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This study revises and improves the chronostratigraphic framework for late Turonian through early Campanian time based on work in the Western Interior U.S. and introduces new methods to better quantify uncertainties associated with the development of such time scales. Building on the unique attributes of the Western Interior Basin, which contains abundant volcanic ash beds and rhythmic strata interpreted to record orbital cycles, we integrate new radioisotopic data of improved accuracy with a recently published astrochronologic framework for the Niobrara Formation. New 40Ar/39Ar laser fusion ages corresponding to eight different ammonite biozones are determined by analysis of legacy samples, as well as newly collected material. These results are complemented by new U-Pb (zircon) chemical abrasion–isotope dilution–thermal ionization mass spectrometry ages from four biozones in the study interval. When combined with published radioisotopic data from the Cenomanian-Turonian boundary, paired 206Pb/238U and 40Ar/39Ar ages spanning Cenomanian to Campanian time support an astronomically calibrated Fish Canyon sanidine standard age of 28.201 Ma. Stage boundary ages are estimated via integration of new radioisotopic data with the floating astrochronology for the Niobrara Formation. The ages are determined by anchoring the long eccentricity bandpass from spectral analysis of the Niobrara Formation to radioisotopic ages with the lowest uncertainty proximal to the boundary, and adding or subtracting time by parsing the 405 k.y. cycles. The new stage boundary age determinations are: 89.75 ± 0.38 Ma for the Turonian-Coniacian, 86.49 ± 0.44 Ma for the Coniacian-Santonian, and 84.19 ± 0.38 Ma for the Santonian-Campanian boundary. The 2σ uncertainties for these estimates include systematic contributions from the radioisotopic measurements, astrochronologic methods, and geologic uncertainties (related to stratigraphic correlation and the presence of hiatuses). The latter geologic uncertainties have not been directly addressed in prior time scale studies and their determination was made possible by critical biostratigraphic observations. Each methodological approach employed in this study—new radioisotopic analysis, stratigraphic correlation, astrochronology, and ammonite and inoceramid biostratigraphy—was critical for achieving the final result.

High-resolution osmium (Os) isotope stratigraphy across the Cenomanian–Turonian Boundary Interval from 6 sections for four transcontinental settings has produced a record of seawater chemistry that demonstrates regional variability as a function of terrestrial and hydrothermal inputs, revealing the impact of palaeoenvironmental processes. In every section the 187Os/188Os profiles show a comparable trend; radiogenic values in the lead up to Oceanic Anoxic Event 2 (OAE 2); an abrupt unradiogenic trend at the onset of OAE 2; an unradiogenic interval during the first part of OAE 2; and a return to radiogenic values towards the end of the event, above the Cenomanian–Turonian boundary. The unradiogenic trend in 187Os/188Os is synchronous in all sections. Previous work suggests that activity of the Caribbean LIP (Large Igneous Province) was the source of unradiogenic Os across the OAE 2 and possibly an instigator of anoxia in the oceans. Here we assess this hypothesis and consider the influence of activity from other LIPs; such as the High Arctic LIP.A brief shift to high radiogenic 187Os/188Os values occurred in the Western Interior Seaway before the onset of OAE 2. We evaluate this trend and suggest that a combination of factors collectively played critical roles in the initiation of OAE 2; differential input of nutrients from continental and volcanogenic sources, coupled with efficient palaeocirculation of the global ocean and epeiric seas, enhanced productivity due to higher nutrient availability, which permitted penecontemporaneous transport of continental and LIP-derived nutrients to trans-equatorial basins.

A central issue in education is how to support the spatial thinking involved in learning science, technol- ogy, engineering, and mathematics (STEM). We investi- gated whether and how the cognitive process of analogical comparison supports learning of a basic spatial concept in geoscience, fault. Because of the high variability in the appearance of faults, it may be difficult for students to learn the category-relevant spatial structure. There is abundant evidence that comparing analogous examples can help students gain insight into important category-defining fea- tures (Gentner in Cogn Sci 34(5):752-775, 2010). Further, comparing high-similarity pairs can be especially effective at revealing key differences (Sagi et al. 2012). Across three experiments, we tested whether comparison of visually similar contrasting examples would help students learn the fault concept. Our main findings were that participants performed better at identifying faults when they (1) com- pared contrasting (fault/no fault) cases versus viewing each case separately (Experiment 1), (2) compared similar as opposed to dissimilar contrasting cases early in learning (Experiment 2), and (3) viewed a contrasting pair of sche- matic block diagrams as opposed to a single block diagram of a fault as part of an instructional text (Experiment 3). These results suggest that comparison of visually similar contrasting cases helped distinguish category-relevant from category-irrelevant features for participants. When such comparisons occurred early in learning, participants were more likely to form an accurate conceptual representation. Thus, analogical comparison of images may provide one powerful way to enhance spatial learning in geoscience and other STEM disciplines.

We develop an intercalibrated astrochronologic and radioisotopic time scale for the Cenomanian-Turonian boundary (CTB) interval near the Global Stratotype Section and Point in Colorado, USA, where orbitally influenced rhythmic strata host bentonites that contain sanidine and zircon suitable for 40Ar/39Ar and U-Pb dating. Paired 40Ar/39Ar and U-Pb ages are determined from four bentonites that span the Vascoceras diartianum to Pseudaspidoceras flexuosum ammonite biozones, utilizing both newly collected material and legacy sanidine samples of J. Obradovich. Comparison of the 40Ar/39Ar and U-Pb results underscores the strengths and limitations of each system, and supports an astronomically calibrated Fish Canyon sanidine standard age of 28.201 Ma. The radioisotopic data and published astrochronology are employed to develop a new CTB time scale, using two statistical approaches: (1) a simple integration that yields a CTB age of 93.89 ± 0.14 Ma (2σ; total radioisotopic uncertainty), and (2) a Bayesian intercalibration that explicitly accounts for orbital time scale uncertainty, and yields a CTB age of 93.90 ± 0.15 Ma (95% credible interval; total radioisotopic and orbital time scale uncertainty). Both approaches firmly anchor the floating orbital time scale, and the Bayesian technique yields astronomically recalibrated radioisotopic ages for individual bentonites, with analytical uncertainties at the permil level of resolution, and total uncertainties below 2‰. Using our new results, the duration between the Cenomanian-Turonian and the Cretaceous-Paleogene boundaries is 27.94 ± 0.16 Ma, with an uncertainty of less than one-half of a long eccentricity cycle.

Delineation of stratigraphic sequences and their component systems tracts in mudrock-dominated facies is generally difficult due to the relatively homogenous, fine-grained nature of the strata. In this study, we apply a multi-proxy analytical approach to a thick Devonian mudrock-dominated succession through detailed analysis of sedimentologic, paleobiologic, and geochemical data through 600 m of mudrock-rich facies. Varied combinations of proxies prove to be most useful in delineating sequence development in anoxic-, dysoxic-, and oxic-dominated mudrock settings, and in mixed mudrock–carbonate and mixed mudrock–sandstone successions. These interpretations are tested against an established sequence stratigraphic framework for 11 Middle to Upper Devonian (mid-Eifelian to lower Famennian) sequences in the Appalachian Basin. The sequences presented here further detail and refine global Devonian T–R cycles Id to IIe of the well known Johnson, Sandberg and Klapper sea-level curve. The usefulness of proxies in delineating depositional sequences and systems tracts varies dependent on depositional, paleoceanographic, paleoecologic, and early diagenetic conditions. Those proxies that show a range of variations in specific settings, such as grain size, degree of bioturbation, and concentrations of TOC and elements/elemental ratios (e.g., CaCO3, Al, Ti, Mg, Sc, Si, Mo, Ni, V; Si/Al and Ti/Al) may help delineate depositional dynamics related to redox conditions, condensation, dilution, and clastic, biologic, and/or authigenic sediment sources. In fine-grained, anoxic-dominated facies, interpreted to represent basinal settings, sequences and systems tracts are best delineated by anoxic-related proxies TOC and Mo. In intermediate, dysoxic-dominated settings, TOC, Mo, bioturbation, and Al remain effective indicators of sequence development. In relatively oxygenated, mudrock-rich and carbonate poor sequences, bioturbation may function as the most effective proxy for recognizing systems tracts. For mixed fine-grained siliciclastic–carbonate successions, concentration and type of CaCO3 (e.g., benthic macroskeletal, pelagic styliolinid/dacryoconarid, and micritic/calcisilt) are useful in identifying position within cycles. In more proximal, carbonate-poor successions, fine- and coarse-grained fractions become increasingly differentiated; these can be distinguished by relatively high Si/Al ratios (Si/Al ≥ ca. 5). Elemental ratios indicative of coarser clastic input (e.g., Si/Al, Zr/Al and Ti/Al) are applicable to identifying position with a sequence, but they may also be affected by input from eolian, volcanogenic, or biogenic sources. In addition, fluxes of siliciclastic, carbonate, and TOC sediment types may dilute the concentration of the others. Multiple lines of evidence should be examined in interpreting relative depth and position within a sequence.

Episodes of widespread distribution of organic-rich marine facies within relatively narrow stratigraphic intervals have been interpreted as Oceanic Anoxic Events (OAEs). There are a number of such intervals in the Upper Cretaceous, some of which also exhibit significant positive excursions in carbon isotope records. As a result, a general model of the relationship between enhanced organic carbon burial and positive C-isotope shifts has emerged. Carbon isotope stratigraphy has, therefore, developed the dual utility of a chronostratigraphic tool and a means to help identify and define intervals of exceptional organic matter burial and preservation. However, variations in regional carbon isotope records of globally documented events suggest that the relationship between organic carbon burial flux and magnitude of carbon isotope excursion may not be entirely straightforward. For example, strata of Coniacian–Santonian age have long been recognized for enhanced organic content in many areas, but the designation of Oceanic Anoxic Event III for this interval has been rather equivocal in the literature, mostly due to the weak positive δ 13 C excursion (≤ 0.5‰). Yet, in some areas organic carbon accumulation rates for Coniacian–Santonian strata are roughly double that of the older OAE II, which is marked by a 2–4‰ shift in δ 13 C. Interestingly, the burial flux of carbonate, which leads to depletion rather than enrichment of the C-isotope signal, is also significantly elevated during Coniacian–Santonian time. In this study, the role of variations in organic carbon and carbonate accumulation rates on carbon isotopic expression are explored through application of data on the composition, accumulation rate, and areal distribution of end member facies types to the development of a global carbon isotopic mass balance model. The results are used to explore the hypothesis that net organic carbon burial during OAE III was not accurately represented in the carbon isotopic record. These results have important implications for interpretation of C-isotope records related to past perturbations of the global carbon cycle.

Ninety-four million years ago, during Ocean Anoxic Event 2, there was a marked increase in the burial of organic carbon in marine sediments. Measurements of stomata in fossil leaves show that the two main pulses of carbon burial were associated with a decline in atmospheric CO2 levels of up to 26%. The Cretaceous Ocean Anoxic Event 2 (about 94 million years ago) is thought to be linked to extensive volcanism, which triggered a biogeochemical chain of events that eventually led to widespread marine anoxia and a remarkable increase in carbon burial in marine sediments1,2,3. It has been suggested that the event was accompanied by a substantial decrease in atmospheric CO2 concentrations4, but the quantification of the drawdown remains controversial5,6. Here we reconstruct atmospheric CO2 concentrations throughout the ocean anoxic event from counts of the stomata in fossil leaves, and use terrestrial carbon isotopes to link the reconstruction to marine records of the event7,8. We find that before the onset of ocean anoxia, atmospheric CO2 concentrations increased by 20% over background levels of 370+100/−70 ppm. This was part of a long-term rise in atmospheric CO2 levels, presumably caused by volcanism, which reached a peak of 500+400/−180 ppm. However, two pulses of extensive carbon burial during the ocean anoxic event, as indicated by positive carbon isotope excursions, are associated with decreased atmospheric CO2 concentrations. We conclude that the sequestration of marine organic carbon led to a decrease in atmospheric CO2 concentration of up to 26% during Ocean Anoxic Event 2.

We have measured iron (Fe) isotope compositions of bulk samples and chemically extracted pyrite in two black shale units: the Oatka Creek Formation (OCF) and the younger Geneseo Formation (GF) deposited in the Appalachian basin during the mid-to-late Devonian. The depositional redox conditions for these units are well established through multiple paleoproxies, including degree of pyritization (DOP) and ratios of total Fe to Al (Fe T /Al), suggesting that both deposits reflect oxygen-deficient environments, but that euxinia (anoxia with hydrogen sulfide in the bottom waters) was more frequent and persistent during deposition of the OCF. Iron isotopes show systematic variations that are consistent with the inferred water column redox conditions. Samples from the OCF yield low and variable bulk Fe isotope compositions (− 0.44‰ to 0.03‰ in δ 56 Fe relative to average igneous rocks) that are inversely correlated with Fe T /Al, whereas bulk δ 56 Fe values ( δ 56 Fe T ) of the GF fall in a narrower range (− 0.09‰ to 0.12‰). δ 56 Fe values of pyrite ( δ 56 Fe Py ) display good correlation with δ 56 Fe T in the OCF, but no such correlation is observed in the GF. The Fe isotope data and other paleo-redox indicators, when viewed collectively, point to a benthic Fe source on the shelf and shelf-to-basin transfer that operated during deposition of the OCF, similar to what has been observed in the modern Black Sea. For the first time this study confirms the strength of Fe isotopes in delineating this Fe enrichment mechanism in the ancient geological record and emphasizes the utility of the Fe isotope proxy for fingerprinting and quantifying ancient biogeochemical cycling of Fe.

Several periods of global ocean anoxia punctuated the Cretaceous period. Marine-sediment chemistry indicates that extensive volcanism at the beginning of Oceanic Anoxic Event 2 released sulphur to the oceans, triggering a biogeochemical cascade that led to enhanced surface productivity and depletion of oxygen in the underlying waters. During the Cretaceous period (∼145–65 million years ago), there were several periods of global ocean anoxia, each lasting less than one million years. These events, known as ocean anoxic events, were marked by significant increases in organic carbon burial1, and are generally attributed to increased primary productivity in surface waters2. The details underpinning the initiation, maintenance and termination of these events, however, remain equivocal. Here we present sulphur isotope data spanning the Ocean Anoxic Event 2 (about 94.5 million years ago) from sedimentary rocks in Colorado that were formed in the Western Interior Seaway; this seaway ran north–south, splitting North America during the Cretaceous. Sulphate levels increased rapidly from relatively low background levels at the onset of the event, most likely from the release of sulphur by massive volcanism, and fell during the anoxic event. We infer that the input of sulphate facilitated increased carbon remineralization, which enhanced nutrient recycling and increased global primary productivity, eventually resulting in widespread ocean anoxia. Our scenario indicates that Ocean Anoxic Event 2 may have persisted until sulphate levels were stabilized by the formation and burial of the sulphur mineral pyrite, which returned primary productivity to background levels. We suggest that fluctuations in sulphate levels may have regulated the marine carbon cycle during past periods of low oceanic sulphate concentration.

Geoscience instructors and textbooks rely on analogy for teaching students a wide range of content, from the most basic concepts to highly complicated systems. The goal of this paper is to connect ...

Molybdenum (Mo) isotopes in ancient sediments are promising recorders of global ocean paleoredox conditions. Organic-rich black shales can be used to reconstruct ancient ocean Mo isotope compositions if these sediments record the isotopic composition of contemporaneous seawater. Comparison of δ 98/95 Mo in two Devonian shale sequences of similar age, the New York Oatka Creek and Geneseo Formations, reveals that this assumption cannot be applied to all organic-rich shales. Although both sequences contain laminated intervals, elevated organic carbon, and enrichments of redox-sensitive metals, the mean δ 98/95 Mo differs systematically between the formations by ~0.59‰. Independent paleoredox indicators reveal that portions of the Oatka Creek Formation were deposited under pervasively euxinic (anoxic and sulfidic) conditions, whereas conditions during deposition of the Geneseo Formation were intermittently euxinic to suboxic (oxygen deficient but not sulfidic in the water column). We infer that reconstruction of ancient ocean δ 98/95 Mo from organic-rich shales requires independent verification of persistent local euxinia. With these considerations in mind, our data point to δ 98/95 Mo in the Devonian oceans ~0.6‰ lighter than in today9s oceans, consistent with expanded anoxia.

Milankovitch-climate theory provides a fundamental framework for the study of ancient climates. Although the identification and quantification of orbital rhythms are commonplace in paleoclimate research, criticisms have been advanced that dispute the importance of an astronomical climate driver. If these criticisms are valid, major revisions in our understanding of the climate system and past climates are required. Resolution of this issue is hindered by numerous factors that challenge accurate quantification of orbital cyclicity in paleoclimate archives. In this study, we delineate sources of noise that distort the primary orbital signal in proxy climate records, and utilize this template in tandem with advanced spectral methods to quantify Milankovitch-forced/paced climate variability in a temperature proxy record from the Vostok ice core (Vimeux and others, 2002). Our analysis indicates that Vostok temperature variance is almost equally apportioned between three components: the precession and obliquity periods (28%), a periodic "100,000" year cycle (41%), and the background continuum (31%). A range of analyses accounting for various frequency bands of interest, and potential bias introduced by the "saw-tooth" shape of the glacial/interglacial cycle, establish that precession and obliquity periods account for between 25 percent to 41 percent of the variance in the 1/10 kyr - 1/100 kyr band, and between 39 percent to 66 percent of the variance in the 1/10 kyr - 1/64 kyr band. These results are approximately two to four times greater than those published by Wunsch (2004) for the same Vostok time series. In all cases, most of the remaining variance is accounted for by the "100,000" year cycle, which is distinct from a background continuum that resembles autoregressive "red noise." Our analysis high- lights the importance of a comprehensive assessment of the climate signals in geologic records, and underscores the significance of orbital forcing/pacing as a primary agent of Pleistocene climate change.

The Turonian–Campanian Niobrara Formation of the Western Interior basin, U.S.A., is characterized by decimeter-scale rhythmic alternations of chalk and marl beds and decameter-scale oscillations of chalky and marly facies. This study applies recent advances in quantitative cyclostratigraphic analysis to high-resolution geophysical records in order to test for an orbital signal. We studied records of the entire formation (~ 85 m) from two wells in Colorado (40° 17′ N, 104° 38′ W; 40° 14′ N, 104° 41′ W). The study utilized high-resolution time series of borehole resistivity measured from Formation MicroImager tools (FMI). Advanced spectral techniques indicate the presence of eccentricity, obliquity, and precession periodicities throughout the study interval. Deviations of bundling ratios in the lower Fort Hays Member from those predicted by simple eccentricity modulation of precession, previously reported in the literature, are likely due to the influence of obliquity. Deconvolution and frequency tracking of orbital components were used to reconstruct sedimentation rates for the Niobrara Formation and thus develop a high-resolution orbital timescale, which permits calculations of accumulation rates for sedimentary components and the duration of inclusive stages, substages, and, in some cases, biozones. The Niobrara Formation in the studied cores appears to be hiatus-free and therefore provides a continuous record of rhythmic deposition for astronomical timescale development. The relationship between carbonate content and sedimentation rate throughout the Niobrara Formation indicates that the decameter-scale oscillations of chalk and marl are dominantly driven by variations in siliciclastic flux (dilution). The duration, derived from the orbital timescale, between the Turonian–Coniacian and Coniacian–Santonian boundaries is 3.40 ± 0.13 myr, which is generally consistent with radiometric estimates for the Coniacian. The orbitally derived duration of the Santonian Stage is 2.39 ± 0.15 myr, which is very similar to estimates based on curve fitting between radiometric dates. Development of high-resolution orbital chronometers provides excellent time-resolution in intervals that lack radiometric-time control or where control points are widely spaced.

The Cenomanian–Turonian interval of the Sevier foredeep, western U.S.A., is examined in order to (1) establish a high-resolution stratigraphic framework for marginal-marine strata of this interval and (2) test for the existence of high-frequency (tens of kyr-scale) cycles of continental runoff or sea-level change predicted by the hemipelagic record and climate models. High rates of sediment accumulation in marginal-marine environments of southwestern Utah (up to 210 m/Myr, compacted) and a northward translation of the major Sevier thrusting made possible the preservation of a highly detailed record of shoreline movements. The coeval Bridge Creek Limestone, linked with the study interval using biostratigraphic and bentonite-stratigraphic data of previous authors, provides an unprecedented, high-resolution orbital time scale. Three orders of transgressive–regressive cycles defined as genetic sequences are identified in the upper Cenomanian (S. gracile and N. juddii Zones) through lower Turonian (W. devonense through M. nodosoides Zones). The longest sequence (S. gracile Zone through V. birchbyi Zone) spans approximately 800 kyr and is penecontemporaneous with the δ13Corg positive excursion that defines Oceanic Anoxic Event II (OAE II). Medium-term and short-term sequences show durations of c. 65–160 kyr and c. 20–40 kyr, respectively. Features suggesting regression due to relative sea-level fall are described from some of the 20–40 kyr cycles in the lowermost S. gracile Zone (possibly including the uppermost M. mosbyense Zone). The data provide the first physical evidence globally of Cenomanian–Turonian changes in shoreline position and relative sea level, whose recurrence interval was as short as a few tens of kyr. These processes provide a viable depositional link between the rhythmic deposition of the Bridge Creek Limestone and the primary orbital forcing of insolation and climate. Although the possible tectonic influence is difficult to unravel, the study area represents an important reference point for climate and oceanographic modeling of the Cenomanian–Turonian greenhouse and OAE II.

Quantification of Milankovitch orbital cyclicity within ancient strata has become a principal tool for refinement of the geologic time scale. However, accurate characterization of the orbital signal in deep time paleoclimate records is commonly challenged by inadequate radiometric time constraints for calibration of the spatial rhythms to temporal periods. This problem can potentially introduce large errors into derivative orbital timescales. In this study we develop a new method for the identification and calibration of orbital cyclicity in cyclostratigraphic records. The method (average spectral misfit, or ASM) yields an objective estimate of the optimal sedimentation rate for a stratigraphic interval that preserves a record of orbital forcing. The technique also provides a formal statistical test for rejecting the null hypothesis (no orbital signal). Application of the method to assess orbital cyclicity in the upper Bridge Creek Limestone Member (Turonian) of the Western Interior Basin highlights the utility of this new cyclostratigraphic tool, and provides a means to independently evaluate conflicting interpretations of the lithologic cycles. Importantly, ASM offers a new consistent standard by which orbital timescales may be compared. Hence, the quality of an orbital timescale can be formally qualified by reporting its average spectral misfit and null hypothesis significance level. This technique will permit improvement of Mesozoic/Cenozoic orbital timescales and extension of orbital time scale development into the Paleozoic, as the method is not dependent upon well-constrained radiometric age data.

Temporal gaps in the stratigraphic record have important implications for the interpretation of stratigraphically rapid geochemical and paleobiologic changes, for the development of high-resolution time scales (e.g., orbital time scales), and for deciphering the response of depositional systems to climatic change and basin evolution (e.g., accommodation space vs. sediment supply). In ancient pelagic and hemipelagic strata, the identification of relatively short hiatus (10 3 –10 6 years) is commonly challenged by the resolution limits of geochronologic techniques. In this study, a quantitative method is developed to identify and quantify hiatuses in strata where Milankovitch orbital cycles are documented. The new method is based on the interpretation of evolutive harmonic analysis (EHA) results and permits estimation of minimum durations for the temporal gaps. Application of this technique to an optical densitometry data series derived from the Cenomanian/Turonian (C/T) Bridge Creek Limestone Member permits quantification of hiatus within the unit and addresses important questions about abrupt changes in biogeochemical proxy records across the boundary interval. Integration of the new method with stratigraphic data and geochemical burial flux estimates permits a detailed reconstruction of the controls on sediment delivery and hiatus generation, including a refined analysis of the orbital-scale linkages between climate and sedimentation. D 2004 Elsevier B.V. All rights reserved.

A new consensus on the processes responsible for organic carbon burial in ancient epeiric seas has emerged. More firmly grounded in the uniformitarian framework of modern oceanography and biogeochemistry, this consensus recognizes the interdependent roles of sedimentation, primary production, and microbial metabolism in favor of earlier end-member models (e.g., “production vs. preservation”). In this study, one of the classic stratigraphic sequences upon which the “preservation” end-member was based is re-interpreted in light of this new consensus. The study employs an extensive new sedimentological–biogeochemical database from cores drilled in western New York. The database spans over 500 m and 15 my of Devonian deposition in the Appalachian basin and provides a framework for comparative study of organic matter burial. The major conclusions are: (1) few organic-rich units were deposited under pervasive anoxic–sulfidic water columns; (2) establishment and breakdown of seasonal thermoclines, on annual or longer timescales, were the predominant mode of stratification; and (3) under such conditions, remineralization of bio-limiting nutrients may have played a key role in organic matter burial by creating a “eutrophication pump.” This pump may have augmented an already rising nutrient inventory such that productivity levels exceeded the threshold required for development of suboxic to anoxic conditions in sediments, and episodically in bottom waters. A final conclusion asserts that the master variable for organic matter accumulation was relative sea-level change, which exerted influence on clastic dilution, preservation, and production processes. Sea-level rise events led to sediment starvation and organic carbon concentration in distal basin sediments, as well as to decreased effectiveness of seasonal mixing and thus longer build-up intervals for remineralized nutrients. Episodic mixing of nutrient-enriched bottom waters led to enhanced production. Ultimately, increased clastic sediment delivery and water column mixing during relative sea-level fall diluted surface sediment organic content such that respiratory demand could be met by increased oxygen supply, thus terminating deposition of strata enriched in organic carbon.

Based on study of modern foraminifera, changes in test porosity of ancient planktic foraminiferal specimens may be interpreted to reflect primarily changes in water temperature. To test this hypothesis, planktic foraminiferal porosity data (percent pore space per unit area) of Hedbergella delrioensis from three localities in the Cretaceous Western Interior basin were analyzed in the context of basin history (e.g., relative sea-level). These data record spatial and temporal changes in water masses at the average living depth of H. delrioensis . A significant increase in porosity between the upper Cenomanian M. mosbyense biozone and lower S. gracile biozone suggests an increase in water mass temperature, reflecting northward advection of a central core of warmer (less dense) water into the central seaway. The spread of this water mass toward the north, and laterally east and west, correlates with an independent record of rising sea-level. In addition to this secular porosity trend, higher frequency fluctuations may reflect transient oceanographic events related to changes in sea-level or circulation. In this study, porosity measurement was improved utilizing a scanning electron microscope and image analysis software. The work confirms prior results that as few as five foraminifera are required to produce a statistical assessment of average sample porosity. Use of pore concentration (number of pores per unit area), previously suggested as a temperature proxy, was not supported. The major interpretative contribution is a temporally precise record of spatial changes in Western Interior water masses.

An integrated lithologic, paleontologic, and multi-proxy geochemical study of the Middle Devonian Oatka Creek Formation, a black shale in the northern Appalachian Basin, indicates that a number of different factors contributed to organic carbon-rich black shale deposition. Conditions leading to this organic-rich sedimentary deposit were ultimately controlled by a relative sealevel rise, dominantly eustatic but with possible contributions from local tectonics, which cut off the supply of carbonate to the basin. Geochemical proxy evidence –such as Mo/Ti, Fe/Ti, Corg, Spy, and δ34Spy –suggests that as sealevel continued to rise after the carbonate supply was cut off, a threshold was crossed at which point conditions in the basin shifted from dominantly anoxic to dominantly euxinic (anoxic-sulfidic bottom waters). Concurrent with the shift to dominantly euxinic conditions, the supply of siliciclastic sediments was cut off, resulting in a condensed horizon, as evidenced by the elemental ratios of Si/Al and K/(Fe+Mg) and the relative concentration of eolian silt as determined petrographically and from scanning electron microscopy. Sediment starvation in the basin appears to have facilitated the biogeochemical (re)cycling of C, N, and P. Specifically, the elemental ratios of C, N, and P and the stable carbon isotope composition of organic matter suggest that the preferential regeneration of P under anoxic conditions (and of N during the oxic phase of oxic/anoxic oscillation) led to enhanced primary production in surface waters, thereby maintaining euxinic conditions in the bottom waters through respiration of settling organic matter. Finally, it is observed that, though conditions in the basin seem to have remained consistently anoxic-sulfidic for some time after the initial shift to euxinic conditions, a progressive increase in siliciclastic sedimentation led to a corresponding decrease in the enrichment of various redox-related elements, illustrating the overriding control that sedimentation can have on geochemical proxy records.

Secular changes in bulk sedimentation have a significant impact on the preservation of climatic signals in rhythmically bedded strata. In this study a quantitative cyclostratigraphic methodology is developed to use these signal distortions as a means to reconstruct sedimentation. The method exploits a moving-window Fourier technique that we designate "evolutive harmonic analysis" (EHA). We calibrate this methodology to a specific set of geologic conditions (i.e., changes in bulk sedimentation) using a series of stratigraphic models and then apply the technique to the Cenomanian-Turonian Greenhorn Formation of the Western Interior basin. Application of EHA to strata that preserve a record of orbital forcing allows the reconstruction of a high-resolution sedimentation history in which bulk sedimentation rates and hiatuses are quantified. Using the data on bulk sedimentation rate, rates of accumulation for geochemical proxies representing detrital, biogenic, and authigenic contributions to the sediment can be calculated. By integrating these quantitative assessments of siliciclastic dilution, primary production, and depositional redox conditions within a high-resolution chronostratigraphic framework, we are able to deconvolve secular trends in hemipelagic sedimentation during the Late Cenomanian-Early Turonian in the Western Interior basin. This integrated quantitative stratigraphic approach allows a reevaluation of hypotheses for the accumulation of excess organic matter in the Western Interior basin during the interval representing Oceanic Anoxic Event II.

Integrated geochemical data suggest that black shale deposition in the Devonian Geneseo Formation of western New York was initiated by the coincidence of siliciclastic starvation and the intensification of seasonal water column stratification and mixing. Once established, however, black shale deposition was maintained through efficient recycling of biolimiting nutrients which enhanced primary productivity. Recycling efficiency was achieved through a positive feedback loop of oscillating benthic redox conditions that enhanced N and P regeneration from sediments, sustained high primary productivity by returning nutrients to the photic zone during mixing, and ensured a downward flux of organic matter that drove or enhanced the episodic development of benthic anoxia during stratification. This feedback was ultimately disrupted by rising siliciclastic influx, which diluted organic matter and restored benthic redox stability. The abrupt overturn of diverse, long-standing Appalachian basin marine communities may have been the result of trophic resource destabilization during Geneseo deposition.

The Late Devonian mass extinction was unusually protracted and ecologically selective, with preferential diversity losses among reefbuilding organisms and tropical, shallow-water faunas in general. We have investigated the link between the extinction’s unique characteristics and changes in biogeochemical cycling through analyses of the δ 13 C and C:N:P atomic ratios of organic matter buried across the Kellwasser Horizons in western New York State. Each horizon is characterized by (1) a long-term, + 4‰–5‰ excursion in δ 13 C, ~3‰ of which occurs within the horizon, and (2) a dramatic increase in the burial ratios of C:N:P, from values of ~100:15:1 to an average of ~5000:170:1. On the basis of these results, we propose that (1) increased efficiency of biolimiting nutrient recycling, resulting from cyclic water column stratification and mixing, promoted eutrophication during Kellwasser deposition in New York, and (2) the isotope excursions represent the composite effect of long-term, global organic C burial, and local changes in photosynthetic C isotope fractionation related to nutrient availability. This eutrophication model forges a mechanistic link between proposed Late Devonian climatic cooling and the selective demise of taxa likely to have been narrowly adapted to oligotrophic conditions.

This chapter provides an overview of the fundamental concepts and tools that have been applied to study organic carbon-rich facies and the history of changes in marine redox conditions that they uniquely record. It first develops a conceptual model for the processes and feedbacks involved during the deposition of fine-grained organic carbon-rich black shales and illustrates how geochemical proxies are developed from detrital, biogenic, and authigenic fluxes and processes. The proxies reviewed include some recently introduced metal isotope proxies (isotope systematics of Mo, Fe, and Re–Os), in addition to elemental and compound concentration/accumulation proxies (trace metal, biomarker, CaCO 3 , etc.) and stable isotopes (C and S) that have been in use for many years. A vast amount of recent work utilizing redox proxies has focused on Precambrian strata to reveal fundamental changes in the ocean–atmosphere system that influenced the evolution of life on the early Earth. Highlights from recent publications on Precambrian strata, along with selected studies of Phanerozoic and modern anoxic deposits (i.e., oceanic anoxic events and sapropels), are updated in this revision to provide a broad review of the concepts and methods used to study organic carbon-rich deposits.

Premise of the study Epiphyllous bryophytes are a highly characteristic feature of many humid tropical forest ecosystems. In contrast to the extensive fossil record for the leaves of their host plants, the record is virtually nonexistent for the epiphylls themselves, despite a fossil record for mosses that begins in the Middle Carboniferous Period, 330 million years ago. Methods Epifluorescence optical microscopy, scanning electron microscopy, and atomic force microscopy were employed to investigate an intimate association between a newly discovered epiphyllous moss and a Lauraceae plant host from the middle Cretaceous. Key results We describe the oldest fossil specimen of an epiphyllous moss, Bryiidites utahensis gen. et sp. nov., identified from an individual specimen only 450 µm long, situated on an approximately one millimeter square fossil leaf fragment. The moss epiphyll is exquisitely preserved as germinating spores and short-celled protonemata with transverse and oblique cross-walls closely matching those of extant epiphyllous mosses on the surface of the plant-leaf hosts. Conclusions The extension of the epiphyll record back to the middle Cretaceous provides fossil evidence for the appearance of epiphyllous mosses during the diversification of flowering plants, at least 95 million years ago. It also provides substantive evidence for a tropical maritime climate in central North America during the middle Cretaceous.

[1] An analysis of orbitally influenced climate-sensitive sedimentation is conducted across a meridional transect of the proto-North Atlantic, to reconstruct the behavior of the carbon cycle and climate during Oceanic Anoxic Event 2 (OAE 2; ∼94 Ma). Midlatitude to near equatorial sites spanning deep-ocean to shelf environments are evaluated using a new inverse method for the identification and calibration of orbital influence in ancient strata. The results yield consistent independent astrochronologies at each location—compatible with the recently published radioisotopic/astrochronologic time scale at the Cenomanian-Turonian GSSP—and thus provide a unified high-resolution temporal context for evaluation of the event. The proto-North Atlantic astrochronologies document an amplification of obliquity power during OAE 2, indicating that obliquity was the dominant pacemaker of organic matter accumulation by the latter portion of the event. The strong obliquity signal suggests an influence of high-latitude climate processes across midlatitude deep-ocean to tropical shelf environments. Changes in oceanic circulation during OAE 2 represent a likely mechanism for the propagation of a high latitude signal, and proxy data from Demerara Rise reveal the development of a new intermediate water mass source synchronous with the record of obliquity amplification. We hypothesize that a dense high-latitude water mass displaced a pre-existing nutrient-rich anoxic deep-water layer in the proto-North Atlantic (augmented by enhanced volcanism), driving an episode of high productivity that lasted 700–800 ka. The development and intensification of the new intermediate/deep water source was potentially driven by eustatic sea level rise, which created expansive seaways in mid to high latitudes.

Because of the need to understand the links and feedbacks of the carbon cycle during times of global greenhouse conditions numerous studies have focused on the Cretaceous climate. Much of what we know about this warm period in Earth's history comes from the study of ocean sediments recovered from both ocean drill cores and marine sediments exposed at the Earth's surface. In contrast, there are few studies of Cretaceous terrestrial sediments. The Songliao basin located in northeast China offers a unique opportunity to understand Cretaceous paleoclimate of terrestrial settings because it contains a nearly complete record of lacustrine sediments deposited throughout the Cretaceous and there is an active drilling program to recover core from this paleolake. We present carbon, oxygen and strontium isotopic data from ostracods collected from two drill cores (SK-1 (N) and SK-1 (S)) that cover a time interval that extends from the Turonian through the Maastrichtian. These data record robust isotopic trends with numerous carbon and oxygen isotope shifts that are both rapid and long-term. We tentatively interpret this record to reflect changes in both global climate and regional basin evolution. In the Turonian and Coniacian Qingshankou Formation we observe several carbon isotope shifts that appear to be correlative to marine isotopic records based upon timing and magnitude of the isotopic changes. We suggest that the carbon isotope record in the Songliao basin reflect the decrease in carbon isotope ratios following the strong positive excursion at the Cenomanian/Turonian boundary, a positive isotope excursion in the late Turonian, and the negative isotope shift that occurs at the Turonian/Coniacian boundary. Upward in the section, however, the marine and Songliao isotopic records diverge as sediment sources shift from the southwest, east and north to more northerly. Strontium isotopes record the change in source region as they increase markedly between the Coniacian/Santonian Yaojia and Santonian/Campanian Nenjiang Formations. As this is the first isotopic record of the Songliao basin we are cautious about many of our interpretations of these isotopic data. Yet it is clear that with tighter age constraints and focused sampling centered on many of the isotope excursions presented here will allow for a deeper understanding of the terrestrial response to changing climate and the hydrologic response to basin evolution.

Upper Cretaceous limestone-shale couplets developed within the late transgressive stage of the Greenhorn cyclothem may be correlated from carbonate-dominated (basinal) sequences in central Kansas and Colorado westward to clastic cycles in southern Utah. Six such basinal couplets have been traced to corresponding upward-coarsening progradational cycles developed on the western margin of the Western Interior basin. In the central basin in Colorado and Kansas, these sedimentary cycles are represented by limestone-shale and marlstone-shale couplets ∼0.5-1.0 m in thickness. More calcareous parts of these couplets may be correlated westward into condensed, fossiliferous concretion and shell beds in proximal offshore lithofacies of Arizona and Utah. These concretion and shell beds are physically traceable farther landward (westward) into bioturbated, fossil-rich, transgressive lag deposits that bound 10- to 20-m-thick coarsening-upward progradational strand-plain deposits (parasequences) in southwestern Utah. Thus, the progra-dational phase of parasequence deposition correlates with accumulation of clay-rich sediment in the central basin, and the transgressive phase is characterized by reduced terrigenous input and deposition of carbonate-rich sediment. We consider Milankovitch-style orbital forcing of climate and tectonically induced fluctuations in rates of foredeep basin subsidence as possible forcing mechanisms for these basinwide events. Based on the widespread distribution of the limestone-shale couplets, as well as on estimated sedimentation rates and geochronology, it has been widely speculated that these carbonate cycles reflect Milankovitch cycles with periodicities on the order of 20 k.y. to 100 k.y. If so, then stratigraphic data suggest that orbital forcing of climate affected eustasy and/or sediment input and biogenic production in the Western Interior basin. Alternatively, thrusting events in the Sevier orogenic belt may have produced episodic changes in the rates of foredeep basin subsidence and consequent changes in base level, which could have controlled the deposition of the Greenhorn parasequences and carbonate cycles. In either case, correlation of these units demonstrates a consistent basinwide sedimentary response to high-frequency base level or sediment input changes in the Western Interior epicontinental basin.

Organic-carbon-rich strata or "black shales," including dark gray to black, laminated, carbonaceous mudrocks characterized by impoverished benthonic faunas, or devoid of metazoan life, have long intrigued geol­ ogists, both because of their widespread distribution at certain times in the past and their early recognition as potential hydrocarbon source rocks. One of the major obsessions of many early workers, to the mid1900s, was the application of uniformitarian principles to depositional models for black shales. Thus, although it was recognized that some black shale units were unusually widespread, many early workers sought less extensive, modern analogues for environments of formation of dark-colored, rela

[1] In this study the controls on organic carbon burial and molybdenum accumulation in ancient strata are investigated through an integration of biogeochemical modeling and data analysis. Critical aspects of the study are employment of a biogeochemical model for organic matter degradation to explore the controls on pore water sulfide generation and authigenic molybdenum accumulation and use of accumulation rate data for Mo, OC, CaCO3, Fe, and Ti to reconstruct organic matter remineralization processes. The model results form a conceptual framework for the interpretation of primary production estimates and geochemical burial fluxes (calculated on the basis of a high-resolution orbital timescale) for the Cenomanian-Turonian (C-T) oceanic anoxic event 2 interval in the Western Interior basin. The results of this study suggest that the strong correlation between source rock development and intervals of transgression in the geologic record (such as the C-T) could reflect a confluence of biogeochemical processes within sediments that obviates the need for large-magnitude changes in primary production levels or oceanographic conditions (such as prolonged periods of stable water column stratification).

The Tinajitas and Peas Blancas Limestones are thin bioclastic carbonate units of Eocene age in the foreland fold and thrust belt of eastern Venezuela. They represent the only limestone deposits in the otherwise siliciclastic Cenozoic succession of the eastern Venezuelan shelf. The geometry of the carbonate units suggests a coast-parallel, bar-like form; the particle population (larger benthic forams, glauconite, and algal balls) indicates shallow-marine provenance; and there is a regional unconformity with major lacuna landward from the location of the limestone units. We interpret these carbonates as the capping deposit of a coast-parallel ridge on the continental shelf that rose and sank in the Eocene. We propose that this transient ridge was an elastic forebulge that migrated landward across the shelf and craton hinge of the South American continent because of a brief episode of Eocene subduction of northern South American lithosphere. Such subduction may have been caused by closure between the North and South American plates. The bulge had an amplitude of at least 800 m and migrated landward at least 50–100 km. This distance suggests the minimum closure between the plates at the subduction zone. Supporting evidence comes from facies suites of the passive-margin wedge, now juxtaposed in the thrust and fold belt. The most seaward facies suite contains no evident Eocene lacuna or limestone; these facies were at too great a depth for the passing bulge to affect sedimentation. In the shallower intermediate facies, which contains the Tinajitas–Peas Blancas Limestones, the passing ridge brought the seabed to depths of carbonate production, but not to shoal. In the most landward facies of the passive-margin wedge and on the platform, the ridge caused emergence and erosion, producing the Eocene unconformity of large coast-parallel extent. The resubsidence of the margin in the Oligocene is attributed to passage of a foreland basin, which trailed the forebulge.

The limestone/marlstone bedding couplets of the Bridge Creek Limestone Member, Cenomanian-Turonian Greenhorn Formation, were analyzed by applying spectral techniques to high-resolution lithologic and geochemical data from a core. The results suggest that the Bridge Creek contains a complex record of orbital cyclicity. The dominant signal appears to be obliquity, but signals corresponding to precession and eccentricity were also observed. The development of the bedding couplets is interpreted to have resulted from a combination of factors, including insolation-controlled changes in higher-latitude precipitation leading to dilution/redox cycles, and in lower-latitude evaporation, leading to changes in surface water conditions and productivity cycles in the calcareous plankton. The data nterpreted to reflect redox cycles appear to be more strongly influenced by obliquity, and show a weak precessional signal. In contrast, trends in the carbonate record show the opposite response. The complex bedding pattern observed in the Bridge Creek Limestone is interpreted to result from the competing influences of different orbital cycles expressed through different pathways of the depositional system, and was also affected by changes in sedimentation rates related to relative sea level fluctuations, aperiodic dilution by volcanic ash, and changes in organic-matter production and redox conditions related to a global "oceanic anoxic event". These factors complicate cycle analysis in the lower part of the member but leave a relatively undisturbed record in the upper Bridge Creek Limes one.

A combination of biostratigraphic markers (ammonites, inoceramid bivalves) and carbon isotope excursions is employed to establish a high-resolution correlation between the middle to late Cenomanian successions of the Western Interior Basin (USA) and the Anglo-Paris Basin (southern UK). Sequences identified from sedimentologic criteria in the Pueblo succession and elsewhere in the Western Interior Basin are shown to coincide precisely with globally recognized sea-level events and were therefore under eustatic control. This evidence refutes arguments that Cenomanian sequences in the Western Interior Basin were formed by local tectonic events. The interaction of longer-term tectonic movements and more rapid eustatic change may have simply enhanced the amount of erosion associated with sequence boundaries. A crossplot of radiometric ages derived from North American bentonites against an orbitally tuned time scale developed in the Anglo-Paris Basin provides support for the argument that the sequences were controlled by the 405-k.y.-long eccentricity cycle. © 2008 The Geological Society of America.

The stagnant basin model that is widely applied to organic-rich dark shales and laminated marls and chalks has led to the view that sea-bottom conditions during accumulation were persistently anaerobic to dysaerobic. There are however, clear indications from the presence of trace and body fossils, that the conditions were much more variable. First, there are individuals and communities adapted to lower-than-normal oxygen levels: microbial mats and “flat clams” (Inoceramidae,Posidoniidae,Monotidae, and small Pectinidae). Second, there are event populations, bedding-plane concentrations of many individuals of few species that flourished during short periods (months to years?) of higher or lower oxygenation. Third, there are often transported assemblages of taxa from adjacent shelf or slope environments or from the mass mortality of pelagic organisms.

Previous time scales for the Cenomanian-Turonian boundary (CTB) interval containing Oceanic Anoxic Event II (OAE II) vary by a factor of three. In this paper we present a new orbital time scale for the CTB stratotype established independently of radiometric, biostratigraphic, or geochemical data sets, update revisions of CTB biostratigraphic zonation, and provide a new detailed carbon isotopic record for the CTB study interval. The orbital time scale allows an independent assessment of basal biozone ages relative to the new CTB date of 93.55 Ma (GTS04). The d 13 Corg data document the abrupt onset of OAE II, significant variability in d 13 Corg values, and values enriched to almost 222‰. These new data underscore the difficulty in defining OAE II termination. Using the new isotope curve and time scale, estimates of OAE II duration can be determined and exported to other sites based on integration of well-established chemostratigraphic and biostratigraphic datums. The new data will allow more accurate calculations of biogeochemical and paleobiologic rates across the CTB.

Many ancient rhythmic hemipelagic sequences have been interpreted to record orbital variations, but the exact nature of the climatic and depositional transfer functions responsible for this link remains poorly understood. Two-dimensional numerical simulations were used to explore selected aspects of orbital signal distortion in linked siliciclastic and hemipelagic systems. The models suggest that transfer of multiorder (e.g., 20, 100, and 400 k.y.) oscillations in relative sea level into the hemipelagic record produces an inherent amplitude distortion of the shorter-period (e.g., 20 k.y.) cycle. This distortion gives rise to amplitude modulation (AM), which is qualitatively similar to AM of orbitally driven changes in insolation (e.g., eccentricity modulation of precession-driven cycles). However, unlike the orbitally driven AM, synthesized AM is distinctly phase shifted relative to the stratigraphic record of the long-period (e.g., 100 k.y., 400 k.y.) cycle as a result of sealevel‐driven changes in the storage capacity of nearshore through alluvial parts of the source siliciclastic system. Hence, multiorder changes in sea level can leave a distinct AM signature in dilution-affected hemipelagic records, thus making hemipelagic rhythms due to eccentricity-forced sea-level changes distinguishable from other types of orbitally driven hemipelagic cyclicity.

The first systematic test for a predictive relationship between organic carbon content and stratigraphic hierarchy in a deep-water slope to basin-floor deposit was performed. The studied section includes the Pipeline Shale, the Brushy Canyon Formation, and the lower part of the Cherry Canyon Formation of the Delaware Mountain Group, West Texas. This interval represents one large-scale, 3rd-order genetic sequence within which 4th- and 5th-order stratigraphic cycles are recognized. Samples of fine-grained facies throughout the section were collected from outcrop and analyzed for organic carbon content and hydrogen index. Degree of pyritization was also determined for a subset of the samples. The results indicate that organic enrichment is closely correlated to the stratigraphic hierarchy at the 3rd-, 4th-, and 5th-order levels. The data suggest that quantity and quality of preserved organic matter are controlled by changes in bulk sedimentation rate (dilution vs. condensation), which affect organic matter inputs to the sediment, as well as the balance between (1) burial and preservation of organic matter and (2) its degradation on the sea floor during times of sediment starvation.

Questions concerning the application of established biofacies models to mid-Cretaceous black shales prompted a study of diversity characteristics in a fauna from the Late Cenomanian Hartland Shale Member, Western Interior basin. Numerical faunal data are used to assess species abundance patterns, and a new method of analyzing diversity is introduced that incorporates species richness, Shannon index, and equitability into a single plot. In addition, numerical simulations designed to emulate the sampling of species-abundance distributions are used to improve data interpretation. The study illustrates how measured diversity results from the combination of primary ecological controls and sampling effects. Proximal offshore assemblages are characterized by high diversity and log series species-abundance patterns, interpreted as truncated (incompletely sampled) log-normal distributions. Primary ecological controls include variable physical/chemical parameters, biological factors such as predation and competition, and intermediate disturbance frequency. Distal offshore assemblages are characterized by low diversity with patterns of species abundance resembling geometric series. These are interpreted as truncated log-series distributions (sampling effect) that reflect dominance of multiple opportunists, abundant resources in a dysoxic environment, and high disturbance frequency. The data are used to develop an ecological model for diversity levels in basinal black shale facies based on the interplay of recruitment, growth rate, tolerance to low oxygen and sulphide, and disturbance frequency (due to fluctuations of the redox boundary). Although certain taxa (chiefly Inoceramidae) evolved highly opportunistic life strategies to exploit basinal paleoenvironments, it was the unpredictable interaction of these four factors that determined diversity patterns. Analysis of Hartland Shale biofacies illustrates the difficulties in applying a strictly linear relationship between paleo-oxygen levels and diversity.