CD179: Deep-sea biology of the Portguese canyons


Daily diary

Tuesday 2 May 2006

Raquel writes:

"Yesterday was the halfway point of the cruise. We stopped at Lisbon to exchange some scientists for new ones: Sarah, John and Veerle left us, as did Terry and Ian, and we got Doug, Nina, Emily and Jeroen instead. It was a lovely sunny day although a bit too hot, which made you realise the big temperature difference between land and sea. You could really feel the heat coming off the land! Luckily, we weren’t there for long and were soon steaming back out to the canyons. Wasting as little time as possible, we got a successful megacore in for the night watch, and deployed the amphipod trap with some fresh mackerel in there this time, after learning the lesson from the unsuccessful smoked kipper episode!

Today started with finishing off the second megacore of the night watch which overlapped with the day watch, and we worked in a big team to finish it off quickly. Later on that afternoon we had a piston core deployment which went very well. Although the trigger core returned empty, we managed to recover around 6 m of densely laminated mud. It was a very interesting core because it showed a gradual transition between infrequent but large sandy turbidity current events to much more frequent smaller (and therefore thinner) turbidites (Figure 1). We had textbook examples of several sedimentary features, including flame structures (Figure 2- where sand gets loaded by mud and the sand gets pushed up into flame-like structures), basal erosion leading to mud being incorporated into the flow (also Figure 2- which over time leads to what are known as rip-up clasts, or clasts of mud in the base of the flow), irregular erosive contacts, grading within the beds, and a small but impressive clayey-shelly-gravelly debris flow to top it all off (Figure 3). So I was very happy with it! :o)

1) Although the core is not complete,you can see the increase in number of turbidites up the core with time (from top right to bottom left) and decrease in their size.

2) The top of this sandy turbidite is on the left and the bottom is on the right. Note the clear flame structure on the top contact, and the erosive bottom contact incorporating mud into the flow

3) The clay-, gravel- and shell-rich debris flow at the bottom of section 1 (also seen in Figure 1)

As I write this, the switch from day to night watch has taken place in the middle of a megacore recovery, so the day watch obtained the cores from the lander and sliced each one into several 1, 2 and 5 cm packages. Then the night watch came out and sieved these packages to two grain-size fractions to look at the different macrofauna species living in the top 20 cm of the canyon thalwegs. Overall, a very successful and sunny day!

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April 2006
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