David A. Fike

Oxidation of the Ediacaran Ocean

Oxygenation of the Earth’s surface is increasingly thought to have occurred in two steps. The first step, which occurred ∼2,300 million years (Myr) ago, involved a significant increase in atmospheric oxygen concentrations and oxygenation of the surface ocean. A further increase in atmospheric oxygen appears to have taken place during the late Neoproterozoic period (∼800–542 Myr ago). This increase may have stimulated the evolution of macroscopic multicellular animals and the subsequent radiation of calcified invertebrates, and may have led to oxygenation of the deep ocean. However, the nature and timing of Neoproterozoic oxidation remain uncertain. Here we present high-resolution carbon isotope and sulphur isotope records from the Huqf Supergroup, Sultanate of Oman, that cover most of the Ediacaran period (∼635 to ∼548 Myr ago). These records indicate that the ocean became increasingly oxygenated after the end of the Marinoan glaciation, and they allow us to identify three distinct stages of oxidation. When considered in the context of other records from this period, our data indicate that certain groups of eukaryotic organisms appeared and diversified during the second and third stages of oxygenation. The second stage corresponds with the Shuram excursion in the carbon isotope record and seems to have involved the oxidation of a large reservoir of organic carbon suspended in the deep ocean, indicating that this event may have had a key role in the evolution of eukaryotic organisms. Our data thus provide new insights into the oxygenation of the Ediacaran ocean and the stepwise restructuring of the carbon and sulphur cycles that occurred during this significant period of Earth’s history.

Fossil steroids record the appearance of Demospongiae during the Cryogenian period

The Neoproterozoic era (1,000–542 Myr ago) was an era of climatic extremes and biological evolutionary developments culminating in the emergence of animals (Metazoa) and new ecosystems. Here we show that abundant sedimentary 24-isopropylcholestanes, the hydrocarbon remains of C30 sterols produced by marine demosponges, record the presence of Metazoa in the geological record before the end of the Marinoan glaciation (∼635 Myr ago). These sterane biomarkers are abundant in all formations of the Huqf Supergroup, South Oman Salt Basin, and, based on a new high-precision geochronology, constitute a continuous 100-Myr-long chemical fossil record of demosponges through the terminal Neoproterozoic and into the Early Cambrian epoch. The demosponge steranes occur in strata that underlie the Marinoan cap carbonate (>635 Myr ago). They currently represent the oldest evidence for animals in the fossil record, and are evidence for animals pre-dating the termination of the Marinoan glaciation. This suggests that shallow shelf waters in some late Cryogenian ocean basins (>635 Myr ago) contained dissolved oxygen in concentrations sufficient to support basal metazoan life at least 100 Myr before the rapid diversification of bilaterians during the Cambrian explosion. Biomarker analysis has yet to reveal any convincing evidence for ancient sponges pre-dating the first globally extensive Neoproterozoic glacial episode (the Sturtian, ∼713 Myr ago in Oman).

A Stratified Redox Model for the Ediacaran Ocean

Oceans Before the Dawn Although the fossil record shows a clear explosion of animal diversity in the oceans at the beginning of the Cambrian period (∼542 million years ago), the evolutionary pressures driving this increase in diversity remain unclear. The likely scenario involves drastic changes in the distribution of oxygen in ocean basins, but global glaciations and poor preservation of rocks from this time often prevent a clear picture of global ecology before the dawn of animal life. Li et al. (p. 80, published online 11 February; see the Perspective by Narbonne) characterized the geochemical makeup of sedimentary rocks from south China that indicate alternating layers of sulfide- and iron-rich (i.e., sulfate-limited) anoxic waters in the several million years leading up to the Cambrian. These conditions may have set the stage for an eventual increase in productivity of photosynthetic life that oxygenated the oceans and allowed for rapid animal evolution. Geological records in China indicate that ocean chemistry may explain the delay in life’s biggest diversification period. The Ediacaran Period (635 to 542 million years ago) was a time of fundamental environmental and evolutionary change, culminating in the first appearance of macroscopic animals. Here, we present a detailed spatial and temporal record of Ediacaran ocean chemistry for the Doushantuo Formation in the Nanhua Basin, South China. We find evidence for a metastable zone of euxinic (anoxic and sulfidic) waters impinging on the continental shelf and sandwiched within ferruginous [Fe(II)-enriched] deep waters. A stratified ocean with coeval oxic, sulfidic, and ferruginous zones, favored by overall low oceanic sulfate concentrations, was maintained dynamically throughout the Ediacaran Period. Our model reconciles seemingly conflicting geochemical redox conditions proposed previously for Ediacaran deep oceans and helps to explain the patchy temporal record of early metazoan fossils.

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.

Aeolian processes at the Mars Exploration Rover Meridiani Planum landing site

The martian surface is a natural laboratory for testing our understanding of the physics of aeolian (wind-related) processes in an environment different from that of Earth. Martian surface markings and atmospheric opacity are time-variable, indicating that fine particles at the surface are mobilized regularly by wind. Regolith (unconsolidated surface material) at the Mars Exploration Rover Opportunitys landing site has been affected greatly by wind, which has created and reoriented bedforms, sorted grains, and eroded bedrock. Aeolian features here preserve a unique record of changing wind direction and wind strength. Here we present an in situ examination of a martian bright wind streak, which provides evidence consistent with a previously proposed formational model for such features. We also show that a widely used criterion for distinguishing between aeolian saltation- and suspension-dominated grain behaviour is different on Mars, and that estimated wind friction speeds between 2 and 3 m s-1, most recently from the northwest, are associated with recent global dust storms, providing ground truth for climate model predictions.

The earliest Cambrian record of animals and ocean geochemical change

The Cambrian diversification of animals was long thought to have begun with an explosive phase at the start of the Tommotian Age. Recent stratigraphic discoveries, however, suggest that many taxa appeared in the older Nemakit-Daldynian Age, and that the diversification was more gradual. We map lowest Cambrian (Nemakit-Daldynian through Tommotian) records of δ 13 C CaCO 3 variability from Siberia, Mongolia, and China onto a Moroccan U/Pb–δ 13 C CaCO 3 age model constrained by five U/Pb ages from interbedded volcanic ashes. The δ 13 C CaCO 3 correlations ignore fossil tie points, so we assume synchroneity in δ 13 C trends rather than synchroneity in first appearances of animal taxa. We present new δ 13 C org , 87 Sr/ 86 Sr, uranium, and vanadium data from the same carbonate samples that define the Moroccan δ 13 C CaCO 3 curve. The result is a new absolute time line for first appearances of skeletal animals and for changes in the carbon, strontium, and redox chemistry of the ocean during the Nemakit-Daldynian and Tommotian ages at the beginning of the Cambrian. The time line suggests that the diversification of skeletal animals began early in the Nemakit-Daldynian, with much of the diversity appearing by the middle of the age. Fossil first appearances occurred in three pulses, with a small pulse in the earliest Nemakit-Daldynian (ca. 540–538 Ma), a larger pulse in the mid- to late Nemakit-Daldynian (ca. 534–530 Ma), and a moderate pulse in the Tommotian (ca. 524–522 Ma). These pulses are associated with rapid reorganizations of the carbon cycle, and are superimposed on long-term increases in sea level and the hydrothermal flux of Sr.

Soils of Eagle Crater and Meridiani Planum at the Opportunity Rover Landing Site

The soils at the Opportunity site are fine-grained basaltic sands mixed with dust and sulfate-rich outcrop debris. Hematite is concentrated in spherules eroded from the strata. Ongoing saltation exhumes the spherules and their fragments, concentrating them at the surface. Spherules emerge from soils coated, perhaps from subsurface cementation, by salts. Two types of vesicular clasts may represent basaltic sand sources. Eolian ripples, armored by well-sorted hematite-rich grains, pervade Meridiani Planum. The thickness of the soil on the plain is estimated to be about a meter. The flatness and thin cover suggest that the plain may represent the original sedimentary surface.

Spatial grain size sorting in eolian ripples and estimation of wind conditions on planetary surfaces: Application to Meridiani Planum, Mars

The landscape seen by the Mars Exploration Rover (MER) Opportunity at Meridiani Planum is dominated by eolian (wind-blown) ripples with concentrated surface lags of hematitic spherules and fragments. These ripples exhibit profound spatial grain size sorting, with well-sorted coarse-grained crests and poorly sorted, generally finer-grained troughs. These ripples were the most common bed form encountered by Opportunity in its traverse from Eagle Crater to Endurance Crater. Field measurements from White Sands National Monument, New Mexico, show that such coarse-grained ripples form by the different transport modes of coarse- and fine-grain fractions. On the basis of our field study, and simple theoretical and experimental considerations, we show how surface deposits of coarse-grained ripples can be used to place tight constraints on formative wind conditions on planetary surfaces. Activation of Meridiani Planum coarse-grained ripples requires a wind velocity of 70 m/s (at a reference elevation of 1 m above the bed). From images by the Mars Orbiter Camera (MOC) of reversing dust streaks, we estimate that modern surface winds reach a velocity of at least 40 m/s and hence may occasionally activate these ripples. The presence of hematite at Meridiani Planum is ultimately related to formation of concretions during aqueous diagenesis in groundwater environments; however, the eolian concentration of these durable particles may have led to the recognition from orbit of this environmentally significant landing site.

Micron-scale mapping of sulfur cycling across the oxycline of a cyanobacterial mat: a paired nanoSIMS and CARD-FISH approach.

The metabolic activities of microbial mats have likely regulated biogeochemical cycling over most of Earths history. However, the relationship between metabolic activity and the establishment of isotopic geochemical gradients in these mats remains poorly constrained. Here we present a parallel microgeochemical and microbiological study of micron-scale sulfur cycling within hypersaline microbial mats from Guerrero Negro, Baja California Sur, Mexico. Dissolved sulfide within the mats was captured on silver discs and analyzed for its abundance and δ34S isotopic composition using high-resolution secondary ion mass spectrometry (nanoSIMS). These results were compared to sulfide and oxygen microelectrode profiles. Two-dimensional microgeochemical mapping revealed well-defined laminations in sulfide concentration (on scales from 1 to 200 μm), trending toward increased sulfide concentrations at depth. Sulfide δ34S decreased from ∼+10‰ to −20‰ in the uppermost 3 mm and oscillated repeatedly between −10‰ and −30‰ down to a depth of 8 mm. These variations are attributed to spatially variable bacterial sulfate reduction within the mat. A parallel examination of the spatial distribution of known sulfate-reducing bacteria within the family Desulfobacteraceae was conducted using catalyzed reporter deposition fluorescence in situ hybridization. Significant concentrations of Desulfobacteraceae were observed in both oxic and anoxic zones of the mat and occurred in several distinct layers, in large aggregates and heterogeneously dispersed as single cells throughout. The spatial distribution of these microorganisms is consistent with the variation in sulfide concentration and isotopic composition we observed. The parallel application of the methodologies developed here can shed light on micron-scale sulfur cycling within microbially dominated sedimentary environments.

Superheavy pyrite (δ34Spyr > δ34SCAS) in the terminal Proterozoic Nama Group, southern Namibia: A consequence of low seawater sulfate at the dawn of animal life

Sulfur isotope analysis (δ^(34)S) of well-preserved carbonates spanning an ~10 Ma interval of the terminal Proterozoic Nama Group reveals that disseminated pyrite is consistently enriched in ^(34)S relative to coeval seawater sulfate as preserved in carbonate-associated sulfate (CAS). This observation is not consistent with the current paradigm for interpreting the geologic record of sulfur isotopes, which assumes that pyrite δ^(34)S (δ^(34)S_(pyr)) will be equal to or less than co-occurring CAS δ^(34)S (δ^(34)S_(CAS)) due to the kinetic isotope effect of bacterial sulfate reduction (BSR) that favors the lighter isotope of sulfur (^(32)S) during sulfur-oxygen bond breakage. Although the precise mechanism of pyrite sulfur isotope enrichment is debatable, our combined observations of extremely ^(34)S-enriched pyrite, low bulk-rock concentrations of sulfur from CAS and pyrite, and high-frequency fluctuations in δ^(34)S_(CAS) and δ^(34)S_(pyr) throughout the Nama Group carbonates point to very low concentrations of sulfate in portions of the terminal Proterozoic ocean. The additional occurrence of ^(34)S-enriched pyrite in contemporaneous terminal Proterozoic sections from Poland and Canada reveal that low seawater sulfate may have been widespread in the oceans at this time. However, the absence of such extremely 34S-enriched pyrite from well-preserved, coeval carbonate sections in Oman suggests that such conditions were not globally uniform. Low, geographically varied concentrations of marine sulfate in terminal Proterozoic time are consistent with elevated, geographically varied concentrations of reactive, nonpyritized iron in marine shales recently reported for this interval, suggesting that high dissolved Fe(II) and low O_2 persisted in a range of marine facies as late as 543 Ma -— tens of millions of years after the origin of animals, yet prior to their major diversification in Early Cambrian time.