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Walvis Ridge MV1203 Expedition

Daily Science Report 9 -- Sunday, 19 February 2012


After just over a week on the R/V Melville we have named nine new seamounts, six of which have been dredged for rocks. Since we are sailing onboard the R/V Melville and are dredging the 'whale' ridge (Walvis is the Afrikaanse word for whale), we have decided to name seamounts after characters in Moby Dick. For example, Ishmael, Ahab, and Pequod. Once this list has been exhausted, naming will continue with names of whales, old whaling ships, former whaling ports and other words relating to whales and the history of whaling.

Dredging involves towing a large, heavy iron dredge bucket behind the ship on a very long cable that can extend down to 5 km depth, although typically we have been starting at 2-3 km water depth. The first six dredges of the cruise, although on the whole somewhat disappointing, have yielded several rocks that may be useful for determining the geochemistry and age of each seamount. During the very first dredge the dredge bucket became stuck and was broken off. The ship carries six dredge buckets onboard, so a new dredge was attached to the wire. The second and third dredges brought up a handful of useful rocks, although mostly only dark manganese rinds (more about these in a later post). The fourth and fifth dredges were more successful and we spent many hours sawing and describing rocks. Unfortunately many of these rocks are quite old and thus not well-preserved. Over time seawater can 'alter' rocks, which means that the original mineralogy and chemistry of the rock is changed, often into weaker, fractured pieces and soft clays. These are not useful because they no longer record the conditions under which the rocks originally formed, which is what we are interested in.

Some unusual rocks and samples were collected in all of the dredges. Several small pieces of coral and an entire horn coral were retrieved. On one dredge, a starfish was found in the bucket. In several places pumice (a highly vesicular, light-colored rock) was found. This is very unusual as such rocks tend to come from volcanoes in volcanic arcs, such as Japan and New Zealand. The Walvis ridge is not located anywhere near one of these arcs. The closest possible arc location is the South Sandwich Islands, which are located roughly 2000 km to the Southwest of the Walvis Ridge. One possible explanation is that these pumice pieces may have been carried by currents and eventually deposited in the South Atlantic.

Susan Schnur