New insights into the degassing dynamics of Lago Albano (Colli Albani volcano, Rome, Italy) during the last three decades (1989-2019) DMITRI ROUWET (1), GIANCARLO TAMBURELLO (1), GIOVANNI CHIODINI (1), GIOVANNELLA PECORAINO (2), MONIA PROCESI (3), TULLIO RICCI (3), STEFANIA VENTURI (4, 5), ALESSANDRO SANTI (4), JACOPO CABASSI (5), ORLANDO VASELLI (4, 5), FRANCO TASSI (4, 5) & ANTONIO COSTA (1) ABSTRACT With this study a nine-year hiatus (May 2010-April 2019) in the quantification of the CO 2 content of Lago Albano by our working group has been resolved through the acquisition and analysis from two new field campaigns. Based on a CO 2 budget analysis the dynamics of CO 2 degassing throughout the past thirty years (1989-2019) is detailed and quantified. The decreasing CO 2 content (expressed as dissolved inorganic carbon, DIC) in the lake, since the co-seismic CO 2 input during the 1989-1990 seismic swarm beneath Colli Albani volcano, was accelerated at lake bottom layers (-140 m to bottom, near -160 m) in the 4-5 years after the swarm, continued afterwards at lower depths (-125 to -95 m), and seems to have reached steady-state conditions during recent years. The peculiar lake basin morphology has control on the degassing dynamics. The low chemical gradients detected during the April 2019 survey have induced near-zero degassing conditions, and arguably stopped the gas-self lifting process: Lago Albano might not become CO 2 -free in the future. This finding has implications for gas hazard when the next seismic swarm will hit the area. The updated degassing model also takes into account the lake level drop, and hence the volume decrease of Lago Albano, caused by excessive well pumping for anthropic purposes. This volume decrease appears to have a destabilizing effect on the degassing dynamics, which renders Lago Albano’s gas release less predictable in the future. Enhanced gas surveys (high-frequency and fine-scale spatial measurements) are needed to shed light on how Lago Albano degasses in this quiescent stage during the Anthropocene. A submersible infra-red detector to directly measure in-lake dissolved CO 2 concentrations, applied satisfactorily during this study, is an adapted instrument to do so. KEY WORDS: Volcanic lake, degassing dynamics, dissolved CO 2 , lake stability, limnology, hazard assessment INTRODUCTION After the deadly Lake Nyos gas burst in August 1986 in Cameroon (KLING et alii, 1987; TANYILEKE et alii, 2019) Lago Albano, on the border of Colli Albani volcanic complex (e.g. GIORDANO et alii, 2006), became the focus of several studies aimed at investigating its CO 2 degassing dynamics in the late eighties-early nineties (IWGCL Newsletter; MARTINI et alii, 1994; PEDRESCHI, 1995). In 1989- 1990, a seismic swarm beneath Colli Albani resulted in the injection of a significant amount of CO 2 into Lago Albano’s deepest water layers (AMATO et alii., 1994; CARAPEZZA et alii, 2008, CHIODINI et alii, 2012). These two almost synchronous events warned about the potential hazard assessment of Europe’s deepest volcanic lake, similar to Lake Nyos (i.e. “Nyos-bias”, ROUWET et alii, 2019). Nevertheless, the sudden seismically triggered CO 2 recharge below Lago Albano in 1989-1990 did not turn out to be sufficiently voluminous to cause CO 2 supersaturation of the bottom waters and consequent sudden gas release. Moreover, it is known that temperate climates protect volcanic lakes from accumulating CO 2 for prolonged periods, as yearly lake turnover facilitates partial CO 2 release in winter when cold, dense CO 2 -free surface waters sink into deeper water layers (CHIODINI et alii, 2012). ROUWET et alii (2019) proposed that Lago Albano is instead an “anti-Nyos-type” lake, for being suddenly recharged with CO 2 (vs. a continuous CO 2 input at Lake Nyos; EVANS et alii, 1993; KUSAKABE, 2017), and with a periodical CO 2 release during winter overturn events (vs. sudden gas bursts at Lake Nyos following CO 2 saturation; KUSAKABE, 2015, 2017). The recent review of anomalous CO 2 degassing related to six seismic crises during the 19 th and 20 th centuries near Colli Albani has demonstrated that three out of six crises led to anomalous degassing, regardless of the magnitude and duration of the seismic crisis (ROUWET et alii, 2019). In terms of hazard, CO 2 can suddenly be released from Lago Albano during seismic crises (1) when near-critical CO 2 concentrations dominate in deep water layers, or (2) when well-localized vents massively inject CO 2 into the lake to punch through any physical-chemical lake stratification. It is hence of great importance to monitor the CO 2 saturation state of deep layers since a seismic crisis can occur at any time. Within these perspectives, this study provides a new data set on water and dissolved gas chemistry (chemical and isotopic composition – δ 13 C DIC , δ 13 C CO2 , δ 13 C CH4 , 3 He/ 4 He ratios), and physicochemical parameters along a vertical profile in correspondence with the maximum depth of Lago Albano for April and June 2019. These measurements, which in the past were routinely acquired (CIONI et alii, 2003; CARAPEZZA et alii, 2008), are the first publicly available since May 2010, after an hiatus of almost nine years since Ital. J. Geosci., Vol. 140, No. 1 (2021), pp. , 11 figs., 2 tabs. (https://doi.org/10.3301/IJG.2020.19) © Società Geologica Italiana, Roma 2021 (1) Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Bo- logna, Italy. (2) Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Palermo, Italy. (3) Istituto Nazionale di Geofisica e Vulcanologia, Sezione di Roma1, Italy. (4) University of Florence, Department of Earth Sciences, Flor- ence, Italy. (5) Istituto di Geoscienze e Georisorse, Consiglio Nazionale delle Ricerche (CNR-IGG), Florence, Italy. Corresponding authors e-mail: dmitri.rouwet@ingv.it Accepted manuscript