HEAT FLOW (G. Delisle, M. Zeibig)

Equipment: BGR employs currently two different types of marine heat flow probes – a conventional probe, built after the so-called violin-bow concept and a second probe, specially designed for employment in hard ground situations. As it turned out, the sediments adjacent to the east of Kamchatka Peninsula along the eastern flank of the trench are characterized by rather hard top sediments, which excluded the use of the conventional type marine heat flow probe. For this reason the BGR-“hard ground” heat flow probe turned out to be an indispensable instrument during this cruise.

The “hard ground” heat flow probe features a 2.2 m long sensor rod made of steel with a diameter of 2 cm mounted along the long axis of a cage and held in position by a special mechanism to prevent bending during penetration of hard ground sediments. The necessary force to press the sensor rod into the sediments is provided by a cylinder, which houses lead plates with a total weight of 600 kg and an electronic unit within a pressure vessel with a total weight of additional 144 kg. The purpose of the electronic unit housed in the pressure vessel is to control the data transfer and the measurements. All measured data are transferred via cable in real time online to a laptop PC on board.

All measured data are recorded, stored, digitized and monitored by so-called “intelligent sensor modules” (ISM) installed in the pressure vessel. This technology relies on immediate digitization and downloads of measured values in the memory and enables us to improve the accuracy of measurement to ∼0.002K. All recorded values are sent to an analogue-multiplexer and then to a 16bit-converter. The high accuracy and linearity during A/D-transformation is achieved by the application of the sigma delta method. To further improve the accuracy of the measurements, an arithmetic mean of 20 consecutive measurements per sensor is formed and then accepted as one single measured value.

All specific modules, which control the configuration, linearization and scaling data in the ISM-module, are stored in an EPROM. Storage and display of the measured data is done via a special computer code, stored on PC.

To achieve optimum thermistor calibration, the heat flow probe is stopped slightly above the seafloor on the down trip. After a time period of typically less than 2 minutes, thermal stabilization within ∼0.001K is obtained at all thermistors. It is assumed that the thermistors measure identical seawater temperatures. Recalibration of all thermistors is achieved by using one thermistor as the master sensor, whose measured value is used to calibrate the data measured by the other thermistors.

Following this procedure, the probe is lowered with a velocity of 0.3 m s-1, until penetration of the seafloor by the sensor rod is achieved. The thermal gradient in the sediments is measured continuously for a time period of typically 8 minutes. After this period, the frictional heat component caused by the penetration of the rod into the sediments has decayed to negligible values. Thereafter, a constant electric current is sent through the heating wire for the measurement of the in-situ thermal conductivity (λ). The temperature increase in the metallic rod is inversely proportional to the in-situ thermal conductivity of the adjacent sediments. We have measured the linear T-increase after initial heat-up of the assemblage and will derive λ from this curve.

A patent has been issued for this particular design.

5.4.1. Site selection

Permission to carry out heat flow measurements were restricted by Russian authorities to areas, designated as Area 13 and 14. These areas cover the north-eastern flank and parts of the central portion of the Meiji Seamount. It was therefore attempted to define points of measurement along a profile extending upslope from the lower flank to the near top of the Meiji Seamount. The key intention of this profile was to determine the variability of heat flow in this area. The site selection had to be carried out with limited prior knowledge of the hardness of the sea floor sediments at the sites, since due to time constraints no prior gravity or piston core experiments were carried out at identical positions. However the obvious

relative hardness of the recovered sediments by the first piston core stations (KL9, KL12, KL14), together with estimates based on the Parasound images did cause us to deploy the BGR-hard ground heat flow probe. Later on, the deployment of the conventional heat flow probe of BGR would have been possible in the softer surface sediments on top of the Meiji Seamount. However, to save ship’s time, any equipment change was avoided.

5.4.2. First results

Four heat flow measurements were carried out in Area 13 (north-western flank of Meiji Seamount), where we have measured consistently highly linear temperature gradients (Fig.

5.4.1.). This result implies a priori a high data quality achieved by the employed equipment.

At all four data points we have measured high temperature gradients (and therefore high heat flow in excess of 100 mW/m²) - implying the presence of a regional positive heat flow anomaly.

Fig. 5.4.1.: Thermal gradient determined at position 53° 52'; 163° 48'.

In contrast, the three measurements in Area 14 have consistently shown a slight and systematic deviation of the measured subsurface temperatures from linearity (Fig. 5.4.2.).

Fig. 5.4.2.: Typical measurement in Area 14: Slight deviations of temperature points from linearity were observed at all three stations.

In principle several potential processes can be listed as cause for this deviation: a) erosional/depositional processes, b) periodic fluctuations of sea floor temperatures throughout the year, or c) near surface sediment compaction upon impact of the instrument on the seafloor. Erosional /depositional processes influencing only modestly the near surface temperature distribution can be considered as unlikely. Due to the shallow penetration of the

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observed temperature fluctuation any periodic fluctuation of sea floor temperatures would have to be short-periodic (days/weeks), which represents alike an unlike scenario. The last aspect is considered to be the most likely cause for the observed sinusoidal deviation from linearity. The deviation was only observed in relatively soft sediments (Area 14 vs. Area 13).

It is plausible that the impact upon harder sediments in Area 13 was insufficient to initiate vertical mass movement associated with compaction.

Irrespective of this point, all three data points in Area 14 show considerably lower temperature gradients (and therefore lower heat flow) as the four data points in Area 13.

(Compaction by instrument impact tends to artificially slightly increase the temperature gradient and heat flow, as the sediment temperatures thereafter raise over a shorter distance.)

Approximate heat flow values were calculated with an assumed uniform thermal conductivity value of 0.9 Wm-1K-1. In-situ thermal conductivity values were measured at all stations with the conventional heating experiment. The results will be evaluated in a second step after return to BGR. The heating experiment was regularly performed for a time period of 8 minutes.

Table 5.4.1. shows a summary of the measured heat flow values (based on an approximation of λ) gained during the cruise.

Station T-gradient

(K/m)

Thermal conductivity

(W/mK)

Water depth (m)

Heat flow (mW/m²)

Area Lat. N Long. E

HF17 0.131 0.9 5285 118.7 13 54° 163° 20'

HF25 0.112 0.9 4994 101 13 54 °5' 163° 37'

HF27 0.187 0.9 5471 168.4 13 54° 9.4' 163° 36.6'

HF29 0.268 0.9 3891 241.5 13 53° 52' 163° 48'

HF34 0.108 0.9 2996 97.5 14 53° 15.45' 164° 17.5'

HF36 0.058 0.9 3223 52.4 14 53° 7.16' 164° 34.4'

HF38 0.066 0.9 3205 60.1 14 53° 11.3' 165° 5.48'

Table 5.4.1.: Summary of measured heat flow values.

Fig. 5.4.3. shows the measured heat flow values in Area 13 in context with the geologic situation. By chance three of the four points of measurement were placed at what appears to be in positions near major faults according to detailed seafloor mapping by the SIMRAD E 120 system of RV SONNE. Notably, the three highest heat flow values were measured near fault positions, while the lowest value was obtained in a ridge position away from faults.

Fig. 5.4.3.: Measured heat flow values in Area 13.

This result can be considered to be an indication that the observed high heat flow is caused by fault-controlled upward migrating fluid flow.

Fig. 5.4.4. shows the heat flow distribution in the whole area with the inclusion of our measurements. Our 4 data points of Area 13 define the major positive heat flow anomaly around 54°N, 163° 30’ E, while our three measurements in Area 14 are located on the eastern flank of this anomaly:

They are positioned within the area of steady decline of heat flow towards the centre of the Meiji Seamount. It appears that high heat flow is concentrated along the north-western flank of the seamount.

The shape of the anomaly is not necessarily indicative of a shallow emplaced magma body as cause of the anomaly. A preferable interpretation of the observed heat flow distribution is given by the assumption of fluid circulation in highly fractured rock within the descending limb of the Meiji Seamount, associated with convective upward transport of heat from depth.

This aspect will be further studied with the help of a numerical model on conductive/convective heat transport in the area of investigation.

Fig. 5.4.4.: Heat flow distribution offshore Kamchatka.