DOI: 10.1594/pangaea.883808
title: Filchner Ice Shelf cavity noble gas (helium and neon) measurements

description: To obtain noble gas samples from the water column below an ice shelf, we have constructed a new gas-tight in-situ water sampler to prevent the water samples from the sub-glacial cavity from gas-fractionation and degassing due to freezing and expansion. The novel instrument contains a degreased soft copper tube (diameter 10 mm, length 90 cm) allowing a sea-water sample of ~44 g as usually used for oceanic noble gas samples. Both ends are connected with high pressure ball valves with gas-tight swagelok© connectors to it at both ends. Additional intake and outtake funnels extend the tube at both ends of the sampling device to increase the water flux for flushing and rinsing the copper tube. The instrument is mounted in an aluminum cylinder (diameter 100 mm, length 1510 mm, weight 8 kg) to protect it while it is moved through the bore hole (diameter approx. 250 mm). The instrument is lowered through the bore hole into the water underneath the ice shelf on a 3.25 mm wire together with a CTD measuring continuously temperature, salinity and pressure. While lowering, the valves are kept open and are spanned with connecting wires and a steel spring. When the instrument is at target depth, the rinsing of the sampler is ensured by moving it frequently up and down for a few meters (we tested it under lab-conditions and found moving it ten times one meter up and down results in &gt;99% water exchange in the tube). As a messenger weight deployed along the wire hits a piston, it releases a spanner wheel held back by a metal trigger arm, which releases the steel spring to close the valves. As soon as the instrument is recovered at the surface, the copper tube with the water sample is mechanically squeezed and sealed with steel clamps and screws at both ends near the valves. The finally clamped sample is removed from the valves and the aluminum cylinder. The copper tube is rinsed with fresh water and stored away, whereas the valves, spring, and aluminum cylinder can be used again. The water sampler was deployed below the Filchner Ice Shelf on six hot-water drilled bore holes, at each site at various depths, in austral summer 2015/16 (FISP South) and 2016/17 (FISP North). The water samplers were clamped onto the CTD wire and positioned at desired target depths using the CTD pressure readings as a reference. Samples were usually taken during the up-cast. Before releasing the messenger, the samplers were moved approximately 8-10 times up and down over a distance of about 2 m, in order to ensure proper flushing of the sample tube with the ambient water at the sample depth. The noble gas samples were analyzed in the IUP Bremen noble gas mass spectrometry lab HELIS. The samples are processed in a first step with an ultra high vacuum gas extraction system. For analysis of the noble gas isotopes [3He, 4He, 20Ne] the extracted gas is transferred into a fully automated mass spectrometric system equipped with a two-stage cryogenic trap system. The system is regularly calibrated using atmospheric air standards. Also measurement of blanks and linearity are done (reproducibility better ±0.2%, accuracy better ±0.5%). For details see Sültenfuß et al., 2009, doi:10.1080/10256010902871929. Seawater properties under FIS (and within each borehole) were measured using a compact CTD system consisting of a SBE49 attached to a SBE36 Power Data Interface Module with a Deck Unit at the surface. Temperature and salinity related to the noble gas measurements were obtained from the CTD up-casts by identifying the depths of the sampler release in the CTD profiles. The Theta and S values were read from the CTD up-cast profile using the known distance to the CTD instrument and, hence, the position of the sampler in the water column. Acknowledgment: We like to thank the British Antarctic Survey (BAS) and the Alfred Wegener Institute (AWI) for supporting the FISP drilling campaigns 2015/16 and 2016/17 by logistics and funding. We acknowledge the work of Gerhard Fraas, who designed an early version of a noble-gas in-situ water sampler. This work was partly funded by the Deutsche Forschungsgemeinschaft (DFG, SPP 1158 Antarktisforschung, grant HU 1544/4 to O. Huhn).