Thursday, February 4, 2010

Seasonal algal blooms recorded in an Antarctic sediment core

For the past two days we have been working on collecting a very unique set of cores. We are currently coring into the Adelie Drift, in a small basin on the Antarctic shelf with a water depth of about 1000 m. The basin is filled with organic sediments composed of more than 90 % diatoms, other microscopic algae and zooplankton. That type of sediment is called diatom ooze. In the photo, you can see a part of a core that we processed today. Each light-colored layer represents the cysts left by a spring bloom of microscopic algae, predominantly diatoms. These diatoms bloom when daylight lengthens and when the sea ice starts to break up in the spring, so that nutrients are released into the surface waters. The dark layers in the core are created in the autumn and winter, when the days shorten, the sea freezes over, and eventually the polar night takes over. Besides diatoms, we also found fish bones in the dark layers in the cores, predominantly vertebrae. Each light and dark layer forms a couplet representing one year. Like tree rings, changes in the surface climate conditions affect the thickness and composition of each layer. The Adelie Drift probably contains about 10,000 of these laminae (layers) going back 10,000 years. This is a very valuable record documenting short-term climate change in a very remote place on our planet.

The coring of these sediments is a bit different from the drilling we were doing at the previous site. We are using an APC piston coring system now (see drilling equipment class in an earlier post). It works like a giant syringe where watery mud is extracted out of the sea floor. The first couple of cores were very soupy and accompanied by a very powerful smell of rotten eggs (H2S). We also had some issues with exploding cores from all the gases that were released out of the sediment: there is a huge pressure difference between atmospheric pressure here on deck and the weight of 1000 m of water pressing on the sediment there below. Here is a photo of what is left of a core liner after the core exploded. Luckily no one got hurt, because everyone is wearing protective safety gear.

We were treated with another beautiful day today as well. It is almost unreal: we are in Antarctica and it is supposed to be very cold and windy, but it was just around freezing and in the sun it felt quite pleasant. We did feel the catabatic wind come down from the ice-sheet when the sun was setting, but it wasn't too bad at all. The catabatic wind, is a typical wind of the polar regions, and it develops when air cools over the surface of an ice sheet, becomes dense, and flows out to the coast. The sun is setting at around 11 PM. It does not get completely dark because the sun is only a few degrees below the horizon and the sunset becomes a sunrise within 2 hours. In the photo below the surface above the iceberg is the top of the ice sheet. It is odd to see the sun set above an ice sheet, but it is a magnificent experience!

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