White Paper Title: 
Tidal-scale variations in plumes and their implications for estimating vent-scale heat fluxes

The patterns of bending and variations in entrainment rates observed over a 24 hour time series of acoustic images of the plume rising from the north tower of Grotto Vent, which was reported in Rona et al. (2006), suggest a strong influence of the tidal cycle on plumes but also significant complexity. This complexity comes in several ways: 1) The bending of the plumes is stronger in the southern direction. 2) The timing of the bending of the plumes does not seem to be in phase with the tidal cycle. 3) The relationship of plume strength, plume bending and entrainment rate is not entirely consistent.

While more data is needed to verify the above interpretation (see Rona's white paper on acquiring data), the context and implications for the behavior of the rising plume merits consideration: Grotto Vent, like many of the vents on the Endeavour segment of the Juan de Fuca Ridge, is a large (10+ m tall) mound, which has many places discharging very hot fluids, including black smokers, flanges, beehives, and white smokers as well as diffuse fluids emanating from every surface. Furthermore, Grotto Vent is actually 2 separate mounds in close proximity: the taller but narrower north tower, which has the strongest black smokers based on backscatter intensity in acoustic images, and shorter but wider and roughly peanut shaped south mound, which seems to mostly diffuse flow with a few black smokers. The direction of bending may influence which sources merge into the main plume(s), which would change the rise rate and the impact of the tidal currents on entrainment and bending leading to the observed complexity.

In contrast, on the EPR, there are single plumes discharging from isolated black smokers, or at least that's what many discussions imply. In the only acoustic imaging data taken on the EPR (Rona et al., 1991; Bemis et al., 2002), we saw what started as two very separate black smoker plumes (4m separation at base) merge at 18 mab. So even here, relationships between source, tide and plume are complex.

An effective conceptual model relating the behavior of the rising plume to its source(s) needs to account for a) the size of the area that contributes heat to plume, b) the multiplicity of heat sources, c) the non-linear addition of momentum when plumes merge (which results in an increase in the rise rate greater than might be anticipated), and d) the response of the plume to changes in tidal current speed and direction.

I propose a new conceptual model: First, the vent or mound is a diffuser plate (a grate with flow and heat supplied more or less uniformly from below), where it is anticipated that heat comes from an area rather than a point source. Second, when multiple mounds are sufficiently nearby each other, their plumes will interact when tidal currents bring them in close proximity but not necessarily otherwise. Thus interactions are variable over time as well as space. Neither of these aspects makes the calculation of heat flux for a vent field easier -- more data is needed to judge variability in time and to account for interactions. Everything we've learned within the scope of Ridge 2000 suggests that our models and methods of heat flux estimation are too simplistic (see B Lavelle's comments and animation on the R2K 2010 Community meeting wiki site).

The integration of other data and observations should help turn this qualitative model into a quantitative model. And in any case, it should provide a context or stimulus for discussion of how to integrate data and models that aim to estimate heat and chemical fluxes. The merging affects may also have implications for models of how event plumes form.