2 Materials and Methods
To gain a better understanding of what kinds of microbes thrive in oligotrophic, subterranean environments and the traces they leave behind, we analyzed an interdisciplinary set of samples collected in Icelandic lava tubes. Lava tubes are isolated, fragile, and potentially dangerous environments, and as such their exploration requires much care, planning, and specialized equipment.
2.1 Icelandic lava tubes
During a field expedition in the summer of 2018, we explored four lava tubes in two distinct regions of Iceland: the southern Eldhraun lava field and the northern Ódáðahraun lava field. The caves, marked with black stars on Figure 1, are labeled B and C in the Eldhraun lava field (basaltic, intermediate, and acidic lavas since 1100 AD), and H and R in the Ódáðahraun lava field (basaltic, intermediate, and acidic lava flows older than 1100 AD). The moss-covered Eldhraun lava field is part of the Laki lava flow originating from the Laki (Skaftfir Fires) fissure eruption in southern Iceland near the older Mt. Laki hyaloclastite formation during 1783 and 1784. The eruption produced 14.7 ± 1.0 km3 of basaltic lava flow, with 2.6% of the erupted volume in the form of tephra and ash (Thordarson & Self, 1993). The Ódáðahraun lava field is a sandy lava desert 5 000 km2in size, composed of many older lava flows sometimes covered by recent pumice deposits (Sheth, 2018). The oldest lava flow originated 10-12 thousand years ago, and the youngest lava flow is from the Holuhraun eruption in 2014-2015 (Hannesdóttir et al., 2010). Within this huge expanse sits Mt. Herðubreið, a classic table mountain, with hyaloclastite formations under its lava cap. Countless lava tubes abound in the Eldhraun and Ódáðahraun lava fields, many of which have been explored by the Icelandic Speleological Society (ISS), who have noted and archived their locations, and worked extensively to keep them pristine and protected from human contamination.