TERRESTRIAL BENTHIC MATS AS SITES OF O2 ACCUMULATION ON ARCHEAN EARTH

An Archean terrestrial biosphere may have produced microenvironments containing free O₂ in an otherwise anoxic world based on observations of a newly discovered modern analog in Lake Fryxell, Antarctica. In Lake Fryxell, cyanobacterial photosynthesis produces an “oasis” of free O₂ in benthic mats below an anoxic water column. Rates of photosynthesis are slow due to low illumination, but O₂ production seasonally exceeds consumption and loss to the surrounding environment. As a result, tens of micromols per liter of O₂ accumulate within the photosynthetically active mat during the summer. These transient O₂ oases provide the first known modern analog for formation of O₂ oases during Archean time, prior to oxidation of Earth’s atmosphere at about 2.4 Ga.

We hypothesize that once the first cyanobacteria evolved, they produced localized O₂ oases in benthic mats analogous to those in Lake Fryxell, specifically, in environments with little mixing, e.g. seasonally stratified lakes. Widespread development of O₂ oases may have initially been restricted due to the high sensitivity of photosystem II to photon damage, which likely limited O₂ production to relatively low light environments. As the efficiency of photosytem II increased and cyanobacteria evolved mechanisms to reduce photoinhibition, O₂ oases expanded within benthic mats and into terrestrial water columns and the oceans, eventually leading to oxidation of the suraface oceans and atmosphere. The presence of O₂ oases in benthic mats in terrestrial aquatic systems like those in Lake Fryxell may account for geological evidence for oxidative sulfate mineral weathering on land as early as 2.8 Ga.