15 June, 2000

Ocean Going Rock Hounds

Dredge, what an unlikable term. It sounds harsh, and in many ways dredging is a harsh way to sample living organisms, as well as non-living objects at the bottom of the ocean.

Dredging is rarely used by itself for scientific purposes, because it samples only who or what is home at the time of the dredge. In conjunction with other sampling methods, dredging can give a scientist a general idea of the characteristics of a given area.

Basically there are two major types of dredging operations. When living organisms are sampled using this method it is called trawling. Trawling involves dragging a device designed to collect organisms near the bottom of the ocean behind a ship. It may also collect benthic creatures that live in the upper few centimeters of the mud that covers the ocean floor. One type of trawl is called an otter trawl. It has "tickler chains", chains that drag over the ocean floor to stir up organisms. It is also composed of a "tickler board", which channels the organisms into the collecting unit.

Today, we tested the ability of the ship to run a rock dredge. Before we let out the dredge, the scientists double checked the wenches and used an instrument known as SEABEAM to map the floor of the ocean. A rugged terrain could cause the dredge to get hung up, a dangerous proposition when it is attached to the ship by heavy wire cable. To avoid such an event, SEABEAM was used. Basically, the instrumentation sends out a "ping" sound from the ship. Computers on the ship record the time of the ping and the return of its echo from the bottom of the ocean floor. Based on the amount of time it takes the sound to travel to the bottom and back, the depth of the ocean can be determined. As I mentioned earlier, it was important for us to find a fairly level area to dredge. We found such an area with a depth of 1,000 meters (well over 3,000 feet). Rock dredging is done for similar reasons as the trawl. Its purpose is to give an idea of the general geologic makeup of an area, in conjunction with other surveying techniques, like core sampling. This type of survey provides a window into the geologic history and processes of the area, which can answer basic questions. How old are the rocks? Has the area been influenced by volcanism? What classes of rocks are present? For the dredging operation we performed today, we collected a mixture of rock classes. Both sedimentary (made up of sediment "glued" together) and metamorphic (rocks in which the chemical and crystal structure has change due to intense heat and pressure, one rock changing into another) rocks were found. One could guess that the sedimentary rocks, which normally form in shallow water, have been transported to the 1,000-meter depths by the large icebergs, which calve off of glaciers from the enormous ice sheet of Greenland.

Along with the rocks, a few samples of an unknown living organism were found. We also brought up a bit of mud, which was very fined grained, like slip used by pottery makers. During the next leg of the Healy science trials, we will be delving back into the deep to do another dredge and getting knee deep in mud! I can't wait!

A HUGE iceberg we passed while working on the deck. (9)

A SEABEAM image of the ocean floor, the ship, and the ship's path. (1)

Sifting through the mud in search of samples. (7)

Sending the rock dredge down. (3)

An unknown organism brought up from 1,000 meters below the ocean's surface! (8)

Spraying water to separate the items collected from the fine-grained mud. (6)

Bringing the dredge up from the ocean floor. (4)

Dumping the contents of the dredge on deck. (5)