Insights into tide-related variability at seafloor hydrothermal vents from time-series temperature measurements

Thermocouple/thermistor array packages and an in situ gamma detector were deployed in 1994 at two vent sites on the northern Cleft Segment of the Juan de Fuca Ridge. Continuous records of fluid temperatures were obtained in four separate locations over a period of 5.5 months, and these data were supplemented by current meter observations made ∼2.5 km to the south within the axial valley. Temperatures measured at a location of focused high temperature flow showed that: (1) the maximum temperature in the chimney was stable and did not exhibit tide-related variability; (2) temperatures within the chimney wall were variable on time-scales of minutes, indicating rapid shifts in amounts of cold seawater or hot vent fluid flowing across chimney walls; and (3) the stable maximum temperature within the chimney conduit was ∼9°C less than the maximum fluid temperature recorded at the vent site during the same time interval, and thus stable high temperatures within chimneys are not necessarily indicative of the maximum temperature within the vent structure or the hydrothermal system. Time-series records from areas of diffuse flow indicate modulation of temperature and total radioactivity by tidally induced changes in bottom currents. Spectra of the current meter record and temperature records are similar, with spectral peaks observed at 12.4 h, 16–17 h (inertial peak) and 4–5 days. Phases between maxima in current, tide and temperature records are consistent with temperature changes resulting from periodic shifts in currents from north to south, and the subsequent northward or southward advection of warm fluids venting from multiple local sources. Periodic (12.4 h) variability of temperature was also recorded by a thermocouple buried ∼1 cm within one of the deposits and is likely a result of periodic variations in the temperature at the boundary of the highly conductive sulfide deposit. The time-series results presented demonstrate the need for measuring and considering the effects of local currents when investigating causes of temporal variability within seafloor hydrothermal systems.