Monday, March 16, 2015

Coring for Crystals


8 March, 2015
19o58.830' N 94o10.587' W
1859 m Depth

Sometimes interesting findings come when the outlook seems least promising. Meteor had completed two TV sled tows over a potentially important site. The swath mapping showed several large circular features that resembled craters and a series of channel-like canyons with bright acoustic reflectors near the seafloor. We thought that there might be extensive asphalts and possibly exposed salt at the seafloor. However, the TV sled results were disappointing. We towed the sled 1.5 m above the seafloor for several hours in two different places without seeing any communities or definitive indication of hydrocarbon seepage. The site seemed really inactive.

The gravity core has to be rotated to bring it onboard after
collecting a core. It is a tricky job to handle the 20' pipe and
the huge weight - especially when the seas are rough.
Weather conditions were still too rough for the ROV diving so Chief Scientist Gerhard Bohrmann scheduled a gravity core targeting one of the areas of high reflectivity. A gravity corer is essentially a huge pipe with a massive weight as the top. Lowered to just above the seafloor and then allowed to sink into the bottom, a spring-like "core catcher" at the end of the pipe retains the round section of sediment that the pipe has penetrated. The German-style gravity corer is an impressive beast - a 12 cm diameter, 6 m long (5 in x 20 ft) pipe with a 2 ton stack of lead disks at the top. We lowered the gravity core 1,860 m down to the bottom and allowed it to penetrate. Then we hauled it back up again - over 3 hours for the round trip. Alas, when we inspected the core back on deck it had only penetrated about 60 cm (less than 2 ft) into the seafloor. Once again, it seemed like our sampling efforts had been defeated. But when the end of the core was examined, Gerhard discovered what had stopped the corer. The end of the core was packed with huge crystals of gypsum. The corer had been stopped by a solid layer of crystals just under the surface of the sediment!

Gerhard Bohrmann, chief scientist, pulling a section of the
gravity corer out for inspection.
This is a fascinating result. The mounds that we are investigating throughout the Chapopote region are formed by salt domes - 170 million year old salt deposits that are forced upward by the weight of accumulating sediments. Gerhard explained that when the salt comes close to the seafloor it dissolves into seawater. When all the common salt (NaCl) has dissolved, what remains is a less water-soluble salt, gypsum (CaSo4). So this layer of crystal, which must be very thick to stop a 2 ton gravity corer, demonstrates that salt was indeed near the seafloor at this site. Gypsum is also a classic cap-rock for petroleum reservoirs. In all our combined years of coring, non of us on board had seen gypsum layers like the ones we found at this seemingly inactive site.


Huge gypsum (CaSo4) crystals recovered from the end of Meteor's first gravity
core of the cruise (with a 5 peso coin for scale). The layer of crystal must have
been really thick to stop the gravity corer. Layers of rock like this are perfect
capping formations for sealing in oil and gas reservoirs. It is remarkable to
find it so close to the surface.


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