November 18, Into Orca Basin

The heavy duty duo

Dive 4647. Today is the first dive into Orca Basin, a brine-filled open ocean basin of 400 square kilometers, 2400 m deep at the deepest spot. The bottom 200 meters are anoxic and hypersaline, similar as some Red Sea deep brines. The Orca Basin brines are so dense that Alvin cannot dive very far into them; therefore the sub is loaded with ballast, Pilot Sean Kelley, and two substantial observers, biogeochemists Vladimir Samarkin of the University of Georgia, and Ian MacDonald, who has been diving in Orca Basin twenty years ago while finishing his PhD. Who says you can never dive into the same brine twice? Read Ian’s first hard account of his dive below.

Diving through the brine interface

Alvin sinks fast, 40 meters per second instead of the usual 20 or 25 meters, and soon reaches the pycnocline of Orca Basin – where Alvin passes through delicate stratified layers of fine particles hanging in the water column – and then it stops. The fully weighted sub has neutral buoyancy in the heavy brine, and cannot go deeper. The deepest spot is 2215 meters. From here, Alvin lowers the brine snake (the brine sampler hose) and reaches 2235 m depth.

A pickled tree

A brine profile spanning almost 65 meters across the pycnocline is collected, and then there is some time  for sightseeing. Orca Basin acts as a natural sediment trap for all kinds of strange things; an ancient tree is half-buried in the steep southern slope of the basin, a little above the pycnocline. On his previous dive, Ian collected a brine-pickled, apparently dead crab, which promptly revived when Alvin carried it into less saline waters.

Rich is at a loss for words

Bottle six, the brine bottle from the interface at 2215 meters depth, undergoes strange deformations as it returns to the surface; one end of the brine tube (the white plastic tube) blows up very gradually, and its inner diameter increases until it is much too wide for its cap (the gray disk within the white tube). Rich Viso, the soft-spoken fluid transport expert from Coastal Carolina University, is flabbergasted. Brines are not supposed to do that…

Regardless, we are doing two CTD casts this evening for a more detailed look at the water column and the pycnocline of Orca Basin. With respect to microbiology, Orca Basin is almost entirely unstudied; a great model system for deep, stratified anoxic brines is waiting!

black bilgewater cores of Mordor

Well, the beautiful model system strikes back. The first Orca Basin multicorer comes up at ten in the evening, with eight cores full of the blackest, bubbliest mud that I have ever seen. Be careful with open flames! No cigarettes (that is verboten anyway). Even weirder, this creamy soft gunk does not smell at all. The MUC team (previously introduced) jumps into action: core sectioning, sampling, porewater centrifugation and geochemical assays keep us busy until 3:30 am, always in the hope that there is actually a gradient that can be measured. Oh well, one can always sleep in Alvin…

Here, the original report from Ian MacDonald, with first-hand background info on Orca Basin.

Yesterday I had a dive in the Orca Basin.  This is the huge
brine-filled “lake” I dived with ALVIN into back in 1990.  It reminded me how
much I like being in the submarine as opposed to looking at the video
screens while a robot drives around looking.  Even tho the seating in
ALVIN is awful (I have recently developed tendinitis in my right knee
from sitting in a weird angle to wrap myself around the pilot while
looking out the window.), and you really don’t have a good  view,
there is an immediacy to the experience that is irreplaceable.

Orca is 2489m deep; a 200-m layer of super salty brine (7x the
salinity of seawater) fills the basin like grenadine  in
the bottom of a cocktail glass.  In a profile, the salinity starts to
gradually rise at about 2150.  At 2200, the salinity shoots up and by
2250 it reaches its 7x maximum.  Last time, we only got to 2215, with
a maximum salinity of about 70.  For my dive, the ALVIN crew put extra
weight on the submarine and took on extra ballast water on the way
down to try and penetrate the layer.  We laid bets on how much deeper
than the record we’d get (my bet 2280).  Going down, we screamed along
at 40 m/min–way faster than the usual 20 m/min.  But when  we hit
2190, the water went murky and we started slowing down; finally we
screeched to a halt at 2215m.  The water was thick with gelatinous
strands of particles–something you’d never notice from an ROV–but
they were motionless.

We lowered our brine snake–so called because it looks like a
plumber’s snake–down an additional 20 m to collect samples of the
deeper brine, but no amount of driving would force the submarine
lower.  We were hundreds of m from the steep southern edge of the
basin.  We drove over on autopilot until we came to a steeply angle
wall.  It was covered in a uniform coating of fine sediments.  As we
drove along, the slope steepened and the fine sediment disappeared,
leaving clay formations like buttresses.  Suddenly we saw a mass of
tubes protruding from the side of the wall–almost like the roots of a
tree.  We drove up to look more closely, and it was a tree, it’s trunk
hidden under the mud, but roots and branches sticking out into the
water.  How old we wondered, and how did it get here 150 miles from
the closest land?

We pumped out ballast water and dropped one of our dive 100 lb
weights–which in regular seawater would have made up really buoyant.
It the brine world, it made the submarine rise up a whopping 22 m.
But that 22 m made a huge difference.  All of the sudden there was
life on the bottom–fish and the burrows of worms–and the water was
full of swimmers.  Little amphipods swimming in backward summer-salts,
delicate garnet jellies with fringes of tentacles, and, my favorite,
slender copepods with long fringed antenna.

We repeated the weight dropping and sampling twice more–rising about
15 m each time–and each time it the character of the water and the
bottom changed radically over just that short space.  That’s very
unusual for the deep sea where typically it takes many tens or
hundreds of meters to notice a change.  These are changes I noticed
because of being in the submarine where the tiny details are
observable and I could feel myself in direct contact with this strange
and fascinating world.