  Ocean-atmosphere chemistry on Earth has undergone dramatic evolutionary
changes through its long history, with potentially significant ramifications
for the emergence and long-term stability of atmospheric biosignatures. Though
a great deal of work has centered on refining our understanding of false
positives for remote life detection, much less attention has been paid to the
possibility of false negatives, that is, cryptical biospheres that are
widespread and active on a planet's surface but are ultimately undetectable or
difficult to detect in the composition of a planet's atmosphere. Here, we
summarize recent developments from geochemical proxy records and Earth system
models that provide insight into the long-term evolution of the most readily
detectable potential biosignature gases on Earth - oxygen (O2), ozone (O3), and
methane (CH4). We suggest that the canonical O2-CH4 disequilibrium biosignature
would perhaps have been challenging to detect remotely during Earth's ~4.5
billion year history and that in general atmospheric O2/O3 levels have been a
poor proxy for the presence of Earth's biosphere for all but the last ~500
million years. We further suggest that detecting atmospheric CH4 would have
been problematic for most of the last ~2.5 billion years of Earth's history.
More broadly, we stress that internal oceanic recycling of biosignature gases
will often render surface biospheres on ocean-bearing silicate worlds cryptic,
with the implication that the planets most conducive to the development and
maintenance of a pervasive biosphere will often be challenging to characterize
via conventional atmospheric biosignatures.
