Interglacial dust, ocean fertilisation and Neoproterozoic earth oxygenation

Abstract

Feedback processes that drove Earth's second major increase in ocean–atmosphere oxygen levels during the Neoproterozoic are poorly constrained. Variability in seawater redox over geological timescales is commonly linked to changes in the biogeochemical cycling of P and thus the rate of primary production and generation of photosynthetic oxygen. In the modern surface ocean, an important source of bioessential P and micronutrients (Fe, Cu, Co, Zn, Mo, Cr and Ni) is aeolian dust derived from deserts and arid, post‐glacial landscapes. It is interpreted herein that glacial retreat following the Sturtian (ca 717 to 660 Ma) and Marinoan (ca 650 to 635 Ma) snowball glaciations provided copious dust to the global ocean. Correlation of interglacial siltstone successions in palaeogeographical context suggests that such dust accumulation was diachronous and concentrated in the palaeo‐horse latitudes (30° N and 30° S). Delivery of this dust from continents is likely reflected in the steep increase in global radiogenic Sr isotope values (⁸⁷Sr/⁸⁶Sr) in post‐Sturtian carbonates, and changes in the δ¹⁸O signatures of Cryogenian zircons derived from subducted marine sediments. Accumulation of sedimentary organic matter also peaked during interglacial periods, suggesting a causal link between glaciation, aeolian dust and primary production. This relationship implies windblown dust was an important source of P and micronutrients for an evolving biological pump that stimulated primary production, enhanced burial of organic carbon and increased ocean–atmosphere oxygen concentrations. Thus, delivery of aeolian dust to the global ocean was likely critical for sustaining Earth's second major increase in oxygen. Sequestration of atmospheric CO₂ in organic‐rich siltstones and shales is also interpreted to have been an important negative feedback process, which together with silicate weathering, prevented runaway greenhouse conditions during interglacial periods. The oxygen produced by this aeolian marine biological pump may have helped pave the way for the evolution of multicellular animals in the Ediacaran.

INTRODUCTION

The onset of Neoproterozoic ocean–atmosphere oxygenation began in the Tonian (ca 750 Ma) and is thought to have been linked to snowball glaciations (Kirschivink, 1992; Hoffman & Schrag, 2002; Canfield, 2005; Holland, 2006, 2009; Och & Shields-Zhou, 2012; Hoffman et al., 2017). Whether this was a single protracted event or a series of smaller, asynchronous episodes this period is considered the second major stepwise increase in oxygen after the Palaeoproterozoic Great Oxidation Event (Och & Shields-Zhou, 2012; Lyons, 2014; Sahoo et al., 2016; Goddéris et al., 2017; Lyons et al., 2021). By the Ediacaran, (ca 580 Ma) oxic marine environments had expanded to concentrate bioavailable P at the seafloor (Brasier & Callow, 2007; Nelson et al., 2010; Papineau, 2010; Pufahl & Groat, 2017) and to stimulate the evolution of multicellular animals (Logan et al., 1995; Canfield et al., 2008; Och & Shields-Zhou, 2012; Wood et al., 2015; Tostevin et al., 2016; Boag et al., 2018; Bowyer et al., 2020, 2023; Lyons et al., 2021; Sperling et al., 2021; Stockey et al., 2024; Walton et al., 2023; Kaiho et al., 2024).

In modern open ocean systems, oxygen generated through primary production in surface waters is largely controlled by nutrient input from aeolian dust derived from deserts and glacial rock flour (Shinn et al., 2000; Garrison et al., 2003, 2006; Mahowald et al., 2008, 2014; Jickells et al., 2014; Marticorena, 2014; Muhs et al., 2014). This silt and clay are an important external source of bioessential P, which is thought to limit photosynthetic oxygen production over geological timescales (Föllmi, 1996; Tyrrell, 1999; Moore et al., 2013; Falkowski, 2014; Jickells et al., 2014).

Aeolian dust transport and deposition are, however, seldom considered when attempting to understand the relationship between Neoproterozoic nutrient cycling, primary production and ocean–atmosphere oxygenation. Modelling suggests that increased dust input to the atmosphere was several times higher in the Neoproterozoic than the Phanerozoic because of the snowball glaciations, strengthened wind circulation and absence of terrestrial plants (Hoffman et al., 2017; Liu et al., 2020). With fluvially derived nutrients restricted to coastal environments (Poulton & Raiswell, 2002), the copious silt and clay generated by physical weathering and glacial grinding during the Sturtian (ca 717 to 660 Ma) and Marinoan (ca 654 to 632 Ma) glaciations (Kennedy et al., 2006; Tosca et al., 2010) were likely critical for fertilising the Neoproterozoic open ocean (Liu et al., 2020).

A compilation of published petrographic and sedimentologic data of Cryogenian and Ediacaran interglacial siltstone successions, including those with strong evidence of an aeolian origin (e.g. pits and grooves on grain surfaces, grain size distribution and absence of correlative fluvial deposits), as well as the global correlation of these stratigraphic sections in a palaeogeographical context are herein presented for the first time. As on the modern Earth, palaeogeography suggests that Neoproterozoic oceanic fertilisation was focussed in the arid subtropical highs of the horse latitudes at 30° N and 30° S where the Hadley and Ferrell atmospheric cells converged (Webster, 2004) and dust deposition dominated. Outside of these windy high-pressure belts, carbonate accumulation was common. Accumulation of sedimentary organic matter preserved as black shale, graphite and hydrocarbons (Condie et al., 2001; Ghori et al., 2009; Le Heron & Craig, 2012) suggests that nutrients carried in windblown rock flour generated during the Sturtian and Marinoan glaciations not only formed thick organic-rich siltstones and black shales but were also critical for stimulating sustained photosynthetic oxygen production during interglacial periods. Additionally, an increase in radiogenic ⁸⁷Sr/⁸⁶Sr values in marine carbonates (Veizer, 1989; Halverson et al., 2007; Melezhik et al., 2009; Chen et al., 2022) and anomalous δ¹⁸O values in inherited zircons from subducted sediments (Sundell et al., 2024) are consistent with an increase in the supply of continentally derived sediment to the Cryogenian oceans, possibly driven by interglacial aeolian dust.

This synthesis paper presents new connections between Neoproterozoic siltstone deposition and aeolian nutrient input to yield novel insights into the processes that drove Neoproterozoic oxygenation. Important links are established between deglaciation, atmospheric circulation, aeolian dust mobilisation, nutrient cycling, ocean fertilisation and widespread production of photosynthetic oxygen during the dawn of multicellular animals. This collection of interconnected processes worked concomitantly with other Earth system feedbacks to regulate Neoproterozoic ocean–atmosphere oxygenation and is herein termed the aeolian marine biological pump (AMBP).

METHODS

Thirty published stratigraphic intervals through Neoproterozoic siltstone successions were compared, using the most recent interpretations and age constraints from the literature (Figure 1; Table 1 and references therein). Stratigraphic sections were anchored in space and time on the Neoproterozoic palaeogeographical reconstructions that reflect the tectonic setting of the Amazon and São Francisco cratons in relation to the Bambui Group (Li et al., 2008, 2013). The sections were hung on diamictites and cap carbonates, which are globally recognised marker horizons in Cryogenian and Ediacaran strata (Knoll et al., 2006; Bold et al., 2016; Pu et al., 2016; Park et al., 2019; Xiao & Narbonne, 2020; Shields et al., 2021). This permitted correlation of the Sturtian-Marinoan (Cryogenian Interglacial, ca 660–650 Ma) and Marinoan-Gaskiers (Ediacaran Interglacial, ca 635–580 Ma) periods with the arid zones in the horse latitudes (Figure 2). The first appearance of Ediacaran fossils (Xiao & Narbonne, 2020 and references therein) and the Shuram Negative Carbon Isotope Excursion at ca 574–562 Ma (Canfield et al., 2020; Rooney et al., 2020) were used to refine stratigraphic correlations and to establish approximate age constraints. In general, the glacial–interglacial transition is recorded in stratigraphic sections by glacial diamictites that are blanketed by a cap carbonate (Kirschivink, 1992; James et al., 2001; Hoffman, 2002; Hoffman & Schrag, 2002; Pruss et al., 2010; Arnaud et al., 2011; Hoffman, 2011; Crockford et al., 2016; Xiao & Narbonne, 2020). Conformable deposits above each cap carbonate are an assemblage of either clastic or carbonate lithofacies, depending on palaeolatitude and basin setting.

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TABLE 1 Neoproterozoic siltstone and silt-rich successions with interpreted aeolian provenance deposited during the Cryogenian (ca 650 Ma) and Ediacaran (ca 600 Ma) interglacials.

Formation	Basin name/region	Basin type	Country	Fluvial influence	Features	Depositional environment	References
ca 600 Ma
Pertatataka	Amadeus Basin	Foreland	Australia	✓	OM, Gl	Pelagic-turbidite	Calver and Lindsay (1998) and Grey and Calver (2007)
Brachina	Adelaide Rift	Rift	Australia	✓		Pelagic	Borch et al. (1988), Cowley (2020)
Tanana	Officer Basin	Foreland	Australia	✓		Prodelta, distal delta front	Calver and Lindsay (1998)
Mall Bay	Avalon Peninsula	Foreland	Canada			Contourite/turbidite	Gardiner and Hiscott (1988)
Biri/Bjørånes	Central	Rift	Norway	✓	OM, FPy	Offshore	Bjørlykke (1965), Tucker (1983), Nystuen and Lamminen (2011) and Vidal (1990)
Doushantuo (Mb. II)a	Weng'an	Epeiric sea	China		P, OM, FPy, Gl	Carbonate platform	Jiang et al. (2011) and Zhou et al. (2017)
Canyon	East	Foreland	Greenland			Offshore	Fairchild and Hambrey (1995)
Dracoisen (D-3)	Svalbard	Rift	Norway		Gl	Offshore	Fairchild and Hambrey (1995)
Ola	Zavkhan Terrane	Passive margin	Mongolia		HCI	Carbonate ramp/platform	Bold et al. (2016) and Macdonald et al. (2009)
Hüttenberga	Otavi Mountains	Passive margin	Namibia		OM, HCI	Peritidal	Cui et al. (2018), Hoffman (2011) and Hoffman (2023)
Serra de Santa Helena	São Francisco	Epeiric sea	Brazil		OM, FPy, L, HCI	Deep subtidal	Campos (2017), DaSilva et al. (2022) and Reis et al. (2016)
Ura	Patom Uplift	Foreland	Siberia	✓	OM, FPy, L, HCI	Deep subtidal	Petrov (2018) and Pokrovsky and Bujakaite (2015)
Masirah Bay (Mbr3)	Huqf-Haushi	Passive margin	Oman	✓	Gl	Delta front	Allen and Leather (2006) and Osburn et al. (2014)
ca 650 Ma
Aralka	Amadeus Basin	Foreland	Australia	✓	OM	Basinal	Le Heron and Craig (2012), Preiss (2000) and Verdel and Campbell (2017)
Meramangye	Officer Basin	Foreland	Australia	✓		Shelf	Calver and Lindsay (1998)
Tapley Hill	Adelaide Rift	Rift	Australia	✓	OM, FPy	Basin floor	Le Heron and Craig (2012), Preiss (2000) and Verdel and Campbell (2017)
Brøttum/Bjørånes	Central	Rift	Norway		OM, Fpy	Basinal	Englund (1973), Nystuen and Lamminen (2011) and Vidal (1990)
Arena	East	Rift	Greenland	✓		Basinal, turbidite	Fairchild and Hambrey (1995) and Stouge et al. (2011)
Elbobreen (E3)	NE Svalbard	Rift	Norway			Basinal, turbidite	Fairchild and Hambrey (1995) and Fairchild et al. (2016)
Taishir (Tayshir)a	Zavkhan Terrane	Passive margin	Mongolia		OM, HCI	Carbonate ramp	Bold et al. (2016) and Macdonald et al. (2009)
Datangpo	Weng'an	Passive margin	China		HC, OM	Shelf margin	Bao et al. (2018), Zhang et al. (2011) and Zhu et al. (2019)
Ombaatjie/Aurosa	Otavi Mountains	Passive margin	Namibia		HCI	Carbonate platform	Bechstädt et al. (2009), Hoffman et al. (2021) and Hoffman, 2023)
Carrancas/Macaubas	São Francisco Basin	Rift/Passive margin	Brazil	✓	OM	Shoreface to deep subtidal	De Castro et al. (2019), Pedrosa-Soares et al. (2011) and Uhlein et al. (2016)

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The rate of continental input in seawater estimated, after the ⁸⁷Sr/⁸⁶Sr data from Chen et al. (2022), was split into a pre-Sturtian (3520–662 Ma) and post-Sturtian (650–537 Ma) data series, with a single break at 650 Ma. Only Sr isotope data with values >500 ppm and with ages flagged as high-certainty or moderate-certainty were considered. Linear regression for ⁸⁷Sr/⁸⁶Sr values in both age series was calculated using MS Excel. The slope of regression illustrates the contrast between the rate of radiogenic increase in pre-Sturtian and post-Sturtian times.

Numerical models of Neoproterozoic dust production and modern ocean–atmosphere data were used to assess how the delivery of aeolian derived P and micronutrients stimulated primary production (Figure 3) and consequently, generation of photosynthetic oxygen in the Neoproterozoic surface ocean (Benitez-Nelson, 2000; Mahowald et al., 2005, 2008, 2014; Paytan & McLaughlin, 2007; Moore et al., 2013; De La Rocha & Passow, 2014; Jickells et al., 2014; Liu et al., 2020). It is assumed here that Neoproterozoic sediment could not be trapped on continents by the binding action of roots because land plants only evolved in the Devonian (Davies & Gibling, 2010). Although organic binding by lichen or microbial mats may have occurred in some terrestrial environments (Knauth & Kennedy, 2009; Kump, 2014; Lalonde & Konhauser, 2015), these communities were likely restricted to wet low-lying and coastal areas far removed from continental interiors that were under the influence of the horse latitudes. These assumptions are corroborated by the robust sedimentological record for aeolian depositional processes in the late Neoproterozoic (Eriksson & Simpson, 1998; Eriksson et al., 1998; Allen & Hoffman, 2005; Bose et al., 2012). A strong equator-to-pole temperature gradient during Neoproterozoic interglacial periods is also thought to have invigorated atmospheric circulation (Eriksson & Simpson, 1998; Eriksson et al., 1998; Allen & Hoffman, 2005; Bose et al., 2012), which undoubtedly intensified wind abrasion and transport of glacially derived dust in the horse latitudes. Thus, estimates of aeolian dust and nutrient delivery based on the modern ocean–atmosphere system and presented herein are very conservative when extrapolated to the Neoproterozoic.

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Aeolian dust and siltstone accumulation

Aeolian dust consists of windblown silt and clay derived from the physical and chemical weathering of rocks, volcanic activity, wind abrasion and mechanical grinding of bedrock by glaciers (Smalley, 1966, 1990; Smalley & Krinsley, 1978; Whalley et al., 1982; Pye, 1984, 1995; Hesse, 1994; Assallay et al., 1998; Smith et al., 2002; Muhs, 2006, 2013; Schwamborn et al., 2012; Muhs et al., 2014). Windblown quartz and feldspar grains are characterised by pits, grooves and smoothed faces (Figure 4) formed by aeolian saltation and collision (Brookfield, 2011; Marticorena, 2014; Scheuvens & Kandler, 2014; Vos et al., 2014). The grain size of particles transported by suspension is typically <70 μm, above which entrainment by wind is more difficult (Marticorena, 2014). Saltation of larger particles (ca >100 μm), however, breaks grains across mineral crystal faces (Smalley, 1990; Assallay et al., 1998; Smith et al., 2002; Crompton et al., 2020) to produce clasts that can be carried by windblown suspension (Smalley, 1990; Assallay et al., 1998; Smith et al., 2002; Marticorena, 2014 and references therein; Crompton et al., 2020). The grain size distribution in aeolian deposits depends on the distance from source and is usually unimodal, skewed towards the silt fraction with a relatively uniform decrease in mud-size particles (McGee et al., 2013; Vandenberghe, 2013; Mahowald et al., 2014).

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Glaciated continents are an important source of mineral dust, especially during cyclic glacial advances and retreats, when freeze–thaw cycling and mechanical grinding produce rock flour that is further reworked and transported by wind (Smalley, 1966, 1990; Whalley et al., 1982; Assallay et al., 1998; Smith et al., 2002; Schwamborn et al., 2012; Vandenberghe, 2013, 2022; Marticorena, 2014; Vos et al., 2014). When deposited on land, this mineral dust accumulates as loess, which forms nutrient-rich deposits commonly altered by pedogenic processes to generate fertile soils (Smalley, 1966, 1990; Smalley & Krinsley, 1978; Pye, 1984; Pécsi, 1990; Bristow et al., 2010; Muhs, 2013; Jickells et al., 2014; Sprafke & Obreht, 2016). In the marine realm, aeolian dust accumulates as silt and mud that are either reworked by currents in shallow settings or fall via suspension rain in distal environments to form fine-grained laminated deposits (Dalrymple et al., 1985; Hesse, 1994; Rea, 1994; Hesse & McTainsh, 2003; Stuut, 2014).

Aeolian dust deposition is concentrated in the horse latitudes because trade winds favourable for dispersing fine-grained, desert sediment develop in these arid high-pressure atmospheric belts at ca 30° N and S where Ferrell and Hadley convection cells converge (Webster, 2004). Dust either accumulates in coastal waters or is transported in the stratosphere for thousands of kilometres (Hesse, 1994; Hesse & McTainsh, 2003; Muhs, 2013; Knippertz & Stuut, 2014; Muhs et al., 2014; Stuut, 2014). In situ measurements and modelling of Saharan dust accumulation suggest that sedimentation rates in this region, calculated from drill core, average ca 10 mm/kyr for the past 2000 year of accumulation (McGee et al., 2013). Over a period of ca 10 Myr, which is the estimated duration of Cryogenian interglacial periods (Bao et al., 2018; Park et al., 2019; Zhou et al., 2019), a siltstone succession ca 100 m thick is calculated to have accumulated. This thickness is comparable to many Neoproterozoic interglacial siltstone successions (Figure 1) and supports the notion that silt and clay accumulation increased during the Neoproterozoic (Kennedy et al., 2006; Tosca et al., 2010; Liu et al., 2020).

Neoproterozoic siltstone and palaeogeography

Neoproterozoic siltstones are widely described in the literature (Table 1), but their provenance is seldom reported. When the temporal and palaeogeographical context of all successions are considered collectively (Figure 2), it becomes clear that many siltstones contain attributes diagnostic of an aeolian source (Whalley et al., 1982; Hesse, 1994; Rea, 1994; Assallay et al., 1998; Mahowald et al., 1999; Shi, 2005; Enzel et al., 2010; Schwamborn et al., 2012; Vandenberghe, 2013; Muhs et al., 2014; Vos et al., 2014; Meijer et al., 2020). Pits and grooves on quartz and feldspar grains in siltstone from the Bambui Group in central Brazil (Drummond et al., 2015; DaSilva et al., 2022) and the Tapley Hill Formation in Australia (Figure 4) are compatible with aeolian transport. A windblown origin of the coeval Ura Formation in Siberia is implied by a grain size distribution typical of aeolian silt and clay derived from loess (Petrov, 2018). Based on palaeogeographical reconstructions, these and other Neoproterozoic siltstone successions are interpreted to have accumulated in the horse latitudes (Figure 2). Although rivers were undoubtedly point sources of sediment in some successions, silt accumulation under arid, mid-latitude high pressure belts are compelling evidence for a windblown, rather than riverine origin. Additionally, many of the globally correlative siltstone packages lack associated fluvial lithofacies (Williams et al., 2008; Drummond et al., 2015; Petrov, 2018; DaSilva et al., 2022).

The steep and protracted rise in ⁸⁷Sr/⁸⁶Sr isotope ratios in post-Sturtian carbonate successions, which is 4.5 times higher than in pre-Sturtian carbonates (Figure 5), has traditionally been attributed solely to increased chemical weathering (Veizer, 1989; Kennedy et al., 2006; Halverson et al., 2007; Chen et al., 2022). The new interpretation herein, however, suggests an increased and sustained supply of physically weathered and mineralogically immature fine sediment to the global ocean during Neoproterozoic interglacial periods (Tosca et al., 2010) was also probably responsible for this increase in Sr isotope values. Shifts in δ¹⁸O values in igneous zircons reinforce this interpretation because it suggests that the subduction of continentally derived sediment changes zircon composition in collision-related igneous rocks (Sundell et al., 2024). Because continental and nearshore sediments are typically trapped in accretionary wedges and minimally involved in subduction (Plank & Langmuir, 1998), increased delivery of aeolian silt and clay to the open ocean may be responsible for these changes in zircon composition.

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Palaeogeographical reconstructions (Figure 2) suggest that the correlation of Cryogenian siltstone successions is diachronous as depocentres drifted with time through the horse latitudes during the Sturtian-Marinoan and Marinoan-Gaskiers interglacial periods (Evans, 2000; Evans & Raub, 2011; Li et al., 2013; Wen et al., 2020; Wang et al., 2023a, 2023b, 2023c). Penecontemporaneous carbonate-rich successions are interpreted to have been deposited outside this aeolian dust pathway, which allowed limestone to accumulate without the diluting effect of copious windblown sediment. Increased rainfall in the tropics could have further reduced available dust to the surface ocean around the equator by keeping dust bound to the continents, preventing entrainment by wind (McGee et al., 2013; Marticorena, 2014; Scheuvens & Kandler, 2014; Tegen & Schulz, 2014). Examples of carbonate-rich successions containing thin siltstone units include the phosphatic Doushantuo Formation (China), Nama Group (Namibia), Taishir Formation (Mongolia) and Araras Formation (Brazil). Other correlative Neoproterozoic carbonates such as the Doushantuo (China) accumulated in the horse latitudes, but along the windward side of continents where aeolian dust input was likely minimal, such as the case of the modern Great Barrier Reef (Australia; Hesse & McTainsh, 2003).

Windblown nutrients and the aeolian marine biological pump

Dust from the Sahara Desert is often carried for ca 6300 km through the horse latitudes, reaching the eastern seaboard of North America (Jickells et al., 2005; Baker et al., 2013), Amazon region (Garrison et al., 2003, 2006; Bristow et al., 2010) and the Caribbean (Shinn et al., 2000; Griffin et al., 2001, 2002; Garrison et al., 2003, 2006; Weir-Brush et al., 2004; Swart et al., 2014), stimulating primary production over ca 12,000 km², a vast area of the open ocean (Baker et al., 2006). Because the open ocean is far removed from fluvial discharge and coastal upwelling, aeolian dust is the only external source of bioessential elements (Paytan & McLaughlin, 2007; De La Rocha & Passow, 2014; Stuut, 2014).

Although degradation (remineralisation) of organic matter in the water column releases nutrients, it is often not adequate to overcome the typical nutrient deficiency of surface waters. The accessibility of bioavailable N through nitrogen fixation is also generally not sufficient by itself to stimulate primary productivity (Falkowski, 1997, 2014; Falkowski et al., 1998; Tyrrell, 1999; Mills et al., 2004). Large regions of the modern surface ocean with little dust input are commonly characterised by high N and low chlorophyl (HNLC) concentrations, indicating that primary production is not N limited (Jickells et al., 2014). Aeolian dust contains P and a host of micronutrients (e.g. Fe, Cu, Co, Zn, Mo, Cr and Ni), which when delivered together lessen the impact of nutrient co-limitation on primary productivity (Tyrrell, 1999; Mills et al., 2004; Moore et al., 2013; De La Rocha & Passow, 2014; Jickells et al., 2014; Morel et al., 2014).

Phosphorus and Fe are the two most important bioessential elements carried by aeolian dust (Föllmi, 1996; Benitez-Nelson, 2000; Baker et al., 2006, 2013; Paytan & McLaughlin, 2007; Mahowald et al., 2008; Moore et al., 2013; Jickells et al., 2014). Phosphorus, a building block of DNA, is required for cellular energy transfer and limits biological productivity over geological time scales, given that P is controlled by weathering and erosion of continents (Föllmi, 1996; Pufahl, 2010; Filippelli, 2011). Iron is an important micronutrient and necessary for photosynthesis (Falkowski et al., 1998; Jickells et al., 2005; Moore et al., 2013; Bruland et al., 2014; De La Rocha & Passow, 2014; Falkowski, 2014). Their transport in windblown dust and limiting effect on primary production in the modern surface ocean is well documented for modern systems (Falkowski et al., 1998; Bishop et al., 2002; Jickells et al., 2005, 2014; Mahowald et al., 2005; Baker et al., 2006, 2013; Moore et al., 2013; Falkowski, 2014). Once P and Fe are released from clays and made bioavailable in seawater, picoplankton and phytoplankton blooms develop within hours, increasing photosynthetic oxygen generation in the photic zone (Bishop et al., 2002). For example, a two day dust storm in 2001 increased primary production in the horse latitudes, establishing an efficient AMBP that almost doubled the concentration of particulate organic carbon (POC) in seawater over a 15 day period (Mahowald et al., 1999, 2005, 2014; Bishop et al., 2002; Jickells et al., 2005; Moore et al., 2013).

Nutrients become bioavailable in seawater through desorption on clay surfaces. The ability of clays to release nutrients and other metals is complex and controlled by mineralogy (free surface energy, crystal lattice layering), physical properties (surface to volume ratio of particles, swelling properties), nature of the metal (electrical charge and ionic radius) and environmental conditions (pH, Eh, fluid chemistry, temperature). The type of clay mineral formed during chemical weathering depends primarily on the parent mineral and environment, both having a strong effect on nutrient adsorption and release (Abollino et al., 2008; Liu et al., 2022). For example, P and Cr³⁺ adsorb more easily onto kaolinite than illite or smectite to form a strong metal-clay bond that is broken to release these nutrients in seawater with a pH >8 (Hao et al., 2021, 2022). In productive surface waters, organic matter can change the rate at which nutrients are released because organic carbon also adsorbs metals and develops chemical bonds with clays (Abollino et al., 2008; Liu et al., 2022; Shang et al., 2022; Shang, 2023; Zhao et al., 2023). In these cases, metals are released, and chemical bonds are broken when settling organic matter is microbially degraded either in the water column or upon deposition at the seafloor (Sholkovitz et al., 2012; Bruland et al., 2014; De La Rocha & Passow, 2014; Morel et al., 2014).

Because the export of organic matter from the surface ocean to seafloor drives nutrients downwards, the photic zone is generally nutrient-deficient unless a sustained external source such as aeolian dust is delivered (Falkowski, 2014). Thus, the input of aeolian-derived clay and silt is important for stimulating an efficient biological pump that produces photosynthetic oxygen and sequesters organic C on the seafloor (De La Rocha & Passow, 2014). Based on the increased accumulation of immature fine sediments in the Neoproterozoic (Kennedy et al., 2006; Tosca et al., 2010), it is likely that at no time in Earth history would photosynthetic oxygen production from a highly efficient AMBP have been more acute than the periods between Neoproterozoic glaciations.

DISCUSSION

Glacial retreat at the end of the Sturtian and Marinoan glaciations is interpreted to have exposed subaerially the rock flour produced through tens of millions of years of mechanical weathering and rock grinding (Kennedy et al., 2006; Tosca et al., 2010). Strong winds that swept Neoproterozoic interglacial landscapes are interpreted to have continually transported and exposed fresh rock flour to prevent soil shielding in the horse latitudes, accelerating the rate of silicate weathering of clay minerals and atmospheric CO₂ sequestration (Liu et al., 2020; Pu et al., 2022; Macdonald & Swanson-Hysell, 2023). Chemical weathering of interglacial landscapes drained by rivers, erosion of mountainous terranes and upwelling along ice-free continental margins undoubtedly increased nutrient delivery to the surface ocean and stimulated photosynthetic oxygen production (Campbell & Squire, 2010; Papineau, 2010; Pufahl & Groat, 2017; Laakso et al., 2020; Cañadas et al., 2022). These are, however, geographically restricted processes that focus oxygen production over smaller areas of the ocean than aeolian dust input (Jickells et al., 2005; Paytan & McLaughlin, 2007; Bruland et al., 2014).

Phosphorus and micronutrients (e.g. Fe, Cu, Co, Zn, Mo, Cr and Ni) are interpreted to have been released from minerals in rock flour due to chemical weathering during aeolian transport to form clay minerals that adsorb elements from parent minerals (Siever & Woodford, 1979; Nenes et al., 2011; Baker et al., 2014; Stockdale et al., 2016). Although glacially derived rock flour typically contains more illite and smectite because kaolinite forms during the advanced stages of chemical weathering (Tosca et al., 2010; Hao et al., 2021), aeolian transport accelerates chemical weathering on the surface of grains (Siever & Woodford, 1979; Nenes et al., 2011; Stockdale et al., 2016). The chemical weathering at the grain scale associated with photochemical reactions, such as Fe reduction and acidification, is also thought to have increased the bioavailability of nutrients onto newly formed reactive clays during aeolian transport (Garrison et al., 2003; Nenes et al., 2011; Sholkovitz et al., 2012; Baker et al., 2014; Stockdale et al., 2016).

Neoproterozoic seawater is thought to be largely redox stratified (Canfield et al., 2008; Li et al., 2010; Narbonne, 2010; Xiao & Narbonne, 2020), which likely affected the bioavailability of P. This is because the Fe-(oxyhydr)oxides are interpreted to have precipitated in the oxygenated surface ocean from either Fe²⁺ released from aeolian dust in the open ocean or delivered via upwelling of anoxic ferruginous bottom waters along the margins of favourably positioned continents. As Fe-(oxyhydr)oxides fell via suspension rain just below the oxygen chemocline and dissolved, adsorbed P would have been released and concentrated near the base of the photic zone to sustain primary production. Although the strong affinity of dissolved P for Fe-(oxyhydr)oxide is sometimes thought to have limited P availability in the Neoproterozoic surface ocean (Laakso & Schrag, 2018; Laakso et al., 2020; Dodd et al., 2023), other micronutrients carried by dust (e.g. Cu, Co, Zn, Mo, Cr and Ni) would have also overcome the typical surface water nutrient co-limitation (Mills et al., 2004; Moore et al., 2013; Falkowski, 2014; Jickells et al., 2014), promoting a diverse and sustained phytoplankton community in Neoproterozoic oceans (Butterfield, 2009, 2015b; Cohen et al., 2011; Brocks, 2018; Nettersheim et al., 2019; Porter & Riedman, 2023). With the strong redox gradients that persisted in the surface ocean, the biogeochemical cycles of P and Fe were probably coupled, which would have assisted with maintaining high concentrations of these elements to sustain primary production. Iron redox cycling (Heggie et al., 1990; Ruttenberg & Sulak, 2011; Ruttenberg, 2014) across the oxygen chemocline would have released P adsorbed onto freshly precipitated Fe-(oxyhydr) oxides.

A reducing, acidic atmosphere and redox-stratified open ocean in the Cryogenian and Ediacaran (Fike et al., 2006; Canfield et al., 2008; Och & Shields-Zhou, 2012; Li et al., 2013; Kaiho et al., 2024) would have been ideal for releasing clay-bound nutrients. Strong chemical contrasts between the anoxic atmosphere, a photosynthetically oxygenated surface ocean and anoxic to euxinic intermediate and bottom waters (Canfield et al., 2008; Li et al., 2010) are interpreted to have efficiently stripped adsorbed metals on settling clay particles. Collection of modern aeolian dust, in situ measurements and laboratory dissolution experiments, including increased Fe solubility of mafic minerals in an anoxic atmosphere (Siever & Woodford, 1979) support this idea and demonstrate that the release of nutrients should have been enhanced in Neoproterozoic seawater (Jickells et al., 2005, 2014; Paytan & McLaughlin, 2007; Baker et al., 2013; Moore et al., 2013; Hao et al., 2021, 2022).

A change in plankton community structure from predominantly cyanobacteria to increasing eukaryote zooplankton dominance occurred at the Tonian-Cryogenian boundary and could have contributed to the onset of the Sturtian Glaciation (Figure 5; Riedman & Sadler, 2018; Nettersheim et al., 2019; Brocks et al., 2023; Porter & Riedman, 2023). This radiation of eukaryotes may have been linked to the increased mobilisation of nutrients from the weathering of Large Igneous Provinces (LIPs) emplaced during the breakup of Rodinia, which began at ca 750 Ma (Li et al., 2013; Horton, 2015; Cox et al., 2016; Park et al., 2021; Pu et al., 2022; Macdonald & Swanson-Hysell, 2023). The appearance of vase-shaped and phosphatic scale microfossils at ca 720 Ma also suggests an increase in the delivery of these micronutrients to the ocean as LIPs were progressively exhumed and weathered (Cohen et al., 2011; Horton, 2015; Cox et al., 2016; Pu et al., 2022; Macdonald & Swanson-Hysell, 2023). Increased input of Co, Cu and Ni is also recorded in long-term, concomitant changes in the chemistry of authigenic framboidal pyrite precipitated in accumulating organic-rich sediment (Large et al., 2017). This change in community structure is thought to have caused faster settling rates of organic matter due to the larger size of zooplankton organisms (Robbins et al., 1985; Butterfield, 2009, 2015b; Nettersheim et al., 2019).

Nutrients carried to surface waters by aeolian dust would also favour large eukaryote zooplankton, which together with the ability of sinking aeolian particles to scavenge plankton detritus (Iversen & Ploug, 2010; Jickells et al., 2014; Stuut, 2014), is interpreted to have accelerated sequestration of atmospheric CO₂ during Neoproterozoic interglacial periods. These periods coincide with increased accumulation of sedimentary organic matter (Condie et al., 2001; Ghori et al., 2009; Le Heron & Craig, 2012), making the late Neoproterozoic second only to the early Palaeoproterozoic Great Oxidation Event (at ca 2.4 to 2.2 Ga) for the accumulation of organic-rich strata in the Precambrian (Condie et al., 2001; Canfield, 2005; Canfield et al., 2020). These strata include Neoproterozoic hydrocarbon source rocks in China, Brazil, Oman and Australia (Table 1; Ghori et al., 2009; Grosjean et al., 2009; Le Heron & Craig, 2012; Osburn et al., 2014; Zhu et al., 2019), as well as organic-rich siltstones (Cui et al., 2020; DaSilva et al., 2022) and graphite deposits in metamorphosed areas in mobile belts in Brazil (Pacheco et al., 2021). Similarly, the Palaeoproterozoic Great Oxidation Event is also interpreted in the literature to have been closely linked with the Huronian Glaciation, in addition to suggested links with increased photosynthetic oxygen production during a time when volcanic emissions were becoming more oxidising (Condie et al., 2001; Holland, 2006; Planavsky et al., 2010; Bekker & Holland, 2012; Melezhik et al., 2012; Partin et al., 2013; Lowenstein et al., 2014; Lenton & Daines, 2017; Parnell et al., 2021; Fakhraee et al., 2023).

On the modern Earth, ca 1800 Mt/year of aeolian dust (Table 2) and up to ca 0.0965 Mt of P per year are delivered to the global ocean (Mahowald et al., 2008; Moore et al., 2013; Jickells et al., 2014). Using the Redfield ratio (C:106, N:16, P:1), ca 340,000 Gmol C/year can potentially be sequestered to produce 3200 Gmol/year of photosynthetic oxygen in the surface ocean. One important factor to be taken into consideration in these estimations is that although fungal-lichen ecosystems may have colonised some Precambrian terrestrial environments (Knauth & Kennedy, 2009; Kump, 2014; Lalonde & Konhauser, 2015; see also discussion in Liu et al., 2020), the absence of vascular land plants with roots to anchor sediment likely ensured a steady supply of aeolian dust and nutrients (Dalrymple et al., 1985; Hesse & McTainsh, 2003) to fuel the AMBP in the horse latitudes (Figures 1 and 3).

TABLE 2 Estimates for modern aeolian dust input to the atmosphere and in the Precambrian (ca 720 Ma). Modified from Mahowald et al. (2005) and Liu et al. (2020).

Global dust emission	Reference	Mt/year (=Tg/year)
Modern	Ginoux et al. (2001)	1814
Zender et al. (2003)	1490
Tegen et al. (2004)	1800
Luo et al. (2003)	1650
Mahowald et al. (2005)	1790
Precambrian	Liu et al. (2020)—k = 0.0375a	36,463
Liu et al. (2020)—k = 0.3a	132,303

Considering modern vegetation cover and the recent modelling of the Neoproterozoic atmosphere, which suggests 10–35 times more aeolian dust emission than in modern days (Liu et al., 2020), the potential carbon sequestration and oxygen production is conservative. These calculations suggest that nutrients carried by aeolian dust promote the conversion of CO₂ into photosynthetic oxygen and sedimentary organic matter in such copious quantities that the effects of AMBP should not be ignored when modelling Neoproterozoic oxygenation and climate. An AMBP that efficiently exported organic C to the seafloor not only produced oxygen that diffused into the atmosphere, but together with widespread silicate weathering on continents (Hoffman & Schrag, 2002), probably sequestered enough atmospheric CO₂ to usher in the Sturtian and Marinoan glaciations at ca 717 and 650 Ma (Butterfield, 2009, 2015b; Lenton et al., 2014; Lenton & Daines, 2017).

Such an increase in oxygen levels during the Neoproterozoic is well documented in the chemostratigraphic record (Logan et al., 1995; Canfield, 2005; Fike et al., 2006; Holland, 2006, 2009; Canfield et al., 2008; Scott et al., 2008; Och & Shields-Zhou, 2012; Partin et al., 2013). The appearance of the first true phosphorite giants also strongly supports geochemical evidence for a rise in ocean–atmosphere oxygen (Pufahl & Groat, 2017). Deposition of the aerially extensive Doushantuo Formation (China) phosphorites starting at ca 620 Ma required the establishment of a suboxic seafloor in the full spectrum of shelf environments (Nelson et al., 2010; Pufahl, 2010; Pufahl & Hiatt, 2012; Pufahl & Groat, 2017; Zhang et al., 2019). This style of phosphorite accumulation is interpreted to record the ventilation of progressively more distal marine environments, which suggests the locus of phosphogenesis expanded and deepened through the water column with time (Nelson et al., 2010; Pufahl & Groat, 2017).

Although recent interpretations based on U and Mo isotopes (Sperling et al., 2015; Chen et al., 2018; Zhang et al., 2018; Cao et al., 2020; Tostevin & Mills, 2020) and chlorophyl maturation (Kaiho et al., 2024) refute an oxygenation event in the Neoproterozoic, the implications of post-depositional alteration on statistical trends observed in these data are largely unknown. Additionally, biological fractionation limits the use of U isotopes as a redox proxy (Stylo et al., 2015) and the documented changes of original U and Mo isotopes are seldom interpreted in a paragenetic context. When considering these issues, the wealth of geochemical proxy data that suggests otherwise, the appearance of phosphorite giants in the stratigraphic record and the new evidence presented herein for AMBP-produced oxygen, together present a compelling case for Neoproterozoic oxygenation.

CONCLUSIONS

Siltstones deposited during the Neoproterozoic contain an underappreciated record of climate, nutrient cycling and ocean–atmosphere oxygenation. During interglacial periods between snowball glaciations, aeolian transport of nutrient-rich, mineral dust is interpreted to have fuelled surface ocean productivity, driving the onset of Earth's second major stepwise increase in oxygen. This accumulation of silt was focussed on the horse latitudes where widespread sequestration of sedimentary organic matter created a long-term C sink, which together with sustained photosynthetic oxygen production, permitted the eventual ventilation of deeper marine environments. In addition, sequestration of atmospheric CO₂ was probably an important negative feedback process, which together with silicate weathering, prevented runaway greenhouse conditions during interglacial periods.

If these conclusions are correct, then a fascinating collection of feedbacks, manifest as the AMBP, has been discovered. Neoproterozoic snowball glaciations generated extensive amounts of mineral dust, which was subsequently blown into the world ocean, stimulating primary production. The sustained increase in photosynthetic oxygen production combined with sequestration of resultant sedimentary organic matter beneath the seafloor likely assisted with the progressive ventilation of the late Neoproterozoic oceans. Such oxygenation may have also affected the carbon cycle (e.g. carbon isotope excursions) and helped precondition seawater for the eventual evolution of multicellular animals in the Ediacaran.

ACKNOWLEDGEMENTS

Comments from Chief Editor P. Swart, in addition to those from M. Wallace, K. Konhauser and two anonymous reviewers greatly improved the manuscript. S. Beyer and A. Dobosz are gratefully acknowledged for technical support in the Queen's Facility for Isotope Research (QFIR). E. M. Guimaraes is thanked for insightful discussions and field support. Scholarships to LGdS were awarded by Queen's University, Geological Survey of Brazil, International Association of Sedimentologists and International Conference for Ediacaran and Cambrian Stratigraphy (ICECS 2018). Research was supported through NSERC Discovery Grants to PKP and NPJ.

CONFLICT OF INTEREST STATEMENT

The authors declare no conflict of interest.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from the corresponding author upon reasonable request.

References

Abollino, O., Giacomino, A., Malandrino, M. & Mentasti, E. (2008) Interaction of metal ions with montmorillonite and vermiculite. Applied Clay Science, 38, 227–236.

Allen, P.A. & Hoffman, P.F. (2005) Extreme winds and waves in the aftermath of a Neoproterozoic glaciation. Nature, 433, 123–127.

Allen, P. & Leather, J. (2006) Post‐Marinoan marine siliciclastic sedimentation: The Masirah Bay Formation, Neoproterozoic Huqf Supergroup of Oman. Precambrian Research, 144(3–4), 167–198. [DOI: https://dx.doi.org/10.1016/j.precamres.2005.10.006]

Zhu, G.‐Y., Li, T.‐T., Zhao, K., Zhang, Z.‐Y., Chen, W.‐Y., Yan, H.‐H., Zhang, K.‐J. & Chi, L.‐X. (2019) Excellent source rocks discovered in the Cryogenian interglacial deposits in South China: geology, geochemistry, and hydrocarbon potential. Precambrian Research, 333, [eLocator: 105455].