Oceanography 540--Marine Geological Processes--Winter Quarter 2001

Periodic Phenomena in Vent Environments

One of the more intriguing observations made in vent environments is that temperatures and flow show periodic fluctuations inferred to be tidally forced. At present there is no accepted explanation of this relationship. Indeed, there are question whether the observations reprsent basic characteristics of vent processes or simply reflect the influence of tidal currents on the measurement apparatus.

Tidal Forcing

For a comprehensive discussion of tidal generation refer to a standard physical oceanography text, e.g. (46). The periodic characteristics of the tide are related to the interaction of astronomical periodic behavior controlling the spacing and orientation of the Earth, Moon, and Sun:
Earth spin24 h
Moon's orbit27.32158 d
Earth's orbit365.2422 d
Moon's nodal precesion18.613 y
Variation of Moon's perigee8.861 y
Variation of Earth's perihelion20940 y

The forcing can be related to a set of additive tidal constituents (366 of them), the most important of these being:
K (cm)
M2main lunar semidiurnal12.421 h24.233
K1soli-lunar diurnal23.93 h14.156
S2main solar semidurnal12 h11.284
O1main lunar diurnal25.82 h10.051
P1main solar diurnal24.07 h4.684
Q1elliptical lunar diurnal26.87 h4.684
N2elliptical lunar semidiurnal12.66 h4.640
Mflunar fortnightly13.66 d4.174

Tidal analysis involves interpreting long duration records to establish the phase and amplitude of these constituents at a particular location. Harmonic analysis is used, i.e., constructing a least squares fit to waveforms of known frequency. For details see (47) pg 397-402.

Figure 1 shows the tidal forcing due to the first four constitutents all with phase of zero. The matlab script, tidebuild.m illustrates the progressive contributions of the components starting with the dominant M2 tide.


Figure 1. Tidal forcing from M2, K1, S2 and O1 tidal constituents.

Tidal Currents

In order for sea lvel to change fluid must move from place to place giving rise to tidal currents. In a general sense, tidal currents are related to the first derivative of the tidal amplitude. In the open ocean these currents are rotary. To construct models for tidal currents, records of current direction and speed are interpretted as the additive effects of the underlying tidal constituents.

In the vent environment there is a strong influence of topographic forcing and of frictional boundary effects, especially when there is a well defined rift valley. As an examle consider the character of current in the axial valley near Main Endeavour Field, Juan de Fuca Ridge:


Earth Tides

In response to the gravitation fields of the Moon and Sun, the Earth deforms because it has a certain degree of elasticity. The amplitudes of motion are much smaller than for the water tide, of order 30 cm and in practice only the first four tidal constituents are considered when analyzing the earth tide.


Several mechanism have been suggested that would connect tidal forcing to vent behavior:

Some Representative Observations

As an example of periodic behavior consider these observations from Schultz et al. (47), taken with an electromagnetic flow appartus deployed at the "Peanut" structure of Main Endeavour Field. The appartus consists of a vertical tube sealed by a gasket to the surface of a vent structure, with measurements made of the flow through and temperature in this tube.

Figure 3. From (47)


Figure 4. From (47)

The power spectra clearly show an influence of the M2 tide in all of the records. However an analyis of the coherence between the records demonstrates that most of the coherence between these records is at periods >12 h arguing that while tidal signal are evident, most of the variability is related to long period, low frequency phenomena.

This time series is filtered to pass low frequencies:


Figure 5. From (47)

There appears to be an inverse relatinship between temperature and flow, perhaps with a small lag. There are some plausible physical explanations for this inverse relationship but such explanation are at this point ad hoc in nature.

Tidally related variability is also observed in high temperature, smoker style vents. These data are from the "Cannaport" vent at MEF:

This example is representative of many records where a 12 hr periodicity is present, though not easily interpretable in terms of making a connection between forcing and response.

A more intriguing (and perhpas unique) record comes from observations made in 1995 at MEF at the vent structure named "Puffer", a vent poised on the critical curve (379°C at 220 bars):

The strong excursions in temperature come with a period ~12.4 h, the M2 period, and the strongest excursion is in phase with the full Moon. However in a long time series taken in the following summer only two excursions were recorded. In a dive program conducted the following summer, the pattern was again detected and an ALVIN dive conducted to measure temperature and collect fluids before, during, and after an event. A ~20°C excursion was observed. The entrained fluid, surprisingly, was not seawater, but is consistent with being the conjugate brine of the relatively fresh venting fluids, conductively cooled. The mechanism by which these fluids are entrained is likely due to cracking events of some kind, but it is unclear whether they are induced by the Earth tide or the pressure flucations due to the water tide.

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