This is Laurel Childress reporting in on gravity cores. As part of my research to investigate the offshore and down-canyon transport of organic carbon we will be collecting several gravity and multi-cores from the upper, middle, and lower reaches of Astoria Canyon. Gravity cores are a tool we use to collect sediment from the deep sea. We lower a 10′ core tube on a line to the bottom and a set of weights drives the tube into the ocean floor. Unlike the multicorer, which deploys up to eight short tubes at a time, the gravity core brings back one long core.
The first gravity core was collected Wednesday, September 21 at Station AC20. This station is located near the bottom of the canyon, and is the gravity core taken furthest from shore at 48 nautical miles offshore the mouth of the Columbia River. Coring was conducted in approximately 1650 meters water depth, however due to a mixture of very fine grained surface mud, large swells, and previous failed attempts to multi-core we were unsure of the recovery. The result was initially perplexing, as we obviously had good penetration, but did not seem to have recovered much sediment (only 81 cm). We split the core in two with one half to be archived at OSU and the other half photographed, described, sub-sampled and frozen for later analysis.
Upon examining the split core the reason for our short core became immediately apparent; multiple layers of very coarse, up to pebble sized material. Based on the history of the canyon and we suspect these are turbidite layers, most likely associated with a major event in the Columbia River basin. Turbidites are rocks, sediment, and other debris that gets shaken loose during an earthquake or similar major event and tumble down slope. Turbidite layers are useful because they are distinct events in the geological record that can be correlated across regions giving a time thumbprint to the sediment. Further coring in the upper reaches of the canyon will occur in the coming days and will hopefully enlighten us further on the offshore reach of such event layers.