Day 6: Great success with new corer
Wednesday, August 28, 2013
Today's ROV operations resulted in a plethora of samples: 20 rock samples, 14 short sediment push cores, and six long piston cores. The objective of today's dive was to sample the thick sedimentary section and underlying lava flows on the northwestern caldera rim. We have made several efforts over the last seven years to sample this section at other points around the caldera rim and had many difficulties. We could only recover a fraction of the full thickness of the deposit: the core tubes might be inserted two meters into the sediment but would only recover 25 centimeters of core.
Several years ago we used a device that causes the core to vibrate to reduce friction while being inserted (vibracore), but we substituted clear core tubes in this device earlier in this expedition. We could see that the sediment was shaken up during vibration and redistributed vertically through the core, and got poor recovery. Using one-meter cores that were pushed into the sediment by the ROV manipulator arm resulted in better, but still poor, recovery.
But now, by employing a newly developed piston core designed by MBARI Mechanical Engineer Francois Cazevane (see his log), we are confident that we have recovered the complete sedimentary section exposed on the northwest caldera rim today. Unlike the traditional push core, the piston core develops suction that keeps the top of the sedimentary surface near the seafloor, so underlying units don't compress and act as a plug or wedge that simply pile drives sediment out of the way at depth.
Pure-black volcanic sand layers on Axial's rim are highly angular and moderately sorted making them highly compressible and difficult to recover. Furthermore, the very loosely bound nature of the black sand makes it susceptible to falling out of the bottom of the core, but our thick core-catchers seem to keep the material inside the core. Due to strong suction developed between the piston and the upper sediment surface, the upper few centimeters become disturbed and redistributed. To overcome this pitfall, we also collected short cores that preserve the primary features of the uppermost 20 centimeters. Almost all of our long piston cores recovered over 50 centimeters of material, nearly their full insertion depth, with the longest being about 65 centimeters. This is a huge improvement from the 20-30 centimeters we generally recovered in the past. Overall a breakthrough from our previous Axial coring woes!
The ROV swing-arm rack of piston cores, after we've sampled six sites on the northwest rim of Axial Volcano. Some cores have visible layers of black volcanic sand. None of the sediments here were as deep as the deepest sediment section we've found at Axial, which is over on the northeast rim and two meters thick (but we had failed to collect it all). Hopefully we have succeeded now in sampling the full depth of this deposit on the norhtwest rim.
Concentric ring faults near the rim of the caldera are unstable zones along which the caldera wall may continue to collapse. The sediment off to the right in this photo blankets the flanks of this volcano and is our target for coring.
So why are these cores so important you might ask? Well the sedimentary section on Axial seamount contains alternating layers of muddy volcanic sand, pure volcanic sand, and hydrothermal sand units. The associations of these distinct layers, their physical characteristics, and chemical compositions contain important clues about explosive eruptions on Axial and the evolution of the volcano. Furthermore, the geologic community's understanding of deep-sea volcanic explosivity is still in its infancy, and the cores collected from Axial will offer many opportunities for studies that will expand our knowledge of explosive volcanism along the mid-ocean ridge system.
Ryan is on the after-dive wax-tipped rock corer deployment team, and proudly shows off a perfect sample of black volcanic glass embedded in surfboard wax in the nose cone.
(A) Drained, collapsed lava channel once was full of molten lava under the cooling crust of an inflated lobate flow. As the flow progressed, lava drained downhill. The level in the channel subsided and the lobate flow roof collapsed, leaving lava veneer and shelves of subsequent chilled roof levels stuck to the walls as it drained, like bathtub rings. (B) Closeup of lava shelves shows lava drips hanging from a shelf, vaguely reminiscent of teeth on a monster. Red laser dots are 29 centimeters apart for scale.
— Ryan Portner