Stephen R. Meyers

Orbital time scale and new C-isotope record for Cenomanian-Turonian boundary stratotype

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.

Intercalibration of radioisotopic and astrochronologic time scales for the Cenomanian-Turonian boundary interval, Western Interior Basin, USA

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.

Quantification of deep-time orbital forcing by average spectral misfit

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.

Integrating 40Ar/39Ar, U-Pb, and astronomical clocks in the Cretaceous Niobrara Formation, Western Interior Basin, USA

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.

Resolving Milankovitchian controversies: The Triassic Latemar Limestone and the Eocene Green River Formation

Although orbital forcing is commonly proposed as the driver of ancient sedimentary rhythms, the lack of adequate independent time control (radio isotopic data) to calibrate these cycles has stood as a major challenge to evaluation of the hypothesis. Here I apply a new statistical approach to evaluate cyclicity in two historically important rhythmic units for which orbital forcing has been proposed: the Triassic Latemar Limestone (Dolomites, Italy) and the Eocene Green River Formation (Wyoming, USA). A major advance of the new method is its explicit evaluation of the null hypothesis of no orbital signal. The null hypothesis can be rejected with a high degree of confi dence in the Latemar Limestone (probability <0.30%) and Green River Formation (probability <0.07%). The analyses also resolve controversies about the specifi c orbital calibrations at each site. Both data series reveal the expected precession, obliquity, and eccentricity orbital components, and yield astrochronologies that are consistent with proposed radio isotopic based time scales.

Resolving Milankovitch: Consideration of signal and noise

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 highlights 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.

Testing the astronomical time scale for oceanic anoxic event 2, and its extension into Cenomanian strata of the Western Interior Basin (USA)

The development of integrated astronomical and radioisotopic time scales from rhythmic strata of the Western Interior Basin (WIB) has played a fundamental role in the refi nement of Late Cretaceous chronostratigraphy. In this study, X-ray fl uorescence (XRF) core scanning is utilized to develop a new elemental data set for cyclostratigraphic investigation of CenomanianTuronian strata in the WIB, using material from the Aristocrat-Angus-12-8 core (northcentral Colorado). The XRF data set yields the fi rst continuous 5-mm-resolution analysis of lithogenic, biogenic, and syngeneticauthigenic proxies through the uppermost Lincoln Limestone Member, the Hartland Shale Member, and the Bridge Creek Limestone Member, including oceanic anoxic event 2 (OAE 2). The 40 Ar/ 39 Ar ages from ashes in three biozones, including a new age from the Dunveganoceras pondi biozone (uppermost Lincoln Limestone Member), provide geochronologic constraints for the cyclostratigraphic analysis. Astrochronologic testing of the 5-mm-resolution XRF weight percent CaCO3 data via average spectral misfi t analysis yields strong evidence for astronomical infl uence on climate and sedimentation. Results from the Bridge Creek Limestone Member are consistent with the previously published astrochronology from the U.S. Geological Survey #1 Portland core (central Colorado), and identifi cation of an astronomical signal in the underlying Hartland Shale Member now permits extension of the WIB astrochronology into the earlier Cenomanian, prior to OAE 2. High rates of sedimentation in the Angus core during the interval of OAE 2 initiation, as compared to the Portland core, allow recognition of a strong precessional control on bedding development. As a consequence, the new results provide a rare high-resolution chronometer for the onset of OAE 2, and the timing of proposed hydrothermal trace metal enrichment as observed in the 5 mm XRF data.

Testing astronomically tuned age models

Astrochronology is fundamental to many paleoclimate studies, but a standard statistical test has yet to be established for validating stand-alone astronomically tuned time scales (those lacking detailed independent time control) against their astronomical insolation tuning curves. Shackleton et al. (1995) proposed that the modulation of precessions amplitude by eccentricity can be used as an independent test for the successful tuning of paleoclimate data. Subsequent studies have demonstrated that eccentricity-like amplitude modulation can be artificially generated in random data, following astronomical tuning. Here we introduce a new statistical approach that circumvents the problem of introducing amplitude modulations during tuning and data processing, thereby allowing the use of amplitude modulations for astronomical time scale evaluation. The method is based upon the use of the Hilbert transform to calculate instantaneous amplitude following application of a wide band precession filter and subsequent low-pass filtering of the instantaneous amplitude to extract potential eccentricity modulations. Statistical significance of the results is evaluated using phase-randomized surrogates that preserve the power spectrum structure of the data but have randomized amplitude modulations. Application of the new testing algorithm to two astronomically tuned data sets demonstrates the efficacy of the technique and confirms the presence of astronomical signals. Additionally, it is demonstrated that a minimal tuning approach using (at maximum) one precession cycle per ~100 kyr eccentricity cycle does not introduce systematic frequency modulations, even when a narrow band-pass filter is applied, allowing direct comparison of data amplitudes and orbital eccentricity.

Axial obliquity control on the greenhouse carbon budget through middle- to high-latitude reservoirs

Carbon sources and sinks are key components of the climate feedback system, yet their response to external forcing remains poorly constrained, particularly for past greenhouse climates. Carbon-isotope data indicate systematic, million-year-scale transfers of carbon between surface reservoirs during and immediately after the Late Cretaceous thermal maximum (peaking in the Cenomanian-Turonian, circa 97–91 million years, Myr, ago). Here we calibrate Albian to Campanian (108–72 Myr ago) high-resolution carbon isotope records with a refined chronology and demonstrate how net transfers between reservoirs are plausibly controlled by ~1 Myr changes in the amplitude of axial obliquity. The amplitude-modulating terms are absent from the frequency domain representation of insolation series and require a nonlinear, cumulative mechanism to become expressed in power spectra of isotope time series. Mass balance modeling suggests that the residence time of carbon in the ocean-atmosphere system is—by itself—insufficient to explain the Myr-scale variability. It is proposed that the astronomical control was imparted by a transient storage of organic matter or methane in quasi-stable reservoirs (wetlands, soils, marginal zones of marine euxinic strata, and potentially permafrost) that responded nonlinearly to obliquity-driven changes in high-latitude insolation and/or meridional insolation gradients. While these reservoirs are probably underrepresented in the geological record due to their quasi-stable character, they might have provided an important control on the dynamics and stability of the greenhouse climate.

Basin-scale cyclostratigraphy of the Green River Formation, Wyoming

The fl uviolacustrine Wilkins Peak Member of the Eocene Green River Formation preserves repetitive sedimentary facies that have been interpreted as an orbitally induced climate signal. However, previous quantitative studies of cyclicity in this member have used oil-yield data derived from single locations. Here, macrostratigraphy is used to quantitatively describe the spatiotemporal patterns of three different lithofacies associations from 8 to 12 localities that span much of the basin. Macrostratigraphic time series demonstrate that there is a reciprocal basinscale relationship between carbonate-rich lacustrine facies and siliciclastic-rich alluvial facies. Spectral analyses identify statistically signifi cant periods (≥90% confi dence level) in basin-scale sedimentation that are consistent with Milankovitch-predicted orbital periodicities, with a particularly strong ~100 k.y. cycle expressed in all lithofacies associations. Numerous non-Milankovitch periods are also recognized, indicating complex depositional responses to orbital forcing, autocyclic controls on sedimentation, or harmonic artifacts. Although fl uctuations in Lake Gosiute water level did affect basin-scale patterns of sedimentation, they are not directly related to the 100 k.y. short-eccentricity cycle, as previously supposed. Instead, 100 k.y. cycles are principally recorded by the recurrence of alluvial environments, which exerted a dominant control on basin-scale patterns of sedimentation generally. Thus, the hydrologic controls on lake level that have been classically linked to short-eccentricity actually occurred at finer temporal scales (<100 k.y.). Understanding the complex links between orbital forcing and sedimentation in the Wilkins Peak Member is facilitated by analysis of time series that refl ect spatial as well as temporal variability in stratigraphic data. Macrostratigraphy is, therefore, promising as an analytical tool for basin-scale cyclostratigraphy.