SUB-BASEMENT FOSSIL SOIL CORED FROM THE NORTH PACIFIC (ODP, LEG 197): A MODEL OF THEIR FORMATION AND USE AS ANALOGUES FOR A DEEP SUBSURFACE BIOSPHERE ON MARS

The detection of past/ present life buried deep beneath the surface of a planet is a primary step to constrain the presence and evolution of life on that planet. We propose that volcanically-derived fossil soils, which are deeply buried (down to ~310 mbsf), receive no sunlight, and are primarily anoxic and nutrient-limited, represent a novel potential subsurface biosphere on Earth that may be used as a model to test for extant and extinct life on Mars.

Late Paleocene-early Eocene paleosoil, or fossil soils were drilled from the volcanic basement during the ODP Leg 197 (Emperor Seamounts, north Pacific) at Nintoku Seamount and Koko Seamount. The cm-to- meter-thick soil units are rich in Fe-oxides/oxy-hydroxides and were deeply buried beneath the eruptive sub-basement.

The fossil soils contain very low but measurable amounts of total organic carbon (TOC = 0.01-0.12 %wt, ±0.02%, N=36; and ultra-low N-tot content (0.0031% to 0.0054%). Furthermore, the organic Carbon and Nitrogen have stable isotope values more negative (i.e., d13C-org. = –25.3‰ to –26.1‰ and d15N-tot = –9.5‰ to +2.5‰) than those of exposed Hawaiian counterparts (i.e., d13C-org = –22.1‰ to –23.0‰; and d15N-tot up to +8.5‰).

This suggest that variable sources of organics (land plants/ primary secondary bacterial) were preserved in a deeply buried isolated system for ~>54 Ma. Specifically, burial rates up to ten times higher than the average subsidence rates (e.g., present day ~2.5 mm/years) produced conditions for the isolation of these fossil soils from the ocean and the atmosphere in a present-day sub-basement setting. We suggest here that this feature would make appropriate the use of fossil soil as plausible analogues for a Deep Subsurface Biosphere on Mars.