Low-Temperature Hydrothermal Plumes in the Near-Bottom Boundary Layer at Endeavour Segment, Juan de Fuca Ridge

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of both high-temperature (Veirs et al., 2006) and diffuse (Kinoshita et al., 1998) plumes within the water column, and have been observed to affect temperature in the immediate vicinity of diffuse vents (Little et al., 1988;Tivey et al., 2002;Sheirer at al., 2006).Here, we describe recent measurements that reveal in greater detail the important role that tidal advection plays in modulating the BBL environment near diffuse hydrothermal plumes.
From 2000-2003, the Thermal Grid project (Johnson et al., 2002; see   et al., 2012, in this issue).Although some fraction of the diffuse effluent becomes entrained relatively quickly into nearby plumes from high-temperature sources, a number of studies suggest that a significant portion flows laterally as discrete low-level plumes that remain detectable downstream for considerable distances (Trivett and Williams, 1994;Kinoshita et al., 1998, Veirs et al., 2006).pers. comm., 2011), and is comparable to an estimate of heat flowing laterally below 75 m altitude (Veirs et al., 2006).Venting at sites in between the principal vent fields, such as those we sampled at Beach, Clam Bed, and Raven, as well as other low-temperature sites yet to be discovered, augment the total diffuse hydrothermal heat flow from the segment.While these observations, made over short windows of several days, provide insight into the effects of tidal currents on diffuse plumes in the near-bottom  2 but for a deployment at the base of the talus slope about 34 m north of Grotto/Lobo vent, with the velocity sensor (white "ski-pole" end) sitting above the north flank of a small mound venting shimmering water and covered with tubeworms.The (usually) upper thermistor (blue) was unintentionally wrapped around the current meter and recorded values at a lower altitude than 0.5 m (green). in this location, the tide does not reverse the prevailing northward current, but periods of greatest heat flux are observed when the current is weakest, less than 1 cm s -1 to the north, and the buoyant plume can rise more vertically.The record mean heat flux is 3.4 ± 1.1 kw m -2 .However, it is likely that the instrument did not sample the maximum heat flux-a temperature reading at the top of the mound reached 35°-40°c.

Figure 1 )
Figure 1) used MAVS3 acoustic current meters, equipped with one-meter-long thermistor strings, to collect multiday time series of near-bottom temperature, vertical temperature gradient, threecomponent velocity, and turbulent heat flux.Sixteen low-temperature vent sites were sampled (see examples in Figures 2 to 4), ranging from South Main Endeavour to High Rise vent fields, along with a central axial valley control site where the near-bottom temperature is homogeneous within the thermistors' accuracy of 0.02°C.Current

Figure 1 .
Figure 1.Map of the area of Endeavour Segment of the Juan de Fuca Ridge studied during the 2000-2003 Thermal Grid project, along with locations of instruments shown in Figures 2-4.The color bar indicates depth (m), determined by a Jason 2 SM2000 swath bathymetry survey using a systematic grid.For more details, seeJohnson et al. (2002).

Figure 2 .
Figure2.Record from a site located about 40 m west-southwest of the black smoker Hulk. on September 30, 2000, the instrument was deployed 7 m along bearing 304° from the diffusely venting crack shown at the left.Time (x-axis) is in GMT.The upper panel shows temperature at the bottom (red), at 0.5 m altitude (green), and at 1 m altitude (blue).Heat flux (center panel) is calculated via direct correlation, using ~ 2 Hz vertical velocity and temperature data, averaged over 17 minute ensembles.data for all panels of the figure are further smoothed with a running mean over 10 ensembles.The lower panel shows horizontal current speed, color-coded by direction (°T = degrees clockwise from north), and vertical velocity (black).at this site, the current record is dominated by the semidiurnal tide with flow alternating direction along 335° ± 9° during four periods of strong flow each day.Vertical velocity is also tidally modulated due to reversing flow oriented along a sloping bottom.water from the diffuse source is swept past the sensor when the tidal current is to the northwest, resulting in a 0.05-0.2°cincrease in temperature throughout the BBL, and vertical turbulent heat flux values of 0.1-0.5 kw m -2 .The record mean heat flux (with 95% confidence limit error using number of degrees of freedom determined from the integral time scale at the control site) is 0.04 ± 0.04 kw m -2 .

Figure 3 .
Figure3.information same as Figure2but for a deployment at the base of the talus slope about 34 m north of Grotto/Lobo vent, with the velocity sensor (white "ski-pole" end) sitting above the north flank of a small mound venting shimmering water and covered with tubeworms.The (usually) upper thermistor (blue) was unintentionally wrapped around the current meter and recorded values at a lower altitude than 0.5 m (green). in this location, the tide does not reverse the prevailing northward current, but periods of greatest heat flux are observed when the current is weakest, less than 1 cm s -1 to the north, and the buoyant plume can rise more vertically.The record mean heat flux is 3.4 ± 1.1 kw m -2 .However, it is likely that the instrument did not sample the maximum heat flux-a temperature reading at the top of the mound reached 35°-40°c.
to understand the physical controls on lower water column variability.And, many other factors are expected to be at play on different timescales, including tidal variability of the hydrothermal sources themselves, longer-period current variability, and the geophysical processes that control both the slow evolution/decay of diffuse vent systems and their responses to episodic seismic events.Much more detailed discussion of the Thermal Grid project can be found in PhD dissertations by Matt Pruis (2004) and Irene García-Berdeal (2006), which can be accessed from the University of Washington ResearchWorks archive at: https://digital.lib.washington.edu/researchworks.ackNowLEdGEMENTS This work would not have been possible without the scientists and crew of R/V Thompson and ROV Jason 2. Funding was provided by National Science Foundation grants OCE-9911523 and OCE-0085615 and the University of Washington Royalty Research Fund.
Figure 4. information same as Figure 2 but for the strongest diffuse heat flux observed, at a site located approximately 40 m north-northwest of S&M vent, starting on June 23, 2001.The sensor is located 0.5 m above a linear crack oriented along approximately 060°.at this location, the combination of a northward background current and the semidiurnal tide results in only relatively brief periods of southward flow, accompanied by drops of several degrees in bottom temperature and a 50% reduction in the heat flux (record mean = 150 ± 13 kw m -2 ).