Monday 20 June 2022

Evidence for photosynthetic oxygen production in a Neoarchean lake.

The photosynthetic production of oxygen has been a driving force in the evolution of the Earth's surface throughout much of the planet's history. However, quite when this process began remains a mystery. Oxygen has been present in the Earth's atmosphere since the Great Oxidation Event, 2.4 billion years ago, which provides a latest possible date for the onset of photosynthesis, but opinions are divided as to whether this marks the onset of oxygen production by the biological organisms, which then rapidly changed the face of the Earth, or whether photosynthesis had been occurring long before this, but been masked as a signal in the geological record by abundant reducing compounds in Archean environments, which prevented oxygen from building up in the atmosphere. 

Theoretically, if oxygen production did start long before the Great Oxidation Event, then this should have led to the development of local 'oxygen oases', areas where oxygen production was able to overpower any local redox buffering to produce localised oxidative conditions, which might have been preserved in the rock record. Many Archean continental sedimentary deposits show signs of oxidative weathering, despite having apparently been laid down under a reducing atmosphere. A possible cause of this could have been the local production of oxygen in lake environments. A modern analogue for this has been observed in lakes in Antarctica, where photosynthetic Cyanobacteria produce oxygen in benthic microbial mats, beneath an anoxic water column. 

Microbial mats produce distinctive sedimentary structures called Stromatolites; these form when layers of micro-organisms create biofilms on the surface of sediments in shallow water environments. Typically such films are buried by sediments periodically, with a new biofilm forming on the surface. Over time this builds up to a distinctive structure with layers of organic and inorganic material. Since these structures record the environment in which pre-Great Oxidation Event photosynthesis was likely to have occurred, Archean lacustrine Stromatolites have become a target for scientists searching for evidence of such activity.

In a paper published in the journal Geology on 9 May 2022, Dylan Wilmeth of the Department of Earth Sciences at the University of Southern California, and the Laboratoire Géosciences Océan at the Institut Universitaire Européen de la MerStefan Lalonde, also of the Laboratoire Géosciences Océan at the Institut Universitaire Européen de la Mer, William Berelson, also of the Department of Earth Sciences at the University of Southern California, Victoria Petryshyn of the Environmental Studies Program at the University of Southern California, Aaron Celestian, again of the Department of Earth Sciences at the University of Southern California, and the Natural History Museum of Los Angeles CountyNicolas Beukes of the Department of Geology at the University of Johannesburg,  Stanley Awramik of the Department of Earth Science at the University of California, Santa BarbaraJohn Spear of the Department of Civil and Environmental Engineering at the Colorado School of Mines, and Taleen Mahseredjian and Frank Corsetti, again of the Department of Earth Sciences at the University of Southern California, present evidence for the presence of oxygen within microbial mats in a 2.74 billion-year-old palaeolake in the Hartbeesfontein Basin of South Africa.

The Hartbeesfontein Palaeolake formed in a half-graben structure within the Ventersdorp Continental Rift. ItTH has been identified as a lacustrine deposit on the basis of frequent, meter-scale facies shifts and intercalation with subaerial volcanic deposits. The palaeolake deposits also contain numerous Stromatolites, preserved as chert, many of which show exquisitely preserved microbial structures. Also present in the Stromatolites showing this high quality preservation are numerous rounded fenestrae (holes), which are interpreted as having been formed by gas bubbles produced by the activities of microbes living within the mats. These fenestrae are evenly distributed across the structures. Microbes living within mats of this sort can potentially produce a range of gasses (e.g. methane), so the presence of the fenestrae does not necessarily indicate the production of oxygen. 

Hartbeesfontein Basin (South Africa) Stromatolite textures. (A) Location map. Black square in inset represents map location. (B) Domal stromatolitic chert; hammer head is 2 cm tall. Wilmeth et al. (2022).

Wilmeth et al. used Rare Earth Element data to investigate the possible presence of oxygen within the Hartbeestfontein Stromatolites, in particular the distribution of cerium. Cerium levels were found to be anomalously high around fenestrae, and anomalously low in the surrounding laminae, which Wilmeth et al. believe is evidence of the element being scavenged onto oxides forming around the bubbles.

Fenestral oxides. (A), (B) Bubble fenestra with extensive oxides along bottom margin viewed in plane-polarized and reflected light. (C), (D) Fenestral (fen.) oxide viewed in reflected light. (E) Schematic of manganese (Mn) and cerium (Ce) oxidation and sorption onto fenestral oxides during mat growth. Qtz strom., quartz Stromatolite layers. Wilmeth et al. (2022).

Three distinctive assemblages of oxides could be observed within the Hartbeestfontein Stromatolites; within the bubble fenestrae, within the laminae, and on erosional surfaces. The fenestrae oxides appear orange and white under reflected light, and are found at the contact between the walls of the fenestrae and the megaquartz filling of the interior, implying that they were deposited early, before the emplacement of the quartz cement. Examination of this oxide layer with an electron microprobe found it to be rich in manganese, and the minerals goethite and titanite, whereas the oxide layers in the Stromatolite laminae were formed of  haematite and goethite, and are black, red, and yellow in colour, often with a metallic lustre. Oxides on recent erosional surfaces are reddish-orange in colour, and dominated by iron compounds.

Laminar and surficial oxides. (A), (B) Laminar oxides viewed under reflected light. Note the difference between laminar and fenestral oxides. (C) Laminar oxide with dissolution zones filled by orange goethite shown in reflected light. (D) Electron microprobe analysis backscatter image of laminar oxide with dissolution zones. (E) Schematic of laminar versus fenestral oxides during mat growth. (F) Surficial oxides formed by recent weathering. Qtz cem., quartz cement. Wilmeth et al. (2022).

All of the oxides present within the Hartbeestfontein Stromatolites are thought to have derived from minerals present in the original Archean microbial mats. However, these deposits have since undergone both  greenschist-grade metamorphism and weathering at the surface, so interpreting the original conditions must be done with care. For example, the haematite minerals present in the laminae of the Stromatolites were probably originally deposited as ferrihydrite or goethite.

Rare Earth Elements such as Cerium tend to be fairly immobile once deposited in rock formations, and not prone to redistribution by metamorphic processes. This makes them a useful tool for geologists wishing to understand the depositional conditions under which ancient strata were laid down. Furthermore, any available cerium within the water column will rapidly be incorporated into any manganese or iron oxides forming.

Cerium anomalies which are believed to have been formed after deposition are known, though these are due to the precipitation of cerium from water running over or through the rock. In the case of the Hartbeestfontein Stromatolites, the raised cerium levels can be observed around fenestrae that were enclosed within chert and recovered from drill cores, making this scenario highly unlikely. 

The presence of areas of both raised cerium (around fenestrae) and lowered cerium (within laminar layers), suggests that a radox boundary was present within the original Stromatolites, and therefore presumably the surrounding water column. Disolved cerium is scavenged from water and deposited onto iron or manganese oxides under oxidising conditions, but dissolves back into solution under reducing conditions. This would imply a shifting redox boundary within the ancient Hartbeestfontain Palaeolake, shifting above and below the microbial mats in response to changing local conditions. 

The deposition of cerium oxides around fenestrae withoin the Hartbeestfontain Stromatolites appears to be indicative of oxgen formation by microbes within the mats from which the Stromatolites formed. Given the highly reducing conditions thought to have been present within most Archean environments, this oxygen is likely to have been consumed by redox reactions long before it was able to make any meaningful impact on the wider lake environment, let alone the world beyond, but nevertheless the presence of these oxides tells us that ancient microbes had begun to produce oxygen by this time.

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