Real-time observation of dispersed hydrothermal plumes using nephelometry: examples from the Mid-Atlantic Ridge

Abstract

As part of the 1984–1985 NOAA VENTS program on the Mid-Atlantic Ridge, nephelometry was used to provide real-time detection and tracking of dispersed hydrothermal plumes. At all nine 1984 study sites, hydrothermal activity was detected by in-situ, real-time nephelometer measurements and later confirmed by dissolved Mn and particulate Fe measurements. These same techniques were employed in a site-specific survey of the Trans-Atlantic Geotraverse (TAG) area in 1985 where large water-column anomalies in turbidity and in dissolved Mn helped lead to the discovery of high-temperature black smokers. The optical response of the nephelometer was to hydrothermally-derived particulate matter. Thus strong correlations existed between the nephelometer readings and total suspended matter ( r = 0.98, n = 34), and particulate Fe ( r = 0.88, n = 32). In addition, digital nephelometer data correlated well with dissolved Mn ( r = 0.88; n = 78) throughout a large concentration range (0.2–31.0 nmol/kg). These data provide good evidence for the utility of in-situ nephelometer measurements for locating and surveying plumes from hydrothermal vents. It also appears possible, within limits, to predict concentrations of in-situ total suspended matter, of particulate Fe and of dissolved Mn.