Australia: The Land Where Time Began

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Recycled Selenium Records Ocean-Atmosphere Oxygenation in Lavas Influenced by Hot Spots

There are consequences for subducted surface signatures that are now stored in the mantle of the Earth. In this paper Yierpan et al. report significant variations in selenium isotopes that are mass-dependent in modern lavas that are hot-spot dependent. These variations are correlated with mantle source enrichment for which the only explanation is by the incorporation of abyssal sediment that has been subducted from a redox-stratified ocean from the Mid-Proterozoic. Selenium geochemical signatures of these sediments have been preserved mostly during long-term recycling and, therefore, may complement the global surface sediment record as ancient archives of oxygen. It is implied further, that a significantly oxygenated ocean-atmosphere system was present throughout the mid-Proterozoic, by combined deep mantle and surface perspectives, together with models that are emerging for atmospheric oxygen based on selenium systematics

Plate tectonics intimately links the surface oxygenation and mantle evolution of the Earth (Campbell & Allen, 2008; Keller & Schoene, 2012; Kump & Barley, 2007; Lee et al., 2016; Smith & Mezger, 2017). The geochemical signals that are archived in the surface sedimentary record typically provide the constraints on the ocean-atmosphere redox evolution over time (Holland, 2006; Large et al., 2019; Lyons, Reinhard & Planavsky, 2014; Poulton & Canfield, 2011; Scott et al., 2008; Sperling et al., 2015). As a result of subduction recycling of the oceanic crust, however, most of the sediments of the deep ocean such as those that are typically deposited beyond the continental slopes are likely to have been obliterated from the geological record (Sperling et al., 2015; Patchett et al., 1984). The interior of the Earth therefore provides complementary clues to secular changes in the surface redox conditions, which impart unique geochemical signatures into the mantle by way of subduction (Anderson et al., 2015; Cabral et al., 2013; Labidi, Cartigny & Moreira, 2013; Liu et al., 2019; Stolper & Bucholz, 2019). In this study selenium (Se) isotopic variation in the oceanic lavas that were hot spot-influenced was used in order to infer the recycled Se isotopic and elemental composition of sediments and pyrites that had been subducted in the mid-Proterozoic. Selenium is a chalcophyle element that is redox-sensitive and exhibits a variety of oxidation states (-2, +4 and +6) in the different geological reservoirs (Johnson & Bullen, 2004). Selenium isotopic and elemental abundances of marine sediments and sedimentary pyrites have emerged as new redox proxies in the ocean-atmosphere system, because of the distinct mobility and large isotopic fractionation between different Se species in low-temperature environments. Large Se isotopic variations, with δ⁸²Se values (deviation in ⁸²Se/⁷⁶Se relative to the standard between ~-3 per mil (‰) and +3‰ are displayed by global marine sediments. From the Precambrian to the Phanerozoic there is a marked shift in sediment average δ⁸²Se towards lighter values (from ~-0.54‰ to -0.17‰), which reflects oxygenation of the deep ocean in the Late Neoproterozoic (Stüeken et al., 2015). In mantle samples δ⁸²Se variability, by contrast, is rather limited though resolvable (~-0.3‰ to +3‰). According to Yierpan et al., selenium isotope signatures should be highly sensitive to the presence of recycled sediments, given the large differences in Se contents between the igneous and surface reservoirs (a difference of ~1 to 2 orders of magnitude), and the lack of isotope fractionation during high-temperature mantle processes that involve Se partitioning between surface and silica melt (Yierpan et al., 2019). Selenium systematics are thus particularly sensitive tracers to study ocean-atmosphere redox evolution that is recorded by the surface and interior of the Earth.

In this study, Se isotope composition of selected samples from the mid-ocean ridge basalt (MORB) glasses from the southern and northern mid-Atlantic Ridge (S- and N-MAR). The presence of recycled surface materials highlighted by localised interaction between the S-MAR and Shona and Discovery hot spots resulted in prominent geochemical heterogeneities in the MORB mantle source (Labidi, Cartigny & Moreira, 2013; Andres, Blichert-Toft & Schilling, 2002; Douglass, Schilling & Fortignie, 1999; le Roux, 2002). The samples that are targeted cover a full spectrum of radiogenic isotope variations that are found in global MORB, that range from highly depleted to enriched basalts with enriched mantle 1 (EM1), LOMU (low μ; μ²³⁸U/²⁰⁴Pb), and HIMU (high μ) affinities.

Origin of Se isotopic variability in the mantle

It is shown by an x² test that mantle domains that are strictly depleted have isotopic compositions that are homogenous with statistical variability that is only subtle. The correlations that have been observed between δ⁸²Se and ⁸⁷Sr/⁸⁶Sr, ¹⁴³Nd/¹⁴⁴Nd, and δ³⁴S in the S-MAR basalts cannot be explained by fractionation during melting of the mantle and/or igneous differentiation (Yierpan, König, Labidi & Schoenberg, 2019). The data of Yierpan et al. require, instead, incorporation of enriched plume components with isotopically heavier Se into the ambient asthenospheric mantle. It is argued by Yierpan et al. that the enrichment of heavy Se isotopes in the S-MAR can be achieved only by the addition of pelagic sediments, which were constrained previously to have a recycling age of the mid-Proterozoic between 1 and 2 Ga (billion years ago) based on radiogenic Pb and stable S (both mass-dependent and mass-independent) isotope systems (Labidi, Cartigny & Moreira, 2013; Andres, Blichert-toft, & Schilling, 2002; Douglass, Schilling & Fortignie, 1999).

1.    Most sediments from the Proterozoic are enriched in ⁸²Se relative to the igneous inventory, which would satisfy readily the positive slope of the Se-Sr isotope mixing relationship.

2.    The recycled components are required by the apparent linearity of this trend to have Se/Sr ratios comparable to the depleted mantle and only pelagic sediments qualify under such criteria as a result of authigenic Se enrichment.

Credence is lent to a broadly linear mixing trend by assessment of alternative mixing models (see below). Other materials that are recycled that carry EM1, LOMU, and HIMU components in the Discovery and Shona plumes [such as ancient oceanic crust, delaminated subcontinental lithospheric mantle, and lower continental crust (Andres, Blichert-Toft & Schilling, 2002; Douglass, Schilling & Fortignie, 1999; le Roux et al., 2002)] have little effect on the Se isotope signature of the MAR mantle source, because either their δ⁸²Se are within the igneous inventory, or their Se contents are comparable to the MORB mantle. This is similar to the case of the systematics of S isotope and highlights close geochemical relationship between Se and S during long-term recycling of crust, in spite of the large differences in the redox potential of these systems in surface environments.

Recycled sediments from a redox-stratified ocean

When interpreted within the framework which had previously been established, of marine Se cycle, Se isotope and elemental systematics of recycled sediments (Large et al., 2019; Johnson & Bullen, 2004; Large et al., 2014; Stüeken et al., 2015), may reveal an average global extent of ocean oxygenation over a broad geological interval of time. These results from pelagic sediments that are subducted giving an integrated view of those that have been continuously been deposited at various depths/redox conditions in the open ocean over the lifetime of seafloor that is subducting [e.g., ~50 to 100 Myr on average (Syracuse & Albers, 2006) versus residence time of Se in the ocean of 10⁴ years. It might also be expected that further homogenisation of Se within the subducted package during storage of mantle for ~1 – 2 Ga is sedimentary sulphides (as the major host of Se) were molten at a range of convective upper mantle conditions (Yierpan, König, Labidi & Schoenberg, 2019; Zhang, von der Handt & Hirshmann, 2918).

The δ⁸²Se – ⁸⁷Sr/⁸⁶Sr of covariation of the MORB data from this study – combined with the model that had been established previously of linear ³⁴S-⁸⁷Sr/⁸⁶Sr relationship (Labidi, Cartigny & Moreira, 2013) and overall similarity between Se and S isotope and elemental behaviour during recycling - allows extrapolation of the content of Se and ⁸²Se of sediment that has been recycled by the use of a linear mixing model. An average Se content of 2.45 ± 0.71 μg/g (1s) was calculated for the recycled sediment by using the well constrained Se/Sr ratio of the depleted mantle and Sr content of recycled sediment. This value is close to the +1s upper bound of the observed sediment average that is essentially identical for the 1- to 2-Ga age interval 0.85^+2.12/_-0.61 μg/g, 1s) and the entire Proterozoic (0.56^+2.27/_-.45 μg/g, 1s. This is an argument against a significant, if any, loss of Se (or modification of the Se/Sr ratio) and, therefore, fractionation from bulk sedimentary lithologies during subduction in the mid-Proterozoic and recycling on a large scale in the  mantle, which is reminiscent of that suggested for S (Labidi, Cartigny & Moreira, 2013). When the linear regression was extrapolated to a model composition of 1,5-Ga-old recycled sediment ⁸⁷Sr/⁸⁶Sr = 0.7203 (Labidi, Cartigny & Moreira, 2013; Andres, Blichert-Toft & 2002; Roux et al., 2002; Rehkӓmper & Hofmann, 1997) it yields δ⁸⁷Sr = +1.44 ± 3.9‰ (95% CI). This value, though it is not uncommon in sediments from the mid-Proterozoic, is similar to or heavier than the +1s upper bound of the observed average for 1- to 2-Ga interval (+0.62 ± 0.90‰; 1s, N = 76) or the entire Proterozoic (+0.53 ± 1.13‰; 1s, N = 210). According to Yierpan et al. this is reassuringly consistent with pelagic sediments that are recycled from an ocean that is redox stratified (Poulton & Canfield, 2011), where the subduction of oceanic plate was associated predominantly with sediments from the deep ocean that were deposited on the abyssal seafloor beyond the continental slope settings [e.g., (Patchett et al., 1984)]. Because of near-quantitative reduction of water soluble Se oxygenation under oxic/suboxic conditions at shallower depth of water, these abyssal sediments would be characterised by the highest δ⁸²Se. The applicability and robustness of the simple linear mixing model for characterisation of the recycled sediment reservoir was further assessed in this study. The ⁸²Se-⁸⁷Sr/⁸⁶Sr array within the MORB data range might represent only a small segment of a hyperbolic mixing curve, considering the amount of sediment that was added to the S-MAR mantle in small [up ~1 weight % (wt %) in the model of Yierpan et al.; see also (Labidi, Cartigny & Moreira, 2013; le Roux et al., 2002)]. Besides, Se content and ⁸²Se of the sediment end member cannot be determined independently. However, the permissible range of these 2 variables can be constrained. It is shown by their mixing models with the known range of 1- to 2-Ga-old sediment compositions (mean ± 1s) that a realistic Se content and ⁸²Se of recycled end member in the S-MAR source must both be at least comparable to the +1s upper bound of the sediment averages that have been observed. Strong support is provided by this result for the idea (see above) that there was insignificant Se modification during subduction of the sediment and that recycled sediments reflect dominantly “abyssal” δ⁸²Se signature at the surface. Moreover, all the mixing arrays that are compatible with the MORB data and the observed sediment averages (δ³²Se up to +2s upper bound) of Yierpan et al., lie well within the 95% CI range of the linear extrapolation, which, in turn, implies the sediment composition that is recycled may be estimated adequately by a mixing model that is broadly linear.

The possible ³²Se range of recycled sediment that is observed from δ³²Se relationship at other reasonable ages of recycling between 1 and 2 Ga would, accordingly, also remain realistic for sediments that have been subducted associated with prevalent anoxia in the deep ocean (Poulton & Canfield, 2011) and, therefore, not alter the interpretation of Yierpan et al.

Mantle recycling of atmospheric oxygenation

Large portions of deep marine pyrites from the surface that had been removed by subduction and therefore a significant reservoir of Se was transferred into the deep mantle source of their hot spots. This abyssal Se record that was “lost-and-resurfaced” complements the shallower marine pyrite record that was preserved on the surface of the Earth, which was used for reconstruction of the ocean-atmosphere redox evolution (Large et al., 2019; Large et al., 2014; Mukherjee et al., 2018). Combining the results of this study for the recycled sediment content of Se (2.45 ± 0.71 μg/g; 1s) with the mean pyrite-bound S content of 1- to 2-Ga-old sediments [1.1^+1.4/_-0.6 wt %; 1s, N = 85; (Scott et al., 2008; Poulton, Fralick & Canfield, 2004)] and mean pyrite/matrix Se ratio of ~0.82 observed for sediments of all ages (Large et al., 2014), it was estimated by Yierpan et al. to be a range of Se content between 13⁺⁴/-₄ and 119⁺¹⁵⁵/_-74 μg/g (1s) for the pyrite that was recycled. This range depends on the relative contributions of pyrite and other matrices (organic matter, clay, and other silicates) to the bulk Se budget in global black shales (Large et al., 2014). Because of the greater fraction of organic-bound Se in the sediments of the mid-Proterozoic compared with sediments from the Phanerozoic, where more abundant Se might be incorporated into pyrites after dissimilatory reduction of Se oxyanions in seawater (Stüeken et al., 2015), a more realistic value tends towards the “lower estimate” of Yierpan et al. Different results for the content of Se of recycled pyrite would result from the invoking of nonlinear mixing relationships between the depleted mantle and sediment, and its minimum possible range is calculated at fixed δ⁸²Se values of 1s and 2s above the mean that is observed of 1- to 2-Ga-old sediments. A “lower estimate” of recycled pyrite Se content that is comparable within error to the average sedimentary pyrite that was formed during the 1- to 2-Ga-old interval (19⁺³³/_-12 ɥg/g) or entire Proterozoic [⁺²⁴/_-10 ɥg/g, 1s; (Large et al., 2019; Large et al., 2014; Mukherjee et al., et al., 2018)].

Additional support is lent to the representativeness of Se of surface pyrites, though shallower, for atmospheric modelling, by the notable similarity between the recycled abyssal and surface pyrite Se signatures from different depositional/redox environments (Large et al., 2019). The idea of high atmospheric oxygen levels over an extended interval of time (~50 – 100 Myr; see above) in the mid-Proterozoic [on average, ~30 to 60% present atmospheric level (PAL), is supported, in turn, within the framework of the  Large et al., (Large et al., 2019) model, the concentration estimates of Se in the present study for the recycled abyssal pyrite. Such a high estimate for oxygen in (Large et al., 2019) is in contrast with earlier suggestions that ranged from <0.1% to 10% PAL (Bellefroid et al., 2018; Canfield et al., 2018; Gilleaudeau et al., 2016; Planavsky et al., 2014; Zhang et al., 2016), with the range that is  most widely accepted range of about 0.1 to 10% PAL (Lyons, Reinhard & Planavsky, 2014); note however that it still remains lower than the threshold values that are required to sustain a fully oxygenated abyssal ocean [~70 to 80% PAL; (Alcott, Mills & Poulton, 2019)], which is consistent with the well-established mid-Proterozoic ocean redox structure [e.g., (Poulton & Canfield, 2011)] and the Se isotope signature that is inferred from recycled sediments demonstrated above. It seems these nuanced pictures are not consistent with the static evolution of marine chemistry and low atmospheric oxygen across the middle age of the Earth [~0.8 to 1.8 Ga, aka the “Boring Billion”, that is widely assumed to have been the case (Holland, 2006; Bellefroid et al., 2018; Planavsky et al., 2014; Brasier & Lindsay, 1998; Cawood & Hawkes, 2014). Rather, as has been suggested by models that are emerging recently, it appears that the Earth underwent a dynamic configuration of its surface and interior during this period towards a world that was habitable for the later emergence of complex life (Large et al., 1029; Mukherjee et al., 2018; Doglioni, Pignatti & Coleman, 2016; Hamilton, 2019; Mukherjee & Large, 2020).

Yierpan, A., et al. (2020). "Recycled selenium in hot spot–influenced lavas records ocean-atmosphere oxygenation." Science Advances 6(39): eabb6179.