Great success with new corer
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!
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 Portner