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The world’s clearest marine water, Secchi discs, and the JOIDES Resolution

The center of the South Pacific Gyre purportedly holds the world’s clearest marine water (Claustre et al., 2008). The elliptical center of the gyre between 20°–30°S and 98°–122°W is largely devoid of the suspended solids (i.e., particulate organic matter, plankton tests/frustules, and clay) that limit the transparency of surface marine water (Jerov, 1968; Gordon and McCluney, 1975). A variety of optical measurements performed by oceanographers during the past 20 y confirm the clarity of the gyre’s center (see Claustre et al., 2008). For example, Claustre et al., (1999) and Morel et al. (2007) show that photosynthetic-available radiation, the range of light between 400 and 700 nm used by autotrophic organisms, penetrates to a minimum depth of 100–150 meters below sea level (mbsl). The extremely low abundances of organic matter and planktonic tests/frustules are also confirmed by in situ and remote-sensing ocean color sensors that report maximum seasonal total chlorophyll concentrations of 0.02 mg/m3 for November–January and 0.045 mg/m3 for June–August (Fougnie et al., 2002; Morel and Maritorena, 2001).

The absence of suspended terrigenous solids in the water column is inferred from the exceedingly low accumulation rate of clay particles in seafloor sediment underlying the center of the gyre. Scientific drilling into the seafloor near the center of the gyre recovered 14 m of calcareous nannofossil ooze deposited on 4 Ma basaltic crust (see the “Site U1368” chapter [Expedition 329 Scientists, 2011]). Microfossils showed limited dissolution effects, and thus it is likely that the recovered sediment accurately depicts total sediment flux to the seafloor. Clay content within the ooze is between 0% and 10%. Thus, the maximum decompacted accumulation rate of clay on the seafloor, a proxy for the concentration of clay particles in the overlying water column, is ~0.4 µm/y. Given that the clay particles accumulating on the seafloor during any year during the past 4 m.y. are distributed through a column of water >2500 m thick, it is fair to assume that the concentration of clay in any cubic meter of the water column is practically zero. It should be noted that other scientific coring results have been interpreted to indicate that the flux of clay to the seafloor in the center of the gyre could be zero (Rea et al., 2006).

The Secchi disc is a tool for measuring water transparency. For over a century, the device has been used in pelagic environments to assess surface productivity and overall water purity (Cialdi, 1866; Boguslawski and Krümmel, 1907; Clarke, 1939; Riley, 1957). The disc is continuously lowered into water, and the depth at which visual contact is lost is recorded. The disc is lowered to an unspecified additional depth and then raised until it becomes visually recognized. The depth at which it becomes recognized is also recorded. The average of the depth at which visual contact is lost during lowering and the depth at which it becomes visible during raising is called the Secchi disc transparency (SDT) (Tyler, 1968). Although the method is highly subjective, its widespread and long-term use makes it a staple of hydrologic investigations (Bukata et al., 1988). The results are very useful for tracking temporal changes in water transparency at designated locations that are revisited by a common set of observers (see​programs/​wq/​plants/​management/​joysmanual/​4secchi.html). For example, historical records of Secchi disc measurements have been used to determine how phytoplankton in the North Atlantic have responded to increases in anthropogenic CO2 in the past century (Falkowski and Wilson, 1992). However, the subjectivity of the method and variability in factors that impact overall water transparency makes comparisons of SDTs from different water bodies untenable.

The R/V JOIDES Resolution is an ideal platform for deploying a Secchi disc. The ship regularly circumnavigates the world oceans and positions itself far from coastlines for periods from 2 to 60 days. As an integral component of its drilling mission, the position of the ship is known precisely and heave-related effects are intentionally minimized. Finally, the ship is equipped with hydraulic cranes and winches for raising and lowering a large-scale Secchi disc and a deployable small watercraft for positioning observers near the water surface. Therefore, the JOIDES Resolution represents a near-ideal marine platform for traveling the world oceans and acquiring SDT data.

This paper provides details pertaining to the construction of two new Secchi discs intended for use by staff of the JOIDES Resolution during future expeditions. It also documents the results of the initial deployments of the discs during Expedition 329 in the center of the South Pacific Gyre, the world’s “clearest” marine water.