During the trips of Nov. 2006 and 2007, we took water samples from TL and MCL and measured water temperature and depth with an Automatic Temperature and Depth Recorder. The temperature profiles exhibit small temperature gradients with water depths
>50m. This observation reveals quite
different thermal gradient feature from normal lakes in non-volcanic related lakes. Our observations are similar to the previous surveys of Delmelle et al.
(1998). This is because the thermal input and degassing from the lake bottom causing the fast mixing of the water column. Such a thermal gradient influences the geochemical system of Lake Taal. And, these profiles will be benefit to the dynamic model of heat, geochemical and isotopic budgets.
We have measured the concentrations of Na, K, Mg, Ca, Al, Fe, Mn, Sr, Ba, Co, Ni, Cu, Zn, Pb, As, Ti, Cr, and Cd, and isotopic values of δD,
δ18O and δ87Sr in the water samples.
Our results agree very well with the results of Delmelle et al. (1998) and of Castillo and Gonzales (1976). Tables
1-3 list the data results.
From these results, we have obtained following conclusions:
(1) Filtered and non-filtered samples have similar concentrations, implying that particle phase in the water samples is insignificant. There is no chemical gradient in the water column shown by the similar concentrations between TAL-4 (from 40m deep) and TAL-5 (surface at the same site), which corresponds to the weak thermal gradient shown in Fig. 4a. The water column is mixed very well.
(2) The geothermal fluid (TAL-11) contains much higher concentrations of measured major and minor elements (Na, K, Mg, Ca, Al, Fe, Mn), reflecting input of volcanic water, dissolution of volcanic rocks, and incursion of seawater. TAL-7 represents a mixture of geothermal vent with Lake Taal water.
Although this sample was taken from near shore (~10m away from the island and 2m water depth), we can still see the influence of the geothermal input.
However, the geothermal water is quickly diluted in the lake.
(3) Isotopic composition of Taal Lake bias from MWL indicates strong evaporation, and mixing with groundwater and seawater (Fig. 1a).
(4) Sr isotopic composition of Taal Lake reveals mixing of at least three end-members including surface runoff, hydrothermal fluid and seawater. The δ87Sr value of the volcanic water (TAL-11) is about 1000ppm lower than that of the lake waters. Therefore, δ87Sr may be a good indicator of volcanic activity (Fig. 1b).
(5) High concentrations of major ions in Taal Lake are introduced by both seawater and hydrothermal inputs, so that they could not be used as index of volcanic activity.
(6) Among trace and heavy metal elements, Fe, Mn, Cu and Zn may be used as indicators of volcanic activity.
Our most significant study is on a 120-cm long gravity core, Core TLS2, retrieved from 15-m water depth of Taal Lake. Sixteen AMS 14C dates were made on plant remains at different layers of the core, matching very well with the bomb 14C curve determined in tree rings (Fig. 2). The distribution of the bomb
14C profile allows us to establish the chronology of the core which yields a constant sedimentation rate of 2.04cm/year spanning the past 60 years.
Table 1 Major and minor elemental
contents (ppm) in water samples from Lake Taal. * Data from Delmelle et al., 1998.Table 2 Trace/heavy metal
concentrations (ppm) in water samples of Lake Taal.
Table 3 Stable isotopic compositions in
water samples of Lake Taal. * Data from Delmelle et al., 1998.Figure 1
. (a) δD and δ18O values; and (b) δ87Sr values of water samples from Taal Lake basin.Figure 2: Comparison between the ∆
14C of samples from Core TLS2 (lower panel) and the bomb 14C curve of the atmospheric CO2 (upper panel). The cross symbols with calendar years stand for possible match with the bomb 14C curve of the atmospheric CO2.For this core, we have measured pore water content, weight loss by 0.5NHCl leaching, TOC, TON, δ18O and δ13C of TIC, δ13C of TOC and δ15N of TON, and concentrations of Na, K, Mg, Ca, Fe, Mn, Sr, Ba, Cu, Zn, Pb, Li in the acid-leachable phase. The annual resolution δ18O and δ13C records provide us detailed variations of the lake’s hydrological, biological and sedimentary history. Carbonate was precipitated in isotopic equilibrium with the lake water at ~30oC which is close to the measured water temperature. The δ18O and δ13C of TIC co-vary in the core, because of changes in surface water input and geothermal input. In general, when there is more input surface water, both δ18O and δ13C of the lake goes lighter due to dilution effect. The lake productivity at this time will be lower, and carbonate precipitation is less.
When the lake experiences less surface water input and/or more evaporation, δ18O and δ13C of the lake goes heavier due to the hydrological balance and
increased lake productivity. However, when the volcanic activity increases, significant amount of hydrothermal input and deep CO2 input will lead to increase of lake’s δ18O and δ13C. Both carbonate and organic carbon will decrease due to the influence of volcanic input. This situation was occurred around 1991.
However, if a volcanic eruption causes significant amount of dead carbon from vegetation and organism in and around the lake, the lake’s δ13C will be depleted.
At the time, the δ18O and δ13C of the lake goes the opposite way. The 1965 eruption may be an example of such a case. With the detailed geochemical profiles of Core TLS2, we have found anomalies of high concentrations of Fe, Mn, Cu, K and heavy δ18O and δ13C values of TIC around 1991-1994 when the Taal volcano was active (Fig. 3).
These proxies may be considered as indicators of geochemically monitoring volcanic activity for long-term prediction.
Acknowledgements
We thank everyone for their help in this study, especially during the field trips.
This study has been supported by NSC grants 95WFA0901514 and
97-2628-M-006-014.
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Figure 3. Geochemical proxies in Core TLS2 indicate geothermal activities of Taal Volcano
during the past 60 years. Increased concentrations of K, Cu, Fe and Mn and δ18O values reflect hydrothermal inputs to the lake.References
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