Spatial and Time Series Analysis of Santorini Hydrothermal Waters

Two multi-element preconcentration techniques were applied to the sea-water samples in order to recover simultaneously the transition metal elements of interest from the highly saline matrices. These involved precipitation with rubeanic acid as well as solvent extraction with a diethyl dithiocarbonate reagent. After the preconcentration stage, the samples were analysed by inductively coupled plasma atomic emission spectroscopy, thereby providing multi-element geochemical data for use in the spatial and time series studies.

Results show that Fe, Mn, Zn and Cd are highly enriched in the embayment waters relative to those in the caldera at large, and exhibit considerable variation around the embayment. Absolute values fall within the range of those previously reported, and reflect the degree of mixing of the hydrothermal solutions with normal sea water.

Time series analysis of waters at one vent was undertaken for a period of 7 days in 1982 and 14 days in 1983, both in springtime. During these periods, sampling was carried out on a regular basis. Diurnally the results show that Fe, Mn, Zn and Cd have highest concentrations in the morning and tend to decrease towards the late afternoon. Throughout the 1982 sampling period, the average daily concentrations of Fe and Mn tended to increase, while those of the other elements varied more randomly. Comparing the average element concentrations during both years' sampling it was seen that Zn varied little, but that Fe, Mn and Cd were more than 50% higher in 1983 than in 1982. Geological and meteorological factors are thought to control this variability.

INTRODUCTION

It is well known from laboratory experiments that submarine hydrothermal exhalations encountered at divergent plate boundaries result from sea water-rock interaction under acidic and reducing conditions (Seyfried and Bischoff 1977; Seyfried and Mottl 1982; Rona 1984). Their chemical composition is a function of a number of factors such as the rate of the plate extension, the type of rocks being leached, the nature of the elements being mobilized, the rock-water ratio and the nature, composition and thickness of the sediment column above the basement (Edmond et al. 1979a, b; Von Damm et al. 1985a, b).

Similar processes are also responsible for the formation of hydrothermal solutions released on the sea floor in volcanic island arc regions. In both geotectonic environments the discharge of metal-rich hydrothermal solutions on the sea floor leads to the formation of marine mineral deposits including metal sulphides, iron-rich alumino-silicates, iron oxides and manganese oxides (Rona 1984; Cronan et al. 1982; 1985; Cronan 1986; Von Stackelberg 1985; Varnavas 1989).

The study of these hydrothermal systems not only leads to a better understanding of the processes involved in the genesis of marine mineral deposits, but, in addition, has implications useful in exploration for similar land-based deposits (Varnavas and Cronan 1988).

A considerable number of investigations have been carried out on the Santorini hydrothermal field, some of which include the analysis of hot waters. Smith and Cronan (1975) divided the Santorini hydrothermal field into three major sub-environments:

     (i)   The inner exhalative zone, which comprises the inner parts of the embayments where water depths are generally below 5 m and wave action is limited. Temperatures up to 30° C have been measured in this zone.

     (ii)  The outer exhalative zone which comprises the seaward parts of the embayments with water depths varying between 5 and 30 m. The wave and sea-water circulation here is greater than in the inner exhalative zone.

     (iii) The channel zone is the area between Palaea and Nea Kameni where the water depth is greater than 30 m. The sea-floor sediments in the inner and outer exhalative zones are fine brown-orange iron-rich muds, while in the channel zone they are volcanic sands mixed with shells and other organic detritus.

Chemical analysis of water samples from the inner and outer exhalative zones showed significant enrichments in Fe, Mn, As, Al, Si, Ba, K and Ca and depletions in Na and U relative to normal sea water (Pushkina 1967; 1968; Peeters 1978; Smith and Cronan 1983; Varnavas and Cronan 1988; Halberg et al. 1989). Iron decreased rapidly seawards reaching near normal sea-water values at the boundary between the inner and outer exhalative zones. By contrast, Mn had a lesser gradient, reaching its normal sea-water values at the outer edge of the outer exhalative zone. Zn remained enriched in the outer exhalative zone, while Na was found to increase with increasing pH, and decrease with increasing temperature.

The Santorini metalliferous sediments, formed from precipitation of metals out of the hydrothermal solutions, have attracted the interest of a great number of investigators and more work has been carried out on them than on the waters (Harder 1960; Butuzova 1966; 1969; 1978; Bonatti et al. 1972; Puchelt 1972; Puchelt et al 1973; Petersen and Müller 1978; Schroll 1978; Smith and Cronan 1975; 1978; 1983; Boström and Widenfalk 1984; Varnavas and Cronan 1988; Halberg et al. 1989).

It has been found that Fe is significantly enriched in the sediments close to the vents in the inner exhalative zones and decreases gradually in the outer exhalative zones towards the channel zone. Mo, Hg and, to a lesser extent, Cu are also enriched in the inner exhalative zone sediments, whereas Mn, Zn and V are higher in the outer exhalative zones (Smith and Cronan 1983). More than half of Fe forms minerals such as goethite, siderite and framboidal pyrite. Microbiological processes are thought to participate in the mineral formation (Schroll 1978). Arsenic, although enriched relative to its concentrations in normal sediments, exhibits low values near the vents and increases markedly away from them in both the Palaea Kameni and Nea Kameni embayments. In the channel it is still well above normal values (Varnavas and Cronan 1988). These data support a continuous transport of metals out from the embayments. Manganese and Fe enrichments in surface and subsurface sediments both within and outside the Santorini caldera (Petersen and Müller 1978; Smith and Cronan 1978; Varnavas 1989) may be partly related to transportation of Mn and Fe hydroxides from the embayments out to the open sea.

The petrology and geochemistry of the Santorini metamorphic system have been studied by Davis and Bastas (1978).

Despite the extensive work that has been carried out in the Santorini hydrothermal field, variations in the chemical composition of the hydrothermal waters with time have not previously been studied. In the present work the results of time series analysis of hydrothermal waters undertaken in the Santorini hydrothermal field in 1982 and 1983 are presented, and possible geological, meteorological and oceanographic influences on their chemical variability are discussed. Variations in the composition of hydrothermal waters between different vents have also been studied.

FIELD METHODS

In order to study the areal variations in the chemical composition of Santorini hydrothermal solutions, hot waters were collected in April 1982 from six shallow water hydrothermal vents in the Palaea Kameni embayment. The locations of the vents are shown in Fig. 1. At the deeper sites, sampling was carried out using a reversing water sampling bottle, while at shallow water sites a 500 ml plastic bottle was deployed by hand in order to avoid disturbance of the sea-floor sediment. The distance of the samples from the sea floor was constant in all cases, and it was ensured that hydrothermal gas bubbles were passing into the sampling bottles at the time the water samples were taken.

For the investigations of variations in the chemical composition of Santorini hydrothermal waters with time, hot waters were collected from a strong vent (Vent A in Fig. 1) during the periods from 14.4.1982 until 20.4.1982 and from 14.5.1983 until 27.5.1983. The time of sampling throughout the day is shown in Table 2. All sampling from Vent A for the above purpose was carried out using a bottle which was lowered close to the sea floor and triggered. Its height above the vent was constant.

Immediately after their collection, all water samples were filtered through 0.45 μm Millepore filters and acidified.

ANALYTICAL METHODS

Analyses of the samples was carried out by R. Anderson according to the methods of Anderson (1985) and Anderson et al. (1984). The preconcentration method used was based upon exploratory work by Pahlavanapour et al. (1982). The modified method utilizes rubeanic acid (dithio-oxamide) as the complexing precipitant in conjunction with bismuth as a carrier ion. Reagents used included. (i) rubeanic acid (dithio-oxamide) saturated solution in absolute ethanol; (ii) cresol red indicator: 1% w/v in 0.1M ammonia solution; (iii) concentrated ammonia, hydrochloric acid and nitric acid: Aristar grades used directly or diluted with DIW as appropriate; (These were found to be sufficiently uncontaminated with the metals of interest for direct use in the analysis of our water samples.); (iv) digestion acid: a 50% aqua regia solution containing concentrated hydrochloric acid (37.5 ml) concentrated nitric acid (912.5 ml) and DIW (50 ml); this was used immediately; (v) bismuth solution: 1000 μgml^-1 Spectrosol Bi (III) standard solution; (vi) trace element solutions: 1000 μgml^-1 Spectrosol standard solutions except Ti (IV) which was prepared from its oxalate.

This method worked well for all elements studied except Mn, possibly due to it only forming weak complexes with rubeanic acid. Mn was determined in all samples by a standard solvent extraction technique, which was also used to check the accuracy of the analyses by the rubeanic acid method. Metal determinations were carried out on an ARL 34000 inductively coupled plasma spectrometer (ICPAES). Data processing and instrument control were carried out by a dedicated PDP 11/04 computer. The ICP source was run at a power of 1250 W with a viewing height of 14 mm above the load coil. The nebulizer used was one of the glass concentric type (Meinhardt) taking up 1.0 ml min^-1 of sample with a flow of humidified argon at 1.0 litres min^-1. The sample flow rate was restricted by the use of a flexible polyethylene capillary uptake tube 310 mm long and 0.5 mm i.d. The tip of the nebulizer was washed between samples with 0.5 ml of de-ionized water containing 1% v/v Photo-flo to prevent a build-up of solids at the nebulizer tip, and reduce nebulizer noise. The spray chamber was a double-pass Scott-type and the torch a Fassel-type with argon flows of 121 min^-1 coolant and 0.41 min^-1 plasma gas.

A determination required 20 seconds pre-flush for signal stabilization and 3 x 5 second integrations. Linear calibrations were established for all elements studied.

The preconcentration equipment used comprised a 500 ml Teflon separating funnel, a 0.4 μm 25 mm diameter polycarbonate membrane filter, a 25 mm diameter Swinnex filter holder, a 1 ml Eppendorf pipette tip with 2-3 mm removed from the narrower end, and a waste-water reservoir and vacuum pump.

With the tap closed, a 250 ml water sample was added to the reaction vessel followed by a 250 ml spike of the 1000 μg ml^-1 Bi (III) standard. The reaction vessel was swirled vigorously to disperse the reagent, and this procedure was repeated for each further addition. A 2 ml portion of rubeanic acid solution was added followed by two drops of cresol red indicator. The pH of the contents was adjusted to 8.0-8.5 by the addition in drops of a suitable molarity solution of ammonia with constant swirling. At the end-point the colour changes from yellow to purple. The cap was replaced on the reaction vessel and the mixture set aside for at least one hour before filtration of the contents.

The polycarbonate membrane filter plus precipitate were transferred to a glass culture-tube with Teflon-lined, screw-cap. The precipitate was digested with 1 ml of the 50% aqua regia by warming at 60-80° C for 5-10 minutes until the brown stain on the filter disappeared. The screw-cap was retained during this stage but fractionally loosened to prevent a pressure build-up. A further addition of 1 ml of DIW was made, followed by nebulization into the ICP for quantitative, simultaneous multi-element determination.

RESULTS

The chemical composition of the hydrothermal waters collected in 1982 from six vents in the Palaea Kameni embayment is shown in Table 1 and that of those collected in 1982 and 1983 from Vent A are shown in Table 2.

       Iron:      Iron in the Palaea Kameni hydrothermal vents varies between 2,820 μg/l and 10,100 μg/l, this representing about one thousand- to three thousand-fold enrichment of Fe over normal sea-water values. These values are lower than the Fe concentration in the Nea Kameni hydrothermal waters and are within the range reported previously for the hydrothermal waters of this embayment (Table 1). They are also comparable with the Fe concentration given for hydrothermal vents from other volcanic arcs such as the hot waters from Matupi Harbour (Ferguson and Lambert 1972) and the hot lakes on Hokkaido, Japan (Wauschkuhn et al. 1977; Hariya and Hokkaido 1985). By contrast, they are generally lower than the Fe concentrations in mid-ocean ridge hydrothermal waters such as on the EPR at 21° N, where Fe reaches maximum values of 141,000 μg/l (Von Damm et al. 1985a) and the Red Sea, where the average Fe values have been found to be around 59,000 μg/l (White 1968). However, the concentrations of Fe in the Palaea Kameni vents are comparable with those given for the Guaymas Basin, Gulf of California hydrothermal solutions (Table 1).

It can be seen from Table 1 that significant variations of Fe occur from one vent to another. Smith and Cronan (1983) demonstrated that Fe decreases greatly between the inner exhalative zones and the outer exhalative zones. The present work shows that significant variations of Fe also exist within the inner exhalative zone of Palaea Kameni. Fe tends to be highest in some vents occurring along the northern shoreline of the embayment.

Analysing the variations in Fe with time, it was observed on 14.4.1982 that Fe had its highest value at noon and tended to decrease in the afternoon. In addition, a periodic pattern was observed with a low at about 14.30h and a peak at about 16.00h (Fig. 2). On the following day (24 hours later, 15.4.1982) a similar time variation pattern for Fe was observed except that its absolute values were higher than on the previous day (Fig. 3). On 17.4.1982 Fe continued to increase compared with the previous days, varying between 4,620 μg/l and 7,130 μg/l. In its diurnal variations it tended to increase from 12.00h to about 13.30h when it showed its maximum value, and then from 13.30h to 16.50h it exhibited a cyclical variation pattern and two maxima and two minima.

Throughout the whole 1982 sampling period (14.4.1982-20.4.1982) Fe was seen to increase with time, having its lowest values on 14.4.1982 and highest values on 20.4.1982.

The diurnal variations of Fe in the spring of 1983 are directly comparable with those observed in the spring of 1982 with peaks at about 12.00h, 13.20h and 16.00h and lows at about 12.50h, 14.50h and 16.50h (Fig. 6). However, concentrations of Fe in the waters collected in the spring of 1983 were higher than those collected in 1982 (Fig. 7).

       Manganese:      Manganese in the hydrothermal vents analysed varies between 300 and 612 μg/l, representing about a 20-fold enrichment over normal sea-water values (Riley 1971). As in the case of Fe, Mn values in the Palaea Kameni vents are lower than the Mn values reported for the Nea Kameni vents, but they fall within the Mn range given previously for this embayment. However, in contrast to the case of Fe, the Mn values are significantly lower than concentrations in other volcanic springs such as in Matupi Harbour and the mid-ocean ridge vents (Table 1).

The areal variations of Mn within the inner exhalative zone of Palaea Kameni follow those of Fe, exhibiting the highest values in vents occurring along the northern shoreline of the embayment. However, these vents are characterized by higher Fe/Mn ratios than the remaining vents suggesting a greater supply of excess iron than of excess manganese.

In its 1982 diurnal variations, Mn follows Fe exhibiting a periodic pattern with distinct maxima at about 12.40h, 13.30h and 16.00h and minima at about 13.00h, 14.20h and 17.00h (Fig. 6, 7), with a tendency to decrease in the afternoon. The same diurnal variations were found in the spring of 1983 (Fig. 8) except that on average the absolute values of Mn were higher than those found in 1982 (Fig. 9). The strong similarities existing between the time variations of Fe and Mn are supported by positive correlations found between the two elements in both sets of water samples (1982 and 1983).

       Zinc:      Zinc is highly enriched in the Palaea Kameni hydrothermal waters relative to normal sea water, varying between 8 and 56 μg/l. A Zn enrichment of about 100-fold is found in a vent occurring at the northern shoreline of the embayment, this being comparable with the Zn enrichments reported previously for the Palaea Kameni inner exhalative zone (Smith and Cronan 1983) and being within the Zn variation in Matupi Harbour hydrothermal waters (Table 1). However, it is observed that the vent with the highest concentrations of Fe and Mn is characterized by low Zn values.

In its diurnal variations Zn shows a periodic pattern with maxima near 12.00h, 14.00h and 16.00h and minima at 13.00h, 15.00h and 17.00h (Fig. 10). This pattern was observed both in 1982 and 1983. In both sets of samples no correlation was found between Zn and Fe or Mn. However, Zn, like Fe and Mn, tends to decrease in the afternoon (Fig. 10).

       Cadmium:      No Cd data have been published for the Santorini hydrothermal waters in previous studies. In the vents examined here, Cd varies between 0.5 and 2.3 μg/l, the latter value being about 7.5 times higher than the average Cd value in normal sea water. Also, it is observed that the Palaea Kameni hydrothermal waters contain greater amounts of Cd than the Galapagos Spreading Centre hydrothermal solutions but smaller amounts than East Pacific Rise 21° N and Guaymas Basin hydrothermal springs (Table 1).

Cadmium follows Fe in its geographic and time variations. Highest concentrations are found in vents occurring on the northern shoreline of the embayment, while in its diurnal variations it exhibits a periodic variation pattern with maxima and minima coinciding with those of Fe (Fig. 11). The strong association of Cd with Fe is also demonstrated by a high correlation coefficient found between these two elements.

DISCUSSION

The geochemical data presented in this work demonstrate that there are significant spatial and temporal variations in the composition of Palaea Kameni hydrothermal waters. Iron, Mn and Cd have been found to be most enriched in the vents occurring on the northern shoreline of the embayment, and together with Zn, vary with time.

The process of formation of metal-rich submarine hydrothermal solutions in such environments involves leaching of rocks by sea water under acid and reducing conditions. During this process, the sea water penetrates into rock fractures and on contact with hot rocks is itself heated and becomes enriched in H₂S, presumably by SO₄ = reduction. The H₂S is then thought to be oxidized to sulphuric acid, which subsequently attacks the rocks, leading to the formation of metal-rich hydrothermal solutions (Boström and Widenfalk 1984). On this basis, it could be implied that the geological conditions occurring under the northern shoreline of the embayment are more favourable for a higher degree of leaching than elsewhere. Such conditions could include the presence of greater numbers of faults and fractures than elsewhere under the embayment.

An alternative explanation for the high metal concentrations on the northern shoreline of the Palaea Kameni embayment could be that there is a smaller degree of mixing of the hydrothermal solutions with sea water after their discharge onto the sea floor than elsewhere in the embayment. In this respect, an examination of the meteorological data during the sampling period reveals that the winds were blowing from the north. Under these conditions the northern shore of the embayment would have been protected and therefore the hydrothermal solutions there would have undergone a lower degree of mixing with sea water than elsewhere in the embayment. In addition, the water depth on the northern shoreline of the embayment is shallower than on the opposite shore, which would also have minimized mixing, provided that there was no significant horizontal water circulation.

The investigation of the temporal variations in the chemical composition of the hydrothermal solutions showed the following:

1.  The diurnal metal variations form a periodic pattern.
2. Iron, Mn and Cd behave similarly, exhibiting maxima near 12.00h, 13.30h and 16.00h and minima at 12.50h and 15.00h.
3. Zinc shows a periodic pattern in its diurnal variations but its maxima and minima do not coincide with those of Fe, Mn and Cd.
4. There is a tendency for all metals to decrease in the afternoon, this being most marked in the case of Zn and Cd.
5.  The same diurnal variations were observed both in 1982 and 1983.

In order to interpret the temporal variations in the compositions of the hydrothermal waters, the variations with time of meteorological and oceanographic parameters such as temperature, atmospheric pressure, wind direction and speed, and sea-water levels during the sampling periods have been examined. It is observed that on the 14.4.1982 and 15.4.1982 temperature and atmospheric pressure show periodic variations, exhibiting minima at 24.00h and maxima at 12.00h, decreasing during the sampling period from 12.00h to 17.00h (Fig. 12, 13). It is therefore seen that the maxima in metal concentrations observed at 12.00h coincides with maxima in atmospheric pressure, and their general decrease between 12.00h and 17.00h coincides with a decrease in atmospheric pressure. Possibly atmospheric pressure is affecting the discharge of the hydrothermal solutions.

An examination of sea-water levels in the embayment during the sampling periods reveals that they also exhibit a periodic pattern with a maximum near 12.00h and a minimum near 17.00h (Fig. 12, 13), these coinciding with maxima and minima respectively in metal concentrations. When the sea-water level reaches its maximum, the sea-water movements are at a minimum. Under these conditions, the degree of mixing of the hydrothermal solutions with sea water is low and thus the sampled mixture would be characterized by higher metal concentrations. By contrast, when the sea-water levels reach minimum values, sea-water movements increase, which would lead to a greater dilution of the hydrothermal solutions and lower metal concentrations.

SUMMARY AND CONCLUSIONS

1. The chemical analysis of hydrothermal solutions from Palaea Kameni submarine hydrothermal vents shows that they are markedly enriched in Fe, Mn, Zn and Cd relative to normal sea water.
2. The concentrations of Fe, Mn, Zn and Cd in the hydrothermal waters are comparable with their concentrations in similar hydrothermal vents in other volcanic arcs.
3. Hydrothermal vents on the northern shoreline of the embayment are characterized by higher concentrations of Fe, Mn, Zn and Cd and Fe/Mn ratios, probably as a result of lower mixing of the hydrothermal solutions with sea water than elsewhere in the embayment. This conclusion is supported by the observation that during the sampling period the prevailing direction of the winds was from the north and thus the northern shoreline of the embayment was protected.
4. The chemical composition of the embayment waters was not constant throughout the day but exhibited periodic variations with maxima in the Fe, Mn and Cd values near 12.00h, 13.30h and 16.00h and minima near 12.50h and 15.00h.
5. There is a general tendency for the concentrations of Fe, Mn, Zn and Cd to decrease from 12.00h to 17.00h.
6. The temporal variations in spring 1982 and spring 1983 were identical.
7. The average concentrations of Fe, Mn and Cd were higher in the samples collected in spring 1983 than in those collected in spring 1982.
8. The decrease in Fe, Mn, Zn and Cd from 12.00h to 17.00h coincides with a decrease in atmospheric pressure and in levels of sea water which may affect the flow and mixing of the hydrothermal solutions.

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