1 Scientific RepoRts | 5:12152 | DOi: 10.1038/srep12152 www.nature.com/scientificreports The Kallisti Limnes, carbon dioxide-accumulating subsea pools Richard Camilli 1 , Paraskevi Nomikou 2 , Javier Escartín 3 , Pere Ridao 4 , Angelos Mallios 1,4,5 , Stephanos P. Kilias 2 & Ariadne Argyraki 2 and the Caldera Science Team: Muriel Andreani 6 , Valerie Ballu 3 , Ricard Campos 4 , Christine Deplus 3 , Taoufc Gabsi 4 , Rafael Garcia 4 , Nuno Gracias 4 , Natàlia Hurtós 4 , Lluis Magí 4 , Catherine Mével 3 , Manuel Moreira 3 , Narcís Palomeras 4 , Olivier Pot 3 , David Ribas 4 , Lorraine Ruzié 7 & Dimitris Sakellariou 5 Natural CO 2 releases from shallow marine hydrothermal vents are assumed to mix into the water column, and not accumulate into stratifed seafoor pools. We present newly discovered shallow subsea pools located within the Santorini volcanic caldera of the Southern Aegean Sea, Greece, that accumulate CO 2 emissions from geologic reservoirs. This type of hydrothermal seafoor pool, containing highly concentrated CO 2 , provides direct evidence of shallow benthic CO 2 accumulations originating from sub-seafoor releases. Samples taken from within these acidic pools are devoid of calcifying organisms, and channel structures among the pools indicate gravity driven fow, suggesting that seafoor release of CO 2 at this site may preferentially impact benthic ecosystems. These naturally occurring seafoor pools may provide a diagnostic indicator of incipient volcanic activity and can serve as an analog for studying CO 2 leakage and benthic accumulations from subsea carbon capture and storage sites. Anthropogenic carbon dioxide (CO 2 ) inputs and naturally occurring marine hydrothermal venting can profoundly impact marine food webs and ecosystems 1–3 . Sub-seafoor storage of CO 2 within geological formations is gaining international acceptance as a mechanism for reducing CO 2 emissions and lessening the impacts of ocean acidifcation 4,5 . However, specifc concerns over leakage of CO 2 from sub-seafoor injection sites necessitate feld studies in order to understand associated risks 6,7 . Te Santorini volcanic group and Kolumbo submarine volcano are located along the Hellenic Volcanic Arc in the southern Aegean 8,9 . Te prehistoric Minoan eruption of the Santorini caldera, circa 1627-1600 B.C. 10 , was one of the largest in human history, with a total eruption volume of up to 60 km 3,8 . Tis erup- tion buried prehistoric settlements, including the fourishing Bronze Age city of Akrotiri 10,11 . Subsequent volcanic eruptions formed the Kameni islands, which emerged in 197 B.C. at the center of the caldera. Historical records from the past fve centuries reveal a characteristic interval of 61 ± 68 years between eruptions, with the latest eruption occurring in 1950 12 . In January 2011, following more than six decades of quiescence, the Santorini caldera entered a renewed phase of unrest characterized by sustained seismicity, caldera-wide infation, and increased gas emissions. Spherical Mogi models of deformation from January 2011 to April 2012 suggest a source centered 1.5 km north of Nea Kameni, 4 km beneath the caldera’s Northern Basin 13 (Fig. 1), and a corre- sponding volume increase of 1 to 2 × 10 7 m 3 14,15 . Isotopic analysis of magmatic and thermal decarbona- tion gas emissions indicate increasing mantle CO 2 contribution 16 . Tese variations coincide with episodic 1 Woods Hole Oceanographic Institution, Department of Applied Ocean Physics and Engineering, Woods Hole, MA, 02543, USA. 2 University of Athens, Faculty of Geology and Geoenvironment, Panepistimiopoli Zographou, 15784 Athens, Greece. 3 Institut de Physique du Globe de Paris, CNRS UMR7154, 75238 Paris Cedex 05, France. 4 University of Girona, Computer Vision and Robotics Group, 17071 Girona, Spain. 5 Institute of Oceanography, Hellenic Centre for Marine Research, Anavyssos, Greece. 6 Laboratoire de Géologie de Lyon, UMR 5276, ENS et Université Lyon 1, 69622 Villeurbanne Cedex, France. 7 University of Manchester, School of Earth, Atmospheric and Environmental Science, Oxford Road Manchester M13 9PL: UK. Correspondence and requests for materials should be addressed to R.C. (email: rcamilli@whoi.edu) Received: 25 September 2014 Accepted: 29 May 2015 Published: 16 July 2015 OPEN