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Small disc

  • Date: 13 November 2010
  • Location: 27°55.0024′S, 123°9.6679′W

The first deployment was done at noon in slightly choppy seas. The tarred marlin line was routed through a pulley installed on the end of a Venturo Cranes HT40KXP crane mounted at the starboard aft end of the JOIDES Resolution. The crane was extended to 6.1 m and held ~7.6 m above the sea surface. Three observers were stationed alongside the ship railing (at the same level as the crane) with an additional person reeling out line as needed. The disc was lowered until it was no longer visible by any observer and then reeled back in. A knot was tied in the line where it was wet from immersion. The line was then laid out on the JOIDES Resolution helideck and measured (Fig. F2). We measured 46.5 m from the knot to the Secchi disc.

Large disc

First deployment

  • Date: 16 November 2010
  • Location: 27°54.9916′S, 123°9.6681′W

The first deployment of the 1.2 m disc was a test run to check stability of the large disc and practice the task of hanging the cumbersome object over the side of the ship. With three observers and an operator for the winch, the Secchi disc was deployed to 48 mbsl until the ocean surface conditions (wave chop and glare) made it difficult to make accurate observations. The disc was then retrieved and plans were made to try again when weather conditions were favorable and a larger number of observers could be pressed into service.

Second deployment

  • Date: 18 November 2010
  • Location: 27°54.9920′S 123°9.6561′W

The second deployment of the large disc utilized a dozen observers, including one observer located almost directly above the disc, looking downward (Fig. F3). The disc was deployed ~6 h after sunrise under partly cloudy conditions. The JOIDES Resolution, with its bow pointing east, allowed our observers on the fantail and helideck to have the sun behind them during the period of observation. Once again, however, moderately strong winds created suboptimal surface conditions. In an effort to maximize viewing conditions and reduce surface chop, the JOIDES Resolution master turned off the starboard rear propeller (Fig. F4). Although a lesser amount of chop resulted from this action, the continuing surface disturbances resulting from wind action continued to hinder our ability to decisively resolve the white and black pattern of the Secchi disc depth beyond 61.5 mbsl. To confirm this depth of observation, the disc was lowered an additional 2–3 m and then raised to 61.5 mbsl, where observers were able to positively identify the object.