Creationism in Crisis - How Marine Worms Caused a Burst in Biodiversity - 480 Million Years Before 'Creation Week'

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The Ordovician Biota

Johns Hopkins scientists pinpoint an unlikely hero in evolution: Worms | Hub

Creationists often try to misrepresent the so-called 'Cambrian Explosion' - the relatively rapid radiation of different taxa over a period of some 6 million years when the first hard body parts evolved and by the end of which all the progenitors of the main modern taxa had appeared - as a sudden event, happening almost overnight, where major taxons suddenly appeared without ancestors, as some sort of evidence of intelligent design.

This is nonsense of course, as the 'explosion' took some 6-10 million years and was preceded by the Ediacaran biota, but what they don't seem to realise is that this period was followed fairly soon after by another period of rapid evolution and increased biodiversity in the 'Ordovician Explosion' or the 'Great Ordovician Biodiversification Event' (GOBE) when the same basic forces that produced the Cambrian explosion continued for another 25 million years.

Tell me all about the Great Ordovician Biodiversification Event, please. The Great Ordovician Biodiversification Event (GOBE) was a major evolutionary event that took place approximately 485 to 443 million years ago during the Ordovician period. This event marked a significant increase in biodiversity, particularly among marine life, and was one of the most important radiations of life in Earth's history. Here are the key details about the GOBE:

Background and Timing

• Period The Ordovician period, following the Cambrian period and preceding the Silurian period.
• Duration The GOBE took place over an extended period during the Early to Middle Ordovician, roughly spanning 25 million years.

Environmental Context

• Climate The Ordovician period experienced a relatively warm climate with high sea levels, which led to extensive shallow continental shelves—ideal environments for marine life.
• Geography The supercontinent Gondwana was located near the South Pole, and there were other smaller landmasses scattered around the equator, providing diverse habitats.

Biodiversification

• Marine Life The GOBE is characterized by a dramatic increase in the diversity and complexity of marine organisms. Key groups that diversified during this period include:
  • Brachiopods Marine invertebrates with hard shells on their upper and lower surfaces.
  • Bryozoans Colonial animals that formed intricate skeletal structures.
  • Echinoderms A group that includes starfish, sea urchins, and crinoids.
  • Mollusks Including gastropods (snails), bivalves (clams), and cephalopods (squids and their relatives).
  • Arthropods Particularly trilobites and early forms of crustaceans.
  • Corals Rugose and tabulate corals started to build reef structures.
• Plankton The diversification of phytoplankton and zooplankton provided a crucial base for the marine food web.

Causes

• Ecological Factors Increased habitat complexity due to the development of reefs and other structures provided new niches for organisms to exploit.
• Biotic Interactions The escalation of predator-prey relationships may have driven evolutionary innovations and adaptations.
• Geochemical Changes Increased nutrient availability, possibly due to volcanic activity and weathering of continental rocks, may have supported higher primary productivity.
• Atmospheric and Oceanic Changes Changes in oxygen levels and ocean circulation patterns may have contributed to the diversification.

Impact and Legacy

• Long-Term Effects The GOBE set the stage for modern marine ecosystems. Many of the marine life forms that diversified during this period have descendants that exist today.
• Fossil Record The Ordovician fossil record is rich and provides crucial insights into early marine life and the dynamics of biodiversity.
• Mass Extinction The end of the Ordovician period was marked by a significant mass extinction event, largely due to a major glaciation event that caused sea levels to drop and habitats to shrink. However, the diversification that occurred during the GOBE had a lasting impact on life.

Research and Study

• Paleontology The GOBE is a major focus of paleontological research, with studies examining fossil records, sedimentary deposits, and geochemical evidence to understand the drivers and patterns of diversification.
• Evolutionary Biology Understanding the GOBE helps scientists comprehend the mechanisms of evolutionary radiations and the factors that influence biodiversity.

In summary, the Great Ordovician Biodiversification Event was a pivotal time in the history of life on Earth, marked by a significant increase in the diversity of marine organisms. This event laid the foundation for many of the marine ecosystems that we see today and is a key area of study for understanding the history of life and the processes that drive biodiversity.

Explain the formation of pyrites in marine sediment and how they relate to atmospheric oxygen levels.

The formation of pyrite (FeS₂) in marine sediments is a geochemical process that is closely linked to the levels of oxygen in the atmosphere and oceans. Here’s a detailed explanation:

Formation of Pyrite

1. Iron Supply: Iron (Fe) is supplied to marine sediments primarily through the weathering of continental rocks and is transported to the ocean by rivers.
2. Sulfur Source: Sulfate (SO₄²⁻) is the most common form of sulfur in seawater, originating from the weathering of sulfate minerals and from volcanic activity.
3. Bacterial Sulfate Reduction: In anoxic (oxygen-poor) conditions in marine sediments, sulfate-reducing bacteria (SRB) thrive. These bacteria use sulfate for respiration, producing hydrogen sulfide (H₂S) as a byproduct: \[ \text{SO₄}^{2-} + 2\text{CH}_2\text{O} \rightarrow \text{H}_2\text{S} + 2\text{HCO}_3^- \]
4. Formation of Iron Sulfides: Hydrogen sulfide reacts with iron to form iron sulfide minerals. Initially, this forms as amorphous iron sulfide (FeS), which can further react to form more stable pyrite (FeS₂): \[ \text{Fe}^{2+} + \text{H}_2\text{S} \rightarrow \text{FeS} + 2\text{H}^+ \] \[ \text{FeS} + \text{S} \rightarrow \text{FeS}_2 \]

Relationship with Atmospheric Oxygen Levels

1. Oxygen and Sulfate Availability: Higher atmospheric oxygen levels increase the oxidation of sulfide minerals on land, which enhances the supply of sulfate to the oceans. More sulfate in the ocean supports more bacterial sulfate reduction, hence more pyrite formation in anoxic sediments.
2. Redox Conditions in Sediments: The presence of pyrite in marine sediments indicates low-oxygen or anoxic conditions. When oxygen levels in the ocean and atmosphere are high, the ocean’s deeper waters are well-oxygenated, limiting the extent of anoxic conditions necessary for pyrite formation.
3. Proxy for Ancient Oxygen Levels: Pyrite formation can serve as a proxy for past oxygen levels. Extensive pyrite deposits in ancient sedimentary rocks suggest periods of lower atmospheric and oceanic oxygen levels. Conversely, a decrease in pyrite deposition in marine sediments can indicate rising oxygen levels, as more iron would remain in the oxidized state (ferric iron, Fe³⁺) and less would be available to form pyrite.

Geological Implications

• Ordovician Period: During the Ordovician period, atmospheric oxygen levels were rising, which influenced the redox state of the oceans. As oxygen levels increased, the extent of anoxic conditions in marine environments would have decreased, potentially reducing pyrite formation in certain areas.
• Biodiversification: The changes in oxygen levels and the resulting shifts in marine chemistry could have played a role in the Great Ordovician Biodiversification Event by altering habitats and nutrient availability, promoting the evolution of new life forms.

Conclusion

The formation of pyrite in marine sediments is a complex process dependent on the availability of iron, sulfate, and anoxic conditions, all of which are influenced by atmospheric oxygen levels. By studying pyrite and its geological context, scientists can infer past oxygen levels and better understand the environmental conditions that influenced the evolution of life on Earth.

The result was another massive increase in the variety and complexity of (especially) marine life and in the variety of single-celled plankton, both phytoplankton and zooplankton, which provided a base for the food pyramid that resulted from this biodiversification.

Now scientists led by biologists from John Hopkins University Department of Earth and Planetary Science with colleagues from the University of Maryland's Center for Environmental Science, Horn Point Laboratory, believe they have identified the major environmental changes that triggered this 'explosion', as a chain of events initiated by the activities of burrowing marine worms.

The level of oxygen depends on a complex, reversible series of chemical reactions that result in the formation of iron pyrites in anoxic marine sediment, so pushing the dynamic towards reducing oxygen levels. And this is where burrowing worms play a key part, by disturbing the sediment, allowing oxygen to mix with the pyrites and reverse the reaction, so releasing sulphates into the sea and increasing the activity of the bacteria, which in turn produces more atmospheric oxygen.

By examining core samples of marine sediment from around Chesapeake Bay, the team have worked out how this change in the dynamics of the chemical process caused by burrowing worms probably caused the change in atmospheric oxygen which drove the burst of biodiversity in the Middle Ordovician known to science as the (GOBE).

They have published their findings, open access in the journal Geochimica et Cosmochimica Acta and explained them in a John Hopkins University news release by Hannah Robbins. The evidence was found in iron pyrites:

JOHNS HOPKINS SCIENTISTS PINPOINT AN UNLIKELY HERO IN EVOLUTION: WORMS

Their digging and burrowing sparked a chain of events that released oxygen into the ocean and atmosphere, helping to kick-start what is known as the Great Ordovician Biodiversification Event

One of Earth's most consequential bursts of biodiversity—a 30-million-year period of explosive evolutionary changes spawning innumerable new species—may have the most modest of creatures to thank for the vital stage in life's history: worms.

The digging and burrowing of prehistoric worms and other invertebrates along ocean bottoms sparked a chain of events that released oxygen into the ocean and atmosphere and helped kick-start what is known as the Great Ordovician Biodiversification Event, roughly 480 million years ago, according to new findings Johns Hopkins University researchers published in the journal Geochimica et Cosmochimica Acta.

It's really incredible to think how such small animals, ones that don't even exist today, could alter the course of evolutionary history in such a profound way. With this work, we'll be able to examine the chemistry of early oceans and reinterpret parts of the geological record.

Assistant Professor Maya Gomes, senior author Department of Earth and Planetary Sciences
Johns Hopkins University, Baltimore, MD, USA.

To better understand how changes in oxygen levels influenced large-scale evolutionary events, Gomes and her research team updated models that detail the timing and pace of increasing oxygen over hundreds of millions of years.

They examined the relationship between the mixing of sediment caused, in part, by digging worms with a mineral called pyrite, which plays a key role in oxygen buildup. The more pyrite that forms and becomes buried under the mud, silt, or sand, the more oxygen levels rise.

Researchers measured pyrite from nine sites along a Maryland shoreline of the Chesapeake Bay that serves as a proxy for early ocean conditions. Sites with even just a few centimeters of sediment mixing held substantially more pyrite than those without mixing and those with deep mixing.

The findings challenge previous assumptions that the relationship between pyrite and sediment mixing remained the same across habitats and through time, Gomes said.

Conventional wisdom held that as animals churned up sediments by burrowing in the ocean floor, newly unearthed pyrite would have been exposed to and destroyed by oxygen in the water, a process that would ultimately prevent oxygen from accumulating in the atmosphere and ocean. Mixed sediments have been viewed as evidence that oxygen levels were holding steady.

The new data suggests that a small amount of sediment mixing in water with very low levels of oxygen would have exposed buried pyrite, sulfur, and organic carbon to just enough oxygen to kick-start the formation of more pyrite.

It's kind of like Goldilocks. The conditions have to be just right. You have to have a little bit of mixing to bring the oxygen into the sediment, but not so much that the oxygen destroys all the pyrite and there's no net buildup.

Kalev Hantsoo, first author
Department of Earth and Planetary Sciences
Johns Hopkins University, Baltimore, MD, USA.

When the researchers applied this new relationship between pyrite and the depth of sediment mixing to existing models, they found oxygen levels stayed relatively flat for millions of years and then rose during the Paleozoic era, with a steep rise occurring during the Ordovician period.

The extra oxygen likely contributed to the Great Ordovician Biodiversification Event, when new species rapidly flourished, the researchers said.

There's always been this question of how oxygen levels relate to the moments in history where evolutionary forces are ramped up and you see a greater diversity of life on the planet. The Cambrian period also had a massive speciation event, but the new models allow us to rule out oxygen and focus on other things that may have driven evolution during that time.

Assistant Professor Maya Gome.

Details of this technical experiment are given in the team's paper in published their findings, open access in the journal Geochimica et Cosmochimica Acta:

Abstract
The early Paleozoic Era (∼540–420 Ma) was an interval of profound biogeochemical changes including increasing oxygen (O₂) and the onset of bioturbation (sediment mixing by animals). It is hypothesized that incipient bioturbation caused a monotonic decrease in sedimentary burial of pyrite (FeS₂), which would have slowed atmospheric O₂ accumulation. However, pyrite accumulation can exhibit complex responses to dynamic, low-O₂ environmental conditions. To assess pyrite burial in a potential modern analogue to early Paleozoic environments, we collected sediment cores from the Chesapeake Bay, an estuary with multiple gradients in sulfate concentration, hypoxia intensity, organic carbon flux and lability, and bioturbation. Results indicate that pyrite accumulation is maximized not under strong sulfate depletion in highly reducing sediments, but rather in sediments that occupy the mid-range of sulfate–chloride ratios. This probably occurs through efficient replenishment of pore water sulfate and/or through the generation of sulfur redox intermediates, which promote pyrite formation via the polysulfide reaction pathway. In light of these results and in contrast to earlier models, we hypothesize that mild early Paleozoic bioturbation temporarily increased pyrite burial efficiency by stimulating higher sulfate reduction rates and increasing sedimentary sulfide retention. Compiled sulfur and carbon data from a geochemical database indicate that median sulfur-carbon ratios of fine-grained marine siliciclastic rocks increased from the Ediacaran through the Ordovician, then decreased and became much less variable from the Silurian onward. Thus, the Cambrian and Ordovician Periods may constitute a distinct interval of the Proterozoic-Phanerozoic transition in which bioturbation temporarily accelerated O₂ buildup. This transition probably ended in the Silurian, when pO₂ rose to sufficient levels to homogenize sedimentary carbon–sulfur cycling.

1. Introduction
The timing of Earth system oxygenation and its links to biological evolution are central questions in geobiology (Cole et al., 2020, Sperling et al., 2022). Oxygen (O₂) accumulation at Earth’s surface has primarily resulted from the reduction of carbon dioxide (CO₂) via oxygenic photosynthesis and the reduction of sulfate (SO₄^2–) via microbial sulfate reduction, followed by reduced carbon and sulfur burial (Holland, 1962, Holland, 1973). Net oxidation of the ocean–atmosphere system occurs if the reduced products of these reactions—organic carbon and sulfide, respectively—are separated from the oxidized products by burial in sediment; otherwise, the reverse reactions will consume the oxidized species (Garrels and Perry, 1974). Sedimentary pyrite (FeS₂) is the largest stable reservoir of sulfide in crustal sediments (Rickard and Luther, 2007), although organosulfur compounds can also become a significant component of reduced sulfur in association with sulfur redox intermediates (Riedinger et al., 2017), or in localities with very high organic carbon content (Zaback et al., 1993). Given that the reduction of sulfate followed by sedimentary pyrite burial has been one of the two major sources of ocean–atmosphere oxygenation over Earth’s history (Berner, 1982), increases in the global rate of pyrite precipitation and burial in Earth’s past would have quickened the pace of O₂ buildup.

The availability of organic carbon, sulfate, and reactive iron have commonly been cited as the primary controls on the rate of sedimentary pyrite accumulation (Berner, 1984). Pyrite burial in many modern localities is roughly proportional to burial fluxes of total organic carbon (TOC), with a TOC/pyrite-S ratio of 2.8 ± 0.8 attributed to ‘normal marine’ sediments, i.e. sediments that are deposited under oxygenated marine waters with normal ocean salinity of ∼35 (Berner, 1982, Berner and Raiswell, 1983). This relationship exists primarily because organic carbon deposition rates broadly fuel and quantitatively scale with sediment microbial sulfate reduction rates, although it is not only the amount of TOC but also TOC reactivity that plays a role in determining sulfate reduction rates (Meister et al., 2013). Up to 90 % of the sulfide produced by microbial sulfate reduction is reoxidized (Jørgensen, 1982.1), and the amount of sulfide buried as pyrite is influenced by the favorability of three pyrite-forming reactions. Pyrite typically forms by the reaction of iron monosulfide (FeS) with either hydrogen sulfide (H₂S) or polysulfide (Sn^2–, where 2 ≤ n ≤ 8) (Luther, 1991, Rickard and Luther, 1997, Butler et al., 2004); alternatively, pyrite can form via the reaction of ferric hydroxide surface species with dissolved sulfide to yield Fe(II)OH²⁺, which then reacts with sulfide or polysulfide radicals to form pyrite (Wan et al., 2017.1). This latter reaction is called the ferric hydroxide surface (FHS) pathway and is most favorable under high ratios of Fe(III)-oxide minerals to dissolved sulfide (Wan et al., 2017.1).

Polysulfides are a pool of reduced sulfur that forms through the reaction of dissolved bisulfide (HS^–) with elemental sulfur (S₈) and exists in equilibrium with these species (Teder, 1971, Steudel, 2003). Higher pH in anoxic pore water will favor the generation of HS– and polysulfides, while lower pH will favor the generation of H₂S and S₈ (Kamyshny et al., 2003.1, Kamyshny et al., 2004.1). The two FeS-derived pyrite formation pathways follow the form.

S_n^2– + FeS → S_n-1^2– + FeS₂

(1)

H₂S + FeS → H₂ + FeS₂

(2)

with Eq. (1) referred to as the polysulfide pathway (where n commonly ranges from 4 to 6 under relevant environmental conditions; Kamyshny et al., 2004.1) and Eq. (2) referred to as the H₂S pathway. In Eq. (1), n refers to the number of sulfur atoms in a polysulfide molecule.

Although pyrite formation is associated with reducing environments, Eqs. (1), (2) demonstrate that pyrite sulfur (with an oxidation state of −1) is more oxidized than FeS sulfur (with an oxidation state of −2). The oxidative power in Eq. (1) derives from the internal sulfur atoms in polysulfide, which have formal oxidation states of 0. Meanwhile, in Eq. (2), H₂S completes FeS-pyrite oxidation by reducing its hydrogen to molecular hydrogen (H₂) (Rickard, 2012). The FHS pathway involves net oxidation of sulfide via the surfaces of ferric hydroxide minerals, generating sulfide or polysulfide radicals with a −1 oxidation state which can then participate in pyrite nucleation (Wan et al., 2017.1).

It was previously thought that partially oxidized sulfur compounds might be a necessary component of pyrite precipitation (Berner, 1970, Berner, 1974.1), but the demonstration of the H₂S pathway at ambient temperatures (Drobner et al., 1990, Rickard, 1997.1, Schoonen, 2004.2) indicated that pyrite precipitation can occur via a simpler set of reactions, since the presence of H₂S requires only sulfate reduction rather than an oxidative sulfur cycle. Because polysulfide is a mixed-valence sulfur species that requires relatively alkaline conditions and H₂S is a reduced species that exists under relatively acidic conditions, it is generally assumed that the H₂S pathway predominates in strongly reducing and acidic environments while the polysulfide pathway predominates under slightly more oxidized and alkaline conditions (Rickard and Luther, 2007). The FHS pathway has been proposed as a significant reaction in sediments with high fractions of reactive iron and low amounts of sulfide, e.g., in low-salinity systems with high terrigenous sediment loads (Wan et al., 2017.1).

The relative rates of the H₂S and polysulfide reaction pathways are difficult to constrain, but sulfur cycling microorganisms—including sulfate reducing, sulfide oxidizing, and sulfur disproportionating microbes—are critical to both reactions. Although abiotic laboratory experiments have generated reaction rate constants that indicate that the H₂S pathway is substantially faster than the polysulfide pathway (Butler et al., 2004), experiments that include microbial activity suggest roughly equal fluxes of pyrite formation via these two pathways in natural settings (Canfield et al., 1998). The rate limiting step of Eqs. (1), (2) is pyrite nucleation, which requires supersaturated conditions and can be strongly influenced by microbial activity (Schoonen and Barnes, 1991.1, Canfield et al., 1998, Rickard and Luther, 2007, Rickard, 2012). The most obvious influence of microbial sulfur metabolisms on pyrite formation is their generation of H₂S and sulfur redox intermediates; however, microbial biomass itself can also aid in the precipitation of pyrite and FeS (Donald and Southam, 1999, Wacey et al., 2015, Picard et al., 2018, Duverger et al., 2020.1), and microbial interactions can promote pyrite formation (Thiel et al., 2019).

A further complication in deciphering the rate of Eq. (1) is that polysulfide is a highly reactive compound that is sensitive to electron activity (Eh) and proton activity (pH) (Kleinjan et al., 2005a). Furthermore, the generation of polysulfide to fuel Eq. (1) likely depends on the surface areas of S₈ and FeS, which are also difficult to measure (Rickard, 1975). The solubility of S₈ also influences pore water polysulfide concentration, but the rate of reaction of S₈ with the dissolved sulfide-polysulfide pool can vary by ∼6 orders of magnitude depending on whether S₈ is in dissolved, colloidal, or crystalline form (Fossing and Jørgensen, 1990.1, Kamyshny and Ferdelman, 2010, Avetisyan et al., 2019.1). Sulfur-cycling metabolisms can influence this aspect of pyrite precipitation because sulfide oxidizing microbes such as Beggiatoa can generate large amounts of colloidal, water-soluble S₈ globules by encasing them in hydrophilic proteins (Kleinjan et al., 2005.1b, Maki, 2013.1). This hydrophilicity can increase the rate of the reaction of S₈ with HS^– to form polysulfide. Thus, although the rates of Eqs. (1), (2) are difficult to constrain in natural environments, it is apparent that microbial oxidative sulfur cycling plays an important role in determining rates of pyrite formation because of its strong influence on the concentrations of H₂S, S₈, and Sn^2– in sediments.

The response of pyrite precipitation rates to changing sediment and water column conditions is important to our understanding of the early Paleozoic Era, which spans the beginning of the Cambrian Period through the end of the Silurian Period (∼540–420 Ma; Tarhan et al., 2021). The redox proxy record of this interval of Earth history is somewhat ambiguous but generally points to lower atmospheric oxygen concentrations than those of the modern Earth system (Tostevin and Mills, 2020.2, Wei et al., 2021.1), a conclusion that is supported by recent generations of Earth system box models (Krause et al., 2018.1, Lenton et al., 2018.2). The pace of oxygenation in the early Paleozoic is important for contextualizing the remarkable evolutionary changes that occurred in this interval, including the spread of biomineralization (Wood and Zhuravlev, 2012.1), the restructuring of trophic networks (Dunne et al., 2008), the diversification of animal body plans (Knoll and Carroll, 1999.1), and the onset of penetrative bioturbation (Bottjer et al., 2000, Carbone and Narbonne, 2014).

Bioturbation, the physical mixing and fluid exchange of shallow sediments caused by animals (Kristensen et al., 2012.2), has increasingly influenced biogeochemical cycling across the Phanerozoic Eon. In the modern Earth system, bioturbation has a pronounced effect on sedimentary biogeochemistry (Meysman et al., 2006, Deng et al., 2020.3, van de Velde et al., 2020.4). Its advent in the Paleozoic Era has been implicated in changes to the cycling of phosphorus (Boyle et al., 2014.1, Tarhan et al., 2021), iron (van de Velde et al., 2023), and sulfur (Canfield and Farquhar, 2009). The rate at which early bioturbation intensified is a critical factor in understanding its effect on the biogeochemical evolution of Paleozoic Earth. Sediment fabric analysis indicates that the sedimentary mixed layer deepened only gradually from the Cambrian onward, reaching ∼1.5 cm by the Silurian and ∼3 cm by the Devonian—well short of the modern global mixed layer depth of ∼10 cm (Tarhan et al., 2015.1, Boudreau, 1998.1). It is possible that the initial deepening of the sedimentary mixed layer could have had a disproportionately large effect on sediment biogeochemistry, but the likelihood of a nonlinear response to early penetrative bioturbation has been questioned (Cribb et al., 2023.1). On the other hand, sedimentary oxidative sulfur cycling can undergo complex responses to bioturbation, such as ecological turnover between sulfide oxidizing communities of Beggiatoa and cable bacteria (Malkin et al., 2022.1).

Bioturbation has been shown to stimulate sulfate reduction. High sulfate reduction rates are frequently observed in sediments with moderate to strong bioturbation (Goldhaber et al., 1977, Aller and Yingst, 1978, Jørgensen and Parkes, 2010.1, Quintana et al., 2013.2, Jørgensen, 2021.2). Sulfate reduction rates have also been shown to be substantially higher in bioturbated sediments than in nearby or otherwise similar non-bioturbated sediments (Hines and Jones, 1985, Bertics et al., 2010.2, Bertics and Ziebis, 2010.3, Bertics and Ziebis, 2010.3, Bertics et al., 2010.2; cf. Kristensen and Blackburn, 1987; Nielsen et al., 2003.2), including in the Chesapeake Bay (Roden and Tuttle, 1993.1). This probably occurs because bioirrigation (i.e., fluid advection) introduces fresh sulfate to the substrate, while biodiffusion (i.e., solid particle diffusion) simultaneously mixes labile organic matter downward as a reductant to fuel further sulfate reduction (van de Velde and Meysman, 2016). In other words, bioturbation can increase the supply of sulfate to sediments while also increasing microbial sulfate demand.

The effect of bioturbation on sedimentary sulfide retention, i.e. ‘net’ sulfate reduction (Moeslund et al., 1994), is more ambiguous because bioturbation introduces dissolved oxygen and solid oxide compounds to the substrate (Thamdrup et al., 1994.1). Dissolved oxygen can quickly oxidize H₂S, pyrite, and FeS (Lowson, 1982.2, Zhang and Millero, 1993.2, Jeong et al., 2010.4); Fe-oxides oxidize H₂S to ZVS (Poulton et al., 2004.3); and MnO₂ oxidizes pyrite to sulfate, FeS to ZVS, and H₂S to polysulfide (Schippers and Jørgensen, 2001, Avetisyan et al., 2021.3). Moderate bioturbation has minimal effects on the oxygen penetration depth of sediments, but it can increase the thickness of the suboxic layer, typically defined as the zone in which neither O₂ nor dissolved sulfide are present (Bonaglia et al., 2019.2, Cribb et al., 2023.1). Thus, the net effect of oxidant mixing on pyrite formation and retention depends on the mode and intensity of bioturbation. If bioturbation and net mixed layer oxidation are mild, then the addition of Fe^- and Mn^- oxides may promote pyrite precipitation by partially oxidizing sulfides to sulfur redox intermediates, including polysulfides. Under more strongly oxidizing conditions, sulfide compounds (both solid and dissolved) are more likely to be fully reoxidized to sulfate.

Pioneering studies of sedimentary pyrite formation in the modern environment generally focused on ‘end-member’ settings, i.e., sediments with well-developed mixed layers under generally oxic water (Goldhaber et al., 1977, Jørgensen, 1977.1) or non-burrowed sediments underlying permanently sulfidic water (Lyons, 1997.2). However, settings with low rates of bioturbation and fluctuating oxygen concentration may more closely reflect the conditions that prevailed along continental shelves and slopes in the early Paleozoic (Tarhan et al., 2015.1, Pruss and Gill, 2024). Emerging models of early Paleozoic ocean redox evolution point to the establishment of wedge-shaped, fluctuating oxygen minimum zones over mid-shelf sediments, with inner and outer shelf sediments less prone to anoxia (Guilbaud et al., 2018.3). Trace fossil distributions from inner to outer shelf facies corroborate this model (Buatois et al., 2020.5). Combined fossil and geochemical data suggest that oxic-euxinic oscillations occurred on sub-millennial timescales in a Cambrian epicontinental sea (Dahl et al.,2019.3), and distinct sediment cores from the same basin (separated by ∼150 km) may reflect different contemporaneous concentrations of water column H₂S between sites (Zhao et al., 2023.2). Localized variability in continental shelf oxygenation is also documented into the Ordovician and Silurian (Edwards et al., 2018.4, Jin et al., 2021.4).

Given the emerging picture of Paleozoic shallow marine redox structures, studies of present-day pyrite formation in more heterogeneous redox regimes—particularly upwelling zones (Böning et al., 2004.4) and semi-restricted hypoxic basins (Figueroa et al., 2023.3, Liu et al., 2021.5)—may provide a better analog for geochemical conditions that prevailed in the early Paleozoic. In this intermediate redox category, the Chesapeake Bay presents a distinctive case of rapid redox fluctuation. Even the most oxygen-stressed areas of the Chesapeake Bay, which feature no bioturbation and undergo months of anoxia/euxinia each year, are still exposed to oxic bottom waters a majority of the time and retain median annual benthic oxygen concentrations of at least 4 mg/L (Table 1). Although these fluctuations are unusual for a modern shallow marine basin, they may have been more common in the early Paleozoic, when lower atmospheric _pO₂ made shallow marine water columns more susceptible to spatiotemporal redox variability (Pruss and Gill, 2024).

For the purposes of understanding the pace of Paleozoic oxygenation, controls on shallow marine pyrite burial play a larger role in determining global pyrite burial fluxes than controls on deep marine pyrite burial. This is because pyrite burial broadly scales with organic carbon burial, and about 85% of modern marine organic carbon burial occurs in shelf and deltaic sediments (Hedges and Keil, 1995); in the Paleozoic world, when pelagic primary productivity was lower than today (Ridgwell and Zeebe, 2005.2), this fraction may have been higher. Furthermore, the majority (76%) of global sulfate reduction occurs in shallow (<200 m) marine environments (Canfield et al., 2005.3). If lower slope environments (200–1000 m) are also included, sulfate reduction in shallow settings constitutes 94% of global sulfate reduction (Canfield et al., 2005.3). In light of the complex relationships between bioturbation, microbial oxidative sulfur cycling, and pyrite precipitation rates, it is important to investigate trends in shallow marine pyrite burial under dynamic, low-oxygen conditions similar to the early Paleozoic.

Hantsoo, Kalev; Gomes, Maya; Brenner, Dana; Cornwell, Jeffrey; Palinkas, Cindy M.; Malkin, Sairah
Trends in estuarine pyrite formation point to an alternative model for Paleozoic pyrite burial Geochimica et Cosmochimica Acta (2024) 374, 51-71; DOI: 10.1016/j.gca.2024.04.018

Copyright: © 2024 The authors.
Published by Elsevier Ltd. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)

Like the butterfly effect in chaos theory - a metaphor for the way a small change in one part of a chaotic system can result in a large change in another part of the system, so in the evolution of life on earth, a small change such as the activity of burrowing worms in disturbing marine sediment, so changing a dynamic chemical process, resulted in the major environmental change that drove the burst of biodiversity that set the science for the eventual evolution of the major taxons of today.

And of course, it almost goes without saying that the Theory of Evolution, which places environmental change as the main cause of biodiversity, is entirely consistent with this finding, so there is much for creationists to ignore, lie about or misrepresent here:

No hint that the scientists found their data impossible to interpret using the TOE and needed to turn to childish creationist superstitions to explain it, and it all took place in that dim and distant past, 480 million years before creationists believe earth existed.

So, all they now need is an explanation of the dating methods and a logical reason to explain how they made 10,000 years or less look like 480 million years, and an explanation for why the scientists show no sign of abandoning the TOE in favour of creationism, as creation cult leader have been telling their followers in imminent - and has been for about the last 50 years or more.

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Ten Reasons To Lose Faith: And Why You Are Better Off Without It

This book explains why faith is a fallacy and serves no useful purpose other than providing an excuse for pretending to know things that are unknown. It also explains how losing faith liberates former sufferers from fear, delusion and the control of others, freeing them to see the world in a different light, to recognise the injustices that religions cause and to accept people for who they are, not which group they happened to be born in. A society based on atheist, Humanist principles would be a less divided, more inclusive, more peaceful society and one more appreciative of the one opportunity that life gives us to enjoy and wonder at the world we live in.

Available in Hardcover, Paperback or ebook for Kindle

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