Biogas (CO₂, O₂, dimethylsulfide) dynamics in spring Antarctic fast ice

Abstract

We studied the temporal variations of CO 2 , O 2 , and dimethylsulfide (DMS) concentrations within three environments (sea-ice brine, platelet ice-like layer, and underlying water) in the coastal area of Adélie Land, Antarctica, during spring 1999 before ice breakup. Temporal changes were different among the three environments, while similar temporal trends were observed within each environment at all stations. The underlying water was always undersaturated in O 2 (around 85%) and oversaturated in CO 2 at the deepest stations. O 2 concentrations increased in sea-ice brine as it melted, reaching oversaturation up to 160% due to the primary production by the sea-ice algae community (chlorophyll a in the bottom ice reached concentrations up to 160 mg L 21 of bulk ice). In parallel, DMS concentrations increased up to 60 nmol L 21 within sea-ice brine and the platelet ice-like layer. High biological activity consumed CO 2 and promoted the decrease of partial pressure of CO 2 (pCO 2 ). In addition, melting of pure ice crystals and calcium carbonate (CaCO 3 ) dissolution promoted the shift from a state of CO 2 oversaturation to a state of marked CO 2 undersaturation (pCO 2 , 30 dPa). On the whole, our results suggest that late spring land fast sea ice can potentially act as a sink of CO 2 and a source of DMS for the neighbouring environments, i.e., the underlying water or/and the atmosphere.Sea ice covers about 7% of Earth's surface at its maximum seasonal extent, representing one of the largest biomes on the planet. For decades, sea ice was assumed to be an impermeable and inert barrier for air-sea exchanges of CO 2 , and global climate models did not include CO 2 exchanges between this compartment and the atmosphere. However, there is a growing body of evidence that sea ice exchanges CO 2 with the atmosphere. While estimating permeation constants of sulfur hexafluoride (SF 6 ) and CO 2 within sea ice, Gosink et al. (1976) stressed that sea ice is a permeable medium for gases. These authors suggested that gas migration through sea ice could be an important factor in winter ocean-atmosphere exchange at sea-ice surface temperature above 210uC. More recently, uptake of atmospheric CO 2 over sea-ice cover has been reported (Semiletov et al. 2004;Delille 2006;Zemmelink et al. 2006) supporting the need to further investigate pCO 2 dynamics in the sea-ice realm and related CO 2 fluxes.Very few studies have been carried out on the dynamics of the carbonate system within natural sea ice. They have generally been aimed at investigating CaCO 3 precipitation or dissolution (Gleitz et al. 1995), or they have focused on measurements of dissolved inorganic carbon (DIC) and total alkalinity (TA) (Anderson and Jones, 1985;Rysgaard et al. 2007) rather than on pCO 2 . As pointed out by 1 Corresponding author (Bruno.Delille@ulg.ac.be).

Freshwater fluxes in the Weddell Gyre: results from δ 18 O

“…For SIM (figure 4c), highest percentages are visible in the surface layers associated with the recent seasonal decrease in sea-ice extent (approx. stations [67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][82][83][84][85]. This reaches a maximum of 1.8% at station 84, equivalent to 0.8 m of sea-ice derived freshwater in an 85 m thick Surface Water (neutral density γ n < 27.55) layer.…”

“…For MET, the salinity endmember was set to 0, whilst for sea-ice a salinity of 5 [68] and a δ 18 O of +1.8 (taken as representative of surface waters in this region adjusted for the fractionation that occurs during freezing [69]) were used. Estimates of 'PO' were formulated from mean phosphate and oxygen measurements taken from Antarctic snow (for the meteoric endmember [70][71][72][73][74] and sea-ice [74][75][76][77][78]). The δ 18 O endmember for MET carries the greatest uncertainty, as it must represent a combination of both local (seasonally varying) precipitation and glacial melt, which can encompass a large variability in δ 18 O signal depending on the exact latitude and elevation at which the precipitation accumulated.…”

Winter observations of CO&lt;sub&gt;2&lt;/sub&gt; exchange between sea ice and the atmosphere in a coastal fjord environment

“…Consequently, most carbon-cycle research has treated ice cover as areas of zero (or very low) exchange (Tison et al, 2002). This view has been challenged by reports of significant fluxes of CO 2 over first and multiyear sea ice during both spring/summer (Delille et al, 2007;Geilfus et al, 2012;Semiletov et al, 2004Semiletov et al, , 2007Zemmelink et al, 2006) and autumn/winter (Else et al, 2011;Geilfus et al, 2013;Miller et al, 2011a, b) and challenged by suggestions of a coupling between the carbonate system in sea ice, the underlying sea water and the atmosphere (Anderson et al, Figure 1. (a) Regional and (b) local overview of field sites in Young Sound, northeast Greenland.…”

Role of sea ice in global biogeochemical cycles: emerging views and challenges

“…These include the metabolic activities (i.e., primary production and respiration) [e.g., Arrigo et al, 2010;Deming, 2010], which produce and absorb biogenic gases such as O 2 , CO 2 and DMS [Gleitz et al, 1995;Glud et al, 2002;Delille et al, 2007;Tison et al, 2010]. They also include carbonate chemistry, which plays an important role in the dynamics of dissolved inorganic carbon (DIC) in sea ice [Delille, 2006;Rysgaard et al, 2007;Miller et al, 2011a;Geilfus et al, 2012], and the precipitation of calcium carbonate (CaCO 3 ) in the form of ikaite crystals as recently observed in Arctic and Antarctic sea ice [Dieckmann et al, 2008;.…”

“…Sea ice itself is permeable when warm enough [e.g., Golden et al, 1998], supporting gas exchanges [e.g., Delille et al, 2007;Nomura et al, 2010;Papakyriakou and Miller, 2011] and acts as a source for some gases, for example DMS [Zemmelink et al, 2008] and potentially Bromine Oxide (BrO) [e.g., Simpson et al, 2007]. Until now, the research community has mainly been interested in the study of biogenic and climatically significant gases (i.e., N 2 O, O 2 , CO 2 , DMS) [e.g., Delille et al, 2007], although there is growing interest in research on other gases such as Br components, which influence polar atmospheric chemistry [e.g., Simpson et al, 2007] and methane (CH 4 ), a strong greenhouse gas that is present in gas bubbles released from anoxic sediments to the water column and sea ice [Shakhova et al, 2009]. A second process of interest is the sinking to depth of CO 2 -rich brine (e.g., richer than seawater), released into the surface ocean during sea ice formation [Rysgaard et al, 2007].…”