Late Paleoproterozoic ocean redox history from sedimentary thallium and uranium isotopes and implications for eukaryotic evolution

Abstract

The most complex domain of life on Earth, eukaryotes, may have evolved as early as ~2.7 billion years ago (Ga), but the timing and trajectory of their evolution is not well understood nor are the environments in which they evolved. As most modern eukaryotes are aerobic, molecular oxygen (O2) may have played an important role in the development and proliferation of early eukaryotes. However, this is an area of continued debate. The oldest evidence for eukaryotic life lies in the ~1.8 Ga sedimentary rock record as organic-walled microfossils which have been counted and classified to determine their abundance and diversity. However, O2 cannot so readily be accounted for in the rock record. Instead, geochemical redox proxies—chemical signatures resulting from interactions with O2—can be employed to deduce past environmental conditions. At ~1.8 Ga, the Earth’s atmosphere may have been weakly oxygenated, and the oceans may have been stratified into weakly oxygenated surface waters, anoxic (O2-free) and sulfidic (euxinic) mid-depths and anoxic and ferruginous (Fe2+-rich) deep waters. However, details about the areas in which the earliest known eukaryotes inhabited remain vague. Here, uranium (U) and thallium (Tl) isotopes in fine-grained, organic-rich sedimentary rocks of the late Paleoproterozoic—some of which host eukaryotic microfossils—are used to determine the extent of marine productivity along continental margins ~1.84 Ga (U isotopes) and if oxygenation was occurring within an intracontinental basin ~1.78–1.64 Ga (Tl isotopes). Ultimately, these findings provide a better understanding of the environments in which the earliest known eukaryotes inhabited. In the modern oceans, seawater U isotope compositions (δ238Usw) respond to changes in the areal proportions of seafloor covered by oxic, anoxic and euxinic bottom waters. Isotopic fractionation primarily occurs in anoxic/euxinic environments during U(VI)→U(IV) reduction, where the reduced sedimentary phase preferentially retains the heavier 238U and seawater preferential retains the lighter 235U. Because of this, at times of greater oxygenation like today where oceans are oxygenated throughout with only small areas of anoxia, δ238Usw (–0.38‰) is similar to the composition of incoming rivers (–0.29‰). In the Proterozoic when oceans featured ferruginous anoxic deep waters, euxinic continental margin mid-depths, and a thin oxic surface layer, it is expected that δ238Usw would be much lighter. However, Proterozoic carbonates, which approximate δ238Usw at the time they were deposited, are not significantly different from modern. This is resolved with new U isotope data from ~1.84 Ga across a drillcore transect in the Animikie Basin (North America) which features black shales deposited from low-O2, ferruginous and euxinic bottom waters. Unlike carbonates which differ from δ238Usw only by a minor diagenetic effect, black shale δ238U may be heavier than δ238Usw as a result of reductive isotopic fractionation. The heaviest δ238U recorded in the Animikie Basin are found in highly productive (high total organic carbon, TOC) euxinic and ferruginous mid-depths (0.15 ± 0.14‰) and dynamic environments at the redoxcline (0.39 ± 0.16‰). In contrast, low-O2 environments near the shoreline and low-TOC ferruginous environments on the deep shelf in the basin featured light δ238U (–0.10 ± 0.25‰) close to the river input composition. These findings reveal that a widely anoxic ocean can yield δ238Usw near-modern if much of it was low-TOC ferruginous deep waters with minimal isotopic fractionation and only small areas on continental margins with highly productive euxinia had large isotopic fractionations. This is useful for quantifying the level of Proterozoic primary productivity, as times with lighter δ238Usw could signify an expansion of highly productive margins. This is observed at ~1.84 Ga in the carbonate record, which indicate enhanced primary productivity triggered by nutrient input from continental weathering or large igneous province volcanism. A large primary productivity signal with no currently known examples of eukaryotes at ~1.84 Ga could be indicative of dominantly anoxygenic photosynthesis that could have suppressed the development of early aerobic eukaryotes. Thallium isotopes in seawater (ε205Tlsw) respond to the amount of manganese (Mn) oxide burial under well-oxygenated bottom waters. The heavier 205Tl isotope is preferentially adsorbed to and oxidized by Mn oxides and seawater preferentially retains the lighter 203Tl isotope. Thus, at times with expansive oxygenation and Mn oxide burial, as at present, ε205Tlsw (–6‱) is lighter than ocean inputs (–2‱). This can be harnessed for ancient ocean oxygenation reconstruction because under most reducing conditions (low-O2, anoxia, euxinia), black shales and organic-rich mudrocks will directly record contemporaneous ε205Tlsw. In the McArthur Basin (Australia), shales from the ~1.78–1.73 Ga Tawallah Group and ~1.65–1.64 Ga McArthur Group reflect ε205Tlsw in an intracratonic basin (not the global ocean) that cover a range near modern input compositions to modern ε205Tlsw: –2.1‱ to –5.5‱. All geological units studied feature sample groups with ε205Tl near input composition (averaging –2.6‱ to –2.9‱), called ‘baseline’ groups which represent periods of minimum intracratonic basin oxygenation. Periods of isotopic excursion to lighter ε205Tlsw were also recorded in each unit from averages of –3.7‱ to –5.2‱, reflecting periods of expanded intracratonic basin oxygenation. Some of the oldest eukaryotic microfossils are found in the McArthur Basin at ~1.78 Ga and ~1.65 Ga, although often these are found in the locally low-O2 or anoxic environments seen in the studied drillcores. These may therefore be stem group eukaryotes which evolved to live in lower O2 conditions but were not direct ancestors to modern aerobic eukaryotes. However, the presence and periodic expansion of oxygenated environments may have harboured aerobic eukaryotes that have yet to be discovered.