A year of mud

5, 4, 3, 2, 1, … 9:13 AM, February 3rd, 2018, the release code is sent. The acoustic release 200 meters below us sends back a faint signal. The rope at the bottom of the surface buoy starts losing tension. The onboard echosounder shows an object rising quickly. All eyes are on the surface waters behind the R/V Sur-Austral. Everyone onboard is silent. Will the sediment trap make its way up to the surface?

After a few moments, a yellow saucer appears in front of the ship. After a year under water, the large float resurfaces. Below it hangs a sequential sediment trap that has been accumulating mud for an entire year. The boat and its crew jump back into action. Within no time the sediment trap is onboard the Sur-Austral, covered in mud.

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The sequential sediment trap after spending a year in Baker fjord.

The 24 bottles in the sediment trap each contain two weeks of sediment accumulation that, together with the surface turbidity sensor data, will allow us to link sediment properties to Baker river discharge. We were hoping to capture a large GLOF (Glacial Lake Outburst Flood) for the Paleo-GLOFs project. Unfortunately, none happened in the last 12 months, which is why, after a day of maintenance to replace batteries, clean sensors and install new bottles, we redeployed the mooring for another year. The sediments that we just collected will allow us to investigate how seasonal differences in annual river discharge are recorded in Baker fjord sediments.

Last year, we also deployed a sediment trap in Steffen fjord for the HYDROPROX and GLADYS projects. This trap was a non-sequential trap located a few kilometers off the mouth of river Huemules. After recovering this trap, we now know that sediment accumulation rate in the fjord is approx. 4cm/year and we are eager to analyze the retrieved “year of mud”. The results will help us understand how sediment delivered from the glacier-fed river Huemules is deposited in the fjord.

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This is what a year of sediment accumulation looks like in Steffen fjord.

To complement our fjord sediment work, we also wanted to sample the main rivers that discharge in the fjords we study. This meant we needed to reach the mouths of the earlier-mentioned river Huemules but also of rivers Bravo and Pascua. For river Huemules, all it took was a quick zodiac ride from the Sur-Austral. For the other two, we needed to take the camioneta (Chilean for pick-up truck) out for a spin towards the Southern Patagonian Icefield. After a beautiful 45-minute ferry crossing, we arrived near sampling location 1 along river Bravo. Seb delicately sampled the finer sediment fraction off of the river bank whilst Loic was struggling with the GPS. Sampling location 2, at the mouth of river Pascua, revealed slightly more difficult to reach. First, the heavy rain of the previous night resulted in a large fallen Nothofagus blocking the only access road. Luckily, the two geolocos were equipped with two highly functional manual chainsaws, aka Swiss knifes. Under pouring rain, we sawed and dragged tree trunks and branches out of the road. After an hour of work, the natural barricade was defeated.

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Above: The fallen Nothofagus barricades the road. Below: After one hour of sawing and removing tree trunks, we could continue our way to river Pascua.

Unknowingly of the upcoming problem, we drove on to the sound of cumbia. When we arrived at a river crossing, the barge that was supposed to take us to the other side, was on the other side, and unoperated. GAME OVER. This means that our Pascua sample will have to wait another year, when we plan to reach the mouth of the river by boat via the fjord, or learn how to operate the river-crossing barge ourselves!

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Above: We are waiting at the rive crossing with the barge on the other side. Below: “Balsa NO operativa” or “Barge unoperated” which meant we could not reach our sampling site.

Despite the rain and the lack of electricity, our week in Tortel was an overall success. We’ll be back next year to sample river Pascua and recover our sediment traps once again.

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