Feb 14 - Autosub and swath
Noon Position : lat 70 14.6 S, long 2 41.1 W
Course made good: Variable
Air temperature @ noon today : 0.1 degrees C
Sea temperature @ noon today : -0.7 degrees C
Wind: Direction E, Force 3
The Fimbulisen Ice Shelf
The scientific aim for this cruise is based around ice. Amongst other things we are hoping to put Autosub (a Remotely Operated Vehicle) underneath an ice shelf. Initially this was due to be the Filchner-Ronne ice shelf, to the west of Halley. However, satellite images suggested that there was a lot of fast ice in that area and it was decided that reaching our site of work would be difficult and consume a lot of valuable time. Instead we have diverted further east and have now arrived at Fimbulisen ice shelf. It is a small, dinky bit of ice compared to the Filchner, but it is named by the Norwegians who first came here and means 'the Great Ice'.
During our approach we encountered a small amount of pack ice. This was nowhere near as solid or as continuous as that which we encountered earlier in the season going to Rothera. A couple of hours made short work of it and we were soon in free water eyeing up the ice cliffs of Fimbalisen (approximately 35metres high).
This area is unknown territory, the information we have has been provided by satellite imagery and is already out of date. The first position that we plotted on our improvised chart showed us to be under a huge tongue of ice, clearly wrong! It turns out that this tongue has broken off and the surrounding inlets have altered position thus distorting the ice edge. The first task then, was to explore the area. We turned and ran transects parallel to the cliff whilst running the swath bathymetry. This has given us accurate data on the depths of the water in this area. Here is an explanation of this part of the science, by Carol Pudsey.
Fig 1 Swath map of part of the continental shelf. Area shaded grey is covered by ice shelf; blue outlines are small ice-covered islands. Contours of the continental slope are from the General Bathymetric Chart of the Oceans (GEBCO). The ship’s track is shown as white lines. A shoal 100-180 m deep runs northwest from one of the islands and the deepest water (>500 m) is in the east under the ice shelf. Red box = fig 2.
To carry out the oceanographic work on the water flowing beneath and near the Fimbul Ice shelf we first need an accurate map of the continental shelf and slope around it. The map we have made so far using the swath bathymetry system (fig 1) is interesting in its own right, as it shows the glacial processes that have shaped the seabed. At the Last Glacial Maximum some 20,000 years ago the East Antarctic Ice Sheet was much larger than today, and extended to the edge of the continental shelf in many places. As at present, most of the ice drainage was probably through a small number of outlets or ice streams. The present-day Jutulstraumen, flowing into the Fimbul Ice Shelf, drains some 200,000 sq km of the East Antarctic Ice Sheet. The Jutul palaeo-ice stream flowed across the continental shelf in approximately the same place as the northward-protruding tongue of the present-day ice shelf (see the passage track chart). On the seabed a series of ridges and grooves, several km long but only a few metres high, clearly shows the direction of grounded ice flow (fig. 2). When the ice retreated it broke up into icebergs, some of which were of deep enough draft to run aground on the shallow parts of the continental shelf. Their plough marks can be seen cutting across the subglacial ridges and grooves.
Fig 2 Shaded-relief image of the eastern end of the map in fig 1. Low ridges and grooves (orange arrows) mark the direction of ice flow; note that the grooves fan out slightly, as the ice left its drainage valley and spread out on the continental shelf. The iceberg scours or plough marks are 5-10 m deep, up to 300 m wide and 2-3 km long.
The James Clark Ross has investigated similar palaeo-ice streams in other areas around the Antarctic Peninsula over the last few years (JR59, JR71, JR104). Computer models of Antarctic glaciation and climate require “ground truth” from as many places as possible around the continent. Only when models fit the present and past conditions correctly can they be used to predict future climate change.
The autosub was deployed for sea trials very early on during the week. These went well and so the yellow submarine undertook it's first mission over the weekend. It was launched and then followed a track 25 kilometres under the ice shelf before returning back to the ship. This was an historic moment, no measurements have ever been taken under an ice shelf before. Data that is going to be collected includes photos of the sea floor and bottom of the ice shelf, current flows, temperature and profiling of the sea bed. I hope to show some of these pictures on a later web-page, once the data has been processed.
For more information on the science check out the science pages from the Southampton Oceanography Centre.
As usual, there are many people involved in the science. Here are photos of just a few of the people on board at the moment.
What a great week we have had! Good weather, good company, good science.....and a lot of good wildlife spotting opportunities.
We have also seen Emperor Penguins and a lot of Minke Whales, but the budding photographers haven't yet managed to capture them. Maybe next week.
So what does the next week hold? We shall be staying at the Fimbulisen ice shelf and continuing with the science above and also doing some mud cores and dredging for beasties on the sea
bottom. Good messy stuff! Until next week.......