ0.8 Gt C-CH4 4 moved into the atmosphere during the glacial±intergla- increase in the CH4 emission due to the lake's overturn and a decrease in the meridional air transport of air cial transition. During glacial periods and ``short time'' masses enriched by nonbacterial methane from the mid- cold events such as the Younger Dryas, the interhemi- latitudes. spheric gradient between poles was practically absent; 13 that is, the hypothetical northern source was blocked. Recent ␦ C values of CH4 collected from two thaw lakes in the Kolyma lowland and measured at U.S. fa- When the climate was warmed the interhemispheric gra- cilities were Ϫ70 to Ϫ72 in summer, and Ϫ65 to Ϫ80 dient increased to ϳ10% in the optimums of interglacial in winter (Zimov et al. 1997b). This value is less than periods and warm stages. Therefore, we can suggest that produced in summer by the Alaskan tundra (Martens et in warm epochs only limnic sediments and taliks un- al. 1992) and less than detected over Barrow (Quay et derneath the northern lakes are able to maintain rela- tively high CH concentration in the north. We also can al. 1991). It implies either that the Siberian CH4 was 4 not as oxidized as in these other environments or that state that during winter, limnic sediment and talik are there was a difference in substrate or a different pathway sole sources of CH4 into the atmosphere, because in of methanogenesis. It was shown also that the hydrogen winter the soil source of CH4 is dormant. isotopic composition of the CH4 was variable, but most To estimate how much organic carbon in CH4 form samples from the northeastern Siberian were low (␦D might be available for anaerobic destruction by lake

ϭϪ370), indicative of a biotic source for CH4, low evolution we use here a simple calculation (Semiletov oxidation rates in the water column, and CH4 production et al. 1996a). Because thaw lakes with ages of about a by fermentation (Zimov et al. 1997b). few thousands years might be underlain by a layer of Our initial results show that total wintertime (Octo- thawed permafrost with a depth ϳ100±200 m or more ber±May) emission of CH4 from lakes into the atmo- (Chekhovskiy and Shamanova 1976; Tomirdiaro 1980), Ϫ2 sphere might be more then 16 g CH4 m (about 8 g the vast organic reservoirs immobilized in permafrost Ϫ2 Ϫ2 m due to autumn overturn and 8.4 g m due to CH4 became available for anaerobic destruction by way of ebullition), ®ve times more than needed to form the CH4 lake evolution. Concentrations of organics in the per- atmospheric maxima over the Arctic, if the Arctic is mafrost of north Siberia vary, usually in range of 0.5%± treated as an isolated box and with negligible oxidation 5% of C by weight [Zvyagintsev (1992)], which is sim- of atmospheric CH4 in winter. ilar to our measurement in the study area. Assuming Thus, our evaluation shows that the northern lakes that the permafrost density is about 2 ϫ 103 kg mϪ3 and plays a signi®cant role in the seasonal dynamics of at- the content of organic C ϳ0.5% by weight, the upper 3 mospheric CH4 that shows a wintertime absolute max- 100-m column of permafrost contains about 10 kg of imum over the Arctic (Steele et al. 1987; Fung et al. organic carbon per square meter. Therefore, the upper 1991; Reeburgh and Crill 1996). layer of tundra and northern taiga (total area ϳ9.4 ϫ

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106 km2) contains no less than 9400 Gt of organic car- limited dataset for the meridional distribution of at- bon, which might became available for biotic activity mospheric CO2 (Bolin and Keelling 1963). This ap- via thaw lake evolution. Due to the existence of ancient proach has been incorporated into the block-diffusion thaw lakes, with ages of 3000±5000 years or more, car- and open-diffusion models of the ventilation of deep bon stock in the clathrate stability zone might be in- waters in the high latitudes (Siegenthaler 1983). Such volved in the recent methane cycle, because such lakes models assume that North Atlantic Deep Water might be underlain by talik extended through permafrost (NADW) rise in the southern high latitudes to the sur- in the CH4 clathrate stability zone. Thaw lakes migrated face, absorb excess CO2, undergo rapid mixing, and across the north Siberian plains during the Holocene descend as a result of a nonlinear density increase during (Tomirdiaro 1980), releasing to the atmosphere a huge mixing. However, this hypothesis has not invariably amount of CH4; half of this CH4 was derived from been con®rmed by sea measurements. For instance, the Pleistocene carbon: CH4 collected from thaw lakes in results of the DOWNWING and MONSOON expedi- winter had an average 14C age of 27 Kyr (Zimov et al. tions in the Southern Ocean are contradictory (Skirrow

1997b). This age indicates that Pleistocene sediments 1975). Other measurements of pCO2 in the atmosphere deposited 20±40 Kyr ago contributed 68%±100% of the and surface layer of the Southern Ocean indicate that

CH4 ¯ux from these lakes. In contrast, CH4 emitted in this region is not a permanent sink of excess atmospheric 14 the summer had an average C age of 9.2 Kyr, indicating CO2, at least during austral summer (Inoue and Sugi- that Pleistocene C fueled 23%±46% of summer me- mura 1986). Analysis of pCO2 data (1970±82) based thanogenesis and thus that more CH4 was produced in on pH±TA technique and O2±pCO2 correlation tech- the younger surface sediments, which are warmer in nique by V. N. Ivanenkov, Yu. I. Lyakhin, and P. N. summer than winter. Thus, about half of current annual Makkaveev shows that net evasion of CO2 should be methanogenesis is fueled by Pleistocene carbon. The during the autumn and winter in the temperate and high current atmospheric CO2 and CH4 burdens are ϳ750 Gt latitudes in both the Northern and Southern Hemi- C-CO2 and ϳ3.6 Gt C-CH4 (Quay et al. 1991). Then spheres (Bordovsky and Ivanenkov 1985; Bordovsky small changes in the current carbon stock of permafrost and Makkaveev 1991). might affect signi®cantly the growth of the main green- The autumn (austral) distribution of ⌬pCO2 is pre- house gases in the arctic/subarctic atmosphere. The main sented in Fig. 4. We use an annual average value of end product of anaerobic destruction of organics in the pCO2 ϭ 350 ␮atm in the atmosphere over the Southern subwater environment is methane; the atmosphere con- Ocean in February±March 1989 that is based on the tent is less than 0.5% of CO2. Therefore conversion of datasets used by Tans et al. (1990) and Erickson (1989). a small part of the carbon buried in permafrost might The resulting distribution of ⌬pCO2 shows that the sur- cause a large change in the growth of atmospheric CH4 face waters of the south Atlantic Ocean west of ϳ7ЊW in comparison with CO2, though a part of CH4 might are undersaturated by CO2 relative to the atmosphere. be oxidized and converted in the form of CO2. Note It is correlated with the early experimental carbonate that we consider a potential source of atmospheric CH4 data discussed by Keeling (1968). The undersaturation as a source of CO2 also, because atmospheric CH4 is by CO2 obtained in this area might be related to either oxidized to CO2 during a few years (Cicerone and Or- physical or biological factors. The governing physical emland 1988). factor might be a cooling of the warm Brazil Current

Our study of CH4 in the surface Arctic waters shows due to movement toward the Antarctic. Based on the that the adjacent Arctic seas are not a signi®cant source recent experimental data of Millero (1995), we found of CH4, whereas the CH4 emission from the limnic en- that a recorded water temperature decrease about 1ЊC vironment play a signi®cant role in the regional CH4 is enough to explain the obtained undersaturation, be- budget. It con®rms the aircraft data of Kelley and Gos- cause a temperature coef®cient provides a decrease of ink (1988) and Harriss et al. (1992), which show a 10%± pCO2 by 4.3% per 1ЊC drop in the water temperature. 15% increase of CH4 over land. It is probable that sim- It is dif®cult to estimate the in¯uence of biological fac- ilar processes were important during previous intergla- tors, because there is a lack of data for primary pro- cial epochs, when the ``pole-to-pole'' gradient of CH4 ductivity in our cruise. We cannot use literature data, was about 10% (Rasmussen and Khalil 1982; Chap- because satellite sensing of pigments has shown that pellaz et al. 1990; Chappellaz et al. 1993), similar to signi®cant large-scale variability exists in the Southern the present gradient. Ocean (Smith and Sakshaug 1990). The difference of

pCO2 shows that the area east of 7ЊW might be con- sidered as a source of CO into the atmosphere, because b. Antarctic 2 the surface waters are supersaturated by CO2.Itisin- Due to lack of representative carbonate data, the teresting that the supersaturation was obtained near the World Ocean was considered for many years to act as ice edge, though many researchers found anomalous a giant pump, removing atmospheric CO2 from the polar high productivity near the ice edge (Bordovsky and Iva- regions and liberating it in warmer latitudes, especially nenkov 1985; Smith and Sakshaug 1990). It should be at equatorial divergence. This concept was based on a noted here that any sea expedition obtains a dataset only

Unauthenticated | Downloaded 09/25/21 10:31 AM UTC 15 JANUARY 1999 SEMILETOV 303 for the restricted area during limited time; that is, the tribution (Fig. 4). This is explained by ``compensation''; distribution of ⌬pCO2 in the study area is a ``photo'' there are higher values of the pCO2 gradient in the of the real situation. Indeed, we have found previously southern part of the study area, but higher values of that mesoscale space±time variability of the CO2 system transfer velocity (K) in the northern part. The mean might change a photo signi®cantly. For instance, our value of K is enhances from south to north due to an detailed investigation of the CO2 system in the mega- increase of the wind stress and diffusion coef®cient (D), poligon area located in the Paci®c subarctic frontal zone in¯uenced strongly by warming. For instance, the real shows that net invasion of CO2 predominates in June, meridional temperature gradient across the study area whereas in August the reverse situation is found due to might be responsible for a 1.5 times increase of K value a change in the mesoscale circulation (Semiletov and near the northern boundary in comparison with the southern boundary, if the wind stress is similar at the Pipko 1991). Our investigation of the CO2 system dur- ing 3 days of drifting near the ice edge in the Davis Sea southern and northern boundaries. shows that the surface water was in a nearly equilibrated The oceanographic CO2 data shows that the distri- condition, with reversal from slight undersaturation (Ϫ6 bution of atmospheric sinks or sources might be varied Ϭ 7 ppm), to supersaturation (ϩ30 Ϭ 36 ppm) that could signi®cantly, which should be recorded in atmospheric 13 be related to the outcropping of deeper waters enriched variations of CO2 and ␦ CinCO2. Nakazawa and Mor- by CO and nutrients. imoto (1997), based on the approach of Ciais et al. 2 (1995a), calculated oceanic CO ¯uxes for a non-ENSO To evaluate the CO ¯ux (F) across an air±water in- 2 2 year: the total CO source at latitudes south of Antarctic terface the ®lm model is used following the equation F 2 convergence (ϳ50Њ±52ЊS) was found. The average lat- ϭ K ϫ ␣⌬P, where K is a measured value determined itudinal distribution of CO ¯uxes based on a worldwide by real hydrometeorological conditions (wind stress, 2 long-term database of pH and TA also indicates that the temperature and etc.), ␣ is solubility of the gas (CO ), 2 aquatory of the Southern Ocean is a small source and ⌬P is partial pressure gradient. Here we use the throughout the year, rather than a sink of atmospheric mean value of CO2 transfer velocity, determined by Er- CO2 (Bordovsky and Makkaveev 1991; P. Makkaveev, ickson (1989) for different latitude belts. We used K ϭ 1998 personal communication). Previous observations Ϫ1 120 cm day for the aquatory south of 60ЊS, K ϭ 240 of pCO show that during winter, cooling of the surface Ϫ1 2 cm day for latitudes between 50Њ and 60ЊS, and K ϭ water reduces pCO , but upwelling and entrainment in- 300 cm dayϪ1 for latitudes between 40Њ and 50ЊS. The 2 crease pCO2 (Chen 1988). These observations suggested distribution of calculated ¯uxes of CO2 between the that the Antarctic surface waters are likely to be in equi- atmosphere and water is presented in Fig. 5, which librium with the atmosphere. shows a general source of atmospheric CO east of 7ЊW 2 An evaluation of the annual mean oceanic CO2 ¯uxes and south of 60ЊS. Because the southern boundary of shows that net transfer between 53ЊS and Antarctic the study area is located near Antarctic divergence we might be reversed from sink (in 1992) to source (in can assume that upwelling of old North-Atlantic Deep 1993), which may indicate a change in the balance be- Waters (NADW) resulted in partial enrichment by CO2 tween photosynthesis and respiration in this area (Ciais for surface Antarctic waters that is mainly a mix of shelf et al. 1995b) or a small oscillation in the present-day Antarctic waters and NADW. This assumption, based large-scale thermohaline circulation pattern of the ocean on our limited dataset, agrees with atmospheric data that (Broecker 1997). indicate a rise in the CO2 value at the edge of Antarctic Comparison of the autumn ⌬pCO2 distribution for (Tans et al. 1990; Conway et al. 1994). Indeed, the Arctic (Figs. 2a,b) and Antarctic (Fig. 4) shows that in annual average concentrations of CO2 monitored at Ant- both regions the water cooling increases invasion of arctic coastal stations Halley Bay (76ЊS, 26ЊW) and atmospheric CO2, but different factors in¯uence super- Palmer Station (65ЊS, 64ЊW) are higher than at the South saturation of the surface waters by CO2. In the Antarctic, Pole (90ЊS) or at midlatitude (30Њ±50ЊS) in the Southern outcropping of NADW enriched by CO2 probably in- Hemisphere stations (Cape Grim, Amsterdam Island), creases pCO2 at the surface; in the Arctic, riverine out- put and upwelling of bottom coastal waters enhance CO where undersaturation of CO2 in surface waters oc- 2 evasion. Note, that near the arctic ice edge a large un- curred. The presence of a CO2 source near Antarctic divergence also agrees with anomalous high concentra- dersaturation by CO2 was obtained in the different sea- sons, but near the antarctic ice edge (in the Davis Sea) tions of phosphate (P-PO4) in the zone of Antarctic divergence and south of it (Keeling 1968), which could the surface water was supersaturated or quasi-equili- brated by CO2 with air, though photosynthesis is in- indicate penetration of NADW (enriched by CO2 and nutrients) in the surface layer. creased near the ice edge in the both regions (Park et Comparison of Figs. 4 and 5 shows a good correlation al. 1974; Rusanov et al. 1979; Smith and Sakshaug 1990). between distribution of ⌬pCO2 and F, because the gra- dient of pCO2 is a driving force for the CO2 exchange between air and water. Likewise, we can see that the 4. Conclusions spatial distribution of the ¯ux, F (Fig. 5), is more ho- The fall study in the Laptev and Chukchi Seas shows mogeneous in comparison with the pCO2 gradient dis- a signi®cant northward decrease in surface pCO2 that

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⌬pCO2 shows that the aquatory of the Arctic Seas is dations and comments and editing of the manuscript. mainly a sink of atmospheric CO2, though the water Special thanks for Patricia Golden, who helped with the near the mouths of the Siberian rivers and coastal sites English version. are supersaturated signi®cantly by CO2. The literature data for pCO2 distribution in the surface waters show also that the Barents, Kara, and Greenland Seas are REFERENCES undersaturated signi®cantly (down to 40%±50%) with Aagaard, K., and E. C. Carmack, 1989: The role of sea ice and other regard to atmospheric CO2. fresh water in the Arctic circulation. J. Geophys. Res., 94 (C10), All-season data obtained in the delta system of the 14 485±14 498. Antonov, V. S., and V. Ya. Morozova, 1957: Total riverine runoff in Lena River and typical northern lakes show that the the Arctic Seas (in Russian). AARI Proc., 208, 13±52. freshwater is supersaturated by CO2 with a drastic in- Bartlett, K. B., P. M. Crill, R. L. Sass, R. C. Harriss, and N. B. Dise, crease in the CO2 value during wintertime. 1992: Methane emissions from tundra environments in the Yu- kon-Kuskokwim Delta, Alaska. J. Geophys. Res., 97, 16 645± The CH4 ef¯ux from the limnic environment in the north plays a signi®cant role in the CH regional budget, 16 660. 4 Blake, D. R., and F.S. Rowland, 1988: Continuing worldwide increase whereas the role of the arctic adjacent seas in regional in tropospheric methane, 1978±1987. Science, 239, 1129±1131. CH4 emission is small. This agrees with the aircraft data, Bolin, B., and C. D. Keeling, 1963: Large-scale atmospheric mixing which show a 10%±15% increase of CH4 over land as deduced from the seasonal and meridional variation of carbon when the airplane is ¯own from the Arctic Basin toward dioxide. J. Geophys. Res., 68, 3899±3920. Bordovsky, O. K., and V. N. Ivanenkov, Eds., 1985: Hydrochemical the south (Kelley and Gosink 1988; Harriss et al. 1992). Processes in the Ocean (in Russian). P. P. Shirshov Institute of Offshore permafrost might add CH4 into the atmo- Oceanology, 122 pp. sphere, though the preliminary data available are not , and P. N. Makkaveev, 1991: CO2 exchange and carbon budget suf®cient to estimate the source. Evolution of the north- in the Paci®c Ocean. Trans. Russ. Acad. Sci., 320, 1470±1473. ern lakes might be considered as an important com- Broecker, W. S., 1997: Thermohaline circulation, the Achilles heel of our climate system: Will manmade CO2 upset the current ponent of the climatic system. balance? Science, 278, 1582±1588. The pCO2 difference between the surface of the Chappellaz, J., J. M. Barnola, D. Raynaud, Y. S. Korotkevich, and Southern Ocean and atmosphere observed in the austral C. Lorius, 1990: Ice-core record of atmospheric methane over autumn shows that the area east of 7ЊW might be a the past 160,000 years. Nature, 335, 127±131. , T. Blunier, D. Raynaud, J. M. Barnola, J. Schwander, and B. source of CO2 into the atmosphere, whereas the area Stauffer, 1993: Synchronous changes in atmospheric CH4 and west of 7ЊW is a net sink of CO2. This is corroborated Greenland climate between 40 and 8 kyr BP. Nature, 336, 443± by literature data that indicate on overestimation of the 445. Chekhovskiy, A. L., and I. I. Shamanova, 1976: Talik formation under role of Antarctic waters as a sink for atmospheric CO2. thermocarst lakes in the West Siberia (in Russian). Proc. PNIIS, 49, 64±86. Acknowledgments. This study was supported by the Chen, C. T. A., 1985: Preliminary observations of oxygen and carbon Russian Fund for Basic Research (Grants 93-05-8258, dioxide of the wintertime Bering Sea Marginal ice zone. Contin. 96-05-66350, 96-05-79143, 97-05-79064), the Inter- Shelf Res., 4, 465±483. national Science Foundation (ISF) and the Joint Pro- , 1988: Summer-winter comparisons of oxygen, nutrients and carbonates in the polar seas. La Mer, 26, 1±11. gram of ISF and RF Government (Grants RJD000 and , 1993: Carbonate chemistry of the wintertime Bering Sea Mar- RJD300), and the Federal Program ``Integration be- ginal ice zone. Contin. Shelf Res., 13, 67±87. tween Academy of Sciences and High School'' (Grant Ciais, P., and Coauthors, 1995a: Partitioning of ocean and land uptake 13 726). Irina Pipko and Svetlana Pugach did the main of CO2 as inferred by C measurements from the NOAA/CMDL work of processing for data and technical preparation Global Air Sampling Network. J. Geophys. Res., 100, 5051± 5070. of the manuscript. We thank Alexander Gukov, Nikolay , P. P. Tans, M. Trolier, J. W. C. White, and R. J. Francey, 1995b:

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