Bradley B. Sageman

A tale of shales: the relative roles of production, decomposition, and dilution in the accumulation of organic-rich strata, Middle–Upper Devonian, Appalachian basin

Abstract 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.

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.

Black shale deposition and faunal overturn in the Devonian Appalachian Basin: Clastic starvation, seasonal water‐column mixing, and efficient biolimiting nutrient recycling

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.

Eutrophication by decoupling of the marine biogeochemical cycles of C, N, and P: A mechanism for the Late Devonian mass extinction

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.

Estuarine circulation in the Turonian Western Interior seaway of North America

To understand the patterns of lithofacies, marine faunas, organic-carbon enrichment, isotopes, and trace elements deposited in the early Turonian Western Interior seaway, we conducted circulation experiments using a three-dimensional, turbulent flow, coastal ocean model driven by GENESIS, a climate model developed at the National Center for Atmospheric Research (NCAR). Circulation and chemical evolution of the seaway waters are computed under the following initial and boundary conditions: (1) paleobathymetry according to a new interpretation of the lithostratigraphy and biostratigraphy; (2) temperatures and salinities of the Boreal and Tethys oceans and adjacent drainage basins based on isotopic data, atmospheric temperatures, and precipitation-evaporation magnitudes computed by GENESIS; and (3) mean annual wind stresses over the seaway computed by GENESIS. Results show that the seaway exported freshened water much like Hudson Bay today. Runoff from eastern drainages exited the seaway as a northern coastal jet; runoff from western drainages exited as a southern coastal jet. Both jets simultaneously drew in surface Tethyan and Boreal waters, creating a strong counterclockwise gyre occupying the entire north-south extent of the seaway. The curious stratal and faunal variations of the early Turonian deposits arise from this gyre and its associated water masses.

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.

Correlation of basinal carbonate cycles to nearshore parasequences in the Late Cretaceous Greenhorn seaway, Western Interior U.S.A.

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.

Eustatic sea-level record for the Cenomanian (Late Cretaceous)—Extension to the Western Interior Basin, USA

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.

When do black shales tell molybdenum isotope tales

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.

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.