Ice Core Drilling Objectives

Sea ice is always a mixture of solid freshwater ice, liquid salty brine and some air bubbles. The relative ratio of these three phases determines the physical properties of the sea ice, and without their proper knowledge, realistic modeling of sea ice in large scale models is impossible. The ice cores that we collected are meant to improve our knowledge of the properties of Antarctic sea ice and to hence guide improved modeling. In these cores, we measured profiles of sea-ice temperature and salinity as well as density and air content. In particular the latter parameters are largely unknown for Antarctic sea ice and would be of great importance for improved modeling of sea-ice thermodynamics.

Work at sea

We have collected sea-ice samples in the Weddell Sea and performed some standard measurements (temperature and salinity profiles) on them as well as a feasibility study concerning the measurement of the air content within the ice under ‘’field work conditions’’ on a ship.

Since ANT-XXVII/2 was a summer cruise the ice cover in the Weddell Sea was much reduced compared to winter conditions. To the west of the Antarctic Peninsula, sea ice was fully absent and there were not many opportunities to take suitable sea-ice samples. Nonetheless, we were able to collect 13 ice cores in total from 5 different sites during the cruise.

As table 3.6.1 shows, the samples were taken from floes mostly close to the ship that could be reached via a short flight with the helicopter. While the helicopter then waited within visual range on the ship, the sampling was done with the help of Fabian Gall and Matthias Monsees or Markus Heckmann. Given suitable ice-floe conditions up to three cores were drilled with a manual ice core drill that was equipped with a fuel-powered motor. At each drilling site, air temperature 50 cm above the snow surface, snow thickness and a snow temperature profile were recorded. The ice temperature of the cores was immediately measured every five centimeters to obtain a profile.

The cores were sawed into disks of five centimeters thickness and the salinity was measured for each disk back on the ship.

Table 3.6.1: Coring positions and floe properties

Preliminary and expected results

One of the main tasks of our ice-core sampling program was to develop and test several methods to measure the volume of the air bubbles enclosed in the ice body.

These methods are based on different principles: direct measurement of the air bubble volume, replacement and buoyancy.

For the methods based on the buoyancy measurements the ice is to be enclosed in a vacuum-sealed bag, which is then hung under water with an additional weight.

After carefully removing the air from the bag with the melted ice in a second step, the difference in buoyancy will indicate the volume of the air. However, this method only works in calm sea and was clearly not usable with swell in the open sea.

The method based on replacement works as follows: A disc of ice in a vacuum-sealed bag is put in a container with a scale attached to the lid. After filling this container with water to the top of the scale the ice can melt inside the bag, which will lead to a drop of the water level due to density change during the phase transition from ice to water.

Then the bag can be opened by means of a long needle introduced into the container through the scale, releasing the air and leading to a further drop in water level allowing to read the volume of the released air.

To apply this method properly, a container is needed which firstly has a wide opening for placing the ice inside and secondly can be closed tightly enough such that the

filled up to the level of the scale.

Two types of direct measurements have been performed on the cruise under different weather and sea conditions.

For the first method, a disc of ice is enclosed in a vacuum-sealed bag. While melting, the air is released from within the ice and forms a distinct air bubble which can be extracted from the bag by means of a syringe. In order to avoid air being lost through the small hole from the needle, it is put through rubber. After reading off the volume of the air content from the scale of the syringe, the conductivity and temperature can be measured for the remaining melted ice in order to determine the salinity.

For the second method, the disc of ice is placed in a water bath under a funnel with an attached scale. Once the whole scale is under water it is closed, such that the air released from the ice while melting can rise into the scale where its volume can then be determined. The salinity of the melted ice can be determined by means of conductivity measurements of the bath before and after melting the ice disc inside.

After a testing phase the latter two methods were selected to be applied on generally two out of the three cores from each drilling site, while the third core was preserved under vacuum in a -20°C environment for later analysis of the air bubble content with established but complex methods that are not suitable for on-ship work. The results were analyzed preliminarily for systematic errors of the respective methods. A thorough analysis including a comparison to the measurements on the third ice core are planned to be performed after the cruise.

4.1 Repeat sections of total carbon dioxide and total alkalinity across