Elsevier

Earth and Planetary Science Letters

Volume 459, 1 February 2017, Pages 28-35
Earth and Planetary Science Letters

Stratification at the Earth's largest hyperacidic lake and its consequences

https://doi.org/10.1016/j.epsl.2016.11.002Get rights and content

Highlights

  • Acidic lakes are not always perfectly mixed reservoirs.

  • 20 yrs of data indicate a systematic lake stratification during rainy seasons.

  • The density-driven rupture of stratification triggers a limnic eruption.

  • New hazard for an hyperacidic volcanic lake: limnic eruption.

  • The stratification favors the build-up of dissolved CO2 below the cold water layer.

Abstract

Volcanic lakes provide windows into the interior of volcanoes as they integrate the heat flux discharged by a magma body and condense volcanic gases. Volcanic lake temperatures and geochemical compositions therefore typically serve as warnings for resumed unrest or prior to eruptions. If acidic and hot, these lakes are usually considered to be too convective to allow any stratification within their waters. Kawah Ijen volcano, featuring the largest hyperacidic lake on Earth (volume of 27 million m3), is less homogeneous than previously thought. Hourly temperature measurements reveal the development of a stagnant layer of cold waters (<30 °C), overlying warmer and denser water (generally above 30 °C and density ∼1.083 kg/m3). Examination of 20 yrs of historical records and temporary measurements show a systematic thermal stratification during rainy seasons. The yearly rupture of stratification at the end of the rainy season causes a sudden release of dissolved gases below the cold water layer which appears to generate a lake overturn, i.e. limnic eruption, and a resonance of the lake, i.e. a seiche, highlighting a new hazard for these extreme reservoirs. A minor non-volcanic event, such as a heavy rainfall or an earthquake, may act as a trigger. The density driven overturn requires specific salinity-temperature conditions for the colder and less saline top water layer to sink into the hot saline water. Spectacular degassing occurs when the dissolved gases, progressively stored during the rainy season due to a weakened diffusion of carbon dioxide in the top layer, are suddenly released. These findings challenge the homogenization assumption at acidic lakes and stress the need to develop appropriate monitoring setups.

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 27×106 m3 lake (170 m deep Caudron et al., 2015a) which has been consistently hyperacidic (pH < 0.5) and hot (T>30 °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 (SO42, 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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