The dataset lists key biogeochemical parameters measured in sea ice during the SIPEX2 voyage, including dissolved and particulate iron and other trace metals, macronutrients (silicic acid, nitrates+nitrite, phosphoric acid and ammonium), iron binding organic ligands, dissolved and particulate organic carbon, Cholophylla, thermodynamics (temperature, salinity, brine volume and Rayleigh number).

All sampling bottles and equipment were decontaminated using trace metal clean techniques. Care was taken at each site to select level ice with homogeneous snow thickness.

At all the stations, the same sampling procedure has been used :

Firstly, snow was collected using acid cleaned low density polyethylene (LDPE) shovels and transferred into acid-cleaned 3.8 l LDPE containers (Nalgene). Snow collected was analysed for temperature, salinity, nutrients, unfiltered and filtered metals. Snow thickness was recorded with a ruler.

Ice cores were collected using a non-contaminating, electropolished, stainless steel sea ice corer (140 mm internal diameter, Lichtert Industry, Belgium) driven by an electric power drill. Ice cores were collected about 10 cm away from each other to minimise between-core heterogeneity.

A first core was dedicated to the temperature, salinity and Chlorophyll a (Chla). To record temperature, a temperature probe (Testo, plus or minus 0.1 degrees C accuracy) was inserted in holes freshly drilled along the core every 5 to 10 cm, depending on its length. Bulk salinity was measured for melted ice sections and for brines using a YSI incorporated Model 30 conductivity meter. Chla is processed on board using a 10 AU fluorometer (turner Designs, sunnyvale California). The total length of this core is cut in sections of 7 cm. The second core is dedicated to the POC/PON (Particulate Organic Carbon/ Particulate Organic Nitrogen), DOC (Dissolved Organic Carbon) and nutrients. Six sections of 7 cm were sub-sampled from this core. The six sections were chosen so that two top, two intermediate and two basal sections. Two cores are taken for the trace metal analysis. Those cores were directly triple bagged in plastic bags (the inner one is milli-Q washed) and frozen at -20degrees C until analysis at the laboratory.

Brine samples were collected by drainage from “sack holes”. Brines and under ice seawater (~1 m deep) were collected in 1 l Nalgene LDPE bottles using an insulated peristaltic pump and acid cleaned C-flex tubing (Cole Palmer). All samples were then transported to the ship as quickly as possible to prevent further freezing. Samples were used to analyse unfiltered and filtered metals, Chla, POC/PON, nutrients and DOC. Filtration for filtered metals was completed on board using a peristaltic pump and a 0.2 microns cartridge filter.

All the unfiltered and filtered metals collected were acidified (2 ppt HCl seastar) and stored at room temperature until analysis at the laboratory.

Nutrients, DOC and filters for POC/PON were stored frozen at -20 degrees C until analysis at Analytical Service Tasmania, Hbart.

Chla filtrations and analysis were completed on board.

The file "SIPEX2 sea ice data" lists key biogeochemical parameters in sea ice cores, snow, brine and underice seawater (1m depth) collected during the SIPEX2 voyage (64.26-65.15S/116.44-120.58E) carried out between the 26th of september and 29th of october 2012.

The acid-cleaning protocols for sample bottles and equipment followed the guidelines of GEOTRACES (www.geotraces.org). Contamination-free ice coring equipment developed by Lannuzel et al. (2006) was used to collect ice cores. Ice cores were triple bagged and stored at -18 degrees C until further processing in the home laboratory. Ice cores were then sectioned under a class-100 laminar flow hood (AirClean 600 PCR workstation, AirClean System) using a medical grade stainless steel bonesaw (Richards Medical), thouroughly rinsed with ultra-high purity water (18.2 MO), and ice sections were then allowed to melt at ambient temperature in acid-cleaned 3 L Polyethylene (PE) containers. Melted sea-ice sections were then homogenized by a gentle shake and filtered through 0.2 microns pore size polycarbonate filters (Sterlitech, 47 mm diameter) using Teflon(R) perfluoroalkoxy (PFA) filtration devices (Savillex, USA) connected to a vacuum pump set on less than 2 bar to obtain the particulate (greater than 0.2 microns) and dissolved (less than 0.2 microns) metal fractions. The collected filtrates (less than 0.2 microns) were acidified to pH 1.8 using Seastar Baseline(R) HCl (Choice Analytical) and stored at ambient temperature until analysis in the home laboratory. The filters retaining the particulate material were stored frozen in acid-clean petri dishes until further processing.

Standard physico-chemical and biological parameters such as sea-ice and snow thicknesses, in situ ice temperature, sea-ice and brine salinities, ice texture, chlorophyll a (Chla), macro-nutrients (nitrate+nitrite (NOx), phosphate (PO43-), silicic acid (Si(OH)4-) and ammonium (NH4+)), dissolved organic carbon (DOC), and particulate organic carbon and nitrogen (POC and PON) were also determined in each sample at Analytical Service Tasmania (Hobart, Australia) within 6 months of sample collection. Dissolved inorganic nutrients were determined using standard colorimetric methodology (Grasshoff et al., 1983) as adapted for flow injection analysis using an auto-analyzer. Theoretical brine volume fractions (Vb/V) were calculated using in situ ice temperatures and bulk ice salinities and relationships from Cox and Weeks (1983). The full ice core length was examined under crossed-polarised light to identify the texture (i.e., columnar vs granular) according to the method of Langway (1958). Preparation of the thin sections took place in a container kept at -25 degrees C. The thin sections were obtained by cutting vertical sections of about 6 mm thick using a band saw. Ice sections were then thinned down using a microtome blade to reach a final thickness of 3 - 4 mm and observed under cross-polarized lights

The acidified filtrates were diluted 5 times, using 2 % v:v ultrapure HNO3 (Seastar Baseline, Choice Analytical) and dissolved metals concentrations were determined directly using sector field inductively coupled plasma magnetic sector mass spectrometry (SF-ICP-MS; Element 2) following the method described in Lannuzel et al. (2014).

Filters retaining particulate material (greater than 0.2 microns) were digested in a mixture of strong, ultrapure acids (750 micro litres 12N HCl, 250 microlitres 40% HF, 250 microlitres 14N HNO3) in 15 mL Teflon(R) perfluoroalkoxy (PFA) (Savillex, USA) on a Teflon coated graphite digestion hot plate housed in a bench-top fume hood (all DigiPREP from SCP Science, France) coupled with HEPA(R) filters to ensure clean air input at 95 degrees C for 12 h, then dry evaporated for 4 h and re-suspended in 2 % v:v HNO3 (Seastar Baseline, Choice Analytical). The procedure was applied to filter blanks and certified reference materials BCR-414 and MESS-3 to verify the recovery of the acid digestion treatment. The concentrations of particulate metals were then determined by SF-ICP-MS (Bowie et al., 2010). Results for procedural blanks, limits of detection and certified reference materials were found fit for purpose.

The file "SIPEX2 TMR data" lists macro-nutrients concentrations, as well as dissolved iron concentrations collected using a Trace Metal Rosette (TMR) deployed over 1000m depth in the sea ice zone. Dissolved iron (DFe) and iron in the 2+ redox state (FeII) in nanomoles per Litre (nmol/L) were measured onboard using FIA-CL technique explained in Schallenberg et al (2015). Standard deviation associated with the analysis of the samples is indicated by "SD".

Dissolved Fe(III):

Dissolved Fe in this study is operationally defined as the Fe fraction that passes through a 0.2 microns filter. A modified flow injection analysis (FIA) method was used to measure dFe that relies on the detection of Fe(III) with the chemiluminescent reagent luminol (de Jong et al., 1998; Obata et al., 1993). Samples and standards were treated with hydrogen peroxide (H2O2; final concentration = 10 micro mols) at least 1 hour prior to measurement to oxidize any Fe(II) that might be present (Lohan et al., 2005). The system buffers the samples in-line to pH = 4 before passing them for 3 minutes through a pre-concentration column packed with 8-hydroxyquinoline chelating resin (8-HQ). A solution of 0.3 M HCl (Seastar) then elutes Fe(III) from the resin and mixes with 0.8 M ammonium hydroxide (NH4OH), 0.1 M H2O2 and 0.3 mM luminol containing 0.3 mM triethylenetetramine (TETA) and 0.02 M sodium carbonate (Na2CO3), yielding an optimum luminol chemiluminescence reaction pH of 9.5. The resulting solution is passed through a ~5 m mixing coil maintained at 35 degrees C before being pumped to the flow cell mounted in front of a photo-detector.

System blanks were 0.014 plus or minus 0.004 nM, yielding a detection limit (3 x blank standard deviation) of 0.013 nM. Results for SAFe reference materials for Fe were in good agreement with consensus values (see Table 1).

Dissolved Fe(II):

Fe(II) was determined by luminol chemiluminescence detection following the approach of Hansard and Landing (2009) but without sample acidification. Sampling began within minutes after the first Niskin bottle (always from the surface) arrived in the clean container. Samples were analyzed within 2 minutes of filtration and were pumped simultaneously with the luminol reagent into a spiral flow cell made of flexible Tygon(TM) tubing (ID = 0.7 mm) that was mounted in front of a photomultiplier tube (Hamamatsu H9319-01) in a custom-made light-tight box. Flow rates for luminol and sample were ~4.5 mL/min. The photomultiplier tube was operated at 900 V with a 200 ms integration time. Photon counts were recorded using FloZF software (GlobalFIA) and were averaged over 10 second intervals with 5 replications for each sample and standard. The relative standard deviation of these repeat measurements was between 1 and 3%.

The luminol recipe for 1 L reagent is as follows: 0.13 g luminol, 0.34 g Na2CO3, 40 mL concentrated NH4OH and 10-12 mL concentrated HCl (Seastar). This results in 0.75 mM luminol with 3.2 mM Na2CO3. The pH of the reagent is adjusted to ~10.0 with small amounts of NH4OH and HCl. It was found that luminol sensitivity increases with age, so batches were prepared well in advance and used up to 3 months later.

Fe(II) calibration curves were obtained with Fe(II) standard additions in the range 0-100 pM. A 10 mM standard of ammonium iron(II) sulfate hexahydrate was prepared fresh in 0.1 M Seastar HCl and considered stable in the fridge for up to a month. From this stock solution, intermediate standards (50 micro mols and 50 nM) were prepared in 0.05 M Seastar HCl no more than 10 minutes prior to measurement. Standards were added to seawater that had been collected at earlier stations in the cruise and been left in the dark for greater than 24 hours.

Previous investigators (e.g., Rose and Waite, 2001) have commented on the light-sensitivity of the luminol reagent, and it is therefore frequently stored in the dark.