Doctoral defence: Sigrid Soomer “Palaeoweathering record of the Archaean–Proterozoic transition in the Imandra–Varzuga Greenstone Belt, north-western Russia”

On 27 May at 13:15 Sigrid Soomer will defend her doctoral thesis “Palaeoweathering record of the Archaean–Proterozoic transition in the Imandra–Varzuga Greenstone Belt, north-western Russia” for obtaining the degree of Doctor of Philosophy (in Geology).

Supervisor:
Research Fellow Peeter Somelar, University of Tartu

Opponent: 
Professor Heinrich Bahlburg, University of Münster (Germany)

Summary
The Archaean to Proterozoic transition is recognized by significant environmental disturbances triggered by the shift from anoxic to oxic surface environments during the Great Oxygenation Event (GOE) at ca. 2.4 Ga. The onset of oxidative weathering is thought to have caused an extensive acid attack in weathering due to oxidation of sulphide minerals in surface rocks. Chemical weathering itself has an important impact on the evolution of the Earth's surface environments by regulating atmospheric CO₂ through a negative feedback mechanism and release of bio-limiting nutrients into the ocean. Therefore, palaeosols and fine-grained terrigenous sediments derived through weathering are important sources of information on environmental-climatic conditions of the past.

The main aims of the theses are to characterize the weathering intensity, determine environmental conditions, estimate the atmospheric pCO₂ using the Kuksha palaeoweathering crust (ca. 2.5–2.45 Ga), and to reconstruct the changes in silicate weathering intensity using the Imandra–Varzuga metasediments spanning across the Archaean–Proterozoic transition.

The Kuksha weathering crust formed under acidic weathering conditions under cool temperate climate with mean annual temperatures ~12–13 °C and precipitation between 700 and 1100 mm yr^–1 similar to modern Mediterranean climate. Despite the high weathering rates the estimated palaeoatmospheric pCO₂ values range only within 1–10 PAL. However, there is no difference in overall silicate weathering intensity indicators between the Seidorechka Sedimentary and Umba Sedimentary formations deposited, respectively, prior to and after the GOE marking the onset of oxidative weathering.

The results of this study suggest that the punctuated acidic weathering, caused by oxidation, did not affect the long-term weathering intensity. Instead, it seems that the overall silicate weathering decreased with the dropping atmospheric pCO₂ levels during the Archaean–Proterozoic transition.