Meta-analysis of Cryogenian through modern quartz microtextures reveals sediment transport histories

Quantitative scanning electron microscopy (SEM) quartz microtextural analysis can reveal the transport histories of modern and ancient sediments. However, because workers identify and count microtextures differently, it is difficult to directly compare quantitative microtextural data analyzed by different workers. As a result, the defining microtextures of certain transport modes and their probabilities of occurrence are not well constrained. We used principal component analysis (PCA) to directly compare modern and ancient aeolian, fluvial, and glacial samples from the literature with 9 new samples from active aeolian and glacial environments. Our results demonstrate that PCA can group microtextural samples by transport mode and differentiate between aeolian and fluvial/glacial transport modes across studies. The PCA ordination indicates that aeolian samples are distinct from fluvial and glacial samples, which are in turn difficult to disambiguate from each other. Ancient and modern sediments are also shown to have quantitatively similar microtextural relationships. Therefore, PCA may be a useful tool to constrain the ambiguous transport histories of some ancient sediment grains. As a case study, we analyzed two samples with ambiguous transport histories from the Cryogenian Bråvika Member (Svalbard). Integrating PCA with field observations, we find evidence that the Bråvika Member facies investigated here includes aeolian deposition and may be analogous to syn-glacial Marinoan aeolian units including the Bakoye Formation in Mali and the Whyalla Sandstone in South Australia

Biological influences on seafloor carbonate precipitation

The sedimentary record reveals first-order changes in the locus of carbonate precipitation through time, documented in the decreasing abundance of carbonate precipitation on the seafloor. This pattern is most clearly recorded by the occurrence of seafloor carbonate crystal fans (bladed aragonite pseudomorphs neomorphosed to calcite or dolomite), which have a distinct temporal distribution, ubiquitous in Archean carbonate platforms, but declining through Proterozoic time and extremely rare in Phanerozoic basins. To understand better the potential influences on this pattern, we built a mathematical framework detailing the effects of organic matter delivery and microbial respiratory metabolisms on the carbonate chemistry of shallow sediments. Two nonunique end-member solutions emerge in which seafloor precipitation is favorable: enhanced anaerobic respiration of organic matter, and low organic matter delivery to the sediment-water interface. This analysis suggests that not all crystal fans reflect a unique set of circumstances; rather there may have been several different geobiological and sedimentary mechanisms that led to their deposition. We then applied this logical framework to better understand the petrogenesis of two distinct crystal fan occurrences—the Paleoproterozoic Beechey Formation, Northwest Territories, Canada, and the middle Ediacaran Rainstorm Member of the Johnnie Formation, Basin and Range, United States—using a combination of high-resolution petrography, micro X-ray fluorescence and wavelength dispersive spectroscopy, C isotopes, and sedimentary context to provide information on geobiological processes occurring at the sediment-water interface. Interestingly, both of these Proterozoic examples are associated with iron-rich secondary mineral assemblages, have elevated trace metal signatures, and sit within maximum flooding intervals, highlighting key commonalities in synsedimentary geobiological processes that led to seafloor carbonate precipitation

The stratigraphic expression of a large negative carbon isotope excursion from the Ediacaran Johnnie Formation, Death Valley

The Rainstorm Member of the Ediacaran Johnnie Formation of the southern Basin and Range, US, records a large negative excursion in carbon isotope ratios of carbonate strata (δ^(13)C_VPDB > −6‰). The character of the excursion raises fundamental questions about whether this isotopic pattern is accurately capturing the time-series behavior of marine dissolved inorganic carbon (DIC) or is a product of diagenesis. To explore this issue, we examined the expression of this isotopic excursion within the Johnnie oolite, a ~2 m thick marker bed, which records the highest rate of change in δ^(13)C. Sedimentologically, the oolite unit is thought to be time-transgressive; if the isotopic excursion reflects time-series behavior of the carbon cycle, its expression in the oolite across the basin should systematically align the sections according to their slight diachroneity. Detailed carbon and oxygen isotopic stratigraphy of the oolite at seven different locations indicates the magnitude of the excursion at the base of the oolite is spatially variable such that after a palinspastic reconstruction the sections align along a systematic north to south gradient in the isotopic data. The oolite preserved in the Old Dad Mountains, the most southerly section measured, is an outlier to this trend (and the most difficult to ordinate accurately in the reconstruction), showing the largest isotopic range between the onset of carbonate deposition and top of the oolite. Several hypotheses are congruent with these data, but the sum of observations is best explained by a scenario wherein the oolite is time-transgressive and deposited in a north to south manner throughout the onset of the isotopic excursion. If correct, this implies that the stratigraphic δ^(13)C pattern reflects time series behavior of marine DIC

A 600-Million-Year Carbonate Clumped-Isotope Record from the Sultanate of Oman

Carbonate clumped-isotope thermometry is a promising technique that has the potential to help decode the significance of the variability of both physical and geochemical compositions of ancient carbonate rocks. This study utilizes a 600-million-year record of marine carbonate rocks from the subsurface and surface of the Sultanate of Oman to explore how burial and exhumation affected the carbonate clumped-isotope thermometer. Samples span 6 km of burial depth, and include calcite and dolomite mineralogies and a range of carbonate rock textures. We find evidence for two broad patterns in the physical and geochemical behavior of carbonate rocks during burial. The first group of carbonates yield water δ^(18)O_(VSMOW) compositions slightly enriched or equal to an expected “ice-free” seawater composition of –1.2‰ and display good to fair textural preservation suggesting that cementation and lithification occurred within tens of meters of the sediment–water interface. Temperatures from the second group sit on the present-day geotherm, yield highly enriched water δ^(18)O_(VSMOW) compositions, and display fair to poor textural preservation. We find no evidence for solid-state reordering in paired analyses of calcites and dolomites. Our results contribute to a growing body of work that indicates that the seawater δ^(18)O_(VSMOW) composition has not changed significantly over 600 Myr and was not –6‰ in the Ediacaran

A 600-Million-Year Carbonate Clumped-Isotope Record from the Sultanate of Oman

Carbonate clumped-isotope thermometry is a promising technique that has the potential to help decode the significance of the variability of both physical and geochemical compositions of ancient carbonate rocks. This study utilizes a 600-million-year record of marine carbonate rocks from the subsurface and surface of the Sultanate of Oman to explore how burial and exhumation affected the carbonate clumped-isotope thermometer. Samples span 6 km of burial depth, and include calcite and dolomite mineralogies and a range of carbonate rock textures. We find evidence for two broad patterns in the physical and geochemical behavior of carbonate rocks during burial. The first group of carbonates yield water δ^(18)O_(VSMOW) compositions slightly enriched or equal to an expected “ice-free” seawater composition of –1.2‰ and display good to fair textural preservation suggesting that cementation and lithification occurred within tens of meters of the sediment–water interface. Temperatures from the second group sit on the present-day geotherm, yield highly enriched water δ^(18)O_(VSMOW) compositions, and display fair to poor textural preservation. We find no evidence for solid-state reordering in paired analyses of calcites and dolomites. Our results contribute to a growing body of work that indicates that the seawater δ^(18)O_(VSMOW) composition has not changed significantly over 600 Myr and was not –6‰ in the Ediacaran

Dilemmas of Sustainability: On Relevance and Critical Reflection in Sustainability Research; A Guide

The concept of sustainability is increasingly losing its contours. This study reflects on the understanding of sustainability used in research and science. Its authors thus enable us to concisely determine not only concrete contributions to sustainability, but also its limits. Based on an analytical understanding of sustainability and the basic structure of practical dilemmas, the book identifies typical conflicts as causes of dilemmas. Meta-criteria of sustainability allow us to identify dilemmas and to clarify and deal with them practically at an early stage. Finally, the study applies these meta-criteria in concrete project contexts by means of guiding questions for reflection.Der Begriff der Nachhaltigkeit verliert zunehmend an Kontur. Vor diesem Hintergrund gibt dieser Leitfaden die Möglichkeit, das jeweils verwendete Verständnis von Nachhaltigkeit in Forschung und Wissenschaft zu reflektieren und damit den konkreten Beitrag zu Nachhaltigkeit, aber auch die verbundenen Grenzen konkret festzustellen. Ausgehend von einem analytischen Nachhaltigkeitsverständnis und der Klärung der Grundstruktur praktischer Dilemmata benennt der Leitfaden typische Konflikte als Ursache von Dilemmata. Meta-Kriterien der Nachhaltigkeit erlauben, Dilemmata-Früherkennung, -Aufklärung und -Bearbeitung zu unterstützen. Diese werden abschließend mittels Reflexionsleitfaden für die Anwendung in konkreten Projektkontexten operationalisiert

A high-resolution record of early Paleozoic climate

The spatial coverage and temporal resolution of the Early Paleozoic paleoclimate record are limited, primarily due to the paucity of well-preserved skeletal material commonly used for oxygen-isotope paleothermometry. Bulk-rock δ¹⁸O datasets can provide broader coverage and higher resolution, but are prone to burial alteration. We assess the diagenetic character of two thick Cambro–Ordovician carbonate platforms with minimal to moderate burial by pairing clumped and bulk isotope analyses of micritic carbonates. Despite resetting of the clumped-isotope thermometer at both sites, our samples indicate relatively little change to their bulk δ¹⁸O due to low fluid exchange. Consequently, both sequences preserve temporal trends in δ¹⁸O. Motivated by this result, we compile a global suite of bulk rock δ¹⁸O data, stacking overlapping regional records to minimize diagenetic influences on overall trends. We find good agreement of bulk rock δ¹⁸O with brachiopod and conodont δ¹⁸O trends through time. Given evidence that the δ¹⁸O value of seawater has not evolved substantially through the Phanerozoic, we interpret this record as primarily reflecting changes in tropical, nearshore seawater temperatures and only moderately modified by diagenesis. Focusing on the samples with the most enriched, and thus likely least-altered, δ¹⁸O values, we reconstruct Late Cambrian warming, Early Ordovician extreme warmth, and cooling around the Early–Middle Ordovician boundary. Our record is consistent with models linking the Great Ordovician Biodiversification Event to cooling of previously very warm tropical oceans. In addition, our high-temporal-resolution record suggests previously unresolved transient warming and climate instability potentially associated with Late Ordovician tectonic events

Reducing Uncertainties in Carbonate Clumped Isotope Analysis Through Consistent Carbonate-Based Standardization

About a decade after its introduction, the field of carbonate clumped isotope thermometry is rapidly expanding because of the large number of possible applications and its potential to solve long‐standing questions in Earth Sciences. Major factors limiting the application of this method are the very high analytical precision required for meaningful interpretations, the relatively complex sample preparation procedures, and the mass spectrometric corrections needed. In this paper we first briefly review the evolution of the analytical and standardization procedures and discuss the major remaining sources of uncertainty. We propose that the use of carbonate standards to project the results to the carbon dioxide equilibrium scale can improve interlaboratory data comparability and help to solve long‐standing discrepancies between laboratories and temperature calibrations. The use of carbonates reduces uncertainties related to gas preparation and cleaning procedures and ensures equal treatment of samples and standards. We present a set of carbonate standards of diverse composition, discuss how they can be used to correct for mass spectrometric biases, and demonstrate that their use significantly improves the comparability among four laboratories. We propose that the use of these standards or of a similar set of carbonate standards will improve the comparability of data across laboratories.publishedVersio

Nacre tablet thickness records formation temperature in modern and fossil shells

Nacre, the iridescent outer lining of pearls and inner lining of many mollusk shells, is composed of periodic, parallel, organic sheets alternating with aragonite (CaCO_3) tablet layers. Nacre tablet thickness (TT) generates both nacre's iridescence and its remarkable resistance to fracture. Despite extensive studies on how nacre forms, the mechanisms controlling TT remain unknown, even though they determine the most conspicuous of nacre's characteristics, visible even to the naked eye. Thermodynamics predicts that temperature (T) will affect both physical and chemical components of biomineralized skeletons. The chemical composition of biominerals is well-established to record environmental parameters, and has therefore been extensively used in paleoclimate studies. The physical structure, however, has been hypothesized but never directly demonstrated to depend on the environment. Here we observe that the physical TT in nacre from modern and fossil shallow-water shells of the bivalves Pinna and Atrina correlates with T as measured by the carbonate clumped isotope thermometer. Based on the observed TT vs. T correlation, we anticipate that TT will be used as a paleothermometer, useful to estimate paleotemperature in shallow-water paleoenvironments. Here we successfully test the proposed new nacre TT thermometer on two Jurassic Pinna shells. The increase of TT with T is consistent with greater aragonite growth rate at higher T, and with greater metabolic rate at higher T. Thus, it reveals a complex, T-dependent biophysical mechanism for nacre formation

Pyrite-walled tube structures in a Mesoproterozoic sediment-hosted metal sulfide deposit

Unusual decimeter-scale structures occur in the sediment-hosted Black Butte Copper Mine Project deposit within lower Mesoproterozoic strata of the Belt Supergroup, Montana. These low domal and stratiform lenses are made up of millimeter-scale, hollow or mineral-filled tubes bounded by pyrite walls. X-ray micro−computed tomography (micro-CT) shows that the tube structures are similar to the porous fabric of modern diffuse hydrothermal vents, and they do not resemble textures associated with the mineralization of known microbial communities. We determined the sulfur isotopic composition of sulfide minerals with in situ secondary ion mass spectrometry (SIMS) and of texture-specific sulfate phases with multicollector−inductively coupled plasma−mass spectrometry (MC-ICP-MS). The sedimentological setting, ore paragenesis, sulfur isotope systematics, and porosity structure of these porous precipitates constrain the site of their formation to above the sediment-water interface where metalliferous hydrothermal fluids vented into the overlying water column. These data constrain the geochemistry of the Mesoproterozoic sediment-water interface and the site of deposition for copper-cobalt-silver mineralization. Metals in the hydrothermal fluids titrated sulfide in seawater to create tortuous fluid-flow conduits. Pyrite that precipitated at the vent sites exhibits large sulfur isotope fractionation (>50‰), which indicates a close association between the vents and sulfate-reducing microbiota. In the subsurface, base metal sulfides precipitated from sulfide formed during the reduction of early diagenetic barite, also ultimately derived from seawater. This model suggests dynamic bottom-water redox conditions at the vent site driven by the interplay between sulfate-reducing organisms and metalliferous fluid effluence