DOI: 10.1594/pangaea.971321
title: Trace gas measurements, basal glacial meltwater fractions, and water ages during RV POLARSTERN cruise PS124, Southern Weddell Sea, 2021

description: In this data set we present trace gas measurements and derived variables such as basal glacial meltwater (GMW) fractions and water ages. The trace gases comprise the lighter noble gasses, i.e., total helium (He) and the excess above solubility equilibrium ΔHe, the 3He/4He ratio reported as δ3He, total neon (Ne) and the excess ΔNe, and the deviation from the equilibrium He/Ne ratio Δ(He/Ne), as well as the transient anthropogenic trace gases chlorofluorocarbon (CFC-12) and sulphur hexafluoride (SF6) concentrations. From these noble gases He and Ne we derived glacial meltwater fractions. From the transient tracers we computed SF6-concentration ages, CFC-12/SF6 ratio ages and TTD mean ages.
description: Hydrographic data:The hydrographic data were adopted from Tippenhauer et al., 2023.Noble gases:Water samples were taken from the CTD/water bottle systems without contact to atmospheric air into 40 ml gas tight copper tubes, which are clamped of at both sides.In the IUP Bremen noble gas lab the samples were pre-processed with a UHV (ultra-high vacuum) gas extraction system. Sample gases are transferred via water vapour into a glass ampoule kept at liquid nitrogen temperature. For analysis of the noble gas isotopes the glass ampoules are connected to a fully automated UHV mass spectrometric system equipped with a two-stage cryogenic system, a quadrupole and a sector-field mass spectrometer. Regularly, the system is calibrated with atmospheric air standards (reproducibility &lt; 0.2%). Measurement of line blanks and linearity are done as well. The performance of the Bremen facility is described in Sültenfuß et al. (2009).Noble gas concentrations are reported in nmol/kg for He and Ne and are additionally converted into excess above solubility equilibrium ΔHe [%] and ΔNe [%] using the solubility function from Weiss, 1971; δ 3He is reported in %. The precision for He is 0.23%, 0.33% for Ne and 0.43% for δ3He (based on 9 replicate measurements).Glacial meltwater fractions:Glacial meltwater fractions were derived using the method described in Huhn et al., 2021. From surface measurements we calculated surface water excess of 0.3 % for He and 0.9 % for Ne. We derived glacial meltwater from He (21) and Ne (23) individually. Due to other sources of He from crustal He and Ne from sea ice formation) we advise to use either He or Ne based glacial meltwater fractions, whichever is greater (25).Anthropogenic tracers:The CFC-12 and SF6 water samples from the CTD-bottle systems were stored in ~220 ml glass ampoules by avoiding contact to the atmosphere during the tapping by a dedicated tubing and rinsing procedure. After sampling, the ampoules are flame sealed after a headspace of pure nitrogen had been applied.The determination of CFC-12 and SF6 concentrations in the IUP Bremen gas chromatography lab is accomplished by purge and trap (cryogenic trapping at -65°C) sample pre-treatment of a precise water volume of 140 ml followed by gas chromatographic separation on a capillary column and electron capture detection (ECD). After thermal desorption the released gases are separated on a pre-column of type Aluminia Bond/CFC, 0.54 mm ID x 3m, and a main column of type Aluminia BOND/CFC, 0.54 mm ID x 30 m. SF6 and CFC-12 are then detected on a micro-ECD.The analytical system is calibrated frequently by analyzing different volumes of known standard gas concentrations. The loss of CFCs and SF6 into the headspace is considered by equilibration between liquid and gas phase under controlled conditions before the sealed ampoules are opened and the volume of the headspace was precisely measured. A more detailed description of the measurement system is given by Bulsiewicz et al. (1998).CFC-12 concentrations are reported in pmol/kg and SF6 in fmol/kg, both reported on SIO98 scale (Prinn et al., 2000). The concentrations were also converted into partial pressure [ppt] using the solubility functions from Warner and Weiss, 1985, and Bullister et al., 2002. The precision of the measurement, based on the comparison of the replicate samples, is 1.3 % or for CFC-12 and 2.5 % for SF6. The accuracy for CFC-12 is 2% and for SF6 is 4%, including errors of calibration, linearity, standard-gas, gas volumes for calibration, water volume, gas loss into the head-space, and calibration scale. The detection limit is 0.002 pmol/kg for CFC-12 and 0.04 fmol/kg for SF6.Water ages:We computed three types of water ages. For all types we assumed a common and constant surface water saturation of 80% for both tracers. In a first order approach we used the SF6 concentration (i.e., the partial pressure) and compared it to the atmospheric tracer history (data from Bullister (2015) up to 2013 for CFC-12 and SF6; afterwards from Dutton, Hall, Montzka, et al. (2022) and Dutton, Hall, Dlugokencky, et al. (2022) for CFC-12 and SF6, respectively), to compute a concentration age, assuming pipe-flow without any mixing. In a higher order approach, we computed a so-called ratio age by comparing the CFC-12/SF6 partial pressure ratio to the atmospheric tracer ratio history, assuming mixing with tracer free, i.e., old water. Finally, we used the Transit Time Distribution (TTD) method (e.g., Huhn et al., 2013) to derive a mean age, regarding one-dimensional advection and dispersion, with a constant advection/dispersion ratio (Peclet number Pe = 1 = constant).Quality flags:All measurements were assigned with a quality flag. These flags are based on issues during the measurements (e.g., observed failure of the instruments, etc.) or if observations are obvious outliers (e.g., negative values or far too high / too low measurements related to other measurements nearby). The flags for the measurements were assigned to the derived values (glacial meltwater fractions, ages) on which they are based.The flags mean: 2 = good measurement or value, 3 = doubtful measurement or value, 4 = bad measurement or value, 6 = mean of replicate measurement, 9 = no measurement or value.