NOTES ON CRUSTS
(observations from acoustic records, submersible dives, and MaxRover)
Side-scan reflectors were usually rock outcrops that could produce strong acoustic returns.
Rocks are black, sometimes with a dull shine. They have irregular surfaces, jagged/angular edges, and a batryoidal-bumpy-irregular-pitted surface (rounded structures perhaps 5 – 10 cm in diameter) and thus were easy to recognize where partially buried beneath sediment. Their appearance is very similar to pillow lavas. In some areas, it seemed as though a thin sediment cover existed on top of buried crust, implying a more extensive distribution in the subsurface on the sea floor than was apparent (this is also implied by rubble piles of dead coral branches – see notes below on fauna).
In general, rocks form what appeared to be massive layers perhaps 50 – 80 cm thick. Outcrop edges were often undercut, implying these rocks form crusts that sit on top of, or are partially buried in, sediment. Some exposures had vertical and horizontal holes anywhere from a few centimeters to tens of centimeters in diameter, but in general the rock appeared dense. No internal layering or bedding was obvious, although in a few areas what appeared to be slabs perhaps a few centimeters thick were seen. The exception was at location S91 where an exposure was perhaps 1.5 m thick with a near-vertical face. Here vague irregular lenses were present, from decimeters to centimeters in thickness that varied laterally. It is not clear what has caused the vertical cut in the outcrop.
An upper thin (thickness no more than a few centimeters perhaps) brown – yellowish/brown crust was sometimes noticed that may incorporate black clasts (difficult to tell). This upper crust is not always present, suggesting either it is readily altered to black after vent activity has ceased, or is not always produced during venting. Sometimes this crust appeared highly porous with holes a centimeter or so in diameter, suggesting perhaps relict venting pipes.
Rock is very hard. Impact with the submersible manipulator arm did nothing – no breaking, chipping, etc. – and sampling could not be done. There were no small fragments loose on the sea floor to collect (the manipulator arm is difficult to use for this application). At location S91 a sediment sample was taken that contains granule and sand-sized black grains presumably derived from the crusts. The host mud is medium-brown silty clay.
Outcrops of crusts come in various sizes. Some were larger than the footprint of the submersible, others smaller. Side-scan traces suggest that these may form extensive and thick exposures. Outcrops did not seem aligned, although this is difficult to determine from the limited range of view from the submersible, nor could this be determined from acoustic records. Shapes of outcrops also varied from linear to nearly circular to irregular.
What appeared to be active venting was seen at one site. Here cloudy water could be seen with turbulence as indicated by suspended material and some shimmering. Turbulence appeared somewhat constant but in pulses. Venting fluid seemed to remain in the depression suggesting a denser water than ambient bottom sea water. Venting was from a depression perhaps a meter in diameter and 50-70 cm deep, with vertical sides. This hole was surrounded and within a black crust that appeared to be perhaps 10-20 cm thick, the surrounding surface crust irregular in shape and distance from the hole. The bottom of the depression also appeared to be a crust, as though this depression represented a collapse feature in a pre-existing crust (or, alternatively, that crust was actively forming on the floor of the depression). Such large depressions were not noticed at any other exposures in the area, suggesting perhaps that this was an unusual feature; smaller holes seen in the yellowish-brown crust may represent more typical venting conduits.
Murky, cloudy water in an undercut notch noticed at location S91 might indicate active venting there. Alternatively the water may simply represent suspended sediment raised by a large grouper fish that was in the notch.
Fauna surrounding the active vent, and seen on other crust outcrops included: shrimp (dense populations around the active vent; sparse elsewhere as at outcrops), serpulid worm tubes (usually on outcrops but also found on dead coral fragments; they are grey-white tubes about 5 mm in diameter and a few cm long); grouper fish (one was seen to eat a shrimp at the active vent site); coral (rare, except at station S91 where large [~1 m2 or larger] accumulations of intertwined dead branches were seen, a few with a one or two live corals growing on them); and sponges (rare). Of these, the large population of shrimp seemed the best indicator of active venting. In general it appears that sessile animals and plants were simply using outcrops and rubble piles as a substrate rather than indicating active venting of fluids. This is suggested at location S91 where dead coral branches form distinct rubble piles, presumably outlining areas where the substrate is close to the sea floor, now repopulated by only a few new live corals. It is not clear what has caused the mass mortality of these corals. Rubble piles are sometimes surrounded by scattered broken branches, some partially buried in sediment.
A mystery at the active vent was that both shrimp and groupers were in the cloudy water. If this water was methane-rich, it is curious that it was not toxic to these animals. An alternate suggestion might be that the venting fluid is a brine.
Bottom current activity was clear from: scour pits around outcrops; the undercutting of outcrops; coarser sediment adjacent to some exposures suggesting a lag deposit; debris caught against some outcrops; and, the prompt dispersal of suspended sediment raised when towed equipment (such as the Max Rover) hit the sea floor. Velocities appear to be only on the order of a few cm/sec. Velocities are not adequate to remove sediment from mounds surrounding burrows, to form sedimentary structures such as ripple marks; strong currents at the time of the dives were not noticeable from navigating the submersible nor from the tilt of delicate stick-like organic stalks and gorgonian fans. The fine-grained texture of the silty clay sampled at station S91 also indicates a lack of vigorous bottom currents.
These comments are the collective thoughts of all in the scientific party.
