Geochemical insight into differences in the physical structures and dynamics of two adjacent maar lakes at Mt. Vulture volcano (southern Italy)

Abstract

[1] We report on the first geochemical investigation of the Monticchio maar lakes (Mt. Vulture volcano, southern Italy) covering an annual cycle that aimed at understanding the characteristic features of the physical structures and dynamics of the two lakes. We provide the first detailed description of the lakes based on high-resolution conductivity-temperature-depth (CTD) profiles, chemical and isotopic (H and O) compositions of the water, and the amounts of dissolved gases (e.g., He, Ar, CH 4 , and CO 2 ). The combined data set reveals that the two lakes, which are separated by less than 200 m, exhibit different dynamics: one is a meromictic lake, where the waters are rich in biogenic and mantle-derived gases, while the other is a monomictic lake, which exhibits complete turnover of the water in winter and the release of dissolved gases. Our data strongly suggest that the differences in the dynamics of the two lakes are due to different density profiles affected by dissolved solutes, mainly Fe, which is strongly enriched in the deep water of the meromictic lake. A conceptual model of water balance was constructed based on the correlation between the chemical composition of the water and the hydrogen isotopic signature. Gas-rich groundwaters that feed both of the lakes and evaporation processes subsequently modify the water chemistry of the lakes. Our data highlight that no further potential hazardous accumulation of lethal gases is expected at the Monticchio lakes. Nevertheless, geochemical monitoring is needed to prevent the possibility of vigorous gas releases that have previously occurred in historical time.

Mantle CO2 degassing at Mt. Vulture volcano (Italy): Relationship between CO2 outgassing of volcanoes and the time of their last eruption

“…Similar 3 He/ 4 He values, measured in fluids from around Mt. Vulture, indicate that the deep volcanic system is still degassing (Caracausi et al, 2009(Caracausi et al, , 2013bChiodini et al, 2000a). The two maar lakes act as collectors of mantle-derived He and CO 2 (Caracausi et al, 2013b and references therein) and in the last few centuries they have shown evidence of several sudden releases of mantle-derived fluids, which had accumulated in the crust and/or volatiles which had outgassed from magma (Caracausi et al, 2009 and references therein).…”

“…Hence geochemical evidences support that an active outgassing of mantlederived volatiles from magmatic intrusion at depth occurs in the region of the Mt. Vulture volcano (Caracausi et al, 2013a(Caracausi et al, , 2013b. This is strongly supported by recent geophysical investigations that highlighted: 1) the presence of magmatic intrusions between 5 and 20 km at depth under the Mt.…”

“…The chemical composition of free gases and gas flow rates are considered for pools, and C tot and water flow rate are used for the maar lakes (Caracausi et al, 2013b). Most of the sampling sites fall within the MAB including also the sites above fluvio-lacustrine sediments.…”

“…2; Appendix A), of which 79 are from Paternoster (2005) and Parisi et al (2011aParisi et al ( , 2011b, and 60 are from this work. Data from eight pools located on the volcanic edifice (this work) and two maar lakes (Caracausi et al, 2013b;Nicolosi, 2012) are also evaluated. Most of investigated sites fall within the volcanic deposits while only a few samples, located in the southern area, are inside the fluvio-lacustrine sediments (Fiumara di Atella Super-synthem), that is considerated a local extension of the volcanic aquifer with a relatively low groundwater circulation (Parisi et al, 2011a).…”

“…Isotopic analyses of carbon (δ 13 C tot ) were performed using an AP 2003 IRMS mass spectrometer on C tot after its precipitation, following Caracausi et al (2013b). Carbonate species were precipitated as SrCO 3 by adding SrCl 2 and NaOH in solid state.…”