Weak Hydrothermal Carbonation of the Ongeluk Volcanics

Shibuya et al. Progress in Earth and Planetary Science (2017) 4:31 Progress in Earth and DOI 10.1186/s40645-017-0145-6 Planetary Science RESEARCHARTICLE Open Access Weak hydrothermal carbonation of the Ongeluk volcanics: evidence for low CO2 concentrations in seawater and atmosphere during the Paleoproterozoic global glaciation Takazo Shibuya1,2,3*, Tsuyoshi Komiya4, Ken Takai1,2,3, Shigenori Maruyama5,7 and Michael J. Russell6 Abstract It was previously revealed that the total CO2 concentration in seawater decreased during the Late Archean. In this paper, to assess the secular change of total CO2 concentration in seawater, we focused on the Paleoproterozoic era when the Earth experienced its first recorded global glaciation. The 2.4 Ga Ongeluk Formation outcrops in the Kaapvaal Craton, South Africa. The formation consists mainly of submarine volcanic rocks that have erupted during the global glaciation. The undeformed lavas are mostly carbonate-free but contain rare disseminated calcites. The 13 carbon isotope ratio of the disseminated calcite (δ Ccc vs. VPDB) ranges from − 31.9 to − 13.2 ‰. The relatively low 13 13 δ Ccc values clearly indicate that the carbonation was partially contributed by C-depleted CO2 derived from 13 decomposition of organic matter beneath the seafloor. The absence of δ Ccc higher than − 13.2‰ is consistent 13 with the exceptionally C-depleted CO2 in the Ongeluk seawater during glaciation. The results suggest that carbonation occurred during subseafloor hydrothermal circulation just after the eruption of the lavas. Previously, it was reported that the carbonate content in the uppermost subseafloor crust decreased from 3.2 to 2.6 Ga, indicating a decrease in total CO2 concentration in seawater during that time. However, the average CO2 (as carbonate) content in the Ongeluk lavas (< 0.001 wt%) is much lower than those of 2.6 Ga representatives and even of modern equivalents. This finding suggests that the total CO2 concentration in seawater further decreased during the period between 2.6 and 2.4 Ga. Thus, the very low content of carbonate in the Ongeluk lavas is probable evidence for the extremely low CO2 concentration in seawater during the global glaciation. Considering that the carbonate content of the subseafloor crusts also shows a good correlation with independently estimated atmospheric pCO2 levels through the Earth history, it seem highly likely that the low carbonate content in the Ongeluk lavas reflects the low atmospheric pCO2 at that time. We conclude that the continuous decrease in CO2 concentration of seawater/atm. from 3.2 Ga was one of the contributing factors to the Paleoproterozoic global glaciation. Keywords: Paleoproterozoic global glaciation, Ongeluk volcanics, Carbonation, Carbon and oxygen isotopes, Seawater/ atmosphere CO2 level * Correspondence: [email protected] 1Department of Subsurface Geobiological Analysis and Research (D-SUGAR), Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan 2Research and Development Center for Submarine Resources, Japan Agency for Marine-Earth Science and Technology (JAMSTEC), 2-15 Natsushima-cho, Yokosuka 237-0061, Japan Full list of author information is available at the end of the article © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Shibuya et al. Progress in Earth and Planetary Science (2017) 4:31 Page 2 of 13 Introduction settings (mid-ocean ridge or rift basin) underwent strong Because the Sun was approximately 30% less luminous carbonation caused by intense interaction with ambient at 4.6 Ga (Gough 1981), the mean temperature of the CO2-rich seawater (Nakamura and Kato 2004; Shibuya Earth’s surface would have been below the freezing point et al. 2007a, 2012). Furthermore, the lower carbonate of seawater if the albedo and atmospheric composition content in the 2.6 Ga greenstones compared to those in of the early Earth had been the same as at the present the 3.5 and 3.2 Ga greenstones indicated that the CO2 (Sagan and Mullen 1972). However, the occurrence of concentration in seawater decreased during Late pillow lavas and sedimentary rocks clearly indicate the Archean period—possibly caused by the formation and presence of liquid water on the early Earth (Sagan and breakdown of the supercontinent at around 2.7 Ga (Shi- Mullen 1972). This “faint young Sun paradox” has been buya et al. 2013a). However, there are no data for the explained by positing a greenhouse effect to counteract carbonation of subseafloor crust after 2.6 Ga, although the lower solar luminosity (e.g., Kasting 1987, 1993; the data from the DSDP/ODP drill cores have been ob- Sleep and Zahnle 2001). Yet, at present, this paradox has tained from the Phanerozoic crusts (Alt and Teagle not yet been entirely resolved because the geologic rec- 1999; Gillis and Coogan 2011). ord suggests that CO2 levels in the Archean atmosphere In this paper, we focus on the Paleoproterozoic era potentially exerted enough of a greenhouse effect to when global climate drastically cooled to produce an ice avoid Earth freezing for contemporary albedos, whereas age, though it reverted thereafter (Evans et al. 1997; the presence of other greenhouse gases such as CH4 and Kirschvink et al. 2000; Harada et al. 2015). In this era, OCS was also reported (e.g., Pavlov et al. 2003; Ueno et the atmospheric CO2 level was temporarily insufficient al. 2009). Furthermore, the changes in albedos of the to keep the Earth’s surface above the freezing point of early Earth also had the potential to compensate the water. Previously, a number of triggers for the Paleopro- faint young Sun (Rosing et al. 2010). However, compre- terozoic global glaciation have been proposed: (1) the hensive geological studies provide evidence suggesting collapse of a methane greenhouse caused by the global that atmospheric CO2 levels in the early Earth were sub- rise of oxygen over the period ~2.5–2.2 Ga (Kopp et al. stantially higher than at present: e.g., > 100 PAL at 3.8– 2005), (2) the removal of atmospheric CO2 via increased 1.8 Ga (Ohmoto et al. 2004), > 100 PAL at 3.5–3.2 Ga silicate weathering of the amalgamated supercontinent (Lowe and Tice 2004), at > 7 PAL bar (25 °C) at 3.2 Ga at ~2.5 Ga (Young 2013) and/or of the newly created vo- (Hessler et al. 2004), 23×/÷3 PAL at 2.2 Ga (Sheldon luminous continental flood basalts at 2.45 Ga (Melezhik 2006), 30–190 PAL at 2.15 Ga, and various other esti- 2006), and (3) the shutdown of volcanic CO2 emissions mates for 2.77–1.85 Ga (Kanzaki and Murakami 2015). resulting from a global magmatic lull that lasted from The putative CO2-rich atmosphere in the early Earth 2.45 to 2.2 Ga (Condie et al. 2009). All these models are would also have kept oceans acidic and CO2- and metal- premised upon low atmospheric CO2 levels. To test this rich (Konhauser et al. 2007). Such CO2-rich seawater supposed prerequisite and to reveal the secular change provided seafloor hydrothermal vent environments that of CO2 concentration in seawater after 2.6 Ga, we inves- would sustain the emergence and early evolution of life tigated the carbonation of the Paleoproterozoic submar- (Takai et al. 2006; Russell et al. 2010, 2014; Shibuya et al. ine volcanics in the Ongeluk Formation, Transvaal 2015, 2016). Furthermore, chemical exchange reactions Supergroup, South Africa. Based on the lines of geo- would have had particular characteristics as the then logical evidence, the Ongeluk volcanics are considered CO2-rich seawater interfaced oceanic lithosphere (e.g., to have erupted during the global glaciation (Kirschvink intense CO2 fixation into the crust) (Nakamura and Kato et al. 2000; Kopp et al. 2005). Therefore, it is expected 2004; Rouchon and Orberger 2008) resulting in the gen- that the degree of carbonation of the Ongeluk volcanics eration of novel high-temperature hydrothermal fluids qualitatively reflects the CO2 concentration in seawater (e.g.., alkaline, silica-rich, metal-poor fluid, and Mg-rich and atmosphere during that glaciation. fluid) (Shibuya et al. 2010; Ueda et al. 2016). These pro- cesses would have played a significant role in controlling Methods the chemistry of early oceans. Therefore, the secular The Ongeluk formation change of CO2 levels in the atmosphere and seawater is The Griqualand West Sequence in South Africa mainly important for understanding the evolution of surface en- comprises Late Archean to Paleoproterozoic sediment- vironments and life on Earth. ary rocks and intercalating volcanic rocks deposited on In this context, the degree of carbonation of green- the Kaapvaal Craton (Beukes and Smit 1987; Cornell et stones has been considered to qualitatively reflect CO2 al. 1996). In the Griqualand West Sequence, the Trans- concentrations in ancient seawater (Kitajima et al. 2001; vaal Supergroup is subdivided into three groups by Nakamura and Kato 2004; Hofmann and Harris 2008). major unconformities; Ghaap Group, Postmasburg The 3.5 and 3.2 Ga greenstones in extensional geological Group and Olifantshoek Group in stratigraphicaclly Shibuya et al. Progress in Earth and Planetary Science (2017) 4:31 Page 3 of 13 ascending order. The Paleoproterozoic Ongeluk Forma- (Evans et al. 1997; Kirschvink et al. 2000; Hoffman 2013; tion belongs to the Postmasburg Group that mainly Gumsley et al. 2017). Furthermore, the Hotazel Forma- consists of diamictite, volcanic rocks, terrigenous clastic tion, consisting predominantly of manganese formation/ rocks, banded iron formations, manganese formations, banded iron formation which includes dropstones at its and shallow marine dolostones (Beukes and Smit 1987).

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