Devon B. Cole

Evolution of the global phosphorus cycle

The macronutrient phosphorus is thought to limit primary productivity in the oceans on geological timescales. Although there has been a sustained effort to reconstruct the dynamics of the phosphorus cycle over the past 3.5 billion years, it remains uncertain whether phosphorus limitation persisted throughout Earth’s history and therefore whether the phosphorus cycle has consistently modulated biospheric productivity and ocean–atmosphere oxygen levels over time. Here we present a compilation of phosphorus abundances in marine sedimentary rocks spanning the past 3.5 billion years. We find evidence for relatively low authigenic phosphorus burial in shallow marine environments until about 800 to 700 million years ago. Our interpretation of the database leads us to propose that limited marginal phosphorus burial before that time was linked to phosphorus biolimitation, resulting in elemental stoichiometries in primary producers that diverged strongly from the Redfield ratio (the atomic ratio of carbon, nitrogen and phosphorus found in phytoplankton). We place our phosphorus record in a quantitative biogeochemical model framework and find that a combination of enhanced phosphorus scavenging in anoxic, iron-rich oceans and a nutrient-based bistability in atmospheric oxygen levels could have resulted in a stable low-oxygen world. The combination of these factors may explain the protracted oxygenation of Earth’s surface over the last 3.5 billion years of Earth history. However, our analysis also suggests that a fundamental shift in the phosphorus cycle may have occurred during the late Proterozoic eon (between 800 and 635 million years ago), coincident with a previously inferred shift in marine redox states, severe perturbations to Earth’s climate system, and the emergence of animals.

A shale-hosted Cr isotope record of low atmospheric oxygen during the Proterozoic

The emergence and expansion of animal life on Earth represents a dramatic shift in the structure and complexity of the biosphere. A lack of firm constraints on surface oxygen levels during the mid-Proterozoic has resulted in heated debate as to whether the rise and earliest diversification of animals was directly linked to a change in environmental oxygen levels or, instead, simply reflects the timing of innovations in gene expression and developmental regulation and was independent of a direct environmental trigger. Here, we present chromium (Cr) isotope data from marine black shales that provide evidence for minimal Cr oxidation throughout the mid-Proterozoic leading up to the diversification of eukaryotes and the rise of animals during the late Neoproterozoic. This observation requires very low background oxygen levels (<1% of present atmospheric levels). Accepting previously proposed estimates of p O2 levels needed to induce Cr isotope fractionation, our data provide support for the persistence of an Earth system in which baseline atmospheric p O2 would have been low enough to inhibit the diversification of animals until ca. 800 Ma. More generally, evidence for a delayed rise of atmospheric oxygen strongly suggests that environmental factors have played a fundamental role in controlling the emergence and expansion of complex life on Earth.

No evidence for high atmospheric oxygen levels 1,400 million years ago

Zhang et al. (1) recently proposed atmospheric oxygen levels of ∼4% present atmospheric levels (PAL) based on modeling a paleoenvironment reconstructed from trace metal and biomarker data from the 1,400 Ma Xiamaling Formation in China. Intriguingly, this pO2 level is above the threshold oxygen requirements of basal animals and clashes with evidence for atmospheric oxygen levels <<1% PAL in the mid-Proterozoic (2). However, there are fundamental problems with the inorganic and organic geochemical work presented by Zhang et al. (1).

Integrated geochemical-petrographic insights from component-selective δ238U of Cryogenian marine carbonates

Emerging geochemical proxies have improved our understanding of the broad-scale history of Earth’s oxygenation. However, paleoredox work does not always include extensive consideration of sample preservation and paleoenvironmental setting. This is particularly an issue with marine carbonates, which although being potentially ideal ocean redox archives, are commonly altered during diagenesis. Here we provide new insight into the robustness of uranium isotopes (238U/235U ratios: δ238U values) as paleoredox tracers by determining texture-specific δ238U values from a well-described Cryogenian (Balcanoona) reef complex in South Australia. We found high variability in δ238U values between different carbonate components, even within a single sample. Petrographically, the best-preserved components from the Balcanoona reef are marine cements, which have a mean δ238U value of −0.23‰, essentially unfractionated from riverine inputs. These values are interpreted as reflecting a marine system with widespread anoxic and iron-rich settings. Less-well-preserved phases have δ238U values spanning almost the entire extent of the documented isotopic range. This integrated petrographic-geochemical work demonstrates the need for petrographic analysis and careful sample selection on a case-by-case basis in future carbonate metal isotope geochemistry.

PASSIVE TRANSGRESSION: REMARKABLE PRESERVATION AND SPATIAL DISTRIBUTION OF UPPERMOST DEVONIAN (FAMENNIAN) MARGINAL AND NEARSHORE MARINE FACIES AND FAUNA OF WESTERN LAURENTIA

Abstract The stratigraphic record of shallow-water shoreline paleoenvironments is characterized by significant facies heterogeneity and laterally discontinuous stratal geometries. In contrast, we investigate a uniquely extensive and microstratigraphically spatially uniform interval of upper Famennian (Upper Devonian) marginal marine strata in the Rocky Mountains. This transgressive deposit (< 5 m thick) rests on a depositional sequence boundary, and is composed of a thin, discontinuous basal transgressive sandstone bed, two thin fossiliferous shale beds, and several meters of oncolite-bearing carbonate wackestone. The lower shale is a consistently thin (~ 0.1 m) bed with a fossil fauna of spinicaudatans (clam shrimp or conchostracans), which are extant, bivalved, chitinous, benthic crustaceans that live in fresh to brackish water, in ephemeral ponds, estuaries, and other shoreline settings. Isotopic data for the oncolitic unit record deposition in an epicontinental seaway with restricted circulation and locally brackish conditions. Sulfur isotope data may also reflect short-term changes in redox conditions, consistent with our interpretation of temporary hypoxia during deposition of the spinicaudatan bed, based on its faunal assemblages. The remarkably large areal extent (~ 1600 km north to south, and ~ 1000 km east to west) of this uniformly thin interval with marginal marine fauna is one of the most unusual paleoecological events of the latest Devonian in Laurentia. It is considered an artifact of exceptional depositional processes including passive transgression (i.e., little or no wave or tidal ravinement) along with rapid opportunistic takeover of habitats during transgression. The radiation, possibly associated with temporary hypoxia, was aided by the spinicaudatan reproductive strategy of numerous offspring, high growth rate, and rapid reproduction.

UV radiation limited the expansion of cyanobacteria in early marine photic environments

Prior to atmospheric oxygenation, ecosystems were exposed to higher UV radiation fluxes relative to modern surface environments. Iron–silica mineral coatings have been evoked as effective UV radiation shields in early terrestrial settings. Here we test whether similar protection applied to planktonic cyanobacteria within the Archean water column. Based on experiments done under Archean seawater conditions, we report that Fe(III)–Si-rich precipitates absorb up to 70% of incoming UV-C radiation, with a reduction of <20% in photosynthetically active radiation flux. However, we demonstrate that even short periods of UV-C irradiation in the presence of Fe(III)–Si precipitates resulted in high mortality rates, and suggest that these effects would have persisted throughout much of the photic zone. Our findings imply that despite the shielding properties of Fe(III)–Si-rich precipitates in the early water column, UV radiation would continue to limit cyanobacterial expansion and likely had a greater effect on Archean ecosystem structure before the formation of an ozone layer.The means by which planktonic cyanobacteria were able to persist through the Archean despite high fluxes of UV radiation are unclear. Here, the authors show that Fe(III)-Si rich precipitates in the Archean photic zone could have provided early planktonic cyanobacteria an effective shield against UV-C radiation.

Ecological Expansion and Extinction in the Late Ediacaran: Weighing the Evidence for Environmental and Biotic Drivers

The Ediacara Biota, Earths earliest communities of complex, macroscopic, multicellular organisms, appeared during the late Ediacaran Period, just prior to the Cambrian Explosion. Ediacara fossil assemblages consist of exceptionally preserved soft-bodied forms of enigmatic morphology and affinity which nonetheless represent a critical stepping-stone in the evolution of complex animal ecosystems. The Ediacara Biota has historically been divided into three successive Assemblages-the Avalon, the White Sea, and the Nama. Although the oldest (Avalon) Assemblage documents the initial appearance of several groups of Ediacara taxa, the two younger (White Sea and Nama) Assemblages record a particularly striking suite of ecological innovations, including the appearance of diverse Ediacara body plans-in tandem with the rise of bilaterian animals-as well as the emergence of novel ecological strategies such as movement, sexual reproduction, biomineralization, and the development of dense, heterogeneous benthic communities. Many of these ecological innovations appear to be linked to adaptations to heterogeneous substrates and shallow and energetic marine settings. In spite of these innovations, the majority of Ediacara taxa disappear by the end of the Ediacaran, with interpretations for this disappearance historically ranging from the closing of preservational windows to environmentally or biotically mediated extinction. However, in spite of the unresolved affinity and eventual extinction of individual Ediacara taxa, these distinctive ecological strategies persist across the Ediacaran-Cambrian boundary and are characteristic of younger animal-dominated communities of the Phanerozoic. The late Ediacaran emergence of these strategies may, therefore, have facilitated subsequent radiations of the Cambrian. In this light, the Ediacaran and Cambrian Periods, although traditionally envisioned as separate worlds, are likely to have been part of an ecological and evolutionary continuum.