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.