Hydrography and flow in the rift valley of the Mid-Atlantic Ridge

Abstract

Slow-spreading mid-ocean ridges such as the Mid-Atlantic Ridge are characterized by deep axial rift valleys which are isolated from the water on the ridge flanks. Topographic effects therefore have a significant impact on the rift-valley hydrography and dynamics but little is known about the details. Known processes of global importance acting near the axes of midocean ridges include high rates of diapycnal mixing associated with the rough topography and high-temperature hydrothermal circulation, a major source for a number of chemical constituents of the ocean.

Physical data sets from the rift valley of two connected segments of the Mid-Atlantic Ridge, which include the largest known hydrothermal vent field of the Atlantic, were analyzed to investigate the segment-scale hydrography, dynamics and geothermal fluxes. The data include two quasi-synoptic hydrographic and particle plume surveys (one year apart) and one-year-long records from an array of moored current meters.

The hydrographic properties of the rift-valley water were similar during the two surveys, suggesting a stable state characterized by inflow from the eastern ridge flank, unidirectional along-segment flow (directly observed during an entire year), and monotonic along-valley hydrographic gradients consistent with high rates of diapycnal mixing. Geothermal processes do not appear to contribute significantly to these patterns. The data contain signatures
of a range of dynamical processes consistent with high rates of diapycnal mixing, including hydraulically controlled sill flows, topographic lee waves and high-energy tidal flows.

The spatial distribution of the light-scattering anomalies associated with the dispersing hydrothermal particle plume are consistent with the dynamical observations. Close to the vent field the particle distribution is highly inhomogeneous but density-averaged profiles indicate that the mean plume is Gaussian in depth. To quantify the fluxes associated with the hydrothermal plume the corresponding hydrographic anomalies were determined. The complexity of the hydrography within the rift valley precludes the application of "standard" methods ho that a new method had to be developed resulting in the first quantitative hydrographic anomaly measurements of an Atlantic hydrothermal plume. The hydrographic and particle anomalies of this plume are linearly correlated, indicating that the particles behave conservatively in the near field.

Estimates for the heat flux associated with the hydrothermal plume were derived using two established methods, one based on plume-rise modeling and the other on the advection of heat anomalies away from the vent field. Height-of-rise modeling yields values which are an order of magnitude too low because the plume model relies on a point-source assumption which is violated by the geometry of the vent field. The uncertainties associated with the advection method are particularly small at the site studied because of the uni-directionality of the flow field, the small uncertainties of the hydrographic anomaly measurements, and the Gaussian shape of the averaged near-field plume. The resulting estimate for the heat flux associated with the particle plume is 2.5 GW.

Mass and heat budgets of the rift valley indicate that high diapycnal diffusivities are required to account for the hydrographic observations and suggest that a portion of the water flowing along the rift valley may be lost to the overlying water column. Inspired by the observations a simple analytical and numerical model for the flow within the rift valley was developed. The results indicate that the rift valley acts as an efficient low-pass filter with characteristic time scales of weeks to months, providing a plausible explanation for the persistence of the along-segment flow.

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