Stable isotope composition of dissolved inorganic carbon and particulate organic carbon in sea ice from the Ross Sea, Antarctica

Abstract

[1] We examined controls on the carbon isotopic composition of sea ice brines and organic matter during cruises to the Ross Sea, Antarctica in November/December 1998 and November/December 2006. Brine samples were analyzed for salinity, nutrients, total dissolved inorganic carbon (SCO 2 ), and the 13 C/ 12 C ratio of SCO 2 (d 13 C SCO 2 ). Particulate organic matter from sea ice cores was analyzed for percent particulate organic carbon (POC), percent total particulate nitrogen (TPN), and stable carbon isotopic composition (d 13 C POC ). SCO 2 in sea ice brines ranged from 1368 to 7149 mmol kg −1 , equivalent to 1483 to 2519 mmol kg −1 when normalized to 34.5 psu salinity (sSCO 2 ), the average salinity of Ross Sea surface waters. Sea ice primary producers removed up to 34% of the available SCO 2 , an amount much higher than the maximum removal observed in sea ice free water. Carbonate precipitation and CO 2 degassing may reduce sSCO 2 by a similar amount (e.g., 30%) in the most hypersaline sea ice environments, although brine volumes are low in very cold ice that supports these brines. Brine d 13 C SCO 2 ranged from −2.6 to +8.0‰ while d 13 C POC ranged from −30.5 to −9.2‰. Isotopic enrichment of the SCO 2 pool via net community production accounts for some but not all carbon isotopic enrichment of sea ice POC. Comparisons of sSCO 2 , d 13 C SCO 2 , and d 13C POC within sea ice suggest that p (the net photosynthetic fractionation factor) for sea ice algae is ∼8‰ smaller than the p observed for phytoplankton in open water regions of the Ross Sea. These results have implications for modeling of carbon uptake and transformation in the ice-covered ocean and for reconstruction of past sea ice extent based on stable isotopic composition of organic matter in sediment cores.

Macro-nutrient concentrations in Antarctic pack ice: Overall patterns and overlooked processes

“…Silicic acid is mainly required for the growth of diatoms, with likely minor consumption by silicoflagellates and radiolarians, to form their cell walls (i.e., biogenic silica, bSiO 2 ) (Tréguer and De La Rocha, 2013). Despite the growing recognition of the role played by flagellates in sea ice habitats (Caron and Gast, 2010;Torstensson et al, 2015), diatoms are thought to dominate primary production in Antarctic sea ice (Arrigo et al, 2010). Silicon is therefore a potentially limiting element for diatom-based primary production in sea ice (as suggested for Arctic sea ice by Gosselin et al, 1990, andSmith et al, 1990).…”

“…During a recent Baltic sea ice study, where direct and salinity-buffered melting were compared, no a In parentheses are the number of cores or sackholes with no data for ammonium concentration. b ANTARKTIS V/2; ANTARKTIS X/3; ROAV98; NBP06-08; SIMBA (Dieckmann et al, 1991;Gleitz and Thomas, 1993;Gleitz et al, 1995;Arrigo et al, 2001;Munro et al, 2010). c WEPOLEX; ANTARKTIS VIII/2; AURORA2003-V1; OSO 08-09 (Clarke and Ackley, 1984;Becquevort et al, 2009;Fransson et al, 2011).…”

“…The general procedure consists of collecting a core, dividing it into sections (average section thickness = 0.09 ± 0.04 m) using a clean stainless steel saw, followed by the melting of the ice sections in the dark at ambient temperature or < 4°C (see Miller et al, 2015). Nutrient concentrations in brine were obtained either from measurements in samples extracted from centrifuged ice sections (n = 152; average section thickness = 0.14 ± 0.05 m) (Arrigo et al, 2003;Munro et al, 2010), or using the sackhole technique (n = 93), i.e., drilling partial core holes to a desired depth within the ice and allowing brines from the surrounding ice to drain into the hole (Gleitz et al, 1995). These methods have been shown to be robust for nutrient sampling (Miller et al, 2015).…”

“…Sea ice is a semi-solid medium permeated by a network of channels and pores that are variably connected with seawater (Weeks, 2010). These brine-filled spaces can be heavily colonized by a sympagic (ice-associated) microbial community that is taxonomically diverse and metabolically active (Arrigo et al, 2010;Caron and Gast, 2010;Deming, 2010;and references therein). During its incorporation into the ice, the microbial community is exposed to major biogeochemical and physical changes in its habitat, including fluctuations in temperature, salinity, dissolved oxygen, light, pH, the surrounding organic matrix, and nutrients.…”

“…By contrast, dissolved silicon [mostly silicic acid; Si (OH) 4 ] is a macro-nutrient predominantly used by diatoms to build their frustules (biogenic silica; bSiO 2 ) and to a lesser extent by radiolarians and silicoflagellates (Tréguer and De La Rocha, 2013). Since primary production in sea ice is thought to be dominated by diatoms (Arrigo et al, 2010), Si(OH) 4 has also been suggested as a potentially limiting nutrient in this ecosystem (Gosselin et al, 1990;Smith et al, 1990).…”