Woodward W. Fischer

Low Mid-Proterozoic atmospheric oxygen levels and the delayed rise of animals

Low oxygen limited the rise of animals Oxygen levels in Earths early atmosphere had an important influence on the evolution of complex life. Planavsky et al. analyzed the isotopic signature of chromium in sedimentary rocks from across the globe—a proxy for past oxygen levels. Oxygen levels in the mid-Proterozoic (1.6 billion to 900 million years ago) were very low: less than 0.1% of the modern atmosphere. These low levels were probably below the minimum oxygen requirements for the earliest animals, delaying their emergence and diversification. Science, this issue p. 635 Oxygen levels in Earth’s early atmosphere were often less than 1% of modern levels. The oxygenation of Earth’s surface fundamentally altered global biogeochemical cycles and ultimately paved the way for the rise of metazoans at the end of the Proterozoic. However, current estimates for atmospheric oxygen (O2) levels during the billion years leading up to this time vary widely. On the basis of chromium (Cr) isotope data from a suite of Proterozoic sediments from China, Australia, and North America, interpreted in the context of data from similar depositional environments from Phanerozoic time, we find evidence for inhibited oxidation of Cr at Earth’s surface in the mid-Proterozoic (1.8 to 0.8 billion years ago). These data suggest that atmospheric O2 levels were at most 0.1% of present atmospheric levels. Direct evidence for such low O2 concentrations in the Proterozoic helps explain the late emergence and diversification of metazoans.

The Magnitude and Duration of Late Ordovician–Early Silurian Glaciation

Carbonate isotopes reveal a link between past ocean temperatures and mass extinction. Understanding ancient climate changes is hampered by the inability to disentangle trends in ocean temperature from trends in continental ice volume. We used carbonate “clumped” isotope paleothermometry to constrain ocean temperatures, and thereby estimate ice volumes, through the Late Ordovician–Early Silurian glaciation. We find tropical ocean temperatures of 32° to 37°C except for short-lived cooling by ~5°C during the final Ordovician stage. Evidence for ice sheets spans much of the study interval, but the cooling pulse coincided with a glacial maximum during which ice volumes likely equaled or exceeded those of the last (Pleistocene) glacial maximum. This cooling also coincided with a large perturbation of the carbon cycle and the Late Ordovician mass extinction.

An Iron Shuttle for Deepwater Silica in Late Archean and Early Paleoproterozoic Iron Formation

Iron formations are typically thinly bedded or laminated sedimentary rocks containing 15% or more of iron and a large proportion of silica (commonly > 40%). In the ca. 2590-2460 Ma Campbellrand-Kuruman Complex, Transvaal Supergroup, South Africa, iron formation occurs as a sediment-starved deepwater facies distal to carbonates and shales. Iron minerals, primarily siderite, define the lamination. The silica primarily occurs as thin beds and nodules of diagenetic chert (now microcrystalline quartz), filling pore space and replacing iron formation minerals and co-occurring deepwater lithologies. Mechanisms proposed to explain precipitation of the iron component of iron formation include photosynthetic oxygen production, anoxygenic photosynthesis, abiotic photochemistry, and chemoautotrophy using Fe(II) as an electron donor. The origin and mechanism of silica precipitation in these deposits have received less attention. Here we present a conceptual model of iron formation that offers insight into the deposition of silica. The model hinges on the proclivity of dissolved silica to adsorb onto the hydrous surfaces of ferric oxides. Soluble ferrous iron is oxidized in the surface ocean to form ferric hydroxides, which precipitate. Fe(OH)_3 binds silica and sinks from the surface ocean along with organic matter, shuttling silica to basinal waters and sediments. Fe(III) respiration in the sediments returns the majority of iron to the water column but also generates considerable alkalinity in pore waters, driving precipitation of siderite from Fe2+ and respiration-influenced CO2. Silica liberated during iron reduction becomes concentrated in pore fluids and is ultimately precipitated as diagenetic mineral phases. This model explains many of the mineralogical, textural, and environmental features of Late Archean and earliest Paleo-proterozoic iron formation.

Wave-Modified Turbidites: Combined-Flow Shoreline and Shelf Deposits, Cambrian, Antarctica

Sandstone tempestite beds in the Starshot Formation, cen- tral Transantarctic Mountains, were deposited in a range of shoreline to shelf environments. Detailed sedimentological analysis indicates that these beds were largely deposited by wave-modified turbidity currents. These currents are types of combined flows in which storm-generated waves overprint flows driven by excess-weight forces. The interpreta- tion of the tempestites of the Starshot Formation as wave-dominated turbidites rests on multiple criteria. First, the beds are generally well graded and contain Bouma-like sequences. Like many turbidites, the soles display abundant well-developed flutes. They also contain thick divisions of climbing-ripple lamination. The lamination, however, is dominated by convex-up and sigmoidal foresets, which are geometries identical to those produced experimentally in current-dominated com- bined flows in clear water. Finally, paleocurrent data support a tur- bidity-current component of flow. Asymmetric folds in abundant con- volute bedding reflect liquefaction and gravity-driven movement and hence their orientations indicate the downslope direction at the time of deposition. The vergence direction of these folds parallels paleocur- rent readings of flute marks, combined-flow ripples, and a number of other current-generated features in the Starshot event beds, indicating that the flows were driven down slope by gravity. The wave component of flow in these beds is indicated by the presence of small- to large- scale hummocky cross-stratification and rare small two-dimensional ripples. Wave-modified turbidity currents differ from deep-sea turbidity cur- rents in that they may not be autosuspending and some proportion of the turbulence that maintains these flows comes from storm waves. Such currents are formed in modern shoreline environments by a com- bination of storm waves and downwelling sediment-laden currents. They may also be formed as a result of oceanic floods, events in which intense sediment-laden fluvial discharge creates a hyperpycnal flow. Event beds in the Starshot Formation may have formed from such a mechanism. Oceanic floods are formed in rivers of small to medium size in areas of high relief, commonly on active margins. The Starshot Formation and the coeval Douglas Conglomerate are clastic units that formed in response to uplift associated with active tectonism. Sedi- mentological and stratigraphic data suggest that coarse alluvial fans formed directly adjacent to a marine basin. The geomorphic conditions were therefore likely conducive to rapid fluvial discharge events asso- ciated with storms. The abundance of current-dominated combined- flow ripples at the tops of many Starshot beds indicates that excess- weight forces were dominant throughout deposition of many of these beds.

Manganese-oxidizing photosynthesis before the rise of cyanobacteria

The emergence of oxygen-producing (oxygenic) photosynthesis fundamentally transformed our planet; however, the processes that led to the evolution of biological water splitting have remained largely unknown. To illuminate this history, we examined the behavior of the ancient Mn cycle using newly obtained scientific drill cores through an early Paleoproterozoic succession (2.415 Ga) preserved in South Africa. These strata contain substantial Mn enrichments (up to ∼17 wt %) well before those associated with the rise of oxygen such as the ∼2.2 Ga Kalahari Mn deposit. Using microscale X-ray spectroscopic techniques coupled to optical and electron microscopy and carbon isotope ratios, we demonstrate that the Mn is hosted exclusively in carbonate mineral phases derived from reduction of Mn oxides during diagenesis of primary sediments. Additional observations of independent proxies for O2—multiple S isotopes (measured by isotope-ratio mass spectrometry and secondary ion mass spectrometry) and redox-sensitive detrital grains—reveal that the original Mn-oxide phases were not produced by reactions with O2, which points to a different high-potential oxidant. These results show that the oxidative branch of the Mn cycle predates the rise of oxygen, and provide strong support for the hypothesis that the water-oxidizing complex of photosystem II evolved from a former transitional photosystem capable of single-electron oxidation reactions of Mn.

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.

Large wind ripples on Mars: A record of atmospheric evolution

Wind blowing over sand on Earth produces decimeter-wavelength ripples and hundred-meter– to kilometer-wavelength dunes: bedforms of two distinct size modes. Observations from the Mars Science Laboratory Curiosity rover and the Mars Reconnaissance Orbiter reveal that Mars hosts a third stable wind-driven bedform, with meter-scale wavelengths. These bedforms are spatially uniform in size and typically have asymmetric profiles with angle-of-repose lee slopes and sinuous crest lines, making them unlike terrestrial wind ripples. Rather, these structures resemble fluid-drag ripples, which on Earth include water-worked current ripples, but on Mars instead form by wind because of the higher kinematic viscosity of the low-density atmosphere. A reevaluation of the wind-deposited strata in the Burns formation (about 3.7 billion years old or younger) identifies potential wind-drag ripple stratification formed under a thin atmosphere.

CARBONATES IN SKELETON-POOR SEAS: NEW INSIGHTS FROM CAMBRIAN AND ORDOVICIAN STRATA OF LAURENTIA

Abstract Calcareous skeletons evolved as part of the greater Ediacaran–Cambrian diversification of marine animals. Skeletons did not become permanent, globally important sources of carbonate sediment, however, until the Ordovician radiation. Representative carbonate facies in a Series 3 (510–501 Ma) Cambrian to Tremadocian succession from western Newfoundland, Canada, and Ordovician successions from the Ibex area, Utah, USA, show that, on average, Cambrian and Tremadocian carbonates contain much less skeletal material than do post-Tremadocian sediments. Petrographic point counts of skeletal abundance within facies and proportional facies abundance in measured sections suggest that later Cambrian successions contain on average <5% skeletal material by volume, whereas the skeletal content of post-Tremadocian Ordovician sections is closer to ∼15%. A compilation of carbonate stratigraphic sections from across Laurentia confirms that post-Tremadocian increase in skeletal content is a general pattern and not unique to the two basins studied. The long interval (∼40 myr) between the initial Cambrian appearance of carbonate skeletons and the subsequent Ordovician diversification of heavily skeletonized organisms provides an important perspective on the Ordovician radiation. Geochemical data increasingly support the hypothesis that later Cambrian oceans were warm and, in subsurface water masses, commonly dysoxic to anoxic. We suggest that surface waters in such oceans would have been characterized by relatively low saturation states for calcite and aragonite. Mid-Ordovician cooling would have raised oxygen concentrations in subsurface water masses, establishing more highly oversaturated surface waters. If correct, these links could provide a proximal trigger for the renewed radiation of heavily skeletonized invertebrates and algae.

Biogeochemistry: Life before the rise of oxygen

The discovery of molecular fossils in 2.7-billion-year-old rocks prompted a re-evaluation of microbial evolution, and of the advent of photosynthesis and rise of atmospheric oxygen. That discovery now comes into question.

Sulfur isotopes of organic matter preserved in 3.45-billion-year-old stromatolites reveal microbial metabolism

The 3.45-billion-year-old Strelley Pool Formation of Western Australia preserves stromatolites that are considered among the oldest evidence for life on Earth. In places of exceptional preservation, these stromatolites contain laminae rich in organic carbon, interpreted as the fossil remains of ancient microbial mats. To better understand the biogeochemistry of these rocks, we performed microscale in situ sulfur isotope measurements of the preserved organic sulfur, including both Δ33S and . This approach allows us to tie physiological inference from isotope ratios directly to fossil biomass, providing a means to understand sulfur metabolism that is complimentary to, and independent from, inorganic proxies (e.g., pyrite). Δ33S values of the kerogen reveal mass-anomalous fractionations expected of the Archean sulfur cycle, whereas values show large fractionations at very small spatial scales, including values below -15‰. We interpret these isotopic patterns as recording the process of sulfurization of organic matter by H2S in heterogeneous mat pore-waters influenced by respiratory S metabolism. Positive Δ33S anomalies suggest that disproportionation of elemental sulfur would have been a prominent microbial process in these communities.