  We investigate atmospheric responses of modeled hypothetical Earth-like
planets in the habitable zone of the M-dwarf AD Leonis to reduced oxygen (O2),
removed biomass (dead Earth), varying carbon dioxide (CO2) and surface relative
humidity (sRH). Results suggest large O2 differences between the reduced O2 and
dead scenarios in the lower but not the upper atmosphere. Ozone (O3) and
nitrous oxide (N2O) also show this behavior. Methane depends on hydroxyl (OH),
its main sink. Abiotic production of N2O occurs in the upper layers.
Chloromethane (CH3Cl) decreases everywhere on decreasing biomass. Changing CO2
(from x1 to x100 present atmospheric level (PAL)) and surface relative humidity
(sRH) (from 0.1 percent to 100 percent) does not influence CH3Cl as much as
lowering biomass. Therefore, CH3Cl can be considered a good biosignature.
Changing sRH and CO2 has a greater influence on temperature than O2 and biomass
alone. Changing the biomass produces ~6 kilometer (km) in effective height (H)
in transmission compared with changing CO2 and sRH ( about 25km). In
transmission O2 is discernible at 0.76 microns for greater than 0.1 PAL. The O3
9.6 micron band was weak for the low O2 runs and difficult to discern from dead
Earth, however O3 at 0.3 microns could serve as an indicator to distinguish
between reduced O2 and dead Earth. Spectral features of N2O and CH3Cl
corresponded to some km H. CH4 could be detectable tens of parsecs away with
ELT except for the 10-4 and 10-6 PAL O2 scenarios. O2 is barely detectable for
the 1 PAL O2 case and unfeasible at lower abundances.
