Monday, 25 February 2013

17 - Geostrauphic wonders

A rare fish at the vent site
As our time at the deepest vents draws to a close, we reflect on what we have learned during these past nights. I say ‘nights’ as this is how it feels; immersed in the perpetual darkness of the abyssal depths, cameras for our eyes, the only light is that being reflected back as Isis picks out the colourful hues and dark shadows of the Beebe Vent Field.

The colours at the vent site are stunning
As geologists, we have wandered the furthest from the main vent site, mapping the surrounding hills and valleys. Here, we have found a landscape of volcanoes and lavas. Pelagic sediment sits like a recent snow-fall, picking out the texture and detail of the rocky surface in brilliant white filaments of chalk. Striking like dark streaks across this alpine scene are black fissures: cracks forming the plate boundary where the seafloor drops vertically away into the darkness below. Not even our most powerful lights can penetrate here, leaving the bottom a mysterious realm of shadows and gloom.

At the astonishing depth of 5080 metres, in the bottom of the deepest valley, we find a carpet of bright orange mud. Such a startling contrast in colour means only one thing: even down here, the minerals falling out from the distant vent field are having a profound impact. The orange colour is the result of iron. Spewed from the hydrothermal waters at over 400°C, the iron oxidises rapidly and falls as rust onto the seabed below. The accumulations here speak of thousands of years of fall-out.

Up slope, the seafloor shows signs of past catastrophes. Sink holes appear where earthquakes have shifted the rocks below and the rusty sediment has sunk to fill the resulting holes. A little further up the slope we are met by huge blocks of sulfide perched precariously on top of each other, teetering on the brink of the abyss. Some are as large as a bus, massive blocky lumps, with rusty scree in between, from which shimmering water seeps.

The colours here are amongst the most amazing sights: oranges and reds from the abundant iron, but also peacock hues of green, blue and purple: sure signs that copper is also in abundance. In places, green ‘stalagmites’ cling precariously to the rocky overhangs. Formed from a copper mineral called ‘atacamite’, here the copper is literally leaking out of the rock. This is an amazing sight to us as it confirms one of the hypotheses that bought us here: that the hydrothermal minerals at these depths and high temperatures will be rich in copper. Back on the ship, these rocks are indeed like peacocks: their vibrant colours attract everyone’s attentions and, for the first time, compete on an even footing with the biology for being the most photogenic.

The science party relaxing at sunset on deck after completion of dives.

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