The mineral dolomite (CaMg(CO3)2) forms in only small quantities in modern oceans, cannot be precipitated abiotically from unmodified seawater in laboratory experiments, yet comprises much of the carbonate rock record. The challenge of explaining the apparent temporal discrepancy in dolomite, the “dolomite problem,” has fascinated carbonate sedimentologists for centuries. Yet, this pursuit has lacked a quantitative tabulation of dolomite in the rock record. Here, we use the North American rock record, as archived in Macrostrat, to assemble a record of dolomite abundance through geologic time. The completeness and age resolution of our dataset allow us to compare dolomite abundance with environmental variables, including stromatolite abundance, evaporite occurrences, sea level, glaciation, and temperature. We use these comparisons to test the assumption that the bulk of the geologic dolomite record was formed via secondary diagenetic processes. We find no monotonic decrease in abundance with age-the expected result if late diagenesis affects the bulk of the record. Dolomite was just as abundant during the first half of the Paleozoic as it was during most of the Neoproterozoic, a challenge to canonical thinking. We show that a number of dolomite precipitation mechanisms known from modern environments and experimentally grown dolomite can explain many of the patterns we observe in the North American dolomite record. Perhaps dolomite is not such a problem after all.

Dolomite (CaMg(CO3)2) forms in minor quantities in modern environments yet comprises most of the Precambrian carbonate rock record. Precambrian dolomites are often fine-grained and fabric-retentive and are interpreted to have precipitated as primary cements or formed as early diagenetic replacements of CaCO3. Detailed physical and chemical characterization of these dolomites could inform their origin and relevance for paleoenvironmental reconstruction. Here, we use synchrotron radiation to produce a nanometer-resolution crystal orientation map of one exquisitely-preserved ooid deposited at the onset of the Shuram carbon isotope excursion (~574 Ma). The crystal orientation map reveals small (~10μm) acicular, radially-oriented crystals grouped into bundles of similarly-oriented crystals with varying optical properties. We interpret that this dolomite formed via primary, spherulitic precipitation during ooid growth in shallow marine waters. This result provides additional evidence that the physicochemical properties of late Precambrian oceans promoted dolomite precipitation and supports a primary origin for the Shuram excursion.

The Ediacaran-aged Shuram excursion was the last and largest of the Neoproterozoic negative carbon isotope anomalies. Recognized in stratigraphic successions around the globe, it precedes diverse evidence for macroscopic, multicellular life, and follows the Cryogenian global glaciations and Ediacaran Gaskiers glaciation. Hypotheses for the cause of the Shuram excursion can be broadly grouped into those that argue for post-depositional diagenetic alteration of the carbon isotope record and those that argue the extremely low δ13C values reflect a primary perturbation to the carbon cycle. Given the timing and magnitude of this event, distinguishing between these disparate hypotheses, or combining them, is critical for reconstructing the environmental conditions under which complex life evolved on Earth. We test specific predictions of each model using a range of stratigraphic observations and micro- and macro-analytical techniques. We find that the type sections in Oman where the Shuram excursion was first described are well preserved and contain a range of features difficult to reconcile with a post-depositional origin. However, many salient features are consistent with an extreme warming event coupled to a carbon cycle perturbation, analogous to the Paleocene-Eocene Thermal Maximum (PETM), and increased middle Ediacaran volcanism. We propose that cooling associated with the recovery was critical for origination rates of macroscopic soft-bodied organisms.

The habitability and ecology of Earth is fundamentally shaped by surface temperature, but the temperature history of our planet is not easily reconstructed, especially before the evolution of early biomineralizing animals. This work presents a billion-year-long, high-resolution, mineral-specific record of oxygen isotope measurements in shallow marine rocks. Clumped isotope paleothermometry results from four minerals resolves previous ambiguity in seawater oxygen isotope composition and confirms that long-term cooling punctuated by short-lived temperature extremes are dominant components of this record. We consider post-depositional effects by comparing Phanerozoic rock and fossil records, and identify temporal and spatial controls on alteration. Furthermore, this record is suggestive of key differences in dolomite (CaMg(CO3)2) formation processes between the Neoproterozoic (1000–538.8 Ma) and Phanerozoic (538.8–0 Ma), consistent with previous suggestions based on petrographic and sedimentological observations. This record, when viewed alongside the fossil record, suggests temperature change is tightly coupled to extinction and origination in the history of life and carbon cycle perturbations over the last billion years.

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.

Increased adoption and improved methodology in carbonate clumped isotope thermometry has greatly enhanced our ability to interrogate a suite of Earth-system processes. However, interlaboratory discrepancies in quantifying Increased use and improved methodology of carbonate clumped isotope thermometry has greatly enhanced our ability to interrogate a suite of Earth-system processes. However, inter-laboratory discrepancies in quantifying carbonate clumped isotope (Δ47) measurements persist, and their specific sources remain unclear. To address inter-laboratory differences, we first provide consensus values from the clumped isotope community for four carbonate standards relative to heated and equilibrated gases with 1,819 individual analyses from 10 laboratories. Then we analyzed the four carbonate standards along with three additional standards, spanning a broad range of δ47 and Δ47 compositions, for a total of 5,329 analyses on 25 individual mass spectrometers from 22 different laboratories. Treating three of the materials as known standards and the other four as unknowns, we find that the use of carbonate reference materials is a robust method for standardization that yields inter-laboratory discrepancies entirely consistent with in-laboratory analytical uncertainties. Carbonate reference materials, along with measurement and data processing practices described herein, provide the carbonate clumped isotope community with a robust approach to achieve inter-laboratory agreement as we continue to use and improve this powerful geochemical tool. We propose that carbonate clumped isotope data normalized to the carbonate reference materials described in this publication should be reported as Δ47 (I-CDES) for Intercarb-Carbon Dioxide Equilibrium Scale.

The oldest recognized proxies for low atmospheric oxygen are massive iron-rich deposits. Following the rise of oxygen ~2.4 billion years ago, massive iron formations largely disappear from the geologic record, only to reappear in a pulse ~1.88 Ga, which has been attributed to passive margin transgressions, changing ocean chemistry triggered by intense volcanism, or lowered atmospheric oxygen levels. The North American Gogebic Range has exposures of both volcanics and iron formation, providing an ideal field locality to interrogate the relationship between the lithologies and investigate triggers for this pulse of iron formation. To determine the environmental context and key factors driving post-GOE iron formation deposition, we made detailed observations of the stratigraphy and facies relationships and present updated mapping relationshipsof the Gogebic Range Ironwood Iron Formation and the Emperor Volcanics. This work expands existing mine datasets and logs to constrain variations in stratigraphy. Our results are the first to quantitatively constrain thickness variations along the entire Gogebic range and tie them to syn-sedimentary faulting along listric normal faults and half grabens. Furthermore, our datasets suggest that initiation of major local volcanism does not coincide with iron formation deposition, thus, local intense volcanism cannot be invoked as a causal trigger. Finally, the possibility of iron formation deposition in a shallow water environment suggests that the post-GOE iron formation pulse may not reflect global marine transgressions, but instead a chemocline shallowing due to decreased atmospheric oxygen.

Pre- and syn-glacial low-latitude carbonate sediments of the Elbobreen Formation, NE Svalbard, preserve evidence for dramatic climate changes associated with Cryogenian glaciations (720–635 Ma). We combine carbonate stable (δ13C, δ18O) and clumped isotope (Δ47) geochemistry with petrographic observations to assess the source of carbonate within glacial facies of the Petrovbreen Member and their environmental significance. Calcite Δ47 temperatures reflect solid-state reordering under burial temperatures, whereas dolomites record lower temperatures that vary with depositional facies. Pre-glacial dolomites have Δ47 temperatures from 48–73°C, with a reconstructed fluid δ18O value of +0.6‰ (VSMOW) in the coldest sample. Glacial dolomites comprise: (1) detrital carbonate clasts similar to pre-glacial strata in stable isotope composition, Δ47 temperature, and petrographic textures; and (2) autochthonous dolomicrite and re-worked dolomicrite clasts with heavier δ18O values and colder Δ47 temperatures of 19–44 °C. Measured dolomite temperatures likely include a component of diagenetic alteration that elevated the sample temperature above that imparted at deposition. The statistically significant difference in temperatures between precipitated matrix and re-worked detrital clasts in diamictite indicates that matrix samples preserve some component of carbonate that records early temperature differences either reflecting the primary sediments or early dolomitization and shallow lithification. The higher source fluid δ18O values in glacial carbonates is consistent with an active hydrological cycle, either through local evaporation or growth of continental ice sheets sourced from evaporation of seawater. Continued hydrological cycling and 20–30 °C offsets in temperature between glacial and non-glacial conditions constrain carbonate depositional environments in this first Cryogenian glaciation.

The potential for carbonate clumped isotope thermometry to independently constrain both the formation temperature (T∆47 ) of carbonate minerals and fluid oxygen isotope composition allows insight into long-standing questions in the Earth sciences, but remaining discrepancies between calibration schemes hamper interpretation of T∆47 measurements. To address discrepancies between calibrations, we designed and analyzed a sample suite (41 total samples) with broad applicability across the geosciences, with an exceptionally wide range of formation temperatures, precipitation methods, and mineralogies. We see no statistically significant offset between sample types, although comparison of calcite and dolomite remains inconclusive. When data are reduced identically, the regression defined by this study is nearly identical to that defined by four previous calibration studies that used carbonate-based standardization; we combine these data to present a composite carbonate-standardized regression equation. Agreement across a wide range of temperature and sample types demonstrates a unified, broadly applicable clumped isotope thermometer calibration.

Granule- to cobble- sized clasts in the Glen Torridon region of Gale crater on Mars were studied using data captured by NASA's Mars Science Laboratory Curiosity rover between sols 2302 and 2593. The morphology and composition of clasts have the potential to reveal the nature and extent of erosional processes acting in a region. In this study, measurements of shape, size, texture and element abundance of unconsolidated clasts within lower Glen Torridon were compiled. Eight primary clast types were identified, all of which are sedimentary and can be compositionally linked to local bedrock, suggesting relatively short transport distances. Several clast types exhibit signs of aeolian abrasion, such as facets, pits, flutes and grooves. These results indicate that clasts are primarily the product of bedrock degradation followed by extensive aeolian wear.

Carbonate rocks on continental crust are one of Earth’s largest reservoirs of CO2 and yet the controls on their volume through time are poorly understood. Here we quantify temporal changes in preserved continental carbonate rocks over the last billion years in both global and North America-specific datasets within paleogeographic context. We find the preserved area of continental carbonate rocks increases by ~175% across the Neoproterozoic-Phanerozoic boundary ca. 539 million years ago, coincident with the rise of macroscopic, multicellular life and the evolutionary innovation of carbonate biomineralization in shallow water reefs. We demonstrate that crustal loading from carbonate sediments on one tropical paleo-continent (North America) contributes to an increase in continent-scale accommodation in the early Phanerozoic, expanding shallow marine environments. We predict this feedback between enhanced carbonate accumulation and subsidence was an important component of the termination of the Great Unconformity. These results are combined into a new conceptual model that links the changes in preserved carbonate rock volumes to the evolutionary innovation of carbonate biomineralization in a range of complex organisms. Our model implies evolutionary controls on the carbonate rock reservoir enhanced CO2 sequestration at the beginning of the Phanerozoic, with consequences for Earth’s carbon cycle, climate and habitability.

Detrital zircon records of provenance are used to reconstruct paleogeography, sediment sources, and tectonic configuration. Recognition of the biases in detrital zircon records that result from hydraulic sorting of sediment and the initial characteristics of zircons in source regions (e.g., size and abundance) has added new complexity and caution in the interpretation of these records. In this study, we examine the role of transport process and sediment sorting in these records. We begin our analysis by investigating the influence of grain size and transport process in biasing detrital zircon provenance records in an idealized sedimentary system. Our modeling results show that settling and selective entrainment can leave distinct, process-dependent fingerprints in detrital zircon spectra if initial size variation between source zircon populations exists. We then consider a case study: a detrital zircon record from Ediacaran to Terreneuvian Death Valley. We focus on the Rainstorm Member, which is geochemically, mineralogically, and sedimentologically unusual. In addition to Earth’s largest negative carbon isotope excursion (the Shuram excursion), the Rainstorm Member also contains anachronistic carbonate structures and a detrital mineral suite enriched in heavy minerals. We evaluate the detrital zircon provenance record of the Rainstorm Member, and find that, despite its unusual character, the provenance of the unit is similar to other units in the succession, with substantial input from Yavapai-Matzatzal provinces. Size and density measurements of heavy and light density components of the deposit suggest that its enriched heavy mineral suite is best explained through concentration by selective entrainment and winnowing. We find that our detrital zircon dataset is susceptible to hydrodynamic fractionation, so that grain size exerts influence on its provenance record, in particular for large Grenville-aged (1.0-1.2 Ga) grains.