When the Challenger set out in 1872 on its four-year scientific expedition to traverse the oceans of the world, it was major news. Nowadays it requires the landing on a distant planet, or one of those recurrent atom smashing feats of the physicists, to capture the popular imagination. In the meantime, oceanography has continued its steady march forward, and it now involves equipment and techniques just as sophisticated as those employed by NASA.
The techniques employed follow the same principles as those employed by the Challenger crew, only improved through modern sophistication. Sampling of the ocean waters was carried out in the Challenger by lowering equipment fitted with thermometers. Use now is made of CTD’s (standing for conductivity, temperature and depth). It consists of a cylindrical metal frame that is lowered to the ocean depths by a ship crane. The circumference of the cylinder is fitted with Niskin bottles in the formation of a rosette.
The Niskin bottle is designed to collect samples of ocean water. They consist of cylinders with caps open at both ends. The caps contain weights or messengers which are designed to snap at a certain depth (determined by pressure) thus closing the caps. In this way samples are collected at various depths while the CTD is lowered into the ocean.
Even before the samples are recovered, scientists on board the ship are able to take various readings of the ocean water at different depths. These are accomplished through sensors fitted to the CTDs, and transmitted back via the lowering cable to the on-board computer. The conductivity of the water measures the salinity, while the pressure measures the depth. Other measurements include temperature, oxygenation and pH.
More sophisticated measures include transmissivity, which gives a measure of the floating particles and therefore measures turbidity. PAR measures the radiation available for photosynthesis, while fluorescence measures the level of photosynthesis itself. Sometimes scientists may use these measures to determine where to sample the water from.
A sediment trap is able to examine a wider volume of water. Sediment is allowed to fall into a white metal ring, through a cone into the cod-end, the collection point. Such sediment is called ocean snow. The snow catcher is an oversized Niskin bottle that may retrieve greater volumes of water. The sediment is then allowed to settle at the bottom before its investigation. Such sediment usually includes marine excreta, dead plankton and algae.
Even larger volumes of water may be sampled using the McLane pump. Filters are present at the mouth of the pump where suspended particles gather. These filters are then retrieved for examination. This technique is useful to determine the level of particulate carbon and trace metals.
Extensive examination of the ocean floor would be carried out by the marine geologist and would follow many of the same techniques as on dry land. However, the shallow surface of the ocean bed is usually under the purview of the oceanographer. Frames containing the sampling instruments are lowered to the ocean floor hydraulically. The camel-grab collects samples a few centimeters deep, while the Van Veen grab sampler may penetrate 20 cm. Borers retrieve a column of sediment many meters long and drilling is employed to reach further depths. Benthic landers sit on the ocean bed and transmit information for long-term observation.
Sometimes technology need not improve at all. The Challenger used metal buckets to dredge samples from the ocean floor. Such dredges are still employed in the shallower depths. The only improvements are to ensure that the samples obtained suffer minimal damage and disturbance.
