Virtual SEES Seminar -- Prof. Kurt Konhauser, University of Alberta, Canada Title: How Cyanobacteria Oxygenated Earth

Abstract: A remarkably coherent ensemble of evidence points to a significant accumulation of atmospheric oxygen for the first time in Earth's history beginning ca. 2.45 Ga, the so-called Great Oxidation Event (GOE). Briefly, this includes the disappearance of detrital pyrite, uranitite and siderite from fluvial and deltaic deposits, an increase in the retention of iron in paleosols, an enrichment of Cr and U in iron formations, and perhaps most importantly, the disappearance of sedimentary sulphur isotope mass-independent anomalies indicative of atmospheric SO2 processing in the absence of appreciable ozone. However, several trace element and isotopic proxies have recently suggested oxidative weathering hundreds of millions of years earlier1-2. The superposition of pre-GOE signals for oxidative weathering at a time of global anoxia represents a conundrum for which the most straightforward explanation is that pre-GOE oxidative weathering is the result of intense O2 generation – and immediate consumption – at sub-meter scales by benthic oxygenic photosynthesis in the terrestrial realm3. Despite the absence of a UV-protective ozone layer in the Archean, a terrestrial phototrophic biosphere may have existed in various sheltered environments, including biological soil crusts and freshwater microbial mats covering riverbed, lacustrine, and estuarine sediments. An intriguing question that follows from this hypothesis is if cyanobacteria were conceivably metabolising at modern rates on land by perhaps 3.0 Ga, what happened in the hundreds of million years between the first, rare signals of oxidative weathering and the first significant accumulation of atmospheric oxygen, i.e., the GOE? While the exact confluence of factors controlling the success of Earth's earliest oxygenic phototrophs remains an open question4, several factors may have depressed areal coverage or photosynthetic efficiency of cyanobacteria, and thus masked their potential presence prior to the GOE, including the lack of colonisable surface area for oxidative weathering.

[1] Stueken et al (2012), Nature Geoscience 5, 722-725.

[2] Planavsky et al (2014), Nature Geoscience 7, 283-286.

[3] Lalonde and Konhauser (2015), PNAS 112, 995-1000.

[4] Planavsky et al (2021), Nature Reviews: Earth and Environment 2:123-139.