Stratification at the Earth's largest hyperacidic lake and its consequences
Introduction
Hot lakes located in the crater of active volcanoes generally provide valuable information of the state of a volcano (Hurst et al., 2012), as the thermal energy and mass injected is intercepted by the lake rather than passing directly to the atmosphere (Terada et al., 2011). Among the 474 lakes listed within the new database of volcanic lakes, VOLADA (Rouwet et al., 2014), a substantial proportion belongs to the hot acidic lake class. These exceptional natural manifestations of volcanic activity are generally considered to be extremely dynamic and perfectly mixed systems, due to a wide and powerful permanent thermal plume at the lake bottom (Rouwet et al., 2014). This is thought to prevent the development of heterogeneities and/or stratification of lake waters within the lake volume, as commonly observed in ‘Nyos-type’ lakes (Rouwet et al., 2014 and references therein). Christenson (1994) specifically ascertained the behavior of Ruapehu volcanic lake during a quiescent period in February 1991. The lake showed little stratification and he concluded that the development of a gas enriched water column was unlikely. Assuming lake volumes are perfectly homogeneous, heat and mass balance approaches based on in-situ measurements can be applied to estimate mass and thermal fluxes emitted from the volcanic vents into the bottom of the lake, as an indication of the state of the underlying volcano (e.g., Pasternack and Varekamp, 1997).
The acidity and/or difficulty of access to these lakes usually hinder a high frequency of data collection. Even acidic lakes which have been monitored for decades (Ruapehu (New Zealand) Dibble, 1974, Christenson, 1994, Christenson et al., 2010, Yugama (Kusatsu-Shirane, Japan) Ohba et al., 1994, Ohba et al., 2008, Poás (Costa Rica) Rowe et al., 1992, Martinez et al., 2000, Rouwet et al., 2016) are generally sampled in the best case on a monthly basis and from the lake surface at the shores. Assumed to be representative of the whole lake, this classic approach has been demonstrated to be helpful for volcano monitoring efforts (e.g., Hurst et al., 1991, Ohba et al., 2008, Rouwet and Tassi, 2011.
Here, we first question the homogeneity assumption for acidic lakes by comparing a unique high-resolution temperature dataset (1-h data measured between May 2010 and July 2012, at depth greater than 2 m) recorded at the largest hyperacidic lake on Earth (Kawah Ijen) with weekly measurements. To better constrain the relevance and the understanding of this dataset, additional parameters such as lake water density and level, chemical composition of the top-17 cm of the lake, meteorological data, thermal infrared images of the lake, satellite measurements and historical records are discussed below. Our results highlight the dramatic heterogeneity of shallow lake waters, particularly during rainy seasons when a stratification develops. The implications and associated hazards are discussed at the end.
Section snippets
Background
Kawah Ijen volcano (East Java, Indonesia, Fig. 1a–c) is potentially one of the most dangerous volcanoes in Indonesia. The crater hosts a lake (170 m deep Caudron et al., 2015a) which has been consistently hyperacidic (pH < 0.5) and hot ( °C) for at least 50 yrs (Caudron et al., 2015b). Numerous areas with bubble activity were observed using an echo sounder (Caudron et al., 2016). According to Takano et al. (2004), the lake is chemically homogeneous. However, the first
Instruments and methods
Several sensors were immersed into the Kawah Ijen volcanic lake. Due to the extreme conditions, the instruments are all installed on the lake shores. An acid-proof buoy was installed in 2011, but sank in less than a year and instruments were never recovered. A temperature sensor (iButton, accuracy of 0.5 °C, resolution of 0.625 °C) has been located since June 2010 close to the western shore (DAM, Fig. 1d), at a depth of ∼5 m. At exactly the same location, a Troll 500 device was installed
Results
Both lake temperature (small dots, Fig. 2b) and level (Fig. 2c) recordings followed the same trend: they rose in mid-December 2010, two months after the first significant rainfalls of the season (Fig. 2a). The lake level reached a maximum in early May 2011, then decreased until December 2011. The temperatures measured from the surface (blue squares and triangles, Fig. 2b) and at greater depths (gray dots, Fig. 2b) were only similar between October 2010 and January 2011. After mid-January 2011,
Discussion
The results indicate significant temperature differences (up to 15 °C) during the rainy season between measurements performed at the surface and at larger depths. This raises questions regarding the spatial and temporal extension of the stratification, and ultimately the reliability of in-situ and remote surface lake temperature estimates to assess the state of volcanic activity.
Different densities are expected if a stratification of lake waters exists. Density measured using a pycnometer
Modeling of the overturn
We now investigate the conditions at which the overturn may occur. Fig. 6 shows how the density of the lake water varies with salinity (in g/L of Total Dissolved Solids (TDS)) for a temperature between 10 and 39.5 °C. The temperature range for the lake water (31–39.5 °C), as measured in September 2014 (Table A.1), is delimited by the red solid lines in Fig. 6. Lake overturn is density driven, hence, the top cold meteoric water layer sinks into the hot underlying lake water when the density of
Conclusion
Volcanic lake temperatures and variations in chemical compositions have generally served as warnings for recurrent unrest or prior to eruptions in the past (Tonini et al., 2016). An increase in heat and gas flux translates into an overall increase in lake temperatures and anion concentrations (, Cl−, F−), the lake playing the role of a calorimeter in the system (e.g., Rouwet and Tassi, 2011). Hourly temperature measurements at the largest hyperacidic lake on Earth demonstrate the
Acknowledgments
We wish to thank the reviewers and particularly J. Varekamp who provided detailed and stimulating remarks. We also acknowledge the Editor, T. Mather, who addressed constructive comments which led us to develop the modeling of the overturn, but also greatly improved the quality of the manuscript. We dedicate this study to Bruno Capaccioni who tragically passed away during the writing of this manuscript. We are grateful to CVGHM support on the field and in Bandung, particularly to the observers
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