CO₂ drawdown and cooling at the onset of the Great Oxidation Event recorded in 2.45 Ga paleoweathering crust

Abstract

The Archean atmosphere is thought to have been devoid of oxygen but, instead, containing high concentrations of greenhouse gases, such as CO₂ and possibly CH₄, that were required to keep the Earth surface environments warm enough for maintenance of liquid water under faint-young-Sun conditions. Earlier studies have suggested that the CO₂ concentrations must have been as high as over 1000 times present atmospheric levels (PAL) to overcome the effects of weaker solar luminosity. However, more recent studies of Precambrian paleosols imply existence of at least tenfold-lower, but still high pCO₂ levels. The appearance of minute amounts of atmospheric O₂ at the transition of Archean and Proterozoic is well documented, however, the bulk composition of the atmosphere and its dynamics at this crucial time is still widely debated. Different paleoclimate proxies of the ~2.45 Ga Kuksha paleoweathering crust suggest its formation in a cool, temperate climate. Paleoatmospheric pCO₂ estimates by geochemical and isotopic mass-balance models suggest remarkably low paleoatmospheric CO₂ levels, with the best guess estimates of atmospheric CO₂ between 1 and 10 PAL. We speculate that the Kuksha paleoweathering crust either predates, or partly overlaps with, the onset of Huronian glaciation and corresponds to a time interval of low CO₂ level during the first recorded large glaciation in Earth history.

Introduction

Atmospheric carbon dioxide concentration is an important factor in control of the Earth's surface temperatures and chemical processes (Broecker, 2018; Dessert et al., 2003; Kasting, 1987). High atmospheric CO₂ concentration lowers the pH of the meteoric precipitation and by greenhouse effect increases air temperature, both leading to intensified weathering (Kanzaki and Murakami, 2018a, Kanzaki and Murakami, 2018b). 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 (Dessert et al., 2003; Kump et al., 2000; Walker et al., 1981) and release of bio-limiting nutrients into the ocean (Hao et al., 2017; Filippelli, 2011). On a long term the level of atmospheric CO₂ acts as a climate thermostat, keeping Earth environments in the range where water is mostly present in liquid form and surface temperatures are favorable for life (Kump et al., 2000).

Elevated CO₂ levels (or other greenhouse gases such as methane) and the enhanced greenhouse effect have played an important role in early Earth history by compensating for the faint-young-Sun effect (Feulner, 2012; Haqq-Misra et al., 2008; Sagan and Mullen, 1972; Walker, 1982). Models of stellar evolution have led to interpretations that the Sun was 20–25% less luminous in the Archean as compared to modern (Wolf and Toon, 2014; Ozaki et al., 2018), and became 6% less luminous during the Proterozoic (Donnadieu et al., 2004) suggesting that Earth's surface would have been in a frozen state under present atmospheric composition (von Paris et al., 2008). However, there is ample evidence that the surface and ocean temperatures of the Archean were the same, or perhaps even warmer as compared to present day (Knauth and Lowe, 2003; Robert and Chaussidon, 2006; Hren et al., 2009; Blake et al., 2010). Although the need for high CO₂ levels in the Archean has been debated (e.g., Rosing et al., 2010) to overcome the lower solar luminosity, it has been suggested that CO₂ concentrations had to have been at least 100–1000 times that of the present level (Kasting, 1993) in order to maintain the temperatures above the water freezing point.

Direct estimates of the Archean-Proterozoic pCO₂ levels are mainly limited to weathering crusts (paleosols) that are capable of recording the paleoatmospheric composition (e.g., Rye and Holland, 1998; Holland, 2006, Holland, 2009; Sheldon, 2006; Sheldon and Tabor, 2009; Kanzaki and Murakami, 2015, Kanzaki and Murakami, 2018a, Kanzaki and Murakami, 2018b). Paleoatmospheric pCO₂ estimates in the Archean and Proterozoic (Kanzaki and Murakami, 2015, Kanzaki and Murakami, 2018b; Sheldon, 2006) generally agree with, or are lower than, modeled atmospheric composition (Kasting, 2010). However, Precambrian paleosols have a relatively low preservation potential and are commonly preserved as denudation surfaces rather than as complete weathering profiles (Peters and Gaines, 2012). As a result, the pCO₂ estimates obtained from paleosols can vary by more than an order of magnitude (Sheldon, 2006).

The focus of this paper is on pCO₂ modeling at the beginning of Proterozoic using a recently described c. 2.50–2.44 Ga paleoweathering crust formed on basaltic parent rock of the Kuksha Volcanic Formation in the Imandra-Varzuga Greenstone Belt of the Kola Peninsula (Soomer et al., 2019). This weathering crust was formed during a critical time period in Earth history at the Archean-to-Proterozoic transition when sequential global events paved the way for the establishment of the aerobic atmosphere of the modern-type Earth (Bekker et al., 2004, Melezhik et al., 2005; Holland, 2006). The key event was the shift from anoxic to oxic surface environmental conditions, termed the “Great Oxidation Event” (GOE; Holland, 2006; Lyons et al., 2014). The age of the GOE is bracketed between ca. 2.4 and 2.3 Ga (e.g., Bekker et al., 2004) but Gumsley et al. (2017) have recently suggested onset of the GOE between ca. 2.46 and 2.426 Ga. The GOE was also associated with the initiation of the first low-latitude Huronian glaciations between 2.45 and 2.22 Ga (Young, 2019; Evans et al., 1997; Bekker et al., 2004). The atmospheric composition and environmental conditions before and after the GOE are not well-constrained and are thus still highly debated (Bekker et al., 2004; Canfield, 2005; Kaufman et al., 2007; Lyons et al., 2014; Planavsky et al., 2014; Zahnle and Catling, 2014). The Kuksha paleoweathering crust, immediately predating and/or overlapping in age with the GOE thus, could provide important insights into the atmospheric-climatic conditions of this key time interval. The aim of this paper is first, to estimate the atmospheric pCO₂ concentrations at the time of the weathering and, secondly, to determine environmental conditions (mean annual precipitation – MAP; mean annual temperature – MAT) during the formation of the Kuksha weathering crust at the beginning of the GOE using geochemical climofunctions.