222Rn and 226Ra: indicators of sea-ice effects on air-sea gas exchange KENT A . FANNING and LINDA M. TORRES Fanning. K. A . & Torres. L. M. 1991: I2'Rn and ""Ra: indicators of sea-ice effects on air-sea gas exchange. Pp. 51-58 in Sakshaug. E . , Hopkins. C. C. E . & Oritsland. N . A . (eds.): Proceedings of the Pro Mare Symposium on Polar Marine Ecology, Trondhcim. 12-16 May 1990. Polar Research 10(1). I2'Rn and 226Ra distributions beneath the sea ice of the Barents Sea revealed that ice cover has varied effects on air-sea gas exchange. Twice, once in late summer and once in late winter, seawater samples from the top meter below drill holes had zzrRn activities that were not lower than their '"Ra activities. indicating the existence of secular equilibrium and a negligible net exchange of lr2Rn and other gases with the atmosphere. However. seawater in the upper 20-85 m usually exhibited at least somc zzrRn depletion; 22'Rn-to-2'bRa activity ratios tended to have 'ice-free' values (0.3-0.9) in the summer and values between 0.9 and 1.0 in the winter. Integrated '*'Rn depletions and piston velocities in both seasons typically fell in the lower 25% of the ranges for ice-free seawater. suggesting that a moderate but far from total reduction in gas exchange is normally caused by ice cover and/or meltwater. The results demonstrate that sea-ice interference with the oceanic uptake of atmospheric gases such as COz is not well understood and needs further investigation. Kent A . Fanning and Linda M. Torres, Department of Marine Science, Uniuersit.v of South Florida, Sf. Petersburg, Florida 33701, USA Introduction The global significance of the effects of sea-ice cover on air-gas exchange is unresolved at present. A complete absence of atmospheric-C02 penetration through sea ice of the Weddell Sea was invoked to help explain why sinking Antarctic Bottom Water appears t o carry away only 15% of the anthropogenic excess C 0 2 that enters the surface ocean (Poisson & Chen 1987). A partial rather than a complete reduction of gas exchange across sea ice was implied by water-mass studies that found freons F-11 and F-12 to be 20-37% undersaturated in the surface Arctic Ocean (Kry- sell & Wallace 1988; Wallace et al. 1987). These studies emphasized the consequences of high- latitude gas exchange on oceanic solutes but they did not determine gas-exchange parameters for the air-seawater (or air-ice-seawater) interface. 222Rn is a quantitative tracer of air-sea gas exchange on time scales of a few days (Broecker & Peng 1971, 1974, 1982; Peng et al. 1979; Sme- thie et al. 1985; Glover & Reeburgh 1987). In a parcel of seawater isolated from exchange with the atmosphere or underlying sediments, activi- ties of 222Rn (A222) and its parent 226Ra (A?2h) will become equal and attain a condition called secular equilibrium. Near the sea surface in the ocean's mixed layer, secular equilibrium is dis- rupted by the preferential escape of radon to the atmosphere, and the ratio of A222 to is thus <1.0. A222:A226 ratios in ice-free oceanic mixed layers are typically 0.5-0.9, averaging 0.76 (50.09) in GEOSECS studies (Peng et al. 1979) and 0.75 (50.07) in Transient Tracers' studies (Smethie et al. 1985). The lowest observed ratios are -0.3 in the Bering Sea (Glover & Reeburgh 1987). Piston uelocify, which is defined as the thickness of a sea water column exchanging gas with the atmosphere per unit time (Broecker & Peng 1982), ranges over nearly two orders of magnitude in ice-free seawater (Table 1). The work reported here was a reconnaisance of 222Rn and 226Ra distributions near sea ice. Our objectives were to obtain new insights into the mechanisms of air-sea gas exchange in ice-bearing waters and to estimate the 222Rn depletions and piston velocities in the presence of sea ice for comparison to values for the same exchange par- ameters in the absence of sea ice. Materials and methods The work was done in April 1986 (late winter), and September 1988 (late summer), aboard the Norwegian Coast Guard icebreaker K/V ANDENES during the Norwegian Pro Mare program on the 5 2 K . A . Fanning & L . ,M. Torres Table 1 Comparison ot w i t h 2z2Rn gas exchange parameters elsewhere in the ocean :Rn gas exchange parameters in the Barents S C J i n late winter (April) and late summer (September) Barents Sea Elsewhere Late winter Late summer Whole Tropical Bcring hydrostaiions ice station5 ocean Atlantic Sea (Ref 1 ) (Ref 2) (Ref. 3 ) 43 52 7 3 Depth of !'?Rn depletion ( m ) Total ?'!Rn depletion (dpm m ') Value( s ) Average Piston velocity (m d I ) Value( s) Average Piston velocity (20°C) Value(s) Average 110 63 10-20 15-20 48 ( 2 2 6 ) - - 280 610 3Jo-680 3jo-450 190-3610 330-2190 - - - - - - 960 1239 0.6 1 . 5 1.3-2.6 0 9-1.2 0.5-12.7 1.4-6.9 0.2-4.9 - - - - 2 8 3.6 2.2 1 . 2 2.8 2.5-5.1 1 9 - 2 . 4 0.6-18.6 1.1-6.0 - - - - - 3 3 3 . 1 - References: ( I ) Peng et al (1979) (2) Smcthic et al (1985). ( 3 ) Glover & Reeburgh (1987) Barents Sea. This scheduling permitted a range of sea-ice effects to be examined because of the large seasonal variations in Barents Sea ice cover. The winter cruise (Fig. 1 ) investigated the entire water column beneath or adjacent to close- packed winter ice (>90% surface coverage). Sea- 10" 20" 30" 40' 50"E 1 " " " " ' " " " ' " ' 1 8 0 " N c X ice Slation a2 * ?V 'Ice Sialion a 3 X ice Stailon L5 m t 1 @ = Winter Slalions (19061 I F I R I . Map of the Barents Sea showing the station locations during the late -,inter cruise of April 1986. and the late summcr cruise of September 1988. Approximate ice limits are also s h o s n . water i n natural or ship-created open areas was sampled with 30-liter Niskin bottles to within 10 m of the bottom. Hydrographic parameters were determined with a CTD or discrete sampling. At an ice station ( H l ) , a helicopter served as transport for collecting seawater below unbroken ice more than 50 miles from the vessel. A small ( - 10 cm) hole was drilled into the ice, and vacua in gas-tight, 20-liter PVC sampling bottles were used to suck seawater from beneath the ice through weighted tubing (I.D.: 6-12 mm) (Fig. The late summer cruise (Fig. 1) consisted entirely of ice stations at which the 1986 ice- station protocol was followed (Fig. 2). Surface ice coverage within the pack was ~ 7 0 % . 2zzRn activities were determined aboard the vessel after using the 20-liter PVC bottles as z22Rn-extraction chambers (Mathieu et al. 1988) (Fig. 2 ) . Overall 222Rn extraction and counting efficiency was 85%. After the extractions, the seawater samples were slowly passed through col- umns of Mn-oxide-coated acrylic fiber to extract 22hRa (Reid et al. 1979). Later. the columns were sealed in gas-washing bottles so that the 226Ra activities of the original samples could be deter- mined from the z22Rn ingrown after >lmonth of storage i n accordance with the tests and rec- ommendations of Moore (1981). Relative stand- ard deviations of replicate 226Ra analyses averaged 6%. 2 ) . Indicators of sea-ice effects 53 SIGHT TUBf 1 a e v A +=? HOLE DRILLED WITH AUGER -1, TOP OF ICE WEIGHTED SAMPLE TUBE dI1 INLET VALVE GAS DISPERSION FRlT TO OUTLET VALVE i 'BOTTOM OF ICE INLET VALVE ARROWS SHOW PATH OF HELIUM D C Fig. 2. Diagram of the deployment and use of 20-liter PVC radon sampling and extraction bottles showing: A ) an overall view. B) a cutaway view. C) a bottle in sampling position for sucking seawater through weighted tubing lowered into drill holes at ice stations. and D) a radon extraction. Vacua in the bottles were also used to suck seawater from 30-liter Niskin bottles at Pro Marc hydrostations. face water. Out of sixteen samplings of the upper 85 m, ten had A222:A226 values higher than the normal range (0.5-0.9), and six had ratios within it. Five of those (two each at hydrostations 43 and 52 and one at ice station H1) exceeded 0.8. These results suggest that the extensive ice cover retarded the degree of degassing. One sample in Results The late winter cruise of 1986 Upper portions of the winter z2zRn and 226Ra profiles (Fig. 3) approached secular equilibrium much more closely than reported for ice-free sur- V I L 3 ? 2. i? A C T IV IT Y ( g ) 3 0 1 0 1 0 0 - - 2 0 0 - - 30 0 - - 4 0 0 E n n v I I- W a > 1 ' t R a l * 5 I ? < I : v ) I 1 I l l 1 - A C T IV IT Y ( g ) 0 1 0 2 0 3 0 2 0 0 30 0 R a I I 0 4 0 0 0 1 0 2 0 3 0 4 0 S A L IN IT Y ( % o ) S TA TI O N 4 3 S H IP -M A D E O P E N IN G IN T H IN I C E 1 0 2 0 3 0 4 0 S A L IN IT Y ( % o ) S TA TI O N 5 2 S TA TI O N 3 7 O P E N W A TE R N E A R I C E E D G E O P E N W A TE R W IT H IN I C E , C E N T R A L B A N K 1 0 2 0 3 0 4 0 S A L IN IT Y ( '3 0 0 ) IC E S TA . H 1 F IR . 3 Pr of ile s of ' "R n ac tiv ity a nd " 'R a ac tiv ity a nd s al in it y fo r th c B ar cn ts S ea c ru is c in l ar e w in te r. A pr il 1 98 6 (h yd ro st at io ns 4 3. 5 2 an d 37 a nd i cc s ta ti on H I) . D cp th s w er e m ea su rc d w ith r cf er cn cc t o th e se a su rf ac c at h yd ro st at io ns o r to t he i ce s ur fa ce a t ic e rt at io ns . S ca w at cr r os e to w ith in 5 c m o f th c ic e su rf ac c in a ll d ri ll ho le s. C T D s al in ity p ro fi le s ar e sh ow n as s ol id l in cs w it ho ut d at a po in ts : di sc re te s al in it y m ca w re m en ts a re I nd ic at ed b y so lid c ir cl es ( 0 ) . Indicators of sea-ice effects 5 5 0 - - - 10- - the upper 20 meters (the 1-m sample at ice station H1) showed clear evidence for secular equilib- rium, i.e. no reduction in below Another 1-meter sample (at hydrostation 43) had an A z 2 ? value 4% below its A,,, value. This sample was close t o secular equilibrium, but could also have experienced a small amount of recent exchange. To o u r knowledge, the water column at hydrostation 43 comes the closest to exhibiting no **'Rn loss to t h e atmosphere of all oceanic water columns studied to date. Its lowest A 2 2 2 : A 2 2 6 value was 0.93 at 1 0 m ; all others in the upper 100 m fell between that value and 1.0. The strongest winter 222Rn depletions were a t hydrostation 37 which had also apparently under- gone a recent water-column turnover. A layer in the upper 20 m with a 40-70% 222Rn surplus was both overlain and underlain by seawater with -20% z22Rn depletion. Large near-bottom '"Rn enrichments at hydrostations 43 and 52 and the small near-bottom enrichment at hydrostation 37 indicated that Barents Sea sediments, as Bering Sea sediments (Glover & Reeburgh 1987) and other shelf sediments (Fanning et al. 1982), emit 222Rn and label near-bottom water with an A222 value greater than its A226 value. Thus the 222Rn surplus in the seawater at 7-15111 denotes its recent near-bottom origin, and the 222Rn deple- tion between 40 and 7 8 m denotes the recent arrival of air-exposed surface water. The water column had little density structure to provide stability, being isohaline (34.5%0 salinity) and almost isothermal (-1.7 t o -13°C). In addition the water was much shallower than at the other hydrostations (135 m vs. 300-400 m), and the sea surface was ice-free. These three conditions prob- ably led to t h e overturn and the increased out- gassing relative t o t h e other hydrostations. Depths of the 222Rn-deficient zones beneath the late-winter ice were taken as the depths at which secular equilibrium appeared or were esti- mated by linear interpolation between the great- est depth showing a 222Rn depletion and the next *. % : - i d i \ + /!la-226 4 ) : I . I ! I i l SALINITY : I - & 0 Fig. 4. Profiles of 222Rn activity and **"Ra activity and salinity for the Barents Sea cruise in late summer. September 1988 (ice stations 2 and 3). Depths were measured with reference to the sea surface at hydrostations or to the ice surface at ice stations. Seawater rosc to within 5 em of the ice surface in all drill holes. Discretc salinity measurements are indicated hy solid circles (0). ICE STA. 2 10 20 30 0 10 20 30 SALINITY (%o) ICE STA. 3 10 20 30 1 Rn-222 i 2o - 0 10 20 30 SALINITY (%o) 56 K . A. Funrirng c(: L . M . Torres depth sampled (Table 1 ) . The zones extended deeper (60-110 m ) than usually observed (Peng et al. 1979) and reached the depletion depths found after storms (Fanning et al. 1982). Salinities at hydrostations 43 and 52 had slight increases from -34.YTr above 5 0 m to -35.1% below 100 m . However the density increases that might have resulted were partially offset by temperature increases from - 1 to - 2°C above 50 m to - 1 .o"C below 100rn. resulting in a weak water-column stability that allowed deeper penetration of gas- exchange processes. T h e largest pycnocline that was found occurred between 100 and 120m at hydrostation 43 where u, increased by only 0.12. By contrast, ut changes over 20-meter-thick pycnoclines in the Bering Sea were -8 times larger (fig. 3d in Glover & Reeburgh 1987). beneath ice station 3 (Fig. 4). showed no reduction of A,?? below A?,,. Since the water was obviously meltwater (salinity = 6.1%), a reasonable explanation is that i t was confined to the upper portion of an ice-walled cavity. Other- wise. the mixing processes that were producing and distributing strongly '??Rn-deficient waters throughout the region should have destroyed the strong halocline between 1 and 2 m along with the associated secular equilibrium. Apparently. the combination of solid ice walls and meltwater was capable of restricting '"Rn loss enough to maintain secular equilibrium despite the sub- stantial air-sea "'Rn exchange indicated by the normal-to-extensive '??Rn depletions in the top meter beneath ice stations 2 and 5. The late sunirner cricise of I988 Discussion Profiles for this cruise indicated much greater individual '"Rn depletions (Fig. 4). Most of the ---Rn-'2hRa data pairs from ice stations 2 and 3 had activity ratios i n the normal range: 0.5-0.9. Two had very low ratios of 0.30 and 0.37 ( t h e 2-m and 3-m values, respectively. from ice station 2) Not shown is a 1-meter sample taken at ice station 5 (see Fig. 1) with = 2 . 8 d p m (lOOOL)-'. = 0.38. These low values confirm previous Bering Sea findings that water parcels at high-latitude can undergo considerable ?:?Rn loss during the sum- mer (Glover & Reeburgh 1987). The summertime "'Rn losses occurred in ice- melt-stabilized seawater. North of 79" N in the region containing the three ice stations (Fig. 1 ) . Pro Mare C T D casts detected a surface layer in which temperatures were usually less than -1.4"C. and salinities ranged from 3 3 . 0 % ~ to less than 1 w C r (Fig. 4 ) . T h e layer was 2 0 m thick at most locations. Clearly identifiable thermoclines and haloclines and strong pycnoclines lay between the layer and deeper waters having temperatures between - 1 and 0°C and salinities >34Cr. T h e greater openness of the summer ice pack (s70Qr coverage) and steady winds from the northwest quadrant at 3-10 m s - ' apparently enhanced the degassing in the open areas and the downward and horizontal mixing of the ??'Rn-deficient waters in the meltwater layer One example of under-ice secular equilibrium was found during the summer cruise. As at the wintertime ice station H 1 in Fig. 3. 1-meter water 77- = 7.4 dpm (100 L ) - ' . and A ? ? ? : ,,, ---Rn depletions and piston velocities for both summer and winter were calculated by integrating depth profiles (Smethie e t al. 1985) for all stations in Figs. 3 and 4 except hydrostation 37 which, due to turnover. was obviously not at steady state. Comparisons were then made to parameters either published in or calculated from previous studies (Table 1). For hydrostations 43 and 52, integrations were made between the surface and the estimated depths of depletion (see above). Time and equipment constraints prevented a more detailed sampling of the water column, resulting in fairly large uncertainties i n the measured thicknesses of the z'rRn depletion zones at hydrostations 43 and 52. T h e consequent errors in the depletions and piston velocities for those stations in Table 1 are estimated t o be up to 18% for hydrostations 43 and 30% for hydrostation 52. These values fall in the ranges reported by Smethie e t al. (1985). Because the difficulty of sampling through drill holes precluded the precise determination of depletion depths during the summer cruise, the maximum summertime depletion depth was assumed to be the maximum thickness of the meltwater layer (20 m ) . This assumption was employed because pycnoclines at the base of the meltwater were 20-40 times the largest 1986 win- ter pycnocline and 2-5 times the Bering Sea pycnoclines found by Glover & Reeburgh (1987). Thus. parameters for ice stations 2 and 3 in Table 1 are shown with ranges. Low values are based on the average depletions down t o the greatest Indicators of sea-ice effects 57 regions o n the underside of the ice. T h e ice struc- tures that produce the isolation are unknown. Normally, even in winter, breaks or other weak- nesses in the ice cover seem to be present in sufficient abundance that the integrated gas exchange over the surface water column is slightly less than observed in ice-free seawater. The impli- cation for the possible role of seawater as a sink for anthropogenic COz is that sea ice is a 'porous' barrier t o the uptake of C 0 2 by high-latitude surface waters having a p C 0 2 below the atmospheric value. Poisson & Chen's (1987) assumption of n o gas exchange across Weddell Sea ice during the formation of Antarctic Bottom Water is thus open to question, although the lack of an appreciable p C 0 2 gradient across Weddell Sea ice may mean that an impermeable ice barrier is not required t o explain the low amounts of anthropogenic COz in Antarctic Bottom Water. Until direct studies of '"Rn beneath Weddell Sea ice are performed in winter, the uncertainty regarding the influence of Antarctic sea ice on the fate of atmospheric COz will remain. Ackriowledgemenrs. - Pro Mare provided logistical support. Financial support was provided by the Univcrsity of South Florida Faculty Rcsearch and Creative Scholarship Fund. the American-Scandinavian Foundation. the U . S . National Science Foundation (via grant INT 8900496 to Scripps Institution o f Oceanography and grant OCE WI33Y2 to the University of South Florida), and Pro Mare. C. Rooth supplied valuable comments and suggestions. depth sampled, and high values on the assumption that those depletions persisted t o 20 meters. Barents-Sea l Z 2 R n depletions and piston vel- ocities fit within the ranges of those parameters found elsewhere in ice-free seawater. T h e overall range of Barents Sea z22Rn depletions (280- 680 dpm m-I) falls in the lower 14% of the whole- ocean range and the lower 19% of the tropical- Atlantic range. T h e overall range of Barents Sea piston velocities normalized to 20°C (1.2-5.1 m d & ' ) falls in the lower 25% of the whole-ocean range, but the highest normalized Barents Sea piston velocity is only 15% below the highest tropical-Atlantic value. Although these com- parisons clearly indicate that sea ice restricts gas exchange, Barents Sea parameters are far from the zeros t o be expected if sea-ice cover routinely permitted no gas exchange. as required by the Antarctic excess-COz model of Poisson & Chen (1987). Our zz2Rn and 226Ra results a r e much more consistent with the weaker restrictions on gas-exchange implied by the moderate freon undersaturations in the Arctic Ocean (Krysell & Wallace 1988; Wallace e t al. 1987). Even a t ice- covered hydrostation 43, the upper third of the water column had A 2 2 2 : A z 2 6 ratios that were uniformly, albeit just slightly, less than unity. Interestingly, integrated Barents Sea '"Rn depletions and piston velocities varied little between summer and winter, suggesting that the overall effect of sea ice o n air-sea gas exchange is roughly the same in both seasons (Table 1). In summer, individual percent depletions were high, but ice meltwater was present to constrain the depth of exchange. In winter, low water-column stability permitted the deeper penetration of 222Rn outgassing t o 60-110 m, but the greater ice cover appeared t o restrict the degrees of deple- tion. Probably there was a deep convection fol- lowed by a horizontal advection of ??'Rn-depleted water associated with the leads that occupy at least 1% of a sea-ice region, even in winter (Smith et al. 1990). The distributions of 222Rn depletions and enrichments at hydrostation 37 (Fig. 3) sug- gest that the process in those leads might begin with the turnover of a low-stability water column. 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