Temporal variability and tidal modulation of hydrothermal exit-fluid temperatures at the Lucky Strike deep-sea vent field, Mid-Atlantic Ridge

We deployed autonomous temperature sensors at black smoker chimneys, cracks, and diffuse flow areas at the Lucky Strike hydrothermal field (Mid-Atlantic Ridge, similar to 37 degrees 17' N) between summer 2009 and summer 2012 and contemporaneously measured tidal pressures and currents as part of the long-term MoMAR experiment to monitor hydrothermal activity. We classify the temperature data according to the hydrogeologic setting of the measurement sites: a high-temperature regime (>190 degrees C) representing discharge of essentially unmixed, primary hydrothermal fluids through chimneys, an intermediate-temperature regime (10-100 degrees C) associated with mixing of primary fluids with cold pore fluids discharging through cracks, and a low-temperature regime (<10 degrees C) associated with a thermal boundary layer forming over bacterial mats associated with diffuse outflow of warm fluids. Temperature records from all the regimes exhibit variations at semi-diurnal tidal periods, and cross-spectral analyses reveal that high-temperature discharge correlates to tidal pressure while low-temperature discharge correlates to tidal currents. Intermediate-temperature discharge exhibits a transitional behavior correlating to both tidal pressure and currents. Episodic perturbations, with transient temperature drops of up to similar to 150 degrees C, which occur in the high-temperature and intermediate-temperature records, are not observed on multiple probes (including nearby probes at the same site), and they are not correlated with microearthquake activity, indicating that the perturbation mechanism is highly localized at the measurement sites within the hydrothermal structures. The average temperature at a given site may increase or decrease at annual time scales, but the average temperature of the hydrothermal field, as a whole, appears to be stable over our 3 year observation period.