Antarctic Voyage: On the shelf at last!

By Guest Author 01/03/2013

Date: 28/2/2013
Location: 66.170212°S, 147.947581°E
Weather: Cloudy with the odd sunny spell, a light breeze and very cold
Sea state: Calm and icy

Sea ice image from 26th February 2013. [NASA MODIS satellite]

Sunny, clear days definitely make this region look less formidable and give it a magical glow.

They also provide us with good sea ice images, clear of cloud, from the NASA satellite MODIS. Every day we scrutinize the new satellite image that is sent through to the ship in the hope that the area near the moorings has opened up enough to get in and retrieve them. No luck so far.

A few days ago, after a beautiful, cloudless day, we got a new image that did show an opening up of the sea ice over to the east of the Mertz Glacier, although unfortunately nowhere near the moorings. So we have made a bee-line (dodging a few ice streams) for this opening and are steaming south over the shelf to get some samples before we have to turn around and start heading home.

Iceberg scours from the multibeam. [Helen Bostock]

As we came up the slope and on to the shelf we started to see iceberg scours in the multibeam data. The iceberg scours are caused by the keel of the deep icebergs scraping along the sea floor. The scours criss-cross in different directions. As we get further on to the shelf we also see glacial lineations (lines). These are different from the scours as they are more regular and only flow perpendicular to the shelf (generally south to north). These are relics from when sea level was lower and the Mertz and Ninnis Glaciers flowed all the way out across the continental shelf. Previously, detailed mapping in this area has also found evidence of glacier grounding lines (where the snout of the glacier has been stuck on the bottom), and moraines (mounds of sediment pushed up at the side and the end of the glacier).

However, the main reason for making this effort to get up on to the shelf is to study biological productivity. While there are lots of nutrients out in the open ocean for the phytoplankton to use, they are limited by the lack of iron. In this region iron is sourced from the coastal sediments and local upwelling (see blog post 25: The biological pump – the importance of microscopic phytoplankton). Therefore, there are usually higher concentrations of phytoplankton in the surface waters over the shelf than on the slope. This increase in phytoplankton productivity will draw down the CO2 (and the nutrients) from the surface waters, which is what the chemists are measuring (see blog post 13: The carbon team).  We can also see evidence for this increased phytoplankton productivity across the shelf when we have been filtering (see blog post 25: The biological pump – the importance of microscopic phytoplankton).

Now that we are on the shelf we are able to use the underwater GoPro video camera, which has a special housing and lights that allows it to go down to water depths of less than 2000 m (unfortunately on the slope all of our stations were deeper than this). The camera is attached to the CTD frame, so the video footage is collected opportunistically at the same time as the water data. We get video of the water column, sediment on the sea floor, and also any deep-sea creatures that live down there. After the first few stations we have already seen krill swimming in the water, sea pens, sea stars, and a couple of sponges (more on the underwater critters in a future blog post).

For the above-water photographers there has also been an increase in wildlife again. We have even seen some Emperor penguins, as well as the more common Adelie penguins, and there was a pod of up to 24 orcas around the ship last night.

Emperor Penguin. [Adrian Bass]
Orcas. [Adrian Bass]

Some of you may have noticed that we crossed the Antarctic Circle yesterday. This is the southern-most latitude where, on the longest day in summer, the sun stays above the horizon.  Unfortunately we don’t have 24 hours daylight as we are well past the summer solstice. This makes it a lot harder for the officers on the bridge, trying to steer the ship in between the sea ice in the dark.