Collecting core samples
August 14, 2013
This morning, we woke up to surprisingly nice weather: the wind was down and the sun out. The waves were still in the two meter range, but much better than yesterday. During the night, the ship had moved to our work site for the morning, the southwest side of the caldera (site C). The crew deployed the remotely operated vehicle (ROV) at 6:23 a.m. and it reached the seafloor at a depth of 1,540 meters (5,052 feet) about an hour later. We did two short dives today in order to view two different sections of the caldera wall for Tom Kwasnitschka’s photogrammetry, and to core more of the thick clastic deposits on the caldera rim—for which we continue to tune our coring method.
We have spent a good portion of four of our first five ROV dives at Axial Seamount trying various tubes, catchers, and techniques to improve core recovery of a one-to-two-meter thick sediment blanket on the rims around the caldera. We have tried to collect in this section several times in the past because the sediment contains evidence of volcanic, hydrothermal, and tectonic activity at the volcano over the last perhaps 30,000 years. The sediment consists primarily of black basalt glass fragments that are mostly sand- to coarse-sand sized. The sand commonly occurs as thick layers of almost pure glass that are uniform in composition (and therefore from single eruptions).
On older volcanoes, such deposits become lithified (turned to rock) as the glass begins to alter to clays that cement the grains together. These samples can be collected using the manipulators, just like lava samples are collected. Here at Axial, the sands are simply loose grains, which is what makes the sediment so difficult to recover in cores.
We collected cores in 30-centimeter polycarbonate tubes and in two-meter aluminum vibracore tubes in 2006, and again in 2009. Despite trying different core catchers and insertion methods, the longest cores recovered were only about 20 to 25 centimeters long. In 2011, we tried one-meter polycarbonate tubes as push cores and recovered up to about 50 to 55 centimeters of the sediment, or nearly half the section present. Radiocarbon dating on calcite skeletons of microfossils in the cores has established that the sediment is 6,000 to 31,000 years old at the bottoms of cores. The oldest age may represent the time when the caldera first formed.
This year we returned with more one-meter polycarbonate push cores as well as new two-meter polycarbonate tubes to try in the vibracorer. The clear tubes allow us to see how much core is recovered while the dive is still underway and methods can then be varied to try to increase the recovery. The vibracorer, with several varieties of tubes and interior treatments, still did not recover cores any longer than the polycarbonate cores that were pushed into the sediment using the manipulator on the ROV. However, we now have several half-meter cores from four new locations around the caldera and plan to add a few more during Leg 3 of the expedition. The layers and compositions of the glasses in all these cores will be compared to understand how the glass particles form, are transported, and are deposited.
The two dives today made for a quick turn-around right at lunchtime, but we were more streamlined about it than we were on our first dive-day when we did the same song and dance. Then, in the evening, we collected three wax-tipped cores to collect lava samples from the northernmost flow arm of the 2011 eruption. The lab is quickly filling up with sediment and rock samples that will keep our team of geologists busy in the lab for many months.
— Dave Clague and François Cazenave