Biogeochemical composition of natural sea ice brines from the Weddell Sea during early austral summer

Abstract

Sea ice brines were collected from a single floe composed of different ice types in the western Weddell Sea in December 2004. The chemical composition of the brines (temperature: 23.4uC to 22.1uC; salinity: 40-63) was examined on seven occasions over 25 days with measurements of dissolved oxygen, dissolved inorganic macronutrients (nitrate plus nitrite, ammonium, phosphorus [DIP], and silicic acid), pH, total alkalinity (A T ), dissolved organic carbon (DOC) and nitrogen (DON), total dissolved inorganic carbon (C T ), and the stable isotopic composition of C T (d 13 C T ). The in situ pH ranged from 8.41-8.82 on the seawater scale, dissolved oxygen from 212-604 mmol kg 21 , nitrate from 0.1-3.1 mmol kg 21 , ammonium 0.1-2.4 mmol kg 21 , DIP 0.4-2.0 mmol kg 21 , silicic acid 4-80 mmol kg 21 , A T 2,690-4,620 meq kg 21 , DOC 115-359 mmol kg 21 , DON 8-26 mmol kg 21 , C T 2,090-3,550 mmol kg 21 , and d 13 C T +2.9%-+6.4%. Compared with the chemical composition of surface oceanic water (salinity of 34), the brines had elevated pH, reduced concentrations of dissolved inorganic macronutrients (including carbon), especially dissolved inorganic nitrogen, and were mostly supersaturated with dissolved oxygen with respect to equilibrium with air, whereas the C T was considerably enriched in 13 C. The chemical composition of the brines was consistent with internal biological productivity, but there was a lack of a distinctive and uniform relationship among the major dissolved inorganic nutrients typically used for describing biological activity. This was interpreted as the result of varying stoichiometry of biological activity within a very small spatial scale. Modification by abiotic processes was a potential contributing factor, such as degassing acting on the dissolved oxygen concentration. Carbonate mineral formation, acting on A T and C T , was not evident in brines from first-year ice but was apparent in brine from second-year ice.Sea ice covers 13% of the surface of the earth at its maximum extent, equivalent to the areal extent of deserts and tundra. The largest expanse of sea ice occurs in the Southern Ocean, totalling 18.8 3 10 16 km 2 of 0.5-0.6 m mean thickness at its maximum extent in September (Comiso 2003). It is an ecologically diverse habitat and has been increasingly recognized for its key role in global biogeochemical cycles, including contribution to the primary productivity of the Southern Ocean (Arrigo 2003;Arrigo and Thomas 2004) and the ventilation of the deep ocean at high latitudes (Francois et al. 1997;Stephens and Keeling 2000).The chemical composition of sea ice is primarily a function of salinity and temperature, but it is modified further by productivity of the internal microbial assemblages recruited from the surface seawater during sea ice formation (Brierley and Thomas 2002;Thomas and Dieckmann 2002;Rysgaard et al. 2007). Primary production is a major carbon sink within sea ice, resulting in the fixation of 30-70 Tg C yr 21 into biomass in the firstand multi-year ice pack of the Sou...

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.…”

Sea ice and snow thickness and physical properties of an ice floe in the western Weddell Sea and their changes during spring warming

“…As will be shown later with the ice-core analyses, most thick ice was SYI, while the thin ice was FYI. The sediment trap and biogeochemistry site (site 9b; Papadimitriou et al, 2007) was rather level with a low standard deviation, too, but the total thickness was quite high, indicating thick FYI (Table 1). This will be discussed with the presentation of the ice-core analyses.…”

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

“…For some high-biomass Antarctic sea ice microhabitats, potential algal growth limitation by depleted CO 2 , elevated brine pH, and oxygen supersaturation has been reported [Gleitz et al, 1995;. In contrast, heterotrophic processes, e.g., bacterial activity, heterotrophic grazing and excretion, as well as viral lysis of host cells, can result in nutrient remineralization within the sea ice [Gleitz et al, 1995;Papadimitriou et al, 2007;Riedel et al, 2008;. Large spatial variability in ice algal activity is found from submeter to large scales [Rysgaard et al, 2001;Deal et al, 2011], due to processes that operate at micro-(individual pores), meso-(snow distribution) and macro-(nutrient input) scales.…”