has a substantial impact on the composition of the overlying atmosphere; as the permafrost thaws, a significant amount of old

Journal of Marine Systems 66 (2007) 204226www.elsevier.com/locate/jmarsysterrestrial carbon becomes available for biogeochemical cycling and oxidation to CO2. The Arctic Ocean's role in determiningregional CO2 balance has been ignored, because of its small size (only 4% of the world ocean area) and because its continuoussea-ice cover is considered to impede gaseous exchange with the atmosphere so efficiently that no global climate models includeCO2 exchange over sea-ice. In this paper we show that: (1) the Arctic shelf seas (the Laptev and East-Siberian seas) may becomea strong source of atmospheric CO2 because of oxidation of bio-available eroded terrestrial carbon and river transport; (2) theChukchi Sea shelf exhibits the strong uptake of atmospheric CO2; (3) the sea-ice melt ponds and open brine channels form animportant spring/summer air CO2 sink that also must be included in any Arctic regional CO2 budget. Both the direction andamount of CO2 transfer between air and sea during open water season may be different from transfer during freezing andthawing, or during winter when CO2 accumulates beneath Arctic sea-ice; (4) direct measurements beneath the sea ice gave twoinitial results. First, a drastic pCO2 decrease from 410 atm to 288 atm, which was recorded in FebruaryMarch beneath thefast ice near Barrow using the SAMI-CO2 sensor, may reflect increased photosynthetic activity beneath sea-ice just after polarsunrise. Second, new measurements made in summer 2005 beneath the sea ice in the Central Basin show relatively high valuesof pCO2 ranging between 425 atm and 475 atm, values, which are larger than the mean atmospheric value in the Arctic insummertime. The sources of those high values are supposed to be: high rates of bacterial respiration, import of the UpperCarbonate chemistry dynamics and carbon dioxide fluxes acrossthe atmosphereicewater interfaces in the Arctic Ocean:

Pacific sector of the Arctic

Igor P. Semiletov a,b,, Irina I. Pipko b, Irina Repina c, Natalia E. Shakhova a,b

a International Arctic Research Center/University of Alaska Fairbanks, Fairbanks, AK, USAb V.I.Il'ichov Pacific Oceanological Institute, Far Eastern Branch of Russian Academy of Sciences, Vladivostok, Russia

c A.M. Obukhov Institute of Atmospheric Physics, Russian Academy of Sciences, Moscow, Russia

Received 14 November 2005; accepted 27 May 2006Available online 18 October 2006

Abstract

Climatic changes in the Northern Hemisphere have led to remarkable environmental changes in the Arctic Ocean, which issurrounded by permafrost. These changes include significant shrinking of sea-ice cover in summer, increased time between sea-ice break-up and freeze-up, and Arctic surface water freshening and warming associated with melting sea-ice, thawingpermafrost, and increased runoff. These changes are commonly attributed to the greenhouse effect resulting from increasedatmospheric carbon dioxide (CO2) concentration and other non-CO2 radiatively active gases (methane, nitrous oxide). Thegreenhouse effect should be most pronounced in the Arctic where the largest air CO2 concentrations and wintersummervariations in the world for a clean background environment were detected. However, the airlandshelf interaction in the Arctic Corresponding author. International Arctic Research Center/University of Alaska Fairbanks, Fairbanks, AK, USA.E-mail address: igorsm@iarc.uaf.edu (I.P. Semiletov).

0924-7963/$ - see front matter 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.jmarsys.2006.05.012

aluerces.

on cy

Mariin extent, but it is surrounded by permafrost (Fig. 1),which is being degraded at an increasing rate underconditions of warming which are most pronounced inSiberia and Alaska (ACIA, 2004). Onshore permafrost

geological body; a significant portion of onshorepermafrost was submerged by seawater during Holo-cene transgression and became a part of the shallowSiberian shelf (Romanovskii et al., 2000). A similaratmosphere (ACIA, 2004). The Arctic Ocean is smallapparent, as a result of several major field studies, thatairlandsea interactions in the Arctic have a substan-tial impact on the composition of the overlying

progress. As a result, a huge amount of sub-sea andcoastal OOC may be involved in modern biogeochem-ical cycling in the form of CO2 and CH4. Note that theonshore and offshore Siberian permafrost is a unitaryHalocline Water (UHW) from the Chukchi Sea (CS) where vfrom the Lena river plume, or any combination of these sou 2006 Elsevier B.V. All rights reserved.

Keywords: Arctic Ocean; Atmospheresea icewater interaction; CarbFluxes; Climate change

1. Background

Completing the balance sheet for the global carbonbudget is a task at the forefront of the natural sciences.The ocean acts as a major sink for the CO2 released to theatmosphere by fossil fuel combustion, land use change,and cement production (IPCC, 2001). A recent synthesisestimates that 48% of fossil fuel CO2 now resides in theocean (Sabine et al., 2004). Our present understanding ofthe temporal and spatial distribution of the net CO2 fluxinto or out of the ocean is derived from a combination offield data, which are limited by sparse temporal andspatial coverage, and model results (Takahashi et al.,1997; Feely et al., 2001; Takahashi et al., 2002). Labo-ratories all around the world contributed to a very largeand internally consistent global CO2 data set, whichcompiled up to 940,000 measurements in the Atlantic,Pacific, Indian and Southern Oceans (Takahashi et al.,2002). Since the variation of pCO2 in the surface water ofthe World Ocean is much greater than the atmosphericpCO2 seasonal variability of about 20 atm in remoteuncontaminated marine air, the direction and magnitudeof the CO2 flux through the airsea interface are regu-lated primarily by changes in the oceanic pCO2 (Feely etal., 2001), while the drivers of airsea ice fluxes are nowunder discussion (SOLAS Newsletter, 2005) A numberof questions remain, however, about the continentalmargin carbon cycle. The dominant terms affecting shelfcarbon dynamics vary considerably by region, with areasof both large net positive and negative airsea CO2fluxes. A global picture of the coastal carbon cycle willonly emerge through a network of more intensive obser-vations taken across themost diverse set of environmentspossible.

Over the past couple of decades it has become

I.P. Semiletov et al. / Journal ofcontains a huge amount of ancient organic matter thats of pCO2 range between 400 and 600 atm, a contribution

cle; Carbonate system; Carbon dioxide; Siberian shelf; Alaskan shelf;

might be involved in current biogeochemical cyclingdue to thermokarst, coastal erosion, and an increase insummer thawing of the permafrost. The upper 100 mlayer of tundra and northern taiga contains at least9400 Gt of organic carbon that might become availablefor biotic activity via thermokarst and thaw lakedevelopment and transport by streams and rivers intothe ocean (Semiletov, 1999a). It is known that the Arcticregion contains a huge amount of organic carbon buriednot only inland but also within the Arctic Oceansedimentary basin (called Arctic carbon hyper pool,Gramberg et al., 1983). More than 90% of all organiccarbon burial occurs via sediment deposition on deltas,continental shelves, and upper continental slopes(Hedges et al. 1999), and a significant portion oforganic carbon withdrawal occurs over the Siberianshelf (Fahl and Stein 1999; Bauch et al. 2000). Areas ofArctic shelf and slope, containing a major proportion ofthe organic carbon pool, represent more than 86.5% ofthe Arctic Ocean sedimentary basin (Gramberg et al.,2000). It was considered until recently that, due toslightly negative annual temperatures within the watercolumn and the lid type coverage of shelf sediments bysub-sea permafrost, old organic carbon (OOC) buried onthe Siberian Arctic shelf is completely prevented frombecoming involved in the modern carbon cycle.Moreover, it was stated that OOC buried in permafrostis not bio-available (Romankevich and Vetrov, 2001).Nevertheless, our early study conducted in the LaptevSea (199498) showed that pCO2 anomalies in waterare related somehow to degradation of permafrost andoxidation of OOC in the coastal zone (Semiletov, 1999a,b). Using a modeling approach (Romanovsky andHubberten, 2001) it has been found that thawing ofthe sub-sea permafrost in the Laptev Sea is already in

205ne Systems 66 (2007) 204226mechanism for OOC involvement in the modern carbon

Mar206 I.P. Semiletov et al. / Journal ofcycle may be found onshore, via thaw lake developmentand coastal erosion (Semiletov et al., 1996; Zimov et al.,1997; Semiletov, 2000), and offshore, via degradation ofsub-sea permafrost (through both thermokarst formationand simple thawing) under geothermal and river heatingeffects (Romanovskii and Hubberten, 2001; Semiletovet al., 2005).

However, the Arctic Ocean's role in determiningregional CO2 balance has been ignored, because conti-nuous sea-ice cover is considered to impede gaseousexchange with the atmosphere so efficiently that noglobal climate models include CO2 exchange over sea-ice (Tison et al., 2002). Pioneering measurements byGosink and Kelley in the 19601970s (Gosink et al.,1976; Kelley and Gosink, 1979) strongly indicate thatgas migration through sea ice is an important factor inoceanatmosphere winter communication particularlywhen the surface temperature is N10 C. Limited datashow that the Arctic rivers are supersaturated by CO2 ascompared to the atmosphere because of degradation ofsoil organic material (Kelley, 1970; Makkaveev, 1994;Semiletov, 1999a,b); therefore the riversea mixingzone functions as a source of atmospheric CO2. Riverstransport large quantities of both dissolved and parti-culate matter and in this sense they may be regarded as

Fig. 1. Circum-Arctic map of terresine Systems 66 (2007) 204226carriers of a wide variety of chemical signals to the seamargin. Some studies show that coastal erosion plays amajor role in the landshelf transport of the particulateorganic matter (POM) which is bioavailable (Guo et al.,2004); in contrast, dissolved organic carbon is notbioavailable (Dittmar and Kattner, 2003). Marine nitro-gen and phosphorus cycles are linked to the carboncycle. The degradation of POM causes the formation ofanomalously high pCO2 and nutrient concentrations inthe near shore zone at highly eroded coastal ice-com-plexes enriched by old terrestrial organics (Semiletov,1999a,b; Semiletov et al., 2005). Recycling of limitingnutrients from old terrestrial organic matter could in-fluence the element stoichiometry (and new produc-tion) (Codispoti and Richards, 1968; Cauwet andSidorov, 1996; Jones et al., 1998; Kattner et al., 1999).Furthermore, it is clear that since the nature and extent ofice cover is changing in the Arctic, a sound quantitativeunderstanding of these interactions is essential fordeveloping the ability to predict the future state of theatmosphere in the Arctic, and how that state relates toglobal climate change driven by the greenhouse effect.However, our understanding of the associated CO2 ex-change processes that occur at relevant interfaces in theArctic is weak.

trial and sub sea permafrost.

MariSome papers consider the Arctic Ocean to be a sinkfor atmospheric CO2, because the cold, relatively lowsalinity surface layers have the potential to take upatmospheric CO2 (Takahashi et al., 1997). Howeverevaluations of annual CO2 uptake from the atmospherediffer by almost one order of magnitude, from 27 Tg CCO2 (Anderson et al., 1998) up to 176 Tg CCO2(Lyakhin and Rusanov, 1983), because of a lack ofrepresentative data. Until now, very limited data on thecarbon chemistry of the Chukchi, East Siberian, andLaptev seas were available. Arctic marine regions aresuggested to have a potential for CO2 uptake as a resultof both seasonally high production and high seawatersolubility of CO2 (Miller et al., 1999). The solubility ofCO2 in seawater increases with decreasing temperature,which is an important process in the oceans. So, thesimplest mechanism by which the Arctic ocean may actas a CO2 sink is via deep-water formation in the coldpolar seas: surface water sinks and thus transports CO2to greater depths. Existing studies have shown that highlatitude summer ocean surface waters display excep-tionally low CO2 concentration: the degree of that un-dersaturation varies dramatically from about 100 atmin the Greenland Sea (Miller et al., 1999) down to 150200 atm in the Chukchi, Laptev, and Kara Seas, exceptin coastal zones (Kelley, 1970; Semiletov, 1999a; Pipkoet al., 2002). These studies confirm a potentially im-portant sink for atmospheric carbon (Chen et al., 1990;Broecker and Peng, 1992). However, aircraft samplingof atmospheric CO2 shows that the Arctic Ocean itself isa significant source of CO2 in the winter and a sink inthe summer. Both high altitude flights (Bolin andKeeling, 1963) and the low altitude data (Kelley andGosink, 1979) support these conclusions. Shipboarddata also show that the low summer CO2 concentrationin the surface waters is rapidly replaced with high tovery high CO2 partial pressure in fall and winter (Kelleyet al., 1968; Pipko et al., 2002). The atmospheric CO2concentration begins to rise at Point Barrow in late Au-gust, months before it does at mid-latitudes (late October),and reaches a worldwide high (for clean backgroundconditions) in late May (Conway et al., 1994; Ciais et al.,1995), months after northern hemisphere anthropogenicspace heating has ceased, and lower latitude photosyn-thesis has recommenced. A seasonal oscillation is clearlyevident, probably produced principally by land plants andthe Arctic Ocean and sea ice. At the end of summer it maybe related to two factors: (1) enhancedCO2 emission fromthe shallow shelf because of an increase in the mixinglayer depth and consequent enrichment of the surfacelayer byCO , and (2) an imbalance between enhanced soil

I.P. Semiletov et al. / Journal of2

respiration and tundra/taiga photosynthetic uptake(Zimov et al., 1993, 1996; Semiletov, 1995). Thereforebudgeting CO2 fluxes of the Arctic Ocean require to takeinto account the yearly cycle of CO2 fluxes rather than thelimited summer data set alone. Studying carbon cycling inthe Arctic marginal seas should have a high scientificpriority, especially in the coastal zone where the redis-tribution of carbon between terrestrial and marine envi-ronments occurs and its exchange with the atmospherehas not been characterized. Productivity measurements ofphytoplankton blooms in the perennial pack ice (Pome-roy, 1997) and the dense ice algae over the Arctic Basinand continental shelves (Nansen, 1906; Horner andAlexander, 1972; Melnikov, 1989; Gosselin et al., 1997)show large variation in primary and secondary productionover an extensive area important to biological activity andcarbon deposition or export. The direction and amount ofgeneral CO2 transfer between the air and sea during theopen water season, freeze-up, and break-up can differbetween the near-shore zones and over the mid/outershelves or the Central Basin. To evaluate the ArcticOcean's effect on the regional CO2 budget, we need toinvestigate the role of the water system and sea ice incarbon pumping between the water and sea ice, andelucidate how this system influences the dynamics ofcarbonate system parameters. Because changes in theArctic might be amplified through numerous feedbacks,we thus see a need to evaluate the role of the sea icewatersystem as both a possible sink and source of atmosphericCO2 in the Arctic.

The oceansea iceatmosphere gas exchange pro-cesses are not clear. However the pioneering measure-ments of gas transfer through the sea ice executed byKelley and Gosink in the late 1970s (Gosink et al., 1976;Kelley and Gosink, 1979) showed that the intensity anddirection of such exchange could be significant in thetotal balance of CO2 above ice covered seas that are asource of CO2 in late fall/winter, and a sink in spring/summer. Recent changes in Arctic climate are leading torising air and permafrost winter temperatures, melting ofsea ice and shrinking of sea ice cover, increased intervalbetween break-up and freeze-up, increased concentra-tion and seasonal amplitudes of atmospheric CO2, andredistribution of water masses and sea ice transport; suchchanges may lead to changes in CO2 exchange across theairsea interface. Ice also strongly affects airsea ex-change of gases by providing a variable cover over theocean a process that might enhance the ability ofseasonally ice-covered seas to capture CO2 (Yager et al.,1995). Some studies over first-year sea ice have shownthat it is not a passive barrier to carbon dioxide exchangebut rather is an active participant in the carbon cycle,

207ne Systems 66 (2007) 204226providing a significant carbon sink, at least at some times

of the year. During spring and summer, melt ponds andopen brine channels in sea ice contribute important airCO2 sinks that have, hitherto, been neglected in Arcticregional CO2 budgets (Semiletov et al., 2004). Thedirection and the amount of CO2 transfer between air andsea may differ between freezing and thawing cycles and,during winter, CO2 can accumulate beneath Arctic sea-ice (Macdonald et al., submitted for publication).Therefore decreased sea-ice extent and lengthening ice-free periods that should lead to a longer period ofphotosynthetic activity, and absorption of air throughleads and melt ponds in late spring and summer could bea significant unappreciated determinant of Arctic CO2balance.

In this paper we present data describing the CO2flux above the airsea interface, the sea ice, andelucidating the dynamics of the carbonate system in theSiberian Arctic seas and Arctic basin. These data wereobtained on expeditions organized by the Far-EasternBranch of Russian Academy of Sciences (FEBRAS)and the International Arctic Research Center (IARC)/University of Alaska Fairbanks in the summers of19962005, and winter of 2002 (Figs. 2, 3). Table 1

2. Description of the study area

The Siberian shelf, with a typical depth of 3070 m,is a controlling area for the flux of freshwater, nutrients,and carbon into the Arctic from the large Siberian rivers.The continental shelf of the East Siberian Sea (ESS) isthe widest and shallowest in the World Ocean, yet it isstill poorly explored. The wide shelf is an importantregion for production and processing of organic matterbefore the material is transported into the Arctic Ocean.This shelf is heavily influenced by the great extent of icecoverage in the ESS; thus, it has a high potential forbeing impacted by global climate change and warmingin the Arctic. The Arctic Ocean receives about 10% ofthe global river discharge and 25 Tg of terrigenousdissolved organic carbon (DOC) each year (Carmack,2000; Stein and Macdonald, 2003). The ESS isinfluenced by water exchange from the eastern LaptevSea (where local shelf waters are diluted mostly by LenaRiver discharge) and by inflow of Pacific waters fromthe Chukchi Sea (CS). Pacific water inflow occursthrough the Bering Strait, crossing the Chukchi shelfand entering the Arctic Basin through Barrow and

208 I.P. Semiletov et al. / Journal of Marine Systems 66 (2007) 204226shows on which expedition/cruises the data used in thispaper were collected. Some pCO2 results obtained beneaththe sea ice near the North Pole and Cape Barrow are alsodiscussed.Fig. 2. The study area: sumHerald Canyons (Weingartner et al., 1999).The shallow ESS shelf exhibits the largest gradients

in all oceanographic parameters observed for the entireArctic Ocean (Semiletov et al., 2005). The highest ratesmer of 19942004.

005;

Mariof coastal erosion and consequent offshore flux oferoded material were also found there (Dudarev andSemiletov, 2001; Stein and Macdonald, 2004). There-fore research in this region will contribute to the largerinternational effort to improve understanding of Arcticecosystem and biogeochemical function. We focus ouropen-sea study in two areas of the Arctic seas: (1) theLaptevwest East-Siberian system, which is stronglyinfluenced by the Lena river inflow, and (2) the eastEast-SiberianChuckchi land-shelf system, which isinfluenced strongly by Pacific water inflow. Some re-sults from measurements of turbulent CO2 fluxes ob-tained along the Russian Arctic shelf slope (September,2005) and near Cape Barrow (June 2002), and sub-icepCO2 data obtained in the vicinity of the North Pole(MayAugust, 2005) and near Cape Barrow (FebruaryApril, 2002) illustrate CO system dynamics in the ice-

Fig. 3. The study area: summer of 2

I.P. Semiletov et al. / Journal of2

covered Arctic Ocean.The most detailed research was done in summer in the

southeastern part of the Laptev Sea and the southwesternpart of the ESS (Figs. 2, 3). The ESS is a vast, shallowshelf that receives most of its particulate supply fromcoastal erosion (Stein and Macdonald, 2004; Semiletovet al., 2005). It was shown that all of the Russian shelvesdepend predominantly on coastal erosion for the supplyof inorganic solids, which then provide themeans to buryorganic carbon (Rachold, et al., 2000; Semiletov, 2001;Rachold, et al., 2004). Based on chemical and hydro-logical data, this shelf may be divided into two domains;the eastern area is strongly influenced by Pacific inflow(Semiletov et al., 2005), whereas the western area isinfluenced strongly by fresh water flux and particulatematerial from coastal erosion (the Lena solids dischargesignal is negligible). We consider the ESS to be a boun-dary area where Pacific and Atlantic waters interact.The ESS is largely unexplored because of its remotelocation and harsh environment. We will focus on des-cribing the variability of the CO2 system in the East-Siberian, Laptev, and Chukchi shallow shelf waters,which is determined by atmospheric forcing, runoff, andinteraction between Pacific and Atlantic water masses.Note that there is no direct significant influence of thenutrient-poor Atlantic Intermediate Water (AIW) onformation of local shelf waters and their thermal regime,because AIW lies outside of the shelf area. Historicaldata show that the AIW can penetrate occasionally ontothe East-Siberian and Chukchi shelves only at horizonsbelow 100 m (Nikiforov and Shpaikher, 1980). Thevariability of surface conditions in the Laptev and East-Siberian seas is determined mainly by atmospheric for-cing and Lena runoff (Semiletov et al., 2005), while theshallow Chukchi Sea (CS) is under the influence of

ocean depth in meters on scale bar.

209ne Systems 66 (2007) 204226Pacific water inflow (Walsh et al., 1989). Among theSiberian rivers the Lena River is a major source of totalsuspended matter and total organic carbon (Cauwet andSidorov, 1996; Gordeev et al., 1996), though the majorportion of the suspended material settles in the deltaregion (Lisitzin, 1994; Rachold et al., 1996). The amountof DOC transported by the Lena River into the LaptevSea is similar to the amount of particulate organic carbontransported via coastal erosion from the Taymir Penin-sula to the CS (Semiletov, 1999b). The coastal zone inthis area plays a significant role in the freshwater budget;processes involved include carbon transport, accumula-tion, and transformation, and seaward export of parti-culate and dissolved materials to offshore shelf/sloperegions. Warming causes thawing of the permafrost,which underlies a substantial fraction of the Arctic; thisprocess could accelerate river discharge and carbonlosses from soils (Savelieva et al., 2000; Freeman et al.,

amete

perio

mber

mber

Septe

stSe

2002

ary

mber

MarTable 1Cruises/expeditions, location, time period data, methods/measured par

Cruise/expedition Location Time

RV Alpha Helix Eastern part of theChukchi Sea,western part of theBeafort Sea

Septe

RV Dunay Southeastern partof the Laptev Sea

Septe

HV Nikolay Kolomeitsev Coastal zone of theBarents, Kara,Laptev, East-Siberianand Chukchi seas

July

RV Professor Khromov Western part of theChukchi Sea

Augu

CO2 flux measurement abovethe fast sea ice

Coastal zone of theChukchi Sea(near Cape Barrow)

June

Ice-tethered measurementfrom fast ice

Coastal zone of theChukchi Sea(near Cape Barrow)

Febru

HV Ivan Kireev Coastal zone of the Septe

210 I.P. Semiletov et al. / Journal of2001). Siberian freshwater discharge to the Arctic Oceanis expected to increase with increasing temperatures(Semiletov et al., 2000; Peterson et al., 2002), potentiallyresulting in greater riverine export of old terrigenousorganic carbon to the ocean. The role of the ESS coastalzone in the transport and fate of freshwater and terrestrialorganic carbon has not been discussed sufficiently, be-cause reliable oceanographic data are lacking; onlyNansen bottles have been used to collect data in the ESS.Changes in river runoff and the fresh water budget areclosely related to the dynamics of the carbonate system(nutrients and dissolved oxygen) (Semiletov et al., 2000).

The CS is a controlling region for the low-salinityPacific water that enters the Arctic Ocean through theBeringStrait (Rudels et al., 1996).Net northward transportthrough the Bering Strait averages 0.8 Sv (Roach et al.,1995). The CS is a broad, shallow shelf that receivesrelatively small amounts of runoff and sediments fromrivers. The freshwater inflow through the Bering Strait,when normalized against a salinity of 34.8, is estimatedto be1670 km3 yr1, which is a greater volume than the

East-Siberian SeaHV Ivan Kireev Southeastern part

of the Laptev Sea,coastal zone of theEast-Siberian Sea

September

Ice-breaker Kapitan Dranytsin;ice-station

Shelf and shelf slopeof the Laptev seas

September

Ice-tethered measurement frommulti-year ice: Russian NorthPole-33 drifting station

Central Arctic Basin MayAugurs and resulting data

d Method/measurement Resultingdata

1996 pHTAT CalculatedpCO2, FCO2

1999 pHNBSAT CalculatedpCO2

mber 2000 pHNBSCT CalculatedpCO2, FCO2

ptember 2002 Air CO2 concentrationpHTAT

CalculatedpCO2, FCO2

Turbulent fluxes of CO2chamber fluxes of CO2head-space technique

MeasuredFCO2, pCO2

March 2003 Direct pCO2 measurementsusing SAMI-CO2 sensor

Measuredsub-ice pCO2

2003 Air CO2 concentration pHTAT Calculated

ine Systems 66 (2007) 204226input of any single river discharging into the Arctic, andhalf of the total 3300 km3 yr1 river runoff for the entireArctic basin (Aagaard and Carmack, 1989). Low-salinity,nutrient-rich Pacific water flows north through the BeringStrait, passes through the CS and enters the deep ArcticOcean at the shelf break (Jones et al., 1998). The PacificOcean-derived waters of the upper halocline enhance thestrong, permanent stratification between a relatively freshsurface layer (0100 m) and underlying saline water ofAtlantic origin in the Arctic Ocean's Canada Basin(Nikiforov and Shpaikher, 1980). The CS shelf sustains itshigh marine productivity partly from nutrient-rich inflowfrom the Pacific Ocean, particularly from the AnadyrCurrent, which feeds the western side of Bering Strait, andpartly from shelf edge exchange and upwelling (Walsh etal., 1989). The primary production is large in this sea(Macdonald et al., submitted for publication), with newproduction estimated at 2600109 mol yr1. AlthoughPacific water is significantly modified in the Bering andChukchi seas (via production and decay of organic matter,mixing with runoff etc.), waters entering the halocline

pCO2, FCO22004 Air CO2 concentration pHTAT Calculated

pCO2, FCO2

2005 Turbulent fluxes of CO2 MeasuredFCO2

st, 2005 Direct pCO2 measurementsusing SAMI-CO2 sensor

Measuredsub-ice pCO2PAR

Marifrom the CS shelf can be traced by their chemicalproperties that reflect their Pacific source (Jones andAnderson, 1986; Salmon andMcRoy, 1994; Cooper et al.,1997; Jones et al., 1998). Our previouswork (Semiletov etal., 1999; Pipko et al., 2002) demonstrates that duringSeptember alone about 2 Tg of CCO2 may be absorbedby the CS due to fall cooling and biological production.Obviously in the spring and summertimewhen the highestprimary production is observed (Walsh et al., 1989) theuptake of atmospheric CO2may bemuch higher. Based onthe difference between the change caused by biologicalactivity and the observed change in total dissolvedinorganic carbon, Kaltin and Anderson (2005) estimatedthe oceanic CO2 uptake by the water passing over theBeringChukchi shelves during one year as 22 Tg of CCO2 which is a value similar to the annual total riverinetransport of dissolved organic carbon into the ArcticOcean (Gordeev, 2000).

3. Methods and data

All cruises and sea ice-based expeditions are listed inTable 1. Values of pH on the NBS (National Bureau ofStandards) scale (pHNBS) were measured during the 1999and 2000 cruises. Precision was about 0.006 pH unit. Theapparent constants for the dissociation of carbonic acid inseawater from Mehrbach et al. (1973) refitted by Millero(1979) were used for calculating carbonate parameters onthe NBS scale for the 19992000 dataset.

On the cruises since 2002 and on the 1996 cruise inthe CS we measured pH at 250.1 C with an ORION8103 Ross electrode on the total hydrogen ion con-centration scale (pHT), using Trisbuffer preparedaccording to Goyet and Dickson (DOE, 1994). Theprecision was 0.002 pH unit. Values of pCO2 forcruises in 1996 and 20022004 were calculated with theCO2 program developed by Lewis and Wallace (1998)as were all other computations for the carbonate systemfor this dataset. The dissociation constants of Mehrbachet al. (1973) as parameterized by Dickson and Millero(1987) and advocated by Lee et al. (2000), Wanninkhofet al. (1999) and McElligott et al. (1998) were used for1996 and 20022004 data.

Total alkalinity (AT) data were obtained by directindicator titration in an open cell using a 665-Dosimatsystem with a precision of 0.1%. Total inorganic carbonconcentrations (CT) were measured by gas chromatogra-phy with a precision of 1% (Weiss, 1981; Semiletov,1992).

Using the ship's Seabird conductivity/temperature/depth (CTD) meter, continuous profiles of conductivity,

I.P. Semiletov et al. / Journal oftemperature, pressure, light transmission, in situ fluo-rescence, and oxygen were made on the downcast withdata averaged over 1dbar intervals. Water samples weretaken using Niskin bottles. An Autosal salinometerreferenced against IAPSO standard sea water also wasused to test the CTD salinity data.

Atmospheric pCO2 concentration was measured usinga Li-Cor-820 non-dispersive infrared analyzer with pre-cision to within 3% with a 15 cm optical bench.

In order to analyze the carbon exchange in the atmo-spheresea system we used two techniques:

1) Calculated CO2 fluxes. The CO2 flux betweenatmosphere and ocean is determined by the difference ofCO2 concentration between the sea and atmosphere (thethermodynamic driving force) and by the rate of exchangeor the transfer velocity (the kinetic parameter) (Matthews,1999). The thermodynamic factor (?pCO2) is determinedmainly by the sea surface temperature, salinity, biologicalproduction, water upwelling and advection. Transfervelocity is a function primarily of wind speed and tempe-rature, but many other processes also influence it,including processes that produce surface organic filmsand chemical enhancement, and some other processes aswell (Wanninkhof, 1992; Matthews, 1999). At present,estimates of gas transfer over the Arctic are based onparameterizations developed and tested in temperate ortropical seas.

Two types of flux parameterization were applied tothe data obtained in September 1996 (Pipko et al.,2002), one using the quadratic relation to wind speed(Wanninkhof, 1992) and the other using the cubicrelation (Wanninkhof and McGillis, 1999):

k 0:31 u2660=Scx0:5

k 0:0283 u3660=Scx0:5

FCO2 K0TkTDpCO2;

where k is gas transfer velocity (cm h 1), u is the windspeed (m s 1), Scx is the Schmidt number for CO2(Wanninkhof, 1992), K0 is the CO2 solubility at the insitu temperature (mol m 3atm 1) and FCO2 is flux ofCO2 through the sea/air interface (mmol m

2 d 1).At present, due to a lack of direct CO2 flux measure-ments in the Arctic Ocean, it is not possible to decidewhich relationship best describes CO2 fluxes.

2) Turbulent CO2 fluxes. CO2 flux can be measuredusing either micrometeorological or enclosure methods.The main advantage of micrometeorological methodsover the alternative enclosure methods is their ability tocontinuously measure the surface exchange of matter andenergy. Thismakes it possible to study both the short-term

211ne Systems 66 (2007) 204226variations (e.g., diurnal cycle) and the long-term balances.

MarThe micrometeorological measurements do not disturbthe surface under investigation and provide fluxes on anecosystem scale, thus partly avoiding the difficult up-scaling problems. The markedly smaller target area ofchamber measurements, however, enables a spatiallydetailed study on different components of the ecosystem,which could complement the micrometeorological mea-surements. The most direct micrometeorological methodis the eddy covariance (EC) technique (Fairal et al., 1997;Edson et al., 1998; Fairal et al., 2000; Baldocchi, 2003). Inthis technique the vertical flux of a scalar constituent isobtained as

F wVcV;

where w is the vertical wind speed and c is the quantity ofinterest (e.g., temperature, humidity or gas concentration).The over bar denotes the time average, and a primedenotes the fluctuation of an instantaneous value from thisaverage, e.g.,

wV ww

Fluxes of CO2 (FCO2), water vapor (LE), and heat (H)were calculated using EC technique equations describedelsewhere (Baldocchi, 2003; Andreas et al., 2005).

Initial measurements of the airsea (airsea and airsea ice) turbulent fluxes of CO2 over the Arctic shelfshelf slope were made onboard Kapitan Dranytsin inSeptember 2005 (Fig. 3). With the EC technique themeasurements are carried out using fast response instru-ments sampling typically at 1020 Hz in order to cover alarge proportion of the frequency range of turbulentvariations. For CO2 flux such measurements are feasibleusing a sonic anemometer and a fast infrared gas analyzer.A LI-7500 sensor coupledwith sonic thermo-anemometer(USA-1, METEC) was used for those measurements. TheEC technique was applied as follows. Vertical wind speedand temperature fluctuations were measured at 10 Hzusing a three-dimensional sonic anemometerthermom-eter (USA-1, METEK, GmbH) aligned into the meanwind. Carbon dioxide and water vapor fluctuations weremeasured at 10 Hzwith a fast response open-path infraredLi-Cor 7500 gas analyzer. Tominimize flow disturbances,the LI-7500 sensor head has a smooth, aerodynamicprofile. In addition, the open pathmeasurement eliminatesthe need for a pump, greatly reducing the overall powerrequirement of the system. The open path analyzer alsoeliminates time delays, pressure drops, and sorption/desorption of water vapor on the tubing employed withclosed path analyzers. In our CO exchange study setup,

212 I.P. Semiletov et al. / Journal of2

momentum and fluxes of sensible and latent heat weremeasured using the EC technique. In September of 2005we measured the turbulent CO2 fluxes above the openwater and ice at an ice station located near the edge of themulti-year sea ice. Earlier wemeasured the CO2 exchangeabove the fast ice in late spring/early summer using bothEC and chamber techniques as described in Semiletov etal. (2004). During spring and summer, melt ponds andopen brine channels in sea ice account for significant airCO2 sinks that have, hitherto, been neglected in Arcticregional CO2 budgets.

New information on pCO2 dynamics beneath sea icewas obtained in summer, MayAugust, 2005, in thecentral Arctic Basin using the Russian North Pole-33drifting station as a platform, where ice-tethered pCO2observations were executed with the autonomous SAMI-CO2 device described by De Grandpre et al. (1999) andequipped with a LI-193 Spherical Underwater QuantumSensor to measure photosynthetically active radiation(PAR). The SAMI-CO2was installed 1.5m below the sea-ice bottom, and measured pCO2 at hourly intervals inquasi-stable water temperature which ranged between1.7 C and 1.4 C. Early, the same instrument wasdeployed beneath the sea ice near Cape Barrow inFebruary 17March 23, 2003 (Semiletov et al., 2004).

4. Results and discussion

4.1. Initial results of recent direct CO2 flux measure-ments above the airwater interface

Measurements of CO2 turbulent flux taken recentlyabove open water over the Laptev Sea shelf slope rangedbetween the negative (invasion) and positive (evasion)values of +1.7 mmol m2 d 1 and 1.2 mmol m2 d1

(Fig. 4A,C,D). Daily mean air temperature was near0 C, while wind speed ranged from 5 to 10 m s1

Comparing distribution of CO2 fluxes with surfacetemperature and salinity (Fig. 5A,B) shows that warmerand fresher water which is probably a riverine plumeacts as a source of CO2, while relatively colder andsaltier water near the ice edge is a sink. In this case wecannot distinguish the thermal effect from the biologicaleffect as it has been done previously (Pipko et al., 2002),because no additional data are available.

These data demonstrate high inhomogeneity invalues of turbulent CO2 fluxes and even in their di-rections. To build up a more general picture of CO2sinks and sources and CO2 exchange over the SiberianArctic shelf we base further discussion on the multi-yearstudy of pCO2 dynamics calculated using the traditionalpH-TALK technique (DOE, 1994), and on calculated

ine Systems 66 (2007) 204226CO2 fluxes.

MariI.P. Semiletov et al. / Journal of4.2. The Arctic Ocean as a sink for atmospheric CO2

Results of our 2002 study (late AugustearlySeptember, RV Professor Khromov) of the carbonatesystem in the CS show that the study area acted as a sinkfor atmospheric CO2; a negative gradient between themeasured air CO2 concentration and calculated value ofpCO2 in the surface water indicates that the sea absorbsCO2 from the air (Fig. 6A). In general, the measureddistribution of the CO2 gradient between air and seaagrees well with our earlier results obtained in the samearea in September 2000 (HV Nikolay Kolomeytsev,Fig. 6B); those data show that the study area acted asa sink for atmospheric CO2. Both figures show aspot of CO2 consumption located between 68N and70N latitudes, which is an area of the highest primaryproduction within the Bering/Chukchi seas (Springer andMcRoy, 1993;Walsh et al., 1989); this finding agreeswellwith the data obtained during the September 1996 cruise(RV Alpha Helix), Fig. 6C. All studies show that the CS

Fig. 4. Position of ice station (marked by star), ship tracks, and vertical CO2 foceanographic stations and transects (1 and 2) across the Laptev Sea shelf slalong the transect 1 (C); the same as for (C) but for transect 2 (D).213ne Systems 66 (2007) 204226acts as a sink for atmospheric CO2 in the summer/fallseason, though different factors can determine the surfacepCO2 distribution (Pipko et al., 2005a). Preliminaryanalysis indicates that in the western part of the CS bio-logical processes dominate, whereas a thermal factor (fallcooling) may be responsible for the general trend in sur-face pCO2 distribution of a decrease toward the ice edge,which has been detected in the eastern portion of the CS,Fig. 8C (Semiletov et al., 1999; Pipko et al., 2002). ThepCO2 data obtained below the mixing layer show thatafter the fall/winter convection the direction of the CO2exchange may be reversed. In late September, the con-centration of chlorophyll in the surface layer was aboutthree-fold lower than in early September, whereas at thesame time pCO2 values in the bottom layer increasedtwofold in this area, indicating intensive decay of organicmatter (Pipko et al., 2002). In other words, in just a fewweeks the biogeochemical situation changed significant-ly. High pCO2 values were found in the north of theshallow study area. Here, bottom water is associated with

lux, mmol m 2 d 1, above the sea surface in September of 2005 (A);ope (B); and vertical CO2 flux, mmol m

2 d 1, above the sea surface

Mar214 I.P. Semiletov et al. / Journal ofUHW. Surprisingly low values of pCO2 were found at thebottom ofBarrowCanyon (Pipko et al., 2002), in the AIW(temperatures between 0.28 and 0.50 C, salinities 34.7034.84). The value of pCO2 was below the equilibrationstate with respect to air, near 300 atm; this was deter-mined by cooling AIW that had been nearly equilibratedwith air during the movement across the Arctic Basin. Avertical profile of T, S, and pCO2 along Barrow Canyon(Fig. 7) illustrates the existence of low pCO2 values inAIW, while relatively high values of pCO2 are found inUHW, ranging between 400 atm and 600 atm.

Our data show high temporalspatial variability ofpCO2 as well as other parameters (T, S, nutrients, oxy-gen, chlorophyll) in Arctic surface waters that changethe direction and value of CO2 exchange across the airsea interface. Meso-scale variability of hydrological andhydrochemical parameters obtained between early andlate September, 1996 (Table 2) is similar to interannualvariability between the same parameters obtained in1996 and 2002. During a period of only three weeks, thedirection of CO2 flux may be changed from uptake ofatmospheric CO2 on September 12 (pCO2=339 atm)to evasion of CO2 on September 27 (pCO2=391 atm).

Fig. 5. Distribution of surface temperature (T), C (A), salinity (S), psu (B), sLaptev Sea: September of 2005.ine Systems 66 (2007) 204226Using the CO2 data obtained in the CS in September of1996 and the quadratic equation, the mean CO2 invasionwas equal to 14.613.1 mmol m2 d1; the magnitudeand direction of gas exchange varied from 43.27 mmolm2 d1 (invasion) to +29.34 mmol m2 d1 (evasion).For the cubic equation the mean value was 10.210.1 mmol m2 d1, with a range from 35.63 mmolm2 d1 to +20.14 mmol m2 d1. In the both cases weused the daily average wind speed. For comparison, usingthe in situ and hourly average wind speed (measured at10 m height) the mean magnitude of fluxes calculatedusing the quadratic equation is 12.2 and 15.5 mmolm2 d1, and using the cubic equation, 8.4 and11.8 mmol m2 d1. This large discrepancy betweencurrent bulk gas transfer relations is due to the lack ofdirect CO2 flux measurements in the Arctic Ocean; as aresult, it is not possible to decide which relationship bestdescribes CO2 fluxes. In thisworkwemade estimations ofairsea transfer based on the difference of CO2 con-centration between the atmosphere and sea, and on therate of exchange or the transfer velocity based on the dailymean values of wind speed when using the cubic equa-tion, which is based onmore data, and for which a weaker

ensible heat flux, W m 1 (C), and latent heat flux, W m 1(D), in the

MariI.P. Semiletov et al. / Journal ofdependence of gas transfer at low wind speed and astronger dependence at high wind speed have been de-monstrated (Wanninkhof and McGillis, 1999).

Interannual variability of the CO2 uptake from atmo-sphere ranges from 17.0 mmol m2 d1 in the rela-tively warm years (2002) to 11.0 mmol m2 d1 and7.7 mmol m2 d1 in the relatively cold years of 1996

Fig. 6. Distribution of pCO2 gradients, atm, in surface waters of the ChukchiAlpha Helix (C).215ne Systems 66 (2007) 204226and 2000 (Table 3). The highest value of pCO2 gradientwas obtained in 2000, but the value of FCO2 wasminimal because of low wind and low water tempera-ture, which affect the Schmidt number for CO2 (Wan-ninkhof, 1992).

Using lower evaluation based on the chamber fluxmeasurement, we find that uptake of CO2 from

Sea: RV Professor Khromov (A), RV Nikolay Kolomeytsev(B), RV

m, (C

216 I.P. Semiletov et al. / Journal of Marine Systems 66 (2007) 204226atmosphere by the fast sea ice surface in the CS in June(Semiletov et al., 2004), when algae blooms appear,ranged roughly between 4 and 6 mmol m2 d1 atsites with water depth=02 cm, and between 6 and51 mmol m2 d1 at sites with water depth=25 cm:these results confirm the turbulent flux measurementsranged roughly between 20 and 40 mmol m2 d1.

Fig. 7. Distribution of T, C (A), S, psu, (B), and pCO2, atThis is significantly higher than the FCO2 values pre-sented in Table 3. We believe that our data, mostlyobtained at the end of the hydrological summer, reflectslow rates of CO2 ocean uptake. Using the data obtainedin 1996 we evaluated the September mmol m2 d1

uptake as 2.41012 g CCO2; this value would be about4 times higher if we used the mean FCO2 value obtainedin 2002 (Table 3). Using the minimal and maximal

Table 2Mean values of hydrological and hydrochemical parameters characterizingAugustSeptember of 1996 and 2002

Year,parameter

T,C

S,

O2,%

AT,mmolkg1

PO43,

mol l

2002 .Aug. 26(n=43)

4.071.81 32.300.52 102.217.7 2.3190.024 1.010

1996 .Sep. 12(n=45)

5.051.25 31.980.49 93.55.7 2.2340.016 1.550

1996 .Sept. 27(n=42)

4.770.70 31.530.53 87.613.6 2.2350.020 2.010values of Table 3, we have estimated that during Au-gustSeptember alone the CS absorbs about 10201012 g C of CO2. That evaluation assumed theuptake in August to be the same as the uptake inSeptember; thus we can consider this crude evaluationas the low evaluation. The upper limit of this evaluationyields a value similar to the estimate made by Kaltin and

) along the Barrow Canyon transect (D) (September 1996).Anderson (2005) for annual uptake of atmospheric CO2in the upper 100 m of water transported over theBeringChukchi shelves. Comparing the FCO2 obtainedfor the CS vs. the mean value of CO2 uptake obtained inthe World Ocean, which ranges between 0.3 and.0.8 mmol m2 d1 (Takahashi et al., 1997; Feely etal., 2001) with more recent mean estimate of 0.6 mmolm2 d1 (Takahashi et al., 2004), we find that the

environmental conditions in the south part of the Chukchi Sea in late

1SiO3,mol l1

NO3,

mol l1pHTin situ

CT,mmol kg1

pCO2,atm

.70 11.8012.67 5.066.45 8.1650.181 2.1260.085 318161

.51 23.009.26 4.786.41 8.0990.089 2.0770.054 33986

.60 28.158.90 6.165.33 8.0490.100 2.1020.050 391127

value

U

757

Mariintensity of atmospheric CO2 uptake is one to two ordersof magnitude higher in the CS.

Comparing the values of CO2 fluxes measured usingthe direct measurements method over the shelf slope ofthe Laptev Sea (see Section 4.1), which ranged from0.4 mmol m2 d1 to 1.2 mmol m2 d1, with theCO2 fluxes obtained in the CS (Table 3), we can see thatthe latter values are one order of magnitude higher. Thedata show that concentration of eroded particulatematerial (PM) in the low-productive (Sorokin andSorokin, 1996) local shelf water of both the East-Siberian and Laptev seas is one order of magnitudehigher than the concentration found in the Pacific-originwater from the CS (Semiletov et al., 2005), whiletransparency increases by one order of magnitude aswater travels from the low-productive waters adjacent tothe Lena river delta (Sorokin and Sorokin, 1996) to thehigh-productive Long Strait/CS (Walsh et al., 1989).Note, that highest macrobenthos biomass was alsoobserved in the Long Strait/CS and adjacent part of theEast-Siberian Sea (Gukov et al., 2005), which is understrong influence of the Pacific derived waters. The mostsignificant increase in transparency and vice versadecrease in the PM value was observed in the hydro-logical frontal zone traveling from the west to the east.Thus we can argue that the change in transparencycaused by the westward increase in PM burden is onereason for low productivity in the Laptev Sea (Sorokinand Sorokin, 1996), even when nutrients are notlimiting. Another reason for low transparency may behigh values of colored dissolved organicmatter (CDOM)which are one order higher in the LS and western part ofESS which is strongly influenced by riverine runoffenriched in humic substances, while the western part ofESS has relatively low CDOM content (Semiletov et al.,

Table 3Mean measured (T, S in the surface layer, wind speed) and calculatedChukchi Sea in late AugustSeptember of 1996, 2000, and 2002

Year, parameter T, C S,

1996 . (n=110) 2.763.11 30.601.212000 . (n=20) 2.361.98 30.391.752002 . (n=32) 6.261.24 30.772.05

I.P. Semiletov et al. / Journal of2006). That may partially explain why the Siberiancoastal zone acts as a source of atmospheric CO2 (seeSection 4.3). In general, our results are agreed withresults described in (Duarte and Augusti, 1999), whichshow that the biota of unproductive ecosystems tends tobe net CO2 sources, whereas the biota of highlyproductive ecosystems acts as a CO2 sink, becausecommunity respiration rates tend to exceed grossprimary production in unproductive aquatic ecosystems,whereas highly productive ecosystems tend to beautotrophic.

To learn more about wintertime dynamics beneaththe sea ice over the shallow CS shelf (near Cape Barrow)in winter we deployed the SAMI-CO2 device fromFebruary 17 to March 23, 2003. The SAMI-CO2 sensorwas installed 2.5 m below the sea-ice bottom, andmeasured pCO2 at half-hourly intervals in quasi-stablewater temperature (Semiletov et al., 2004). In lateFebruary, pCO2 values increased to 400410 atm,while in March a drastic pCO2 decrease from 410 atmto 288 atm was recorded (Fig. 8). This change mayreflect increased photosynthetic activity beneath sea-ice,but additional studies are required to verify thathypothesis. The effects of summer temperature andsolar radiation increase on the sea-ice/sub-ice waterecosystem is expected to enhance Arctic Ocean-scalesinks (Semiletov et al., 2004).

4.3. The Arctic Ocean as a source for atmospheric CO2

Carbon chemistry differs between the CS (whereorganic carbon of marine origin dominates) and theArctic Siberian seas (where eroded terrestrial organiccarbon determines biogeochemical cycling). This isdemonstrated by the spatial distribution of stable isotopemarkers (Naidu et al., 2000; Semiletov et al., 2005). Thehighest contribution of terrestrial organic matter(CTOM) to the shelf sediment (up to 90100%) wasfound in the ESS east from the geochemical frontalzone, which reflects multi-centennial westward pene-tration of the Pacific origin waters (Semiletov et al.,2005). High values of pCO2 throughout the water,especially at the benthic layer (Figs. 9, 10), were foundover the areas affected strongly by coastal erosion

s of pCO2 and CO2 fluxes between atmosphere and surface of the

, m s1 pCO2, atm FCO2, mmol m2 d1

.421.66 11878 11.010.5

.132.42 16130 7.78.3

.612.00 13744 17.012.7

217ne Systems 66 (2007) 204226transport of organic carbon. We suggest that decompo-sition of old terrestrial organic eroded material deter-mines this pCO2 distribution (Semiletov, 1999a,b,2001). Note that data from pyrolysis-GC/MS of thesedimentary organic material (OM) indicated an in-crease in the freshness of the OM from west to eastalong the Siberian Arctic coast (Guo et al., 2004);increasing relative abundance of furfurals (polysacchar-ides) with respect to nitriles was found, with the highest

abundance of OM in the oldest ESS sediments. Avail-able data show that in general the Arctic EasternSiberian coastal zone, influenced by coastal erosion and

riverine run-off, is a source of atmospheric CO2 duringsummer and winter, whereas the CS and the remote(from the coastline) areas of the Laptev Sea tend to be a

Fig. 8. Variability of pCO2 beneath the fast ice near Cape Barrow (the Chukchi Sea): FebruaryMarch of 2003.

218 I.P. Semiletov et al. / Journal of Marine Systems 66 (2007) 204226Fig. 9. Distribution of pCO2, dissolved oxygen, and sum nitrites plus nitratebottom (B) layers.s in the nearshore zone along the Northern Sea Route: surface (A) and

A); an

219I.P. Semiletov et al. / Journal of Marine Systems 66 (2007) 204226sink (Semiletov, 1999a,b; Pipko et al., 2002, 2005a,b).Therefore, the distribution of CO2 exchange between airand sea exhibits a spatialtemporal mosaic pattern ac-ross the different Arctic seas. The transect along theNorthern Sea Route made in the September of 2000(Semiletov, 2001) illustrates the high variability thatexists in Eurasian coastal zone carbonate chemistry(Fig. 9). The greatest Siberian rivers are a source ofCO2-enriched water with pCO2 up to 1000 atm, butthe highest values of pCO2 (N1000 atm) are associatedwith destruction of the eroded OM (Semiletov, 1999a,b;Pipko et al., 2005a,b). The heating effect of the Siberianriver water may cause an increase in rates of coastal(Semiletov et al., 2005) and bottom erosion (Gavrilovet al., 2001), which increases the amount of eroded OM

Fig. 10. The study area of the Laptev Sea expedition-1999 (available for decay. Thus we suggest that pCO2 ano-malies seen in the Laptev Sea (Fig. 9) may be causedby superimposition of two processes: the degradation

Fig. 11. Distribution of CO2 airsea flux (FCO2, mmol m 2 d 1) in the East-S

values are noted up/right.of eroded OM, and the transport of river water enrichedin CO2.

Based on the distribution of the hydrological andhydrochemical data, two areas were identified in theshallow ESS: a Western area that is influenced stronglyby the fresh water flux and PM transport of the coastaleroded material (the Lena solids discharge signal isnegligible), and an Eastern area that is under theinfluence of Pacific derived waters (Semiletov et al.,2005). The CO2 system data obtained in September of2003 and 2004 (Fig. 11, Table 1) reveal the Western areato be a source of atmospheric CO2, while the Easternarea is a sink as was mentioned in (Semiletov, 2001;Pipko et al., 2005a,b). Change in the CO2 flux directionis marked by a solid line, which is significantly shifted

d pCO2 values, atm, in the surface and bottom waters (B).eastward in 2004 from the position measured in 2003.This line agreed with the position of the hydrologicalfrontal zone (FZ), which varied from year to year. The

iberian in September of 2003 (A) and September 2004 (B). Mean FCO2

longitudinal shift of the FZ between Western andEastern areas may reach 10 and more (Semiletov etal., 2005).

Mean values of pCO2, and CO2 fluxes betweenatmosphere and surface water show that in September

sensible heat flux (Fig. 12b) and CO2 flux has beenfound (Fig. 12b). Basic information on the site includesthe air and ice surface temperatures, and total solarradiation obtained during the observations at the icestation (Fig. 13a,b). These measurements demonstrate a

s of

U

4

220 I.P. Semiletov et al. / Journal of Marine Systems 66 (2007) 2042262003 and 2004 the ESS was a source of CO2 into theatmosphere (Table 4). The efflux of CO2 in relativelywarm and windy 2004 was one order of magnitudelarger than in 2003. The fourfold increase in pCO2value in 2004 may be associated with an increased Lenaand Kolyma river discharge volume of nearly one-thirdabove normal (and 30% higher than in 2003), whichcorrelates with significant warming of the whole watercolumn (from 1.35 C up to 2.10 C) in 2004 comparedwith 2003; such an increase in flow increases transportand degradation of the eroded carbon. Using theapproach described by Shakhova et al. (2005) wecalculated the total amount of PM in areas of the ESSthat were used for comparison. This calculation demon-strates 20% increase in PM burden in 2004 comparedto 2003 (Semiletov et al., 2006). Thus, we can assumethat environmental changes caused by warming maycause intensification of the coastal erosion, which joint-ly with increased runoff of river water enriched in CO2may consequently increase the atmospheric CO2 emis-sion from coastal zones.

Additional studies are required to evaluate the inter-annual carbon variability in the highly dynamic coastalenvironment.

4.4. The specific role of the sea ice in CO2 balance

Recent measurements (Table 1, September of 2005)demonstrate atmospheric CO2 uptake ranged betweenFCO2=0.5mmol10

5m2 s1 and3.7mmol105m2

s1 above the annual sea ice over the Laptev Sea outershelf (Fig. 13a). To compare the CO2 fluxes above the seaice with the fluxes above the openwater (Fig. 5A,C,D)werecalculated the CO2 flux above ice in mmol m

2 d1.Rates of CO2 uptake by the sea ice surface were higher(FCO2 ranged between 0.4 mmol m

2 d1 and3.5 mmol m2 d1) than by the water surface near theice-edge along the transect between stations 19 and 36(Fig. 4D). Positive weak air stratification has been ob-served during observations. Similar direction in the

Table 4Measured (T, S in the surface layer, wind speed) and calculated valueSiberian Sea in September of 2003 and 2004

Year, parameter T, C S,

2003 . (n=41) 2.310.96 18.953.88

2004 . (n=54) 2.791.46 16.894.29 5quasi-stable near-zero air temperature, but an ice surfacetemperature that changed synchronously with the incom-ing solar radiation.

Our early measurements of pCO2 in sea-ice brinesand in under-ice water made in June of 2002 on the fastice near Cape Barrow (Semiletov et al., 2004) demons-trated significant CO2 undersaturation of 220280 atmand 130150 atm, respectively, compared to air con-centrations of 365375 atm. Big stationary chamberswith volume 344 l were monitored by taking CO2measurements each day of observation. That chamberdata show a drastic decrease of equilibrium CO2 con-centration (down to 300 atm) in the headspace abovegrowing melt ponds, especially during the last fourmeasurement days, when daily mean temperature roseabove 0 C and melt pond depth increased dramaticallyfrom a few cm to 20 cm. Almost all chamber CO2 fluxmeasurements show atmospheric CO2 uptake by thesea-ice surface (Semiletov et al., 2004). These resultsagree well with the increase of incoming solar radiation,which is absorbed in the melt ponds and beneath the seaice (Makshtas et al., 2005). We suggest that increasedsolar radiation caused enhanced photosynthesis in themelt ponds, ice brines, and sub-ice water, and conse-quently a pCO2 decrease in and beneath the sea ice.

Ice-tethered observations made in the Central Arcticbasin using the Russian North Pole-33 drifting stationshow that values of pCO2 are ranged usually between425 atm and 475 atm with a drop to 375 atm in mid-August; these values are larger than the mean summer-time atmospheric value in the Arctic (340345 atm).The PAR record demonstrated near zero values untilmid-June when the snow and sea ice began to melt andreached its maximum value in July, but no correlationbetween PAR and values of pCO2 has been found. Thesource of those high values may be a combination ofhigh rates of bacterial respiration (Rich et al., 1997) andimport of the UHW from the CS, where values of pCO2range as high as 400600 atm (Pipko et al., 2002). Ourinitial results and those of Gosselin et al. (1997) on

pCO2, and CO2 fluxes between atmosphere and surface of the East-

, m s1 pCO2, atm FCO2, mmol m 2 d1

.010.79 4065 1.01.6.631.86 188130 10.912.6

ptev

221I.P. Semiletov et al. / Journal of Marine Systems 66 (2007) 204226phytoplankton and ice algae production and Rich et al.

Fig. 12. Flux of CO2 (a), and sensible heat (b) at the ice station in the Laflux from the air to the ice surface.(1997) on heterotrophic bacteria indicate that the CentralBasin is not a biological desert, but rather supports an

Fig. 13. Records of the air temperature (solid line) and ice surface temperatuobservation (hours, GMT).active biological community that contributes to the

Sea (September 2005). Negative sign of both fluxes means direction ofcycling of OM. Another explanation for such a pheno-menon involves the Lena river plume. Much of the Lena

re (a); total solar radiation (dashed line) (b) versus the time of

MarRiver water is involved in the Transpolar Current inliquid or frozen phase (Zakharov, 1995; Osterhus andVinje, 1998). High correlation (N0.7) between cumula-tive records of Lena River discharge anomalies, iceconditions in the Siberian seas, and ice export throughFram Strait was found with a time lag of 46 years,representing the time needed for riverine water exten-sion onto the shelf (12 years) and ice transport to FramStrait (23 years) (Semiletov et al., 2000). Surface waterfrom the Laptev Sea shelf, rich in runoff and dissolvedorganic matter (DOM), follows the Gakkel Ridge to-wards Fram Strait (Anderson et al., 1994, 1998). Thiscould explain the high concentration of DOM measuredover the Amerasian shelf and Arctic Basin (Wheeleret al., 1996; Opsahl et al., 1999). Anomalously highvalues of pCO2 detected beneath the sea ice duringthe summertime drifting of the North Pole-33 stationmay indicate a rapid turnover of sub-ice DOM whichis closely connected with high sea-ice production (e.g.Melnikov, 1989; Gosselin et al., 1997).

Airice CO2 fluxes and related dynamics studies inAntarctic pack ice show that the highest oversaturation ofCO2 (pCO2 as high as 915 ppm) appeared in the coldestice in winter (Delille et al., 2005). As the temperaturecrosses the threshold of about 5 C, Antarctic sea icebecomes permeable to gas, and sea ice begins to releaseCO2 to the atmosphere (up to 1.9 mmol m

2 d1). On thewhole, spring and summer Antarctic pack ice appears toact as a CO2 sink with CO2 fluxes ranging from 0 to6 mmol m2 d1. Our early results (Semiletov, 1999a)show that in the fall (March), the open surface water nearand south of the Antarctic divergence is a source of CO2into the atmosphere, which may be related to theupwelling of the North Atlantic Deep Water enriched inCO2 and phosphates (Keeling, 1968; Semiletov, 1995).Release of CO2 from the open water and from sea ice mayexplain the air mean annual CO2 increase over theAntarctic nearshore zone, which was detected at thePalmer and Holly Bay Antarctic stations (Tans et al.,1990). In general, seasonal oscillations in airice ex-change are similar between the Arctic and Antarctic.

There are still more questions than answers about seaice carbon biogeochemistry and its role in atmosphericCO2 fluxes. One of the unresolved questions is:What is amechanism for the prevailing downward CO2 fluxesobserved during the winter in the southern Beaufort Sea(Miller, 2005), while pCO2 values measured within theice were much higher than in the atmosphere, implying aCO2 gradient contrary to the turbulent downward fluxes?A similar question arose from results obtained byZemmelink et al. (2005) in the Australian summer

222 I.P. Semiletov et al. / Journal of(NovemberJanuary) over the Antarctic pack ice: seawater was supersaturated with CO2 resulting in an effluxtowards the atmosphere, while air CO2 in the snow thatcovered the pack ice was undersaturated with respect toatmosphere, leading to a net deposition of CO2. Addi-tional studies are required to understand the CO2 ex-change rates across differently aged and structured sea ice.

5. Summary

1. Ship observations have revealed differences in inten-sity and direction of gas exchange, which depend onthe characteristics of the underlying water masses.Coastal areas, strongly influenced by coastal erosionand the river input of terrestrial carbon (suspendedand dissolved), are the sources of CO2 into theatmosphere. Emission of CO2 from the Arctic coastalzone is influenced by coastal erosion and river runoffwater, which is generally low in transparency andproductivity; erosion and runoff may increase asglobal warming continues. In contrast, surface sea-water in the ice-free highly productive CS shelfduring early spring, summer and early autumn ismostly undersaturated in CO2 and therefore serves asa significant regional sink for atmospheric CO2. Amosaic distribution of CO2 sink and source areas mayform due to outcropping of the UHW, a potential CO2source. The same mosaic distribution of CO2 sinksand sources may result from mixing with AIW, apotential CO2 sink.

2. Measurements of the CO2 system in September of2003 and 2004 suggest that the partitioning of watermasses in the East Siberian Sea is also reflected in thenet direction of CO2 flux, with the western areareleasing CO2 to the atmosphere while the Eastern,Pacific-dominated area is a sink. It was found that thedirection of CO2 flux changes near the frontal zonebetween freshened/source and Pacific/sinkwaters.Position of the frontal zone varies significantly fromyear to year. Mean CO2 fluxes from atmosphere tosurface water were estimated at 11.6 mmol m2 d1

(2003) and 10.912.6 mmol m2 d1 (2004). Thelarger efflux observed in 2004 corresponded withrelatively warm and windy conditions, possibly asso-ciated with increased coastal erosion (20% increase inparticulate material burden) and a 30% increase inLena and Kolyma river discharge for that year.

3. Eddy correlation measurements made above the openwater surface of the Laptev Sea ranged between thenegative (invasion) and positive (evasion) values of+1.7 mmol m2 d1 and 1.2 mmol m2 d1.Comparing the distribution of CO fluxes with surface

ine Systems 66 (2007) 2042262

temperature and salinity shows that warmer and

Marifresher water, which is probably a riverine plume, actsas a source of CO2, while relatively colder and saltierwater near the ice edge is a sink.

4. Flux measurements made on one-year ice in theLaptev Sea and fast ice near Barrow provide someinsights into the influence of sea ice on CO2 exchangebetween atmosphere and sea ice surface.We infer thatin early summer absorption of atmospheric CO2 byice-covered ocean dominates; this agrees with datacollected using aircraft. Our measurements alsosuggest the important role of melt ponds and brinechannels in gas exchange. The sea-ice melt ponds andopen brine channels form an important spring/summer air CO2 sink that also must be included inany Arctic regional CO2 budget; both the directionand amount of CO2 transfer between air and seaduring open water season may be differentfrom transfer during freezing and thawing, or duringwinter when CO2 accumulates beneath Arctic sea-ice.

5. Direct measurements beneath the sea ice gave twoinitial results. First, a drastic pCO2 decrease from410 atm to 288 atm, which was recorded inFebruaryMarch beneath the fast ice near Barrowusing the SAMI-CO2 sensor, may reflect increasedphotosynthetic activity beneath sea-ice just afterpolar sunrise. Second, new measurements made insummer 2005 beneath the sea ice in the CentralBasin show relatively high values of pCO2 rangingbetween 425 atm and 475 atm, values which arelarger than the mean atmospheric value in the Arcticin summertime. The sources of those high values aresupposed to be: high rates of bacterial respiration,import of UHW from the CS where values of pCO2range between 400 and 600 atm, a contributionfrom the Lena River plume, or any combination ofthese sources.

Acknowledgements

We thank Valentine Sergienko, Syun Akasofu,Georgui Golitsyn, and Victor Akulichev for their supportof our work in the Siberian Arctic. This work wassupported by the Far-Eastern Branch of the RussianAcademy of Sciences (RAS), the International ArcticResearch Center (IARC) of the University of AlaskaFairbanks, and by the Cooperative Institute for ArcticResearch through NOAA Cooperative AgreementNA17RJ1224 and the National Science FoundationAgreement No. OPP-0327664, both with the Universityof Alaska. This research was funded in part by theRussian Foundation for Basic Research (No. 04-05-

I.P. Semiletov et al. / Journal of64819 (IPS), No. 05-05-64213 (IIP)), the US NationalScience Foundation (grant OPP-0230455, IPS), and A.M.Obukhov Institute of Atmospheric Physics RAS. Partialsupport also came from the Global Change Program RASHeadquarters No. 13 (N.P. Laverov's program). Contri-bution from Candace O'Connor is highly acknowledged.We acknowledge Alexander Makshtas for his support indeployment of the pCO2 sensor at the Russian North PoleStation-33. We thank both anonymous reviewers for theirdetail and valuable comments.

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