Sedimentary Rock Record Research Articles

Assessment of Hg Speciation Changes in the Sedimentary Rock Record From Thermal Desorption Characteristics

AbstractSedimentary mercury (Hg) has become a widely used proxy for paleo‐volcanic activity. However, scavenging and drawdown of Hg by organic‐matter (OM) and sulfides are important non‐volcanic factors determining variability in such records. Most studies, therefore, normalize total Hg (HgT) to a Hg “host‐phase” proxy (e.g., HgT/TOC for OM, HgT/TS for sulfides), with the dominant host‐phase determined based on the strongest observed (linear) correlations. This approach suffers from various non‐linearities in Hg‐host‐phase behavior and does not account for succession‐level, let alone sample‐level, Hg speciation changes. Thermal desorption characteristics or “profiles” (TDPs) for many Hg species during pyrolysis analysis are well‐established with applications including distinguishing between OM‐bound Hg and different Hg sulfides and oxides in (sub‐)recent sediments. We explore the use of TDPs for geological sediment (rock) samples and illustrate the presence of multiple release phases (Hg species)—correlated to geochemical host‐phase—in (almost) all the 65 analyzed Tithonian (146–145 Ma) silt and mudrock samples. By quantifying the Hg in each release phase for every sample, we find TOC concentration may determine ∼60% of the variability in the first (lower temperature) Hg TDP release phase: a stark difference with the total Hg released from these samples, where ∼20% of variation is explained by TOC variability. TDPs provide insight on sample‐level Hg speciation and demonstrate that, while the common assumption of single‐phase Hg speciation in sedimentary rocks is problematic, differences in Hg speciation can be detected, quantified, and accounted for using commonly applied techniques—opening potential for routine assessment.

Contrasting Recording Efficiency of Chemical Versus Depositional Remanent Magnetization in Sediments

AbstractHow and when sedimentary rocks record Earth's magnetic field is complex. Most studies assume a time‐progressive lock‐in mechanism during sediment deposition called depositional remanent magnetization (DRM). However, magnetic minerals can also form in situ, recording a chemical remanent magnetization (CRM) that is discontinuous in time. Disentangling the two mechanisms represents a major hurdle, and differences in their recording efficiencies remain unexplored. Here, our theoretical solutions demonstrate that CRM intensities exceed DRM by a factor of six when acquired in the same magnetic field. Novel experiments growing greigite (Fe3S4) in sediments and subsequent redeposition under identical magnetic field conditions confirm the predicted difference in recording efficiency. Thus, if left unrecognized, CRM leads to overestimated paleointensity and deserves more attention when interpreting Earth's magnetic history from sedimentary records. Recognition of fundamental differences between CRM and DRM characteristics provide a way forward to distinguish the recording mechanisms through routine laboratory protocols.

Heavy‐Mineral Grain Counting: Counting Techniques, Error Estimation, and the Number of Grains to be Counted

AbstractHeavy‐mineral assemblages of sediments and sedimentary rocks record information regarding provenance, including the source rocks involved, tectonic setting, climatic conditions, and modifications from source to sink. Drawing conclusions on provenance and provenance changes requires robust quantification of individual heavy‐mineral species contents, including error estimates. Nevertheless, it is common practice to count sub‐quantities of grains from aliquots and not considering the bias introduced by (a) counting similar numbers of grains from aliquots containing different total numbers of grains, and (b) using variable counting methods. Consequently, reported heavy‐mineral contents estimated from counting sub‐quantities are affected by errors of unknown extent, making it infeasible to determine whether intra‐ or intersample variations are statistically significant. Based on 65 heavy‐mineral aliquots of variable grain size, mineral species contents, total number of grains, and known composition determined by counting all grains (n = 80,393), here >31 million countings of heavy‐mineral sub‐quantities are simulated using (a) ribbon counting with varying ribbon size, ribbon position, ribbon orientation, total number of counts, and ways of aggregating counts from multiple ribbons, and (b) a newly proposed counting technique called cluster counting. I show that (a) error estimation for a specific aliquot requires a finite population correction; (b) compared to adjacent ribbons, aggregating counts of spatially distant ribbons reduces the error; (c) cluster counting further reduces the error, showing the best fit with theory; and (d) the Wilson score interval enables error calculation as well as the number of grains to be counted to achieve an operator‐specific aim.

Unusual sources of fossil micrometeorites deduced from relict chromite in the small size fraction in ~467 Ma old limestone

ABSTRACTExtraterrestrial chrome spinel and chromite extracted from the sedimentary rock record are relicts from coarse micrometeorites and rarely meteorites. They are studied to reconstruct the paleoflux of meteorites to the Earth and the collisional history of the asteroid belt. Minor element concentrations of Ti and V, and oxygen isotopic compositions of these relict minerals were used to classify the meteorite type they stem from, and thus to determine the relative meteorite group abundances through time. While coarse sediment‐dispersed extraterrestrial chrome‐spinel (SEC) grains from ordinary chondrites dominate through the studied time windows in the Phanerozoic, there are exceptions: We have shown that ~467 Ma ago, 1 Ma before the breakup of the L chondrite parent body (LCPB), more than half of the largest (>63 μm diameter) grains were achondritic and originated from differentiated asteroids in contrast to ordinary chondrites which dominated the meteorite flux throughout most of the past 500 Ma. Here, we present a new data set of oxygen isotopic compositions and elemental compositions of 136 grains of a smaller size fraction (32–63 μm) in ~467 Ma old pre‐LCPB limestone from the Lynna River section in western Russia, that was previously studied by elemental analysis. Our study constitutes the most comprehensive oxygen isotopic data set of sediment‐dispersed extraterrestrial chrome spinel to date. We also introduce a Raman spectroscopy‐based method to identify SEC grains and distinguish them from terrestrial chrome spinel with ~97% reliability. We calibrated the Raman method with the established approach using titanium and vanadium concentrations and oxygen isotopic compositions. We find that ordinary chondrites are approximately three times more abundant in the 32–63 μm fraction than achondrites. While abundances of achondrites compared to ordinary chondrites are lower in the 32–63 μm size fraction than in the >63 μm one, achondrites are approximately three times more abundant in the 32–62 μm fraction than they are in the present flux. We find that the sources of SEC grains vary for different grain sizes, mainly as a result of parent body thermal metamorphism. We conclude that the meteorite flux composition ~467 Ma ago ~1 Ma before the breakup of the LCPB was fundamentally different from today and from other time windows studied in the Phanerozoic, but that in contrast to the large size fraction ordinary chondrites dominated the flux in the small size fraction. The high abundance of ordinary chondrites in the studied samples is consistent with the findings based on coarse extraterrestrial chrome‐spinel from other time windows.

Transgression–regression cycles drive correlations in Ediacaran–Cambrian rock and fossil records

AbstractStrata of the Ediacaran Period (635–538.8 Ma) yield the oldest known fossils of complex, macroscopic organisms in the geologic record. These “Ediacaran-type” macrofossils (known as the Ediacaran biota) first appear in mid-Ediacaran strata, experience an apparent decline through the terminal Ediacaran, and directly precede the Cambrian (538.8–485.4 Ma) radiation of animals. Existing hypotheses for the origin and demise of the Ediacaran biota include: changing oceanic redox states, biotic replacement by succeeding Cambrian-type fauna, and mass extinction driven by environmental change. Few studies frame trends in Ediacaran and Cambrian macroevolution from the perspective of the sedimentary rock record, despite well-documented Phanerozoic covariation of macroevolutionary patterns and sedimentary rock quantity. Here we present a quantitative analysis of North American Ediacaran–Cambrian rock and fossil records from Macrostrat and the Paleobiology Database. Marine sedimentary rock quantity increases nearly monotonically and by more than a factor of five from the latest Ediacaran to the late Cambrian. Ediacaran–Cambrian fossil quantities exhibit a comparable trajectory and have strong (rs > 0.8) positive correlations with marine sedimentary area and volume flux at multiple temporal resolutions. Even so, Ediacaran fossil quantities are dramatically reduced in comparison to the Cambrian when normalized by the quantity of preserved marine rock. Although aspects of these results are consistent with the expectations of a simple fossil preservation–induced sampling bias, together they suggest that transgression–regression and a large expansion of marine shelf environments coincided with the diversification of animals during a dramatic transition that is starkly evident in both the sedimentary rock and fossil records.

Isotopic constraints on lightning as a source of fixed nitrogen in Earth’s early biosphere

Bioavailable nitrogen is thought to be a requirement for the origin and sustenance of life. Before the onset of biological nitrogen fixation, abiotic pathways to fix atmospheric N2 must have been prominent to provide bioavailable nitrogen to Earth’s earliest ecosystems. Lightning has been shown to produce fixed nitrogen as nitrite and nitrate in both modern atmospheres dominated by N2 and O2 and atmospheres dominated by N2 and CO2 analogous to the Archaean Earth. However, a better understanding of the isotopic fingerprints of lightning-generated fixed nitrogen is needed to assess the role of this process on early Earth. Here we present results from spark discharge experiments in N2−CO2 and N2−O2 gas mixtures. Our experiments suggest that lightning-driven nitrogen fixation may have been similarly efficient in the Archaean atmosphere, compared with modern times. Measurements of the isotopic ratio (δ15N) of the discharge-produced nitrite and nitrate in solution show very low values of −6‰ to −15‰ after equilibration with the gas phase with a calculated endmember composition of −17‰. These results are much lower than most δ15N values documented from the sedimentary rock record, which supports the development of biological nitrogen fixation earlier than 3.2 billion years ago. However, some Paleoarchean records (3.7 billion years ago) may be consistent with lightning-derived nitrogen input, highlighting the potential role of this process for the earliest ecosystems. Spark discharge experiments suggest lightning was not the main source of bioavailable nitrogen for the established Archaean biosphere, but could have been significant for Earth’s earliest ecosystems.

Cyclostratigraphy at a tipping point

There are key patterns of variation in the (litho‐) stratigraphical record, which, with advances in computing technology in recent decades, have become amenable to objective numerical analysis. This research has chiefly focused on the search for spatially regular cycles in the sedimentary rock record, since there are theoretical grounds for believing that these would be periodic and thus provide a means of time‐calibrating the stratigraphical records in which they occurred. Less popular lines of research consider the scaling relationships of the tangible, measurable sedimentary rock layers, and of the hiatuses, individually of indeterminate length, that punctuate these layers. Objective source data for all these analyses is provided, on limited (usually decametre) scales, by detailed lithological logging, sampling and dating of sections provided by exposures and cores. An objective global lithostratigraphical database, with larger, kilometre scales, is provided by the widespread practice of electric (wire‐line) logging of deep wells. This involves measurement, at 6‐inch (~15 cm) depth intervals of lithology‐related petrophysical properties. Rapid desk top computer analysis of the hundreds or thousands of readings that comprise these two data series, searches for patterns of variation. This review considers the published evidence for ‘ubiquitous’ cyclicity and its use in chronostratigraphy, despite the contrary claims that stratigraphical records are predominantly random in character.

Impact of temperature increase on the formation of magnetic minerals in shales. The example of Tournemire, France

Remagnetization is a pervasive mechanism affecting all kinds of rocks. Sedimentary rocks are particularly prone to such a mechanism because they can experience a wide range of physical and biogeochemical processes which can lead to partial or total loss of the original paleomagnetic record. Temperature increase is one of key factors affecting the rock magnetic properties in sedimentary rocks. Laboratory experiments on shales in the literature have shown that the magnetic mineralogy changed with temperature as low as 50 °C, and that new magnetic phase(s) are formed. The latter was identified to be mostly iron oxides (e.g., magnetite) and iron sulfides (e.g., pyrrhotite). In this study similar to what previously conducted on samples from the Paris Basin, a laboratory heating experiment is conducted on shale samples from the Tournemire tunnel (SE France) in a temperature range of 50 to 130 °C. This aims to simulate a new exposition of the samples to temperature and to identify the changes in magnetic phases, especially on FeIII-bearing minerals, with temperature. The remanence of the analyzed samples increases stepwise, suggesting an increasing concentration of magnetic minerals above their blocking volume with increasing temperature. Magnetite is the main magnetic carrier present and formed in the heated samples, with possibly pyrrhotite forming at high temperature. A conceptual model of magnetic mineralogy changes with temperature is presented and compared to previous results on claystones from the Paris Basin. The experimental observations support that the paleomagnetic record in sedimentary rocks affected by a later temperature increase during burial should be interpreted carefully. Indeed, remagnetization will likely occur when magnetic minerals record a secondary remanence either growing above their blocking volume or when exposed again to a temperature higher than the maximum burial temperature formerly experienced by the samples.

Lipid biomarker comparison of relict and active microbial mats from the McMurdo Ice Shelf, Antarctica

Earth’s biota leave traces in the form of molecular fossils, typically from the membrane lipids of organisms, preserved in the sedimentary rock record. However, since the environmental conditions prevailing at the time can impact the nature of those remains, it is important to understand the diagenetic processes that occur under contrasting regimes. In this study, we investigated the preservation under cold and arid climatic conditions of intact polar lipids, bacteriohopanepolyols, and quinones preserved in a ∼700-year-old relict, desiccated microbial mat. We compared these to lipid signatures in actively growing counterparts from nearby meltwater ponds of McMurdo Ice Shelf in Antarctica. Substantial differences in lipid distribution were evident between active and relict mats, suggesting that significant diagenetic degradation of lipid biomarkers occurs within a short period, thereby illuminating the selective preservation of certain polar compound classes under cold and dry conditions.

Ancient Siliciclastic-Evaporites as Seen by Remote Sensing Instrumentation with Implications for the Rover-Scale Exploration of Sedimentary Environments on Mars.

Accurate interpretation of the martian sedimentary rock record-and by extension that planet's paleoenvironmental history and potential habitability-relies heavily on rover-based acquisition of textural and compositional data and researchers to properly interpret those data. However, the degree to which this type of remotely sensed information can be unambiguously resolved and accurately linked to geological processes in ancient sedimentary systems warrants further study. In this study, we characterize Mars-relevant siliciclastic-evaporite samples by traditional laboratory-based geological methods (thin section petrography, X-ray diffraction [XRD], backscattered electron imaging, microprobe chemical analyses) and remote sensing methods relevant to martian rover payloads (visible-near-mid infrared reflectance spectroscopy, X-ray fluorescence mapping, XRD). We assess each method's ability to resolve primary and secondary sedimentologic features necessary for the accurate interpretation of paleoenvironmental processes. While the most dominant textures and associated compositions (i.e., bedded gypsum evaporite) of the sample suite are readily identified by a combination of remote sensing techniques, equally important, although more subtle, components (i.e., interbedded windblown silt, meniscus cements) are not resolved unambiguously in bulk samples. However, rover-based techniques capable of coordinating spatially resolved compositional measurements with textural imaging reveal important features not readily detected using traditional assessments (i.e., subtle clay-organic associations, microscale diagenetic nodules). Our findings demonstrate the improved generational capacity of rovers to explore ancient sedimentary environments on Mars while also highlighting the complexities in extracting comprehensive paleoenvironmental information when limited to currently available rover-based techniques. Complete and accurate interpretation of ancient martian sedimentary environments, and by extension the habitability of those environments, likely requires sample return or in situ human exploration.

Geochronology and geochemistry of the Cryogenian sedimentary rocks and Early Paleozoic mafic dykes of the South Qinling belt: Implications for the tectonic evolution of the northwestern South China craton

The late Paleozoic–Mesozoic tectonic evolution of the Central China orogen between the North China and South China cratons was key in the growth of the Asian continent. The preceding Proterozoic–Early Paleozoic geologic history of the Central China orogen is comparatively less well understood yet critical to resolving preconditions for the assembly of Asia. The South Qinling belt located within the Central China orogen exposes a spectacular record of Cryogenian supracrustal sedimentary rocks and Early Paleozoic intermediate-mafic rocks that are related to the development of the northwestern South China craton. In this study, we investigated the Cryogenian sedimentary rocks and Early Paleozoic magmatic dykes/sills in the Daba Shan of the South Qinling belt to provide new constraints on the tectonic evolution of the orogen in the context of the Paleo-Tethyan domain. We report that Cryogenian sedimentary rocks predominantly contain ca. 850–695 Ma detrital zircon grains, which suggests that these short-transported materials were sourced from the northwestern South China craton. Gabbroic dykes that intrude Neoproterozoic rocks have UPb zircon ages ca. 458–446 Ma, which we interpret reflects the timing of regional crustal extension. Geochemical and SrNd isotopic data indicate that Early Paleozoic gabbroic dykes were sourced from the enriched mantle, possibly associated with an upwelling plume that interacted with the lithosphere along the northern margin of the South China craton.

Supercontinent-paced magmatic destabilisation and recratonisation of the Yilgarn Craton

Knowledge of the evolution of ancient cratonic lithospheres underpins our understanding of Precambrian Earth. The Yilgarn Craton has exceptionally well-preserved Archean geology, with juvenile crust formation and major orogenesis concluding in the Neoarchean, and a stabilised upper-crustal architecture developing before 2.42 Ga. However, in an apparent dichotomy, geophysical models resolve lithospheric mantle composition outside the range of xenolith data from Archean regions, indicating the lithospheric mantle has since been extensively refertilised. Post-Archean igneous and sedimentary rocks record a prolonged lithospheric evolution that is not well resolved in datasets recording bulk crustal isotopic evolution. Reconciling these, we combine interpretation of geological and geophysical data to resolve two phases of lithosphere destabilisation driven by major magmatic events at ∼2.06 Ga and at ∼1.08 Ga. During destabilisation, sub-lithospheric and sub-crustal mantle fluxes caused extensive mantle refertilisation. For 200–400 Ma post-refertilisation, distributed sedimentary basins formed during recratonisation of the now denser lithosphere. The timing of these events suggests a relationship with the early stages of supercontinent assembly: Dominant downwelling beneath the assembling supercontinent sustains a sufficiently non-tensile tectonic setting to inhibit lithospheric thinning and breakup and enhances lateral flow of any upwelling mantle. This setting allows widespread intraplate refertilisation to occur while later the assembled supercontinent provides a stable setting allowing thermal re-equilibration and recratonisation to occur. In contrast, lithospheric refertilisation during supercontinent breakup will be more susceptible to density instabilities and recycling in later collisions. Consequently, we suggest that refertilisation of extant cratonic lithosphere may dominantly have occurred during the assembly of supercontinents.

Barform deposits of the Carolyn Shoemaker formation, Gale crater, Mars

ABSTRACT The early environmental history of Mars is encoded in the planet's record of sedimentary rocks. Since 2012, the Curiosity rover has been ascending Mount Sharp, Gale crater's central mound, making detailed observations of sedimentary strata exposed there. The primary depositional setting represented by the rocks examined thus far has been a perennial lake, represented by the mudstones and sandstone lenses of the Murray formation. Here, we report on the sedimentology of outcrops examined in the Carolyn Shoemaker formation, which sits stratigraphically above the Murray formation. We interpret strata exposed in the Glasgow and Mercou members of the Carolyn Shoemaker formation to represent river bars in ancient alluvial and shoreline settings based on sedimentary structures, stratal geometries measured from photogrammetric data, and erosional morphology. The transition from a lacustrine to a fluvial depositional setting records the aggradation and progradation of coastal rivers into what was previously the extent of the Gale lake system. This may have occurred due to the shrinking of the lake over time due to climate-driven changes in the basin water balance, or local three-dimensionality in shoreline evolution, such as the formation of a new sedimentary lobe following a channel switch.

The diagenetic continuum of steroid hydrocarbon transformation from early diagenesis into the oil window

Steroids and their derivatives are ubiquitous in the sedimentary rock record, where they are widely applied as biological or geochemical markers to determine organic facies as well as thermal maturation of bitumen and oils covering the diagenetic to catagenetic stages. The diagenetic continuum from early diagenesis into catagenesis of steroids has not been extensively studied despite the importance of these reactions in influencing the successive continuum of late diagenetic transformations.This study utilizes a stratigraphically constrained transect of 9 wells from the Upper Cretaceous Second White Specks and Belle Fourche formations of south-central Alberta, Canada ranging in thermal maturity from 0.2 %VRE to 0.8 %VRE. Biomarkers were examined from 69 samples from 8 wells and thermal maturity was determined from 87 samples from 9 wells using HAWK programmed pyrolysis analysis. Relative and absolute abundances of regular sterane, diasterene, diasterane, monoaromatic steroid, and triaromatic steroid biomarkers and changes in the proportions of steroid compound classes were determined and evaluated for thermal maturity. Additionally, isomerization ratios within each of the compound classes were examined with increasing thermal maturation.Rearranged diasterenes initially dominate the steroidal hydrocarbons, followed by diasteranes in the late diagenetic and catagenetic stages and at peak oil maturity (0.8 %VRE) diasteranes were most abundant. The relative abundance of a steroid is initially controlled by its rate of production or isomerization from a biological precursor, but at higher thermal maturities selective catagenetic destruction alters the relative abundance of steroid classes, with aromatic steroids and regular steranes being degraded or expelled earliest. Thermal maturity parameters based on the isomerization of regular steranes, and the aromatization of monoaromatic steroids are effective thermal maturity indicators in specific but overlapping ranges beginning at 0.2 %VRE. This work represents a novel examination of a natural thermal maturity transect and helps to consolidate our understanding and application of the diagenetic transformations of steroid compounds.

The influence of tectonic and climatic factors on detrital zircon U[sbnd]Pb age population of Paleozoic metasedimentary rocks in the Lhasa terrane

The origin of the Lhasa terrane in South Tibet remains enigmatic. In this study, we performed new UPb isotopic analyses on detrital zircons from Late Paleozoic metasedimentary rocks in the eastern end of the Lhasa terrane. The detrital zircon ages range from Archean to Paleozoic, with major peaks at ca. 2500 Ma, 1650–1500 Ma, ca. 1170 Ma, ca. 950 Ma, and ca. 580 Ma. Integrated with data from previous studies, the updated database shows that (1) the Early Paleozoic metasedimentary rocks in the Lhasa terrane contain abundant ca. 950 Ma detrital zircons, and (2) the Late Paleozoic metasedimentary rocks in the eastern end of the Lhasa terrane possess a higher proportion of ca. 950 Ma detrital zircons than that in the rest of the Lhasa terrane. While the older (ca. 1170 Ma) Grenvillian detrital zircons could be supplied by multiple sources, the younger (ca. 950 Ma) Grenvillian detrital zircons have a single source from the Indian continent. Thus the above observations suggest that the Lhasa terrane has a strong affinity with Indian Gondwana during the Paleozoic. Abundance change in the distinctive ca. 950 Ma detrital zircons with stratigraphic age and position in the Lhasa terrane is interpreted to have been resulted from tectonic activity and climatic variation. We here emphasize that tectonic and climatic factors played an important role in the detrital zircon record of sedimentary rocks.

Long-term Phanerozoic sea level change from solid Earth processes

The sedimentary rock record suggests that global sea levels may have fluctuated by hundreds of meters throughout Phanerozoic times. Long-term (10–80 Myr) sea level change can be inferred from paleogeographic reconstructions and stratigraphic methods can be used to estimate sea level change over 1–10 Myr in tectonically quiescent regions assumed to be stable. Plate tectonic reconstructions and mantle flow models make it possible to isolate, quantify and estimate the contribution of different solid Earth mechanisms to sea level change through time, including: the volume of water deeper than mid-oceanic ridges, mantle dynamic topography, marine sedimentation, oceanic large igneous province emplacement, deep-water cycle, volume above oceanic trenches and changes in continental area. Although these processes are intrinsically linked, their impact on sea level change is rarely studied in combination, and time-dependent models of long-term eustasy from tectonic and geodynamic processes are in their infancy. Here we couple plate tectonic reconstructions with time-dependent models of past mantle flow and develop a new holistic framework to model sea level change that accounts for the main solid Earth drivers of eustatic rise and fall. We present the first model of the effect of individual solid Earth mechanisms on eustatic change over the past 560 Myr, including time-dependent continental and oceanic dynamic topography, volume above oceanic trenches and changing continental slope. Our results are consistent with the Cretaceous highstand and Paleozoic and early Mesozoic lowstands deduced from stratigraphy, provided that the deep-water cycle contributed ∼240 m of sea level drop since 250 Myr. We find that changes in the volume of water below the depth of mid-ocean ridges are not balanced by changes in the volume of water above subduction zones or by global dynamic topography. Our results confirm that a young seafloor decreased the volume of water below the depth of ridges and contributed +280 m [+90/−0 m] to a sea level high at approximately 120 Ma, and show that changes in dynamic topography were the primary driver (−320 m [+80/−60 m]) of lowering sea level between ∼240–160 Ma when Pangea was assembled. Predicted sea level changes by up to −220/+470 m when Panthalassan mid-ocean ridge spreading rates are varied between 50 mm/yr and 200 mm/yr. We compute sea level estimates for two alternative global tectonic reconstructions and while the results differ (by ∼150 m) between 250-560 Ma, both suggest a sea level fall during Pangea assembly (between ∼380-320 Ma) that is consistent with published constraints.

POTENTIAL ICE CRYSTAL MARKS FROM PENNSYLVANIAN–PERMIAN EQUATORIAL RED-BEDS OF NORTHWEST COLORADO, U.S.A.

ABSTRACTThe Pennsylvanian–Permian Maroon Formation of northwest Colorado is an up to 4,600 m thick succession of mainly siliciclastic continental red-beds deposited in equatorial intermontane basins of the Ancestral Rocky Mountains. Sedimentary surfaces of fluvio-lacustrine to eolian siltstones and fine-grained sandstones from various stratigraphic levels within the Maroon Formation preserve cm-sized straight to gently curved sediment-filled acicular structures referred to five morphological groups: single, branched, stellate, rosette, and bunched. Depositional environment, shape, and size of the structures are most similar to ice crystal marks that result from freezing of water-saturated fine-grained substrate at the sediment-air interface. They differ from other syngenetically produced crystals and crystal pseudomorphs in sedimentary rocks mainly by crystal shape and environmental conditions. The potential ice crystal marks of the Maroon Formation are notable for the fidelity and morphological diversity of the crystal casts and could be a key for the understanding of similar but hitherto often only called enigmatic structures of the sedimentary rock record. The ice crystal mark occurrences in the Maroon Formation suggest that night frost affected lower elevation equatorial areas during the climax of the Late Paleozoic Ice Age and may stimulate research on evolutionary adaptations of early terrestrial biota to overcome significant air temperature fluctuations.

Nitrogen Isotope Discrepancy Between Primary Producers and Sediments in an Anoxic and Alkaline Lake

Nitrogen isotope compositions (δ15N) in sedimentary rocks are extensively used to investigate the biogeochemical nitrogen cycle through geological times. This use relies on the observation that, in modern continental platforms and anoxic basins, surface sediments faithfully record the δ15N of primary producers, assuming that it was similar in the past. Over Earth’s history, however, surface environments experienced profound changes, including the transition of ammonium-dominated to nitrate-dominated waters and the transition from exclusively microbial ecosystems to ecosystems including multicellularity, which make modern environments significantly different compared to earlier ones, potentially invalidating the fundamental assumption that surface sediments faithfully record the δ15N of primary producers. In order to improve our understanding of the nitrogen isotopic information contained in the early Earth’s sedimentary rock record, we investigate here the nitrogen isotope systematics in a microbial, nitrate free and ammonium-rich modern system, the Dziani Dzaha Lake. In this modern system, the δ15N of the reduced dissolved inorganic nitrogen (i.e., NH4+ and NH3) in the water column is close to ∼7‰ . δ15N of suspended particulate matter (SPM) show a similar average value in surface waters (i.e., where SPM is massively composed of active primary producers), but increases up to 14‰ in the deeper part of the water column during periods when it is enriched in dissolved reduced species (i.e., CH4, H2S/HS− and NH4+/NH3). Surface sediments δ15N, with values comprised between 10 and 14 ‰, seem to preferentially record these positive isotopic signatures, rather than those of active primary producers. We propose here that the observed isotopic pattern is mainly linked to the assimilation of ammonium strongly enriched in 15N by isotope exchange with ammonia under basic conditions. Although ammonium assimilation seems here to be responsible for a significant isotopic enrichment due to the basic conditions, in neutral anoxic environments inhabited by similar microbial ecosystems, this process may also significantly impact the δ15N of primary producers towards more negative values. This would have strong implications for our interpretation of the Precambrian sedimentary record as this finding challenges one the fundamental hypotheses underlying the use of sedimentary δ15N in paleo-oceanographic reconstructions, i.e. that surface sediments faithfully record the δ15N of active primary producers in the photic zone.

Recognition of Sedimentary Rock Occurrences in Satellite and Aerial Images of Other Worlds—Insights from Mars

Sedimentary rocks provide records of past surface and subsurface processes and environments. The first step in the study of the sedimentary rock record of another world is to learn to recognize their occurrences in images from instruments aboard orbiting, flyby, or aerial platforms. For two decades, Mars has been known to have sedimentary rocks; however, planet-wide identification is incomplete. Global coverage at 0.25–6 m/pixel, and observations from the Curiosity rover in Gale crater, expand the ability to recognize Martian sedimentary rocks. No longer limited to cases that are light-toned, lightly cratered, and stratified—or mimic original depositional setting (e.g., lithified deltas)—Martian sedimentary rocks include dark-toned examples, as well as rocks that are erosion-resistant enough to retain small craters as well as do lava flows. Breakdown of conglomerates, breccias, and even some mudstones, can produce a pebbly regolith that imparts a “smooth” appearance in satellite and aerial images. Context is important; sedimentary rocks remain challenging to distinguish from primary igneous rocks in some cases. Detection of ultramafic, mafic, or andesitic compositions do not dictate that a rock is igneous, and clast genesis should be considered separately from the depositional record. Mars likely has much more sedimentary rock than previously recognized.

Global investigation of martian sedimentary fan features: Using stratigraphic analysis to study depositional environment

Through the use of high-resolution orbital data, it is possible to study sedimentary fan features on the surface of Mars as archives of past hydrological and climate conditions. Previous studies have commonly interpreted the depositional environment of fan deposits using the geomorphology of the modern fan outcrop alone; however, billions of years of erosion has likely altered the morphology of fan outcrops significantly. Here, we examine the stratigraphy, or internal architecture, of 15 fan deposits across the surface of Mars using orbital outcrop models derived from high-resolution images overlain on digital elevation models. Specifically, we search for a slope break in exposed fan layers, characteristic of a delta deposit and in contrast with an alluvial/fluvial fan that is not expected to have such a slope break. A delta indicates the presence of a downstream standing body of water, providing constraints on the location and abundance of water reservoirs recorded in the sedimentary rock record on Mars. At least one fan outcrop in this study, located in Hargraves crater, shows previously undocumented evidence of a slope break and is thus classified as a delta. Other fan features show some, but not unequivocal evidence of a slope break, but the majority (11) show no evidence of a slope break in their exposed stratigraphy, indicating they cannot be robustly classified as a delta. Of the four fan outcrops with some evidence for deltaic deposition, three are interpreted to have been constructed subsequent to the valley network forming era, including the conclusive delta within Hargraves crater. We also find that all the fan features analyzed in this work have been significantly modified since lithification and have experienced a considerable amount of erosion since initial formation, an important consideration when interpreting the present-day morphology of these landforms on Mars.